Formalization, equivalence and generalization of basic resonance electrical circuits

NASA Astrophysics Data System (ADS)

Penev, Dimitar; Arnaudov, Dimitar; Hinov, Nikolay

2017-12-01

In the work are presented basic resonance circuits, which are used in resonance energy converters. The following resonant circuits are considered: serial, serial with parallel load parallel capacitor, parallel and parallel with serial loaded inductance. For the circuits under consideration, expressions are generated for the frequencies of own oscillations and for the equivalence of the active power emitted in the load. Mathematical expressions are graphically constructed and verified using computer simulations. The results obtained are used in the model based design of resonant energy converters with DC or AC output. This guaranteed the output indicators of power electronic devices.

DGDFT: A massively parallel method for large scale density functional theory calculations.

PubMed

Hu, Wei; Lin, Lin; Yang, Chao

2015-09-28

We describe a massively parallel implementation of the recently developed discontinuous Galerkin density functional theory (DGDFT) method, for efficient large-scale Kohn-Sham DFT based electronic structure calculations. The DGDFT method uses adaptive local basis (ALB) functions generated on-the-fly during the self-consistent field iteration to represent the solution to the Kohn-Sham equations. The use of the ALB set provides a systematic way to improve the accuracy of the approximation. By using the pole expansion and selected inversion technique to compute electron density, energy, and atomic forces, we can make the computational complexity of DGDFT scale at most quadratically with respect to the number of electrons for both insulating and metallic systems. We show that for the two-dimensional (2D) phosphorene systems studied here, using 37 basis functions per atom allows us to reach an accuracy level of 1.3 × 10(-4) Hartree/atom in terms of the error of energy and 6.2 × 10(-4) Hartree/bohr in terms of the error of atomic force, respectively. DGDFT can achieve 80% parallel efficiency on 128,000 high performance computing cores when it is used to study the electronic structure of 2D phosphorene systems with 3500-14 000 atoms. This high parallel efficiency results from a two-level parallelization scheme that we will describe in detail.

Electromagnetic cyclotron-loss-cone instability associated with weakly relativistic electrons

NASA Technical Reports Server (NTRS)

Wong, H. K.; Wu, C. S.; Ke, F. J.; Schneider, R. S.; Ziebell, L. F.

1982-01-01

The amplification of fast extraordinary mode waves at frequencies very close to the electron cyclotron frequency, due to the presence of a population of energetic electrons with a loss-cone type distribution, is studied. Low-energy background electrons are included in the analysis. Two types of loss-cone distribution functions are considered, and it is found that the maximum growth rates for both distribution functions are of the same order of magnitude. When the thermal effects of the energetic electrons are included in the dispersion equation, the real frequencies of the waves are lower than those obtained by using the cold plasma approximation. This effect tends to enhance the growth rate. An idealized case including a parallel electric field such that the distribution function of the trapped energetic electrons is modified is also considered. It is assumed that the parallel electric field can remove the low-energy background electrons away from the source region of radiation. Both these effects increase the growth rate.

Mode Transitions in Hall Effect Thrusters

DTIC Science & Technology

2013-07-01

bM = number of pixels per bin m = spoke order 0m = spoke order m = 0 em = electron mass, 9.1110 -31 kg im = Xe ion mass, 2.18×10 -25...periodogram spectral estimate, Arb Hz -1 eT = electron temperature eT = electron temperature parallel to magnetic field, eV eT  = electron ...Fourier transform of x(t)  = inverse angle from 2D DFT, deg-1  = mean electron energy, eV * = material dependent cross-over energy, eV xy

The calibration of plane parallel ionisation chambers for the measurement of absorbed dose in electron beams of low to medium energies. Part 2: The PTW/MARKUS chamber.

PubMed

Cross, P; Freeman, N

1997-06-01

The purpose of Part 2 study of calibration methods for plane parallel ionisation chambers was to determine the feasibility of using beams of calibration of the MARKUS chamber other than the standard AAPM TG39 reference beams of 60Co and a high energy electron beam (E0 > or = 15 MeV). A previous study of the NACP chamber had demonstrated an acceptable level of accuracy with corresponding spread of -0.5% to +0.8% for its calibration in non-standard situations (medium to low energy electron and photon beams). For non-standard situations the spread in NDMARKUS values was found to be +/-2.5%. The results suggest that user calibrations of the MARKUS chamber in non-standard situations are associated with more uncertainties than is the case with the NACP chamber.

Sheath energy transmission in a collisional plasma with collisionless sheath

DOE PAGES

Tang, Xian-Zhu; Guo, Zehua

2015-10-16

Sheath energy transmission governs the plasma energy exhaust onto a material surface. The ion channel is dominated by convection, but the electron channel has a significant thermal conduction component, which is dominated by the Knudsen layer effect in the presence of an absorbing wall. First-principle kinetic simulations also reveal a robustly supersonic sheath entry flow. The ion sheath energy transmission and the sheath potential are accurately predicted by a sheath model of truncated bi-Maxwellian electron distribution. The electron energy transmission is further enhanced by a parallel heat flux of the perpendicular degrees of freedom.

Solar Wind Electron Interaction with the Dayside Lunar Surface and Crustal Magnetic Fields: Evidence for Precursor Effects

NASA Technical Reports Server (NTRS)

Halekas, Jasper S.; Poppe, A.; Delory, G. T.; Farrell, W. M.; Horanyi, M.

2012-01-01

Electron distributions measured by Lunar Prospector above the dayside lunar surface in the solar wind often have an energy dependent loss cone, inconsistent with adiabatic magnetic reflection. Energy dependent reflection suggests the presence of downward parallel electric fields below the spacecraft, possibly indicating the presence of a standing electrostatic structure. Many electron distributions contain apparent low energy (<100 eV) upwardgoing conics (58% of the time) and beams (12% of the time), primarily in regions with non-zero crustal magnetic fields, implying the presence of parallel electric fields and/or wave-particle interactions below the spacecraft. Some, but not all, of the observed energy dependence comes from the energy gained during reflection from a moving obstacle; correctly characterizing electron reflection requires the use of the proper reference frame. Nonadiabatic reflection may also play a role, but cannot fully explain observations. In cases with upward-going beams, we observe partial isotropization of incoming solar wind electrons, possibly indicating streaming and/or whistler instabilities. The Moon may therefore influence solar wind plasma well upstream from its surface. Magnetic anomaly interactions and/or non-monotonic near surface potentials provide the most likely candidates to produce the observed precursor effects, which may help ensure quasi-neutrality upstream from the Moon.

Parallel Decomposition of the Fictitious Lagrangian Algorithm and its Accuracy for Molecular Dynamics Simulations of Semiconductors.

NASA Astrophysics Data System (ADS)

Yeh, Mei-Ling

We have performed a parallel decomposition of the fictitious Lagrangian method for molecular dynamics with tight-binding total energy expression into the hypercube computer. This is the first time in literature that the dynamical simulation of semiconducting systems containing more than 512 silicon atoms has become possible with the electrons treated as quantum particles. With the utilization of the Intel Paragon system, our timing analysis predicts that our code is expected to perform realistic simulations on very large systems consisting of thousands of atoms with time requirements of the order of tens of hours. Timing results and performance analysis of our parallel code are presented in terms of calculation time, communication time, and setup time. The accuracy of the fictitious Lagrangian method in molecular dynamics simulation is also investigated, especially the energy conservation of the total energy of ions. We find that the accuracy of the fictitious Lagrangian scheme in small silicon cluster and very large silicon system simulations is good for as long as the simulations proceed, even though we quench the electronic coordinates to the Born-Oppenheimer surface only in the beginning of the run. The kinetic energy of electrons does not increase as time goes on, and the energy conservation of the ionic subsystem remains very good. This means that, as far as the ionic subsystem is concerned, the electrons are on the average in the true quantum ground states. We also tie up some odds and ends regarding a few remaining questions about the fictitious Lagrangian method, such as the difference between the results obtained from the Gram-Schmidt and SHAKE method of orthonormalization, and differences between simulations where the electrons are quenched to the Born -Oppenheimer surface only once compared with periodic quenching.

Comparison of the IAEA TRS-398 and AAPM TG-51 absorbed dose to water protocols in the dosimetry of high-energy photon and electron beams

NASA Astrophysics Data System (ADS)

Saiful Huq, M.; Andreo, Pedro; Song, Haijun

2001-11-01

The International Atomic Energy Agency (IAEA TRS-398) and the American Association of Physicists in Medicine (AAPM TG-51) have published new protocols for the calibration of radiotherapy beams. These protocols are based on the use of an ionization chamber calibrated in terms of absorbed dose to water in a standards laboratory's reference quality beam. This paper compares the recommendations of the two protocols in two ways: (i) by analysing in detail the differences in the basic data included in the two protocols for photon and electron beam dosimetry and (ii) by performing measurements in clinical photon and electron beams and determining the absorbed dose to water following the recommendations of the two protocols. Measurements were made with two Farmer-type ionization chambers and three plane-parallel ionization chamber types in 6, 18 and 25 MV photon beams and 6, 8, 10, 12, 15 and 18 MeV electron beams. The Farmer-type chambers used were NE 2571 and PTW 30001, and the plane-parallel chambers were a Scanditronix-Wellhöfer NACP and Roos, and a PTW Markus chamber. For photon beams, the measured ratios TG-51/TRS-398 of absorbed dose to water Dw ranged between 0.997 and 1.001, with a mean value of 0.999. The ratios for the beam quality correction factors kQ were found to agree to within about +/-0.2% despite significant differences in the method of beam quality specification for photon beams and in the basic data entering into kQ. For electron beams, dose measurements were made using direct ND,w calibrations of cylindrical and plane-parallel chambers in a 60Co gamma-ray beam, as well as cross-calibrations of plane-parallel chambers in a high-energy electron beam. For the direct ND,w calibrations the ratios TG-51/TRS-398 of absorbed dose to water Dw were found to lie between 0.994 and 1.018 depending upon the chamber and electron beam energy used, with mean values of 0.996, 1.006, and 1.017, respectively, for the cylindrical, well-guarded and not well-guarded plane-parallel chambers. The Dw ratios measured for the cross-calibration procedures varied between 0.993 and 0.997. The largest discrepancies for electron beams between the two protocols arise from the use of different data for the perturbation correction factors pwall and pdis of cylindrical and plane-parallel chambers, all in 60Co. A detailed analysis of the reasons for the discrepancies is made which includes comparing the formalisms, correction factors and the quantities in the two protocols.

Study of Wave-Particle Interactions for Whistler Mode Waves at Oblique Angles by Utilizing the Gyroaveraging Method

NASA Astrophysics Data System (ADS)

Hsieh, Yi-Kai; Omura, Yoshiharu

2017-10-01

We investigate the properties of whistler mode wave-particle interactions at oblique wave normal angles to the background magnetic field. We find that electromagnetic energy of waves at frequencies below half the electron cyclotron frequency can flow nearly parallel to the ambient magnetic field. We thereby confirm that the gyroaveraging method, which averages the cyclotron motion to the gyrocenter and reduces the simulation from two-dimensional to one-dimensional, is valid for oblique wave-particle interaction. Multiple resonances appear for oblique propagation but not for parallel propagation. We calculate the possible range of resonances with the first-order resonance condition as a function of electron kinetic energy and equatorial pitch angle. To reveal the physical process and the efficiency of electron acceleration by multiple resonances, we assume a simple uniform wave model with constant amplitude and frequency in space and time. We perform test particle simulations with electrons starting at specific equatorial pitch angles and kinetic energies. The simulation results show that multiple resonances contribute to acceleration and pitch angle scattering of energetic electrons. Especially, we find that electrons with energies of a few hundred keV can be accelerated efficiently to a few MeV through the n = 0 Landau resonance.

Anisotropic pitch angle distribution of 50 eV to 50 keV particles at synchronous altitude.

NASA Technical Reports Server (NTRS)

Deforest, S. E.; Mcilwain, C. F.

1972-01-01

At times, the electron pitch angle distributions at synchronous orbit have been observed to be highly anisotropic. In the local morning region, distributions concentrated near 90 deg are often observed in particles of less than approximately 2000 V. This anisotropy decreases with increasing energy from 1 keV to the detector's limit at 50 keV. The time development of anisotropy is consistent with production by pitch angle scattering processes which are not effective on electrons with small velocities parallel to the magnetic field. Another type of distribution has been observed with the low-energy (below 1000 V) electrons concentrated parallel and antiparallel to the magnetic field. These distributions are only seen in the dusk sector, but this may be an orbital artifact.

Determination of Fermi contour and spin polarization of ν = 3 2 composite fermions via ballistic commensurability measurements

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kamburov, D.; Mueed, M. A.; Jo, I.

2014-12-01

We report ballistic transport commensurability minima in the magnetoresistance of nu = 3/2 composite fermions (CFs). The CFs are formed in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to an in-plane, unidirectional periodic potential modulation. We observe a slight asymmetry of the CF commensurability positions with respect to nu = 3/2, which we explain quantitatively by comparing three CF density models and concluding that the nu = 3/2 CFs are likely formed by the minority carriers in the upper energy spin state of the lowest Landau level. Our data also allow us to probe themore » shape and size of the CF Fermi contour. At a fixed electron density of similar or equal to 1.8x10(11) cm(-2), as the quantum well width increases from 30 to 60 nm, the CFs show increasing spin polarization. We attribute this to the enhancement of the Zeeman energy relative to the Coulomb energy in wider wells where the latter is softened because of the larger electron layer thickness. The application of an additional parallel magnetic field (B-parallel to) leads to a significant distortion of the CF Fermi contour as B-parallel to couples to the CFs' out-of-plane orbital motion. The distortion is much more severe compared to the nu = 1/2 CF case at comparable B-parallel to. Moreover, the applied B-parallel to further spin-polarizes the nu = 3/2 CFs as deduced from the positions of the commensurability minima.« less

Wave energy budget analysis in the Earth’s radiation belts uncovers a missing energy

PubMed Central

Artemyev, A.V.; Agapitov, O.V.; Mourenas, D.; Krasnoselskikh, V.V.; Mozer, F.S.

2015-01-01

Whistler-mode emissions are important electromagnetic waves pervasive in the Earth’s magnetosphere, where they continuously remove or energize electrons trapped by the geomagnetic field, controlling radiation hazards to satellites and astronauts and the upper-atmosphere ionization or chemical composition. Here, we report an analysis of 10-year Cluster data, statistically evaluating the full wave energy budget in the Earth’s magnetosphere, revealing that a significant fraction of the energy corresponds to hitherto generally neglected very oblique waves. Such waves, with 10 times smaller magnetic power than parallel waves, typically have similar total energy. Moreover, they carry up to 80% of the wave energy involved in wave–particle resonant interactions. It implies that electron heating and precipitation into the atmosphere may have been significantly under/over-valued in past studies considering only conventional quasi-parallel waves. Very oblique waves may turn out to be a crucial agent of energy redistribution in the Earth’s radiation belts, controlled by solar activity. PMID:25975615

Wave energy budget analysis in the Earth's radiation belts uncovers a missing energy.

PubMed

Artemyev, A V; Agapitov, O V; Mourenas, D; Krasnoselskikh, V V; Mozer, F S

2015-05-15

Whistler-mode emissions are important electromagnetic waves pervasive in the Earth's magnetosphere, where they continuously remove or energize electrons trapped by the geomagnetic field, controlling radiation hazards to satellites and astronauts and the upper-atmosphere ionization or chemical composition. Here, we report an analysis of 10-year Cluster data, statistically evaluating the full wave energy budget in the Earth's magnetosphere, revealing that a significant fraction of the energy corresponds to hitherto generally neglected very oblique waves. Such waves, with 10 times smaller magnetic power than parallel waves, typically have similar total energy. Moreover, they carry up to 80% of the wave energy involved in wave-particle resonant interactions. It implies that electron heating and precipitation into the atmosphere may have been significantly under/over-valued in past studies considering only conventional quasi-parallel waves. Very oblique waves may turn out to be a crucial agent of energy redistribution in the Earth's radiation belts, controlled by solar activity.

Electron acceleration to high energies at quasi-parallel shock waves in the solar corona

NASA Technical Reports Server (NTRS)

Mann, G.; Classen, H.-T.

1995-01-01

In the solar corona shock waves are generated by flares and/or coronal mass ejections. They manifest themselves in solar type 2 radio bursts appearing as emission stripes with a slow drift from high to low frequencies in dynamic radio spectra. Their nonthermal radio emission indicates that electrons are accelerated to suprathermal and/or relativistic velocities at these shocks. As well known by extraterrestrial in-situ measurements supercritical, quasi-parallel, collisionless shocks are accompanied by so-called SLAMS (short large amplitude magnetic field structures). These SLAMS can act as strong magnetic mirrors, at which charged particles can be reflected and accelerated. Thus, thermal electrons gain energy due to multiple reflections between two SLAMS and reach suprathermal and relativistic velocities. This mechanism of accelerating electrons is discussed for circumstances in the solar corona and may be responsible for the so-called 'herringbones' observed in solar type 2 radio bursts.

Generation and evolution of anisotropic turbulence and related energy transfer in a multi-species solar wind

NASA Astrophysics Data System (ADS)

Maneva, Yana; Poedts, Stefaan

2017-04-01

The electromagnetic fluctuations in the solar wind represent a zoo of plasma waves with different properties, whose wavelengths range from largest fluid scales to the smallest dissipation scales. By nature the power spectrum of the magnetic fluctuations is anisotropic with different spectral slopes in parallel and perpendicular directions with respect to the background magnetic field. Furthermore, the magnetic field power spectra steepen as one moves from the inertial to the dissipation range and we observe multiple spectral breaks with different slopes in parallel and perpendicular direction at the ion scales and beyond. The turbulent dissipation of magnetic field fluctuations at the sub-ion scales is believed to go into local ion heating and acceleration, so that the spectral breaks are typically associated with particle energization. The gained energy can be in the form of anisotropic heating, formation of non-thermal features in the particle velocity distributions functions, and redistribution of the differential acceleration between the different ion populations. To study the relation between the evolution of the anisotropic turbulent spectra and the particle heating at the ion and sub-ion scales we perform a series of 2.5D hybrid simulations in a collisionless drifting proton-alpha plasma. We neglect the fast electron dynamics and treat the electrons as an isothermal fluid electrons, whereas the protons and a minor population of alpha particles are evolved in a fully kinetic manner. We start with a given wave spectrum and study the evolution of the magnetic field spectral slopes as a function of the parallel and perpendicular wave¬numbers. Simultaneously, we track the particle response and the energy exchange between the parallel and perpendicular scales. We observe anisotropic behavior of the turbulent power spectra with steeper slopes along the dominant energy-containing direction. This means that for parallel and quasi-parallel waves we have steeper spectral slope in parallel direction, whereas for highly oblique waves the dissipation occurs predominantly in perpendicular direction and the spectral slopes are steeper across the background magnetic field. The value of the spectral slopes depends on the angle of propagation, the spectral range, as well as the plasma properties. In general the dissipation is stronger at small scales and the corresponding spectral slopes there are steeper. For parallel and quasi-parallel propagation the prevailing energy cascade remains along the magnetic field, whereas for initially isotropic oblique turbulence the cascade develops mainly in perpendicular direction.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Harak, B. A. de; Ladino, L.; MacAdam, K. B.

We report measurements of the scattering of electrons by helium atoms in the presence of 1.17 eV photons from a Nd:YAG laser. The incident energy of the electrons was in the range 50-350 eV, and the polarization of the laser was arranged to be parallel to electrons scattered through 135 deg. Energy-shifted peaks corresponding both to one- and two-photon emission were observed. Calculations using the Kroll-Watson approximation are perfectly consistent with the data.

Advanced PIC-MCC simulation for the investigation of step-ionization effect in intermediate-pressure capacitively coupled plasmas

NASA Astrophysics Data System (ADS)

Kim, Jin Seok; Hur, Min Young; Kim, Chang Ho; Kim, Ho Jun; Lee, Hae June

2018-03-01

A two-dimensional parallelized particle-in-cell simulation has been developed to simulate a capacitively coupled plasma reactor. The parallelization using graphics processing units is applied to resolve the heavy computational load. It is found that the step-ionization plays an important role in the intermediate gas pressure of a few Torr. Without the step-ionization, the average electron density decreases while the effective electron temperature increases with the increase of gas pressure at a fixed power. With the step-ionization, however, the average electron density increases while the effective electron temperature decreases with the increase of gas pressure. The cases with the step-ionization agree well with the tendency of experimental measurement. The electron energy distribution functions show that the population of electrons having intermediate energy from 4.2 to 12 eV is relaxed by the step-ionization. Also, it was observed that the power consumption by the electrons is increasing with the increase of gas pressure by the step-ionization process, while the power consumption by the ions decreases with the increase of gas pressure.

Energy limits of electron acceleration in the plasma sheet during substorms: A case study with the Magnetospheric Multiscale (MMS) mission

NASA Astrophysics Data System (ADS)

Turner, D. L.; Fennell, J. F.; Blake, J. B.; Clemmons, J. H.; Mauk, B. H.; Cohen, I. J.; Jaynes, A. N.; Craft, J. V.; Wilder, F. D.; Baker, D. N.; Reeves, G. D.; Gershman, D. J.; Avanov, L. A.; Dorelli, J. C.; Giles, B. L.; Pollock, C. J.; Schmid, D.; Nakamura, R.; Strangeway, R. J.; Russell, C. T.; Artemyev, A. V.; Runov, A.; Angelopoulos, V.; Spence, H. E.; Torbert, R. B.; Burch, J. L.

2016-08-01

We present multipoint observations of earthward moving dipolarization fronts and energetic particle injections from NASA's Magnetospheric Multiscale mission with a focus on electron acceleration. From a case study during a substorm on 02 August 2015, we find that electrons are only accelerated over a finite energy range, from a lower energy threshold at 7-9 keV up to an upper energy cutoff in the hundreds of keV range. At energies lower than the threshold energy, electron fluxes decrease, potentially due to precipitation by strong parallel electrostatic wavefields or initial sources in the lobes. Electrons at energies higher than the threshold are accelerated cumulatively by a series of impulsive magnetic dipolarization events. This case demonstrates how the upper energy cutoff increases, in this case from 130 keV to >500 keV, with each dipolarization/injection during sustained activity. We also present a simple model accounting for these energy limits that reveals that electron energization is dominated by betatron acceleration.

Energy Limits of Electron Acceleration in the Plasma Sheet During Substorms: A Case Study with the Magnetospheric Multiscale (MMS) Mission

NASA Technical Reports Server (NTRS)

Turner, D. L.; Fennell, J. F.; Blake, J. B.; Clemmons, J. H.; Mauk, B. H.; Cohen, I. J.; Jaynes, A. N.; Craft, J. V.; Wilder, F. D.; Baker, D. N.;

Set-up and demonstration of a Low Energy Electron Magnetometer (LEEM)

NASA Technical Reports Server (NTRS)

Rayborn, G. H.

1986-01-01

Described are the design, construction and test results of a Low Energy Electron Magnetometer (LEEM). The electron source is a commercial electron gun capable of providing several microamperes of electron beam. These electrons, after acceleration through a selected potential difference of 100-300 volts, are sent through two 30 degree second-order focussing parallel plate electrostatic analyzers. The first analyzer acts as a monochromator located in the field-free space. It is capable of providing energy resolution of better than 10 to the -3 power. The second analyzer, located in the test field region, acts as the detector for electrons deflected by the test field. The entire magnetometer system is expected to have a resolution of 1 part in 1000 or better.

Relativistic effects in the energy loss of a fast charged particle moving parallel to a two-dimensional electron gas

NASA Astrophysics Data System (ADS)

Mišković, Zoran L.; Akbari, Kamran; Segui, Silvina; Gervasoni, Juana L.; Arista, Néstor R.

2018-05-01

We present a fully relativistic formulation for the energy loss rate of a charged particle moving parallel to a sheet containing two-dimensional electron gas, allowing that its in-plane polarization may be described by different longitudinal and transverse conductivities. We apply our formulation to the case of a doped graphene layer in the terahertz range of frequencies, where excitation of the Dirac plasmon polariton (DPP) in graphene plays a major role. By using the Drude model with zero damping we evaluate the energy loss rate due to excitation of the DPP, and show that the retardation effects are important when the incident particle speed and its distance from graphene both increase. Interestingly, the retarded energy loss rate obtained in this manner may be both larger and smaller than its non-retarded counterpart for different combinations of the particle speed and distance.

Electron acceleration by surface plasma waves in double metal surface structure

NASA Astrophysics Data System (ADS)

Liu, C. S.; Kumar, Gagan; Singh, D. B.; Tripathi, V. K.

2007-12-01

Two parallel metal sheets, separated by a vacuum region, support a surface plasma wave whose amplitude is maximum on the two parallel interfaces and minimum in the middle. This mode can be excited by a laser using a glass prism. An electron beam launched into the middle region experiences a longitudinal ponderomotive force due to the surface plasma wave and gets accelerated to velocities of the order of phase velocity of the surface wave. The scheme is viable to achieve beams of tens of keV energy. In the case of a surface plasma wave excited on a single metal-vacuum interface, the field gradient normal to the interface pushes the electrons away from the high field region, limiting the acceleration process. The acceleration energy thus achieved is in agreement with the experimental observations.

The Strongest Acceleration of >40 keV Electrons by ICME-driven Shocks at 1 au

NASA Astrophysics Data System (ADS)

Yang, Liu; Wang, Linghua; Li, Gang; Wimmer-Schweingruber, Robert F.; He, Jiansen; Tu, Chuanyi; Tian, Hui; Bale, Stuart D.

2018-01-01

We present two case studies of the in-situ electron acceleration during the 2000 February 11 shock and the 2004 July 22 shock, with the strongest electron flux enhancement at 40 keV across the shock, among all the quasi-perpendicular and quasi-parallel ICME-driven shocks observed by the WIND 3DP instrument from 1995 through 2014 at 1 au. We find that for this quasi-perpendicular (quasi-parallel) shock on 2000 February 11 (2004 July 22), the shocked electron differential fluxes at ∼0.4–50 keV in the downstream generally fit well to a double-power-law spectrum, J ∼ E ‑β , with an index of β ∼ 3.15 (4.0) at energies below a break at ∼3 keV (∼1 keV) and β ∼ 2.65 (2.6) at energies above. For both shock events, the downstream electron spectral indices appear to be similar for all pitch angles, which are significantly larger than the index prediction by diffusive shock acceleration. In addition, the downstream electron pitch-angle distributions show the anisotropic beams in the anti-sunward-traveling direction, while the ratio of the downstream over ambient fluxes appears to peak near 90° pitch angles, at all energies of ∼0.4–50 keV. These results suggest that in both shocks, shock drift acceleration likely plays an important role in accelerating electrons in situ at 1 au. Such ICME-driven shocks could contribute to the formation of solar wind halo electrons at energies ≲2 keV, as well as the production of solar wind superhalo electrons at energies ≳2 keV in interplanetary space.

Space charge in nanostructure resonances

NASA Astrophysics Data System (ADS)

Price, Peter J.

1996-10-01

In quantum ballistic propagation of electrons through a variety of nanostructures, resonance in the energy-dependent transmission and reflection probabilities generically is associated with (1) a quasi-level with a decay lifetime, and (2) a bulge in electron density within the structure. It can be shown that, to a good approximation, a simple formula in all cases connects the density of states for the latter to the energy dependence of the phase angles of the eigen values of the S-matrix governing the propagation. For both the Lorentzian resonances (normal or inverted) and for the Fano-type resonances, as a consequence of this eigen value formula, the space charge due to filled states over the energy range of a resonance is just equal (for each spin state) to one electron charge. The Coulomb interaction within this space charge is known to 'distort' the electrical characteristics of resonant nanostructures. In these systems, however, the exchange effect should effectively cancel the interaction between states with parallel spins, leaving only the anti-parallel spin contribution.

Mirror force induced wave dispersion in Alfvén waves

DOE Office of Scientific and Technical Information (OSTI.GOV)

Damiano, P. A.; Johnson, J. R.

2013-06-15

Recent hybrid MHD-kinetic electron simulations of global scale standing shear Alfvén waves along the Earth's closed dipolar magnetic field lines show that the upward parallel current region within these waves saturates and broadens perpendicular to the ambient magnetic field and that this broadening increases with the electron temperature. Using resistive MHD simulations, with a parallel Ohm's law derived from the linear Knight relation (which expresses the current-voltage relationship along an auroral field line), we explore the nature of this broadening in the context of the increased perpendicular Poynting flux resulting from the increased parallel electric field associated with mirror forcemore » effects. This increased Poynting flux facilitates wave energy dispersion across field lines which in-turn allows for electron acceleration to carry the field aligned current on adjacent field lines. This mirror force driven dispersion can dominate over that associated with electron inertial effects for global scale waves.« less

Intense positron beam as a source for production of electron-positron plasma

NASA Astrophysics Data System (ADS)

Stoneking, M. R.; Horn-Stanja, J.; Stenson, E. V.; Pedersen, T. Sunn; Saitoh, H.; Hergenhahn, U.; Niemann, H.; Paschkowski, N.; Hugenschmidt, C.; Piochacz, C.

2016-10-01

We aim to produce magnetically confined, short Debye length electron-positron plasma and test predicted properties for such systems. A first challenge is obtaining large numbers of positrons; a table-top experiment (system size 5 cm) with a temperature less than 5 eV requires about 1010 positrons to have more than 10 Debye lengths in the system. The NEPOMUC facility at the FRM II research reactor in Germany is one of the world's most intense positron sources. We report on characterization (using a retarding field energy analyzer with magnetic field gradient) of the NEPOMUC beam as delivered to the open beam port at various beam energies and in both the re-moderated and primary beam configurations in order to design optimal trapping (and accumulation) schemes for production of electron-positron plasma. The intensity of the re-moderated (primary) beam is in the range 2 -3 x 107 /s (1 - 5 x 108 /s). The re-moderated beam is currently the most promising for direct injection and confinement experiments; it has a parallel energy spread of 15 - 35% and the transverse energy spread is 6 - 15% of the parallel energy. We report on the implications for injection and trapping in a dipole magnetic field as well as plans for beam development, in situ re-moderation, and accumulation. We also report results demonstrating a difference in phosphor luminescent response to low energy positrons versus electrons.

Absolute dose determination in high-energy electron beams: Comparison of IAEA dosimetry protocols

PubMed Central

Sathiyan, S.; Ravikumar, M.

2008-01-01

In this study, absorbed doses were measured and compared for high-energy electrons (6, 9, 12, 16, and 20 MeV) using International Atomic Energy Agency (IAEA), Technical Reports Series No. 277 (TRS), TRS 381, and TRS 398 dosimetry protocols. Absolute dose measurements were carried out using FC65-G Farmer chamber and Nordic Association of Clinical Physicists (NACP) parallel plate chamber with DOSE1 electrometer in WP1-D water phantom for reference field size of 15 × 15 cm2 at 100 cm source-to-surface distance. The results show that the difference between TRS 398 and TRS 381 was about 0.24% to 1.3% depending upon the energy, and the maximum difference between TRS 398 and TRS 277 was 1.5%. The use of cylindrical chamber in electron beam gives the maximum dose difference between the TRS 398 and TRS 277 in the order of 1.4% for energies above 10 MeV (R50 > 4 g/cm2). It was observed that the accuracy of dose estimation was better with the protocols based on the water calibration procedures, as no conversion quantities are involved for conversion of dose from air to water. The cross-calibration procedure of parallel plate chamber with high-energy electron beams is recommended as it avoids pwall correction factor entering into the determination of kQ,Qo. PMID:19893700

Direct determination of k Q factors for cylindrical and plane-parallel ionization chambers in high-energy electron beams from 6 MeV to 20 MeV.

PubMed

Krauss, A; Kapsch, R-P

2018-02-06

For the ionometric determination of the absorbed dose to water, D w , in high-energy electron beams from a clinical accelerator, beam quality dependent correction factors, k Q , are required. By using a water calorimeter, these factors can be determined experimentally and potentially with lower standard uncertainties than those of the calculated k Q factors, which are tabulated in various dosimetry protocols. However, one of the challenges of water calorimetry in electron beams is the small measurement depths in water, together with the steep dose gradients present especially at lower energies. In this investigation, water calorimetry was implemented in electron beams to determine k Q factors for different types of cylindrical and plane-parallel ionization chambers (NE2561, NE2571, FC65-G, TM34001) in 10 cm  ×  10 cm electron beams from 6 MeV to 20 MeV (corresponding beam quality index R 50 ranging from 1.9 cm to 7.5 cm). The measurements were carried out using the linear accelerator facility of the Physikalisch-Technische Bundesanstalt. Relative standard uncertainties for the k Q factors between 0.50% for the 20 MeV beam and 0.75% for the 6 MeV beam were achieved. For electron energies above 8 MeV, general agreement was found between the relative electron energy dependencies of the k Q factors measured and those derived from the AAPM TG-51 protocol and recent Monte Carlo-based studies, as well as those from other experimental investigations. However, towards lower energies, discrepancies of up to 2.0% occurred for the k Q factors of the TM34001 and the NE2571 chamber.

Direct determination of k Q factors for cylindrical and plane-parallel ionization chambers in high-energy electron beams from 6 MeV to 20 MeV

NASA Astrophysics Data System (ADS)

Krauss, A.; Kapsch, R.-P.

2018-02-01

For the ionometric determination of the absorbed dose to water, D w, in high-energy electron beams from a clinical accelerator, beam quality dependent correction factors, k Q, are required. By using a water calorimeter, these factors can be determined experimentally and potentially with lower standard uncertainties than those of the calculated k Q factors, which are tabulated in various dosimetry protocols. However, one of the challenges of water calorimetry in electron beams is the small measurement depths in water, together with the steep dose gradients present especially at lower energies. In this investigation, water calorimetry was implemented in electron beams to determine k Q factors for different types of cylindrical and plane-parallel ionization chambers (NE2561, NE2571, FC65-G, TM34001) in 10 cm  ×  10 cm electron beams from 6 MeV to 20 MeV (corresponding beam quality index R 50 ranging from 1.9 cm to 7.5 cm). The measurements were carried out using the linear accelerator facility of the Physikalisch-Technische Bundesanstalt. Relative standard uncertainties for the k Q factors between 0.50% for the 20 MeV beam and 0.75% for the 6 MeV beam were achieved. For electron energies above 8 MeV, general agreement was found between the relative electron energy dependencies of the k Q factors measured and those derived from the AAPM TG-51 protocol and recent Monte Carlo-based studies, as well as those from other experimental investigations. However, towards lower energies, discrepancies of up to 2.0% occurred for the k Q factors of the TM34001 and the NE2571 chamber.

Structures and Binding Energies of the Naphthalene Dimer in Its Ground and Excited States.

PubMed

Dubinets, N O; Safonov, A A; Bagaturyants, A A

2016-05-05

Possible structures of the naphthalene dimer corresponding to local energy minima in the ground and excited (excimer) electronic states are comprehensively investigated using DFT-D and TDDFT-D methods with a special accent on the excimer structures. The corresponding binding and electronic transition energies are calculated, and the nature of the electronic states in different structures is analyzed. Several parallel (stacked) and T-shaped structures were found in both the ground and excited (excimer) states in a rather narrow energy range. The T-shaped structure with the lowest energy in the excited state exhibits a marked charge transfer from the upright molecule to the base one.

Electron energy distributions measured during electron beam/plasma interactions. [in E region

NASA Technical Reports Server (NTRS)

Jost, R. J.; Anderson, H. R.; Mcgarity, J. O.

1980-01-01

In the large vacuum facility at the NASA-Johnson Space Center an electron beam was projected 20 m parallel to B from a gun with variable accelerating potential (1.0 to 2.5 kV) to an aluminum target. The ionospheric neutral pressure and field were approximated. Beam electron energy distributions were measured directly using an electrostatic deflection analyzer and indirectly with a detector that responded to the X-rays produced by electron impact on the target. At low currents the distribution is sharply peaked at the acceleration potential. At high currents a beam plasma discharge occurs and electrons are redistributed in energy so that the former energy peak broadens to 10-15 percent FWHM with a strongly enhanced low energy tail. At the 10% of maximum point the energy spectrum ranges from less than 1/2 to 1.2 times the gun energy. The effect is qualitatively the same at all pitch angles and locations sampled.

A laboratory study of ion energization by EIC waves and subsequent upstreaming along diverging magnetic field lines

NASA Technical Reports Server (NTRS)

Cartier, S. L.; Dangelo, N.; Merlino, R. L.

1986-01-01

A laboratory study related to energetic upstreaming ions in the ionosphere-magnetosphere system is described. The experiment was carried out in a cesium Q machine plasma with a region of nonuniform magnetic field. Electrostatic ion cyclotron waves were excited by drawing an electron current to a small biased exciter electrode. In the presence of the instability, ions are heated in the direction perpendicular to B. Using a gridded retarding potential ion energy analyzer, the evolution of the ion velocity distribution was followed as the ions passed through the heating region and subsequently flowed out along the diverging B field lines. As expected, the heated ions transfer their energy from perpendicular to parallel motion as they move through the region of diverging B field. Both their parallel thermal energy and the parallel drift energy increase at the expense of the perpendicular energy.

High energy micro electron beam generation using chirped laser pulse in the presence of an axial magnetic field

DOE Office of Scientific and Technical Information (OSTI.GOV)

Akou, H., E-mail: h.akou@nit.ac.ir; Hamedi, M.

2015-10-15

In this paper, the generation of high-quality and high-energy micro electron beam in vacuum by a chirped Gaussian laser pulse in the presence of an axial magnetic field is numerically investigated. The features of energy and angular spectra, emittances, and position distribution of electron beam are compared in two cases, i.e., in the presence and absence of an external magnetic field. The electron beam is accelerated with higher energy and qualified in spatial distribution in the presence of the magnetic field. The presence of an axial magnetic field improves electron beam spatial quality as well as its gained energy throughmore » keeping the electron motion parallel to the direction of propagation for longer distances. It has been found that a 64 μm electron bunch with about MeV initial energy becomes a 20 μm electron beam with high energy of the order of GeV, after interacting with a laser pulse in the presence of an external magnetic field.« less

Electron-acoustic solitons and double layers in the inner magnetosphere: ELECTRON-ACOUSTIC SOLITONS

DOE PAGES

Vasko, I. Y.; Agapitov, O. V.; Mozer, F. S.; ...

2017-05-28

The Van Allen Probes observe generally two types of electrostatic solitary waves (ESW) contributing to the broadband electrostatic wave activity in the nightside inner magnetosphere. ESW with symmetric bipolar parallel electric field are electron phase space holes. The nature of ESW with asymmetric bipolar (and almost unipolar) parallel electric field has remained puzzling. To address their nature, we consider a particular event observed by Van Allen Probes to argue that during the broadband wave activity electrons with energy above 200 eV provide the dominant contribution to the total electron density, while the density of cold electrons (below a few eV)more » is less than a few tenths of the total electron density. We show that velocities of the asymmetric ESW are close to velocity of electron-acoustic waves (existing due to the presence of cold and hot electrons) and follow the Korteweg-de Vries (KdV) dispersion relation derived for the observed plasma conditions (electron energy spectrum is a power law between about 100 eV and 10 keV and Maxwellian above 10 keV). The ESW spatial scales are in general agreement with the KdV theory. We interpret the asymmetric ESW in terms of electron-acoustic solitons and double layers (shocks waves).« less

Electron-acoustic solitons and double layers in the inner magnetosphere: ELECTRON-ACOUSTIC SOLITONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vasko, I. Y.; Agapitov, O. V.; Mozer, F. S.

The Van Allen Probes observe generally two types of electrostatic solitary waves (ESW) contributing to the broadband electrostatic wave activity in the nightside inner magnetosphere. ESW with symmetric bipolar parallel electric field are electron phase space holes. The nature of ESW with asymmetric bipolar (and almost unipolar) parallel electric field has remained puzzling. To address their nature, we consider a particular event observed by Van Allen Probes to argue that during the broadband wave activity electrons with energy above 200 eV provide the dominant contribution to the total electron density, while the density of cold electrons (below a few eV)more » is less than a few tenths of the total electron density. We show that velocities of the asymmetric ESW are close to velocity of electron-acoustic waves (existing due to the presence of cold and hot electrons) and follow the Korteweg-de Vries (KdV) dispersion relation derived for the observed plasma conditions (electron energy spectrum is a power law between about 100 eV and 10 keV and Maxwellian above 10 keV). The ESW spatial scales are in general agreement with the KdV theory. We interpret the asymmetric ESW in terms of electron-acoustic solitons and double layers (shocks waves).« less

X-Ray and TeV Gamma-Ray Emission from Parallel Electron-Positron or Electron-Proton Beams in BL Lacertae Objects

NASA Astrophysics Data System (ADS)

Krawczynski, H.

2007-04-01

In this paper we discuss models of the X-ray and TeV γ-ray emission from BL Lac objects based on parallel electron-positron or electron-proton beams that form close to the central black hole, due to the strong electric fields generated by the accretion disk and possibly also by the black hole itself. Fitting the energy spectrum of the BL Lac object Mrk 501, we obtain tight constraints on the beam properties. Launching a sufficiently energetic beam requires rather strong magnetic fields close to the black hole (~100-1000 G). However, the model fits imply that the magnetic field in the emission region is only ~0.02 G. Thus, the particles are accelerated close to the black hole and propagate a considerable distance before instabilities trigger the dissipation of energy through synchrotron and self-Compton emission. We discuss various approaches to generate enough power to drive the jet and, at the same time, to accelerate particles to ~20 TeV energies. Although the parallel beam model has its own problems, it explains some of the long-standing problems that plague models based on Fermi-type particle acceleration, such as the presence of a very high minimum Lorentz factor of accelerated particles. We conclude with a brief discussion of the implications of the model for the difference between the processes of jet formation in BL Lac-type objects and those in quasars.

X-ray and TeV Gamma-Ray Emission from Parallel Electron-Positron or Electron-Proton Beams in BL Lac Objects

NASA Astrophysics Data System (ADS)

Krawczynski, Henric

2007-04-01

In this contribution we discuss models of the X-rays and TeV gamma-ray emission from BL Lac objects based on parallel electron-positron or electron-proton beams that form close to the central black hole owing to the strong electric fields generated by the accretion disk and possibly also by the black hole itself. Fitting the energy spectrum of the BL Lac object Mrk 501, we obtain tight constrains on the beam properties. Launching a sufficiently energetic beam requires rather strong magnetic fields close to the black hole 100-1000 G. However, the model fits imply that the magnetic field in the emission region is only 0.02 G. Thus, the particles are accelerated close to the black hole and propagate a considerable distance before instabilities trigger the dissipation of energy through synchrotron and self-Compton emission. We discuss various approaches to generate enough power to drive the jet and, at the same time, to accelerate particles to 20 TeV energies. Although the parallel beam model has its own problems, it explains some of the long-standing problems that plague models based on Fermi type particle acceleration, like the presence of a very high minimum Lorentz factor of accelerated particles. We conclude with a brief discussion of the implications of the model for the difference between the processes of jet formation in BL Lac type objects and in quasars.

Energy limits of electron acceleration in the plasma sheet during substorms: A case study with the Magnetospheric Multiscale (MMS) mission

DOE Office of Scientific and Technical Information (OSTI.GOV)

Turner, Drew Lawson; Fennell, J. F.; Blake, J. B.

Here, we present multipoint observations of earthward moving dipolarization fronts and energetic particle injections from NASA's Magnetospheric Multiscale mission with a focus on electron acceleration. From a case study during a substorm on 02 August 2015, we find that electrons are only accelerated over a finite energy range, from a lower energy threshold at ~7–9 keV up to an upper energy cutoff in the hundreds of keV range. At energies lower than the threshold energy, electron fluxes decrease, potentially due to precipitation by strong parallel electrostatic wavefields or initial sources in the lobes. Electrons at energies higher than the thresholdmore » are accelerated cumulatively by a series of impulsive magnetic dipolarization events. This case demonstrates how the upper energy cutoff increases, in this case from ~130 keV to >500 keV, with each dipolarization/injection during sustained activity. We also present a simple model accounting for these energy limits that reveals that electron energization is dominated by betatron acceleration.« less

Energy limits of electron acceleration in the plasma sheet during substorms: A case study with the Magnetospheric Multiscale (MMS) mission

DOE PAGES

Turner, Drew Lawson; Fennell, J. F.; Blake, J. B.; ...

2016-08-01

Here, we present multipoint observations of earthward moving dipolarization fronts and energetic particle injections from NASA's Magnetospheric Multiscale mission with a focus on electron acceleration. From a case study during a substorm on 02 August 2015, we find that electrons are only accelerated over a finite energy range, from a lower energy threshold at ~7–9 keV up to an upper energy cutoff in the hundreds of keV range. At energies lower than the threshold energy, electron fluxes decrease, potentially due to precipitation by strong parallel electrostatic wavefields or initial sources in the lobes. Electrons at energies higher than the thresholdmore » are accelerated cumulatively by a series of impulsive magnetic dipolarization events. This case demonstrates how the upper energy cutoff increases, in this case from ~130 keV to >500 keV, with each dipolarization/injection during sustained activity. We also present a simple model accounting for these energy limits that reveals that electron energization is dominated by betatron acceleration.« less

Scaling of Electron Heating During Magnetic Reconnection

NASA Astrophysics Data System (ADS)

Ohia, O.; Le, A.; Daughton, W. S.; Egedal, J.

2016-12-01

While magnetic reconnection plays a major role in accelerating and heating magnetospheric plasma, it remains poorly understood how the level of particle energization depends on the plasma conditions. Meanwhile, a recent survey of THEMIS magnetopause reconnection observations [Phan et al. GRL 2013] and a numerical study [Shay et al. PoP 2014] found empirically that the electron heating scales with the square of the upstream Alfven speed. Equivalently for weak guide fields, the fractional electron temperature increase is inversely proportional to the upstream electron beta (ratio of electron to magnetic pressure). We present models for symmetric reconnection with moderate [Ohia et al., GRL 2015] or zero guide field that predict the electron bulk heating. In the models, adiabatically trapped electrons gain energy from parallel electric fields in the inflowing region. For purely anti-parallel reconnection, meandering electrons receive additional energy from the reconnection electric field. The predicted scalings are in quantitative agreement with fluid and kinetic simulations, as well as spacecraft observations. Using kinetic simulations, we extend this work to explore how the layer dynamics and electron bulk heating vary as functions of the magnetic shear and plasma and magnetic pressure asymmetry across the reconnection layer. These results are pertinent to recent Magnetospheric Multiscale (MMS) Mission measurements of electron dynamics during dayside magnetopause reconnection.

Nonadiabatic electron response in the Hasegawa-Wakatani equations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stoltzfus-Dueck, T.; Scott, B. D.; Krommes, J. A.

2013-08-15

Tokamak edge turbulence is strongly influenced by parallel electron physics, which relaxes density and potential fluctuations towards electron adiabatic response. Beginning with the paradigmatic Hasegawa-Wakatani equations (HWEs) for resistive tokamak edge turbulence, a unique decomposition of the electric potential (φ) into adiabatic (a) and nonadiabatic (b) portions is derived, based on the requirement that a neither drive nor respond to the parallel current j{sub ∥}. The form of the decomposition clarifies that, at perpendicular scales large relative to the sound radius, the electron adiabatic response controls the nonzonal φ, not the fluctuating density n. Simple energy balance arguments allow onemore » to rigorously bound the ratio of rms nonzonal nonadiabatic fluctuations (b(tilde sign)) relative to adiabatic ones (ã). The role of the vorticity nonlinearity in transferring energy between adiabatic and nonadiabatic fluctuations aids intuitive understanding of self-sustained turbulence in the HWEs. When the normalized parallel resistivity is weak, b(tilde sign) becomes effectively slaved, allowing the reduction to an approximate one-field model that remains valid for strong turbulence. In addition to guiding physical intuition, the one-field reduction should greatly ease further analytical manipulations. Direct numerical simulation of the 2D HWEs confirms the convergence of the asymptotic formula for b(tilde sign)« less

Beam quality corrections for parallel-plate ion chambers in electron reference dosimetry

NASA Astrophysics Data System (ADS)

Zink, K.; Wulff, J.

2012-04-01

Current dosimetry protocols (AAPM, IAEA, IPEM, DIN) recommend parallel-plate ionization chambers for dose measurements in clinical electron beams. This study presents detailed Monte Carlo simulations of beam quality correction factors for four different types of parallel-plate chambers: NACP-02, Markus, Advanced Markus and Roos. These chambers differ in constructive details which should have notable impact on the resulting perturbation corrections, hence on the beam quality corrections. The results reveal deviations to the recommended beam quality corrections given in the IAEA TRS-398 protocol in the range of 0%-2% depending on energy and chamber type. For well-guarded chambers, these deviations could be traced back to a non-unity and energy-dependent wall perturbation correction. In the case of the guardless Markus chamber, a nearly energy-independent beam quality correction is resulting as the effects of wall and cavity perturbation compensate each other. For this chamber, the deviations to the recommended values are the largest and may exceed 2%. From calculations of type-B uncertainties including effects due to uncertainties of the underlying cross-sectional data as well as uncertainties due to the chamber material composition and chamber geometry, the overall uncertainty of calculated beam quality correction factors was estimated to be <0.7%. Due to different chamber positioning recommendations given in the national and international dosimetry protocols, an additional uncertainty in the range of 0.2%-0.6% is present. According to the IAEA TRS-398 protocol, the uncertainty in clinical electron dosimetry using parallel-plate ion chambers is 1.7%. This study may help to reduce this uncertainty significantly.

First principles study of electronic properties, interband transitions and electron energy loss of α-graphyne

NASA Astrophysics Data System (ADS)

Behzad, Somayeh

2016-04-01

The electronic and optical properties of α-graphyne sheet are investigated by using density functional theory. The results confirm that α-graphyne sheet is a zero-gap semimetal. The optical properties of the α-graphyne sheet such as dielectric function, refraction index, electron energy loss function, reflectivity, absorption coefficient and extinction index are calculated for both parallel and perpendicular electric field polarizations. The optical spectra are strongly anisotropic along these two polarizations. For (E ∥ x), absorption edge is at 0 eV, while there is no absorption below 8 eV for (E ∥ z).

Ion and Electron Energization in Guide Field Reconnection Outflows with Kinetic Riemann Simulations and Parallel Shock Simulations

NASA Astrophysics Data System (ADS)

Zhang, Q.; Drake, J. F.; Swisdak, M.

2017-12-01

How ions and electrons are energized in magnetic reconnection outflows is an essential topic throughout the heliosphere. Here we carry out guide field PIC Riemann simulations to explore the ion and electron energization mechanisms far downstream of the x-line. Riemann simulations, with their simple magnetic geometry, facilitate the study of the reconnection outflow far downstream of the x-line in much more detail than is possible with conventional reconnection simulations. We find that the ions get accelerated at rotational discontinuities, counter stream, and give rise to two slow shocks. We demonstrate that the energization mechanism at the slow shocks is essentially the same as that of parallel electrostatic shocks. Also, the electron confining electric potential at the slow shocks is driven by the counterstreaming beams, which tend to break the quasi-neutrality. Based on this picture, we build a kinetic model to self consistently predict the downstream ion and electron temperatures. Additional explorations using parallel shock simulations also imply that in a very low beta(0.001 0.01 for a modest guide field) regime, electron energization will be insignificant compared to the ion energization. Our model and the parallel shock simulations might be used as simple tools to understand and estimate the energization of ions and electrons and the energy partition far downstream of the x-line.

Investigation of low-loss spectra and near-edge fine structure of polymers by PEELS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heckmann, W.

Transmission electron microscopy has changed from a purely imaging method to an analytical method. This has been facilitated particularly by equipping electron microscopes with energy filters and with parallel electron energy loss spectrometers (PEELS). Because of their relatively high energy resolution (1 to 2 eV) they provide information not only on the elements present but also on the type of bonds between the molecular groups. Polymers are radiation sensitive and the molecular bonds change as the spectrum is being recorded. This can be observed with PEEL spectrometers that are able to record spectra with high sensitivity and in rapid succession.

Determination of auroral electrostatic potentials using high- and low-altitude particle distributions

NASA Technical Reports Server (NTRS)

Reiff, P. H.; Collin, H. L.; Craven, J. D.; Burch, J. L.; Winningham, J. D.

1988-01-01

The auroral electrostatic potential differences were determined from the particle distribution functions obtained nearly simultaneously above and below the auroral acceleration region by DE-1 at altitudes 9000-15,000 km and DE-2 at 400-800 km. Three independent techniques were used: (1) the peak energies of precipitating electrons observed by DE-2, (2) the widening of loss cones for upward traveling electrons observed by DE-1, and (3) the energies of upgoing ions observed by DE-1. The assumed parallel electrostatic potential difference calculated by the three methods was nearly the same. The results confirmed the hypothesis that parallel electrostatic fields of 1-10 kV potential drop at 1-2 earth radii altitude are an important source for auroral particle acceleration.

PIC simulations of wave-particle interactions with an initial electron velocity distribution from a kinetic ring current model

DOE PAGES

Yu, Yiqun; Delzanno, Gian Luca; Jordanova, Vania Koleva; ...

2017-07-15

Whistler wave-particle interactions play an important role in the Earth inner magnetospheric dynamics and have been the subject of numerous investigations. By running a global kinetic ring current model (RAM-SCB) in a storm event occurred on Oct 23–24 2002, we obtain the ring current electron distribution at a selected location at MLT of 9 and L of 6 where the electron distribution is composed of a warm population in the form of a partial ring in the velocity space (with energy around 15 keV) in addition to a cool population with a Maxwellian-like distribution. The warm population is likely frommore » the injected plasma sheet electrons during substorm injections that supply fresh source to the inner magnetosphere. These electron distributions are then used as input in an implicit particle-in-cell code (iPIC3D) to study whistler-wave generation and the subsequent wave-particle interactions. Here, we find that whistler waves are excited and propagate in the quasi-parallel direction along the background magnetic field. Several different wave modes are instantaneously generated with different growth rates and frequencies. The wave mode at the maximum growth rate has a frequency around 0.62ω ce, which corresponds to a parallel resonant energy of 2.5 keV. Linear theory analysis of wave growth is in excellent agreement with the simulation results. These waves grow initially due to the injected warm electrons and are later damped due to cyclotron absorption by electrons whose energy is close to the resonant energy and can effectively attenuate waves. The warm electron population overall experiences net energy loss and anisotropy drop while moving along the diffusion surfaces towards regions of lower phase space density, while the cool electron population undergoes heating when the waves grow, suggesting the cross-population interactions.« less

PIC simulations of wave-particle interactions with an initial electron velocity distribution from a kinetic ring current model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yu, Yiqun; Delzanno, Gian Luca; Jordanova, Vania Koleva

Whistler wave-particle interactions play an important role in the Earth inner magnetospheric dynamics and have been the subject of numerous investigations. By running a global kinetic ring current model (RAM-SCB) in a storm event occurred on Oct 23–24 2002, we obtain the ring current electron distribution at a selected location at MLT of 9 and L of 6 where the electron distribution is composed of a warm population in the form of a partial ring in the velocity space (with energy around 15 keV) in addition to a cool population with a Maxwellian-like distribution. The warm population is likely frommore » the injected plasma sheet electrons during substorm injections that supply fresh source to the inner magnetosphere. These electron distributions are then used as input in an implicit particle-in-cell code (iPIC3D) to study whistler-wave generation and the subsequent wave-particle interactions. Here, we find that whistler waves are excited and propagate in the quasi-parallel direction along the background magnetic field. Several different wave modes are instantaneously generated with different growth rates and frequencies. The wave mode at the maximum growth rate has a frequency around 0.62ω ce, which corresponds to a parallel resonant energy of 2.5 keV. Linear theory analysis of wave growth is in excellent agreement with the simulation results. These waves grow initially due to the injected warm electrons and are later damped due to cyclotron absorption by electrons whose energy is close to the resonant energy and can effectively attenuate waves. The warm electron population overall experiences net energy loss and anisotropy drop while moving along the diffusion surfaces towards regions of lower phase space density, while the cool electron population undergoes heating when the waves grow, suggesting the cross-population interactions.« less

Magnitude of parallel pseudo potential in a magnetosonic shock wave

NASA Astrophysics Data System (ADS)

Ohsawa, Yukiharu

2018-05-01

The parallel pseudo potential F, which is the integral of the parallel electric field along the magnetic field, in a large-amplitude magnetosonic pulse (shock wave) is theoretically studied. Particle simulations revealed in the late 1990's that the product of the elementary charge and F can be much larger than the electron temperature in shock waves, i.e., the parallel electric field can be quite strong. However, no theory was presented for this unexpected result. This paper first revisits the small-amplitude theory for F and then investigates the parallel pseudo potential F in large-amplitude pulses based on the two-fluid model with finite thermal pressures. It is found that the magnitude of F in a shock wave is determined by the wave amplitude, the electron temperature, and the kinetic energy of an ion moving with the Alfvén speed. This theoretically obtained expression for F is nearly identical to the empirical relation for F discovered in the previous simulation work.

Multipactor saturation in parallel-plate waveguides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sorolla, E.; Mattes, M.

2012-07-15

The saturation stage of a multipactor discharge is considered of interest, since it can guide towards a criterion to assess the multipactor onset. The electron cloud under multipactor regime within a parallel-plate waveguide is modeled by a thin continuous distribution of charge and the equations of motion are calculated taking into account the space charge effects. The saturation is identified by the interaction of the electron cloud with its image charge. The stability of the electron population growth is analyzed and two mechanisms of saturation to explain the steady-state multipactor for voltages near above the threshold onset are identified. Themore » impact energy in the collision against the metal plates decreases during the electron population growth due to the attraction of the electron sheet on the image through the initial plate. When this growth remains stable till the impact energy reaches the first cross-over point, the electron surface density tends to a constant value. When the stability is broken before reaching the first cross-over point the surface charge density oscillates chaotically bounded within a certain range. In this case, an expression to calculate the maximum electron surface charge density is found whose predictions agree with the simulations when the voltage is not too high.« less

Phase-space dynamics of runaway electrons in magnetic fields

DOE PAGES

Guo, Zehua; McDevitt, Christopher Joseph; Tang, Xian-Zhu

2017-02-16

Dynamics of runaway electrons in magnetic fields are governed by the competition of three dominant physics: parallel electric field acceleration, Coulomb collision, and synchrotron radiation. Examination of the energy and pitch-angle flows reveals that the presence of local vortex structure and global circulation is crucial to the saturation of primary runaway electrons. Models for the vortex structure, which has an O-point to X-point connection, and the bump of runaway electron distribution in energy space have been developed and compared against the simulation data. Lastly, identification of these velocity-space structures opens a new venue to re-examine the conventional understanding of runawaymore » electron dynamics in magnetic fields.« less

Observation of energetic electron confinement in a largely stochastic reversed-field pinch plasma

NASA Astrophysics Data System (ADS)

Clayton, D. J.; Chapman, B. E.; O'Connell, R.; Almagri, A. F.; Burke, D. R.; Forest, C. B.; Goetz, J. A.; Kaufman, M. C.; Bonomo, F.; Franz, P.; Gobbin, M.; Piovesan, P.

2010-01-01

Runaway electrons with energies >100 keV are observed with the appearance of an m =1 magnetic island in the core of otherwise stochastic Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field-pinch plasmas. The island is associated with the innermost resonant tearing mode, which is usually the largest in the m =1 spectrum. The island appears over a range of mode spectra, from those with a weakly dominant mode to those, referred to as quasi single helicity, with a strongly dominant mode. In a stochastic field, the rate of electron loss increases with electron parallel velocity. Hence, high-energy electrons imply a region of reduced stochasticity. The global energy confinement time is about the same as in plasmas without high-energy electrons or an island in the core. Hence, the region of reduced stochasticity must be localized. Within a numerical reconstruction of the magnetic field topology, high-energy electrons are substantially better confined inside the island, relative to the external region. Therefore, it is deduced that the island provides a region of reduced stochasticity and that the high-energy electrons are generated and well confined within this region.

On the Effects of Pickup Ion-driven Waves on the Diffusion Tensor of Low-energy Electrons in the Heliosphere

DOE Office of Scientific and Technical Information (OSTI.GOV)

Engelbrecht, N. Eugene, E-mail: n.eugene.engelbrecht@gmail.com

The effects of Alfvén cyclotron waves generated due to the formation in the outer heliosphere of pickup ions on the transport coefficients of low-energy electrons is investigated here. To this end, parallel mean free path (MFP) expressions are derived from quasilinear theory, employing the damping model of dynamical turbulence. These are then used as inputs for existing expressions for the perpendicular MFP and turbulence-reduced drift coefficient. Using outputs generated by a two-component turbulence transport model, the resulting diffusion coefficients are compared with those derived using a more typically assumed turbulence spectral form, which neglects the effects of pickup ion-generated waves.more » It is found that the inclusion of pickup ion effects greatly leads to considerable reductions in the parallel and perpendicular MFPs of 1–10 MeV electrons beyond ∼10 au, which are argued to have significant consequences for studies of the transport of these particles.« less

Electron trapping in rad-hard RCA IC's irradiated with electrons and gamma rays

NASA Technical Reports Server (NTRS)

Danchenko, V.; Brashears, S. S.; Fang, P. H.

1984-01-01

Enhanced electron trapping has been observed in n-channels of rad-hard CMOS devices due to electron and gamma-ray irradiation. Room-temperature annealing results in a positive shift in the threshold potential far beyond its initial value. The slope of the annealing curve immediately after irradiation was found to depend strongly on the gate bias applied during irradiation. Some dependence was also observed on the electron dose rate. No clear dependence on energy and shielding over a delidded device was observed. The threshold shift is probably due to electron trapping at the radiation-induced interface states and tunneling of electrons through the oxide-silicon energy barrier to fill the radiation-induced electron traps. A mathematical analysis, based on two parallel annealing kinetics, hole annealing and electron trapping, is applied to the data for various electron dose rates.

System and method for generating steady state confining current for a toroidal plasma fusion reactor

DOEpatents

Fisch, Nathaniel J.

1981-01-01

A system for generating steady state confining current for a toroidal plasma fusion reactor providing steady-state generation of the thermonuclear power. A dense, hot toroidal plasma is initially prepared with a confining magnetic field with toroidal and poloidal components. Continuous wave RF energy is injected into said plasma to establish a spectrum of traveling waves in the plasma, where the traveling waves have momentum components substantially either all parallel, or all anti-parallel to the confining magnetic field. The injected RF energy is phased to couple to said traveling waves with both a phase velocity component and a wave momentum component in the direction of the plasma traveling wave components. The injected RF energy has a predetermined spectrum selected so that said traveling waves couple to plasma electrons having velocities in a predetermined range .DELTA.. The velocities in the range are substantially greater than the thermal electron velocity of the plasma. In addition, the range is sufficiently broad to produce a raised plateau having width .DELTA. in the plasma electron velocity distribution so that the plateau electrons provide steady-state current to generate a poloidal magnetic field component sufficient for confining the plasma. In steady state operation of the fusion reactor, the fusion power density in the plasma exceeds the power dissipated in the plasma.

System and method for generating steady state confining current for a toroidal plasma fusion reactor

DOEpatents

Bers, Abraham

1981-01-01

A system for generating steady state confining current for a toroidal plasma fusion reactor providing steady-state generation of the thermonuclear power. A dense, hot toroidal plasma is initially prepared with a confining magnetic field with toroidal and poloidal components. Continuous wave RF energy is injected into said plasma to estalish a spectrum of traveling waves in the plasma, where the traveling waves have momentum components substantially either all parallel, or all anti-parallel to the confining magnetic field. The injected RF energy is phased to couple to said traveling waves with both a phase velocity component and a wave momentum component in the direction of the plasma traveling wave components. The injected RF energy has a predetermined spectrum selected so that said traveling waves couple to plasma electrons having velocities in a predetermined range .DELTA.. The velocities in the range are substantially greater than the thermal electron velocity of the plasma. In addition, the range is sufficiently broad to produce a raised plateau having width .DELTA. in the plasma electron velocity distribution so that the plateau electrons provide steady-state current to generate a poloidal magnetic field component sufficient for confining the plasma. In steady state operation of the fusion reactor, the fusion power density in the plasma exceeds the power dissipated inthe plasma.

Spectral Quadrature method for accurate O ( N ) electronic structure calculations of metals and insulators

DOE PAGES

Pratapa, Phanisri P.; Suryanarayana, Phanish; Pask, John E.

2015-12-02

We present the Clenshaw–Curtis Spectral Quadrature (SQ) method for real-space O(N) Density Functional Theory (DFT) calculations. In this approach, all quantities of interest are expressed as bilinear forms or sums over bilinear forms, which are then approximated by spatially localized Clenshaw–Curtis quadrature rules. This technique is identically applicable to both insulating and metallic systems, and in conjunction with local reformulation of the electrostatics, enables the O(N) evaluation of the electronic density, energy, and atomic forces. The SQ approach also permits infinite-cell calculations without recourse to Brillouin zone integration or large supercells. We employ a finite difference representation in order tomore » exploit the locality of electronic interactions in real space, enable systematic convergence, and facilitate large-scale parallel implementation. In particular, we derive expressions for the electronic density, total energy, and atomic forces that can be evaluated in O(N) operations. We demonstrate the systematic convergence of energies and forces with respect to quadrature order as well as truncation radius to the exact diagonalization result. In addition, we show convergence with respect to mesh size to established O(N 3) planewave results. In conclusion, we establish the efficiency of the proposed approach for high temperature calculations and discuss its particular suitability for large-scale parallel computation.« less

Relationship between field-aligned currents and inverted-V parallel potential drops observed at midaltitudes

NASA Astrophysics Data System (ADS)

Sakanoi, T.; Fukunishi, H.; Mukai, T.

1995-10-01

The inverted-V field-aligned acceleration region existing in the altitude range of several thousand kilometers plays an essential role for the magnetosphere-ionosphere coupling system. The adiabatic plasma theory predicts a linear relationship between field-aligned current density (J∥) and parallel potential drop (Φ∥), that is, J∥=KΦ∥, where K is the field-aligned conductance. We examined this relationship using the charged particle and magnetic field data obtained from the Akebono (Exos D) satellite. The potential drop above the satellite was derived from the peak energy of downward electrons, while the potential drop below the satellite was derived from two different methods: the peak energy of upward ions and the energy-dependent widening of electron loss cone. On the other hand, field-aligned current densities in the inverted-V region were estimated from the Akebono magnetometer data. Using these potential drops and field-aligned current densities, we estimated the linear field-aligned conductance KJΦ. Further, we obtained the corrected field-aligned conductance KCJΦ by applying the full Knight's formula to the current-voltage relationship. We also independently estimated the field-aligned conductance KTN from the number density and the thermal temperature of magnetospheric source electrons which were obtained by fitting accelerated Maxwellian functions for precipitating electrons. The results are summarized as follows: (1) The latitudinal dependence of parallel potential drops is characterized by a narrow V-shaped structure with a width of 0.4°-1.0°. (2) Although the inverted-V potential region exactly corresponds to the upward field aligned current region, the latitudinal dependence of upward current intensity is an inverted-U shape rather than an inverted-V shape. Thus it is suggested that the field-aligned conductance KCJΦ changes with a V-shaped latitudinal dependence. In many cases, KCJΦ values at the edge of the inverted-V region are about 5-10 times larger than those at the center. (3) By comparing KCJΦ with KTN, KCJΦ is found to be about 2-20 times larger than KTN. These results suggest that low-energy electrons such as trapped electrons, secondary and back-scattered electrons, and ionospheric electrons significantly contribute to upward field-aligned currents in the inverted-V region. It is therefore inferred that non adiabatic pitch angle scattering processes play an important role in the inverted-V region. .

[Determination of absorbed dose to water for high energy photon and electron beams--comparison of different dosimetry protocols].

PubMed

Zakaria, Golam Abu; Schütte, Wilhelm

2003-01-01

The determination of absorbed dose to water for high-energy photon and electron beams is performed in Germany according to the dosimetry protocol DIN 6800-2 (1997). At an international level, the main protocols used are the AAPM dosimetry protocol TG-51 (1999) and the IAEA Code of Practice TRS-398 (2000). The present paper systematically compares these three dosimetry protocols, and identifies similarities and differences. The investigations were performed using 4 and 10 MV photon beams, as well as 6, 8, 9, 10, 12 and 14 MeV electron beams. Two cylindrical and two plane-parallel type chambers were used for measurements. In general, the discrepancies among the three protocols were 1.0% for photon beams and 1.6% for electron beams. Comparative measurements in the context of measurement technical control (MTK) with TLD showed a deviation of less than 1.3% between the measurements obtained according to protocols DIN 6800-2 and MTK (exceptions: 4 MV photons with 2.9% and 6 MeV electrons with 2.4%). While only cylindrical chambers were used for photon beams, measurements of electron beams were performed using both cylindrical and plane-parallel chambers (the latter used after a cross-calibration to a cylindrical chamber, as required by the respective dosimetry protocols). Notably, unlike recommended in the corresponding protocols, we found out that cylindrical chambers can be used also for energies from 6 to 10 MeV.

Parallel Electric Field on Auroral Magnetic Field Lines.

NASA Astrophysics Data System (ADS)

Yeh, Huey-Ching Betty

1982-03-01

The interaction of Birkeland (magnetic-field-aligned) current carriers and the Earth's magnetic field results in electrostatic potential drops along magnetic field lines. The statistical distributions of the field-aligned potential difference (phi)(,(PARLL)) were determined from the energy spectra of electron inverted "V" events observed at ionospheric altitude for different conditions of geomagnetic activity as indicated by the AE index. Data of 1270 electron inverted "V"'s were obtained from Low-Energy Electron measurements of the Atmosphere Explorer-C and -D Satellite (despun mode) in the interval January 1974-April 1976. In general, (phi)(,(PARLL)) is largest in the dusk to pre-midnight sector, smaller in the post-midnight to dawn sector, and smallest in the near noon sector during quiet and disturbed geomagnetic conditions; there is a steady dusk-dawn-noon asymmetry of the global (phi)(,(PARLL)) distribution. As the geomagnetic activity level increases, the (phi)(,(PARLL)) pattern expands to lower invariant latitudes, and the magnitude of (phi)(,(PARLL)) in the 13-24 magnetic local time sector increases significantly. The spatial structure and intensity variation of the global (phi)(,(PARLL)) distribution are statistically more variable, and the magnitudes of (phi)(,(PARLL)) have smaller correlation with the AE-index, in the post-midnight to dawn sector. A strong correlation is found to exist between upward Birkeland current systems and global parallel potential drops, and between auroral electron precipitation patterns and parallel potential drops, regarding their mophology, their intensity and their dependence of geomagnetic activity. An analysis of the fine-scale simultaneous current-voltage relationship for upward Birkeland currents in Region 1 shows that typical field-aligned potential drops are consistent with model predictions based on linear acceleration of the charge carriers through an electrostatic potential drop along convergent magnetic field lines to maintain current continuity. In a steady state, this model of simple electrostatic acceleration without anomalous resistivity also predicts observable relations between global parallel currents and parallel potential drops and between global energy deposition and parallel potential drops. The temperature, density, and species of the unaccelerated charge carriers are the relevant parameters of the model. The dusk-dawn -noon asymmetry of the global (phi)(,(PARLL)) distribution can be explained by the above steady-state (phi)(,(PARLL)) process if we associate the source regions of upward Birkeland current carriers in Region 1, Region 2, and the cusp region with the plasma sheet boundary layer, the near-Earth plasma sheet, and the magnetosheath, respectively. The results of this study provide observational information on the global distribution of parallel potential drops and the prevailing process of generating and maintaining potential gradients (parallel electric fields) along auroral magnetic field lines.

Characteristics of a p-Si detector in high energy electron fields.

PubMed

Rikner, G

1985-01-01

Comparison of depth ionization distributions from a silicon semiconductor detector and depth dose curves from a plane parallel ionization chamber show that a semiconductor detector of p-type is well suited for relative electron dosimetry in the energy range of 6 to 20 MeV in Ep,0. Maximum deviations of the order of 1.5 per cent and of 1 mm were obtained down to a phantom depth of about 1 mm. The directional dependence of the detector was about 4 per cent.

Particle-in-cell simulations of the critical ionization velocity effect in finite size clouds

NASA Technical Reports Server (NTRS)

Moghaddam-Taaheri, E.; Lu, G.; Goertz, C. K.; Nishikawa, K. - I.

1994-01-01

The critical ionization velocity (CIV) mechanism in a finite size cloud is studied with a series of electrostatic particle-in-cell simulations. It is observed that an initial seed ionization, produced by non-CIV mechanisms, generates a cross-field ion beam which excites a modified beam-plasma instability (MBPI) with frequency in the range of the lower hybrid frequency. The excited waves accelerate electrons along the magnetic field up to the ion drift energy that exceeds the ionization energy of the neutral atoms. The heated electrons in turn enhance the ion beam by electron-neutral impact ionization, which establishes a positive feedback loop in maintaining the CIV process. It is also found that the efficiency of the CIV mechanism depends on the finite size of the gas cloud in the following ways: (1) Along the ambient magnetic field the finite size of the cloud, L (sub parallel), restricts the growth of the fastest growing mode, with a wavelength lambda (sub m parallel), of the MBPI. The parallel electron heating at wave saturation scales approximately as (L (sub parallel)/lambda (sub m parallel)) (exp 1/2); (2) Momentum coupling between the cloud and the ambient plasma via the Alfven waves occurs as a result of the finite size of the cloud in the direction perpendicular to both the ambient magnetic field and the neutral drift. This reduces exponentially with time the relative drift between the ambient plasma and the neutrals. The timescale is inversely proportional to the Alfven velocity. (3) The transvers e charge separation field across the cloud was found to result in the modulation of the beam velocity which reduces the parallel heating of electrons and increases the transverse acceleration of electrons. (4) Some energetic electrons are lost from the cloud along the magnetic field at a rate characterized by the acoustic velocity, instead of the electron thermal velocity. The loss of energetic electrons from the cloud seems to be larger in the direction of plasma drift relative to the neutrals, where the loss rate is characterized by the neutral drift velocity. It is also shown that a factor of 4 increase in the ambient plasma density, increases the CIV ionization yield by almost 2 orders of magnitude at the end of a typical run. It is concluded that a larger ambient plasma density can result in a larger CIV yield because of (1) larger seed ion production by non-CIV mechanisms, (2) smaller Alfven velocity and hence weak momentum coupling, and (3) smaller ratio of the ion beam density to the ambient ion density, and therefore a weaker modulation of the beam velocity. The simulation results are used to interpret various chemical release experiments in space.

Electron temperature and heat load measurements in the COMPASS divertor using the new system of probes

NASA Astrophysics Data System (ADS)

Adamek, J.; Seidl, J.; Horacek, J.; Komm, M.; Eich, T.; Panek, R.; Cavalier, J.; Devitre, A.; Peterka, M.; Vondracek, P.; Stöckel, J.; Sestak, D.; Grover, O.; Bilkova, P.; Böhm, P.; Varju, J.; Havranek, A.; Weinzettl, V.; Lovell, J.; Dimitrova, M.; Mitosinkova, K.; Dejarnac, R.; Hron, M.; The COMPASS Team; The EUROfusion MST1 Team

2017-11-01

A new system of probes was recently installed in the divertor of tokamak COMPASS in order to investigate the ELM energy density with high spatial and temporal resolution. The new system consists of two arrays of rooftop-shaped Langmuir probes (LPs) used to measure the floating potential or the ion saturation current density and one array of Ball-pen probes (BPPs) used to measure the plasma potential with a spatial resolution of ~3.5 mm. The combination of floating BPPs and LPs yields the electron temperature with microsecond temporal resolution. We report on the design of the new divertor probe arrays and first results of electron temperature profile measurements in ELMy H-mode and L-mode. We also present comparative measurements of the parallel heat flux using the new probe arrays and fast infrared termography (IR) data during L-mode with excellent agreement between both techniques using a heat power transmission coefficient γ  =  7. The ELM energy density {{\\varepsilon }\\parallel } was measured during a set of NBI assisted ELMy H-mode discharges. The peak values of {{\\varepsilon }\\parallel } were compared with those predicted by model and with experimental data from JET, AUG and MAST with a good agreement.

Laser-induced free-free transitions in elastic electron scattering from CO2

NASA Astrophysics Data System (ADS)

Musa, Mohamed; MacDonald, Amy; Tidswell, Lisa; Holmes, Jim; St. Francis Xavier Laser Scattering Lab Team

2011-03-01

This report presents measurements of laser-induced free-free transitions of electrons scattered from CO2 molecules in the ground electronic state at incident electron energies of 3.8 and 5.8 eV under pulsed CO2 laser field. The differential cross section of free-free transitions involving absorption and emission of up to two photons were measured at various scattering angles with the polarization of the laser either parallel with or perpendicular to the the momentum change vector of the scattered electrons. The results of the parallel geometry are found to be in qualitative agreement with the predictions of the Kroll-Watson approximation within the experimental uncertainty whereas those of the perpendicular geometry show marked discrepancy with the Kroll-Watson predictions. This work was supported by the Natural Sciences and Engineering Research Council of Canada and the St. Francis Xavier University Council for Research.

Sub-GeV dark matter detection with electron recoils in carbon nanotubes

NASA Astrophysics Data System (ADS)

Cavoto, G.; Luchetta, F.; Polosa, A. D.

2018-01-01

Directional detection of Dark Matter particles (DM) in the MeV mass range could be accomplished by studying electron recoils in large arrays of parallel carbon nanotubes. In a scattering process with a lattice electron, a DM particle might transfer sufficient energy to eject it from the nanotube surface. An external electric field is added to drive the electron from the open ends of the array to the detection region. The anisotropic response of this detection scheme, as a function of the orientation of the target with respect to the DM wind, is calculated, and it is concluded that no direct measurement of the electron ejection angle is needed to explore significant regions of the light DM exclusion plot. A compact sensor, in which the cathode element is substituted with a dense array of parallel carbon nanotubes, could serve as the basic detection unit.

Electron Scattering by High-Frequency Whistler Waves at Earth's Bow Shock

NASA Technical Reports Server (NTRS)

Oka, M.; Wilson, L. B., III; Phan, T. D.; Hull, A. J.; Amano, T.; Hoshino, M.; Argall, M. R.; Le Contel, O.; Agapitov, O.; Gersham, D. J.;

Electron Scattering by High-frequency Whistler Waves at Earth’s Bow Shock

NASA Astrophysics Data System (ADS)

Oka, M.; Wilson, L. B., III; Phan, T. D.; Hull, A. J.; Amano, T.; Hoshino, M.; Argall, M. R.; Le Contel, O.; Agapitov, O.; Gershman, D. J.; Khotyaintsev, Y. V.; Burch, J. L.; Torbert, R. B.; Pollock, C.; Dorelli, J. C.; Giles, B. L.; Moore, T. E.; Saito, Y.; Avanov, L. A.; Paterson, W.; Ergun, R. E.; Strangeway, R. J.; Russell, C. T.; Lindqvist, P. A.

2017-06-01

Electrons are accelerated to non-thermal energies at shocks in space and astrophysical environments. While different mechanisms of electron acceleration have been proposed, it remains unclear how non-thermal electrons are produced out of the thermal plasma pool. Here, we report in situ evidence of pitch-angle scattering of non-thermal electrons by whistler waves at Earth’s bow shock. On 2015 November 4, the Magnetospheric Multiscale (MMS) mission crossed the bow shock with an Alfvén Mach number ˜11 and a shock angle ˜84°. In the ramp and overshoot regions, MMS revealed bursty enhancements of non-thermal (0.5-2 keV) electron flux, correlated with high-frequency (0.2-0.4 {{{Ω }}}{ce}, where {{{Ω }}}{ce} is the cyclotron frequency) parallel-propagating whistler waves. The electron velocity distribution (measured at 30 ms cadence) showed an enhanced gradient of phase-space density at and around the region where the electron velocity component parallel to the magnetic field matched the resonant energy inferred from the wave frequency range. The flux of 0.5 keV electrons (measured at 1 ms cadence) showed fluctuations with the same frequency. These features indicate that non-thermal electrons were pitch-angle scattered by cyclotron resonance with the high-frequency whistler waves. However, the precise role of the pitch-angle scattering by the higher-frequency whistler waves and possible nonlinear effects in the electron acceleration process remains unclear.

Compton Scattering Cross Sections in Strong Magnetic Fields: Advances for Neutron Star Applications

NASA Astrophysics Data System (ADS)

Eiles, Matthew; Gonthier, P. L.; Baring, M. G.; Wadiasingh, Z.

2013-04-01

Various telescopes including RXTE, INTEGRAL and Suzaku have detected non-thermal X-ray emission in the 10 - 200 keV band from strongly magnetic neutron stars. Inverse Compton scattering, a quantum-electrodynamical process, is believed to be a leading candidate for the production of this intense X-ray radiation. Magnetospheric conditions are such that electrons may well possess ultra-relativistic energies, which lead to attractive simplifications of the cross section. We have recently addressed such a case by developing compact analytic expressions using correct spin-dependent widths and Sokolov & Ternov (ST) basis states, focusing specifically on ground state-to-ground state scattering. However, inverse Compton scattering can cool electrons down to mildly-relativistic energies, necessitating the development of a more general case where the incoming photons acquire nonzero incident angles relative to the field in the rest frame of the electron, and the intermediate state can be excited to arbitrary Landau levels. In this paper, we develop results pertaining to this general case using ST formalism, and treating the plethora of harmonic resonances associated with various cyclotron transitions between Landau states. Four possible scattering modes (parallel-parallel, perpendicular-perpendicular, parallel-perpendicular, and perpendicular-parallel) encapsulate the polarization dependence of the cross section. We present preliminary analytic and numerical investigations of the magnitude of the extra Landau state contributions to obtain the full cross section, and compare these new analytic developments with the spin-averaged cross sections, which we develop in parallel. Results will find application to various neutron star problems, including computation of Eddington luminosities in the magnetospheres of magnetars. We express our gratitude for the generous support of the Michigan Space Grant Consortium, of the National Science Foundation (REU and RUI), and the NASA Astrophysics Theory and Fundamental Program.

Effects of a parallel resistor on electrical characteristics of a piezoelectric transformer in open-circuit transient state.

PubMed

Chang, Kuo-Tsai

2007-01-01

This paper investigates electrical transient characteristics of a Rosen-type piezoelectric transformer (PT), including maximum voltages, time constants, energy losses and average powers, and their improvements immediately after turning OFF. A parallel resistor connected to both input terminals of the PT is needed to improve the transient characteristics. An equivalent circuit for the PT is first given. Then, an open-circuit voltage, involving a direct current (DC) component and an alternating current (AC) component, and its related energy losses are derived from the equivalent circuit with initial conditions. Moreover, an AC power control system, including a DC-to-AC resonant inverter, a control switch and electronic instruments, is constructed to determine the electrical characteristics of the OFF transient state. Furthermore, the effects of the parallel resistor on the transient characteristics at different parallel resistances are measured. The advantages of adding the parallel resistor also are discussed. From the measured results, the DC time constant is greatly decreased from 9 to 0.04 ms by a 10 k(omega) parallel resistance under open output.

Reversible electron heating vs. wave-particle interactions in quasi-perpendicular shocks

NASA Technical Reports Server (NTRS)

Veltri, P.; Mangeney, A.; Scudder, J. D.

1992-01-01

The energy necessary to explain the electron heating in quasi-perpendicular collisionless shocks can be derived either from the electron acceleration in the d.c. cross shock electric potential, or by the interactions between the electrons and the waves existing in the shock. A Monte Carlo simulation has been performed to study the electron distribution function evolution through the shock structure, with and without particle diffusion on waves. This simulation has allowed us to clarify the relative importance of the two possible energy sources; in particular it has been shown that the electron parallel temperature is determined by the d.c. electromagnetic field and not by any wave-particle-induced heating. Wave particle interactions are effective in smoothing out the large gradients in phase space produced by the 'reversible' motion of the electrons, thus producing a 'cooling' of the electrons.

Anomalous pinch of turbulent plasmas driven by the magnetic-drift-induced Lorentz force through the Stokes-Einstein relation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Shaojie, E-mail: wangsj@ustc.edu.cn

It is found that the Lorentz force generated by the magnetic drift drives a generic plasma pinch flux of particle, energy and momentum through the Stokes-Einstein relation. The proposed theoretical model applies for both electrons and ions, trapped particles, and passing particles. An anomalous parallel current pinch due to the electrostatic turbulence with long parallel wave-length is predicted.

Outrunning damage: Electrons vs X-rays-timescales and mechanisms.

PubMed

Spence, John C H

2017-07-01

Toward the end of his career, Zewail developed strong interest in fast electron spectroscopy and imaging, a field to which he made important contributions toward his aim of making molecular movies free of radiation damage. We therefore compare here the atomistic mechanisms leading to destruction of protein samples in diffract-and-destroy experiments for the cases of high-energy electron beam irradiation and X-ray laser pulses. The damage processes and their time-scales are compared and relevant elastic, inelastic, and photoelectron cross sections are given. Inelastic mean-free paths for ejected electrons at very low energies in insulators are compared with the bioparticle size. The dose rate and structural damage rate for electrons are found to be much lower, allowing longer pulses, reduced beam current, and Coulomb interactions for the formation of smaller probes. High-angle electron scattering from the nucleus, which has no parallel in the X-ray case, tracks the slowly moving nuclei during the explosion, just as the gain of the XFEL (X-ray free-electron laser) has no parallel in the electron case. Despite reduced damage and much larger elastic scattering cross sections in the electron case, leading to not dissimilar elastic scattering rates (when account is taken of the greatly increased incident XFEL fluence), progress for single-particle electron diffraction is seen to depend on the effort to reduce emittance growth due to Coulomb interactions, and so allow formation of intense sub-micron beams no larger than a virus.

Outrunning damage: Electrons vs X-rays—timescales and mechanisms

PubMed Central

Spence, John C. H.

2017-01-01

Toward the end of his career, Zewail developed strong interest in fast electron spectroscopy and imaging, a field to which he made important contributions toward his aim of making molecular movies free of radiation damage. We therefore compare here the atomistic mechanisms leading to destruction of protein samples in diffract-and-destroy experiments for the cases of high-energy electron beam irradiation and X-ray laser pulses. The damage processes and their time-scales are compared and relevant elastic, inelastic, and photoelectron cross sections are given. Inelastic mean-free paths for ejected electrons at very low energies in insulators are compared with the bioparticle size. The dose rate and structural damage rate for electrons are found to be much lower, allowing longer pulses, reduced beam current, and Coulomb interactions for the formation of smaller probes. High-angle electron scattering from the nucleus, which has no parallel in the X-ray case, tracks the slowly moving nuclei during the explosion, just as the gain of the XFEL (X-ray free-electron laser) has no parallel in the electron case. Despite reduced damage and much larger elastic scattering cross sections in the electron case, leading to not dissimilar elastic scattering rates (when account is taken of the greatly increased incident XFEL fluence), progress for single-particle electron diffraction is seen to depend on the effort to reduce emittance growth due to Coulomb interactions, and so allow formation of intense sub-micron beams no larger than a virus. PMID:28653018

Proton-impact ionization cross sections of adenine measured at 0.5 and 2.0 MeV by electron spectroscopy

NASA Astrophysics Data System (ADS)

Iriki, Y.; Kikuchi, Y.; Imai, M.; Itoh, A.

2011-11-01

Double-differential ionization cross sections (DDCSs) of vapor-phase adenine molecules (C5H5N5) by 0.5- and 2.0-MeV proton impact have been measured by the electron spectroscopy method. Electrons ejected from adenine were analyzed by a 45∘ parallel-plate electrostatic spectrometer over an energy range of 1.0-1000 eV at emission angles from 15∘ to 165∘. Single-differential cross sections (SDCSs) and total ionization cross sections (TICSs) were also deduced. It was found from the Platzman plot, defined as SDCSs divided by the classical Rutherford knock-on cross sections per target electron, that the SDCSs at higher electron energies are proportional to the total number of valence electrons (50) of adenine, while those at low-energy electrons are highly enhanced due to dipole and higher-order interactions. The present results of TICS are in fairly good agreement with recent classical trajectory Monte Carlo calculations, and moreover, a simple analytical formula gives nearly equivalent cross sections in magnitude at the incident proton energies investigated.

Ion Beam Neutralization Using FEAs and Mirror Magnetic Fields

NASA Astrophysics Data System (ADS)

Nicolaescu, Dan; Sakai, Shigeki; Gotoh, Yasuhito; Ishikawa, Junzo

2011-01-01

Advanced implantation systems used for semiconductor processing require transportation of ion beams which are quasi-parallel and have low energy, such as (11B+,31P+,75As+) with energy in the range Eion = 200-1000 eV. Compensation of ion beam divergence may be obtained through electron injection and confinement in regions of non-uniform magnetic fields. Field emitter arrays with special properties are used as electron sources. The present study shows that electron confinement takes place in regions of gradient magnetic field, such as nearby analyzing, collimator and final energy magnets of the ion beam line. Modeling results have been obtained using Opera3D/Tosca/Scala. In regions of gradient magnetic field, electrons have helical trajectories which are confined like a cloud inside curved "magnetic bottles". An optimal range of positions with respect to the magnet for placing electron sources in gradient magnetic field has been shown to exist.

Collisionless dissipation processes in quasi-parallel shocks. [in solar wind

NASA Technical Reports Server (NTRS)

Quest, K. B.; Forslund, D. W.; Brackbill, J. U.; Lee, K.

1983-01-01

The evolution of collisionless, quasi-parallel shocks (the angle between the shock normal and the upstream magnetic field being less than 45 deg) is examined using two dimensional particle simulations. Reflected ions upstream from the shock are observed with average guiding center velocity and gyrational energy which agree well with the prediction of simple specular reflection. Strong ion heating through the shock ramp is apparently caused by large amplitude whistler turbulence. A flux of suprathermal electrons is also the magnetic field direction. Much stronger ion heating occurs in the shock than electron heating. The relevance of this work to the earth's bow shock is discussed.

Observations of ionospheric electron beams in the plasma sheet.

PubMed

Zheng, H; Fu, S Y; Zong, Q G; Pu, Z Y; Wang, Y F; Parks, G K

2012-11-16

Electrons streaming along the magnetic field direction are frequently observed in the plasma sheet of Earth's geomagnetic tail. The impact of these field-aligned electrons on the dynamics of the geomagnetic tail is however not well understood. Here we report the first detection of field-aligned electrons with fluxes increasing at ~1 keV forming a "cool" beam just prior to the dissipation of energy in the current sheet. These field-aligned beams at ~15 R(E) in the plasma sheet are nearly identical to those commonly observed at auroral altitudes, suggesting the beams are auroral electrons accelerated upward by electric fields parallel (E([parallel])) to the geomagnetic field. The density of the beams relative to the ambient electron density is δn(b)/n(e)~5-13% and the current carried by the beams is ~10(-8)-10(-7) A m(-2). These beams in high β plasmas with large density and temperature gradients appear to satisfy the Bohm criteria to initiate current driven instabilities.

Controllable spin polarization and spin filtering in a zigzag silicene nanoribbon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Farokhnezhad, Mohsen, E-mail: Mohsen-farokhnezhad@physics.iust.ac.ir; Esmaeilzadeh, Mahdi, E-mail: mahdi@iust.ac.ir; Pournaghavi, Nezhat

2015-05-07

Using non-equilibrium Green's function, we study the spin-dependent electron transport properties in a zigzag silicene nanoribbon. To produce and control spin polarization, it is assumed that two ferromagnetic strips are deposited on the both edges of the silicene nanoribbon and an electric field is perpendicularly applied to the nanoribbon plane. The spin polarization is studied for both parallel and anti-parallel configurations of exchange magnetic fields induced by the ferromagnetic strips. We find that complete spin polarization can take place in the presence of perpendicular electric field for anti-parallel configuration and the nanoribbon can work as a perfect spin filter. Themore » spin direction of transmitted electrons can be easily changed from up to down and vice versa by reversing the electric field direction. For parallel configuration, perfect spin filtering can occur even in the absence of electric field. In this case, the spin direction can be changed by changing the electron energy. Finally, we investigate the effects of nonmagnetic Anderson disorder on spin dependent conductance and find that the perfect spin filtering properties of nanoribbon are destroyed by strong disorder, but the nanoribbon retains these properties in the presence of weak disorder.« less

Streamer properties and associated x-rays in perturbed air

NASA Astrophysics Data System (ADS)

Köhn, C.; Chanrion, O.; Babich, L. P.; Neubert, T.

2018-01-01

Streamers are ionization waves in electric discharges. One of the key ingredients of streamer propagation is an ambient gas that serves as a source of free electrons. Here, we explore the dependence of streamer dynamics on different spatial distributions of ambient air molecules. We vary the spatial profile of air parallel and perpendicular to the ambient electric field. We consider local sinusoidal perturbations of 5%-100%, as induced from discharge shock waves. We use a cylindrically symmetric particle-in-cell code to simulate the evolution of bidirectional streamers and compare the electron density, electric field, streamer velocity and electron energy of streamers in uniform air and in perturbed air. In all considered cases, the motion is driven along in decreasing air density and damped along increasing air density. Perturbations of at most 5%-10% change the velocity differences by up to approximately 40%. Perturbations perpendicular to the electric field additionally squeeze or branch streamers. Air variations can thus partly explain the difference of velocities and morphologies of streamer discharges. In cases with large perturbations, electrons gain energies of up to 30 keV compared to 100 eV in uniformly distributed air. For such perturbations parallel to the ambient electric field, we see the spontaneous initiation of a negative streamer; for perpendicular perturbations, x-rays with energies of up to 20 keV are emitted within 0.17 ns.

Theory of energy and power flow of plasmonic waves on single-walled carbon nanotubes

NASA Astrophysics Data System (ADS)

Moradi, Afshin

2017-10-01

The energy theorem of electrodynamics is extended so as to apply to the plasmonic waves on single-walled carbon nanotubes which propagate parallel to the axial direction of the system and are periodic waves in the azimuthal direction. Electronic excitations on the nanotube surface are modeled by an infinitesimally thin layer of free-electron gas which is described by means of the linearized hydrodynamic theory. General expressions of energy and power flow associated with surface waves are obtained by solving Maxwell and hydrodynamic equations with appropriate boundary conditions. Numerical results for the transverse magnetic mode show that energy, power flow, and energy transport velocity of the plasmonic waves strongly depend on the nanotube radius in the long-wavelength region.

Investigation of indirect excitons in bulk 2H-MoS2 using transmission electron energy-loss spectroscopy

NASA Astrophysics Data System (ADS)

Habenicht, Carsten; Schuster, Roman; Knupfer, Martin; Büchner, Bernd

2018-05-01

We have investigated indirect excitons in bulk 2H-MoS2 using transmission electron energy-loss spectroscopy. The electron energy-loss spectra were measured for various momentum transfer values parallel to the and directions of the Brillouin zone. The results allowed the identification of the indirect excitons between the valence band K v and conduction band Λc points, the Γv and K c points as well as adjacent K v and points. The energy-momentum dispersions for the K v-Λc, Γv-K c and K v1- excitons along the line are presented. The former two transitions exhibit a quadratic dispersion which allowed calculating their effective exciton masses based on the effective mass approximation. The K v1- transition follows a more linear dispersion relationship.

Acceleration of auroral electrons in parallel electric fields

NASA Technical Reports Server (NTRS)

Kaufmann, R. L.; Walker, D. N.; Arnoldy, R. L.

1976-01-01

Rocket observations of auroral electrons are compared with the predictions of a number of theoretical acceleration mechanisms that involve an electric field parallel to the earth's magnetic field. The theoretical models are discussed in terms of required plasma sources, the location of the acceleration region, and properties of necessary wave-particle scattering mechanisms. We have been unable to find any steady state scatter-free electric field configuration that predicts electron flux distributions in agreement with the observations. The addition of a fluctuating electric field or wave-particle scattering several thousand kilometers above the rocket can modify the theoretical flux distributions so that they agree with measurements. The presence of very narrow energy peaks in the flux contours implies a characteristic temperature of several tens of electron volts or less for the source of field-aligned auroral electrons and a temperature of several hundred electron volts or less for the relatively isotropic 'monoenergetic' auroral electrons. The temperature of the field-aligned electrons is more representative of the magnetosheath or possibly the ionosphere as a source region than of the plasma sheet.

Electron energy recovery system for negative ion sources

DOEpatents

Dagenhart, William K.; Stirling, William L.

1982-01-01

An electron energy recovery system for negative ion sources is provided. The system, employs crossed electric and magnetic fields to separate the electrons from ions as they are extracted from a negative ion source plasma generator and before the ions are accelerated to their full kinetic energy. With the electric and magnetic fields oriented 90.degree. to each other, the electrons are separated from the plasma and remain at approximately the electrical potential of the generator in which they were generated. The electrons migrate from the ion beam path in a precessing motion out of the ion accelerating field region into an electron recovery region provided by a specially designed electron collector electrode. The electron collector electrode is uniformly spaced from a surface of the ion generator which is transverse to the direction of migration of the electrons and the two surfaces are contoured in a matching relationship which departs from a planar configuration to provide an electric field component in the recovery region which is parallel to the magnetic field thereby forcing the electrons to be directed into and collected by the electron collector electrode. The collector electrode is maintained at a potential slightly positive with respect to the ion generator so that the electrons are collected at a small fraction of the full accelerating supply voltage energy.

SM91: Observations of interchange between acceleration and thermalization processes in auroral electrons

NASA Technical Reports Server (NTRS)

Pongratz, M.

1972-01-01

Results from a Nike-Tomahawk sounding rocket flight launched from Fort Churchill are presented. The rocket was launched into a breakup aurora at magnetic local midnight on 21 March 1968. The rocket was instrumented to measure electrons with an electrostatic analyzer electron spectrometer which made 29 measurements in the energy interval 0.5 KeV to 30 KeV. Complete energy spectra were obtained at a rate of 10/sec. Pitch angle information is presented via 3 computed average per rocket spin. The dumped electron average corresponds to averages over electrons moving nearly parallel to the B vector. The mirroring electron average corresponds to averages over electrons moving nearly perpendicular to the B vector. The average was also computed over the entire downward hemisphere (the precipitated electron average). The observations were obtained in an altitude range of 10 km at 230 km altitude.

Experimental determination of the effective point of measurement of cylindrical ionization chambers for high-energy photon and electron beams.

PubMed

Huang, Yanxiao; Willomitzer, Christian; Zakaria, Golam Abu; Hartmann, Guenther H

2010-01-01

Measurements of depth-dose curves in water phantom using a cylindrical ionization chamber require that its effective point of measurement is located at the measuring depth. Recommendations for the position of the effective point of measurement with respect to the central axis valid for high-energy electron and photon beams are given in dosimetry protocols. According to these protocols, the use of a constant shift P(eff) is currently recommended. However, this is still based on a very limited set of experimental results. It is therefore expected that an improved knowledge of the exact position of the effective point of measurement will further improve the accuracy of dosimetry. Recent publications have revealed that the position of the effective point of measurement is indeed varying with beam energy, field size and also with chamber geometry. The aim of this study is to investigate whether the shift of P(eff) can be taken to be constant and independent from the beam energy. An experimental determination of the effective point of measurement is presented based on a comparison between cylindrical chambers and a plane-parallel chamber using conventional dosimetry equipment. For electron beams, the determination is based on the comparison of halfvalue depth R(50) between the cylindrical chamber of interest and a well guarded plane-parallel Roos chamber. For photon beams, the depth of dose maximum, d(max), the depth of 80% dose, d(80), and the dose parameter PDD(10) were used. It was again found that the effective point of measurement for both, electron and photon beams Dosimetry, depends on the beam energy. The deviation from a constant value remains very small for photons, whereas significant deviations were found for electrons. It is therefore concluded that use of a single upstream shift value from the centre of the cylindrical chamber as recommended in current dosimetry protocols is adequate for photons, however inadequate for accurate electron beam dosimetry.

PAREMD: A parallel program for the evaluation of momentum space properties of atoms and molecules

NASA Astrophysics Data System (ADS)

Meena, Deep Raj; Gadre, Shridhar R.; Balanarayan, P.

2018-03-01

The present work describes a code for evaluating the electron momentum density (EMD), its moments and the associated Shannon information entropy for a multi-electron molecular system. The code works specifically for electronic wave functions obtained from traditional electronic structure packages such as GAMESS and GAUSSIAN. For the momentum space orbitals, the general expression for Gaussian basis sets in position space is analytically Fourier transformed to momentum space Gaussian basis functions. The molecular orbital coefficients of the wave function are taken as an input from the output file of the electronic structure calculation. The analytic expressions of EMD are evaluated over a fine grid and the accuracy of the code is verified by a normalization check and a numerical kinetic energy evaluation which is compared with the analytic kinetic energy given by the electronic structure package. Apart from electron momentum density, electron density in position space has also been integrated into this package. The program is written in C++ and is executed through a Shell script. It is also tuned for multicore machines with shared memory through OpenMP. The program has been tested for a variety of molecules and correlated methods such as CISD, Møller-Plesset second order (MP2) theory and density functional methods. For correlated methods, the PAREMD program uses natural spin orbitals as an input. The program has been benchmarked for a variety of Gaussian basis sets for different molecules showing a linear speedup on a parallel architecture.

Endosymbiosis and the design of eukaryotic electron transport.

PubMed

Berry, Stephan

2003-09-30

The bioenergetic organelles of eukaryotic cells, mitochondria and chloroplasts, are derived from endosymbiotic bacteria. Their electron transport chains (ETCs) resemble those of free-living bacteria, but were tailored for energy transformation within the host cell. Parallel evolutionary processes in mitochondria and chloroplasts include reductive as well as expansive events: On one hand, bacterial complexes were lost in eukaryotes with a concomitant loss of metabolic flexibility. On the other hand, new subunits have been added to the remaining bacterial complexes, new complexes have been introduced, and elaborate folding patterns of the thylakoid and mitochondrial inner membranes have emerged. Some bacterial pathways were reinvented independently by eukaryotes, such as parallel routes for quinol oxidation or the use of various anaerobic electron acceptors. Multicellular organization and ontogenetic cycles in eukaryotes gave rise to further modifications of the bioenergetic organelles. Besides mitochondria and chloroplasts, eukaryotes have ETCs in other membranes, such as the plasma membrane (PM) redox system, or the cytochrome P450 (CYP) system. These systems have fewer complexes and simpler branching patterns than those in energy-transforming organelles, and they are often adapted to non-bioenergetic functions such as detoxification or cellular defense.

Whistlers, Helicons, Lower Hybrid Waves: the Physics of RF Wave Absorption Without Cyclotron Resonances

NASA Astrophysics Data System (ADS)

Pinsker, R. I.

2014-10-01

In hot magnetized plasmas, two types of linear collisionless absorption processes are used to heat and drive noninductive current: absorption at ion or electron cyclotron resonances and their harmonics, and absorption by Landau damping and the transit-time-magnetic-pumping (TTMP) interactions. This tutorial discusses the latter process, i.e., parallel interactions between rf waves and electrons in which cyclotron resonance is not involved. Electron damping by the parallel interactions can be important in the ICRF, particularly in the higher harmonic region where competing ion cyclotron damping is weak, as well as in the Lower Hybrid Range of Frequencies (LHRF), which is in the neighborhood of the geometric mean of the ion and electron cyclotron frequencies. On the other hand, absorption by parallel processes is not significant in conventional ECRF schemes. Parallel interactions are especially important for the realization of high current drive efficiency with rf waves, and an application of particular recent interest is current drive with the whistler or helicon wave at high to very high (i.e., the LHRF) ion cyclotron harmonics. The scaling of absorption by parallel interactions with wave frequency is examined and the advantages and disadvantages of fast (helicons/whistlers) and slow (lower hybrid) waves in the LHRF in the context of reactor-grade tokamak plasmas are compared. In this frequency range, both wave modes can propagate in a significant fraction of the discharge volume; the ways in which the two waves can interact with each other are considered. The use of parallel interactions to heat and drive current in practice will be illustrated with examples from past experiments; also looking forward, this tutorial will provide an overview of potential applications in tokamak reactors. Supported by the US Department of Energy under DE-FC02-04ER54698.

Enhanced pyroelectric and piezoelectric properties of PZT with aligned porosity for energy harvesting applications† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta00967d Click here for additional data file.

PubMed Central

Zhang, Yan; Xie, Mengying; Roscow, James; Bao, Yinxiang; Zhou, Kechao

2017-01-01

This paper demonstrates the significant benefits of exploiting highly aligned porosity in piezoelectric and pyroelectric materials for improved energy harvesting performance. Porous lead zirconate (PZT) ceramics with aligned pore channels and varying fractions of porosity were manufactured in a water-based suspension using freeze-casting. The aligned porous PZT ceramics were characterized in detail for both piezoelectric and pyroelectric properties and their energy harvesting performance figures of merit were assessed parallel and perpendicular to the freezing direction. As a result of the introduction of porosity into the ceramic microstructure, high piezoelectric and pyroelectric harvesting figures of merits were achieved for porous freeze-cast PZT compared to dense PZT due to the reduced permittivity and volume specific heat capacity. Experimental results were compared to parallel and series analytical models with good agreement and the PZT with porosity aligned parallel to the freezing direction exhibited the highest piezoelectric and pyroelectric harvesting response; this was a result of the enhanced interconnectivity of the ferroelectric material along the poling direction and reduced fraction of unpoled material that leads to a higher polarization. A complete thermal energy harvesting system, composed of a parallel-aligned PZT harvester element and an AC/DC converter, was successfully demonstrated by charging a storage capacitor. The maximum energy density generated by the 60 vol% porous parallel-connected PZT when subjected to thermal oscillations was 1653 μJ cm–3, which was 374% higher than that of the dense PZT with an energy density of 446 μJ cm–3. The results are beneficial for the design and manufacture of high performance porous pyroelectric and piezoelectric materials in devices for energy harvesting and sensor applications. PMID:28580142

Computation of the free energy due to electron density fluctuation of a solute in solution: A QM/MM method with perturbation approach combined with a theory of solutions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Suzuoka, Daiki; Takahashi, Hideaki, E-mail: hideaki@m.tohoku.ac.jp; Morita, Akihiro

2014-04-07

We developed a perturbation approach to compute solvation free energy Δμ within the framework of QM (quantum mechanical)/MM (molecular mechanical) method combined with a theory of energy representation (QM/MM-ER). The energy shift η of the whole system due to the electronic polarization of the solute is evaluated using the second-order perturbation theory (PT2), where the electric field formed by surrounding solvent molecules is treated as the perturbation to the electronic Hamiltonian of the isolated solute. The point of our approach is that the energy shift η, thus obtained, is to be adopted for a novel energy coordinate of the distributionmore » functions which serve as fundamental variables in the free energy functional developed in our previous work. The most time-consuming part in the QM/MM-ER simulation can be, thus, avoided without serious loss of accuracy. For our benchmark set of molecules, it is demonstrated that the PT2 approach coupled with QM/MM-ER gives hydration free energies in excellent agreements with those given by the conventional method utilizing the Kohn-Sham SCF procedure except for a few molecules in the benchmark set. A variant of the approach is also proposed to deal with such difficulties associated with the problematic systems. The present approach is also advantageous to parallel implementations. We examined the parallel efficiency of our PT2 code on multi-core processors and found that the speedup increases almost linearly with respect to the number of cores. Thus, it was demonstrated that QM/MM-ER coupled with PT2 deserves practical applications to systems of interest.« less

Parallel-processing with surface plasmons, a new strategy for converting the broad solar spectrum

NASA Technical Reports Server (NTRS)

Anderson, L. M.

1982-01-01

A new strategy for efficient solar-energy conversion is based on parallel processing with surface plasmons: guided electromagnetic waves supported on thin films of common metals like aluminum or silver. The approach is unique in identifying a broadband carrier with suitable range for energy transport and an inelastic tunneling process which can be used to extract more energy from the more energetic carriers without requiring different materials for each frequency band. The aim is to overcome the fundamental 56-percent loss associated with mismatch between the broad solar spectrum and the monoenergetic conduction electrons used to transport energy in conventional silicon solar cells. This paper presents a qualitative discussion of the unknowns and barrier problems, including ideas for coupling surface plasmons into the tunnels, a step which has been the weak link in the efficiency chain.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pratapa, Phanisri P.; Suryanarayana, Phanish; Pask, John E.

We present the Clenshaw–Curtis Spectral Quadrature (SQ) method for real-space O(N) Density Functional Theory (DFT) calculations. In this approach, all quantities of interest are expressed as bilinear forms or sums over bilinear forms, which are then approximated by spatially localized Clenshaw–Curtis quadrature rules. This technique is identically applicable to both insulating and metallic systems, and in conjunction with local reformulation of the electrostatics, enables the O(N) evaluation of the electronic density, energy, and atomic forces. The SQ approach also permits infinite-cell calculations without recourse to Brillouin zone integration or large supercells. We employ a finite difference representation in order tomore » exploit the locality of electronic interactions in real space, enable systematic convergence, and facilitate large-scale parallel implementation. In particular, we derive expressions for the electronic density, total energy, and atomic forces that can be evaluated in O(N) operations. We demonstrate the systematic convergence of energies and forces with respect to quadrature order as well as truncation radius to the exact diagonalization result. In addition, we show convergence with respect to mesh size to established O(N 3) planewave results. In conclusion, we establish the efficiency of the proposed approach for high temperature calculations and discuss its particular suitability for large-scale parallel computation.« less

Influence of a parallel magnetic field on the microwave photoconductivity in a high-mobility two-dimensional electron system

NASA Astrophysics Data System (ADS)

Yang, C. L.; Du, R. R.; Pfeiffer, L. N.; West, K. W.

2006-07-01

Using a two-axis magnet, we have studied experimentally the influence of a parallel magnetic field (B//) on microwave-induced resistance oscillations (MIROs) and zero-resistance states (ZRS) previously discovered in a high-mobility two-dimensional electron system. We have observed a strong suppression of MIRO/ZRS by a modest B//˜1T . In Hall bar samples, magnetoplasmon resonance (MPR) has also been observed concurrently with the MIRO/ZRS. In contrast to the suppression of MIRO/ZRS, the MPR peak is apparently enhanced by B// . These findings cannot be explained by a simple modification of single-particle energy spectrum and/or scattering parameters by B// .

Observations of Magnetosphere-Ionosphere Coupling Processes in Jupiter's Downward Auroral Current Region

NASA Astrophysics Data System (ADS)

Clark, G. B.; Mauk, B.; Allegrini, F.; Bagenal, F.; Bolton, S. J.; Bunce, E. J.; Connerney, J. E. P.; Ebert, R. W.; Gershman, D. J.; Gladstone, R.; Haggerty, D. K.; Hospodarsky, G. B.; Kotsiaros, S.; Kollmann, P.; Kurth, W. S.; Levin, S.; McComas, D. J.; Paranicas, C.; Rymer, A. M.; Saur, J.; Szalay, J. R.; Tetrick, S.; Valek, P. W.

2017-12-01

Our view and understanding of Jupiter's auroral regions are ever-changing as Juno continues to map out this region with every auroral pass. For example, since last year's Fall AGU and the release of publications regarding the first perijove orbit, the Juno particles and fields teams have found direct evidence of parallel potential drops in addition to the stochastic broad energy distributions associated with the downward current auroral acceleration region. In this region, which appears to exist in an altitude range of 1.5-3 Jovian radii, the potential drops can reach as high as several megavolts. Associated with these potentials are anti-planetward electron angle beams, energetic ion conics and precipitating protons, oxygen and sulfur. Sometimes the potentials within the downward current region are structured such that they look like the inverted-V type distributions typically found in Earth's upward current region. This is true for both the ion and electron energy distributions. Other times, the parallel potentials appear to be intermittent or spatially structured in a way such that they do not look like the canonical diverging electrostatic potential structure. Furthermore, the parallel potentials vary grossly in spatial/temporal scale, peak voltage and associated parallel current density. Here, we present a comprehensive study of these structures in Jupiter's downward current region focusing on energetic particle measurements from Juno-JEDI.

Electron precipitation in the post midnight sector of the auroral zones. [on the Explorer 40 satellite

NASA Technical Reports Server (NTRS)

Frank, L. A.; Saflekos, N. A.; Ackerson, K. L.

1975-01-01

Comprehensive measurements of the angular distributions and energy spectra of electron intensities with electrostatic analyzer arrays on board the low-altitude satellite Injun 5 are reported. These are for the post-midnight sector of the auroral zones during the high-intensity events accompanying magnetic substorms. Precipitation features on closed terrestrial field lines well equatorward of the trapping boundary for energetic electrons with E greater than 45 keV were examined. No evidences of maxima in the differential energy spectra or of strongly field-aligned currents which are indicative of quasi-static electric fields aligned parallel to the geomagnetic field were found. Precipitation of low-energy electron intensities fluctuated on time scales greater than 2 seconds as viewed at the satellite position. This precipitation was characterized by isotropy for all pitch angles outside the atmospheric backscatter cone.

Electron heating and energy inventory during asymmetric reconnection in a laboratory plasma

NASA Astrophysics Data System (ADS)

Yoo, J.; Na, B.; Jara-Almonte, J.; Yamada, M.; Ji, H.; Roytershteyn, V.; Argall, M. R.; Fox, W.; Chen, L. J.

2017-12-01

Electron heating and the energy inventory during asymmetric reconnection are studied in the Magnetic Reconnection Experiment (MRX) [1]. In this plasma, the density ratio is about 8 across the current sheet. Typical features of asymmetric reconnection such as the large density gradients near the low-density-side separatrices, asymmetric in-plane electric field, and bipolar out-of-plane magnetic field are observed. Unlike the symmetric case [2], electrons are also heated near the low-density-side separatrices. The measured parallel electric field may explain the observed electron heating. Although large fluctuations driven by lower-hybrid drift instabilities are also observed near the low-density-side separatrices, laboratory measurements and numerical simulations reported here suggest that they do not play a major role in electron energization. The average electron temperature increase in the exhaust region is proportional to the incoming magnetic energy per an electron/ion pair but exceeds the scaling of the previous space observations [3]. This discrepancy is explained by differences in the boundary condition and system size. The profile of electron energy gain from the electric field shows that there is additional electron energy gain associated with the electron diamagnetic current besides a large energy gain near the X-line. This additional energy gain increases electron enthalpy, not the electron temperature. Finally, a quantitative analysis of the energy inventory during asymmetric reconnection is conducted. Unlike the symmetric case where the ion energy gain is about twice more than the electron energy gain [4], electrons and ions obtain a similar amount of energy during asymmetric reconnection. [1] J. Yoo et al., accepted for a publication in J. Geophys. Res. [2] J. Yoo et al., Phys. Plasmas 21, 055706 (2014). [3] T. Phan et al., Geophys. Res. Lett. 40, 4475 (2013). [4] M. Yamada et al., Nat. Comms. 5, 4474 (2014).

Acceleration of cosmic rays by turbulence during reconnection events

NASA Astrophysics Data System (ADS)

Drake, Jim

2007-05-01

A Fermi-like model for energetic electron production during magnetic reconnection is described that converts a substantial fraction of released magnetic energy into energetic electrons [1]. Magnetic reconnection with a guide field leads to the growth and dynamics of multiple magnetic islands rather than a single large x-line. Electrons trapped within islands gain energy as they reflect from ends of contracting magnetic islands. The resulting rate of energy gain dominates that from parallel electric fields. The pressure from energetic electrons rises rapidly until the rate of electron energy gain balances the rate of magnetic energy release, establishing for the first time a link between the energy gain of electrons and the released magnetic energy. The energetic particle pressure therefore throttles the rate of reconnection. A transport equation for the distribution of energetic particles, including their feedback on island contraction, is obtained by averaging over the particle interaction with many islands. The steady state solutions in reconnection geometry result from convective losses balancing the Fermi drive. At high energy distribution functions take the form of a powerlaw whose spectral index depends only on the initial electron β, lower (higher) β producing harder (softer) spectra. The spectral index matches that seen in recent Wind spacecraft observations in the Earth's magnetotail. Harder spectra are predicted for the low β conditions of the solar corona or other astrophysical systems. Ions can be similarly accelerated if they are above an energy threshold. 1. J. F. Drake, M. Swisdak, H. Che and M. Shay, Nature 443, 553, 2006.

Introducing PROFESS 2.0: A parallelized, fully linear scaling program for orbital-free density functional theory calculations

NASA Astrophysics Data System (ADS)

Hung, Linda; Huang, Chen; Shin, Ilgyou; Ho, Gregory S.; Lignères, Vincent L.; Carter, Emily A.

2010-12-01

Orbital-free density functional theory (OFDFT) is a first principles quantum mechanics method to find the ground-state energy of a system by variationally minimizing with respect to the electron density. No orbitals are used in the evaluation of the kinetic energy (unlike Kohn-Sham DFT), and the method scales nearly linearly with the size of the system. The PRinceton Orbital-Free Electronic Structure Software (PROFESS) uses OFDFT to model materials from the atomic scale to the mesoscale. This new version of PROFESS allows the study of larger systems with two significant changes: PROFESS is now parallelized, and the ion-electron and ion-ion terms scale quasilinearly, instead of quadratically as in PROFESS v1 (L. Hung and E.A. Carter, Chem. Phys. Lett. 475 (2009) 163). At the start of a run, PROFESS reads the various input files that describe the geometry of the system (ion positions and cell dimensions), the type of elements (defined by electron-ion pseudopotentials), the actions you want it to perform (minimize with respect to electron density and/or ion positions and/or cell lattice vectors), and the various options for the computation (such as which functionals you want it to use). Based on these inputs, PROFESS sets up a computation and performs the appropriate optimizations. Energies, forces, stresses, material geometries, and electron density configurations are some of the values that can be output throughout the optimization. New version program summaryProgram Title: PROFESS Catalogue identifier: AEBN_v2_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEBN_v2_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 68 721 No. of bytes in distributed program, including test data, etc.: 1 708 547 Distribution format: tar.gz Programming language: Fortran 90 Computer: Intel with ifort; AMD Opteron with pathf90 Operating system: Linux Has the code been vectorized or parallelized?: Yes. Parallelization is implemented through domain composition using MPI. RAM: Problem dependent, but 2 GB is sufficient for up to 10,000 ions. Classification: 7.3 External routines: FFTW 2.1.5 ( http://www.fftw.org) Catalogue identifier of previous version: AEBN_v1_0 Journal reference of previous version: Comput. Phys. Comm. 179 (2008) 839 Does the new version supersede the previous version?: Yes Nature of problem: Given a set of coordinates describing the initial ion positions under periodic boundary conditions, recovers the ground state energy, electron density, ion positions, and cell lattice vectors predicted by orbital-free density functional theory. The computation of all terms is effectively linear scaling. Parallelization is implemented through domain decomposition, and up to ˜10,000 ions may be included in the calculation on just a single processor, limited by RAM. For example, when optimizing the geometry of ˜50,000 aluminum ions (plus vacuum) on 48 cores, a single iteration of conjugate gradient ion geometry optimization takes ˜40 minutes wall time. However, each CG geometry step requires two or more electron density optimizations, so step times will vary. Solution method: Computes energies as described in text; minimizes this energy with respect to the electron density, ion positions, and cell lattice vectors. Reasons for new version: To allow much larger systems to be simulated using PROFESS. Restrictions: PROFESS cannot use nonlocal (such as ultrasoft) pseudopotentials. A variety of local pseudopotential files are available at the Carter group website ( http://www.princeton.edu/mae/people/faculty/carter/homepage/research/localpseudopotentials/). Also, due to the current state of the kinetic energy functionals, PROFESS is only reliable for main group metals and some properties of semiconductors. Running time: Problem dependent: the test example provided with the code takes less than a second to run. Timing results for large scale problems are given in the PROFESS paper and Ref. [1].

Investigation of indirect excitons in bulk 2H-MoS2 using transmission electron energy-loss spectroscopy.

PubMed

Habenicht, Carsten; Schuster, Roman; Knupfer, Martin; Büchner, Bernd

2018-05-23

We have investigated indirect excitons in bulk 2H-MoS 2 using transmission electron energy-loss spectroscopy. The electron energy-loss spectra were measured for various momentum transfer values parallel to the [Formula: see text] and [Formula: see text] directions of the Brillouin zone. The results allowed the identification of the indirect excitons between the valence band K v and conduction band Λ c points, the Γ v and K c points as well as adjacent K v and [Formula: see text] points. The energy-momentum dispersions for the K v -Λ c , Γ v -K c and K v1 -[Formula: see text] excitons along the [Formula: see text] line are presented. The former two transitions exhibit a quadratic dispersion which allowed calculating their effective exciton masses based on the effective mass approximation. The K v1 -[Formula: see text] transition follows a more linear dispersion relationship.

Megavolt parallel potentials arising from double-layer streams in the Earth's outer radiation belt.

PubMed

Mozer, F S; Bale, S D; Bonnell, J W; Chaston, C C; Roth, I; Wygant, J

2013-12-06

Huge numbers of double layers carrying electric fields parallel to the local magnetic field line have been observed on the Van Allen probes in connection with in situ relativistic electron acceleration in the Earth's outer radiation belt. For one case with adequate high time resolution data, 7000 double layers were observed in an interval of 1 min to produce a 230,000 V net parallel potential drop crossing the spacecraft. Lower resolution data show that this event lasted for 6 min and that more than 1,000,000 volts of net parallel potential crossed the spacecraft during this time. A double layer traverses the length of a magnetic field line in about 15 s and the orbital motion of the spacecraft perpendicular to the magnetic field was about 700 km during this 6 min interval. Thus, the instantaneous parallel potential along a single magnetic field line was the order of tens of kilovolts. Electrons on the field line might experience many such potential steps in their lifetimes to accelerate them to energies where they serve as the seed population for relativistic acceleration by coherent, large amplitude whistler mode waves. Because the double-layer speed of 3100  km/s is the order of the electron acoustic speed (and not the ion acoustic speed) of a 25 eV plasma, the double layers may result from a new electron acoustic mode. Acceleration mechanisms involving double layers may also be important in planetary radiation belts such as Jupiter, Saturn, Uranus, and Neptune, in the solar corona during flares, and in astrophysical objects.

Empirical valence bond models for reactive potential energy surfaces: a parallel multilevel genetic program approach.

PubMed

Bellucci, Michael A; Coker, David F

2011-07-28

We describe a new method for constructing empirical valence bond potential energy surfaces using a parallel multilevel genetic program (PMLGP). Genetic programs can be used to perform an efficient search through function space and parameter space to find the best functions and sets of parameters that fit energies obtained by ab initio electronic structure calculations. Building on the traditional genetic program approach, the PMLGP utilizes a hierarchy of genetic programming on two different levels. The lower level genetic programs are used to optimize coevolving populations in parallel while the higher level genetic program (HLGP) is used to optimize the genetic operator probabilities of the lower level genetic programs. The HLGP allows the algorithm to dynamically learn the mutation or combination of mutations that most effectively increase the fitness of the populations, causing a significant increase in the algorithm's accuracy and efficiency. The algorithm's accuracy and efficiency is tested against a standard parallel genetic program with a variety of one-dimensional test cases. Subsequently, the PMLGP is utilized to obtain an accurate empirical valence bond model for proton transfer in 3-hydroxy-gamma-pyrone in gas phase and protic solvent. © 2011 American Institute of Physics

Monte Carlo studies of thermalization of electron-hole pairs in spin-polarized degenerate electron gas in monolayer graphene

NASA Astrophysics Data System (ADS)

Borowik, Piotr; Thobel, Jean-Luc; Adamowicz, Leszek

2018-02-01

Monte Carlo method is applied to the study of relaxation of excited electron-hole (e-h) pairs in graphene. The presence of background of spin-polarized electrons, with high density imposing degeneracy conditions, is assumed. To such system, a number of e-h pairs with spin polarization parallel or antiparallel to the background is injected. Two stages of relaxation: thermalization and cooling are clearly distinguished when average particles energy < E> and its standard deviation σ _E are examined. At the very beginning of thermalization phase, holes loose energy to electrons, and after this process is substantially completed, particle distributions reorganize to take a Fermi-Dirac shape. To describe the evolution of < E > and σ _E during thermalization, we define characteristic times τ _ {th} and values at the end of thermalization E_ {th} and σ _ {th}. The dependence of these parameters on various conditions, such as temperature and background density, is presented. It is shown that among the considered parameters, only the standard deviation of electrons energy allows to distinguish between different cases of relative spin polarizations of background and excited electrons.

Improved treatment of exact exchange in Quantum ESPRESSO

DOE PAGES

Barnes, Taylor A.; Kurth, Thorsten; Carrier, Pierre; ...

2017-01-18

Here, we present an algorithm and implementation for the parallel computation of exact exchange in Quantum ESPRESSO (QE) that exhibits greatly improved strong scaling. QE is an open-source software package for electronic structure calculations using plane wave density functional theory, and supports the use of local, semi-local, and hybrid DFT functionals. Wider application of hybrid functionals is desirable for the improved simulation of electronic band energy alignments and thermodynamic properties, but the computational complexity of evaluating the exact exchange potential limits the practical application of hybrid functionals to large systems and requires efficient implementations. We demonstrate that existing implementations ofmore » hybrid DFT that utilize a single data structure for both the local and exact exchange regions of the code are significantly limited in the degree of parallelization achievable. We present a band-pair parallelization approach, in which the calculation of exact exchange is parallelized and evaluated independently from the parallelization of the remainder of the calculation, with the wavefunction data being efficiently transformed on-the-fly into a form that is optimal for each part of the calculation. For a 64 water molecule supercell, our new algorithm reduces the overall time to solution by nearly an order of magnitude.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barnes, Taylor A.; Kurth, Thorsten; Carrier, Pierre

Here, we present an algorithm and implementation for the parallel computation of exact exchange in Quantum ESPRESSO (QE) that exhibits greatly improved strong scaling. QE is an open-source software package for electronic structure calculations using plane wave density functional theory, and supports the use of local, semi-local, and hybrid DFT functionals. Wider application of hybrid functionals is desirable for the improved simulation of electronic band energy alignments and thermodynamic properties, but the computational complexity of evaluating the exact exchange potential limits the practical application of hybrid functionals to large systems and requires efficient implementations. We demonstrate that existing implementations ofmore » hybrid DFT that utilize a single data structure for both the local and exact exchange regions of the code are significantly limited in the degree of parallelization achievable. We present a band-pair parallelization approach, in which the calculation of exact exchange is parallelized and evaluated independently from the parallelization of the remainder of the calculation, with the wavefunction data being efficiently transformed on-the-fly into a form that is optimal for each part of the calculation. For a 64 water molecule supercell, our new algorithm reduces the overall time to solution by nearly an order of magnitude.« less

Symmetry Breaking by Parallel Flow Shear

NASA Astrophysics Data System (ADS)

Li, Jiacong; Diamond, Patrick

2015-11-01

Plasma rotation is important in reducing turbulent transport, suppressing MHD instabilities, and is beneficial to confinement. Intrinsic rotation without an external momentum input is of interest for its plausible application on ITER. k∥ spectrum asymmetry is required for residual Reynolds stress that drives the intrinsic rotation. Parallel flows are reported in linear devices without magnetic shear. In CSDX, parallel flows are mostly peaked in the core [Thakur et al., 2014]; more robust flows and reversed profiles are seen in PANTA [Oldenburger, et al. 2012]. A novel mechanism for symmetry breaking in momentum transport is proposed. Magnetic shear or mean flow profile are not required. A seed parallel flow shear (PFS) sets the sign of residual stress by selecting certain modes to grow faster. The resulted spectrum imbalance leads to a nonzero residual stress, which further drives a parallel flow with ∇n as the free energy source, adding to the shear until saturated by diffusion. Balanced flow gradient is set by Π∥Res /χϕ . Residual stress is calculated for ITG turbulence and collisional drift wave turbulence where electron-ion and electron-neutral collisions are discussed and compared. Numerical simulation is proposed for testing the effect of PFS.

Parallel 3-D numerical simulation of dielectric barrier discharge plasma actuators

NASA Astrophysics Data System (ADS)

Houba, Tomas

Dielectric barrier discharge plasma actuators have shown promise in a range of applications including flow control, sterilization and ozone generation. Developing numerical models of plasma actuators is of great importance, because a high-fidelity parallel numerical model allows new design configurations to be tested rapidly. Additionally, it provides a better understanding of the plasma actuator physics which is useful for further innovation. The physics of plasma actuators is studied numerically. A loosely coupled approach is utilized for the coupling of the plasma to the neutral fluid. The state of the art in numerical plasma modeling is advanced by the development of a parallel, three-dimensional, first-principles model with detailed air chemistry. The model incorporates 7 charged species and 18 reactions, along with a solution of the electron energy equation. To the author's knowledge, a parallel three-dimensional model of a gas discharge with a detailed air chemistry model and the solution of electron energy is unique. Three representative geometries are studied using the gas discharge model. The discharge of gas between two parallel electrodes is used to validate the air chemistry model developed for the gas discharge code. The gas discharge model is then applied to the discharge produced by placing a dc powered wire and grounded plate electrodes in a channel. Finally, a three-dimensional simulation of gas discharge produced by electrodes placed inside a riblet is carried out. The body force calculated with the gas discharge model is loosely coupled with a fluid model to predict the induced flow inside the riblet.

Monte Carlo MP2 on Many Graphical Processing Units.

PubMed

Doran, Alexander E; Hirata, So

2016-10-11

In the Monte Carlo second-order many-body perturbation (MC-MP2) method, the long sum-of-product matrix expression of the MP2 energy, whose literal evaluation may be poorly scalable, is recast into a single high-dimensional integral of functions of electron pair coordinates, which is evaluated by the scalable method of Monte Carlo integration. The sampling efficiency is further accelerated by the redundant-walker algorithm, which allows a maximal reuse of electron pairs. Here, a multitude of graphical processing units (GPUs) offers a uniquely ideal platform to expose multilevel parallelism: fine-grain data-parallelism for the redundant-walker algorithm in which millions of threads compute and share orbital amplitudes on each GPU; coarse-grain instruction-parallelism for near-independent Monte Carlo integrations on many GPUs with few and infrequent interprocessor communications. While the efficiency boost by the redundant-walker algorithm on central processing units (CPUs) grows linearly with the number of electron pairs and tends to saturate when the latter exceeds the number of orbitals, on a GPU it grows quadratically before it increases linearly and then eventually saturates at a much larger number of pairs. This is because the orbital constructions are nearly perfectly parallelized on a GPU and thus completed in a near-constant time regardless of the number of pairs. In consequence, an MC-MP2/cc-pVDZ calculation of a benzene dimer is 2700 times faster on 256 GPUs (using 2048 electron pairs) than on two CPUs, each with 8 cores (which can use only up to 256 pairs effectively). We also numerically determine that the cost to achieve a given relative statistical uncertainty in an MC-MP2 energy increases as O(n 3 ) or better with system size n, which may be compared with the O(n 5 ) scaling of the conventional implementation of deterministic MP2. We thus establish the scalability of MC-MP2 with both system and computer sizes.

Solvent-assisted multistage nonequilibrium electron transfer in rigid supramolecular systems: Diabatic free energy surfaces and algorithms for numerical simulations

NASA Astrophysics Data System (ADS)

Feskov, Serguei V.; Ivanov, Anatoly I.

2018-03-01

An approach to the construction of diabatic free energy surfaces (FESs) for ultrafast electron transfer (ET) in a supramolecule with an arbitrary number of electron localization centers (redox sites) is developed, supposing that the reorganization energies for the charge transfers and shifts between all these centers are known. Dimensionality of the coordinate space required for the description of multistage ET in this supramolecular system is shown to be equal to N - 1, where N is the number of the molecular centers involved in the reaction. The proposed algorithm of FES construction employs metric properties of the coordinate space, namely, relation between the solvent reorganization energy and the distance between the two FES minima. In this space, the ET reaction coordinate zn n' associated with electron transfer between the nth and n'th centers is calculated through the projection to the direction, connecting the FES minima. The energy-gap reaction coordinates zn n' corresponding to different ET processes are not in general orthogonal so that ET between two molecular centers can create nonequilibrium distribution, not only along its own reaction coordinate but along other reaction coordinates too. This results in the influence of the preceding ET steps on the kinetics of the ensuing ET. It is important for the ensuing reaction to be ultrafast to proceed in parallel with relaxation along the ET reaction coordinates. Efficient algorithms for numerical simulation of multistage ET within the stochastic point-transition model are developed. The algorithms are based on the Brownian simulation technique with the recrossing-event detection procedure. The main advantages of the numerical method are (i) its computational complexity is linear with respect to the number of electronic states involved and (ii) calculations can be naturally parallelized up to the level of individual trajectories. The efficiency of the proposed approach is demonstrated for a model supramolecular system involving four redox centers.

Beam impingement angle effects on secondary electron emission characteristics of textured pyrolytic graphite

NASA Technical Reports Server (NTRS)

Curren, A. N.; Jensen, K. A.

1984-01-01

Experimentally determined values of true secondary electron emission and relative values of reflected primary electron yield for untreated and ion-textured pyrolytic graphite over a range of primary electron energy levels and electron beam impingement angles are presented. Information required to develop high efficiency multistage depressed collectors (MDC's) for microwave amplifier traveling-wave tubes for space communication and aircraft applications is provided. To attain the highest possible MDC efficiencies, the electrode surfaces must have low secondary electron emission characteristics. Pyrolytic graphite, a chemically vapor-deposited material, is a particularly promising candidate for this application. The pyrolytic graphite surfaces studied were tested over a range of primary electron beam energies and beam impingement angles from 200 to 2000 eV and direct (0 deg) to near-grazing angles (85 deg), respectively. Surfaces both parallel to and normal to the planes of material deposition were examined. The true secondary electron emission and reflected primary electron yield characteristics of the pyrolytic graphite surfaces are compared to those of sooted control surfaces.

Highly parallel implementation of non-adiabatic Ehrenfest molecular dynamics

NASA Astrophysics Data System (ADS)

Kanai, Yosuke; Schleife, Andre; Draeger, Erik; Anisimov, Victor; Correa, Alfredo

2014-03-01

While the adiabatic Born-Oppenheimer approximation tremendously lowers computational effort, many questions in modern physics, chemistry, and materials science require an explicit description of coupled non-adiabatic electron-ion dynamics. Electronic stopping, i.e. the energy transfer of a fast projectile atom to the electronic system of the target material, is a notorious example. We recently implemented real-time time-dependent density functional theory based on the plane-wave pseudopotential formalism in the Qbox/qb@ll codes. We demonstrate that explicit integration using a fourth-order Runge-Kutta scheme is very suitable for modern highly parallelized supercomputers. Applying the new implementation to systems with hundreds of atoms and thousands of electrons, we achieved excellent performance and scalability on a large number of nodes both on the BlueGene based ``Sequoia'' system at LLNL as well as the Cray architecture of ``Blue Waters'' at NCSA. As an example, we discuss our work on computing the electronic stopping power of aluminum and gold for hydrogen projectiles, showing an excellent agreement with experiment. These first-principles calculations allow us to gain important insight into the the fundamental physics of electronic stopping.

Geometrization of the Dirac theory of the electron

NASA Technical Reports Server (NTRS)

Fock, V.

1977-01-01

Using the concept of parallel displacement of a half vector, the Dirac equations are generally written in invariant form. The energy tensor is formed and both the macroscopic and quantum mechanic equations of motion are set up. The former have the usual form: divergence of the energy tensor equals the Lorentz force and the latter are essentially identical with those of the geodesic line.

Monte Carlo study of electron relaxation in graphene with spin polarized, degenerate electron gas in presence of electron-electron scattering

NASA Astrophysics Data System (ADS)

Borowik, Piotr; Thobel, Jean-Luc; Adamowicz, Leszek

2017-12-01

The Monte Carlo simulation method is applied to study the relaxation of excited electrons in monolayer graphene. The presence of spin polarized background electrons population, with density corresponding to highly degenerate conditions is assumed. Formulas of electron-electron scattering rates, which properly account for electrons presence in two energetically degenerate, inequivalent valleys in this material are presented. The electron relaxation process can be divided into two phases: thermalization and cooling, which can be clearly distinguished when examining the standard deviation of electron energy distribution. The influence of the exchange effect in interactions between electrons with parallel spins is shown to be important only in transient conditions, especially during the thermalization phase.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cebe, M; Pacaci, P; Mabhouti, H

Purpose: In this study, the two available calculation algorithms of the Varian Eclipse treatment planning system(TPS), the electron Monte Carlo(eMC) and General Gaussian Pencil Beam(GGPB) algorithms were used to compare measured and calculated peripheral dose distribution of electron beams. Methods: Peripheral dose measurements were carried out for 6, 9, 12, 15, 18 and 22 MeV electron beams of Varian Triology machine using parallel plate ionization chamber and EBT3 films in the slab phantom. Measurements were performed for 6×6, 10×10 and 25×25cm{sup 2} cone sizes at dmax of each energy up to 20cm beyond the field edges. Using the same filmmore » batch, the net OD to dose calibration curve was obtained for each energy. Films were scanned 48 hours after irradiation using an Epson 1000XL flatbed scanner. Dose distribution measured using parallel plate ionization chamber and EBT3 film and calculated by eMC and GGPB algorithms were compared. The measured and calculated data were then compared to find which algorithm calculates peripheral dose distribution more accurately. Results: The agreement between measurement and eMC was better than GGPB. The TPS underestimated the out of field doses. The difference between measured and calculated doses increase with the cone size. The largest deviation between calculated and parallel plate ionization chamber measured dose is less than 4.93% for eMC, but it can increase up to 7.51% for GGPB. For film measurement, the minimum gamma analysis passing rates between measured and calculated dose distributions were 98.2% and 92.7% for eMC and GGPB respectively for all field sizes and energies. Conclusion: Our results show that the Monte Carlo algorithm for electron planning in Eclipse is more accurate than previous algorithms for peripheral dose distributions. It must be emphasized that the use of GGPB for planning large field treatments with 6 MeV could lead to inaccuracies of clinical significance.« less

THEMIS Observations of the Magnetopause Electron Diffusion Region: Large Amplitude Waves and Heated Electrons

NASA Technical Reports Server (NTRS)

Tang, Xiangwei; Cattell, Cynthia; Dombeck, John; Dai, Lei; Wilson, Lynn B. III; Breneman, Aaron; Hupack, Adam

2013-01-01

We present the first observations of large amplitude waves in a well-defined electron diffusion region based on the criteria described by Scudder et al at the subsolar magnetopause using data from one Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite. These waves identified as whistler mode waves, electrostatic solitary waves, lower hybrid waves, and electrostatic electron cyclotron waves, are observed in the same 12 s waveform capture and in association with signatures of active magnetic reconnection. The large amplitude waves in the electron diffusion region are coincident with abrupt increases in electron parallel temperature suggesting strong wave heating. The whistler mode waves, which are at the electron scale and which enable us to probe electron dynamics in the diffusion region were analyzed in detail. The energetic electrons (approx. 30 keV) within the electron diffusion region have anisotropic distributions with T(sub e(right angle))/T(sub e(parallel)) > 1 that may provide the free energy for the whistler mode waves. The energetic anisotropic electrons may be produced during the reconnection process. The whistler mode waves propagate away from the center of the "X-line" along magnetic field lines, suggesting that the electron diffusion region is a possible source region of the whistler mode waves.

An experimental study of recombination and polarity effect in a set of customized plane parallel ionization chambers.

PubMed

Kron, T; McNiven, A; Witruk, B; Kenny, M; Battista, J

2006-12-01

Plane parallel ionization chambers are an important tool for dosimetry and absolute calibration of electron beams used for radiotherapy. Most dosimetric protocols require corrections for recombination and polarity effects, which are to be determined experimentally as they depend on chamber design and radiation quality. Both effects were investigated in electron beams from a linear accelerator (Varian 21CD) for a set of four tissue equivalent plane parallel ionization chambers customized for the present research by Standard Imaging (Madison WI). All four chambers share the same design and air cavity dimensions, differing only in the diameter of their collecting electrode and the corresponding width of the guard ring. The diameters of the collecting electrodes were 2 mm, 4 mm, 10 mm and 20 mm. Measurements were taken using electron beams of nominal energy 6 to 20 MeV in a 10 cm x 10 cm field size with a SSD of 100 cm at various depths in a Solid Water slab phantom. No significant variation of recombination effect was found with radiation quality, depth of measurement or chamber design. However, the polarity effect exceeded 5% for the chambers with small collecting electrode for an effective electron energy below 4 MeV at the point of measurement. The magnitude of the effect increased with decreasing electron energy in the phantom. The polarity correction factor calculated following AAPM protocol TG51 ranged from approximately 1.00 for the 20.0 mm chamber to less than 0.95 for the 2 mm chamber at 4.1 cm depth in a electron beam of nominally 12 MeV. By inverting the chamber it could be shown that the polarity effect did not depend on the polarity of the electrode first traversed by the electron beam. Similarly, the introduction of an air gap between the overlying phantom layer and the chambers demonstrated that the angular distribution of the electrons at the point of measurement had a lesser effect on the polarity correction than the electron energy itself. The magnitude of the absolute difference between charge collected at positive and negative polarity was found to correlate with the area of the collecting electrode which is consistent with the explanation that differences in thickness of the collecting electrodes and the number of electrons stopped in them contribute significantly to the polarity effect. Overall, the polarity effects found in the present study would have a negligible effect on electron beam calibration at a measurement depth recommended by most calibration protocols. However, the present work tested the corrections under extreme conditions thereby aiming at greater understanding of the mechanism underlying the correction factors for these chambers. This may lead to better chamber design for absolute dosimetry and electron beam characterization with less reliance on empirical corrections.

Theory and simulations of current drive via injection of an electron beam in the ACT-1 device

DOE Office of Scientific and Technical Information (OSTI.GOV)

Okuda, H.; Horton, R.; Ono, M.

1985-02-01

One- and two-dimensional particle simulations of beam-plasma interaction have been carried out in order to understand current drive experiments that use an electron beam injected into the ACT-1 device. Typically, the beam velocity along the magnetic field is V = 10/sup 9/ cm/sec while the thermal velocity of the background electrons is v/sub t/ = 10/sup 8//cm. The ratio of the beam density to the background density is about 10% so that a strong beam-plasma instability develops causing rapid diffusion of beam particles. For both one- and two- dimensional simulations, it is found that a significant amount of beam andmore » background electrons is accelerated considerably beyond the initial beam velocity when the beam density is more than a few percent of the background plasma density. In addition, electron distribution along the magnetic field has a smooth negative slope, f' (v/sub parallel/) < 0, for v/ sub parallel/ > 0 extending v/sub parallel/ = 1.5 V approx. 2 V, which is in sharp contrast to the predictions from quasilinear theory. An estimate of the mean-free path for beam electrons due to Coulomb collisions reveals that the beam electrons can propagate a much longer distance than is predicted from a quasilinear theory, due to the presence of a high energy tail. These simulation results agree well with the experimental observations from the ACT-1 device.« less

Development of broad bandwidth nonlinear spectroscopies for characterization of electronic states in materials systems

NASA Astrophysics Data System (ADS)

Mehlenbacher, Randy D.

Carbon nanotubes are an interesting class of materials with many exceptional properties that make them appealing for optoelectronic devices. Their optical properties, particularly when cast in thin films, are not well understood. In this thesis, I describe the development of spectroscopic techniques for measuring energy and charge transport processes in thin films of semiconducting carbon nanotubes. Using transient absorption spectroscopy, I observe energy transport on two time scales in these films, with 20% of nanotubes transferring energy to smaller bandgap nanotubes within 300 fs. After 3 ps, 70% of the photoexcitation resides on small bandgap nanotubes. To study the complete landscape of energy transport in thin films of carbon nanotubes, I developed two dimensional white light spectroscopy (2D-WL). In 2D-WL spectroscopy, a broadband, white light supercontinuum is used to both excite and probe the sample. This technique has a bandwidth spanning > 500-1500 nm, a far broader bandwidth than previously reported in 2D electronic spectra. I take advantage of this large bandwidth to study the interactions and evolution of S1 and S2 excitons in a thin film of carbon nanotubes. I find that energy transfers between S1 excitons on a 2 ps time scale and occurs by a non-Forster energy transfer mechanism. In contrast, the energy in the S2 states redistributes on an ultrafast time scale, <100 fs, and undergoes autoionization producing free electrons and holes. I use 2D-WL spectroscopy to study the electronic states in thin films of bare, semiconducting carbon nanotubes. In these films, energy transfer occurs in <100 fs between bare carbon nanotubes and this energy transfer is between parallel nanotubes. By taking advantage of the laser pulse polarization for each interaction, I resolve otherwise difficult to observe couplings between electronic states. To facilitate data interpretation, the orientational response for isotropic two dimensional samples to polarized electric fields is developed. Using polarization control 2D-WL spectroscopy, I measure the coupling between nanotube S1 transitions and radial breathing modes. The doped tubes form trions with transition dipoles that are not parallel to the S1 transition and energy transfer from the S1 exciton to the trion occurs within 1 ps.

Observations of Large-Amplitude, Parallel, Electrostatic Waves Associated with the Kelvin-Helmholtz Instability by the Magnetospheric Multiscale Mission

NASA Technical Reports Server (NTRS)

Wilder, F. D.; Ergun, R. E.; Schwartz, S. J.; Newman, D. L.; Eriksson, S.; Stawarz, J. E.; Goldman, M. V.; Goodrich, K. A.; Gershman, D. J.; Malaspina, D.;

The Structure and Properties of 0.1 - 100 keV Electron Distributions Over Jupiter's Polar Aurora Region and their Contribution to Polar Aurora Emissions

NASA Astrophysics Data System (ADS)

Ebert, R. W.; Allegrini, F.; Bagenal, F.; Bolton, S. J.; Chae, K.; Connerney, J. E. P.; Clark, G. B.; Gladstone, R.; Hue, V.; Kurth, W. S.; Levin, S.; Louarn, P.; Mauk, B.; McComas, D. J.; Paranicas, C.; Saur, J.; Reno, C.; Szalay, J. R.; Thomsen, M. F.; Valek, P. W.; Weidner, S.; Wilson, R. J.

2017-12-01

In addition to the main emissions in the north and south, Jupiter's auroral emissions also include polar, satellite-related, and other features. Here we present observations from Juno's Jovian Auroral Distributions Experiment (JADE) of 0.1 - 100 keV electrons in Jupiter's polar aurora region during the spacecraft's northern and southern polar passes bounding PJ1 (27 August 2016), PJ3 (11 December 11 2016), PJ4 (2 February 2017), PJ5 (27 March 2017), PJ6 (19 May 2017), and PJ7 (11 July 2017). Specifically, we focus on the spatial structure, energy and pitch angle distributions, and energy flux and spectra of these electrons. The observations reveal regions containing magnetic field aligned beams of bi-directional electrons having broad energy distributions interspersed between beams of upward electrons with narrow, peaked energy distributions, regions void of these electrons, and regions dominated by penetrating radiation, with penetrating radiation being most common. The electrons show evidence of acceleration via parallel electric fields (inverted-V structures) and via stochastic processes (bi-directional distributions). The inverted-V structures identified to date were observed from 1.4 - 2.9 RJ and had spatial scales of 100s to 1000s of kilometers along Juno's trajectory. The upward energy flux of the electron distributions was typically greater than the downward energy flux and their contribution to producing Jupiter's polar aurora emissions will be discussed.

A Model for Dissipation of Solar Wind Magnetic Turbulence by Kinetic Alfvén Waves at Electron Scales: Comparison with Observations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schreiner, Anne; Saur, Joachim, E-mail: schreiner@geo.uni-koeln.de

In hydrodynamic turbulence, it is well established that the length of the dissipation scale depends on the energy cascade rate, i.e., the larger the energy input rate per unit mass, the more the turbulent fluctuations need to be driven to increasingly smaller scales to dissipate the larger energy flux. Observations of magnetic spectral energy densities indicate that this intuitive picture is not valid in solar wind turbulence. Dissipation seems to set in at the same length scale for different solar wind conditions independently of the energy flux. To investigate this difference in more detail, we present an analytic dissipation modelmore » for solar wind turbulence at electron scales, which we compare with observed spectral densities. Our model combines the energy transport from large to small scales and collisionless damping, which removes energy from the magnetic fluctuations in the kinetic regime. We assume wave–particle interactions of kinetic Alfvén waves (KAWs) to be the main damping process. Wave frequencies and damping rates of KAWs are obtained from the hot plasma dispersion relation. Our model assumes a critically balanced turbulence, where larger energy cascade rates excite larger parallel wavenumbers for a certain perpendicular wavenumber. If the dissipation is additionally wave driven such that the dissipation rate is proportional to the parallel wavenumber—as with KAWs—then an increase of the energy cascade rate is counterbalanced by an increased dissipation rate for the same perpendicular wavenumber, leading to a dissipation length independent of the energy cascade rate.« less

A Model for Dissipation of Solar Wind Magnetic Turbulence by Kinetic Alfvén Waves at Electron Scales: Comparison with Observations

NASA Astrophysics Data System (ADS)

Schreiner, Anne; Saur, Joachim

2017-02-01

In hydrodynamic turbulence, it is well established that the length of the dissipation scale depends on the energy cascade rate, I.e., the larger the energy input rate per unit mass, the more the turbulent fluctuations need to be driven to increasingly smaller scales to dissipate the larger energy flux. Observations of magnetic spectral energy densities indicate that this intuitive picture is not valid in solar wind turbulence. Dissipation seems to set in at the same length scale for different solar wind conditions independently of the energy flux. To investigate this difference in more detail, we present an analytic dissipation model for solar wind turbulence at electron scales, which we compare with observed spectral densities. Our model combines the energy transport from large to small scales and collisionless damping, which removes energy from the magnetic fluctuations in the kinetic regime. We assume wave-particle interactions of kinetic Alfvén waves (KAWs) to be the main damping process. Wave frequencies and damping rates of KAWs are obtained from the hot plasma dispersion relation. Our model assumes a critically balanced turbulence, where larger energy cascade rates excite larger parallel wavenumbers for a certain perpendicular wavenumber. If the dissipation is additionally wave driven such that the dissipation rate is proportional to the parallel wavenumber—as with KAWs—then an increase of the energy cascade rate is counterbalanced by an increased dissipation rate for the same perpendicular wavenumber, leading to a dissipation length independent of the energy cascade rate.

MMS Observation of Magnetic Reconnection in the Turbulent Magnetosheath

NASA Astrophysics Data System (ADS)

Vörös, Z.; Yordanova, E.; Varsani, A.; Genestreti, K. J.; Khotyaintsev, Yu. V.; Li, W.; Graham, D. B.; Norgren, C.; Nakamura, R.; Narita, Y.; Plaschke, F.; Magnes, W.; Baumjohann, W.; Fischer, D.; Vaivads, A.; Eriksson, E.; Lindqvist, P.-A.; Marklund, G.; Ergun, R. E.; Leitner, M.; Leubner, M. P.; Strangeway, R. J.; Le Contel, O.; Pollock, C.; Giles, B. J.; Torbert, R. B.; Burch, J. L.; Avanov, L. A.; Dorelli, J. C.; Gershman, D. J.; Paterson, W. R.; Lavraud, B.; Saito, Y.

2017-11-01

In this paper we use the full armament of the MMS (Magnetospheric Multiscale) spacecraft to study magnetic reconnection in the turbulent magnetosheath downstream of a quasi-parallel bow shock. Contrarily to the magnetopause and magnetotail cases, only a few observations of reconnection in the magnetosheath have been reported. The case study in this paper presents, for the first time, both fluid-scale and kinetic-scale signatures of an ongoing reconnection in the turbulent magnetosheath. The spacecraft are crossing the reconnection inflow and outflow regions and the ion diffusion region (IDR). Inside the reconnection outflows D shape ion distributions are observed. Inside the IDR mixing of ion populations, crescent-like velocity distributions and ion accelerations are observed. One of the spacecraft skims the outer region of the electron diffusion region, where parallel electric fields, energy dissipation/conversion, electron pressure tensor agyrotropy, electron temperature anisotropy, and electron accelerations are observed. Some of the difficulties of the observations of magnetic reconnection in turbulent plasma are also outlined.

The energy balance and pressure in the solar transition zone for network and active region features

NASA Technical Reports Server (NTRS)

Nicolas, K. R.; Bartoe, J.-D. F.; Brueckner, G. E.; Vanhoosier, M. E.

1979-01-01

The electron pressure and energy balance in the solar transition zone are determined for about 125 network and active region features on the basis of high spectral and spatial resolution extreme ultraviolet spectra. Si III line intensity ratios obtained from the Naval Research Laboratory high-resolution telescope and spectrograph during a rocket flight are used as diagnostics of electron density and pressure for solar features near 3.5 x 10 to the 4th K. Observed ratios are compared with the calculated dependence of the 1301 A/1312 A and 1301 A/1296 A line intensity ratios on electron density, temperature and pressure. Electron densities ranging from 2 x 10 to the 10th/cu cm to 10 to the 12th/cu cm and active region pressures from 3 x 10 to the 15th to 10 to the 16th/cu cm K are obtained. Energy balance calculations reveal the balance of the divergence of the conductive flux and turbulent energy dissipation by radiative energy losses in a plane-parallel homogeneous transition zone (fill factor of 1), and an energy source requirement for a cylindrical zone geometry (fill factor less than 0.04).

Measurements of hot electrons in the Extrap T1 reversed-field pinch

NASA Astrophysics Data System (ADS)

Welander, A.; Bergsåker, H.

1998-02-01

The presence of an anisotropic energetic electron population in the edge region is a characteristic feature of reversed-field pinch (RFP) plasmas. In the Extrap T1 RFP, the anisotropic, parallel heat flux in the edge region measured by calorimetry was typically several hundred 0741-3335/40/2/011/img1. To gain more insight into the origin of the hot electron component and to achieve time resolution of the hot electron flow during the discharge, a target probe with a soft x-ray monitor was designed, calibrated and implemented. The x-ray emission from the target was measured with a surface barrier detector covered with a set of different x-ray filters to achieve energy resolution. A calibration in the range 0.5-2 keV electron energy was performed on the same target and detector assembly using a 0741-3335/40/2/011/img2 cathode electron gun. The calibration data are interpolated and extrapolated numerically. A directional asymmetry of more than a factor of 100 for the higher energy electrons is observed. The hot electrons are estimated to constitute 10% of the total electron density at the edge and their energy distribution is approximated by a half-Maxwellian with a temperature slightly higher than the central electron temperature. Scalings with plasma current, as well as correlations with local 0741-3335/40/2/011/img3 measurements and radial dependences, are presented.

Enhanced quasi-static particle-in-cell simulation of electron cloud instabilities in circular accelerators

NASA Astrophysics Data System (ADS)

Feng, Bing

Electron cloud instabilities have been observed in many circular accelerators around the world and raised concerns of future accelerators and possible upgrades. In this thesis, the electron cloud instabilities are studied with the quasi-static particle-in-cell (PIC) code QuickPIC. Modeling in three-dimensions the long timescale propagation of beam in electron clouds in circular accelerators requires faster and more efficient simulation codes. Thousands of processors are easily available for parallel computations. However, it is not straightforward to increase the effective speed of the simulation by running the same problem size on an increasingly number of processors because there is a limit to domain size in the decomposition of the two-dimensional part of the code. A pipelining algorithm applied on the fully parallelized particle-in-cell code QuickPIC is implemented to overcome this limit. The pipelining algorithm uses multiple groups of processors and optimizes the job allocation on the processors in parallel computing. With this novel algorithm, it is possible to use on the order of 102 processors, and to expand the scale and the speed of the simulation with QuickPIC by a similar factor. In addition to the efficiency improvement with the pipelining algorithm, the fidelity of QuickPIC is enhanced by adding two physics models, the beam space charge effect and the dispersion effect. Simulation of two specific circular machines is performed with the enhanced QuickPIC. First, the proposed upgrade to the Fermilab Main Injector is studied with an eye upon guiding the design of the upgrade and code validation. Moderate emittance growth is observed for the upgrade of increasing the bunch population by 5 times. But the simulation also shows that increasing the beam energy from 8GeV to 20GeV or above can effectively limit the emittance growth. Then the enhanced QuickPIC is used to simulate the electron cloud effect on electron beam in the Cornell Energy Recovery Linac (ERL) due to extremely small emittance and high peak currents anticipated in the machine. A tune shift is discovered from the simulation; however, emittance growth of the electron beam in electron cloud is not observed for ERL parameters.

Low-Altitude Satellite Measurements of Pulsating Auroral Electrons

NASA Technical Reports Server (NTRS)

Samara, M.; Michell, R. G.; Redmon, R. J.

2015-01-01

We present observations from the Defense Meteorological Satellite Program and Reimei satellites, where common-volume high-resolution ground-based auroral imaging data are available. These satellite overpasses of ground-based all-sky imagers reveal the specific features of the electron populations responsible for different types of pulsating aurora modulations. The energies causing the pulsating aurora mostly range from 3 keV to 20 keV but can at times extend up to 30 keV. The secondary, low-energy electrons (<1 keV) are diminished from the precipitating distribution when there are strong temporal variations in auroral intensity. There are often persistent spatial structures present inside regions of pulsating aurora, and in these regions there are secondary electrons in the precipitating populations. The reduction of secondary electrons is consistent with the strongly temporally varying pulsating aurora being associated with field-aligned currents and hence parallel potential drops of up to 1 kV.

Planning and Prototyping for a Storage Ring Measurement of the Proton Electric Dipole Moment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Talman, Richard

2015-07-01

Electron and proton EDM's can be measured in "frozen spin" (with the beam polarization always parallel to the orbit, for example) storage rings. For electrons the "magic" kinetic energy at which the beam can be frozen is 14.5 MeV. For protons the magic kinetic energy is 230 MeV. The currently measured upper limit for the electron EDM is much smaller than the proton EDM upper limit, which is very poorly known. Nevertheless, because the storage ring will be an order of magnitude cheaper, a sensible plan is to first build an all-electric electron storage ring as a prototype. Such anmore » electron ring was successfully built at Brookhaven, in 1954, as a prototype for their AGS ring. This leaves little uncertainty concerning the cost and performance of such a ring. (This is documentedin one of the Physical Review papers mentioned above.)« less

Kinetic-scale fluctuations resolved with the Fast Plasma Investigation on NASA's Magnetospheric Multiscale mission.

NASA Astrophysics Data System (ADS)

Gershman, D. J.; Figueroa-Vinas, A.; Dorelli, J.; Goldstein, M. L.; Shuster, J. R.; Avanov, L. A.; Boardsen, S. A.; Stawarz, J. E.; Schwartz, S. J.; Schiff, C.; Lavraud, B.; Saito, Y.; Paterson, W. R.; Giles, B. L.; Pollock, C. J.; Strangeway, R. J.; Russell, C. T.; Torbert, R. B.; Moore, T. E.; Burch, J. L.

2017-12-01

Measurements from the Fast Plasma Investigation (FPI) on NASA's Magnetospheric Multiscale (MMS) mission have enabled unprecedented analyses of kinetic-scale plasma physics. FPI regularly provides estimates of current density and pressure gradients of sufficient accuracy to evaluate the relative contribution of terms in plasma equations of motion. In addition, high-resolution three-dimensional velocity distribution functions of both ions and electrons provide new insights into kinetic-scale processes. As an example, for a monochromatic kinetic Alfven wave (KAW) we find non-zero, but out-of-phase parallel current density and electric field fluctuations, providing direct confirmation of the conservative energy exchange between the wave field and particles. In addition, we use fluctuations in current density and magnetic field to calculate the perpendicular and parallel wavelengths of the KAW. Furthermore, examination of the electron velocity distribution inside the KAW reveals a population of electrons non-linearly trapped in the kinetic-scale magnetic mirror formed between successive wave peaks. These electrons not only contribute to the wave's parallel electric field but also account for over half of the density fluctuations within the wave, supplying an unexpected mechanism for maintaining quasi-neutrality in a KAW. Finally, we demonstrate that the employed wave vector determination technique is also applicable to broadband fluctuations found in Earth's turbulent magnetosheath.

Dimer formation of perylene: An ultracold spectroscopic and computational study

NASA Astrophysics Data System (ADS)

Birer, Ö.; Yurtsever, E.

2015-10-01

The electronic spectra of perylene inside helium nanodroplets recorded by the depletion method are presented. The results show two broad peaks in addition to sharp monomer vibronic transitions due to dimer formation. In order to understand the details of the spectra, first the dimer formation is studied by DFT and SCS-MP2 calculations and then the electronic spectra are calculated at the minima of the potential energy surface (PES). Theoretical calculations show that there are two low-lying energetically degenerate dimer structures; namely a parallel displaced one and a rotated stacked one. PES around these minima is very flat with a number of local minima at higher energies which at the experimental temperatures cannot be populated. Even though thermodynamically these two structures are equally populated, dynamical considerations point out that in helium droplet the parallel displaced geometry is encouraged by the natural alignment of the molecules due to the acquired angular momentum following the pick-up process. The calculated spectrum of the parallel displaced geometry predicts the positions of the dimer transitions within 30 nm of the experimental spectrum. Furthermore, the difference between the two dimer transitions is accurately predicted to be about 25 nm while the experimental difference was about 20 nm. Such a small difference could only be detected due to the ultracold conditions helium nanodroplets provided.

Parallelization of a Monte Carlo particle transport simulation code

NASA Astrophysics Data System (ADS)

Hadjidoukas, P.; Bousis, C.; Emfietzoglou, D.

2010-05-01

We have developed a high performance version of the Monte Carlo particle transport simulation code MC4. The original application code, developed in Visual Basic for Applications (VBA) for Microsoft Excel, was first rewritten in the C programming language for improving code portability. Several pseudo-random number generators have been also integrated and studied. The new MC4 version was then parallelized for shared and distributed-memory multiprocessor systems using the Message Passing Interface. Two parallel pseudo-random number generator libraries (SPRNG and DCMT) have been seamlessly integrated. The performance speedup of parallel MC4 has been studied on a variety of parallel computing architectures including an Intel Xeon server with 4 dual-core processors, a Sun cluster consisting of 16 nodes of 2 dual-core AMD Opteron processors and a 200 dual-processor HP cluster. For large problem size, which is limited only by the physical memory of the multiprocessor server, the speedup results are almost linear on all systems. We have validated the parallel implementation against the serial VBA and C implementations using the same random number generator. Our experimental results on the transport and energy loss of electrons in a water medium show that the serial and parallel codes are equivalent in accuracy. The present improvements allow for studying of higher particle energies with the use of more accurate physical models, and improve statistics as more particles tracks can be simulated in low response time.

Plasma and energetic particle structure of a collisionless quasi-parallel shock

NASA Technical Reports Server (NTRS)

Kennel, C. F.; Scarf, F. L.; Coroniti, F. V.; Russell, C. T.; Smith, E. J.; Wenzel, K. P.; Reinhard, R.; Sanderson, T. R.; Feldman, W. C.; Parks, G. K.

1983-01-01

The quasi-parallel interplanetary shock of November 11-12, 1978 from both the collisionless shock and energetic particle points of view were studied using measurements of the interplanetary magnetic and electric fields, solar wind electrons, plasma and MHD waves, and intermediate and high energy ions obtained on ISEE-1, -2, and -3. The interplanetary environment through which the shock was propagating when it encountered the three spacecraft was characterized; the observations of this shock are documented and current theories of quasi-parallel shock structure and particle acceleration are tested. These observations tend to confirm present self consistent theories of first order Fermi acceleration by shocks and of collisionless shock dissipation involving firehouse instability.

Energetic particles in laboratory, space and astrophysical plasmas

NASA Astrophysics Data System (ADS)

McClements, K. G.; Turnyanskiy, M. R.

2017-01-01

Some recent studies of energetic particles in laboratory, space and astrophysical plasmas are discussed, and a number of common themes identified. Such comparative studies can elucidate the underlying physical processes. For example microwave bursts observed during edge localised modes (ELMs) in the mega amp spherical tokamak (MAST) can be attributed to energetic electrons accelerated by parallel electric fields associated with the ELMs. The very large numbers of electrons known to be accelerated in solar flares must also arise from parallel electric fields, and the demonstration of energetic electron production during ELMs suggests close links at the kinetic level between ELMs and flares. Energetic particle studies in solar flares have focussed largely on electrons rather than ions, since bremsstrahlung from deka-keV electrons provides the best available explanation of flare hard x-ray emission. However ion acceleration (but not electron acceleration) has been observed during merging startup of plasmas in MAST with dimensionless parameters similar to those of the solar corona during flares. Recent measurements in the Earth’s radiation belts demonstrate clearly a direct link between ion cyclotron emission (ICE) and fast particle population inversion, supporting the hypothesis that ICE in tokamaks is driven by fast particle distributions of this type. Shear Alfvén waves in plasmas with beta less than the electron to ion mass ratio have a parallel electric field that, in the solar corona, could accelerate electrons to hard x-ray-emitting energies; an extension of this calculation to plasmas with Alfvén speed arbitrarily close to the speed of light suggests that the mechanism could play a role in the production of cosmic ray electrons.

Electron-impact-ionization dynamics of S F6

NASA Astrophysics Data System (ADS)

Bull, James N.; Lee, Jason W. L.; Vallance, Claire

2017-10-01

A detailed understanding of the dissociative electron ionization dynamics of S F6 is important in the modeling and tuning of dry-etching plasmas used in the semiconductor manufacture industry. This paper reports a crossed-beam electron ionization velocity-map imaging study on the dissociative ionization of cold S F6 molecules, providing complete, unbiased kinetic energy distributions for all significant product ions. Analysis of these distributions suggests that fragmentation following single ionization proceeds via formation of S F5 + or S F3 + ions that then dissociate in a statistical manner through loss of F atoms or F2, until most internal energy has been liberated. Similarly, formation of stable dications is consistent with initial formation of S F4 2 + ions, which then dissociate on a longer time scale. These data allow a comparison between electron ionization and photoionization dynamics, revealing similar dynamical behavior. In parallel with the ion kinetic energy distributions, the velocity-map imaging approach provides a set of partial ionization cross sections for all detected ionic fragments over an electron energy range of 50-100 eV, providing partial cross sections for S2 +, and enables the cross sections for S F4 2 + from S F+ to be resolved.

Auger mediated positron sticking on graphene and highly oriented pyrolytic graphite

NASA Astrophysics Data System (ADS)

Chirayath, V. A.; Chrysler, M.; McDonald, A.; Lim, Z.; Shastry, K.; Gladen, R.; Fairchild, A.; Koymen, A.; Weiss, A.

Positron annihilation induced Auger electron spectroscopy (PAES) measurements on 6-8 layers graphene grown on polycrystalline copper and the measurements on a highly oriented pyrolytic graphite (HOPG) sample have indicated the presence of a bound surface state for positrons. Measurements carried out with positrons of kinetic energies lower than the electron work function for graphene or HOPG have shown emission of low energy electrons possible only through the Auger mediated positron sticking (AMPS) process. In this process the positron makes a transition from a positive energy scattering state to a bound surface state. The transition energy is coupled to a valence electron which may then have enough energy to get ejected from the sample surface. The positrons which are bound to surface state are highly localized in a direction perpendicular to surface and delocalized parallel to it which makes this process highly surface sensitive and can thus be used for characterizing graphene or graphite surfaces for open volume defects and surface impurities. The measurements have also shown an extremely large low energy tail for the C KVV Auger transition at 263eV indicative of another physical process for low energy emission. This work was supported by NSF Grant No. DMR 1508719 and DMR 1338130.

Towards green high capacity optical networks

NASA Astrophysics Data System (ADS)

Glesk, I.; Mohd Warip, M. N.; Idris, S. K.; Osadola, T. B.; Andonovic, I.

2011-09-01

The demand for fast, secure, energy efficient high capacity networks is growing. It is fuelled by transmission bandwidth needs which will support among other things the rapid penetration of multimedia applications empowering smart consumer electronics and E-businesses. All the above trigger unparallel needs for networking solutions which must offer not only high-speed low-cost "on demand" mobile connectivity but should be ecologically friendly and have low carbon footprint. The first answer to address the bandwidth needs was deployment of fibre optic technologies into transport networks. After this it became quickly obvious that the inferior electronic bandwidth (if compared to optical fiber) will further keep its upper hand on maximum implementable serial data rates. A new solution was found by introducing parallelism into data transport in the form of Wavelength Division Multiplexing (WDM) which has helped dramatically to improve aggregate throughput of optical networks. However with these advancements a new bottleneck has emerged at fibre endpoints where data routers must process the incoming and outgoing traffic. Here, even with the massive and power hungry electronic parallelism routers today (still relying upon bandwidth limiting electronics) do not offer needed processing speeds networks demands. In this paper we will discuss some novel unconventional approaches to address network scalability leading to energy savings via advance optical signal processing. We will also investigate energy savings based on advanced network management through nodes hibernation proposed for Optical IP networks. The hibernation reduces the network overall power consumption by forming virtual network reconfigurations through selective nodes groupings and by links segmentations and partitionings.

Molecular Sticker Model Stimulation on Silicon for a Maximum Clique Problem

PubMed Central

Ning, Jianguo; Li, Yanmei; Yu, Wen

2015-01-01

Molecular computers (also called DNA computers), as an alternative to traditional electronic computers, are smaller in size but more energy efficient, and have massive parallel processing capacity. However, DNA computers may not outperform electronic computers owing to their higher error rates and some limitations of the biological laboratory. The stickers model, as a typical DNA-based computer, is computationally complete and universal, and can be viewed as a bit-vertically operating machine. This makes it attractive for silicon implementation. Inspired by the information processing method on the stickers computer, we propose a novel parallel computing model called DEM (DNA Electronic Computing Model) on System-on-a-Programmable-Chip (SOPC) architecture. Except for the significant difference in the computing medium—transistor chips rather than bio-molecules—the DEM works similarly to DNA computers in immense parallel information processing. Additionally, a plasma display panel (PDP) is used to show the change of solutions, and helps us directly see the distribution of assignments. The feasibility of the DEM is tested by applying it to compute a maximum clique problem (MCP) with eight vertices. Owing to the limited computing sources on SOPC architecture, the DEM could solve moderate-size problems in polynomial time. PMID:26075867

Parallel Douglas-Kroll Energy and Gradients in NWChem. Estimating Scalar Relativistic Effects Using Douglas-Kroll Contracted Basis Sets.

DOE Office of Scientific and Technical Information (OSTI.GOV)

De Jong, Wibe A.; Harrison, Robert J.; Dixon, David A.

A parallel implementation of the spin-free one-electron Douglas-Kroll(-Hess) Hamiltonian (DKH) in NWChem is discussed. An efficient and accurate method to calculate DKH gradients is introduced. It is shown that the use of standard (non-relativistic) contracted basis set can produce erroneous results for elements beyond the first row elements. The generation of DKH contracted cc-pVXZ (X = D, T, Q, 5) basis sets for H, He, B - Ne, Al - Ar, and Ga - Br will be discussed.

Large Scale Many-Body Perturbation Theory calculations: methodological developments, data collections, validation

NASA Astrophysics Data System (ADS)

Govoni, Marco; Galli, Giulia

Green's function based many-body perturbation theory (MBPT) methods are well established approaches to compute quasiparticle energies and electronic lifetimes. However, their application to large systems - for instance to heterogeneous systems, nanostructured, disordered, and defective materials - has been hindered by high computational costs. We will discuss recent MBPT methodological developments leading to an efficient formulation of electron-electron and electron-phonon interactions, and that can be applied to systems with thousands of electrons. Results using a formulation that does not require the explicit calculation of virtual states, nor the storage and inversion of large dielectric matrices will be presented. We will discuss data collections obtained using the WEST code, the advantages of the algorithms used in WEST over standard techniques, and the parallel performance. Work done in collaboration with I. Hamada, R. McAvoy, P. Scherpelz, and H. Zheng. This work was supported by MICCoM, as part of the Computational Materials Sciences Program funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division and by ANL.

Algorithm for fast event parameters estimation on GEM acquired data

NASA Astrophysics Data System (ADS)

Linczuk, Paweł; Krawczyk, Rafał D.; Poźniak, Krzysztof T.; Kasprowicz, Grzegorz; Wojeński, Andrzej; Chernyshova, Maryna; Czarski, Tomasz

2016-09-01

We present study of a software-hardware environment for developing fast computation with high throughput and low latency methods, which can be used as back-end in High Energy Physics (HEP) and other High Performance Computing (HPC) systems, based on high amount of input from electronic sensor based front-end. There is a parallelization possibilities discussion and testing on Intel HPC solutions with consideration of applications with Gas Electron Multiplier (GEM) measurement systems presented in this paper.

InP and GaAs characterization with variable stoichiometry obtained by molecular spray

NASA Technical Reports Server (NTRS)

Massies, J.; Linh, N. T.; Olivier, J.; Faulconnier, P.; Poirier, R.

1979-01-01

Both InP and GaAs surfaces were studied in parallel. A molecular spray technique was used to obtain two semiconductor surfaces with different superficial compositions. The structures of these surfaces were examined by electron diffraction. Electron energy loss was measured spectroscopically in order to determine surface electrical characteristics. The results are used to support conclusions relative to the role of surface composition in establishing a Schottky barrier effect in semiconductor devices.

Electronic energy transfer through non-adiabatic vibrational-electronic resonance. II. 1D spectra for a dimer.

PubMed

Tiwari, Vivek; Jonas, David M

2018-02-28

Vibrational-electronic resonance in photosynthetic pigment-protein complexes invalidates Förster's adiabatic framework for interpreting spectra and energy transfer, thus complicating determination of how the surrounding protein affects pigment properties. This paper considers the combined effects of vibrational-electronic resonance and inhomogeneous variations in the electronic excitation energies of pigments at different sites on absorption, emission, circular dichroism, and hole-burning spectra for a non-degenerate homodimer. The non-degenerate homodimer has identical pigments in different sites that generate differences in electronic energies, with parameters loosely based on bacteriochlorophyll a pigments in the Fenna-Matthews-Olson antenna protein. To explain the intensity borrowing, the excited state vibrational-electronic eigenvectors are discussed in terms of the vibrational basis localized on the individual pigments, as well as the correlated/anti-correlated vibrational basis delocalized over both pigments. Compared to those in the isolated pigment, vibrational satellites for the correlated vibration have the same frequency and precisely a factor of 2 intensity reduction through vibrational delocalization in both absorption and emission. Vibrational satellites for anti-correlated vibrations have their relaxed emission intensity reduced by over a factor 2 through vibrational and excitonic delocalization. In absorption, anti-correlated vibrational satellites borrow excitonic intensity but can be broadened away by the combination of vibronic resonance and site inhomogeneity; in parallel, their vibronically resonant excitonic partners are also broadened away. These considerations are consistent with photosynthetic antenna hole-burning spectra, where sharp vibrational and excitonic satellites are absent. Vibrational-excitonic resonance barely alters the inhomogeneously broadened linear absorption, emission, and circular dichroism spectra from those for a purely electronic excitonic coupling model. Energy transfer can leave excess energy behind as vibration on the electronic ground state of the donor, allowing vibrational relaxation on the donor's ground electronic state to make energy transfer permanent by removing excess energy from the excited electronic state of the dimer.

Electronic energy transfer through non-adiabatic vibrational-electronic resonance. II. 1D spectra for a dimer

NASA Astrophysics Data System (ADS)

Tiwari, Vivek; Jonas, David M.

2018-02-01

Vibrational-electronic resonance in photosynthetic pigment-protein complexes invalidates Förster's adiabatic framework for interpreting spectra and energy transfer, thus complicating determination of how the surrounding protein affects pigment properties. This paper considers the combined effects of vibrational-electronic resonance and inhomogeneous variations in the electronic excitation energies of pigments at different sites on absorption, emission, circular dichroism, and hole-burning spectra for a non-degenerate homodimer. The non-degenerate homodimer has identical pigments in different sites that generate differences in electronic energies, with parameters loosely based on bacteriochlorophyll a pigments in the Fenna-Matthews-Olson antenna protein. To explain the intensity borrowing, the excited state vibrational-electronic eigenvectors are discussed in terms of the vibrational basis localized on the individual pigments, as well as the correlated/anti-correlated vibrational basis delocalized over both pigments. Compared to those in the isolated pigment, vibrational satellites for the correlated vibration have the same frequency and precisely a factor of 2 intensity reduction through vibrational delocalization in both absorption and emission. Vibrational satellites for anti-correlated vibrations have their relaxed emission intensity reduced by over a factor 2 through vibrational and excitonic delocalization. In absorption, anti-correlated vibrational satellites borrow excitonic intensity but can be broadened away by the combination of vibronic resonance and site inhomogeneity; in parallel, their vibronically resonant excitonic partners are also broadened away. These considerations are consistent with photosynthetic antenna hole-burning spectra, where sharp vibrational and excitonic satellites are absent. Vibrational-excitonic resonance barely alters the inhomogeneously broadened linear absorption, emission, and circular dichroism spectra from those for a purely electronic excitonic coupling model. Energy transfer can leave excess energy behind as vibration on the electronic ground state of the donor, allowing vibrational relaxation on the donor's ground electronic state to make energy transfer permanent by removing excess energy from the excited electronic state of the dimer.

Magnetic helicity conservation and inverse energy cascade in electron magnetohydrodynamic wave packets.

PubMed

Cho, Jungyeon

2011-05-13

Electron magnetohydrodynamics (EMHD) provides a fluidlike description of small-scale magnetized plasmas. An EMHD wave propagates along magnetic field lines. The direction of propagation can be either parallel or antiparallel to the magnetic field lines. We numerically study propagation of three-dimensional (3D) EMHD wave packets moving in one direction. We obtain two major results. (1) Unlike its magnetohydrodynamic (MHD) counterpart, an EMHD wave packet is dispersive. Because of this, EMHD wave packets traveling in one direction create opposite-traveling wave packets via self-interaction and cascade energy to smaller scales. (2) EMHD wave packets traveling in one direction clearly exhibit inverse energy cascade. We find that the latter is due to conservation of magnetic helicity. We compare inverse energy cascade in 3D EMHD turbulence and two-dimensional (2D) hydrodynamic turbulence.

New Insights into the Nature of Turbulence in the Earth's Magnetosheath Using Magnetospheric MultiScale Mission Data

NASA Astrophysics Data System (ADS)

Breuillard, H.; Matteini, L.; Argall, M. R.; Sahraoui, F.; Andriopoulou, M.; Le Contel, O.; Retinò, A.; Mirioni, L.; Huang, S. Y.; Gershman, D. J.; Ergun, R. E.; Wilder, F. D.; Goodrich, K. A.; Ahmadi, N.; Yordanova, E.; Vaivads, A.; Turner, D. L.; Khotyaintsev, Yu. V.; Graham, D. B.; Lindqvist, P.-A.; Chasapis, A.; Burch, J. L.; Torbert, R. B.; Russell, C. T.; Magnes, W.; Strangeway, R. J.; Plaschke, F.; Moore, T. E.; Giles, B. L.; Paterson, W. R.; Pollock, C. J.; Lavraud, B.; Fuselier, S. A.; Cohen, I. J.

2018-06-01

The Earth’s magnetosheath, which is characterized by highly turbulent fluctuations, is usually divided into two regions of different properties as a function of the angle between the interplanetary magnetic field and the shock normal. In this study, we make use of high-time resolution instruments on board the Magnetospheric MultiScale spacecraft to determine and compare the properties of subsolar magnetosheath turbulence in both regions, i.e., downstream of the quasi-parallel and quasi-perpendicular bow shocks. In particular, we take advantage of the unprecedented temporal resolution of the Fast Plasma Investigation instrument to show the density fluctuations down to sub-ion scales for the first time. We show that the nature of turbulence is highly compressible down to electron scales, particularly in the quasi-parallel magnetosheath. In this region, the magnetic turbulence also shows an inertial (Kolmogorov-like) range, indicating that the fluctuations are not formed locally, in contrast with the quasi-perpendicular magnetosheath. We also show that the electromagnetic turbulence is dominated by electric fluctuations at sub-ion scales (f > 1 Hz) and that magnetic and electric spectra steepen at the largest-electron scale. The latter indicates a change in the nature of turbulence at electron scales. Finally, we show that the electric fluctuations around the electron gyrofrequency are mostly parallel in the quasi-perpendicular magnetosheath, where intense whistlers are observed. This result suggests that energy dissipation, plasma heating, and acceleration might be driven by intense electrostatic parallel structures/waves, which can be linked to whistler waves.

On the wall perturbation correction for a parallel-plate NACP-02 chamber in clinical electron beams.

PubMed

Zink, K; Wulff, J

2011-02-01

In recent years, several Monte Carlo studies have been published concerning the perturbation corrections of a parallel-plate chamber in clinical electron beams. In these studies, a strong depth dependence of the relevant correction factors (p(wall) and P(cav)) for depth beyond the reference depth is recognized and it has been shown that the variation with depth is sensitive to the choice of the chamber's effective point of measurement. Recommendations concerning the positioning of parallel-plate ionization chambers in clinical electron beams are not the same for all current dosimetry protocols. The IAEA TRS-398 as well as the IPEM protocol and the German protocol DIN 6800-2 interpret the depth of measurement within the phantom as the water equivalent depth, i.e., the nonwater equivalence of the entrance window has to be accounted for by shifting the chamber by an amount deltaz. This positioning should ensure that the primary electrons traveling from the surface of the water phantom through the entrance window to the chamber's reference point sustain the same energy loss as the primary electrons in the undisturbed phantom. The objective of the present study is the determination of the shift deltaz for a NACP-02 chamber and the calculation of the resulting wall perturbation correction as a function of depth. Moreover, the contributions of the different chamber walls to the wall perturbation correction are identified. The dose and fluence within the NACP-02 chamber and a wall-less air cavity is calculated using the Monte Carlo code EGSnrc in a water phantom at different depths for different clinical electron beams. In order to determine the necessary shift to account for the nonwater equivalence of the entrance window, the chamber is shifted in steps deltaz around the depth of measurement. The optimal shift deltaz is determined from a comparison of the spectral fluence within the chamber and the bare cavity. The wall perturbation correction is calculated as the ratio between doses for the complete chamber and a wall-less air cavity. The high energy part of the fluence spectra within the chamber strongly varies even with small chamber shifts, allowing the determination of deltaz within micrometers. For the NACP-02 chamber a shift deltaz = -0.058 cm results. This value is independent of the energy of the primary electrons as well as of the depth within the phantom and it is in good agreement with the value recommended in the German dosimetry protocol. Applying this shift, the calculated wall perturbation correction as a function of depth is varying less than 1% from zero up to the half value depth R50 for electron energies in the range of 6-21 MeV. The remaining depth dependence can mainly be attributed to the scatter properties of the entrance window. When neglecting the nonwater equivalence of the entrance window, the variation of p(wall) with depth is up to 10% and more, especially for low electron energies. The variation of the wall perturbation correction for the NACP-02 chamber in clinical electron beams strongly depends on the positioning of the chamber. Applying a shift deltaz = -0.058 cm toward the focus ensures that the primary electron spectrum within the chamber bears the largest resemblance to the fluence of a wall-less cavity. Hence, the influence of the chamber walls on the perturbation correction can be separated out and the residual variation of p(wall) with depth is minimized.

Two-fluid description of wave-particle interactions in strong Buneman turbulence

NASA Astrophysics Data System (ADS)

Che, H.

2014-06-01

To understand the nature of anomalous resistivity in magnetic reconnection, we investigate turbulence-induced momentum transport and energy dissipation while a plasma is unstable to the Buneman instability in force-free current sheets. Using 3D particle-in-cell simulations, we find that the macroscopic effects generated by wave-particle interactions in Buneman instability can be approximately described by a set of electron fluid equations. We show that both energy dissipation and momentum transport along electric current in the current layer are locally quasi-static, but globally dynamic and irreversible. Turbulent drag dissipates both the streaming energy of the current sheet and the associated magnetic energy. The net loss of streaming energy is converted into the electron component heat conduction parallel to the magnetic field and increases the electron Boltzmann entropy. The growth of self-sustained Buneman waves satisfies a Bernoulli-like equation that relates the turbulence-induced convective momentum transport and thermal momentum transport. Electron trapping and de-trapping drive local momentum transports, while phase mixing converts convective momentum into thermal momentum. The drag acts like a micro-macro link in the anomalous heating processes. The decrease of magnetic field maintains an inductive electric field that re-accelerates electrons, but most of the magnetic energy is dissipated and converted into the component heat of electrons perpendicular to the magnetic field. This heating process is decoupled from the heating of Buneman instability in the current sheets. Ion heating is weak but ions play an important role in assisting energy exchanges between waves and electrons. Cold ion fluid equations together with our electron fluid equations form a complete set of equations that describes the occurrence, growth, saturation and decay of the Buneman instability.

Evolution of the axial electron cyclotron maser instability, with applications to solar microwave spikes

NASA Technical Reports Server (NTRS)

Vlahos, Loukas; Sprangle, Phillip

1987-01-01

The nonlinear evolution of cyclotron radiation from streaming and gyrating electrons in an external magnetic field is analyzed. The nonlinear dynamics of both the fields and the particles are treated fully relativistically and self-consistently. The model includes a background plasma and electrostatic effects. The analytical and numerical results show that a substantial portion of the beam particle energy can be converted to electromagnetic wave energy at frequencies far above the electron cyclotron frequency. In general, the excited radiation can propagate parallel to the magnetic field and, hence, escape gyrothermal absorption at higher cyclotron harmonics. The high-frequency Doppler-shifted cyclotron instability can have saturation efficiencies far higher than those associated with well-known instabilities of the electron cyclotron maser type. Although the analysis is general, the possibility of using this model to explain the intense radio emission observed from the sun is explored in detail.

F"orster-type mechanism of the redox-driven proton pump

NASA Astrophysics Data System (ADS)

Mourokh, Lev; Smirnov, Anatoly; Nori, Franco

2007-03-01

We propose a model to describe an electronically-driven proton pump in the cytochrome c oxidase (CcO). We examine the situation when the electron transport between the two sites embedded into the inner membrane of the mitochondrion occurs in parallel with the proton transfer from the protonable site that is close to the negative (inner) side of the membrane to the other protonable site located nearby the positive (outer) surface of the membrane. In addition to the conventional electron and proton tunnelings between the sites, the Coulomb interaction between electrons and protons localized on the corresponding sites leads to so-called F"orster transfer, i.e. to the process when the simultaneous electron and proton tunnelings are accompanied by the resonant energy transfer between the electrons and protons. Our calculations based on reasonable parameters have demonstrated that the F"orster process facilitates the proton pump at physiological temperatures. We have examined the effects of an electron voltage build-up, external temperature, and molecular electrostatics driving the electron and proton energies to the resonant conditions, and have shown that these parameters can control the proton pump operation.

Observations of wave-particle interactions in the flux pile-up region of asymmetric reconnection

NASA Astrophysics Data System (ADS)

Argall, M. R.; Paulson, K. W.; Ahmadi, N.; Matsui, H.; Torbert, R. B.; Alm, L.; Le Contel, O.; Khotyaintsev, Y. V.; Wilder, F. D.; Turner, D. L.; Strangeway, R. J.; Schwartz, S. J.; Magnes, W.; Giles, B. L.; Lindqvist, P. A.; Ergun, R.; Mauk, B.; Leonard, T. W.

2017-12-01

Recent observations have shown electron energization to >100keV with simultaneous whistler wave activity in the vicinity of the dayside reconnection site. We investigate one possible mechanism for producing these energetic particles. Counter-streaming electrons from the magnetosphere enter the diffusion region and are scattered to all pitch angles (PAs) by strong field-line curvature. As the electrons flow outward into the exhaust, they remagnetize and are focused toward 90° at mirror points within the flux pile-up region. This effect, combined with heating mechanisms in the EDR, produces a temperature anisotropy, while the weak magnetic field lowers the resonant energy into the bulk energy of the plasma. In the end, whistler waves are produced near 100Hz with a wave normal angle of 20°. Simultaneous with the waves, the Electron Drift Instrument observes particle flux modulations of 0° and 180° PA, 500 eV electrons. Multi-spacecraft analysis and Liouville mapping techniques allow us to determine the parallel wave current, potential, and associated energy dissipation. Bursts of 100keV electrons are observed and may interact with the whistler waves.

Ion gyroradius effects on particle trapping in kinetic Alfven waves along auroral field lines

DOE Office of Scientific and Technical Information (OSTI.GOV)

Damiano, P. A.; Johnson, J. R.; Chaston, C. C.

In this study, a 2-D self-consistent hybrid gyrofluid-kinetic electron model is used to investigate Alfven wave propagation along dipolar magnetic field lines for a range of ion to electron temperature ratios. The focus of the investigation is on understanding the role of these effects on electron trapping in kinetic Alfven waves sourced in the plasma sheet and the role of this trapping in contributing to the overall electron energization at the ionosphere. This work also builds on our previous effort by considering a similar system in the limit of fixed initial parallel current, rather than fixed initial perpendicular electric field.more » It is found that the effects of particle trapping are strongest in the cold ion limit and the kinetic Alfven wave is able to carry trapped electrons a large distance along the field line yielding a relatively large net energization of the trapped electron population as the phase speed of the wave is increased. However, as the ion temperature is increased, the ability of the kinetic Alfven wave to carry and energize trapped electrons is reduced by more significant wave energy dispersion perpendicular to the ambient magnetic field which reduces the amplitude of the wave. This reduction of wave amplitude in turn reduces both the parallel current and the extent of the high-energy tails evident in the energized electron populations at the ionospheric boundary (which may serve to explain the limited extent of the broadband electron energization seen in observations). Here, even in the cold ion limit, trapping effects in kinetic Alfven waves lead to only modest electron energization for the parameters considered (on the order of tens of eV) and the primary energization of electrons to keV levels coincides with the arrival of the wave at the ionospheric boundary.« less

Ion gyroradius effects on particle trapping in kinetic Alfven waves along auroral field lines

DOE PAGES

Damiano, P. A.; Johnson, J. R.; Chaston, C. C.

2016-11-10

In this study, a 2-D self-consistent hybrid gyrofluid-kinetic electron model is used to investigate Alfven wave propagation along dipolar magnetic field lines for a range of ion to electron temperature ratios. The focus of the investigation is on understanding the role of these effects on electron trapping in kinetic Alfven waves sourced in the plasma sheet and the role of this trapping in contributing to the overall electron energization at the ionosphere. This work also builds on our previous effort by considering a similar system in the limit of fixed initial parallel current, rather than fixed initial perpendicular electric field.more » It is found that the effects of particle trapping are strongest in the cold ion limit and the kinetic Alfven wave is able to carry trapped electrons a large distance along the field line yielding a relatively large net energization of the trapped electron population as the phase speed of the wave is increased. However, as the ion temperature is increased, the ability of the kinetic Alfven wave to carry and energize trapped electrons is reduced by more significant wave energy dispersion perpendicular to the ambient magnetic field which reduces the amplitude of the wave. This reduction of wave amplitude in turn reduces both the parallel current and the extent of the high-energy tails evident in the energized electron populations at the ionospheric boundary (which may serve to explain the limited extent of the broadband electron energization seen in observations). Here, even in the cold ion limit, trapping effects in kinetic Alfven waves lead to only modest electron energization for the parameters considered (on the order of tens of eV) and the primary energization of electrons to keV levels coincides with the arrival of the wave at the ionospheric boundary.« less

Conductance spectra of asymmetric ferromagnet/ferromagnet/ferromagnet junctions

NASA Astrophysics Data System (ADS)

Pasanai, K.

2017-01-01

A theory of tunneling spectroscopy of ferromagnet/ferromagnet/ferromagnet junctions was studied. We applied a delta-functional approximation for the interface scattering properties under a one-dimensional system of a free electron approach. The reflection and transmission probabilities were calculated in the ballistic regime, and the conductance spectra were then calculated using the Landauer formulation. The magnetization directions were set to be either parallel (P) or anti-parallel (AP) alignments, for comparison. We found that the conductance spectra was suppressed when increasing the interfacial scattering at the interfaces. Moreover, the electron could exhibit direct transmission when the thickness was rather thin. Thus, there was no oscillation in this case. However, in the case of a thick layer the conductance spectra oscillated, and this oscillation was most prominent when the middle layer thickness increased. In the case of direct transmission, the conductance spectra of P and AP systems were definitely suppressed with increased exchange energy of the middle ferromagnet. This also refers to an increase in the magnetoresistance of the junction. In the case of oscillatory behavior, the positions of the resonance peaks were changed as the exchange energy was changed.

Parallel Energy Transport in Detached DIII-D Divertor Plasmas

NASA Astrophysics Data System (ADS)

Leonard, A. W.; Lore, J. D.; Canik, J. M.; McLean, A. G.; Makowski, M. A.

2017-10-01

A comparison of experiment and modeling of detached divertor plasmas is examined in the context of parallel energy transport. Experimental estimates of power carried by electron thermal conduction versus plasma convection are experimentally inferred from power balance measurements of radiated power and target plate heat flux combined with Thomson scattering measurements of the Te profile along the divertor leg. Experimental profiles of Te exhibit relatively low gradients with Te < 15 eV from the X-point to the target implying transport dominated by convection. In contrast, fluid modeling with SOLPS produces sharp Te gradients for Te > 3 eV, characteristic of transport dominated by electron conduction through the bulk of the divertor. This discrepancy with experimental transport dominated by convection and modeling by conduction has significant implications for the radiative capacity of divertor plasmas and may explain at least part of the difficulty for fluid modeling to obtain the experimentally observed radiative losses. Comparisons are also made for helium plasmas where the match between experiment and modeling is much better. Work supported by the US DOE under DE-FC02-04ER54698.

Calculation of Vertical and Horizontal Mobilities in InAs/GaSb Superlattices (Postprint)

DTIC Science & Technology

2011-10-13

width 2a and GaSb having width 2b, with the period = 2a + 2b. For energies near the band gap edges, the carrier wave function can be approximated by a...online) Electron energy bands along the growth direction for three combinations of InAs/ GaSb layer widths. For typical carrier densities, at low...Fermi energies , parallel masses, and band gaps from the 8×8 EFA model. Sheet carrier Calculated Measured Calculated InAs GaSb concentration per period

On the p(dis) correction factor for cylindrical chambers.

PubMed

Andreo, Pedro

2010-03-07

The authors of a recent paper (Wang and Rogers 2009 Phys. Med. Biol. 54 1609) have used the Monte Carlo method to simulate the 'classical' experiment made more than 30 years ago by Johansson et al (1978 National and International Standardization of Radiation Dosimetry (Atlanta 1977) vol 2 (Vienna: IAEA) pp 243-70) on the displacement (or replacement) perturbation correction factor p(dis) for cylindrical chambers in 60Co and high-energy photon beams. They conclude that an 'unreasonable normalization at dmax' of the ionization chambers response led to incorrect results, and for the IAEA TRS-398 Code of Practice, which uses ratios of those results, 'the difference in the correction factors can lead to a beam calibration deviation of more than 0.5% for Farmer-like chambers'. The present work critically examines and questions some of the claims and generalized conclusions of the paper. It is demonstrated that for real, commercial Farmer-like chambers, the possible deviations in absorbed dose would be much smaller (typically 0.13%) than those stated by Wang and Rogers, making the impact of their proposed values negligible on practical high-energy photon dosimetry. Differences of the order of 0.4% would only appear at the upper extreme of the energies potentially available for clinical use (around 25 MV) and, because lower energies are more frequently used, the number of radiotherapy photon beams for which the deviations would be larger than say 0.2% is extremely small. This work also raises concerns on the proposed value of pdis for Farmer chambers at the reference quality of 60Co in relation to their impact on electron beam dosimetry, both for direct dose determination using these chambers and for the cross-calibration of plane-parallel chambers. The proposed increase of about 1% in p(dis) (compared with TRS-398) would lower the kQ factors and therefore Dw in electron beams by the same amount. This would yield a severe discrepancy with the current good agreement between electron dosimetry based on an electron cross-calibrated plane-parallel chamber (against a Farmer) or on a directly 60Co calibrated plane-parallel chamber, which is not likely to be in error by 1%. It is suggested that the influence of the 60Co source spectrum used in the simulations may not be negligible for calculations aimed at an uncertainty level of 0.1%.

LETTER TO THE EDITOR: On the pdis correction factor for cylindrical chambers

NASA Astrophysics Data System (ADS)

Andreo, Pedro

2010-03-01

The authors of a recent paper (Wang and Rogers 2009 Phys. Med. Biol. 54 1609) have used the Monte Carlo method to simulate the 'classical' experiment made more than 30 years ago by Johansson et al (1978 National and International Standardization of Radiation Dosimetry (Atlanta 1977) vol 2 (Vienna: IAEA) pp 243-70) on the displacement (or replacement) perturbation correction factor pdis for cylindrical chambers in 60Co and high-energy photon beams. They conclude that an 'unreasonable normalization at dmax' of the ionization chambers response led to incorrect results, and for the IAEA TRS-398 Code of Practice, which uses ratios of those results, 'the difference in the correction factors can lead to a beam calibration deviation of more than 0.5% for Farmer-like chambers'. The present work critically examines and questions some of the claims and generalized conclusions of the paper. It is demonstrated that for real, commercial Farmer-like chambers, the possible deviations in absorbed dose would be much smaller (typically 0.13%) than those stated by Wang and Rogers, making the impact of their proposed values negligible on practical high-energy photon dosimetry. Differences of the order of 0.4% would only appear at the upper extreme of the energies potentially available for clinical use (around 25 MV) and, because lower energies are more frequently used, the number of radiotherapy photon beams for which the deviations would be larger than say 0.2% is extremely small. This work also raises concerns on the proposed value of pdis for Farmer chambers at the reference quality of 60Co in relation to their impact on electron beam dosimetry, both for direct dose determination using these chambers and for the cross-calibration of plane-parallel chambers. The proposed increase of about 1% in pdis (compared with TRS-398) would lower the kQ factors and therefore Dw in electron beams by the same amount. This would yield a severe discrepancy with the current good agreement between electron dosimetry based on an electron cross-calibrated plane-parallel chamber (against a Farmer) or on a directly 60Co calibrated plane-parallel chamber, which is not likely to be in error by 1%. It is suggested that the influence of the 60Co source spectrum used in the simulations may not be negligible for calculations aimed at an uncertainty level of 0.1%.

Lead zirconate titanate nanowire textile nanogenerator for wearable energy-harvesting and self-powered devices.

PubMed

Wu, Weiwei; Bai, Suo; Yuan, Miaomiao; Qin, Yong; Wang, Zhong Lin; Jing, Tao

2012-07-24

Wearable nanogenerators are of vital importance to portable energy-harvesting and personal electronics. Here we report a method to synthesize a lead zirconate titanate textile in which nanowires are parallel with each other and a procedure to make it into flexible and wearable nanogenerators. The nanogenerator can generate 6 V output voltage and 45 nA output current, which are large enough to power a liquid crystal display and a UV sensor.

Cyclotron maser instability and its applications

NASA Astrophysics Data System (ADS)

Wu, C. S.

The possible application of cyclotron maser theory to a variety of radio sources is considered, with special attention given to the theory of auroral kilometric radiation (AKR) of Wu and Lee (1979). The AKR model assumes a loss-cone distribution function for the reflected electrons, along with the depletion of low-energy electrons by the parallel electric field. Other topics considered include fundamental AKR, second-harmonic AKR, the generation of Z-mode radiation, and the application of maser instability to other sources than AKR.

On the modulation of the Jovian decametric radiation by Io. I - Acceleration of charged particles

NASA Technical Reports Server (NTRS)

Smith, R. A.; Goertz, C. K.

1978-01-01

A steady-state analysis of the current circuit between Io and the Jovian ionosphere is performed, assuming that the current is carried by electrons accelerated through potential double layers in the Io flux tube. The circuit analysis indicates that electrons may be accelerated up to energies of several hundred keV. Several problems associated with the formation of double layers are also discussed. The parallel potential drops decouple the flux tube from the satellite's orbital motion.

Particle in cell/Monte Carlo collision analysis of the problem of identification of impurities in the gas by the plasma electron spectroscopy method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kusoglu Sarikaya, C.; Rafatov, I., E-mail: rafatov@metu.edu.tr; Kudryavtsev, A. A.

2016-06-15

The work deals with the Particle in Cell/Monte Carlo Collision (PIC/MCC) analysis of the problem of detection and identification of impurities in the nonlocal plasma of gas discharge using the Plasma Electron Spectroscopy (PLES) method. For this purpose, 1d3v PIC/MCC code for numerical simulation of glow discharge with nonlocal electron energy distribution function is developed. The elastic, excitation, and ionization collisions between electron-neutral pairs and isotropic scattering and charge exchange collisions between ion-neutral pairs and Penning ionizations are taken into account. Applicability of the numerical code is verified under the Radio-Frequency capacitively coupled discharge conditions. The efficiency of the codemore » is increased by its parallelization using Open Message Passing Interface. As a demonstration of the PLES method, parallel PIC/MCC code is applied to the direct current glow discharge in helium doped with a small amount of argon. Numerical results are consistent with the theoretical analysis of formation of nonlocal EEDF and existing experimental data.« less

Electron emission perpendicular to the polarization direction in laser-assisted XUV atomic ionization

NASA Astrophysics Data System (ADS)

Gramajo, A. A.; Della Picca, R.; Arbó, D. G.

2017-08-01

We present a theoretical study of ionization of the hydrogen atom due to an XUV pulse in the presence of an infrared (IR) laser with both fields linearly polarized in the same direction. In particular, we study the energy distribution of photoelectrons emitted perpendicularly to the polarization direction. As we previously showed in Gramajo et al. [Phys. Rev. A 94, 053404 (2016), 10.1103/PhysRevA.94.053404] for parallel emission, by means of a very simple semiclassical model which considers electron trajectories born at different ionization times, the electron energy spectrum can be interpreted as the interplay of intra- and intercycle interferences. However, contrary to the case of parallel emission the intracycle interference pattern stems from the coherent superposition of four electron trajectories giving rise to (i) interference of electron trajectories born during the same half cycle (intra-half-cycle interference) and (ii) interference between electron trajectories born during the first half cycle with those born during the second half cycle (inter-half-cycle interference). The intercycle interference is responsible for the formation of the sidebands. We also show that the destructive inter-half-cycle interference for the absorption and emission of an even number of IR laser photons is responsible for the characteristic sidebands in the perpendicular direction separated by twice the IR photon energy. This contrasts with the emission along the polarization axis (all sideband orders are present) since intra-half-cycle interferences do not exist in that case. The intracycle interference pattern works as a modulation of the sidebands and, in the same way, it is modulated by the intra-half-cycle interference pattern. We analyze the dependence of the energy spectrum on the laser intensity and the time delay between the XUV pulse and the IR laser. Finally, we show that our semiclassical simulations are in very good agreement with quantum calculations within the strong-field approximation and the numerical solution of the time-dependent Schrödinger equation, giving rise to nonzero emission, in contraposition to other theories.

Extrinsic Rashba spin-orbit coupling effect on silicene spin polarized field effect transistors

NASA Astrophysics Data System (ADS)

Pournaghavi, Nezhat; Esmaeilzadeh, Mahdi; Abrishamifar, Adib; Ahmadi, Somaieh

2017-04-01

Regarding the spin field effect transistor (spin FET) challenges such as mismatch effect in spin injection and insufficient spin life time, we propose a silicene based device which can be a promising candidate to overcome some of those problems. Using non-equilibrium Green’s function method, we investigate the spin-dependent conductance in a zigzag silicene nanoribbon connected to two magnetized leads which are supposed to be either in parallel or anti-parallel configurations. For both configurations, a controllable spin current can be obtained when the Rashba effect is present; thus, we can have a spin filter device. In addition, for anti-parallel configuration, in the absence of Rashba effect, there is an intrinsic energy gap in the system (OFF-state); while, in the presence of Rashba effect, electrons with flipped spin can pass through the channel and make the ON-state. The current voltage (I-V) characteristics which can be tuned by changing the gate voltage or Rashba strength, are studied. More importantly, reducing the mismatch conductivity as well as energy consumption make the silicene based spin FET more efficient relative to the spin FET based on two-dimensional electron gas proposed by Datta and Das. Also, we show that, at the same conditions, the current and {{I}\\text{on}}/{{I}\\text{off}} ratio of silicene based spin FET are significantly greater than that of the graphene based one.

NWChem: A comprehensive and scalable open-source solution for large scale molecular simulations

NASA Astrophysics Data System (ADS)

Valiev, M.; Bylaska, E. J.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Van Dam, H. J. J.; Wang, D.; Nieplocha, J.; Apra, E.; Windus, T. L.; de Jong, W. A.

2010-09-01

The latest release of NWChem delivers an open-source computational chemistry package with extensive capabilities for large scale simulations of chemical and biological systems. Utilizing a common computational framework, diverse theoretical descriptions can be used to provide the best solution for a given scientific problem. Scalable parallel implementations and modular software design enable efficient utilization of current computational architectures. This paper provides an overview of NWChem focusing primarily on the core theoretical modules provided by the code and their parallel performance. Program summaryProgram title: NWChem Catalogue identifier: AEGI_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEGI_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Open Source Educational Community License No. of lines in distributed program, including test data, etc.: 11 709 543 No. of bytes in distributed program, including test data, etc.: 680 696 106 Distribution format: tar.gz Programming language: Fortran 77, C Computer: all Linux based workstations and parallel supercomputers, Windows and Apple machines Operating system: Linux, OS X, Windows Has the code been vectorised or parallelized?: Code is parallelized Classification: 2.1, 2.2, 3, 7.3, 7.7, 16.1, 16.2, 16.3, 16.10, 16.13 Nature of problem: Large-scale atomistic simulations of chemical and biological systems require efficient and reliable methods for ground and excited solutions of many-electron Hamiltonian, analysis of the potential energy surface, and dynamics. Solution method: Ground and excited solutions of many-electron Hamiltonian are obtained utilizing density-functional theory, many-body perturbation approach, and coupled cluster expansion. These solutions or a combination thereof with classical descriptions are then used to analyze potential energy surface and perform dynamical simulations. Additional comments: Full documentation is provided in the distribution file. This includes an INSTALL file giving details of how to build the package. A set of test runs is provided in the examples directory. The distribution file for this program is over 90 Mbytes and therefore is not delivered directly when download or Email is requested. Instead a html file giving details of how the program can be obtained is sent. Running time: Running time depends on the size of the chemical system, complexity of the method, number of cpu's and the computational task. It ranges from several seconds for serial DFT energy calculations on a few atoms to several hours for parallel coupled cluster energy calculations on tens of atoms or ab-initio molecular dynamics simulation on hundreds of atoms.

Study of electrostatic electron cyclotron parallel flow velocity shear instability in the magnetosphere of Saturn

NASA Astrophysics Data System (ADS)

Kandpal, Praveen; Pandey, R. S.

2018-05-01

In the present paper, the study of electrostatic electron cyclotron parallel flow velocity shear instability in presence of perpendicular inhomogeneous DC electric field has been carried out in the magnetosphere of Saturn. Dimensionless growth rate variation of electron cyclotron waves has been observed with respect to k⊥ ρe for various plasma parameters. Effect of velocity shear scale length (Ae), inhomogeneity (P/a), the ratio of ion to electron temperature (Ti/Te) and density gradient (ɛnρe) on the growth of electron cyclotron waves in the inner magnetosphere of Saturn has been studied and analyzed. The mathematical formulation and computation of dispersion relation and growth rate have been done by using the method of characteristic solution and kinetic approach. This theoretical analysis has been done taking the relevant data from the Cassini spacecraft in the inner magnetosphere of Saturn. We have considered ambient magnetic field data and other relevant data for this study at the radial distance of ˜4.82-5.00 Rs. In our study velocity shear and ion to electron temperature ratio have been observed to be the major sources of free energy for the electron cyclotron instability. The inhomogeneity of electric field caused a small noticeable impact on the growth rate of electrostatic electron cyclotron instability. Density gradient has been observed playing stabilizing effect on electron cyclotron instability.

Sounding rocket study of auroral electron precipitation

DOE Office of Scientific and Technical Information (OSTI.GOV)

McFadden, J.P.

1985-01-01

Measurement of energetic electrons in the auroral zone have proved to be one of the most useful tools in investigating the phenomena of auroral arc formation. This dissertation presents a detailed analysis of the electron data from two sounding rocket campaigns and interprets the measurements in terms of existing auroral models. The Polar Cusp campaign consisted of a single rocket launched from Cape Parry, Canada into the afternoon auroral zone at 1:31:13 UT on January 21, 1982. The results include the measurement of a narrow, magnetic field aligned electron flux at the edge of an arc. This electron precipitation wasmore » found to have a remarkably constant 1.2 eV temperature perpendicular to the magnetic field over a 200 to 900 eV energy range. The payload also made simultaneous measurements of both energetic electrons and 3-MHz plasma waves in an auroral arc. Analysis has shown that the waves are propagating in the upper hybrid band and should be generated by a positive slope in the parallel electron distribution. A correlation was found between the 3-MHz waves and small positive slopes in the parallel electron distribution but experimental uncertainties in the electron measurement were large enough to influence the analysis. The BIDARCA campaign consisted of two sounding rockets launched from Poker Flat and Fort Yukon, Alaska at 9:09:00 UT and 9:10:40 UT on February 7, 1984.« less

BerkeleyGW: A massively parallel computer package for the calculation of the quasiparticle and optical properties of materials and nanostructures

NASA Astrophysics Data System (ADS)

Deslippe, Jack; Samsonidze, Georgy; Strubbe, David A.; Jain, Manish; Cohen, Marvin L.; Louie, Steven G.

2012-06-01

BerkeleyGW is a massively parallel computational package for electron excited-state properties that is based on the many-body perturbation theory employing the ab initio GW and GW plus Bethe-Salpeter equation methodology. It can be used in conjunction with many density-functional theory codes for ground-state properties, including PARATEC, PARSEC, Quantum ESPRESSO, SIESTA, and Octopus. The package can be used to compute the electronic and optical properties of a wide variety of material systems from bulk semiconductors and metals to nanostructured materials and molecules. The package scales to 10 000s of CPUs and can be used to study systems containing up to 100s of atoms. Program summaryProgram title: BerkeleyGW Catalogue identifier: AELG_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AELG_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Open source BSD License. See code for licensing details. No. of lines in distributed program, including test data, etc.: 576 540 No. of bytes in distributed program, including test data, etc.: 110 608 809 Distribution format: tar.gz Programming language: Fortran 90, C, C++, Python, Perl, BASH Computer: Linux/UNIX workstations or clusters Operating system: Tested on a variety of Linux distributions in parallel and serial as well as AIX and Mac OSX RAM: (50-2000) MB per CPU (Highly dependent on system size) Classification: 7.2, 7.3, 16.2, 18 External routines: BLAS, LAPACK, FFTW, ScaLAPACK (optional), MPI (optional). All available under open-source licenses. Nature of problem: The excited state properties of materials involve the addition or subtraction of electrons as well as the optical excitations of electron-hole pairs. The excited particles interact strongly with other electrons in a material system. This interaction affects the electronic energies, wavefunctions and lifetimes. It is well known that ground-state theories, such as standard methods based on density-functional theory, fail to correctly capture this physics. Solution method: We construct and solve the Dyson's equation for the quasiparticle energies and wavefunctions within the GW approximation for the electron self-energy. We additionally construct and solve the Bethe-Salpeter equation for the correlated electron-hole (exciton) wavefunctions and excitation energies. Restrictions: The material size is limited in practice by the computational resources available. Materials with up to 500 atoms per periodic cell can be studied on large HPCs. Additional comments: The distribution file for this program is approximately 110 Mbytes and therefore is not delivered directly when download or E-mail is requested. Instead a html file giving details of how the program can be obtained is sent. Running time: 1-1000 minutes (depending greatly on system size and processor number).

Observation of a variable sub-THz radiation driven by a low energy electron beam from a thermionic rf electron gun

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smirnov, A. V.; Agustsson, R.; Berg, W. J.

We report observations of an intense sub-THz radiation extracted from a ~3 MeV electron beam with a flat transverse profile propagating between two parallel oversized copper gratings with side openings. Low-loss radiation outcoupling is accomplished using a horn antenna and a miniature permanent magnet separating sub-THz and electron beams. A tabletop experiment utilizes a radio frequency thermionic electron gun delivering a thousand momentum-chirped microbunches per macropulse and an alpha-magnet with a movable beam scraper producing sub-mm microbunches. The radiated energy of tens of micro-Joules per radio frequency macropulse is demonstrated. The frequency of the radiation peak was generated and tunedmore » across two frequency ranges: (476–584) GHz with 7% instantaneous spectrum bandwidth, and (311–334) GHz with 38% instantaneous bandwidth. In this study, the prototype setup features a robust compact source of variable frequency, narrow bandwidth sub-THz pulses.« less

Performance of GAASP/GAAS Superlattice Photocathodes in High Energy Experiments using Polarized Electrons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brachmann, A.; Clendenin, J.E.; Maruyama, T.

2006-02-27

The GaAsP/GaAs strained superlattice photocathode structure has proven to be a significant advance for polarized electron sources operating with high peak currents per microbunch and relatively low duty factor. This is the characteristic type of operation for SLAC and is also planned for the ILC. This superlattice structure was studied at SLAC [1], and an optimum variation was chosen for the final stage of E-158, a high-energy parity violating experiment at SLAC. Following E-158, the polarized source was maintained on standby with the cathode being re-cesiated about once a week while a thermionic gun, which is installed in parallel withmore » the polarized gun, supplied the linac electron beams. However, in the summer of 2005, while the thermionic gun was disabled, the polarized electron source was again used to provide electron beams for the linac. The performance of the photocathode 24 months after its only activation is described and factors making this possible are discussed.« less

Electron Transport Properties of Ge nanowires

NASA Astrophysics Data System (ADS)

Hanrath, Tobias; Khondaker, Saiful I.; Yao, Zhen; Korgel, Brian A.

2003-03-01

Electron Transport Properties of Ge nanowires Tobias Hanrath*, Saiful I. Khondaker, Zhen Yao, Brian A. Korgel* *Dept. of Chemical Engineering, Dept. of Physics, Texas Materials Institute, and Center for Nano- and Molecular Science and Technology University of Texas at Austin, Austin, Texas 78712-1062 e-mail: korgel@mail.che.utexas.edu Germanium (Ge) nanowires with diameters ranging from 6 to 50 nm and several micrometer in length were grown via a supercritical fluid-liquid-solid synthesis. Parallel electron energy loss spectroscopy (PEELS) was employed to study the band structure and electron density in the Ge nanowires. The observed increase in plasmon peak energy and peak width with decreasing nanowire diameter is attributed to quantum confinement effects. For electrical characterization, Ge nanowires were deposited onto a patterned Si/SiO2 substrate. E-beam lithography was then used to form electrode contacts to individual nanowires. The influence of nanowire diameter, surface chemistry and crystallographic defects on electron transport properties were investigated and the comparison of Ge nanowire conductivity with respect to bulk, intrinsic Ge will be presented.

Observation of a variable sub-THz radiation driven by a low energy electron beam from a thermionic rf electron gun

DOE PAGES

Smirnov, A. V.; Agustsson, R.; Berg, W. J.; ...

2015-09-29

We report observations of an intense sub-THz radiation extracted from a ~3 MeV electron beam with a flat transverse profile propagating between two parallel oversized copper gratings with side openings. Low-loss radiation outcoupling is accomplished using a horn antenna and a miniature permanent magnet separating sub-THz and electron beams. A tabletop experiment utilizes a radio frequency thermionic electron gun delivering a thousand momentum-chirped microbunches per macropulse and an alpha-magnet with a movable beam scraper producing sub-mm microbunches. The radiated energy of tens of micro-Joules per radio frequency macropulse is demonstrated. The frequency of the radiation peak was generated and tunedmore » across two frequency ranges: (476–584) GHz with 7% instantaneous spectrum bandwidth, and (311–334) GHz with 38% instantaneous bandwidth. In this study, the prototype setup features a robust compact source of variable frequency, narrow bandwidth sub-THz pulses.« less

Fast imaging with inelastically scattered electrons by off-axis chromatic confocal electron microscopy.

PubMed

Zheng, Changlin; Zhu, Ye; Lazar, Sorin; Etheridge, Joanne

2014-04-25

We introduce off-axis chromatic scanning confocal electron microscopy, a technique for fast mapping of inelastically scattered electrons in a scanning transmission electron microscope without a spectrometer. The off-axis confocal mode enables the inelastically scattered electrons to be chromatically dispersed both parallel and perpendicular to the optic axis. This enables electrons with different energy losses to be separated and detected in the image plane, enabling efficient energy filtering in a confocal mode with an integrating detector. We describe the experimental configuration and demonstrate the method with nanoscale core-loss chemical mapping of silver (M4,5) in an aluminium-silver alloy and atomic scale imaging of the low intensity core-loss La (M4,5@840  eV) signal in LaB6. Scan rates up to 2 orders of magnitude faster than conventional methods were used, enabling a corresponding reduction in radiation dose and increase in the field of view. If coupled with the enhanced depth and lateral resolution of the incoherent confocal configuration, this offers an approach for nanoscale three-dimensional chemical mapping.

Experimental signatures of the inverted phase in InAs/GaSb coupled quantum wells

NASA Astrophysics Data System (ADS)

Karalic, Matija; Mueller, Susanne; Mittag, Christopher; Pakrouski, Kiryl; Wu, QuanSheng; Soluyanov, Alexey A.; Troyer, Matthias; Tschirky, Thomas; Wegscheider, Werner; Ensslin, Klaus; Ihn, Thomas

2016-12-01

Transport measurements are performed on InAs/GaSb double quantum wells at zero and finite magnetic fields applied parallel and perpendicular to the quantum wells. We investigate a sample in the inverted regime where electrons and holes coexist, and compare it with another sample in the noninverted semiconducting regime. The activated behavior in conjunction with a strong suppression of the resistance peak at the charge neutrality point in a parallel magnetic field attest to the topological hybridization gap between electron and hole bands in the inverted sample. We observe an unconventional Landau level spectrum with energy gaps modulated by the magnetic field applied perpendicular to the quantum wells. This is caused by a strong spin-orbit interaction provided jointly by the InAs and the GaSb quantum wells.

Characterization of beryllium-boron-bearing materials by Parallel Electron Energy-Loss Spectroscopy (PEELS)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Garvie, L.A.J.; Buseck, P.R.; Rez, P.

The stoichiometry of a set of commercial and laboratory-synthesized Be-B-bearing materials was determined by parallel electron energy-loss spectroscopy (PEELS). The Be and B K edges are well separated in energy, allowing easy determination of the elemental ratios. PEELS analyses of materials with reported compositions of {open_quotes}Be{sub 2}B{close_quotes} and {open_quotes}Be{sub 2}B{sub 3}{close_quotes} provided stoichiometries near Be{sub 2.8}B and Be{sub 4}B{sub 5}, respectively. We confirmed an earlier report of Be{sub 0.6({+-}0.1)} B{sub 5.9({+-}0.7)}C, a material with the {alpha}-rhombohedral B structure. We also synthesized and characterized Be{sub 0.5({+-}0.1)}B{sub 4.9({+-}0.3)}N, a new ternary material with the {alpha}-rhombohedral B structure with refined hexagonal cell parametersmore » of a{sub h} = 5.487(5) {alpha} and c{sub h} = 12.486(14) {angstrom}. By comparison of the features at the core-loss edges of Be{sub 0.5({+-}0.1)} B{sub 4.9({+-}0.3)}N, we conclude the most N forms N{sub 2} pairs between the icosahedra and Be substitutes for B within the icosahedra.« less

Van Allen Probes observations of intense parallel Poynting flux associated with magnetic dipolarization, conjugate discrete auroral arcs, and energetic particle injection

NASA Astrophysics Data System (ADS)

Wygant, J. R.; Thaller, S. A.; Breneman, A. W.; Tian, S.; Cattell, C. A.; Chaston, C. C.; Mozer, F.; Bonnell, J. W.; Kistler, L. M.; Mouikis, C.; Hudson, M. K.; Claudepierre, S. G.; Fennell, J. F.; Reeves, G. D.; Baker, D. N.; Donovan, E.; Spanswick, E.; Kletzing, C.

2015-12-01

We present measurements from the Van Allen Probes, in the near Earth tail, at the outer boundary of the plasma sheet, of a magnetic dipolarization/injection event characterized by unusually strong earthward poynting flux flowing along magnetic field lines with amplitudes of 200 mW/m2 lasting ~ 1 minute. The Poynting flux was conjugate to a 30 km wide discrete auroral arc observed by the THEMIS auroral array. The observations were obtained at 5.8 Re in the pre-midnight sector during the main phase of a geomagnetic storm on 5/01/2013. This brief interval transferred more electromagnetic energy (at the spacecraft position) than that transferred during entire remainder of the main phase of the storm. The parallel Poynting flux coincided with a local section of the "cross tail current sheet" which generated the dipolarization signature. The latitudinal width of the arc, mapped along magnetic field lines, provides an estimate of the spatial scale of the Poynting flux, the electric fields, and the current sheets (parallel and perpendicular). It is estimated that the latitudinal width of the Poynting flux "sheet" was ~600 km or ~1-2 H+ inertial lengths. An estimate of the ∫E·dl across the current sheet along the direction normal to the plasma sheet is ~20-40 kilovolts. The "normal" to the plasma sheet component of the electric field (~70 mV/m) strongly dominated the azimuthal component(which is reponsible for drift energetization). The dipolarization event resulted in the local dispersion-less injection of electrons between 50 keV and ~2 MeV at the Van Allen Probe position. The injection event involved brief (factor of two) local spike in ~2 MeV electron fluxes. Measurements from the Los Alamos geosynchronous spacecraft, displaced eastward from the Van Allen probes, provided evidence for dispersive energy-time electron signatures consistent with injection and energization at the RBSP position. The Poynting flux also coincided with the energy peak in the up-flowing dispersive ion energy-time profile and the onset of earthward ExB convection. A similar injection event during the storm on 6/1/2013 will be discussed.

Signal enhancement due to high-Z nanofilm electrodes in parallel plate ionization chambers with variable microgaps.

PubMed

Brivio, Davide; Sajo, Erno; Zygmanski, Piotr

2017-12-01

We developed a method for measuring signal enhancement produced by high-Z nanofilm electrodes in parallel plate ionization chambers with variable thickness microgaps. We used a laboratory-made variable gap parallel plate ionization chamber with nanofilm electrodes made of aluminum-aluminum (Al-Al) and aluminum-tantalum (Al-Ta). The electrodes were evaporated on 1 mm thick glass substrates. The interelectrode air gap was varied from 3 μm to 1 cm. The gap size was measured using a digital micrometer and it was confirmed by capacitance measurements. The electric field in the chamber was kept between 0.1 kV/cm and 1 kV/cm for all the gap sizes by applying appropriate compensating voltages. The chamber was exposed to 120 kVp X-rays. The current was measured using a commercial data acquisition system with temporal resolution of 600 Hz. In addition, radiation transport simulations were carried out to characterize the dose, D(x), high-energy electron current, J(x), and deposited charge, Q(x), as a function of distance, x, from the electrodes. A deterministic method was selected over Monte Carlo due to its ability to produce results with 10 nm spatial resolution without stochastic uncertainties. Experimental signal enhancement ratio, SER(G) which we defined as the ratio of signal for Al-air-Ta to signal for Al-air-Al for each gap size, was compared to computations. The individual contributions of dose, electron current, and charge deposition to the signal enhancement were determined. Experimental signals matched computed data for all gap sizes after accounting for several contributions to the signal: (a) charge carrier generated via ionization due to the energy deposited in the air gap, D(x); (b) high-energy electron current, J(x), leaking from high-Z electrode (Ta) toward low-Z electrode (Al); (c) deposited charge in the air gap, Q(x); and (d) the decreased collection efficiency for large gaps (>~500 μm). Q(x) accounts for the electrons below 100 eV, which are regarded as stopped by the radiation transport code but which can move and form electron current in small gaps (<100 μm). While the total energy deposited in the air gap increases with gap size for both samples, the average high-energy current and deposited charge are moderately decreasing with the air gap. When gap sizes are smaller than ~20 μm, the contribution to signal from dose approaches zero while contributions from high-energy current and deposited charges give rise to an offset signal. The measured signal enhancement ratio (SER) was 40.0 ± 5.0 for the 3 μm gap and rapidly decreasing with gap size down to 9.9 ± 1.2 for the 21 μm gap and to 6.6 ± 0.3 for the 100 μm gap. The uncertainties in SER were mostly due to uncertainties in gap size and data acquisition system. We developed an experimental method to determine the signal enhancement due to high-Z nanolayers in parallel plate ionization chambers with micrometer spatial resolution. As the water-equivalent thicknesses of these air gaps are 3 nm to 10 μm, the method may also be applicable for nanoscopic spatial resolution of other gap materials. The method may be extended to solid insulator materials with low Z. © 2017 American Association of Physicists in Medicine.

Low-dose electron energy-loss spectroscopy using electron counting direct detectors.

PubMed

Maigné, Alan; Wolf, Matthias

2018-03-01

Since the development of parallel electron energy loss spectroscopy (EELS), charge-coupled devices (CCDs) have been the default detectors for EELS. With the recent development of electron-counting direct-detection cameras, micrographs can be acquired under very low electron doses at significantly improved signal-to-noise ratio. In spectroscopy, in particular in combination with a monochromator, the signal can be extremely weak and the detection limit is principally defined by noise introduced by the detector. Here we report the use of an electron-counting direct-detection camera for EEL spectroscopy. We studied the oxygen K edge of amorphous ice and obtained a signal noise ratio up to 10 times higher than with a conventional CCD.We report the application of electron counting to record time-resolved EEL spectra of a biological protein embedded in amorphous ice, revealing chemical changes observed in situ while exposed by the electron beam. A change in the fine structure of nitrogen K and the carbon K edges were recorded during irradiation. A concentration of 3 at% nitrogen was detected with a total electron dose of only 1.7 e-/Å2, extending the boundaries of EELS signal detection at low electron doses.

Magnetospheric Multiscale Observations of the Electron Diffusion Region of Large Guide Field Magnetic Reconnection

NASA Technical Reports Server (NTRS)

Eriksson, S.; Wilder, F. D.; Ergun, R. E.; Schwartz, S. J.; Cassak, P. A.; Burch, J. L.; Chen, Li-Jen; Torbert, R. B.; Phan, T. D.; Lavraud, B.;

Suppressing Thermal Energy Drift in the LLNL Flash X-Ray Accelerator Using Linear Disk Resistor Stacks

DTIC Science & Technology

2011-06-01

induction accelerator with a voltage output of 18MeV at a current of 3kA. The electron beam is focused onto a tantalum target to produce X-rays. The... capacitors in each bank, half of which are charged in parallel positively, and the other half are negatively charged in parallel. The charge voltage can...be varied from ±30kV to ±40kV. The Marx capacitors are fired in series into the Blumleins with up to 400kV 2µS output. Figure 1 FXR Pulsed Power

Electron doping evolution of the magnetic excitations in NaFe 1-xCo xAs

DOE PAGES

Carr, Scott V.; Zhang, Chenglin; Song, Yu; ...

2016-06-13

We use time-of-flight (TOF) inelastic neutron scattering (INS) spectroscopy to investigate the doping dependence of magnetic excitations across the phase diagram of NaFe 1-xCo xAs with x = 0, 0.0175, 0.0215, 0.05, and 0.11. The effect of electron-doping by partially substituting Fe by Co is to form resonances that couple with superconductivity, broaden and suppress low energy (E 80 meV) spin excitations compared with spin waves in undoped NaFeAs. However, high energy (E > 80 meV) spin excitations are weakly Co-doping dependent. Integration of the local spin dynamic susceptibility "(!) of NaFe 1-xCo xAs reveals a total fluctuating moment ofmore » 3.6 μ2 B/Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in the Cooverdoped nonsuperconducting NaFe0.89Co0.11As suggests that Fermi surface nesting is responsible for low-energy spin excitations. These results parallel Ni-doping evolution of spin excitations in BaFe 2-xNi xAs 2, confirming the notion that low-energy spin excitations coupling with itinerant electrons are important for superconductivity, while weakly doping dependent high-energy spin excitations result from localized moments.« less

Nonlinear electronic excitations in crystalline solids using meta-generalized gradient approximation and hybrid functional in time-dependent density functional theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sato, Shunsuke A.; Taniguchi, Yasutaka; Department of Medical and General Sciences, Nihon Institute of Medical Science, 1276 Shimogawara, Moroyama-Machi, Iruma-Gun, Saitama 350-0435

2015-12-14

We develop methods to calculate electron dynamics in crystalline solids in real-time time-dependent density functional theory employing exchange-correlation potentials which reproduce band gap energies of dielectrics; a meta-generalized gradient approximation was proposed by Tran and Blaha [Phys. Rev. Lett. 102, 226401 (2009)] (TBm-BJ) and a hybrid functional was proposed by Heyd, Scuseria, and Ernzerhof [J. Chem. Phys. 118, 8207 (2003)] (HSE). In time evolution calculations employing the TB-mBJ potential, we have found it necessary to adopt the predictor-corrector step for a stable time evolution. We have developed a method to evaluate electronic excitation energy without referring to the energy functionalmore » which is unknown for the TB-mBJ potential. For the HSE functional, we have developed a method for the operation of the Fock-like term in Fourier space to facilitate efficient use of massive parallel computers equipped with graphic processing units. We compare electronic excitations in silicon and germanium induced by femtosecond laser pulses using the TB-mBJ, HSE, and a simple local density approximation (LDA). At low laser intensities, electronic excitations are found to be sensitive to the band gap energy: they are close to each other using TB-mBJ and HSE and are much smaller in LDA. At high laser intensities close to the damage threshold, electronic excitation energies do not differ much among the three cases.« less

Tunneling magnetoresistance tuned by a vertical electric field in an AA-stacked graphene bilayer with double magnetic barriers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Dali, E-mail: wangdali@mail.ahnu.edu.cn; National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093; Jin, Guojun, E-mail: gjin@nju.edu.cn

2013-12-21

We investigate the effect of a vertical electric field on the electron tunneling and magnetoresistance in an AA-stacked graphene bilayer modulated by the double magnetic barriers with parallel or antiparallel configuration. The results show that the electronic transmission properties in the system are sensitive to the magnetic-barrier configuration and the bias voltage between the graphene layers. In particular, it is found that for the antiparallel configuration, within the low energy region, the blocking effect is more obvious compared with the case for the parallel configuration, and even there may exist a transmission spectrum gap which can be arbitrarily tuned bymore » the field-induced interlayer bias voltage. We also demonstrate that the significant discrepancy between the conductance for both parallel and antiparallel configurations would result in a giant tunneling magnetoresistance ratio, and further the maximal magnetoresistance ratio can be strongly modified by the interlayer bias voltage. This leads to the possible realization of high-quality magnetic sensors controlled by a vertical electric field in the AA-stacked graphene bilayer.« less

Effect of Electron Seeding on Experimentally Measured Multipactor Discharge Threshold

NASA Astrophysics Data System (ADS)

Noland, Jonathan; Graves, Timothy; Lemon, Colby; Looper, Mark; Farkas, Alex

2012-10-01

Multipactor is a vacuum phenomenon in which electrons, moving in resonance with an externally applied electric field, impact material surfaces. If the number of secondary electrons created per primary electron impact averages more than unity, the resonant interaction can lead to an electron avalanche. Multipactor is a generally undesirable phenomenon, as it can cause local heating, absorb power, or cause detuning of RF circuits. In order to increase the probability of multipactor initiation, test facilities often employ various seeding sources such as radioactive sources (Cesium 137, Strontium 90), electron guns, or photon sources. Even with these sources, the voltage for multipactor initiation is not certain as parameters such as material type, RF pulse length, and device wall thickness can all affect seed electron flux and energy in critical gap regions, and hence the measured voltage threshold. This study investigates the effects of seed electron source type (e.g., photons versus beta particles), material type, gap size, and RF pulse length variation on multipactor threshold. In addition to the experimental work, GEANT4 simulations will be used to estimate the production rate of low energy electrons (< 5 keV) by high energy electrons and photons. A comparison of the experimental fluxes to the typical energetic photon and particle fluxes experienced by spacecraft in various orbits will also be made. Initial results indicate that for a simple, parallel plate device made of aluminum, there is no threshold variation (with seed electrons versus with no seed electrons) under continuous-wave RF exposure.

Electron Jet of Asymmetric Reconnection

NASA Technical Reports Server (NTRS)

Khotyaintsev, Yu. V.; Graham, D. B.; Norgren, C.; Eriksson, E.; Li, W.; Johlander, A.; Vaivads, A.; Andre, M.; Pritchett, P. L.; Retino, A.;

Kinetic Alfven turbulence: Electron and ion heating by particle-in-cell simulations

NASA Astrophysics Data System (ADS)

Gary, S. P.; Hughes, R. S.; Wang, J.; Parashar, T. N.

2017-12-01

Three-dimensional particle-in-cell simulations of the forward cascade of decaying kinetic Alfvén turbulence have been carried out as an initial-value problem on a collisionless, homogeneous, magnetized, electron-ion plasma model with betae = betai =0.50 and mi/me=100 where subscripts e and i represent electrons and ions respectively. Initial anisotropic narrowband spectra of relatively long wavelength modes with approximately gyrotropic distributions in kperp undergo a forward cascade to broadband spectra of magnetic fluctuations at shorter wavelengths. Maximum electron and ion heating rates are computed as functions of the initial fluctuating magnetic field energy density eo on the range 0.05 < eo < 0.50. In contrast to dissipation by whistler turbulence, the maximum ion heating rate due to kinetic Alfvén turbulence is substantially greater than the maximum electron heating rate. Furthermore, ion heating as well as electron heating due to kinetic Alfvén turbulence scale approximately with eo. Finally, electron heating leads to anisotropies of the type T||e> Tperpe where the parallel and perpendicular symbols refer to directions parallel and perpendicular, respectively, to the background magnetic field, whereas the heated ions remain relatively isotropic. This implies that, for the range of eo values considered, the Landau wave-particle resonance is a likely heating mechanism for the electrons and may also contribute to ion heating.

Analysis of Plane-Parallel Electron Beam Propagation in Different Media by Numerical Simulation Methods

NASA Astrophysics Data System (ADS)

Miloichikova, I. A.; Bespalov, V. I.; Krasnykh, A. A.; Stuchebrov, S. G.; Cherepennikov, Yu. M.; Dusaev, R. R.

2018-04-01

Simulation by the Monte Carlo method is widely used to calculate the character of ionizing radiation interaction with substance. A wide variety of programs based on the given method allows users to choose the most suitable package for solving computational problems. In turn, it is important to know exactly restrictions of numerical systems to avoid gross errors. Results of estimation of the feasibility of application of the program PCLab (Computer Laboratory, version 9.9) for numerical simulation of the electron energy distribution absorbed in beryllium, aluminum, gold, and water for industrial, research, and clinical beams are presented. The data obtained using programs ITS and Geant4 being the most popular software packages for solving the given problems and the program PCLab are presented in the graphic form. A comparison and an analysis of the results obtained demonstrate the feasibility of application of the program PCLab for simulation of the absorbed energy distribution and dose of electrons in various materials for energies in the range 1-20 MeV.

Unique spin-polarized transmission effects in a QD ring structure

NASA Astrophysics Data System (ADS)

Hedin, Eric; Joe, Yong

2010-10-01

Spintronics is an emerging field in which the spin of the electron is used for switching purposes and to communicate information. In order to obtain spin-polarized electron transmission, the Zeeman effect is employed to produce spin-split energy states in quantum dots which are embedded in the arms of a mesoscopic Aharonov-Bohm (AB) ring heterostructure. The Zeeman splitting of the QD energy levels can be induced by a parallel magnetic field, or by a perpendicular field which also produces AB-effects. The combination of these effects on the transmission resonances of the structure is studied analytically and several parameter regimes are identified which produce a high degree of spin-polarized output. Contour and line plots of the weighted spin polarization as a function of electron energy and magnetic field are presented to visualize the degree of spin-polarization. Taking advantage of these unique parameter regimes shows the potential promise of such devices for producing spin-polarized currents.

Fast-particle energy loss to a quasi-one dimensional electron gas

NASA Astrophysics Data System (ADS)

Kushwaha, Manvir S.; Zielinski, P.

2000-03-01

A theoretical investigation has been made of the fast-particle energy-loss to a quasi-one-dimensional electron gas (Q1DEG) within the framework of the random-phase-approximation (RPA). For this purpose, we use an exact analytical expression for the inverse dielectric function, which knows no bound as regards the subband occupancy, and the parabolic potential well to characterize the lateral confinement. Three geometries are considered: the fast-particle moving parallel to, being specularly reflected from, and shooting through the Q1DEG. The illustrative numerical examples in all the three geometries lead us to infer that the dominant contribution to the loss peaks comes from the intra- and inter-subband collective excitations.^1 We argue that the high resolution electron energy loss spectroscopy (HREELS) could prove to be a potential alternative of the existing optical (Raman or FIR) spectroscopies.^2 ^1 M.S. Kushwaha and P. Zielinski, Solid State Commun. 112, 605(1999). ^2 M.S. Kushwaha and P. Zielinski, Unpublished.

Cascade model of gamma-ray bursts: Power-law and annihilation-line components

NASA Technical Reports Server (NTRS)

Harding, A. K.; Sturrock, P. A.; Daugherty, J. K.

1988-01-01

If, in a neutron star magnetosphere, an electron is accelerated to an energy of 10 to the 11th or 12th power eV by an electric field parallel to the magnetic field, motion of the electron along the curved field line leads to a cascade of gamma rays and electron-positron pairs. This process is believed to occur in radio pulsars and gamma ray burst sources. Results are presented from numerical simulations of the radiation and photon annihilation pair production processes, using a computer code previously developed for the study of radio pulsars. A range of values of initial energy of a primary electron was considered along with initial injection position, and magnetic dipole moment of the neutron star. The resulting spectra was found to exhibit complex forms that are typically power law over a substantial range of photon energy, and typically include a dip in the spectrum near the electron gyro-frequency at the injection point. The results of a number of models are compared with data for the 5 Mar., 1979 gamma ray burst. A good fit was found to the gamma ray part of the spectrum, including the equivalent width of the annihilation line.

Positron lifetime spectrometer using a DC positron beam

DOEpatents

Xu, Jun; Moxom, Jeremy

2003-10-21

An entrance grid is positioned in the incident beam path of a DC beam positron lifetime spectrometer. The electrical potential difference between the sample and the entrance grid provides simultaneous acceleration of both the primary positrons and the secondary electrons. The result is a reduction in the time spread induced by the energy distribution of the secondary electrons. In addition, the sample, sample holder, entrance grid, and entrance face of the multichannel plate electron detector assembly are made parallel to each other, and are arranged at a tilt angle to the axis of the positron beam to effectively separate the path of the secondary electrons from the path of the incident positrons.

Probing energy transfer events in the light harvesting complex 2 (LH2) of Rhodobacter sphaeroides with two-dimensional spectroscopy.

PubMed

Fidler, Andrew F; Singh, Ved P; Long, Phillip D; Dahlberg, Peter D; Engel, Gregory S

2013-10-21

Excitation energy transfer events in the photosynthetic light harvesting complex 2 (LH2) of Rhodobacter sphaeroides are investigated with polarization controlled two-dimensional electronic spectroscopy. A spectrally broadened pulse allows simultaneous measurement of the energy transfer within and between the two absorption bands at 800 nm and 850 nm. The phased all-parallel polarization two-dimensional spectra resolve the initial events of energy transfer by separating the intra-band and inter-band relaxation processes across the two-dimensional map. The internal dynamics of the 800 nm region of the spectra are resolved as a cross peak that grows in on an ultrafast time scale, reflecting energy transfer between higher lying excitations of the B850 chromophores into the B800 states. We utilize a polarization sequence designed to highlight the initial excited state dynamics which uncovers an ultrafast transfer component between the two bands that was not observed in the all-parallel polarization data. We attribute the ultrafast transfer component to energy transfer from higher energy exciton states to lower energy states of the strongly coupled B850 chromophores. Connecting the spectroscopic signature to the molecular structure, we reveal multiple relaxation pathways including a cyclic transfer of energy between the two rings of the complex.

SUPRATHERMAL ELECTRONS AT SATURN'S BOW SHOCK

DOE Office of Scientific and Technical Information (OSTI.GOV)

Masters, A.; Dougherty, M. K.; Sulaiman, A. H.

2016-07-20

The leading explanation for the origin of galactic cosmic rays is particle acceleration at the shocks surrounding young supernova remnants (SNRs), although crucial aspects of the acceleration process are unclear. The similar collisionless plasma shocks frequently encountered by spacecraft in the solar wind are generally far weaker (lower Mach number) than these SNR shocks. However, the Cassini spacecraft has shown that the shock standing in the solar wind sunward of Saturn (Saturn's bow shock) can occasionally reach this high-Mach number astrophysical regime. In this regime Cassini has provided the first in situ evidence for electron acceleration under quasi-parallel upstream magneticmore » conditions. Here we present the full picture of suprathermal electrons at Saturn's bow shock revealed by Cassini . The downstream thermal electron distribution is resolved in all data taken by the low-energy electron detector (CAPS-ELS, <28 keV) during shock crossings, but the higher energy channels were at (or close to) background. The high-energy electron detector (MIMI-LEMMS, >18 keV) measured a suprathermal electron signature at 31 of 508 crossings, where typically only the lowest energy channels (<100 keV) were above background. We show that these results are consistent with the theory in which the “injection” of thermal electrons into an acceleration process involves interaction with whistler waves at the shock front, and becomes possible for all upstream magnetic field orientations at high Mach numbers like those of the strong shocks around young SNRs. A future dedicated study will analyze the rare crossings with evidence for relativistic electrons (up to ∼1 MeV).« less

Suprathermal Electrons at Saturn's Bow Shock

NASA Astrophysics Data System (ADS)

Masters, A.; Sulaiman, A. H.; Sergis, N.; Stawarz, L.; Fujimoto, M.; Coates, A. J.; Dougherty, M. K.

2016-07-01

The leading explanation for the origin of galactic cosmic rays is particle acceleration at the shocks surrounding young supernova remnants (SNRs), although crucial aspects of the acceleration process are unclear. The similar collisionless plasma shocks frequently encountered by spacecraft in the solar wind are generally far weaker (lower Mach number) than these SNR shocks. However, the Cassini spacecraft has shown that the shock standing in the solar wind sunward of Saturn (Saturn's bow shock) can occasionally reach this high-Mach number astrophysical regime. In this regime Cassini has provided the first in situ evidence for electron acceleration under quasi-parallel upstream magnetic conditions. Here we present the full picture of suprathermal electrons at Saturn's bow shock revealed by Cassini. The downstream thermal electron distribution is resolved in all data taken by the low-energy electron detector (CAPS-ELS, <28 keV) during shock crossings, but the higher energy channels were at (or close to) background. The high-energy electron detector (MIMI-LEMMS, >18 keV) measured a suprathermal electron signature at 31 of 508 crossings, where typically only the lowest energy channels (<100 keV) were above background. We show that these results are consistent with the theory in which the “injection” of thermal electrons into an acceleration process involves interaction with whistler waves at the shock front, and becomes possible for all upstream magnetic field orientations at high Mach numbers like those of the strong shocks around young SNRs. A future dedicated study will analyze the rare crossings with evidence for relativistic electrons (up to ˜1 MeV).

Electron Heating and Acceleration in a Reconnecting Magnetotail

NASA Astrophysics Data System (ADS)

El-Alaoui, M.; Zhou, M.; Lapenta, G.; Berchem, J.; Richard, R. L.; Schriver, D.; Walker, R. J.

2017-12-01

Electron heating and acceleration in the magnetotail have been investigated intensively. A major site for this process is the reconnection region. However, where and how the electrons are accelerated in a realistic three-dimensional X-line geometry is not fully understood. In this study, we employed a three-dimensional implicit particle-in-cell (iPIC3D) simulation and large-scale kinetic (LSK) simulation to address these problems. We modeled a magnetotail reconnection event observed by THEMIS in an iPIC3D simulation with initial and boundary conditions given by a global magnetohydrodynamic (MHD) simulation of Earth's magnetosphere. The iPIC3D simulation system includes the region of fast outflow emanating from the reconnection site that drives dipolarization fronts. We found that current sheet electrons exhibit elongated (cigar-shaped) velocity distributions with a higher parallel temperature. Using LSK we then followed millions of test electrons using the electromagnetic fields from iPIC3D. We found that magnetotail reconnection can generate power law spectra around the near-Earth X-line. A significant number of electrons with energies higher than 50 keV are produced. We identified several acceleration mechanisms at different locations that were responsible for energizing these electrons: non-adiabatic cross-tail drift, betatron and Fermi acceleration. Relative contributions to the energy gain of these high energy electrons from the different mechanisms will be discussed.

Accurate ab initio binding energies of the benzene dimer.

PubMed

Park, Young Choon; Lee, Jae Shin

2006-04-20

Accurate binding energies of the benzene dimer at the T and parallel displaced (PD) configurations were determined using the single- and double-coupled cluster method with perturbative triple correction (CCSD(T)) with correlation-consistent basis sets and an effective basis set extrapolation scheme recently devised. The difference between the estimated CCSD(T) basis set limit electronic binding energies for the T and PD shapes appears to amount to more than 0.3 kcal/mol, indicating the PD shape is a more stable configuration than the T shape for this dimer in the gas phase. This conclusion is further strengthened when a vibrational zero-point correction to the electronic binding energies of this dimer is made, which increases the difference between the two configurations to 0.4-0.5 kcal/mol. The binding energies of 2.4 and 2.8 kcal/mol for the T and PD configurations are in good accord with the previous experimental result from ionization potential measurement.

Nonlinear ELM simulations based on a nonideal peeling–ballooning model using the BOUT++ code

DOE PAGES

Xu, X. Q.; Dudson, B. D.; Snyder, P. B.; ...

2011-09-23

A minimum set of equations based on the peeling–ballooning (P–B) model with nonideal physics effects (diamagnetic drift, E × B drift, resistivity and anomalous electron viscosity) is found to simulate pedestal collapse when using the BOUT++ simulation code, developed in part from the original fluid edge code BOUT. Linear simulations of P–B modes find good agreement in growth rate and mode structure with ELITE calculations. The influence of the E × B drift, diamagnetic drift, resistivity, anomalous electron viscosity, ion viscosity and parallel thermal diffusivity on P–B modes is being studied; we find that (1) the diamagnetic drift and Emore » × B drift stabilize the P–B mode in a manner consistent with theoretical expectations; (2) resistivity destabilizes the P–B mode, leading to resistive P–B mode; (3) anomalous electron and parallel ion viscosities destabilize the P–B mode, leading to a viscous P–B mode; (4) perpendicular ion viscosity and parallel thermal diffusivity stabilize the P–B mode. With addition of the anomalous electron viscosity under the assumption that the anomalous kinematic electron viscosity is comparable to the anomalous electron perpendicular thermal diffusivity, or the Prandtl number is close to unity, it is found from nonlinear simulations using a realistic high Lundquist number that the pedestal collapse is limited to the edge region and the ELM size is about 5–10% of the pedestal stored energy. Furthermore, this is consistent with many observations of large ELMs. The estimated island size is consistent with the size of fast pedestal pressure collapse. In the stable α-zones of ideal P–B modes, nonlinear simulations of viscous ballooning modes or current-diffusive ballooning mode (CDBM) for ITER H-mode scenarios are presented.« less

Coherence of a spin-polarized electron beam emitted from a semiconductor photocathode in a transmission electron microscope

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kuwahara, Makoto, E-mail: kuwahara@esi.nagoya-u.ac.jp; Saitoh, Koh; Tanaka, Nobuo

2014-11-10

The brightness and interference fringes of a spin-polarized electron beam extracted from a semiconductor photocathode excited by laser irradiation are directly measured via its use in a transmission electron microscope. The brightness was 3.8 × 10{sup 7 }A cm{sup −2 }sr{sup −1} for a 30-keV beam energy with the polarization of 82%, which corresponds to 3.1 × 10{sup 8 }A cm{sup −2 }sr{sup −1} for a 200-keV beam energy. The resulting electron beam exhibited a long coherence length at the specimen position due to the high parallelism of (1.7 ± 0.3) × 10{sup −5 }rad, which generated interference fringes representative of a first-order correlation using an electron biprism. The beam also had amore » high degeneracy of electron wavepacket of 4 × 10{sup −6}. Due to the high polarization, the high degeneracy and the long coherence length, the spin-polarized electron beam can enhance the antibunching effect.« less

Traveling wave tube and method of manufacture

NASA Technical Reports Server (NTRS)

Vancil, Bernard K. (Inventor)

2004-01-01

A traveling wave tube includes a glass or other insulating envelope having a plurality of substantially parallel glass rods supported therewithin which in turn support an electron gun, a collector and an intermediate slow wave structure. The slow wave structure itself provides electrostatic focussing of a central electron beam thereby eliminating the need for focussing magnetics and materially decreasing the cost of construction as well as enabling miniaturization. The slow wave structure advantageously includes cavities along the electron beam through which the r.f. energy is propagated, or a double, interleaved ring loop structure supported by dielectric fins within a ground plane cylinder disposed coaxially within the glass envelope.

Quasi-parallel whistler mode waves observed by THEMIS during near-earth dipolarizations

NASA Astrophysics Data System (ADS)

Le Contel, O.; Roux, A.; Jacquey, C.; Robert, P.; Berthomier, M.; Chust, T.; Grison, B.; Angelopoulos, V.; Sibeck, D.; Chaston, C. C.; Cully, C. M.; Ergun, B.; Glassmeier, K.-H.; Auster, U.; McFadden, J.; Carlson, C.; Larson, D.; Bonnell, J. W.; Mende, S.; Russell, C. T.; Donovan, E.; Mann, I.; Singer, H.

2009-06-01

We report on quasi-parallel whistler emissions detected by the near-earth satellites of the THEMIS mission before, during, and after local dipolarization. These emissions are associated with an electron temperature anisotropy α=T⊥e/T||e>1 consistent with the linear theory of whistler mode anisotropy instability. When the whistler mode emissions are observed the measured electron anisotropy varies inversely with β||e (the ratio of the electron parallel pressure to the magnetic pressure) as predicted by Gary and Wang (1996). Narrow band whistler emissions correspond to the small α existing before dipolarization whereas the broad band emissions correspond to large α observed during and after dipolarization. The energy in the whistler mode is leaving the current sheet and is propagating along the background magnetic field, towards the Earth. A simple time-independent description based on the Liouville's theorem indicates that the electron temperature anisotropy decreases with the distance along the magnetic field from the equator. Once this variation of α is taken into account, the linear theory predicts an equatorial origin for the whistler mode. The linear theory is also consistent with the observed bandwidth of wave emissions. Yet, the anisotropy required to be fully consistent with the observations is somewhat larger than the measured one. Although the discrepancy remains within the instrumental error bars, this could be due to time-dependent effects which have been neglected. The possible role of the whistler waves in the substorm process is discussed.

Quantitative analysis of interfacial chemistry in TiC/Ti composite using electron-energy-loss spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gu, M.; Jiang, W.; Zhang, G.

Due to titanium carbide`s physical and elastic properties, titanium carbide particles are widely used as a reinforcement in titanium-alloy-based composites. Previous studies have shown that no obvious reaction products were detected on the interface region in TiC/Ti alloy systems; instead, a nonstoichiometric region in the TiC particle between the Ti{sub 6}Al{sub 4}V alloy and the stoichiometric TiC was found. However, the nature and the extent of the nonstoichiometric zone have not been quantitatively described. The present communication reports some results of a parallel electron-energy-loss spectroscopy (PEELS) study on a 10 vol pct TiC-particle-reinforced IMI-829 metal-matrix composite.

Interference effects on guided Cherenkov emission in silicon from perpendicular, oblique, and parallel boundaries

NASA Astrophysics Data System (ADS)

Couillard, M.; Yurtsever, A.; Muller, D. A.

2010-05-01

Waveguide electromagnetic modes excited by swift electrons traversing Si slabs at normal and oblique incidence are analyzed using monochromated electron energy-loss spectroscopy and interpreted using a local dielectric theory that includes relativistic effects. At normal incidence, sharp spectral features in the visible/near-infrared optical domain are directly assigned to p -polarized modes. When the specimen is tilted, s -polarized modes, which are completely absent at normal incidence, become visible in the loss spectra. In the tilted configuration, the dispersion of p -polarized modes is also modified. For tilt angles higher than ˜50° , Cherenkov radiation, the phenomenon responsible for the excitation of waveguide modes, is expected to partially escape the silicon slab and the influence of this effect on experimental measurements is discussed. Finally, we find evidence for an interference effect at parallel Si/SiO2 interfaces, as well as a delocalized excitation of guided Cherenkov modes.

Plasma and energetic particle structure upstream of a quasi-parallel interplanetary shock

NASA Technical Reports Server (NTRS)

Kennel, C. F.; Scarf, F. L.; Coroniti, F. V.; Russell, C. T.; Wenzel, K.-P.; Sanderson, T. R.; Van Nes, P.; Smith, E. J.; Tsurutani, B. T.; Scudder, J. D.

1984-01-01

ISEE 1, 2 and 3 data from 1978 on interplanetary magnetic fields, shock waves and particle energetics are examined to characterize a quasi-parallel shock. The intense shock studied exhibited a 640 km/sec velocity. The data covered 1-147 keV protons and electrons and ions with energies exceeding 30 keV in regions both upstream and downstream of the shock, and also the magnitudes of ion-acoustic and MHD waves. The energetic particles and MHD waves began being detected 5 hr before the shock. Intense halo electron fluxes appeared ahead of the shock. A closed magnetic field structure was produced with a front end 700 earth radii from the shock. The energetic protons were cut off from the interior of the magnetic bubble, which contained a markedly increased density of 2-6 keV protons as well as the shock itself.

Wave-Particle Interactions Involving Correlated Electron Bursts and Whistler Chorus in Earth's Radiation Belts

NASA Astrophysics Data System (ADS)

Echterling, N.; Schriver, D.; Roeder, J. L.; Fennell, J. F.

2017-12-01

During the recovery phase of substorm plasma injections, the Van Allen Probes commonly observe events of quasi-periodic energetic electron bursts correlating with simultaneously detected upper-band, whistler-mode chorus emissions. These electron bursts exhibit narrow ranges of pitch angles (75-80° and 100-105°) and energies (20-40 keV). Electron cyclotron harmonic (ECH) emissions are also commonly detected, but typically do not display correlation with the electron bursts. To examine sources of free energy and the generation of these wave emissions, an observed electron velocity distribution on January 13, 2013 is used as the starting condition for a particle in cell (PIC) simulation. Effects of temperature anisotropy (perpendicular temperature greater than parallel temperature), the presence of a loss cone and a cold electron population on the generation of whistler and ECH waves are examined to understand wave generation and nonlinear interactions with the particle population. These nonlinear interactions produce energy diffusion along with strong pitch angle scattering into the loss cone on the order of milliseconds, which is faster than a typical bounce period of seconds. To examine the quasi-periodic nature of the electron bursts, a loss-cone recycling technique is implemented to model the effects of the periodic emptying of the loss cone and electron injection on the growth of whistler and ECH waves. The results of the simulations are compared to the Van Allen Probe observations to determine electron acceleration, heating and transport in Earth's radiation belts due to wave-particle interactions.

K-shell X-ray transition energies of multi-electron ions of silicon and sulfur

NASA Astrophysics Data System (ADS)

Beiersdorfer, P.; Brown, G. V.; Hell, N.; Santana, J. A.

2017-10-01

Prompted by the detection of K-shell absorption or emission features in the spectra of plasma surrounding high mass X-ray binaries and black holes, recent measurements using the Livermore electron beam ion trap have focused on the energies of the n = 2 to n = 1 K-shell transitions in the L-shell ions of lithiumlike through fluorinelike silicon and sulfur. In parallel, we have made calculations of these transitions using the Flexible Atomic Code and the multi-reference Møller-Plesset (MRMP) atomic physics code. Using this code we have attempted to produce sets of theoretical atomic data with spectroscopic accuracy for all the L-shell ions of silicon and sulfur. We present results of our calculations for oxygenlike and fluorinelike silicon and compare them to the recent electron beam ion trap measurements as well as previous calculations.

K-shell X-ray transition energies of multi-electron ions of silicon and sulfur

DOE PAGES

Beiersdorfer, P.; Brown, G. V.; Hell, N.; ...

2017-04-20

Prompted by the detection of K-shell absorption or emission features in the spectra of plasma surrounding high mass X-ray binaries and black holes, recent measurements using the Livermore electron beam ion trap have focused on the energies of the n = 2 to n = 1 K-shell transitions in the L-shell ions of lithiumlike through fluorinelike silicon and sulfur. In parallel, we have made calculations of these transitions using the Flexible Atomic Code and the multi-reference Møller-Plesset (MRMP) atomic physics code. Using this code we have attempted to produce sets of theoretical atomic data with spectroscopic accuracy for all themore » L-shell ions of silicon and sulfur. Here, we present results of our calculations for oxygenlike and fluorinelike silicon and compare them to the recent electron beam ion trap measurements as well as previous calculations.« less

Advances in molecular quantum chemistry contained in the Q-Chem 4 program package

NASA Astrophysics Data System (ADS)

Shao, Yihan; Gan, Zhengting; Epifanovsky, Evgeny; Gilbert, Andrew T. B.; Wormit, Michael; Kussmann, Joerg; Lange, Adrian W.; Behn, Andrew; Deng, Jia; Feng, Xintian; Ghosh, Debashree; Goldey, Matthew; Horn, Paul R.; Jacobson, Leif D.; Kaliman, Ilya; Khaliullin, Rustam Z.; Kuś, Tomasz; Landau, Arie; Liu, Jie; Proynov, Emil I.; Rhee, Young Min; Richard, Ryan M.; Rohrdanz, Mary A.; Steele, Ryan P.; Sundstrom, Eric J.; Woodcock, H. Lee, III; Zimmerman, Paul M.; Zuev, Dmitry; Albrecht, Ben; Alguire, Ethan; Austin, Brian; Beran, Gregory J. O.; Bernard, Yves A.; Berquist, Eric; Brandhorst, Kai; Bravaya, Ksenia B.; Brown, Shawn T.; Casanova, David; Chang, Chun-Min; Chen, Yunqing; Chien, Siu Hung; Closser, Kristina D.; Crittenden, Deborah L.; Diedenhofen, Michael; DiStasio, Robert A., Jr.; Do, Hainam; Dutoi, Anthony D.; Edgar, Richard G.; Fatehi, Shervin; Fusti-Molnar, Laszlo; Ghysels, An; Golubeva-Zadorozhnaya, Anna; Gomes, Joseph; Hanson-Heine, Magnus W. D.; Harbach, Philipp H. P.; Hauser, Andreas W.; Hohenstein, Edward G.; Holden, Zachary C.; Jagau, Thomas-C.; Ji, Hyunjun; Kaduk, Benjamin; Khistyaev, Kirill; Kim, Jaehoon; Kim, Jihan; King, Rollin A.; Klunzinger, Phil; Kosenkov, Dmytro; Kowalczyk, Tim; Krauter, Caroline M.; Lao, Ka Un; Laurent, Adèle D.; Lawler, Keith V.; Levchenko, Sergey V.; Lin, Ching Yeh; Liu, Fenglai; Livshits, Ester; Lochan, Rohini C.; Luenser, Arne; Manohar, Prashant; Manzer, Samuel F.; Mao, Shan-Ping; Mardirossian, Narbe; Marenich, Aleksandr V.; Maurer, Simon A.; Mayhall, Nicholas J.; Neuscamman, Eric; Oana, C. Melania; Olivares-Amaya, Roberto; O'Neill, Darragh P.; Parkhill, John A.; Perrine, Trilisa M.; Peverati, Roberto; Prociuk, Alexander; Rehn, Dirk R.; Rosta, Edina; Russ, Nicholas J.; Sharada, Shaama M.; Sharma, Sandeep; Small, David W.; Sodt, Alexander; Stein, Tamar; Stück, David; Su, Yu-Chuan; Thom, Alex J. W.; Tsuchimochi, Takashi; Vanovschi, Vitalii; Vogt, Leslie; Vydrov, Oleg; Wang, Tao; Watson, Mark A.; Wenzel, Jan; White, Alec; Williams, Christopher F.; Yang, Jun; Yeganeh, Sina; Yost, Shane R.; You, Zhi-Qiang; Zhang, Igor Ying; Zhang, Xing; Zhao, Yan; Brooks, Bernard R.; Chan, Garnet K. L.; Chipman, Daniel M.; Cramer, Christopher J.; Goddard, William A., III; Gordon, Mark S.; Hehre, Warren J.; Klamt, Andreas; Schaefer, Henry F., III; Schmidt, Michael W.; Sherrill, C. David; Truhlar, Donald G.; Warshel, Arieh; Xu, Xin; Aspuru-Guzik, Alán; Baer, Roi; Bell, Alexis T.; Besley, Nicholas A.; Chai, Jeng-Da; Dreuw, Andreas; Dunietz, Barry D.; Furlani, Thomas R.; Gwaltney, Steven R.; Hsu, Chao-Ping; Jung, Yousung; Kong, Jing; Lambrecht, Daniel S.; Liang, WanZhen; Ochsenfeld, Christian; Rassolov, Vitaly A.; Slipchenko, Lyudmila V.; Subotnik, Joseph E.; Van Voorhis, Troy; Herbert, John M.; Krylov, Anna I.; Gill, Peter M. W.; Head-Gordon, Martin

2015-01-01

A summary of the technical advances that are incorporated in the fourth major release of the Q-Chem quantum chemistry program is provided, covering approximately the last seven years. These include developments in density functional theory methods and algorithms, nuclear magnetic resonance (NMR) property evaluation, coupled cluster and perturbation theories, methods for electronically excited and open-shell species, tools for treating extended environments, algorithms for walking on potential surfaces, analysis tools, energy and electron transfer modelling, parallel computing capabilities, and graphical user interfaces. In addition, a selection of example case studies that illustrate these capabilities is given. These include extensive benchmarks of the comparative accuracy of modern density functionals for bonded and non-bonded interactions, tests of attenuated second order Møller-Plesset (MP2) methods for intermolecular interactions, a variety of parallel performance benchmarks, and tests of the accuracy of implicit solvation models. Some specific chemical examples include calculations on the strongly correlated Cr2 dimer, exploring zeolite-catalysed ethane dehydrogenation, energy decomposition analysis of a charged ter-molecular complex arising from glycerol photoionisation, and natural transition orbitals for a Frenkel exciton state in a nine-unit model of a self-assembling nanotube.

Unraveling the excitation mechanisms of highly oblique lower-band chorus waves

DOE PAGES

Li, Wen; Mourenas, D.; Artemyev, A. V.; ...

2016-08-17

Excitation mechanisms of highly oblique, quasi-electrostatic lower band chorus waves are investigated using Van Allen Probes observations near the equator of the Earth's magnetosphere. Linear growth rates are evaluated based on in situ, measured electron velocity distributions and plasma conditions and compared with simultaneously observed wave frequency spectra and wave normal angles. Accordingly, two distinct excitation mechanisms of highly oblique lower band chorus have been clearly identified for the first time. The first mechanism relies on cyclotron resonance with electrons possessing both a realistic temperature anisotropy at keV energies and a plateau at 100–500 eV in the parallel velocity distribution.more » The second mechanism corresponds to Landau resonance with a 100–500 eV beam. In both cases, a small low-energy beam-like component is necessary for suppressing an otherwise dominating Landau damping. In conclusion, our new findings suggest that small variations in the electron distribution could have important impacts on energetic electron dynamics.« less

Gyrofluid modeling and phenomenology of low-βe Alfvén wave turbulence

NASA Astrophysics Data System (ADS)

Passot, T.; Sulem, P. L.; Tassi, E.

2018-04-01

A two-field reduced gyrofluid model including electron inertia, ion finite Larmor radius corrections, and parallel magnetic field fluctuations is derived from the model of Brizard [Brizard, Phys. Fluids B 4, 1213 (1992)]. It assumes low βe, where βe indicates the ratio between the equilibrium electron pressure and the magnetic pressure exerted by a strong uniform magnetic guide field, but permits an arbitrary ion-to-electron equilibrium temperature ratio. It is shown to have a noncanonical Hamiltonian structure and provides a convenient framework for studying kinetic Alfvén wave turbulence, from magnetohydrodynamics to sub-de scales (where de holds for the electron skin depth). Magnetic energy spectra are phenomenologically determined within energy and generalized cross-helicity cascades in the perpendicular spectral plane. Arguments based on absolute statistical equilibria are used to predict the direction of the transfers, pointing out that, within the sub-ion range, the generalized cross-helicity could display an inverse cascade if injected at small scales, for example by reconnection processes.

The elusive S{sub 2} state, the S{sub 1}/S{sub 2} splitting, and the excimer states of the benzene dimer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Balmer, Franziska A.; Trachsel, Maria A.; Leutwyler, Samuel

We observe the weak S{sub 0} → S{sub 2} transitions of the T-shaped benzene dimers (Bz){sub 2} and (Bz-d{sub 6}){sub 2} about 250 cm{sup −1} and 220 cm{sup −1} above their respective S{sub 0} → S{sub 1} electronic origins using two-color resonant two-photon ionization spectroscopy. Spin-component scaled (SCS) second-order approximate coupled-cluster (CC2) calculations predict that for the tipped T-shaped geometry, the S{sub 0} → S{sub 2} electronic oscillator strength f{sub el}(S{sub 2}) is ∼10 times smaller than f{sub el}(S{sub 1}) and the S{sub 2} state lies ∼240 cm{sup −1} above S{sub 1}, in excellent agreement with experiment. The S{sub 0}more » → S{sub 1} (ππ{sup ∗}) transition is mainly localized on the “stem” benzene, with a minor stem → cap charge-transfer contribution; the S{sub 0} → S{sub 2} transition is mainly localized on the “cap” benzene. The orbitals, electronic oscillator strengths f{sub el}(S{sub 1}) and f{sub el}(S{sub 2}), and transition frequencies depend strongly on the tipping angle ω between the two Bz moieties. The SCS-CC2 calculated S{sub 1} and S{sub 2} excitation energies at different T-shaped, stacked-parallel and parallel-displaced stationary points of the (Bz){sub 2} ground-state surface allow to construct approximate S{sub 1} and S{sub 2} potential energy surfaces and reveal their relation to the “excimer” states at the stacked-parallel geometry. The f{sub el}(S{sub 1}) and f{sub el}(S{sub 2}) transition dipole moments at the C{sub 2v}-symmetric T-shape, parallel-displaced and stacked-parallel geometries are either zero or ∼10 times smaller than at the tipped T-shaped geometry. This unusual property of the S{sub 0} → S{sub 1} and S{sub 0} → S{sub 2} transition-dipole moment surfaces of (Bz){sub 2} restricts its observation by electronic spectroscopy to the tipped and tilted T-shaped geometries; the other ground-state geometries are impossible or extremely difficult to observe. The S{sub 0} → S{sub 1}/S{sub 2} spectra of (Bz){sub 2} are compared to those of imidazole ⋅ (Bz){sub 2}, which has a rigid triangular structure with a tilted (Bz){sub 2} subunit. The S{sub 0} → S{sub 1}/ S{sub 2} transitions of imidazole-(benzene){sub 2} lie at similar energies as those of (Bz){sub 2}, confirming our assignment of the (Bz){sub 2} S{sub 0} → S{sub 2} transition.« less

Lateral terahertz hot-electron bolometer based on an array of Sn nanothreads in GaAs

NASA Astrophysics Data System (ADS)

Ponomarev, D. S.; Lavrukhin, D. V.; Yachmenev, A. E.; Khabibullin, R. A.; Semenikhin, I. E.; Vyurkov, V. V.; Ryzhii, M.; Otsuji, T.; Ryzhii, V.

2018-04-01

We report on the proposal and the theoretical and experimental studies of the terahertz hot-electron bolometer (THz HEB) based on a gated GaAs structure like the field-effect transistor with the array of parallel Sn nanothreads (Sn-NTs). The operation of the HEB is associated with an increase in the density of the delocalized electrons due to their heating by the incoming THz radiation. The quantum and the classical device models were developed, the quantum one was based on the self-consistent solution of the Poisson and Schrödinger equations, the classical model involved the Poisson equation and density of states omitting quantization. We calculated the electron energy distributions in the channels formed around the Sn-NTs for different gate voltages and found the fraction of the delocalized electrons propagating across the energy barriers between the NTs. Since the fraction of the delocalized electrons strongly depends on the average electron energy (effective temperature), the proposed THz HEB can exhibit an elevated responsivity compared with the HEBs based on more standard heterostructures. Due to a substantial anisotropy of the device structure, the THz HEB may demonstrate a noticeable polarization selectivity of the response to the in-plane polarized THz radiation. The features of the THz HEB might be useful in their practical applications in biology, medicine and material science.

Electron Spectral Breaking Caused by Magnetic Reconnection in Impulsive Flare Events

NASA Astrophysics Data System (ADS)

Tan, Lun C.

2018-05-01

Using data from the Wind/3D Plasma and Energetic Particle (3DP) instrument, we have analyzed the energy spectral difference of low-energy electrons between the “impulsive” and “gradual” solar energetic particle (SEP) events during solar cycle 23. Since simulations reveal that in the exhaust of magnetic reconnection sites, electrons could form a beam structure in which the parallel speed is limited by the electron Alfvén speed (V Ae), their spectral steepening should be observable at the electron energy E e, corresponding to V Ae. In addition, the analysis of transversely oscillating coronal loops shows that in the loop-top region, where the reconnection site is located, V Ae corresponds to E e < 15 keV. We hence search for the spectral steepening of electrons in this E e range. In our search we have taken the effect of local particle acceleration at reconnecting current sheets into consideration. The effect may occur in the solar wind and impact the observed time-intensity profiles of SEPs. Our analysis shows that in the impulsive flare event, the electron spectral steepening occurs at E e = 7 ± 2 keV, whereas no steepening is seen in the gradual event. Therefore, the comparison between the impulsive and gradual SEP event lists provided by this work could be important for future investigations of particle acceleration in the corona and the solar wind.

Core level electron energy-loss spectra of minerals: pre-edge fine structures at the oxygen K-edge . Comment on ``Water in minerals detectable by electron energy-loss spectroscopy EELS'' by R. Wirth, Phys Chem Minerals (1997) 24:561-568

NASA Astrophysics Data System (ADS)

van Aken, P. A.; Liebscher, B.; Styrsa, V. J.

In a recent paper entitled ``Water in minerals detectable by electron energy-loss spectroscopy EELS'' by R. Wirth, it has been claimed that OH-- and H2O-bearing minerals exhibit a characteristic peak in the ELNES spectra at about 528 eV prior to the onset of the O K-edge at 532 eV, which could be used for (semi-)quantitative determination of water- or OH-contents on a nanometer scale. It is shown here by parallel electron energy-loss spectroscopy (PEELS) recorded in a transmission electron microscope (TEM) that O K-pre-edge peaks with very high intensities may also exist in water-free compounds and minerals, in particular when they contain transition metals. These spectral features arise from covalent mixing of the metal and oxygen states, which introduces oxygen p character in unoccupied states of mainly metal character. The point is illustrated by the comparison of hematite (α-Fe2O3) and lepidocrocite (γ-FeOOH) O K-edge PEELS spectra which exhibit similar intensities of the pre-edge peak, despite of their grossly different OH- contents. As a consequence, the general validity of the method proposed by Wirth is questioned.

Physics of the diffusion region in the Magnetospheric Multiscale era

NASA Astrophysics Data System (ADS)

Chen, L. J.; Hesse, M.; Wang, S.; Ergun, R.; Bessho, N.; Burch, J. L.; Giles, B. L.; Torbert, R. B.; Gershman, D. J.; Wilson, L. B., III; Dorelli, J.; Pollock, C. J.; Moore, T. E.; Lavraud, B.; Strangeway, R. J.; Russell, C. T.; Khotyaintsev, Y. V.; Le Contel, O.; Avanov, L. A.

2016-12-01

Encounters of reconnection diffusion regions by the Magnetospheric Multiscale (MMS) mission during its first magnetopause scan are studied in combination with theories and simulations. The goal is to understand by first-principles how stored magnetic energy is converted into plasma thermal and bulk flow energies via particle energization, mixing and interaction with waves. The magnetosheath population having much higher density than the magnetospheric plasma is an outstanding narrator for and participant in the magnetospheric part of the diffusion region. For reconnection with negligible guide fields, the accelerated magnetosheath population (for both electrons and ions) is cyclotron turned by the reconnected magnetic field to form outflow jets, and then gyrotropized downstream. Wave fluctuations are reduced in the central electron diffusion region (EDR) and do not dominate the energy conversion there. For an event with a significant guide field to magnetize the electrons, wave fluctuations at the lower hybrid frequency dominate the energy conversion in the EDR, and the fastest electron outflow is established dominantly by a strong perpendicular electric field via the ExB flow in one exhaust and by time-of-flight effects along with parallel electric field acceleration in the other. Whether the above features are common threads to magnetopause reconnection diffusion regions is a question to be further examined.

Electron magnetic reconnection without ion coupling in Earth's turbulent magnetosheath

NASA Astrophysics Data System (ADS)

Phan, T. D.; Eastwood, J. P.; Shay, M. A.; Drake, J. F.; Sonnerup, B. U. Ö.; Fujimoto, M.; Cassak, P. A.; Øieroset, M.; Burch, J. L.; Torbert, R. B.; Rager, A. C.; Dorelli, J. C.; Gershman, D. J.; Pollock, C.; Pyakurel, P. S.; Haggerty, C. C.; Khotyaintsev, Y.; Lavraud, B.; Saito, Y.; Oka, M.; Ergun, R. E.; Retino, A.; Le Contel, O.; Argall, M. R.; Giles, B. L.; Moore, T. E.; Wilder, F. D.; Strangeway, R. J.; Russell, C. T.; Lindqvist, P. A.; Magnes, W.

2018-05-01

Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region1,2. On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed3-5. Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region6. In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales7-11. However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth's turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling.

Electron magnetic reconnection without ion coupling in Earth's turbulent magnetosheath.

PubMed

Phan, T D; Eastwood, J P; Shay, M A; Drake, J F; Sonnerup, B U Ö; Fujimoto, M; Cassak, P A; Øieroset, M; Burch, J L; Torbert, R B; Rager, A C; Dorelli, J C; Gershman, D J; Pollock, C; Pyakurel, P S; Haggerty, C C; Khotyaintsev, Y; Lavraud, B; Saito, Y; Oka, M; Ergun, R E; Retino, A; Le Contel, O; Argall, M R; Giles, B L; Moore, T E; Wilder, F D; Strangeway, R J; Russell, C T; Lindqvist, P A; Magnes, W

2018-05-01

Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region 1,2 . On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed 3-5 . Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region 6 . In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales 7-11 . However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth's turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling.

Causes of High-temperature Superconductivity in the Hydrogen Sulfide Electron-phonon System

NASA Astrophysics Data System (ADS)

Degtyarenko, N. N.; Mazur, E. A.

The electron and phonon spectra, as well as the density of electron and phonon states of the stable orthorhombic structure of hydrogen sulfide (SH2) at pressures 100-180 GPa have been calculated. It is found that the set of parallel planes of hydrogen atoms is formed at pressure ∼175 GPa as a result of structural changes in the unit cell of the crystal under pressure. There should be complete concentration of hydrogen atoms in these planes. As a result the electron properties of the system acquire a quasi-two-dimensional character. The features of in phase and antiphase oscillations of hydrogen atoms in these planes leading to two narrow high-energy peaks in the phonon density of states are investigated.

Reasons for high-temperature superconductivity in the electron-phonon system of hydrogen sulfide

NASA Astrophysics Data System (ADS)

Degtyarenko, N. N.; Mazur, E. A.

2015-08-01

We have calculated the electron and phonon spectra, as well as the densities of the electron and phonon states, of the stable orthorhombic structure of hydrogen sulfide SH2 in the pressure interval 100-180 GPa. It is found that at a pressure of 175 GPa, a set of parallel planes of hydrogen atoms is formed due to a structural modification of the unit cell under pressure with complete accumulation of all hydrogen atoms in these planes. As a result, the electronic properties of the system become quasi-two-dimensional. We have also analyzed the collective synphase and antiphase vibrations of hydrogen atoms in these planes, leading to the occurrence of two high-energy peaks in the phonon density of states.

A determination of relativistic shock jump conditions using Monte Carlo techniques

NASA Technical Reports Server (NTRS)

Ellison, Donald C.; Reynolds, Stephen P.

1991-01-01

Monte Carlo techniques are used, assuming isotropic elastic scattering of all particles, to calculate jump conditions in parallel relativistic collisionless shocks in the absence of Fermi acceleration. The shock velocity and compression ratios are shown for arbitrary flow velocities and for any upstream temperature. Both single-component electron-positron plasma and two-component proton-electron plasmas are considered. It is shown that protons and electrons must share energy, directly or through the mediation of plasma waves, in order to satisfy the basic conservation conditions, and the electron and proton temperatures are determined for a particular microscopic, kinetic-theory model, namely, that protons always scatter elastically. The results are directly applicable to shocks in which waves of scattering superthermal particles are absent.

Energetic Electrons in Dipolarization Events: Spatial Properties and Anisotropy

NASA Technical Reports Server (NTRS)

Birn, J.; Runov, A.; Hesse, M.

2014-01-01

Using the electromagnetic fields of an MHD simulation of magnetotail reconnection, flow bursts, and dipolarization, we further investigate the acceleration of electrons to suprathermal energies. Particular emphasis is on spatial properties and anisotropies as functions of energy and time. The simulation results are compared with Time History of Events and Macroscale Interactions during Substorms observations. The test particle approach successfully reproduces several observed injection features and puts them into a context of spatial maps of the injection region(s): a dominance of perpendicular anisotropies farther down the tail and closer to the equatorial plane, an increasing importance of parallel anisotropy closer to Earth and at higher latitudes, a drop in energy fluxes at energies below approximately 10 keV, coinciding with the plasma density drop, together with increases at higher energy, a triple peak structure of flux increases near 0 deg, 90 deg, and 180 deg, and a tendency of flux increases to extend to higher energy closer to Earth and at lower latitudes. We identified the plasma sheet boundary layers and adjacent lobes as a main source region for both increased and decreased energetic electron fluxes, related to the different effects of adiabatic acceleration at high and low energies. The simulated anisotropies tend to exceed the observed ones, particularly for perpendicular fluxes at high energies. The most plausible reason is that the MHD simulation lacks the effects of anisotropy-driven microinstabilities and waves, which would reduce anisotropies.

Face classification using electronic synapses

NASA Astrophysics Data System (ADS)

Yao, Peng; Wu, Huaqiang; Gao, Bin; Eryilmaz, Sukru Burc; Huang, Xueyao; Zhang, Wenqiang; Zhang, Qingtian; Deng, Ning; Shi, Luping; Wong, H.-S. Philip; Qian, He

2017-05-01

Conventional hardware platforms consume huge amount of energy for cognitive learning due to the data movement between the processor and the off-chip memory. Brain-inspired device technologies using analogue weight storage allow to complete cognitive tasks more efficiently. Here we present an analogue non-volatile resistive memory (an electronic synapse) with foundry friendly materials. The device shows bidirectional continuous weight modulation behaviour. Grey-scale face classification is experimentally demonstrated using an integrated 1024-cell array with parallel online training. The energy consumption within the analogue synapses for each iteration is 1,000 × (20 ×) lower compared to an implementation using Intel Xeon Phi processor with off-chip memory (with hypothetical on-chip digital resistive random access memory). The accuracy on test sets is close to the result using a central processing unit. These experimental results consolidate the feasibility of analogue synaptic array and pave the way toward building an energy efficient and large-scale neuromorphic system.

Face classification using electronic synapses.

PubMed

Yao, Peng; Wu, Huaqiang; Gao, Bin; Eryilmaz, Sukru Burc; Huang, Xueyao; Zhang, Wenqiang; Zhang, Qingtian; Deng, Ning; Shi, Luping; Wong, H-S Philip; Qian, He

2017-05-12

Conventional hardware platforms consume huge amount of energy for cognitive learning due to the data movement between the processor and the off-chip memory. Brain-inspired device technologies using analogue weight storage allow to complete cognitive tasks more efficiently. Here we present an analogue non-volatile resistive memory (an electronic synapse) with foundry friendly materials. The device shows bidirectional continuous weight modulation behaviour. Grey-scale face classification is experimentally demonstrated using an integrated 1024-cell array with parallel online training. The energy consumption within the analogue synapses for each iteration is 1,000 × (20 ×) lower compared to an implementation using Intel Xeon Phi processor with off-chip memory (with hypothetical on-chip digital resistive random access memory). The accuracy on test sets is close to the result using a central processing unit. These experimental results consolidate the feasibility of analogue synaptic array and pave the way toward building an energy efficient and large-scale neuromorphic system.

Localized Electron Heating by Strong Guide-Field Magnetic Reconnection

NASA Astrophysics Data System (ADS)

Guo, Xuehan; Sugawara, Takumichi; Inomoto, Michiaki; Yamasaki, Kotaro; Ono, Yasushi; UTST Team

2015-11-01

Localized electron heating of magnetic reconnection was studied under strong guide-field (typically Bt 15Bp) using two merging spherical tokamak plasmas in Univ. Tokyo Spherical Tokamak (UTST) experiment. Our new slide-type two-dimensional Thomson scattering system documented for the first time the electron heating localized around the X-point. The region of high electron temperature, which is perpendicular to the magnetic field, was found to have a round shape with radius of 2 [cm]. Also, it was localized around the X-point and does not agree with that of energy dissipation term Et .jt . When we include a guide-field effect term Bt / (Bp + αBt) for Et .jt where α =√{ (vin2 +vout2) /v∥2 } , the energy dissipation area becomes localized around the X-point, suggesting that the electrons are accelerated by the reconnection electric field parallel to the magnetic field and thermalized around the X-point. This work was supported by JSPS A3 Foresight Program ``Innovative Tokamak Plasma Startup and Current Drive in Spherical Torus,'' a Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS) Fellows 15J03758.

Atomic scale structure and chemistry of interfaces by Z-contrast imaging and electron energy loss spectroscopy in the STEM

DOE Office of Scientific and Technical Information (OSTI.GOV)

McGibbon, M.M.; Browning, N.D.; Chisholm, M.F.

The macroscopic properties of many materials are controlled by the structure and chemistry at the grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. The high-resolution Z-contrast imaging technique in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition can be interpreted intuitively. This direct image allows the electron probe to be positioned over individual atomic columns for parallel detection electron energy loss spectroscopy (PEELS) at a spatial resolution approaching 0.22nm. The bonding information which can bemore » obtained from the fine structure within the PEELS edges can then be used in conjunction with the Z-contrast images to determine the structure at the grain boundary. In this paper we present 3 examples of correlations between the structural, chemical and electronic properties at materials interfaces in metal-semiconductor systems, superconducting and ferroelectric materials.« less

Limitations of silicon diodes for clinical electron dosimetry.

PubMed

Song, Haijun; Ahmad, Munir; Deng, Jun; Chen, Zhe; Yue, Ning J; Nath, Ravinder

2006-01-01

This work investigates the relevance of several factors affecting the response of silicon diode dosemeters in depth-dose scans of electron beams. These factors are electron energy, instantaneous dose rate, dose per pulse, photon/electron dose ratio and electron scattering angle (directional response). Data from the literature and our own experiments indicate that the impact of these factors may be up to +/-15%. Thus, the different factors would have to cancel out perfectly at all depths in order to produce true depth-dose curves. There are reports of good agreement between depth-doses measured with diodes and ionisation chambers. However, our measurements with a Scantronix electron field detector (EFD) diode and with a plane-parallel ionisation chamber show discrepancies both in the build-up and in the low-dose regions, with a ratio up to 1.4. Moreover, the absolute sensitivity of two diodes of the same EFD model was found to differ by a factor of 3, and this ratio was not constant but changed with depth between 5 and 15% in the low-dose regions of some clinical electron beams. Owing to these inhomogeneities among diodes even of the same model, corrections for each factor would have to be diode-specific and beam-specific. All these corrections would have to be determined using parallel plane chambers, as recommended by AAPM TG-25, which would be unrealistic in clinical practice. Our conclusion is that in general diodes are not reliable in the measurement of depth-dose curves of clinical electron beams.

In Situ Observation of Intermittent Dissipation at Kinetic Scales in the Earth's Magnetosheath

NASA Astrophysics Data System (ADS)

Chasapis, Alexandros; Matthaeus, W. H.; Parashar, T. N.; Wan, M.; Haggerty, C. C.; Pollock, C. J.; Giles, B. L.; Paterson, W. R.; Dorelli, J.; Gershman, D. J.; Torbert, R. B.; Russell, C. T.; Lindqvist, P.-A.; Khotyaintsev, Y.; Moore, T. E.; Ergun, R. E.; Burch, J. L.

2018-03-01

We present a study of signatures of energy dissipation at kinetic scales in plasma turbulence based on observations by the Magnetospheric Multiscale mission (MMS) in the Earth’s magnetosheath. Using several intervals, and taking advantage of the high-resolution instrumentation on board MMS, we compute and discuss several statistical measures of coherent structures and heating associated with electrons, at previously unattainable scales in space and time. We use the multi-spacecraft Partial Variance of Increments (PVI) technique to study the intermittent structure of the magnetic field. Furthermore, we examine a measure of dissipation and its behavior with respect to the PVI as well as the current density. Additionally, we analyze the evolution of the anisotropic electron temperature and non-Maxwellian features of the particle distribution function. From these diagnostics emerges strong statistical evidence that electrons are preferentially heated in subproton-scale regions of strong electric current density, and this heating is preferentially in the parallel direction relative to the local magnetic field. Accordingly, the conversion of magnetic energy into electron kinetic energy occurs more strongly in regions of stronger current density, a finding consistent with several kinetic plasma simulation studies and hinted at by prior studies using lower resolution Cluster observations.

Low cost electronic ultracapacitor interface technique to provide load leveling of a battery for pulsed load or motor traction drive applications

DOEpatents

King, Robert Dean; DeDoncker, Rik Wivina Anna Adelson

1998-01-01

A battery load leveling arrangement for an electrically powered system in which battery loading is subject to intermittent high current loading utilizes a passive energy storage device and a diode connected in series with the storage device to conduct current from the storage device to the load when current demand forces a drop in battery voltage. A current limiting circuit is connected in parallel with the diode for recharging the passive energy storage device. The current limiting circuit functions to limit the average magnitude of recharge current supplied to the storage device. Various forms of current limiting circuits are disclosed, including a PTC resistor coupled in parallel with a fixed resistor. The current limit circuit may also include an SCR for switching regenerative braking current to the device when the system is connected to power an electric motor.

On the instability and energy flux of lower hybrid waves in the Venus plasma mantle

NASA Technical Reports Server (NTRS)

Strangeway, R. J.; Crawford, G. K.

1993-01-01

Waves generated near the lower hybrid resonance frequency by the modified two stream instability have been invoked as a possible source of energy flux into the topside ionosphere of Venus. These waves are observed above the ionopause in a region known as the plasma mantle. The plasma within the mantle appears to be a mixture of magnetosheath and ionospheric plasmas. Since the magnetosheath electrons and ions have temperatures of several tens of eV, any instability analysis of the modified two stream instability requires the inclusion of finite electron and ion temperatures. Finite temperature effects are likely to reduce the growth rate of the instability. Furthermore, the lower hybrid waves are only quasi-electrostatic, and the energy flux of the waves is mainly carried by parallel Poynting flux. The magnetic field in the mantle is draped over the ionopause. Lower hybrid waves therefore cannot transport any significant wave energy to lower altitudes, and so do not act as a source of additional heat to the topside ionosphere.

Electronic transport in armchair graphene nanoribbon under double magnetic barrier modulation

NASA Astrophysics Data System (ADS)

Wang, Haiyan; Wu, Chao; Xie, Fang; Zhang, Xiaojiao; Zhou, Guanghui

2018-03-01

We present a theoretical investigation of the transport properties and the magnetoresistance effect in armchair graphene nanoribbons (AGNRs) under modulation by two magnetic barriers. The energy levels are found to be degenerate for a metallic AGNR but are not degenerate for a semiconducting AGNR. However, the conductance characteristics show quantized plateaus in both the metallic and semiconducting cases. When the magnetization directions of the barriers change from parallel to antiparallel, the conductance plateau in the metallic AGNR shows a degenerate feature due to matching between the transport modes in different regions. As the barrier height increases, the conductance shows more oscillatory behavior with sharp peaks and troughs. Specifically, the initial position of nonzero conductance for the metallic AGNR system moves towards a higher energy regime, which indicates that an energy gap has been opened. In addition, the magnetoresistance ratio also shows plateau structures in certain specific energy regions. These results may be useful in the design of electron devices based on AGNR nanostructures.

Ion and electron dynamics generating the Hall current in the exhaust far downstream of the reconnection x-line

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fujimoto, Keizo, E-mail: keizo.fujimoto@nao.ac.jp; Takamoto, Makoto

2016-01-15

We have investigated the ion and electron dynamics generating the Hall current in the reconnection exhaust far downstream of the x-line where the exhaust width is much larger than the ion gyro-radius. A large-scale particle-in-cell simulation shows that most ions are accelerated through the Speiser-type motion in the current sheet formed at the center of the exhaust. The transition layers formed at the exhaust boundary are not identified as slow mode shocks. (The layers satisfy mostly the Rankine-Hugoniot conditions for a slow mode shock, but the energy conversion hardly occurs there.) We find that the ion drift velocity is modifiedmore » around the layer due to a finite Larmor radius effect. As a result, the ions are accumulated in the downstream side of the layer, so that collimated ion jets are generated. The electrons experience two steps of acceleration in the exhaust. The first is a parallel acceleration due to the out-of-plane electric field E{sub y} which has a parallel component in most area of the exhaust. The second is a perpendicular acceleration due to E{sub y} at the center of the current sheet and the motion is converted to the parallel direction. Because of the second acceleration, the electron outflow velocity becomes almost uniform over the exhaust. The difference in the outflow profile between the ions and electrons results in the Hall current in large area of the exhaust. The present study demonstrates the importance of the kinetic treatments for collisionless magnetic reconnection even far downstream from the x-line.« less

First principles study of optical properties of molybdenum disulfide: From bulk to monolayer

NASA Astrophysics Data System (ADS)

Hieu, Nguyen N.; Ilyasov, Victor V.; Vu, Tuan V.; Poklonski, Nikolai A.; Phuc, Huynh V.; Phuong, Le T. T.; Hoi, Bui D.; Nguyen, Chuong V.

2018-03-01

In this paper, we theoretically study the optical properties of both bulk and monolayer MoS2 using first-principles calculations. The optical characters such as: dielectric function, optical reflectivity, and electron energy-loss spectrum of MoS2 are observed in the energy region from 0 to 15 eV. At equilibrium state the dielectric constant in the parallel E∥ x and perpendicular E∥ z directions are of 15.01 and 8.92 for bulk while they are 4.95 and 2.92 for monolayer MoS2, respectively. In the case of bulk MoS2, the obtained computational results for both real and imaginary parts of the dielectric constant are in good agreement with the previous experimental data. In the energy range from 0 to 6 eV, the dielectric functions have highly anisotropic, whereas they become isotropic when the energy is larger than 7 eV. For the adsorption spectra and optical reflectivity, both the collective plasmon resonance and (π + σ) electron plasmon peaks are observed, in which the transition in E∥ x direction is accordant with the experiment data more than the transition in E∥ z direction is. The refractive index, extinction index, and electron energy-loss spectrum are also investigated. The observed prominent peak at 23.1 eV in the energy-loss spectra is in good agreement with experiment value. Our results may provide a useful potential application for the MoS2 structures in electronic and optoelectronic devices.

Charge transport in metal oxides: A theoretical study of hematite α-Fe2O3

NASA Astrophysics Data System (ADS)

Iordanova, N.; Dupuis, M.; Rosso, K. M.

2005-04-01

Transport of conduction electrons and holes through the lattice of α-Fe2O3 (hematite) is modeled as a valence alternation of iron cations using ab initio electronic structure calculations and electron transfer theory. Experimental studies have shown that the conductivity along the (001) basal plane is four orders of magnitude larger than the conductivity along the [001] direction. In the context of the small polaron model, a cluster approach was used to compute quantities controlling the mobility of localized electrons and holes, i.e., the reorganization energy and the electronic coupling matrix element that enter Marcus' theory. The calculation of the electronic coupling followed the generalized Mulliken-Hush approach using the complete active space self-consistent field method. Our findings demonstrate an approximately three orders of magnitude anisotropy in both electron and hole mobility between directions perpendicular and parallel to the c axis, in good accord with experimental data. The anisotropy arises from the slowness of both electron and hole mobilities across basal oxygen planes relative to that within iron bilayers between basal oxygen planes. Interestingly, for elementary reaction steps along either of the directions considered, there is only less than one order of magnitude difference in mobility between electrons and holes, in contrast to accepted classical arguments. Our findings indicate that the most important quantity underlying mobility differences is the electronic coupling, albeit the reorganization energy contributes as well. The large values computed for the electronic coupling suggest that charge transport reactions in hematite are adiabatic in nature. The electronic coupling is found to depend on both the superexchange interaction through the bridging oxygen atoms and the d-shell electron spin coupling within the Fe-Fe donor-acceptor pair, while the reorganization energy is essentially independent of the electron spin coupling.

Geometric and electronic structures of silicon-sodium binary clusters. I. Ionization energy of SinNam

NASA Astrophysics Data System (ADS)

Kishi, Reiko; Iwata, Suehiro; Nakajima, Atsushi; Kaya, Koji

1997-08-01

Sodium doped silicon clusters (SinNam; 1⩽n⩽14, 1⩽m⩽5) produced by two types of laser vaporization were studied. The adsorption of Na atoms on the Sin clusters leads the substantial lowering of the ionization energy, Ei, of SinNam clusters. Their reactivity toward NO molecules was measured with a fast flow reactor, and the anticorrelation between the Eis and the reactivity was clearly observed; species having low Ei exhibit high reactivity and vice versa. Moreover, the clear parallelism between the Eis of SinNa and the EAs of Sin is found. This is consistent with the fact that the structure of SinNa clusters keeps the frame of the corresponding Sin cluster unchanged and that the electronic structure of SinNa is similar to that of the corresponding negative ion Sin-. In addition to the experimental studies, the geometries, adsorption energies, and vertical ionization energies of SinNa (n=1-7) were investigated with ab initio MO calculations including electron correlation; The Møller-Plesset perturbation theory was used and the configuration interaction (CI) calculation was carried out, particularly for a diatomic molecule, SiNa.

A parallel algorithm for Hamiltonian matrix construction in electron-molecule collision calculations: MPI-SCATCI

NASA Astrophysics Data System (ADS)

Al-Refaie, Ahmed F.; Tennyson, Jonathan

2017-12-01

Construction and diagonalization of the Hamiltonian matrix is the rate-limiting step in most low-energy electron - molecule collision calculations. Tennyson (1996) implemented a novel algorithm for Hamiltonian construction which took advantage of the structure of the wavefunction in such calculations. This algorithm is re-engineered to make use of modern computer architectures and the use of appropriate diagonalizers is considered. Test calculations demonstrate that significant speed-ups can be gained using multiple CPUs. This opens the way to calculations which consider higher collision energies, larger molecules and / or more target states. The methodology, which is implemented as part of the UK molecular R-matrix codes (UKRMol and UKRMol+) can also be used for studies of bound molecular Rydberg states, photoionization and positron-molecule collisions.

Geminga's tails: a pulsar bow shock probing the interstellar medium.

PubMed

Caraveo, P A; Bignami, G F; DeLuca, A; Mereghetti, S; Pellizzoni, A; Mignani, R; Tur, A; Becker, W

2003-09-05

We report the X-ray Multimirror Mission-Newton European Photon Imaging Camera observation of two elongated parallel x-ray tails trailing the pulsar Geminga. They are aligned with the object's supersonic motion, extend for approximately 2', and have a nonthermal spectrum produced by electron-synchrotron emission in the bow shock between the pulsar wind and the surrounding medium. Electron lifetime against synchrotron cooling matches the source transit time over the x-ray features' length. Such an x-ray detection of a pulsar bow shock (with no Halpha emission) allows us to gauge the pulsar electron injection energy and the shock magnetic field while constraining the angle of Geminga's motion and the local matter density.

Real-time electron dynamics for massively parallel excited-state simulations

NASA Astrophysics Data System (ADS)

Andrade, Xavier

The simulation of the real-time dynamics of electrons, based on time dependent density functional theory (TDDFT), is a powerful approach to study electronic excited states in molecular and crystalline systems. What makes the method attractive is its flexibility to simulate different kinds of phenomena beyond the linear-response regime, including strongly-perturbed electronic systems and non-adiabatic electron-ion dynamics. Electron-dynamics simulations are also attractive from a computational point of view. They can run efficiently on massively parallel architectures due to the low communication requirements. Our implementations of electron dynamics, based on the codes Octopus (real-space) and Qball (plane-waves), allow us to simulate systems composed of thousands of atoms and to obtain good parallel scaling up to 1.6 million processor cores. Due to the versatility of real-time electron dynamics and its parallel performance, we expect it to become the method of choice to apply the capabilities of exascale supercomputers for the simulation of electronic excited states.

Monte Carlo study of si diode response in electron beams.

PubMed

Wang, Lilie L W; Rogers, David W O

2007-05-01

Silicon semiconductor diodes measure almost the same depth-dose distributions in both photon and electron beams as those measured by ion chambers. A recent study in ion chamber dosimetry has suggested that the wall correction factor for a parallel-plate ion chamber in electron beams changes with depth by as much as 6%. To investigate diode detector response with respect to depth, a silicon diode model is constructed and the water/silicon dose ratio at various depths in electron beams is calculated using EGSnrc. The results indicate that, for this particular diode model, the diode response per unit water dose (or water/diode dose ratio) in both 6 and 18 MeV electron beams is flat within 2% versus depth, from near the phantom surface to the depth of R50 (with calculation uncertainty <0.3%). This suggests that there must be some other correction factors for ion chambers that counter-balance the large wall correction factor at depth in electron beams. In addition, the beam quality and field-size dependence of the diode model are also calculated. The results show that the water/diode dose ratio remains constant within 2% over the electron energy range from 6 to 18 MeV. The water/diode dose ratio does not depend on field size as long as the incident electron beam is broad and the electron energy is high. However, for a very small beam size (1 X 1 cm(2)) and low electron energy (6 MeV), the water/diode dose ratio may decrease by more than 2% compared to that of a broad beam.

Radiation transport in kinetic simulations and the influence of photoemission on electron current in self-sustaining discharges

DOE PAGES

Fierro, Andrew S.; Moore, Christopher Hudson; Scheiner, Brett; ...

2017-01-12

A kinetic description for electronic excitation of helium for principal quantum number nmore » $$\\leqslant $$ 4 has been included into a particle-in-cell (PIC) simulation utilizing direct simulation Monte Carlo (DSMC) for electron-neutral interactions. The excited electronic levels radiate state-dependent photons with wavelengths from the extreme ultraviolet (EUV) to visible regimes. Photon wavelengths are chosen according to a Voigt distribution accounting for the natural, pressure, and Doppler broadened linewidths. This method allows for reconstruction of the emission spectrum for a non-thermalized electron energy distribution function (EEDF) and investigation of high energy photon effects on surfaces, specifically photoemission. A parallel plate discharge with a fixed field (i.e. space charge neglected) is used to investigate the effects of including photoemission for a Townsend discharge. When operating at a voltage near the self-sustaining discharge threshold, it is observed that the electron current into the anode is higher when including photoemission from the cathode than without even when accounting for self-absorption from ground state atoms. As a result, the photocurrent has been observed to account for as much as 20% of the total current from the cathode under steady-state conditions.« less

Electron attachment to antipyretics: Possible implications of their metabolic pathways

NASA Astrophysics Data System (ADS)

Pshenichnyuk, Stanislav A.; Modelli, Alberto

2012-06-01

The empty-level structures and formation of negative ion states via resonance attachment of low-energy (0-15 eV) electrons into vacant molecular orbitals in a series of non-steroidal anti-inflammatory drugs (NSAIDs), namely aspirin, paracetamol, phenacetin, and ibuprofen, were investigated in vacuo by electron transmission and dissociative electron attachment (DEA) spectroscopies, with the aim to model the behavior of these antipyretic agents under reductive conditions in vivo. The experimental findings are interpreted with the support of density functional theory calculations. The negative and neutral fragments formed by DEA in the gas phase display similarities with the main metabolites of these commonly used NSAIDs generated in vivo by the action of cytochrome P450 enzymes, as well as with several known active agents. It is concluded that xenobiotic molecules which possess pronounced electron-accepting properties could in principle follow metabolic pathways which parallel the gas-phase dissociative decay channels observed in the DEA spectra at incident electron energies below 1 eV. Unwanted side effects as, e.g., hepatoxicity or carcinogenicity produced by the NSAIDs under study in human organism are discussed within the "free radical model" framework, reported earlier to describe the toxic action of the well-known model toxicant carbon tetrachloride.

MgF2 monolayer as an anti-reflecting material

NASA Astrophysics Data System (ADS)

Mahida, H. R.; Singh, Deobrat; Sonvane, Yogesh; Gupta, Sanjeev K.; Thakor, P. B.

2017-02-01

The single-layer atomic sheet of magnesium fluoride (MgF2) having 1H and 1T phase structure (hexagonal and tetragonal phase) has been calculated by density functional theory (DFT). Further, we have investigated the structural, electronic and optical properties such as frequency dependent dielectric function, absorption spectra, energy loss spectra, reflectivity, refractive index and optical conductivity of monolayer MgF2 for the direction of parallel and perpendicular electric field polarizations. Our results suggest that monolayer MgF2 provides promising applications in anti-reflection coatings, high-reflective systems and in opto-electronic materials.

TEM characterization of nanodiamond thin films.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Qin, L.-C.; Zhou, D.; Krauss, A. R.

The microstructure of thin films grown by microwave plasma-enhanced chemical vapor deposition (MPCVD) from fullerene C{sub 60} precursors has been characterized by scanning electron microscopy (SEM), selected-area electron diffraction (SAED), bright-field electron microscopy, high-resolution electron microscopy (HREM), and parallel electron energy loss spectroscopy (PEELS). The films are composed of nanosize crystallites of diamond, and no graphitic or amorphous phases were observed. The diamond crystallite size measured from lattice images shows that most grains range between 3-5 nm, reflecting a gamma distribution. SAED gave no evidence of either sp2-bonded glassy carbon or sp3-bonded diamondlike amorphous carbon. The sp2-bonded configuration found inmore » PEELS was attributed to grain boundary carbon atoms, which constitute 5-10% of the total. Occasionally observed larger diamond grains tend to be highly faulted.« less

The Microphysics Explorer (MPEX) Mission: A Small Explorer Mission to Investigate the Role of Small Scale Non-Linear Time Domain Structures (TDS) and Waves in the Energization of Electrons and Energy Flow in Space Plasmas.

NASA Astrophysics Data System (ADS)

Wygant, J. R.

2016-12-01

Evidence has accumulated that most energy conversion structures in space plasmas are characterized by intense small-scale size electric fields with strong parallel components, which are prime suspects in the rapid and efficient bulk acceleration of electrons. The proposed MPEX mission will provide, for the first time, 1 ms measurements of electrons capable of resolving the acceleration process due to these small-scale structures. These structures include Time Domain Structures (TDS) which are often organized into wave trains of hundreds of discrete structures propagating along magnetic fields lines. Recent measurements in the near Earth tail on auroral field lines indicate these wave trains are associated with electron acceleration in layers of strong energy flow in the form of particle energy flux and Poynting flux. Also coincident are kinetic Alfven waves which may be capable of driving the time domain structures or directly accelerating electrons. Other waves that may be important include lower hybrid wave packets, electron cyclotron waves, and large amplitude whistler waves. High time resolution field measurements show that such structures occur within dayside and tail reconnection regions, at the bow shock, at interplanetary shocks, and at other structures in the solar wind. The MPEX mission will be a multiphase mission with apogee boosts, which will explore all these regions. An array of electron ESAs will provide a 1 millisecond measurement of electron flux variations with nearly complete pitch angle coverage over a programmable array of selected energy channels. The electric field detector will provide measurement a fully 3-D measurement of the electric field with the benefit of an extremely large ratio of boom length to spacecraft radius and an improved sensor design. 2-D ion distribution functions will be provided by ion mass spectrometer and energetic electrons will be measured by a solid-state telescope.

Nongyrotropic Electrons in Guide Field Reconnection

NASA Technical Reports Server (NTRS)

Wendel, D. E.; Hesse, M.; Bessho, N.; Adrian, M. L.; Kuznetsova, M.

2016-01-01

We apply a scalar measure of nongyrotropy to the electron pressure tensor in a 2D particle-in-cell simulation of guide field reconnection and assess the corresponding electron distributions and the forces that account for the nongyrotropy. The scalar measure reveals that the nongyrotropy lies in bands that straddle the electron diffusion region and the separatrices, in the same regions where there are parallel electric fields. Analysis of electron distributions and fields shows that the nongyrotropy along the inflow and outflow separatrices emerges as a result of multiple populations of electrons influenced differently by large and small-scale parallel electric fields and by gradients in the electric field. The relevant parallel electric fields include large-scale potential ramps emanating from the x-line and sub-ion inertial scale bipolar electron holes. Gradients in the perpendicular electric field modify electrons differently depending on their phase, thus producing nongyrotropy. Magnetic flux violation occurs along portions of the separatrices that coincide with the parallel electric fields. An inductive electric field in the electron EB drift frame thus develops, which has the effect of enhancing nongyrotropies already produced by other mechanisms and under certain conditions producing their own nongyrotropy. Particle tracing of electrons from nongyrotropic populations along the inflows and outflows shows that the striated structure of nongyrotropy corresponds to electrons arriving from different source regions. We also show that the relevant parallel electric fields receive important contributions not only from the nongyrotropic portion of the electron pressure tensor but from electron spatial and temporal inertial terms as well.

Mn-silicide nanostructures aligned on massively parallel silicon nano-ribbons

NASA Astrophysics Data System (ADS)

De Padova, Paola; Ottaviani, Carlo; Ronci, Fabio; Colonna, Stefano; Olivieri, Bruno; Quaresima, Claudio; Cricenti, Antonio; Dávila, Maria E.; Hennies, Franz; Pietzsch, Annette; Shariati, Nina; Le Lay, Guy

2013-01-01

The growth of Mn nanostructures on a 1D grating of silicon nano-ribbons is investigated at atomic scale by means of scanning tunneling microscopy, low energy electron diffraction and core level photoelectron spectroscopy. The grating of silicon nano-ribbons represents an atomic scale template that can be used in a surface-driven route to control the combination of Si with Mn in the development of novel materials for spintronics devices. The Mn atoms show a preferential adsorption site on silicon atoms, forming one-dimensional nanostructures. They are parallel oriented with respect to the surface Si array, which probably predetermines the diffusion pathways of the Mn atoms during the process of nanostructure formation.

Mn-silicide nanostructures aligned on massively parallel silicon nano-ribbons.

PubMed

De Padova, Paola; Ottaviani, Carlo; Ronci, Fabio; Colonna, Stefano; Olivieri, Bruno; Quaresima, Claudio; Cricenti, Antonio; Dávila, Maria E; Hennies, Franz; Pietzsch, Annette; Shariati, Nina; Le Lay, Guy

2013-01-09

The growth of Mn nanostructures on a 1D grating of silicon nano-ribbons is investigated at atomic scale by means of scanning tunneling microscopy, low energy electron diffraction and core level photoelectron spectroscopy. The grating of silicon nano-ribbons represents an atomic scale template that can be used in a surface-driven route to control the combination of Si with Mn in the development of novel materials for spintronics devices. The Mn atoms show a preferential adsorption site on silicon atoms, forming one-dimensional nanostructures. They are parallel oriented with respect to the surface Si array, which probably predetermines the diffusion pathways of the Mn atoms during the process of nanostructure formation.

Langmuir Probe Measurements in an Inductively Coupled GEC Reference Cell Plasma

NASA Technical Reports Server (NTRS)

Ji, J. S.; Kim, J. S.; Cappelli, M. A.; Sharma, S. P.; Arnold, J. O. (Technical Monitor)

1998-01-01

Measurements of electron number density, electron temperature, and electron energy distribution function (EEDF) using a compensated Langmuir probe have been performed on an inductively (transformer ) coupled Gaseous Electronics Conference (GEC) reference cell plasma. The plasma source is operated with CH4, CF4, or their mixtures with argon. The effect of independently driving the electrode supporting the wafer on the probe data is studied. In particular, we find that the plasma structure depends on the phase in addition to the magnitude of the power coupled to the electrode relative to that of the transformer coil. The Langmuir probe is translated in a plane parallel to the electrode to investigate the spatial structure of the plasma. The probe data is also compared with fluid model predictions.

Observations of electron vortex magnetic holes and related wave-particle interactions in the turbulent magnetosheath

NASA Astrophysics Data System (ADS)

Huang, S.; Sahraoui, F.; Yuan, Z.; He, J.; Zhao, J.; Du, J.; Le Contel, O.; Wang, X.; Deng, X.; Fu, H.; Zhou, M.; Shi, Q.; Breuillard, H.; Pang, Y.; Yu, X.; Wang, D.

2017-12-01

Magnetic hole is characterized by a magnetic depression, a density peak, a total electron temperature increase (with a parallel temperature decrease but a perpendicular temperature increase), and strong currents carried by the electrons. The current has a dip in the core region of the magnetic hole and a peak in the outer region of the magnetic hole. There is an enhancement in the perpendicular electron fluxes at 90° pitch angles inside the magnetic hole, implying that the electrons are trapped within it. The variations of the electron velocity components Vem and Ven suggest that an electron vortex is formed by trapping electrons inside the magnetic hole in the circular cross-section. These observations demonstrate the existence of a new type of coherent structures behaving as an electron vortex magnetic hole in turbulent space plasmas as predicted by recent kinetic simulations. We perform a statistically study using high time solution data from the MMS mission. The magnetic holes with short duration (i.e., < 0.5 s) have their cross section smaller than the ion gyro-radius. Superposed epoch analysis of all events reveals that an increase in the electron density and total temperature, significantly increase (resp. decrease) the electron perpendicular (resp. parallel) temperature, and an electron vortex inside the holes. Electron fluxes at 90° pitch angles with selective energies increase in the KSMHs, are trapped inside KSMHs and form the electron vortex due to their collective motion. All these features are consistent with the electron vortex magnetic holes obtained in 2D and 3D particle-in-cell simulations, indicating that the observed the magnetic holes seem to be best explained as electron vortex magnetic holes. It is furthermore shown that the magnetic holes are likely to heat and accelerate the electrons. We also investigate the coupling between whistler waves and electron vortex magnetic holes. These whistler waves can be locally generated inside electron vortex magnetic holes by electron temperature anisotropic instability.

Calculating Potential Energy Curves with Quantum Monte Carlo

NASA Astrophysics Data System (ADS)

Powell, Andrew D.; Dawes, Richard

2014-06-01

Quantum Monte Carlo (QMC) is a computational technique that can be applied to the electronic Schrödinger equation for molecules. QMC methods such as Variational Monte Carlo (VMC) and Diffusion Monte Carlo (DMC) have demonstrated the capability of capturing large fractions of the correlation energy, thus suggesting their possible use for high-accuracy quantum chemistry calculations. QMC methods scale particularly well with respect to parallelization making them an attractive consideration in anticipation of next-generation computing architectures which will involve massive parallelization with millions of cores. Due to the statistical nature of the approach, in contrast to standard quantum chemistry methods, uncertainties (error-bars) are associated with each calculated energy. This study focuses on the cost, feasibility and practical application of calculating potential energy curves for small molecules with QMC methods. Trial wave functions were constructed with the multi-configurational self-consistent field (MCSCF) method from GAMESS-US.[1] The CASINO Monte Carlo quantum chemistry package [2] was used for all of the DMC calculations. An overview of our progress in this direction will be given. References: M. W. Schmidt et al. J. Comput. Chem. 14, 1347 (1993). R. J. Needs et al. J. Phys.: Condensed Matter 22, 023201 (2010).

Evidence from EELS of oxygen in the nucleation layer of a MBE grown III-N HEMT[Electron Energy Loss Spectroscopy, Molecular Beam Epitaxy, High Electron Mobility Transistor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Eustis, T.J.; Silcox, J.; Murphy, M.J.

The presence of oxygen throughout the nominally AlN nucleation layer of a RF assisted MBE grown III-N HEMT was revealed upon examination by Electron Energy Loss Spectroscopy (EELS) in a Scanning Transmission Electron Microscope (STEM). The nucleation layer generates the correct polarity (gallium face) required for producing a piezoelectric induced high mobility two dimensional electron gas at the AlGaN/GaN heterojunction. Only AlN or AlGaN nucleation layers have provided gallium face polarity in RF assisted MBE grown III-N's on sapphire. The sample was grown at Cornell University in a Varian GenII MBE using an EPI Uni-Bulb nitrogen plasma source. The nucleationmore » layer was examined in the Cornell University STEM using Annular Dark Field (ADF) imaging and Parallel Electron Energy Loss Spectroscopy (PEELS). Bright Field TEM reveals a relatively crystallographically sharp interface, while the PEELS reveal a chemically diffuse interface. PEELS of the nitrogen and oxygen K-edges at approximately 5-Angstrom steps across the GaN/AlN/sapphire interfaces reveals the presence of oxygen in the AlN nucleation layer. The gradient suggests that the oxygen has diffused into the nucleation region from the sapphire substrate forming this oxygen containing AlN layer. Based on energy loss near edge structure (ELNES), oxygen is in octahedral interstitial sites in the AlN and Al is both tetrahedrally and octahedrally coordinated in the oxygen rich region of the AlN.« less

Beam dynamics in MABE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Poukey, J.W.; Coleman, P.D.; Sanford, T.W.L.

1985-10-01

MABE is a multistage linear electron accelerator which accelerates up to nine beams in parallel. Nominal parameters per beam are 25 kA, final energy 7 MeV, and guide field 20 kG. We report recent progress via theory and simulation in understanding the beam dynamics in such a system. In particular, we emphasize our results on the radial oscillations and emittance growth for a beam passing through a series of accelerating gaps.

A parallel orbital-updating based plane-wave basis method for electronic structure calculations

NASA Astrophysics Data System (ADS)

Pan, Yan; Dai, Xiaoying; de Gironcoli, Stefano; Gong, Xin-Gao; Rignanese, Gian-Marco; Zhou, Aihui

2017-11-01

Motivated by the recently proposed parallel orbital-updating approach in real space method [1], we propose a parallel orbital-updating based plane-wave basis method for electronic structure calculations, for solving the corresponding eigenvalue problems. In addition, we propose two new modified parallel orbital-updating methods. Compared to the traditional plane-wave methods, our methods allow for two-level parallelization, which is particularly interesting for large scale parallelization. Numerical experiments show that these new methods are more reliable and efficient for large scale calculations on modern supercomputers.

Dehydrogenation of benzene on Pt(111) surface

NASA Astrophysics Data System (ADS)

Gao, W.; Zheng, W. T.; Jiang, Q.

2008-10-01

The dehydrogenation of benzene on Pt(111) surface is studied by ab initio density functional theory. The minimum energy pathways for benzene dehydrogenation are found with the nudge elastic band method including several factors of the associated barriers, reactive energies, intermediates, and transient states. The results show that there are two possible parallel minimum energy pathways on the Pt(111) surface. Moreover, the tilting angle of the H atom in benzene can be taken as an index for the actual barrier of dehydrogenation. In addition, the properties of dehydrogenation radicals on the Pt(111) surface are explored through their adsorption energy, adsorption geometry, and electronic structure on the surface. The vibrational frequencies of the dehydrogenation radicals derived from the calculations are in agreement with literature data.

Dehydrogenation of benzene on Pt(111) surface.

PubMed

Gao, W; Zheng, W T; Jiang, Q

2008-10-28

The dehydrogenation of benzene on Pt(111) surface is studied by ab initio density functional theory. The minimum energy pathways for benzene dehydrogenation are found with the nudge elastic band method including several factors of the associated barriers, reactive energies, intermediates, and transient states. The results show that there are two possible parallel minimum energy pathways on the Pt(111) surface. Moreover, the tilting angle of the H atom in benzene can be taken as an index for the actual barrier of dehydrogenation. In addition, the properties of dehydrogenation radicals on the Pt(111) surface are explored through their adsorption energy, adsorption geometry, and electronic structure on the surface. The vibrational frequencies of the dehydrogenation radicals derived from the calculations are in agreement with literature data.

Extracting Wair from the electron beam measurements of Domen and Lamperti.

PubMed

Tessier, Frédéric; Cojocaru, Claudiu D; Ross, Carl K

2018-01-01

The average energy expended by an energetic electron to create an ion pair in dry air, W air , is a key quantity in radiation dosimetry. Although W air is well established for electron energies up to about 3 MeV, there is limited data for higher energies. The measurements by Domen and Lamperti [Med. Phys. 3, 294-301 (1976)] using electron beams in the energy range from 15 to 50 MeV can, in principle, be used to deduce values for W air , if the electron stopping power of graphite and air are known. A previous analysis of these data revealed an anomalous variation of 2% in W air as a function of the electron energy. We use Monte Carlo simulation techniques to reanalyze the original data and obtain new estimates for W air , and to investigate the source of the reported anomaly. Domen and Lamperti (DL) reported the ratio of the response of a graphite calorimeter to that of a graphite ionization chamber for broad beams of electrons with energies between 15 and 50 MeV and at different depths in graphite (including depths well beyond the range of the primary electrons, i.e., in the bremsstrahlung photon regime). Using a detailed EGSnrc model of the DL apparatus, as well as up-to-date stopping powers, we compute the dose ratio between the ionization chamber cavity and the calorimeter core, for plane-parallel electron beams. This dose ratio, multiplied by the DL measured ratio, provides a direct estimate for W air . Despite an improved analysis of the original work, the extracted values of W air still exhibit an increase as the mean electron energy at the point of measurement decreases below about 15 MeV. This anomalous trend is dubious physically, and inconsistent with extensive data for W air obtained at lower energies. A thorough sensitivity analysis indicates that this trend is unlikely to stem from errors in extrapolation and correction procedures, uncertainties in electron stopping powers, or bias in calorimetry or ionization chamber measurements. However, we find that results are quite sensitive to the intrinsic graphite mass thickness of the detectors and to the incident beam energy. The DL experiment provides data in an energy regime where the electron stopping power is insensitive to the mean excitation energy of graphite - an issue plaguing W air experiments at lower energies. Unfortunately, state-of-the-art scrutiny of the original data cannot explain the anomalous trend in terms of perturbation effects or extrapolation bias. It can only be understood in terms of speculative offsets in graphite mass thickness or beam energy. Therefore higher accuracy measurements for electron energies above 15 MeV are recommended to further resolve the value of W air . © Her Majesty the Queen in Right of Canada 2017. Reproduced with the permission of the Minister of Science.

Controlling nonsequential double ionization of Ne with parallel-polarized two-color laser pulses.

PubMed

Luo, Siqiang; Ma, Xiaomeng; Xie, Hui; Li, Min; Zhou, Yueming; Cao, Wei; Lu, Peixiang

2018-05-14

We measure the recoil-ion momentum distributions from nonsequential double ionization of Ne by two-color laser pulses consisting of a strong 800-nm field and a weak 400-nm field with parallel polarizations. The ion momentum spectra show pronounced asymmetries in the emission direction, which depend sensitively on the relative phase of the two-color components. Moreover, the peak of the doubly charged ion momentum distribution shifts gradually with the relative phase. The shifted range is much larger than the maximal vector potential of the 400-nm laser field. Those features are well recaptured by a semiclassical model. Through analyzing the correlated electron dynamics, we found that the energy sharing between the two electrons is extremely unequal at the instant of recollison. We further show that the shift of the ion momentum corresponds to the change of the recollision time in the two-color laser field. By tuning the relative phase of the two-color components, the recollision time is controlled with attosecond precision.

Non-Thermal Spectra from Pulsar Magnetospheres in the Full Electromagnetic Cascade Scenario

NASA Astrophysics Data System (ADS)

Peng, Qi-Yong; Zhang, Li

2008-08-01

We simulated non-thermal emission from a pulsar magnetosphere within the framework of a full polar-cap cascade scenario by taking the acceleration gap into account, using the Monte Carlo method. For a given electric field parallel to open field lines located at some height above the surface of a neutron star, primary electrons were accelerated by parallel electric fields and lost their energies by curvature radiation; these photons were converted to electron-positron pairs, which emitted photons through subsequent quantum synchrotron radiation and inverse Compton scattering, leading to a cascade. In our calculations, the acceleration gap was assumed to be high above the stellar surface (about several stellar radii); the primary and secondary particles and photons emitted during the journey of those particles in the magnetosphere were traced using the Monte Carlo method. In such a scenario, we calculated the non-thermal photon spectra for different pulsar parameters and compared the model results for two normal pulsars and one millisecond pulsar with the observed data.

Tempest Neoclassical Simulation of Fusion Edge Plasmas

NASA Astrophysics Data System (ADS)

Xu, X. Q.; Xiong, Z.; Cohen, B. I.; Cohen, R. H.; Dorr, M.; Hittinger, J.; Kerbel, G. D.; Nevins, W. M.; Rognlien, T. D.

2006-04-01

We are developing a continuum gyrokinetic full-F code, TEMPEST, to simulate edge plasmas. The geometry is that of a fully diverted tokamak and so includes boundary conditions for both closed magnetic flux surfaces and open field lines. The code, presently 4-dimensional (2D2V), includes kinetic ions and electrons, a gyrokinetic Poisson solver for electric field, and the nonlinear Fokker-Planck collision operator. Here we present the simulation results of neoclassical transport with Boltzmann electrons. In a large aspect ratio circular geometry, excellent agreement is found for neoclassical equilibrium with parallel flows in the banana regime without a temperature gradient. In divertor geometry, it is found that the endloss of particles and energy induces pedestal-like density and temperature profiles inside the magnetic separatrix and parallel flow stronger than the neoclassical predictions in the SOL. The impact of the X-point divertor geometry on the self-consistent electric field and geo-acoustic oscillations will be reported. We will also discuss the status of extending TEMPEST into a 5-D code.

When the lowest energy does not induce native structures: parallel minimization of multi-energy values by hybridizing searching intelligences.

PubMed

Lü, Qiang; Xia, Xiao-Yan; Chen, Rong; Miao, Da-Jun; Chen, Sha-Sha; Quan, Li-Jun; Li, Hai-Ou

2012-01-01

Protein structure prediction (PSP), which is usually modeled as a computational optimization problem, remains one of the biggest challenges in computational biology. PSP encounters two difficult obstacles: the inaccurate energy function problem and the searching problem. Even if the lowest energy has been luckily found by the searching procedure, the correct protein structures are not guaranteed to obtain. A general parallel metaheuristic approach is presented to tackle the above two problems. Multi-energy functions are employed to simultaneously guide the parallel searching threads. Searching trajectories are in fact controlled by the parameters of heuristic algorithms. The parallel approach allows the parameters to be perturbed during the searching threads are running in parallel, while each thread is searching the lowest energy value determined by an individual energy function. By hybridizing the intelligences of parallel ant colonies and Monte Carlo Metropolis search, this paper demonstrates an implementation of our parallel approach for PSP. 16 classical instances were tested to show that the parallel approach is competitive for solving PSP problem. This parallel approach combines various sources of both searching intelligences and energy functions, and thus predicts protein conformations with good quality jointly determined by all the parallel searching threads and energy functions. It provides a framework to combine different searching intelligence embedded in heuristic algorithms. It also constructs a container to hybridize different not-so-accurate objective functions which are usually derived from the domain expertise.

When the Lowest Energy Does Not Induce Native Structures: Parallel Minimization of Multi-Energy Values by Hybridizing Searching Intelligences

PubMed Central

Lü, Qiang; Xia, Xiao-Yan; Chen, Rong; Miao, Da-Jun; Chen, Sha-Sha; Quan, Li-Jun; Li, Hai-Ou

2012-01-01

Background Protein structure prediction (PSP), which is usually modeled as a computational optimization problem, remains one of the biggest challenges in computational biology. PSP encounters two difficult obstacles: the inaccurate energy function problem and the searching problem. Even if the lowest energy has been luckily found by the searching procedure, the correct protein structures are not guaranteed to obtain. Results A general parallel metaheuristic approach is presented to tackle the above two problems. Multi-energy functions are employed to simultaneously guide the parallel searching threads. Searching trajectories are in fact controlled by the parameters of heuristic algorithms. The parallel approach allows the parameters to be perturbed during the searching threads are running in parallel, while each thread is searching the lowest energy value determined by an individual energy function. By hybridizing the intelligences of parallel ant colonies and Monte Carlo Metropolis search, this paper demonstrates an implementation of our parallel approach for PSP. 16 classical instances were tested to show that the parallel approach is competitive for solving PSP problem. Conclusions This parallel approach combines various sources of both searching intelligences and energy functions, and thus predicts protein conformations with good quality jointly determined by all the parallel searching threads and energy functions. It provides a framework to combine different searching intelligence embedded in heuristic algorithms. It also constructs a container to hybridize different not-so-accurate objective functions which are usually derived from the domain expertise. PMID:23028708

Multiple piezo-patch energy harvesters integrated to a thin plate with AC-DC conversion: analytical modeling and numerical validation

NASA Astrophysics Data System (ADS)

Aghakhani, Amirreza; Basdogan, Ipek; Erturk, Alper

2016-04-01

Plate-like components are widely used in numerous automotive, marine, and aerospace applications where they can be employed as host structures for vibration based energy harvesting. Piezoelectric patch harvesters can be easily attached to these structures to convert the vibrational energy to the electrical energy. Power output investigations of these harvesters require accurate models for energy harvesting performance evaluation and optimization. Equivalent circuit modeling of the cantilever-based vibration energy harvesters for estimation of electrical response has been proposed in recent years. However, equivalent circuit formulation and analytical modeling of multiple piezo-patch energy harvesters integrated to thin plates including nonlinear circuits has not been studied. In this study, equivalent circuit model of multiple parallel piezoelectric patch harvesters together with a resistive load is built in electronic circuit simulation software SPICE and voltage frequency response functions (FRFs) are validated using the analytical distributedparameter model. Analytical formulation of the piezoelectric patches in parallel configuration for the DC voltage output is derived while the patches are connected to a standard AC-DC circuit. The analytic model is based on the equivalent load impedance approach for piezoelectric capacitance and AC-DC circuit elements. The analytic results are validated numerically via SPICE simulations. Finally, DC power outputs of the harvesters are computed and compared with the peak power amplitudes in the AC output case.

Automatic Energy Schemes for High Performance Applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sundriyal, Vaibhav

Although high-performance computing traditionally focuses on the efficient execution of large-scale applications, both energy and power have become critical concerns when approaching exascale. Drastic increases in the power consumption of supercomputers affect significantly their operating costs and failure rates. In modern microprocessor architectures, equipped with dynamic voltage and frequency scaling (DVFS) and CPU clock modulation (throttling), the power consumption may be controlled in software. Additionally, network interconnect, such as Infiniband, may be exploited to maximize energy savings while the application performance loss and frequency switching overheads must be carefully balanced. This work first studies two important collective communication operations, all-to-allmore » and allgather and proposes energy saving strategies on the per-call basis. Next, it targets point-to-point communications to group them into phases and apply frequency scaling to them to save energy by exploiting the architectural and communication stalls. Finally, it proposes an automatic runtime system which combines both collective and point-to-point communications into phases, and applies throttling to them apart from DVFS to maximize energy savings. The experimental results are presented for NAS parallel benchmark problems as well as for the realistic parallel electronic structure calculations performed by the widely used quantum chemistry package GAMESS. Close to the maximum energy savings were obtained with a substantially low performance loss on the given platform.« less

An electrostatic deceleration lens for highly charged ions.

PubMed

Rajput, J; Roy, A; Kanjilal, D; Ahuja, R; Safvan, C P

2010-04-01

The design and implementation of a purely electrostatic deceleration lens used to obtain beams of highly charged ions at very low energies is presented. The design of the lens is such that it can be used with parallel as well as diverging incoming beams and delivers a well focused low energy beam at the target. In addition, tuning of the final energy of the beam over a wide range (1 eV/q to several hundred eV/q, where q is the beam charge state) is possible without any change in hardware configuration. The deceleration lens was tested with Ar(8+), extracted from an electron cyclotron resonance ion source, having an initial energy of 30 keV/q and final energies as low as 70 eV/q have been achieved.

New generation of Cherenkov counters

NASA Astrophysics Data System (ADS)

Giomataris, Y.; Charpak, G.; Peskov, V.; Sauli, F.

1992-12-01

Experimental results with a parallel plate avalanche chamber (PPAC) having a CsI photocathode and pad array readout are reported. High gains in excess of 10 5 have been obtained with He gas at atmospheric pressure and traces of CH 4 or CF 4 quencher. Such light gas mixtures extend the transparency for the Cherenkov light to the extreme UV region and allow detector operation with very low sensitivity to the ionization produced by minimum ionizing particles. A hadron blind detector (HBD) is discussed which exploits the broad photon energy bandwidth (≈ 10 eV) and the high Cherenkov threshold ( pπ = 15 GeV). This fast detector, since it has a good spatial resolution, can be used at the future Large Hadron Collider (LHC) or the Superconductivity Super Collider (SSC) either as an efficient electron tagger, rejecting hadrons faking electrons in the calorimeter, or as a pretracker giving fast electron and high-energy muon signature and momentum estimation. Other potential applications in the domain of Cherenkov light detection are also discussed.

ecode - Electron Transport Algorithm Testing v. 1.0

DOE Office of Scientific and Technical Information (OSTI.GOV)

Franke, Brian C.; Olson, Aaron J.; Bruss, Donald Eugene

2016-10-05

ecode is a Monte Carlo code used for testing algorithms related to electron transport. The code can read basic physics parameters, such as energy-dependent stopping powers and screening parameters. The code permits simple planar geometries of slabs or cubes. Parallelization consists of domain replication, with work distributed at the start of the calculation and statistical results gathered at the end of the calculation. Some basic routines (such as input parsing, random number generation, and statistics processing) are shared with the Integrated Tiger Series codes. A variety of algorithms for uncertainty propagation are incorporated based on the stochastic collocation and stochasticmore » Galerkin methods. These permit uncertainty only in the total and angular scattering cross sections. The code contains algorithms for simulating stochastic mixtures of two materials. The physics is approximate, ranging from mono-energetic and isotropic scattering to screened Rutherford angular scattering and Rutherford energy-loss scattering (simple electron transport models). No production of secondary particles is implemented, and no photon physics is implemented.« less

Relaxation of ferroelectric states in 2D distributions of quantum dots: EELS simulation

NASA Astrophysics Data System (ADS)

Cortés, C. M.; Meza-Montes, L.; Moctezuma, R. E.; Carrillo, J. L.

2016-06-01

The relaxation time of collective electronic states in a 2D distribution of quantum dots is investigated theoretically by simulating EELS experiments. From the numerical calculation of the probability of energy loss of an electron beam, traveling parallel to the distribution, it is possible to estimate the damping time of ferroelectric-like states. We generate this collective response of the distribution by introducing a mean field interaction among the quantum dots, and then, the model is extended incorporating effects of long-range correlations through a Bragg-Williams approximation. The behavior of the dielectric function, the energy loss function, and the relaxation time of ferroelectric-like states is then investigated as a function of the temperature of the distribution and the damping constant of the electronic states in the single quantum dots. The robustness of the trends and tendencies of our results indicate that this scheme of analysis can guide experimentalists to develop tailored quantum dots distributions for specific applications.

Spacecraft Observations of a ULF Wave Injected Onto Field Lines by SPEAR

NASA Astrophysics Data System (ADS)

Badman, S. V.; Wright, D. M.; Yeoman, T. K.; Clausen, L. B.; Fear, R. C.; Fazakerley, A. N.; Lucek, E. A.

2008-12-01

SPEAR (Space Exploration by Active Radar) is an ionospheric heating facility situated on Svalbard which is capable of exciting ULF waves on local magnetic field lines. Field-guided ULF waves can interact with the ionospheric Alfvén resonator (IAR) and produce parallel electric fields, which then accelerate electrons along the field line. Detection and study of these waves thus provides information on the properties of the IAR and auroral acceleration processes. We examine an interval from 1 February 2006 when SPEAR was transmitting with a 5 min on-off cycle. During this interval the Cluster spacecraft passed over the heater site. We discuss signatures of the SPEAR-generated wave identified in the Cluster field and electron measurements. One feature of interest is the periodic enhancement of electron fluxes in two broad energy bands (~10-100 eV and ~100-1000 eV) which occur out of phase with each other in the two different energy bands.

Investigation of electron-loss and photon scattering correction factors for FAC-IR-300 ionization chamber

NASA Astrophysics Data System (ADS)

Mohammadi, S. M.; Tavakoli-Anbaran, H.; Zeinali, H. Z.

2017-02-01

The parallel-plate free-air ionization chamber termed FAC-IR-300 was designed at the Atomic Energy Organization of Iran, AEOI. This chamber is used for low and medium X-ray dosimetry on the primary standard level. In order to evaluate the air-kerma, some correction factors such as electron-loss correction factor (ke) and photon scattering correction factor (ksc) are needed. ke factor corrects the charge loss from the collecting volume and ksc factor corrects the scattering of photons into collecting volume. In this work ke and ksc were estimated by Monte Carlo simulation. These correction factors are calculated for mono-energy photon. As a result of the simulation data, the ke and ksc values for FAC-IR-300 ionization chamber are 1.0704 and 0.9982, respectively.

Two-dimensional radiant energy array computers and computing devices

NASA Technical Reports Server (NTRS)

Schaefer, D. H.; Strong, J. P., III (Inventor)

1976-01-01

Two dimensional digital computers and computer devices operate in parallel on rectangular arrays of digital radiant energy optical signal elements which are arranged in ordered rows and columns. Logic gate devices receive two input arrays and provide an output array having digital states dependent only on the digital states of the signal elements of the two input arrays at corresponding row and column positions. The logic devices include an array of photoconductors responsive to at least one of the input arrays for either selectively accelerating electrons to a phosphor output surface, applying potentials to an electroluminescent output layer, exciting an array of discrete radiant energy sources, or exciting a liquid crystal to influence crystal transparency or reflectivity.

Atomic scale structure and chemistry of interfaces by Z-contrast imaging and electron energy loss spectroscopy in the stem

DOE Office of Scientific and Technical Information (OSTI.GOV)

McGibbon, M.M.; Browning, N.D.; Chisholm, M.F.

The macroscopic properties of many materials are controlled by the structure and chemistry at grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted directly without the need for preconceived atomic structure models. Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementarymore » chemical information on an atomic scale. The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface. In this paper we use the complimentary techniques of high resolution Z-contrast imaging and PEELS to investigate the atomic structure and chemistry of a 25{degree} symmetric tilt boundary in a bicrystal of the electroceramic SrTiO{sub 3}.« less

Determination of absorbed dose to water for high-energy photon and electron beams-comparison of the standards DIN 6800-2 (1997), IAEA TRS 398 (2000) and DIN 6800-2 (2006)

PubMed Central

Zakaria, Golam Abu; Schuette, Wilhelm

2007-01-01

For the determination of the absorbed dose to water for high-energy photon and electron beams the IAEA code of practice TRS-398 (2000) is applied internationally. In Germany, the German dosimetry protocol DIN 6800-2 (1997) is used. Recently, the DIN standard has been revised and published as Draft National Standard DIN 6800-2 (2006). It has adopted widely the methodology and dosimetric data of the code of practice. This paper compares these three dosimetry protocols systematically and identifies similarities as well as differences. The investigation was done with 6 and 18 MV photon as well as 5 to 21 MeV electron beams. While only cylindrical chambers were used for photon beams, measurements of electron beams were performed using cylindrical as well as plane-parallel chambers. The discrepancies in the determination of absorbed dose to water between the three protocols were 0.4% for photon beams and 1.5% for electron beams. Comparative measurements showed a deviation of less than 0.5% between our measurements following protocol DIN 6800-2 (2006) and TLD inter-comparison procedure in an external audit. PMID:21217912

Determination of absorbed dose to water for high-energy photon and electron beams-comparison of the standards DIN 6800-2 (1997), IAEA TRS 398 (2000) and DIN 6800-2 (2006).

PubMed

Zakaria, Golam Abu; Schuette, Wilhelm

2007-01-01

For the determination of the absorbed dose to water for high-energy photon and electron beams the IAEA code of practice TRS-398 (2000) is applied internationally. In Germany, the German dosimetry protocol DIN 6800-2 (1997) is used. Recently, the DIN standard has been revised and published as Draft National Standard DIN 6800-2 (2006). It has adopted widely the methodology and dosimetric data of the code of practice. This paper compares these three dosimetry protocols systematically and identifies similarities as well as differences. The investigation was done with 6 and 18 MV photon as well as 5 to 21 MeV electron beams. While only cylindrical chambers were used for photon beams, measurements of electron beams were performed using cylindrical as well as plane-parallel chambers. The discrepancies in the determination of absorbed dose to water between the three protocols were 0.4% for photon beams and 1.5% for electron beams. Comparative measurements showed a deviation of less than 0.5% between our measurements following protocol DIN 6800-2 (2006) and TLD inter-comparison procedure in an external audit.

Photoelectron emission from LiF surfaces by ultrashort electromagnetic pulses

DOE Office of Scientific and Technical Information (OSTI.GOV)

Acuna, M. A.; Gravielle, M. S.; Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires

2011-03-15

Energy- and angle-resolved electron emission spectra produced by incidence of ultrashort electromagnetic pulses on a LiF(001) surface are studied by employing a distorted-wave method named the crystal surface-Volkov (CSV) approximation. The theory makes use of the Volkov phase to describe the action of the external electric field on the emitted electron, while the electron-surface interaction is represented within the tight-binding model. The CSV approach is applied to investigate the effects introduced by the crystal lattice when the electric field is oriented parallel to the surface plane. These effects are essentially governed by the vector potential of the external field, whilemore » the influence of the crystal orientation was found to be negligible.« less

Characterization of Localized Corrosion in an Al-Cu-Li Alloy

NASA Astrophysics Data System (ADS)

Luo, Chen; Zhang, Xinxin; Zhou, Xiaorong; Sun, Zhihua; Zhang, Xiaoyun; Tang, Zhihui; Lu, Feng; Thompson, George E.

2016-05-01

Corrosion behaviors of recently developed 2A97-T6 aluminum-copper-lithium alloy in sodium chloride solution are investigated using scanning electron and transmission electron microscopies in conjunction with electron backscatter diffraction. It has been found that corrosion product rings were established on the alloy surface as early as 5 min during immersion in sodium chloride solution. Meanwhile, hydrogen continuously evolved from within the rings. Pitting corrosion is evident with crystallographic dependant corrosion channel facets mainly parallel to {100} planes. Non-uniform distribution of misorientation in the 2A97 aluminum alloy results in a portion of grains of relatively high stored energy. Such grains were preferentially attacked, serving as local anodes, during the development of crystallographic pitting.

Re-Visiting the Electronic Energy Map of the Copper Dimer by Double-Resonant Four-Wave Mixing

NASA Astrophysics Data System (ADS)

Visser, Bradley; Bornhauser, Peter; Beck, Martin; Knopp, Gregor; Marquardt, Roberto; Gourlaouen, Christophe; van Bokhoven, Jeroen A.; Radi, Peter

2017-06-01

The copper dimer is one of the most studied transition metal (TM) diatomics due to its alkali-metal like electronic shell structure, strongly bound ground state and chemical reactivity. The high electronic promotion energy in the copper atom yields numerous low-lying electronic states compared to TM dimers with d)-hole electronic configurations. Thus, through extensive study the excited electronic structure of Cu_2 is relatively well known, however in practice few excited states have been investigated with rotational resolution or even assigned term symbols or dissociation limits. The spectroscopic methods that have been used to investigate the copper dimer until now have not possessed sufficient spectral selectivity, which has complicated the analysis of the often overlapping transitions. Resonant four-wave mixing is a non-linear absorption based spectroscopic method. In favorable cases, the two-color version (TC-RFWM) enables purely optical mass selective spectral measurements in a mixed molecular beam. Additionally, by labelling individual rotational levels in the common intermediate state the spectra are dramatically simplified. In this work, we report on the rotationally resolved characterization of low-lying electronic states of dicopper. Several term symbols have been assigned unambiguously. De-perturbation studies performed shed light on the complex electronic structure of the molecule. Furthermore, a new low-lying electronic state of Cu_2 is discovered and has important implications for the high-level theoretical structure calculations performed in parallel. In fact, the ab initio methods applied yield relative energies among the electronic levels that are almost quantitative and allow assignment of the newly observed state that is governed by spin-orbit interacting levels.

Plasma potential and electron temperature evaluated by ball-pen and Langmuir probes in the COMPASS tokamak

NASA Astrophysics Data System (ADS)

Dimitrova, M.; Popov, Tsv K.; Adamek, J.; Kovačič, J.; Ivanova, P.; Hasan, E.; López-Bruna, D.; Seidl, J.; Vondráček, P.; Dejarnac, R.; Stöckel, J.; Imríšek, M.; Panek, R.; the COMPASS Team

2017-12-01

The radial distributions of the main plasma parameters in the scrape-off-layer of the COMPASS tokamak are measured during L-mode and H-mode regimes by using both Langmuir and ball-pen probes mounted on a horizontal reciprocating manipulator. The radial profile of the plasma potential derived previously from Langmuir probes data by using the first derivative probe technique is compared with data derived using ball-pen probes. A good agreement can be seen between the data acquired by the two techniques during the L-mode discharge and during the H-mode regime within the inter-ELM periods. In contrast with the first derivative probe technique, the ball-pen probe technique does not require a swept voltage and, therefore, the temporal resolution is only limited by the data acquisition system. In the electron temperature evaluation, in the far scrape-off layer and in the limiter shadow, where the electron energy distribution is Maxwellian, the results from both techniques match well. In the vicinity of the last closed flux surface, where the electron energy distribution function is bi-Maxwellian, the ball-pen probe technique results are in agreement with the high-temperature components of the electron distribution only. We also discuss the application of relatively large Langmuir probes placed in parallel and perpendicularly to the magnetic field lines to studying the main plasma parameters. The results obtained by the two types of the large probes agree well. They are compared with Thomson scattering data for electron temperatures and densities. The results for the electron densities are compared also with the results from ASTRA code calculation of the electron source due to the ionization of the neutrals by fast electrons and the origin of the bi-Maxwellian electron energy distribution function is briefly discussed.

Energy and pitch angle-dispersed auroral electrons suggesting a time-variable, inverted-V potential structure

NASA Astrophysics Data System (ADS)

Arnoldy, R. L.; Lynch, K. A.; Austin, J. B.; Kintner, P. M.

1999-10-01

High temporal resolution electron detectors aboard the PHAZE II rocket flight have shown that the energy-dispersed, field-aligned bursts (FABs) are time coincident with pitch angle-dispersed electrons having energies at the maximum voltage of the inverted-V potential. This modulation of the energetic inverted-V electrons is superimposed upon an energy-diffused background resulting in a peak-to-valley ratio of ~2 for the pitch angle-dispersed electrons. Since the characteristic energy of the FABs, the order of an eV, is considerably less than that of the plasma sheet electrons (the order of a keV) presumably falling through the inverted-V potential to create the discrete aurora, the modulation mechanism has to be independent of the electron temperature. The mechanism must accelerate the cold electrons over a range of energies from the inverted-V energy down to a few tens of eV. It must do this at the same time it is creating a population of hot, pitch angle-dispersed electrons at the inverted-V energy. Both the energy dispersion of the FABs and the pitch angle dispersion of the inverted-V electrons can be used to determine a source height assuming both populations start from the same source region at the same time. These calculations give source heights between 3500 and 5300 km for various events and disagreement between the two methods the order of 20%, which is within the rather substantial error limits of both calculations. A simple mechanism of providing a common start time for both populations of electrons would be a turning on/off of a spatially limited (vertically), inverted-V potential. The energy-dispersed FABs can be reconstructed at rocket altitudes if one assumes that cold electrons are accelerated to an energy determined by how much of the inverted-V potential they fall through when it is turned on. Similarly, the pitch angle-dispersed, inverted-V electrons can be modeled at rocket altitudes if one assumes that the plasma sheet electrons falling through the entire potential drop all start to do so at the same time when the potential is turned on. The FABs seem to fluctuate at either ~10 Hz or near 100 Hz. An important constraint of the on/off mechanism is whether cold electrons (1 eV) can fill the inverted-V volume during the off cycle. The maximum vertical height of the 10 kV potential region for the 10 Hz events would be the order of 100 and 10 km for the 100 Hz events. To get 10 kV, these heights require parallel electric fields of 0.1 and 1 V/m respectively for the 10 and 100 Hz events assuming that the filling is along B from below the inverted-V potential. Alternative mechanisms are also discussed in the light of the data presented.

Parallel computation with molecular-motor-propelled agents in nanofabricated networks.

PubMed

Nicolau, Dan V; Lard, Mercy; Korten, Till; van Delft, Falco C M J M; Persson, Malin; Bengtsson, Elina; Månsson, Alf; Diez, Stefan; Linke, Heiner; Nicolau, Dan V

2016-03-08

The combinatorial nature of many important mathematical problems, including nondeterministic-polynomial-time (NP)-complete problems, places a severe limitation on the problem size that can be solved with conventional, sequentially operating electronic computers. There have been significant efforts in conceiving parallel-computation approaches in the past, for example: DNA computation, quantum computation, and microfluidics-based computation. However, these approaches have not proven, so far, to be scalable and practical from a fabrication and operational perspective. Here, we report the foundations of an alternative parallel-computation system in which a given combinatorial problem is encoded into a graphical, modular network that is embedded in a nanofabricated planar device. Exploring the network in a parallel fashion using a large number of independent, molecular-motor-propelled agents then solves the mathematical problem. This approach uses orders of magnitude less energy than conventional computers, thus addressing issues related to power consumption and heat dissipation. We provide a proof-of-concept demonstration of such a device by solving, in a parallel fashion, the small instance {2, 5, 9} of the subset sum problem, which is a benchmark NP-complete problem. Finally, we discuss the technical advances necessary to make our system scalable with presently available technology.

First Direct Observation of Runaway-Electron-Driven Whistler Waves in Tokamaks

NASA Astrophysics Data System (ADS)

Spong, D. A.; Heidbrink, W. W.; Paz-Soldan, C.; Du, X. D.; Thome, K. E.; Van Zeeland, M. A.; Collins, C.; Lvovskiy, A.; Moyer, R. A.; Austin, M. E.; Brennan, D. P.; Liu, C.; Jaeger, E. F.; Lau, C.

2018-04-01

DIII-D experiments at low density (ne˜1019 m-3 ) have directly measured whistler waves in the 100-200 MHz range excited by multi-MeV runaway electrons. Whistler activity is correlated with runaway intensity (hard x-ray emission level), occurs in novel discrete frequency bands, and exhibits nonlinear limit-cycle-like behavior. The measured frequencies scale with the magnetic field strength and electron density as expected from the whistler dispersion relation. The modes are stabilized with increasing magnetic field, which is consistent with wave-particle resonance mechanisms. The mode amplitudes show intermittent time variations correlated with changes in the electron cyclotron emission that follow predator-prey cycles. These can be interpreted as wave-induced pitch angle scattering of moderate energy runaways. The tokamak runaway-whistler mechanisms have parallels to whistler phenomena in ionospheric plasmas. The observations also open new directions for the modeling and active control of runaway electrons in tokamaks.

Design and performance of the spin asymmetries of the nucleon experiment

NASA Astrophysics Data System (ADS)

Maxwell, J. D.; Armstrong, W. R.; Choi, S.; Jones, M. K.; Kang, H.; Liyanage, A.; Meziani, Z.-E.; Mulholland, J.; Ndukum, L.; Rondón, O. A.; Ahmidouch, A.; Albayrak, I.; Asaturyan, A.; Ates, O.; Baghdasaryan, H.; Boeglin, W.; Bosted, P.; Brash, E.; Brock, J.; Butuceanu, C.; Bychkov, M.; Carlin, C.; Carter, P.; Chen, C.; Chen, J.-P.; Christy, M. E.; Covrig, S.; Crabb, D.; Danagoulian, S.; Daniel, A.; Davidenko, A. M.; Davis, B.; Day, D.; Deconinck, W.; Deur, A.; Dunne, J.; Dutta, D.; El Fassi, L.; Elaasar, M.; Ellis, C.; Ent, R.; Flay, D.; Frlez, E.; Gaskell, D.; Geagla, O.; German, J.; Gilman, R.; Gogami, T.; Gomez, J.; Goncharenko, Y. M.; Hashimoto, O.; Higinbotham, D. W.; Horn, T.; Huber, G. M.; Jones, M.; Kalantarians, N.; Kang, H. K.; Kawama, D.; Keith, C.; Keppel, C.; Khandaker, M.; Kim, Y.; King, P. M.; Kohl, M.; Kovacs, K.; Kubarovsky, V.; Li, Y.; Liyanage, N.; Luo, W.; Mamyan, V.; Markowitz, P.; Maruta, T.; Meekins, D.; Melnik, Y. M.; Mkrtchyan, A.; Mkrtchyan, H.; Mochalov, V. V.; Monaghan, P.; Narayan, A.; Nakamura, S. N.; Nuruzzaman; Pentchev, L.; Pocanic, D.; Posik, M.; Puckett, A.; Qiu, X.; Reinhold, J.; Riordan, S.; Roche, J.; Sawatzky, B.; Shabestari, M.; Slifer, K.; Smith, G.; Soloviev, L.; Solvignon, P.; Tadevosyan, V.; Tang, L.; Vasiliev, A. N.; Veilleux, M.; Walton, T.; Wesselmann, F.; Wood, S. A.; Yao, H.; Ye, Z.; Zhu, L.

2018-03-01

The Spin Asymmetries of the Nucleon Experiment (SANE) performed inclusive, double-polarized electron scattering measurements of the proton at the Continuous Electron Beam Accelerator Facility at Jefferson Lab. A novel detector array observed scattered electrons of four-momentum transfer 2 . 5

Two-dimensional quasi-neutral description of particles and fields above discrete auroral arcs

NASA Technical Reports Server (NTRS)

Newman, A. L.; Chiu, Y. T.; Cornwall, J. M.

1986-01-01

Models are presented for particle distributions, electric fields and currents in an adiabatic treatment of auroral electrostatic potential distributions in order to describe the quiet-time evening auroral arcs featuring both upward and return currents. The models are consistent with current continuity and charge balance requirements for particle populations controlled by adiabatic invariants and quasi-neutrality in the magnetosphere. The effective energy of the cool electron population is demonstrated to have a significant effect on the latitudinal breadth of the auroral electrostatic potential structure and the extent of the penetration of the accelerating potential into the ionosphere. Another finding is that the energy of any parallel potential drop in the lowest few thousand kilometers of the field line is of the same order of magnitude as the thermal energy of the cool electrons. Additional predictions include density cavities along field lines that support large potential drops, and density enhancements along field lines at the edge of an inverted V with a small potential drop.

Heavy ion beam-ionosphere interactions - Electron acceleration

NASA Technical Reports Server (NTRS)

Kaufmann, R. L.; Arnoldy, R. L.; Moore, T. E.; Kintner, P. M.; Cahill, L. J., Jr.

1985-01-01

Moore et al. (1982) described a number of unexpected effects which were observed during the first Argon Release Controlled Study (ARCS 1, or rocket flight 29:014). The present paper provides a description of detailed analyses of the interaction of the argon beam with the ionosphere. An important feature of the considered test was that all detectors and the Ar(+) gun remained attached to the rocket throughout the flight. It is pointed out that the most dramatic effect of ion gun operation on ARCS 1 involved large changes in the fluxes of electrons with energies below about 600 eV. The observations are discussed, taking into account the distribution functions, azimuth dependence, and electron and ion trajectories. Attention is given to the perpendicular ion beam, the parallel ion beam, the acceleration of downgoing and upgoing electrons, and aspects of wave generation.

Beam dynamics in MABE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Poukey, J.W.; Coleman, P.D.; Sanford, T.W.L.

1985-01-01

MABE is a multistage linear electron accelerator which accelerates up to nine beams in parallel. Nominal parameters per beam are 25 kA, final energy 7 MeV, and guide field 20 kG. We report recent progress via theory and simulation in understanding the beam dynamics in such a system. In particular, we emphasize our results on the radial oscillations and emittance growth for a beam passing through a series of accelerating gaps. 12 refs., 8 figs.

Liquid-phase deposition of thin Si films by ballistic electro-reduction

NASA Astrophysics Data System (ADS)

Ohta, T.; Gelloz, B.; Kojima, A.; Koshida, N.

2013-01-01

It is shown that the nanocryatalline silicon ballistic electron emitter operates in a SiCl4 solution without using any counter electrodes and that thin amorphous Si films are efficiently deposited on the emitting surface with no contaminations and by-products. Despite the large electrochemical window of the SiCl4 solution, electrons injected with sufficiently high energies preferentially reduce Si4+ ions at the interface. Using an emitter with patterned line emission windows, a Si-wires array can be formed in parallel. This low-temperature liquid-phase deposition technique provides an alternative clean process for power-effective fabrication of advanced thin Si film structures and devices.

A vortex line for K-shell ionization of a carbon atom by electron impact

NASA Astrophysics Data System (ADS)

Ward, S. J.; Macek, J. H.

2014-10-01

We obtained using the Coulomb-Born approximation a deep minimum in the TDCS for K-shell ionization of a carbon atom by electron impact for the electron ejected in the scattering plane. The minimum is obtained for the kinematics of the energy of incident electron Ei = 1801.2 eV, the scattering angle θf = 4°, the energy of the ejected electron Ek = 5 . 5 eV, and the angle for the ejected electron θk = 239°. This minimum is due to a vortex in the velocity field. At the position of the vortex, the nodal lines of Re [ T ] and Im [ T ] intersect. We decomposed the CB1 T-matrix into its multipole components for the kinematics of a vortex, taking the z'-axis parallel to the direction of the momentum transfer vector. The m = +/- 1 dipole components are necessary to obtain a vortex. We also considered the electron to be ejected out of the scattering plane and obtained the positions of the vortex for different values of the y-component of momentum of the ejected electron, ky. We constructed the vortex line for the kinematics of Ei = 1801.2 eV and θf = 4°. S.J.W. and J.H.M. acknowledge support from NSF under Grant No. PHYS- 0968638 and from D.O.E. under Grant Number DE-FG02-02ER15283, respectively.

Simulating electron wave dynamics in graphene superlattices exploiting parallel processing advantages

NASA Astrophysics Data System (ADS)

Rodrigues, Manuel J.; Fernandes, David E.; Silveirinha, Mário G.; Falcão, Gabriel

2018-01-01

This work introduces a parallel computing framework to characterize the propagation of electron waves in graphene-based nanostructures. The electron wave dynamics is modeled using both "microscopic" and effective medium formalisms and the numerical solution of the two-dimensional massless Dirac equation is determined using a Finite-Difference Time-Domain scheme. The propagation of electron waves in graphene superlattices with localized scattering centers is studied, and the role of the symmetry of the microscopic potential in the electron velocity is discussed. The computational methodologies target the parallel capabilities of heterogeneous multi-core CPU and multi-GPU environments and are built with the OpenCL parallel programming framework which provides a portable, vendor agnostic and high throughput-performance solution. The proposed heterogeneous multi-GPU implementation achieves speedup ratios up to 75x when compared to multi-thread and multi-core CPU execution, reducing simulation times from several hours to a couple of minutes.

EFFECTS OF THE TEMPERATURE ANISOTROPY ON THE MASER INSTABILITY EXCITED BY LOWER ENERGY CUTOFF BEHAVIOR

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tang, J. F.; Wu, D. J.; Yan, Y. H., E-mail: djwu@pmo.ac.cn, E-mail: djwu@pmo.ac.cn

The electron-cyclotron maser (ECM) conventionally driven by velocity anisotropies of energetic electrons trapped in magnetic fields is one of the most important radio-emission mechanisms in astrophysics. Recently, Wu and Tang proposed that a proper lower energy cutoff behavior of power-law electrons can effectively excite the ECM emission. This paper considers effects of temperature anisotropy on this new ECM mechanism. The results show that the growth rates of the ECM emissions increase with {beta}{sub perpendicular0} and {beta}{sub ||0}, where {beta}{sub perpendicular0} and {beta}{sub ||0} are the perpendicular and parallel velocity spreads (in units of the light velocity c) of the energeticmore » electron beam, respectively. Moreover, the growth rates of O1 and X2 modes both sensitively depend on the ratio of the electron-cyclotron frequency to the plasma frequency {Omega} and reach their extremum values at {Omega} {approx_equal} 1.5 for the O1 mode and at {Omega} {approx_equal} 1.0 for the X2 mode. Meanwhile, as the mean velocity of the electron beam {beta}{sub s} (in units of c) increases, the growth rate of the O1 mode remains approximately constant and that of the X2 mode decreases considerably.« less

Numerical computation of solar neutrino flux attenuated by the MSW mechanism

NASA Astrophysics Data System (ADS)

Kim, Jai Sam; Chae, Yoon Sang; Kim, Jung Dae

1999-07-01

We compute the survival probability of an electron neutrino in its flight through the solar core experiencing the Mikheyev-Smirnov-Wolfenstein effect with all three neutrino species considered. We adopted a hybrid method that uses an accurate approximation formula in the non-resonance region and numerical integration in the non-adiabatic resonance region. The key of our algorithm is to use the importance sampling method for sampling the neutrino creation energy and position and to find the optimum radii to start and stop numerical integration. We further developed a parallel algorithm for a message passing parallel computer. By using an idea of job token, we have developed a dynamical load balancing mechanism which is effective under any irregular load distributions

Xyce

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thomquist, Heidi K.; Fixel, Deborah A.; Fett, David Brian

The Xyce Parallel Electronic Simulator simulates electronic circuit behavior in DC, AC, HB, MPDE and transient mode using standard analog (DAE) and/or device (PDE) device models including several age and radiation aware devices. It supports a variety of computing platforms (both serial and parallel) computers. Lastly, it uses a variety of modern solution algorithms dynamic parallel load-balancing and iterative solvers.

The Auroral Field-aligned Acceleration - Cluster Results

NASA Astrophysics Data System (ADS)

Vaivads, A.; Cluster Auroral Team

The four Cluster satellites cross the auroral field lines at altitudes well above most of acceleration region. Thus, the orbit is appropriate for studies of the generator side of this region. We consider the energy transport towards the acceleration region and different mechanisms for generating the potential drop. Using data from Cluster we can also for the first time study the dynamics of the generator on a minute scale. We present data from a few auroral field crossings where Cluster are in conjunction with DMSP satellites. We use electric and magnetic field data to estimate electrostatic po- tential along the satellite orbit, Poynting flux as well as the presence of plasma waves. These we can compare with data from particle and wave instruments on Cluster and on low latitude satellites to try to make a consistent picture of the acceleration region formation in these cases. Preliminary results show close agreement both between in- tegrated potential values at Cluster and electron peak energies at DMSP as well as close agreement between the integrated Poynting flux values at Cluster and the elec- tron energy flux at DMSP. At the end we draw a parallels between auroral electron acceleration and electron acceleration at the magnetopause.

Sci-Thur PM: YIS - 07: Monte Carlo simulations to obtain several parameters required for electron beam dosimetry.

PubMed

Muir, B; Rogers, D; McEwen, M

2012-07-01

When current dosimetry protocols were written, electron beam data were limited and had uncertainties that were unacceptable for reference dosimetry. Protocols for high-energy reference dosimetry are currently being updated leading to considerable interest in accurate electron beam data. To this end, Monte Carlo simulations using the EGSnrc user-code egs_chamber are performed to extract relevant data for reference beam dosimetry. Calculations of the absorbed dose to water and the absorbed dose to the gas in realistic ion chamber models are performed as a function of depth in water for cobalt-60 and high-energy electron beams between 4 and 22 MeV. These calculations are used to extract several of the parameters required for electron beam dosimetry - the beam quality specifier, R 50 , beam quality conversion factors, k Q and k R50 , the electron quality conversion factor, k' R50 , the photon-electron conversion factor, k ecal , and ion chamber perturbation factors, P Q . The method used has the advantage that many important parameters can be extracted as a function of depth instead of determination at only the reference depth as has typically been done. Results obtained here are in good agreement with measured and other calculated results. The photon-electron conversion factors obtained for a Farmer-type NE2571 and plane-parallel PTW Roos, IBA NACP-02 and Exradin A11 chambers are 0.903, 0.896, 0.894 and 0.906, respectively. These typically differ by less than 0.7% from the contentious TG-51 values but have much smaller systematic uncertainties. These results are valuable for reference dosimetry of high-energy electron beams. © 2012 American Association of Physicists in Medicine.

SU-E-T-149: Electron Beam Profile Differences Between Elekta MLCi2 and Elekta Agility Treatment Heads

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, C; Hatcher, C

2014-06-01

Purpose: To report and investigate observed differences in electron beam profiles at various energies/applicators between Elekta MLCi2 and Agility treatment head on Elekta Infinity LINAC Methods: When we upgraded from MLCi2 to Agility on one of our Elekta Infinity LINAC's, electron beam PDDs and profiles were acquired for comparison purpose. All clinical electron energies (6/9/12/15/12/18 MeV) and electron applicators (6/10/14/20/25 square) were included in measurement. PDDs were acquired at 100 SSD in water (PTW MP3 water tank) with a plane-parallel ion chamber (PTW Roos). X and Y Profiles were acquired using IC Profiler (Sun Nuclear Corp.) at 1cm and maximummore » PDD depths (water equivalent). Results: All PDD curves match very well between MLCi2 and Agility treatment head. However, some significant differences on electron profiles were found. On Agility, even after increasing the default auto-tracking offset values for backup diaphragms in Y and MLC in X by 2.8 cm (the maximum allowed change is 3.0 cm), electron profiles still have rounder shoulders comparing to corresponding MLCi2 profiles. This difference is significantly more pronounced at larger applicators (20 and 25 square), for all electron energies. Conclusion: The significant design change between MLCi2 and Agility beam limiting device seems to affect exit electron beam profiles. In IEC1217 X direction, the main change on Agility is the removal of the original MLCi2 X backup diaphragms and replacing it with MLC leaves; In Y direction, the main change is the radius and materials on Y backup diaphragms.« less

Kinetic Studies of Thin Current Sheets at Magnetosheath Jets

NASA Astrophysics Data System (ADS)

Eriksson, E.; Vaivads, A.; Khotyaintsev, Y. V.; Graham, D. B.; Yordanova, E.; Hietala, H.; Markidis, S.; Giles, B. L.; Andre, M.; Russell, C. T.; Le Contel, O.; Burch, J. L.

2017-12-01

In near-Earth space one of the most turbulent plasma environments is the magnetosheath (MSH) downstream of the quasi-parallel shock. The particle acceleration and plasma thermalization processes there are still not fully understood. Regions of strong localized currents are believed to play a key role in those processes. The Magnetospheric Multiscale (MMS) mission has sufficiently high cadence to study these processes in detail. We present details of studies of two different events that contain strong current regions inside the MSH downstream of the quasi-parallel shock. In both cases the shape of the current region is in the form of a sheet, however they show internal 3D structure on the scale of the spacecraft separation (15 and 20 km, respectively). Both current sheets have a normal magnetic field component different from zero indicating that the regions at the different sides of the current sheets are magnetically connected. Both current sheets are boundaries between two different plasma regions. Furthermore, both current sheets are observed at MSH jets. These jets are characterized by localized dynamic pressure being larger than the solar wind dynamic pressure. One current sheet does not seem to be reconnecting while the other shows reconnection signatures. Inside the non-reconnecting current sheet we observe locally accelerated electron beams along the magnetic field. At energies above the beam energy we observe a loss cone consistent with part of the hot MSH-like electrons escaping into the colder solar wind-like plasma. This suggests that the acceleration process within this current sheet is similar to the one that occurs at the bow shock, where electron beams and loss cones are also observed. Therefore, we conclude that electron beams observed in the MSH do not have to originate from the bow shock, but can also be generated locally inside the MSH. The reconnecting current sheet also shows signs of thermalization and electron acceleration processes that are discussed in detail.

Pulse width modulated push-pull driven parallel resonant converter with active free-wheel

DOEpatents

Reass, William A.; Schrank, Louis

2004-06-22

An apparatus and method for high frequency alternating power generation to control kilowatts of supplied power in microseconds. The present invention includes a means for energy storage, push-pull switching means, control electronics, transformer means, resonant circuitry and means for excess energy recovery, all in electrical communication. A push-pull circuit works synchronously with a force commutated free-wheel transistor to provide current pulses to a transformer. A change in the conduction angle of the push-pull circuit changes the amount of energy coupled into the transformer's secondary oscillating circuit, thereby altering the induced secondary resonating voltage. At the end of each pulse, the force commutated free-wheel transistor causes residual excess energy in the primary circuit to be transmitted back to the storage capacitor for later use.

Design and dosimetry of a few leaf electron collimator for energy modulated electron therapy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Al-Yahya, Khalid; Verhaegen, Frank; Seuntjens, Jan

2007-12-15

Despite the capability of energy modulated electron therapy (EMET) to achieve highly conformal dose distributions in superficial targets it has not been widely implemented due to problems inherent in electron beam radiotherapy such as planning dosimetry accuracy, and verification as well as a lack of systems for automated delivery. In previous work we proposed a novel technique to deliver EMET using an automated 'few leaf electron collimator' (FLEC) that consists of four motor-driven leaves fit in a standard clinical electron beam applicator. Integrated with a Monte Carlo based optimization algorithm that utilizes patient-specific dose kernels, a treatment delivery was incorporatedmore » within the linear accelerator operation. The FLEC was envisioned to work as an accessory tool added to the clinical accelerator. In this article the design and construction of the FLEC prototype that match our compact design goals are presented. It is controlled using an in-house developed EMET controller. The structure of the software and the hardware characteristics of the EMET controller are demonstrated. Using a parallel plate ionization chamber, output measurements were obtained to validate the Monte Carlo calculations for a range of fields with different energies and sizes. Further verifications were also performed for comparing 1-D and 2-D dose distributions using energy independent radiochromic films. Comparisons between Monte Carlo calculations and measurements of complex intensity map deliveries show an overall agreement to within {+-}3%. This work confirms our design objectives of the FLEC that allow for automated delivery of EMET. Furthermore, the Monte Carlo dose calculation engine required for EMET planning was validated. The result supports the potential of the prototype FLEC for the planning and delivery of EMET.« less

Classical trajectory studies of gas phase reaction dynamics and kinetics using ab initio potential energy surfaces

NASA Technical Reports Server (NTRS)

Jaffe, Richard L.; Pattengill, Merle D.; Schwenke, David W.

1989-01-01

Strategies for constructing global potential energy surfaces from a limited number of accurate ab initio electronic energy calculations are discussed. Generally, these data are concentrated in small regions of configuration space (e.g., in the vicinity of saddle points and energy minima) and difficulties arise in generating a potential function that is globally well-behaved. Efficient computer codes for carrying out classical trajectory calculations on vector and parallel processors are also described. Illustrations are given from recent work on the following chemical systems: Ca + HF yields CaF + H, H + H + H2 yields H2 + H2, N + O2 yields NO + O and O + N2 yields NO + N. The dynamics and kinetics of metathesis, dissociation, recombination, energy transfer and complex formation processes will be discussed.

Charge Transport in Metal Oxides: A Theoretical Study of Hematite α-Fe2O3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Iordanova, Nellie I.; Dupuis, Michel; Rosso, Kevin M.

2005-04-08

Transport of conduction electrons and holes through the lattice of ??Fe2O3 (hematite) is modeled as a valence alternation of iron cations using ab initio electronic structure calculations and electron transfer theory. Experimental studies have shown that the conductivity along the (001) basal plane is four orders of magnitude larger than the conductivity along the [001] direction. In the context of the small polaron model, a cluster approach was used to compute quantities controlling the mobility of localized electrons and holes, i.e. the reorganization energy and the electronic coupling matrix element that enter Marcus? theory. The calculation of the electronic couplingmore » followed the Generalized Mulliken-Hush approach using the complete active space self-consistent field (CASSCF) method. Our findings demonstrate an approximately three orders of magnitude anisotropy in both electron and hole mobility between directions perpendicular and parallel to the c-axis, in good accord with experimental data. The anisotropy arises from the slowness of both electron and hole mobility across basal oxygen planes relative to that within iron bi-layers between basal oxygen planes. Interestingly, for elementary reaction steps along either of the directions considered, there is only approximately one order of magnitude difference in mobility between electrons and holes, in contrast to accepted classical arguments. Our findings indicate that the most important quantity underlying mobility differences is the electronic coupling, albeit the reorganization energy contributes as well. The large values computed for the electronic coupling suggest that charge transport reactions in hematite are adiabatic in nature. The electronic coupling is found to depend on both the superexchange interaction through the bridging oxygen atoms and the d-shell electron spin coupling within the Fe?Fe donor-acceptor pair, while the reorganization energy is essentially independent of the electron spin coupling.« less

Energy challenges in optical access and aggregation networks.

PubMed

Kilper, Daniel C; Rastegarfar, Houman

2016-03-06

Scalability is a critical issue for access and aggregation networks as they must support the growth in both the size of data capacity demands and the multiplicity of access points. The number of connected devices, the Internet of Things, is growing to the tens of billions. Prevailing communication paradigms are reaching physical limitations that make continued growth problematic. Challenges are emerging in electronic and optical systems and energy increasingly plays a central role. With the spectral efficiency of optical systems approaching the Shannon limit, increasing parallelism is required to support higher capacities. For electronic systems, as the density and speed increases, the total system energy, thermal density and energy per bit are moving into regimes that become impractical to support-for example requiring single-chip processor powers above the 100 W limit common today. We examine communication network scaling and energy use from the Internet core down to the computer processor core and consider implications for optical networks. Optical switching in data centres is identified as a potential model from which scalable access and aggregation networks for the future Internet, with the application of integrated photonic devices and intelligent hybrid networking, will emerge. © 2016 The Author(s).

Experimental and computational studies on stacking faults in zinc titanate

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sun, W.; Ageh, V.; Mohseni, H.

Zinc titanate (ZnTiO{sub 3}) thin films grown by atomic layer deposition with ilmenite structure have recently been identified as an excellent solid lubricant, where low interfacial shear and friction are achieved due to intrafilm shear velocity accommodation in sliding contacts. In this Letter, high resolution transmission electron microscopy with electron diffraction revealed that extensive stacking faults are present on ZnTiO{sub 3} textured (104) planes. These growth stacking faults serve as a pathway for dislocations to glide parallel to the sliding direction and hence achieve low interfacial shear/friction. Generalized stacking fault energy plots also known as γ-surfaces were computed for themore » (104) surface of ZnTiO{sub 3} using energy minimization method with classical effective partial charge potential and verified by using density functional theory first principles calculations for stacking fault energies along certain directions. These two are in qualitative agreement but classical simulations generally overestimate the energies. In addition, the lowest energy path was determined to be along the [451{sup ¯}] direction and the most favorable glide system is (104) 〈451{sup ¯}〉 that is responsible for the experimentally observed sliding-induced ductility.« less

The energy density distribution of an ideal gas and Bernoulli’s equations

NASA Astrophysics Data System (ADS)

Santos, Leonardo S. F.

2018-05-01

This work discusses the energy density distribution in an ideal gas and the consequences of Bernoulli’s equation and the corresponding relation for compressible fluids. The aim of this work is to study how Bernoulli’s equation determines the energy flow in a fluid, although Bernoulli’s equation does not describe the energy density itself. The model from molecular dynamic considerations that describes an ideal gas at rest with uniform density is modified to explore the gas in motion with non-uniform density and gravitational effects. The difference between the component of the speed of a particle that is parallel to the gas speed and the gas speed itself is called ‘parallel random speed’. The pressure from the ‘parallel random speed’ is denominated as parallel pressure. The modified model predicts that the energy density is the sum of kinetic and potential gravitational energy densities plus two terms with static and parallel pressures. The application of Bernoulli’s equation and the corresponding relation for compressible fluids in the energy density expression has resulted in two new formulations. For incompressible and compressible gas, the energy density expressions are written as a function of stagnation, static and parallel pressures, without any dependence on kinetic or gravitational potential energy densities. These expressions of the energy density are the main contributions of this work. When the parallel pressure was uniform, the energy density distribution for incompressible approximation and compressible gas did not converge to zero for the limit of null static pressure. This result is rather unusual because the temperature tends to zero for null pressure. When the gas was considered incompressible and the parallel pressure was equal to static pressure, the energy density maintained this unusual behaviour with small pressures. If the parallel pressure was equal to static pressure, the energy density converged to zero for the limit of the null pressure only if the gas was compressible. Only the last situation describes an intuitive behaviour for an ideal gas.

Current profile modification experiments in EXTRAP T2R

NASA Astrophysics Data System (ADS)

Cecconello, M.; Malmberg, J.-A.; Spizzo, G.; Chapman, B. E.; Gravestjin, R. M.; Franz, P.; Piovesan, P.; Martin, P.; Drake, J. R.

2004-01-01

Pulsed poloidal current drive (PPCD) experiments have been conducted in the resistive shell EXTRAP T2R reversed-field pinch experiment. During the current profile modification phase, the fluctuation level of the m = 1 internally resonant tearing modes decreases, and the velocity of these modes increases. The m = 0 modes are not affected during PPCD, although termination occurs with a burst in the m = 0 amplitude. The PPCD phase is characterized by an increase in the central electron temperature (up to 380 eV) and in the soft x-ray signal. Spectroscopic observations confirm an increase in the central electron temperature. During PPCD, the plasma poloidal beta increases to 14%, and the estimated energy confinement time doubles, reaching 380 µs. The reduction in the fluctuation level and the corresponding increase in the energy confinement time are qualitatively consistent with a reduction in parallel transport along stochastic magnetic field lines.

Magnetic turbulence in a table-top laser-plasma relevant to astrophysical scenarios

NASA Astrophysics Data System (ADS)

Chatterjee, Gourab; Schoeffler, Kevin M.; Kumar Singh, Prashant; Adak, Amitava; Lad, Amit D.; Sengupta, Sudip; Kaw, Predhiman; Silva, Luis O.; Das, Amita; Kumar, G. Ravindra

2017-06-01

Turbulent magnetic fields abound in nature, pervading astrophysical, solar, terrestrial and laboratory plasmas. Understanding the ubiquity of magnetic turbulence and its role in the universe is an outstanding scientific challenge. Here, we report on the transition of magnetic turbulence from an initially electron-driven regime to one dominated by ion-magnetization in a laboratory plasma produced by an intense, table-top laser. Our observations at the magnetized ion scale of the saturated turbulent spectrum bear a striking resemblance with spacecraft measurements of the solar wind magnetic-field spectrum, including the emergence of a spectral kink. Despite originating from diverse energy injection sources (namely, electrons in the laboratory experiment and ion free-energy sources in the solar wind), the turbulent spectra exhibit remarkable parallels. This demonstrates the independence of turbulent spectral properties from the driving source of the turbulence and highlights the potential of small-scale, table-top laboratory experiments for investigating turbulence in astrophysical environments.

Cavity-photon contribution to the effective interaction of electrons in parallel quantum dots

NASA Astrophysics Data System (ADS)

Gudmundsson, Vidar; Sitek, Anna; Abdullah, Nzar Rauf; Tang, Chi-Shung; Manolescu, Andrei

2016-05-01

A single cavity photon mode is expected to modify the Coulomb interaction of an electron system in the cavity. Here we investigate this phenomena in a parallel double quantum dot system. We explore properties of the closed system and the system after it has been opened up for electron transport. We show how results for both cases support the idea that the effective electron-electron interaction becomes more repulsive in the presence of a cavity photon field. This can be understood in terms of the cavity photons dressing the polarization terms in the effective mutual electron interaction leading to nontrivial delocalization or polarization of the charge in the double parallel dot potential. In addition, we find that the effective repulsion of the electrons can be reduced by quadrupolar collective oscillations excited by an external classical dipole electric field.

Dosimetric verification of gated delivery of electron beams using a 2D ion chamber array

PubMed Central

Yoganathan, S. A.; Das, K. J. Maria; Raj, D. Gowtham; Kumar, Shaleen

2015-01-01

The purpose of this study was to compare the dosimetric characteristics; such as beam output, symmetry and flatness between gated and non-gated electron beams. Dosimetric verification of gated delivery was carried for all electron beams available on Varian CL 2100CD medical linear accelerator. Measurements were conducted for three dose rates (100 MU/min, 300 MU/min and 600 MU/min) and two respiratory motions (breathing period of 4s and 8s). Real-time position management (RPM) system was used for the gated deliveries. Flatness and symmetry values were measured using Imatrixx 2D ion chamber array device and the beam output was measured using plane parallel ion chamber. These detector systems were placed over QUASAR motion platform which was programmed to simulate the respiratory motion of target. The dosimetric characteristics of gated deliveries were compared with non-gated deliveries. The flatness and symmetry of all the evaluated electron energies did not differ by more than 0.7 % with respect to corresponding non-gated deliveries. The beam output variation of gated electron beam was less than 0.6 % for all electron energies except for 16 MeV (1.4 %). Based on the results of this study, it can be concluded that Varian CL2100 CD is well suitable for gated delivery of non-dynamic electron beams. PMID:26170552

Massively Parallel Real-Time TDDFT Simulations of Electronic Stopping Processes

NASA Astrophysics Data System (ADS)

Yost, Dillon; Lee, Cheng-Wei; Draeger, Erik; Correa, Alfredo; Schleife, Andre; Kanai, Yosuke

Electronic stopping describes transfer of kinetic energy from fast-moving charged particles to electrons, producing massive electronic excitations in condensed matter. Understanding this phenomenon for ion irradiation has implications in modern technologies, ranging from nuclear reactors, to semiconductor devices for aerospace missions, to proton-based cancer therapy. Recent advances in high-performance computing allow us to achieve an accurate parameter-free description of these phenomena through numerical simulations. Here we discuss results from our recently-developed large-scale real-time TDDFT implementation for electronic stopping processes in important example materials such as metals, semiconductors, liquid water, and DNA. We will illustrate important insight into the physics underlying electronic stopping and we discuss current limitations of our approach both regarding physical and numerical approximations. This work is supported by the DOE through the INCITE awards and by the NSF. Part of this work was performed under the auspices of U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

High-altitude closed magnetic loops at Mars observed by MAVEN

NASA Astrophysics Data System (ADS)

Xu, Shaosui; Mitchell, David; Luhmann, Janet; Ma, Yingjuan; Fang, Xiaohua; Harada, Yuki; Hara, Takuya; Brain, David; Webber, Tristan; Mazelle, Christian; DiBraccio, Gina A.

2017-10-01

With electron and magnetic field data obtained by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we have identified closed magnetic field lines, with both footpoints embedded in the dayside ionosphere, extending up to 6200 km altitude (2.8 $R_m$) into the Martian tail. This topology is deduced from photoelectrons produced in the dayside ionosphere being observed in both parallel and anti-parallel directions along the magnetic field line. At perpendicular pitch angles, cases with either solar wind electrons or photoelectrons have been found, indicative of different formation mechanisms of these closed loops. These large closed loops are predicted by MHD simulations. The case with field-aligned photoelectrons mixed with perpendicular solar wind electrons is likely to be associated with reconnection, while the case with photoelectrons in all directions are probably due to closed field lines being pulled back down tail. We have developed an automated algorithm for distinguishing photoelectrons from solar wind electrons in pitch angle resolved energy spectra. This allows us to systematically analyze the MAVEN database and map the spatial distribution and occurrence rate of these closed magnetic loops, ranging from a few percent to a few tens percent outside of the optical shadow and less than one percent within the wake. These observations can be used to investigate the general magnetic topology in the tail, which is relevant to ion escape, reconnection, and flux ropes.

Reasons for high-temperature superconductivity in the electron–phonon system of hydrogen sulfide

DOE Office of Scientific and Technical Information (OSTI.GOV)

Degtyarenko, N. N.; Mazur, E. A., E-mail: eugen-masur@mail.ru

We have calculated the electron and phonon spectra, as well as the densities of the electron and phonon states, of the stable orthorhombic structure of hydrogen sulfide SH{sub 2} in the pressure interval 100–180 GPa. It is found that at a pressure of 175 GPa, a set of parallel planes of hydrogen atoms is formed due to a structural modification of the unit cell under pressure with complete accumulation of all hydrogen atoms in these planes. As a result, the electronic properties of the system become quasi-two-dimensional. We have also analyzed the collective synphase and antiphase vibrations of hydrogen atomsmore » in these planes, leading to the occurrence of two high-energy peaks in the phonon density of states.« less

The Influence of Shape on the Output Potential of ZnO Nanostructures: Sensitivity to Parallel versus Perpendicular Forces.

PubMed

Cardoso, José; Oliveira, Filipe F; Proenca, Mariana P; Ventura, João

2018-05-22

With the consistent shrinking of devices, micro-systems are, nowadays, widely used in areas such as biomedics, electronics, automobiles, and measurement devices. As devices shrunk, so too did their energy consumptions, opening the way for the use of nanogenerators (NGs) as power sources. In particular, to harvest energy from an object's motion (mechanical vibrations, torsional forces, or pressure), present NGs are mainly composed of piezoelectric materials in which, upon an applied compressive or strain force, an electrical field is produced that can be used to power a device. The focus of this work is to simulate the piezoelectric effect in different ZnO nanostructures to optimize the output potential generated by a nanodevice. In these simulations, cylindrical nanowires, nanomushrooms, and nanotrees were created, and the influence of the nanostructures' shape on the output potential was studied as a function of applied parallel and perpendicular forces. The obtained results demonstrated that the output potential is linearly proportional to the applied force and that perpendicular forces are more efficient in all structures. However, nanotrees were found to have an increased sensitivity to parallel applied forces, which resulted in a large enhancement of the output efficiency. These results could then open a new path to increase the efficiency of piezoelectric nanogenerators.

Modelling excitonic-energy transfer in light-harvesting complexes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kramer, Tobias; Kreisbeck, Christoph

The theoretical and experimental study of energy transfer in photosynthesis has revealed an interesting transport regime, which lies at the borderline between classical transport dynamics and quantum-mechanical interference effects. Dissipation is caused by the coupling of electronic degrees of freedom to vibrational modes and leads to a directional energy transfer from the antenna complex to the target reaction-center. The dissipative driving is robust and does not rely on fine-tuning of specific vibrational modes. For the parameter regime encountered in the biological systems new theoretical tools are required to directly compare theoretical results with experimental spectroscopy data. The calculations require tomore » utilize massively parallel graphics processor units (GPUs) for efficient and exact computations.« less

Ab initio molecular simulations with numeric atom-centered orbitals

NASA Astrophysics Data System (ADS)

Blum, Volker; Gehrke, Ralf; Hanke, Felix; Havu, Paula; Havu, Ville; Ren, Xinguo; Reuter, Karsten; Scheffler, Matthias

2009-11-01

We describe a complete set of algorithms for ab initio molecular simulations based on numerically tabulated atom-centered orbitals (NAOs) to capture a wide range of molecular and materials properties from quantum-mechanical first principles. The full algorithmic framework described here is embodied in the Fritz Haber Institute "ab initio molecular simulations" (FHI-aims) computer program package. Its comprehensive description should be relevant to any other first-principles implementation based on NAOs. The focus here is on density-functional theory (DFT) in the local and semilocal (generalized gradient) approximations, but an extension to hybrid functionals, Hartree-Fock theory, and MP2/GW electron self-energies for total energies and excited states is possible within the same underlying algorithms. An all-electron/full-potential treatment that is both computationally efficient and accurate is achieved for periodic and cluster geometries on equal footing, including relaxation and ab initio molecular dynamics. We demonstrate the construction of transferable, hierarchical basis sets, allowing the calculation to range from qualitative tight-binding like accuracy to meV-level total energy convergence with the basis set. Since all basis functions are strictly localized, the otherwise computationally dominant grid-based operations scale as O(N) with system size N. Together with a scalar-relativistic treatment, the basis sets provide access to all elements from light to heavy. Both low-communication parallelization of all real-space grid based algorithms and a ScaLapack-based, customized handling of the linear algebra for all matrix operations are possible, guaranteeing efficient scaling (CPU time and memory) up to massively parallel computer systems with thousands of CPUs.

Atomic resolution characterization of a SrTiO{sub 3} grain boundary in the STEM

DOE Office of Scientific and Technical Information (OSTI.GOV)

McGibbon, M.M.; Browning, N.D.; Chisholm, M.F.

This paper uses the complementary techniques of high resolution Z-contrast imaging and PEELS (parallel detection electron energy loss spectroscopy) to investigate the atomic structure and chemistry of a 25 degree symmetric tilt boundary in a bicrystal of the electroceramic SrTiO{sub 3}. The gain boundary is composed of two different boundary structural units which occur in about equal numbers: one which contains Ti-O columns and the other without.

Study on acceleration processes of the radiation belt electrons through interaction with sub-packet chorus waves in parallel propagation

NASA Astrophysics Data System (ADS)

Hiraga, R.; Omura, Y.

2017-12-01

By recent observations, chorus waves include fine structures such as amplitude fluctuations (i.e. sub-packet structure), and it has not been verified in detail yet how energetic electrons are efficiently accelerated under the wave features. In this study, we firstly focus on the acceleration process of a single electron: how it experiences the efficient energy increase by interaction with sub-packet chorus waves in parallel propagation along the Earth's magnetic field. In order to reproduce the chorus waves as seen by the latest observations by Van Allen Probes (Foster et al. 2017), the wave model amplitude in our simulation is structured such that when the wave amplitude nonlinearly grows to reach the optimum amplitude, it starts decreasing until crossing the threshold. Once it crosses the threshold, the wave dissipates and a new wave rises to repeat the nonlinear growth and damping in the same manner. The multiple occurrence of this growth-damping cycle forms a saw tooth-like amplitude variation called sub-packet. This amplitude variation also affects the wave frequency behavior which is derived by the chorus wave equations as a function of the wave amplitude (Omura et al. 2009). It is also reasonable to assume that when a wave packet diminishes and the next wave rises, it has a random phase independent of the previous wave. This randomness (discontinuity) in phase variation is included in the simulation. Through interaction with such waves, dynamics of energetic electrons were tracked. As a result, some electrons underwent an efficient acceleration process defined as successive entrapping, in which an electron successfully continues to surf the trapping potential generated by consecutive wave packets. When successive entrapping occurs, an electron trapped and de-trapped (escape the trapping potential) by a single wave packet falls into another trapping potential generated by the next wave sub-packet and continuously accelerated. The occurrence of successive entrapping is influenced by some factors such as the magnitude of wave amplitude or inhomogeneity of the Earth's dipole magnetic field. In addition, an energy range of electrons is also a major factor. In this way, it has been examined in detail how and under which conditions electrons are efficiently accelerated in the formation process of the radiation belts.

Effect of parallel electric fields on the ponderomotive stabilization of MHD instabilities

DOE Office of Scientific and Technical Information (OSTI.GOV)

Litwin, C.; Hershkowitz, N.

The contribution of the wave electric field component E/sub parallel/, parallel to the magnetic field, to the ponderomotive stabilization of curvature driven instabilities is evaluated and compared to the transverse component contribution. For the experimental density range, in which the stability is primarily determined by the m = 1 magnetosonic wave, this contribution is found to be the dominant and stabilizing when the electron temperature is neglected. For sufficiently high electron temperatures the dominant fast wave is found to be axially evanescent. In the same limit, E/sub parallel/ becomes radially oscillating. It is concluded that the increased electron temperature nearmore » the plasma surface reduces the magnitude of ponderomotive effects.« less

Nonlinear Electrostatic Steepening of Whistler Waves: The Guiding Factors and Dynamics in Inhomogeneous Systems

NASA Astrophysics Data System (ADS)

Agapitov, O.; Drake, J. F.; Vasko, I.; Mozer, F. S.; Artemyev, A.; Krasnoselskikh, V.; Angelopoulos, V.; Wygant, J.; Reeves, G. D.

2018-03-01

Whistler mode chorus waves are particularly important in outer radiation belt dynamics due to their key role in controlling the acceleration and scattering of electrons over a very wide energy range. The efficiency of wave-particle resonant interactions is defined by whistler wave properties which have been described by the approximation of plane linear waves propagating through the cold plasma of the inner magnetosphere. However, recent observations of extremely high-amplitude whistlers suggest the importance of nonlinear wave-particle interactions for the dynamics of the outer radiation belt. Oblique chorus waves observed in the inner magnetosphere often exhibit drastically nonsinusoidal (with significant power in the higher harmonics) waveforms of the parallel electric field, presumably due to the feedback from hot resonant electrons. We have considered the nature and properties of such nonlinear whistler waves observed by the Van Allen Probes and Time History of Events and Macroscale Interactions define during Substorms in the inner magnetosphere, and we show that the significant enhancement of the wave electrostatic component can result from whistler wave coupling with the beam-driven electrostatic mode through the resonant interaction with hot electron beams. Being modulated by a whistler wave, the electron beam generates a driven electrostatic mode significantly enhancing the parallel electric field of the initial whistler wave. We confirm this mechanism using a self-consistent particle-in-cell simulation. The nonlinear electrostatic component manifests properties of the beam-driven electron acoustic mode and can be responsible for effective electron acceleration in the inhomogeneous magnetic field.

An explanation for parallel electric field pulses observed over thunderstorms

NASA Astrophysics Data System (ADS)

Kelley, M. C.; Barnum, B. H.

2009-10-01

Every electric field instrument flown on sounding rockets over a thunderstorm has detected pulses of electric fields parallel to the Earth's magnetic field associated with every strike. This paper describes the ionospheric signatures found during a flight from Wallops Island, Virginia, on 2 September 1995. The electric field results in a drifting Maxwellian corresponding to energies up to 1 eV. The distribution function relaxes because of elastic and inelastic collisions, resulting in electron heating up to 4000-5000 K and potentially observable red line emissions and enhanced ISR electron temperatures. The field strength scales with the current in cloud-to-ground strikes and falls off as r -1 with distance. Pulses of both polarities are found, although most electric fields are downward, parallel to the magnetic field. The pulse may be the reaction of ambient plasma to a current pulse carried at the whistler packet's highest group velocity. The charge source required to produce the electric field is very likely electrons of a few keV traveling at the packet velocity. We conjecture that the current source is the divergence of the current flowing at mesospheric heights, the phenomenon called an elve. The whistler packet's effective radiated power is as high as 25 mW at ionospheric heights, comparable to some ionospheric heater transmissions. Comparing the Poynting flux at the base of the ionosphere with flux an equal distance away along the ground, some 30 db are lost in the mesosphere. Another 10 db are lost in the transition from free space to the whistler mode.

Accelerated Electron-Beam Formation with a High Capture Coefficient in a Parallel Coupled Accelerating Structure

NASA Astrophysics Data System (ADS)

Chernousov, Yu. D.; Shebolaev, I. V.; Ikryanov, I. M.

2018-01-01

An electron beam with a high (close to 100%) coefficient of electron capture into the regime of acceleration has been obtained in a linear electron accelerator based on a parallel coupled slow-wave structure, electron gun with microwave-controlled injection current, and permanent-magnet beam-focusing system. The high capture coefficient was due to the properties of the accelerating structure, beam-focusing system, and electron-injection system. Main characteristics of the proposed systems are presented.

Dissipation and particle energization in moderate to low beta turbulent plasma via PIC simulations

NASA Astrophysics Data System (ADS)

Makwana, Kirit; Li, Hui; Guo, Fan; Li, Xiaocan

2017-05-01

We simulate decaying turbulence in electron-positron pair plasmas using a fully-kinetic particle-in-cell (PIC) code. We run two simulations with moderate-to-low plasma β (the ratio of thermal pressure to magnetic pressure). The energy decay rate is found to be similar in both cases. The perpendicular wave-number spectrum of magnetic energy shows a slope between {k}\\perp -1.3 and {k}\\perp -1.1, where the perpendicular (⊥) and parallel (∥) directions are defined with respect to the magnetic field. The particle kinetic energy distribution function shows the formation of a non-thermal feature in the case of lower plasma β, with a slope close to E-1. The correlation between thin turbulent current sheets and Ohmic heating by the dot product of electric field (E) and current density (J) is investigated. Heating by the parallel E∥ · J∥ term dominates the perpendicular E⊥ · J⊥ term. Regions of strong E∥ · J∥ are spatially well-correlated with regions of intense current sheets, which also appear correlated with regions of strong E∥ in the low β simulation, suggesting an important role of magnetic reconnection in the dissipation of low β plasma turbulence.

The structure of the electron diffusion region during asymmetric anti-parallel magnetic reconnection

NASA Astrophysics Data System (ADS)

Swisdak, M.; Drake, J. F.; Price, L.; Burch, J. L.; Cassak, P.

2017-12-01

The structure of the electron diffusion region during asymmetric magnetic reconnection is ex- plored with high-resolution particle-in-cell simulations that focus on an magnetopause event ob- served by the Magnetospheric Multiscale Mission (MMS). A major surprise is the development of a standing, oblique whistler-like structure with regions of intense positive and negative dissipation. This structure arises from high-speed electrons that flow along the magnetosheath magnetic sepa- ratrices, converge in the dissipation region and jet across the x-line into the magnetosphere. The jet produces a region of negative charge and generates intense parallel electric fields that eject the electrons downstream along the magnetospheric separatrices. The ejected electrons produce the parallel velocity-space crescents documented by MMS.

Experimental verification of the role of electron pressure in fast magnetic reconnection with a guide field

DOE PAGES

Fox, W.; Sciortino, F.; v. Stechow, A.; ...

2017-03-21

We report detailed laboratory observations of the structure of a reconnection current sheet in a two-fluid plasma regime with a guide magnetic field. We observe and quantitatively analyze the quadrupolar electron pressure variation in the ion-diffusion region, as originally predicted by extended magnetohydrodynamics simulations. The projection of the electron pressure gradient parallel to the magnetic field contributes significantly to balancing the parallel electric field, and the resulting cross-field electron jets in the reconnection layer are diamagnetic in origin. Furthermore, these results demonstrate how parallel and perpendicular force balance are coupled in guide field reconnection and confirm basic theoretical models ofmore » the importance of electron pressure gradients for obtaining fast magnetic reconnection.« less

Dosimetric characteristics of a MOSFET dosimeter for clinical electron beams.

PubMed

Manigandan, D; Bharanidharan, G; Aruna, P; Devan, K; Elangovan, D; Patil, Vikram; Tamilarasan, R; Vasanthan, S; Ganesan, S

2009-09-01

The fundamental dosimetric characteristics of commercially available metal oxide semiconductor field effect transistor (MOSFET) detectors were studied for clinical electron beam irradiations. MOSFET showed excellent linearity against doses measured using an ion chamber in the dose range of 20-630cGy. MOSFET reproducibility is better at high doses compared to low doses. The output factors measured with the MOSFET were within +/-3% when compared with those measured with a parallel plate chamber. From 4 to 12MeV, MOSFETs showed a large angular dependence in the tilt directions and less in the axial directions. MOSFETs do not show any dose-rate dependence between 100 and 600MU/min. However, MOSFETs have shown under-response when the dose per pulse of the beam is decreased. No measurable effect in MOSFET response was observed in the temperature range of 23-40 degrees C. The energy dependence of a MOSFET dosimeter was within +/-3.0% for 6-18MeV electron beams and 5.5% for 4MeV ones. This study shows that MOSFET detectors are suitable for dosimetry of electron beams in the energy range of 4-18MeV.

The Electronic Structure of Transition Metal Coated Fullerenes

NASA Astrophysics Data System (ADS)

Patton, David C.; Pederson, Mark R.; Kaxiras, Efthimios

1998-03-01

Clusters composed of fullerene molecules with an outer shell of transition metal atoms in the composition C_60M_62 (M being a transition metal) have been produced with laser vaporisation techniques(F. Tast, N. Malinowski, S. Frank, M. Heinebrodt, I.M.L. Billas, and T. P. Martin, Z. Phys D 40), 351 (1997).. We have studied several of these very large systems with a parallel version of the all-electron NRLMOL cluster code. Optimized geometries of the metal encased fullerenes C_60Ti_62 and C_60V_62 are presented along with their HOMO-LUMO gaps, electron affinities, ionization energies, and cohesive energies. We compare the stability of these clusters to relaxed met-car structures (e.g. Ti_8C_12) and to relaxed rocksalt metal-carbide fragments (TiC)n with n=8 and 32. In addition to metal-coated fullerenes we consider the possibility of a trilayered structure consisting of a small shell of metal atoms enclosed by a metal coated fullerene. The nature of bonding in these systems is analyzed by studying the electronic charge distributions.

Directing the path of light-induced electron transfer at a molecular fork using vibrational excitation

NASA Astrophysics Data System (ADS)

Delor, Milan; Archer, Stuart A.; Keane, Theo; Meijer, Anthony J. H. M.; Sazanovich, Igor V.; Greetham, Gregory M.; Towrie, Michael; Weinstein, Julia A.

2017-11-01

Ultrafast electron transfer in condensed-phase molecular systems is often strongly coupled to intramolecular vibrations that can promote, suppress and direct electronic processes. Recent experiments exploring this phenomenon proved that light-induced electron transfer can be strongly modulated by vibrational excitation, suggesting a new avenue for active control over molecular function. Here, we achieve the first example of such explicit vibrational control through judicious design of a Pt(II)-acetylide charge-transfer donor-bridge-acceptor-bridge-donor 'fork' system: asymmetric 13C isotopic labelling of one of the two -C≡C- bridges makes the two parallel and otherwise identical donor→acceptor electron-transfer pathways structurally distinct, enabling independent vibrational perturbation of either. Applying an ultrafast UVpump(excitation)-IRpump(perturbation)-IRprobe(monitoring) pulse sequence, we show that the pathway that is vibrationally perturbed during UV-induced electron transfer is dramatically slowed down compared to its unperturbed counterpart. One can thus choose the dominant electron transfer pathway. The findings deliver a new opportunity for precise perturbative control of electronic energy propagation in molecular devices.

Permanent-magnet energy spectrometer for electron beams from radiotherapy accelerators

DOE Office of Scientific and Technical Information (OSTI.GOV)

McLaughlin, David J.; Shikhaliev, Polad M.; Matthews, Kenneth L.

2015-09-15

Purpose: The purpose of this work was to adapt a lightweight, permanent magnet electron energy spectrometer for the measurement of energy spectra of therapeutic electron beams. Methods: An irradiation geometry and measurement technique were developed for an approximately 0.54-T, permanent dipole magnet spectrometer to produce suitable latent images on computed radiography (CR) phosphor strips. Dual-pinhole electron collimators created a 0.318-cm diameter, approximately parallel beam incident on the spectrometer and an appropriate dose rate at the image plane (CR strip location). X-ray background in the latent image, reduced by a 7.62-cm thick lead block between the pinhole collimators, was removed usingmore » a fitting technique. Theoretical energy-dependent detector response functions (DRFs) were used in an iterative technique to transform CR strip net mean dose profiles into energy spectra on central axis at the entrance to the spectrometer. These spectra were transformed to spectra at 95-cm source to collimator distance (SCD) by correcting for the energy dependence of electron scatter. The spectrometer was calibrated by comparing peak mean positions in the net mean dose profiles, initially to peak mean energies determined from the practical range of central-axis percent depth-dose (%DD) curves, and then to peak mean energies that accounted for how the collimation modified the energy spectra (recalibration). The utility of the spectrometer was demonstrated by measuring the energy spectra for the seven electron beams (7–20 MeV) of an Elekta Infinity radiotherapy accelerator. Results: Plots of DRF illustrated their dependence on energy and position in the imaging plane. Approximately 15 iterations solved for the energy spectra at the spectrometer entrance from the measured net mean dose profiles. Transforming those spectra into ones at 95-cm SCD increased the low energy tail of the spectra, while correspondingly decreasing the peaks and shifting them to slightly lower energies. Energy calibration plots of peak mean energy versus peak mean position of the net mean dose profiles for each of the seven electron beams followed the shape predicted by the Lorentz force law for a uniform z-component of the magnetic field, validating its being modeled as uniform (0.542 ± 0.027 T). Measured Elekta energy spectra and their peak mean energies correlated with the 0.5-cm (7–13 MeV) and the 1.0-cm (13–20 MeV) R{sub 90} spacings of the %DD curves. The full-width-half-maximum of the energy spectra decreased with decreasing peak mean energy with the exception of the 9-MeV beam, which was anomalously wide. Similarly, R{sub 80–20} decreased linearly with peak mean energy with the exception of the 9 MeV beam. Both were attributed to suboptimal tuning of the high power phase shifter for the recycled radiofrequency power reentering the traveling wave accelerator. Conclusions: The apparatus and analysis techniques of the authors demonstrated that an inexpensive, lightweight, permanent magnet electron energy spectrometer can be used for measuring the electron energy distributions of therapeutic electron beams (6–20 MeV). The primary goal of future work is to develop a real-time spectrometer by incorporating a real-time imager, which has potential applications such as beam matching, ongoing beam tune maintenance, and measuring spectra for input into Monte Carlo beam calculations.« less

Permanent-magnet energy spectrometer for electron beams from radiotherapy accelerators.

PubMed

McLaughlin, David J; Hogstrom, Kenneth R; Carver, Robert L; Gibbons, John P; Shikhaliev, Polad M; Matthews, Kenneth L; Clarke, Taylor; Henderson, Alexander; Liang, Edison P

2015-09-01

The purpose of this work was to adapt a lightweight, permanent magnet electron energy spectrometer for the measurement of energy spectra of therapeutic electron beams. An irradiation geometry and measurement technique were developed for an approximately 0.54-T, permanent dipole magnet spectrometer to produce suitable latent images on computed radiography (CR) phosphor strips. Dual-pinhole electron collimators created a 0.318-cm diameter, approximately parallel beam incident on the spectrometer and an appropriate dose rate at the image plane (CR strip location). X-ray background in the latent image, reduced by a 7.62-cm thick lead block between the pinhole collimators, was removed using a fitting technique. Theoretical energy-dependent detector response functions (DRFs) were used in an iterative technique to transform CR strip net mean dose profiles into energy spectra on central axis at the entrance to the spectrometer. These spectra were transformed to spectra at 95-cm source to collimator distance (SCD) by correcting for the energy dependence of electron scatter. The spectrometer was calibrated by comparing peak mean positions in the net mean dose profiles, initially to peak mean energies determined from the practical range of central-axis percent depth-dose (%DD) curves, and then to peak mean energies that accounted for how the collimation modified the energy spectra (recalibration). The utility of the spectrometer was demonstrated by measuring the energy spectra for the seven electron beams (7-20 MeV) of an Elekta Infinity radiotherapy accelerator. Plots of DRF illustrated their dependence on energy and position in the imaging plane. Approximately 15 iterations solved for the energy spectra at the spectrometer entrance from the measured net mean dose profiles. Transforming those spectra into ones at 95-cm SCD increased the low energy tail of the spectra, while correspondingly decreasing the peaks and shifting them to slightly lower energies. Energy calibration plots of peak mean energy versus peak mean position of the net mean dose profiles for each of the seven electron beams followed the shape predicted by the Lorentz force law for a uniform z-component of the magnetic field, validating its being modeled as uniform (0.542 ± 0.027 T). Measured Elekta energy spectra and their peak mean energies correlated with the 0.5-cm (7-13 MeV) and the 1.0-cm (13-20 MeV) R90 spacings of the %DD curves. The full-width-half-maximum of the energy spectra decreased with decreasing peak mean energy with the exception of the 9-MeV beam, which was anomalously wide. Similarly, R80-20 decreased linearly with peak mean energy with the exception of the 9 MeV beam. Both were attributed to suboptimal tuning of the high power phase shifter for the recycled radiofrequency power reentering the traveling wave accelerator. The apparatus and analysis techniques of the authors demonstrated that an inexpensive, lightweight, permanent magnet electron energy spectrometer can be used for measuring the electron energy distributions of therapeutic electron beams (6-20 MeV). The primary goal of future work is to develop a real-time spectrometer by incorporating a real-time imager, which has potential applications such as beam matching, ongoing beam tune maintenance, and measuring spectra for input into Monte Carlo beam calculations.

Ion distribution effects of turbulence on a kinetic auroral arc model

NASA Technical Reports Server (NTRS)

Cornwall, J. M.; Chiu, Y. T.

1982-01-01

An inverted-V auroral arc structure plasma-kinetic model is extended to phenomenologically include the effects of electrostatic turbulence, with k-parallel/k-perpendicular being much less than unity. It is shown that, unless plasma sheet ions are very much more energetic than the electrons, anomalous resistivity is not a large contributor to parallel electrostatic potential drops, since the support of the observed potential drop requires a greater dissipation of energy than can be provided by the plasma sheet. Wave turbulence can, however, be present, with the ion cyclotron turbulence levels suggested by the ion resonance broadening saturation mechanism of Dum and Dupree (1970) being comparable to those observed on auroral field lines. The diffusion coefficient and net growth rate are much smaller than estimates based solely on local plasma properties.

High spatial resolution PEELS characterization of FeAl nanograins prepared by mechanical alloying

DOE Office of Scientific and Technical Information (OSTI.GOV)

Valdre, G.; Botton, G.A.; Brown, L.M.

The authors investigate the nanograin ``chemical`` structure in a nanostructured material of possible industrial application (Fe-Al system) prepared by conventional mechanical alloying via ball milling in argon atmosphere. They restrict themselves to the structural and nanochemical behavior of ball-milled nanocrystalline Fe-Al powders with atomic composition Fe{sub 3}Al, corresponding to a well-known intermetallic compound of the Fe-Al system. Scanning transmission electron microscopy (STEM) equipped with a parallel detection electron energy loss spectrometer (PEELS) has provided an insight on the ``chemical`` structure of both nanograins and their surface at a spatial resolution of better than 1 nm. The energy loss near edgemore » structure of the Al L loss reveals that the Al coordination is similar to a B2 compound and the oxidation of the powder during processing may play a significant role in the stabilization of the intermetallic phases. Conventional transmission electron microscopy (TEM) was used for the structural characterization of the material after the ball milling; powder X-ray diffraction (XRD) aided the investigation.« less

Isotope separation apparatus

DOEpatents

Arnush, Donald; MacKenzie, Kenneth R.; Wuerker, Ralph F.

1980-01-01

Isotope separation apparatus consisting of a plurality of cells disposed adjacent to each other in an evacuated container. A common magnetic field is established extending through all of the cells. A source of energetic electrons at one end of the container generates electrons which pass through the cells along the magnetic field lines. Each cell includes an array of collector plates arranged in parallel or in tandem within a common magnetic field. Sets of collector plates are disposed adjacent to each other in each cell. Means are provided for differentially energizing ions of a desired isotope by applying energy at the cyclotron resonant frequency of the desired isotope. As a result, the energized desired ions are preferentially collected by the collector plates.

Identification of metal s states in Sn-doped anatase by polarisation dependent hard X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Regoutz, A.; Oropeza, F. E.; Poll, C. G.; Payne, D. J.; Palgrave, R. G.; Panaccione, G.; Borgatti, F.; Agrestini, S.; Utsumi, Y.; Tsuei, K. D.; Liao, Y. F.; Watson, G. W.; Egdell, R. G.

2016-03-01

The contributions of Sn 5s and Ti 4s states to the valence band electronic structure of Sn-doped anatase have been identified by hard X-ray photoelectron spectroscopy. The metal s state intensity is strongly enhanced relative to that of O 2p states at high photon energies due to matrix element effects when electrons are detected parallel to the direction of the polarisation vector of the synchrotron beam, but becomes negligible in the perpendicular direction. The experimental spectra in both polarisations are in good agreement with cross section and asymmetry parameter weighted partial densities of states derived from density functional theory calculations.

Whistler Waves Driven by Anisotropic Strahl Velocity Distributions: Cluster Observations

NASA Technical Reports Server (NTRS)

Vinas, A.F.; Gurgiolo, C.; Nieves-Chinchilla, T.; Gary, S. P.; Goldstein, M. L.

2010-01-01

Observed properties of the strahl using high resolution 3D electron velocity distribution data obtained from the Cluster/PEACE experiment are used to investigate its linear stability. An automated method to isolate the strahl is used to allow its moments to be computed independent of the solar wind core+halo. Results show that the strahl can have a high temperature anisotropy (T(perpindicular)/T(parallell) approximately > 2). This anisotropy is shown to be an important free energy source for the excitation of high frequency whistler waves. The analysis suggests that the resultant whistler waves are strong enough to regulate the electron velocity distributions in the solar wind through pitch-angle scattering

Nanowire-based thermoelectric ratchet in the hopping regime

NASA Astrophysics Data System (ADS)

Bosisio, Riccardo; Fleury, Geneviève; Pichard, Jean-Louis; Gorini, Cosimo

2016-04-01

We study a thermoelectric ratchet consisting of an array of disordered nanowires arranged in parallel on top of an insulating substrate and contacted asymmetrically to two electrodes. Transport is investigated in the Mott hopping regime, when localized electrons can propagate through the nanowires via thermally assisted hops. When the electronic temperature in the nanowires is different from the phononic one in the substrate, we show that a finite electrical current is generated even in the absence of driving forces between the electrodes. We discuss the device performance both as an energy harvester, when an excess heat from the substrate is converted into useful power, and as a refrigerator, when an external power is supplied to cool down the substrate.

Effect of germanium concentrations on tunnelling current calculation of Si/Si1-xGex/Si heterojunction bipolar transistor

NASA Astrophysics Data System (ADS)

Hasanah, L.; Suhendi, E.; Khairrurijal

2018-05-01

Tunelling current calculation on Si/Si1-xGex/Si heterojunction bipolar transistor was carried out by including the coupling between transversal and longitudinal components of electron motion. The calculation results indicated that the coupling between kinetic energy in parallel and perpendicular to S1-xGex barrier surface affected tunneling current significantly when electron velocity was faster than 1x105 m/s. This analytical tunneling current model was then used to study how the germanium concentration in base to Si/Si1-xGex/Si heterojunction bipolar transistor influenced the tunneling current. It is obtained that tunneling current increased as the germanium concentration given in base decreased.

Design and dosimetry characteristics of a commercial applicator system for intra-operative electron beam therapy utilizing ELEKTA Precise accelerator.

PubMed

Nevelsky, Alexander; Bernstein, Zvi; Bar-Deroma, Raquel; Kuten, Abraham; Orion, Itzhak

2010-07-19

The design concept and dosimetric characteristics of a new applicator system for intraoperative radiation therapy (IORT) are presented in this work. A new hard-docking commercial system includes polymethylmethacrylate (PMMA) applicators with different diameters and applicator end angles and a set of secondary lead collimators. A telescopic device allows changing of source-to-surface distance (SSD). All measurements were performed for 6, 9, 12 and 18 MeV electron energies. Output factors and percentage depth doses (PDD) were measured in a water phantom using a plane-parallel ion chamber. Isodose contours and radiation leakage were measured using a solid water phantom and radiographic films. The dependence of PDD on SSD was checked for the applicators with the smallest and the biggest diameters. SSD dependence of the output factors was measured. Hardcopies of PDD and isodose contours were prepared to help the team during the procedure on deciding applicator size and energy to be chosen. Applicator output factors are a function of energy, applicator size and applicator type. Dependence of SSD correction factors on applicator size and applicator type was found to be weak. The same SSD correction will be applied for all applicators in use for each energy. The radiation leakage through the applicators is clinically acceptable. The applicator system enables effective collimation of electron beams for IORT. The data presented are sufficient for applicator, energy and monitor unit selection for IORT treatment of a patient.

Evolving Nonthermal Electron Distributions in Simulations of Sgr A*

NASA Astrophysics Data System (ADS)

Chael, Andrew; Narayan, Ramesh

2018-01-01

The accretion flow around Sagittarius A* (Sgr A*), the black hole at the Galactic Center, produces strong variability from the radio to X-rays on timescales of minutes to hours. This rapid, powerful variability is thought to be powered by energetic particle acceleration by plasma processes like magnetic reconnection and shocks. These processes can accelerate particles into non-thermal distributions which do not quickly isothermal in the low densities found around hot accretion flows. Current state-of-the-art simulations of accretion flows around black holes assume either a single-temperature gas or, at best, a two-temperature gas with thermal ions and electrons. We present results from incorporating the self-consistent evolution of a non-thermal electron population in a GRRMHD simulation of Sgr A*. The electron distribution is evolved across space, time, and Lorentz factor in parallel with background thermal ion, electron, and radiation fluids. Energy injection into the non-thermal distribution is modeled with a sub-grid prescription based on results from particle-in-cell simulations of magnetic reconnection. The energy distribution of the non-thermal electrons shows strong variability, and the spectral shape traces the complex interplay between the local viscous heating rate, magnetic field strength, and fluid velocity. Results from these simulations will be used in interpreting forthcoming data from the Event Horizon Telescope that resolves Sgr A*'s sub-mm variability in both time and space.

The microlasertron: An efficient switched-power source of mm wavelength radiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Palmer, R.B.

1986-12-01

An extension of W. Willis' ''Switched Power Linac'' is studied. Pulsed laser light falls on a photocathode wire, or wires, within a simple resonant structure. The resulting pulsed electron current between the wire and the structure wall drives the resonant field, and rf energy is extracted in the mm to cm wavelength range. Various geometries are presented, including one consisting of a simple array of parallel wires over a plane conductor. Results from a one-dimensional simulation are presented.

Evolving non-thermal electrons in simulations of black hole accretion

NASA Astrophysics Data System (ADS)

Chael, Andrew A.; Narayan, Ramesh; Saḑowski, Aleksander

2017-09-01

Current simulations of hot accretion flows around black holes assume either a single-temperature gas or, at best, a two-temperature gas with thermal ions and electrons. However, processes like magnetic reconnection and shocks can accelerate electrons into a non-thermal distribution, which will not quickly thermalize at the very low densities found in many systems. Such non-thermal electrons have been invoked to explain the infrared and X-ray spectra and strong variability of Sagittarius A* (Sgr A*), the black hole at the Galactic Center. We present a method for self-consistent evolution of a non-thermal electron population in the general relativistic magnetohydrodynamic code koral. The electron distribution is tracked across Lorentz factor space and is evolved in space and time, in parallel with thermal electrons, thermal ions and radiation. In this study, for simplicity, energy injection into the non-thermal distribution is taken as a fixed fraction of the local electron viscous heating rate. Numerical results are presented for a model with a low mass accretion rate similar to that of Sgr A*. We find that the presence of a non-thermal population of electrons has negligible effect on the overall dynamics of the system. Due to our simple uniform particle injection prescription, the radiative power in the non-thermal simulation is enhanced at large radii. The energy distribution of the non-thermal electrons shows a synchrotron cooling break, with the break Lorentz factor varying with location and time, reflecting the complex interplay between the local viscous heating rate, magnetic field strength and fluid velocity.

First Direct Observation of Runaway-Electron-Driven Whistler Waves in Tokamaks

DOE PAGES

Spong, D. A.; Heidbrink, W. W.; Paz-Soldan, C.; ...

2018-04-11

DIII-D experiments at low density (n e ~10 19 m -3) have directly measured whistler waves in the 100– 200 MHz range excited by multi-MeV runaway electrons. Whistler activity is correlated with runaway intensity (hard x-ray emission level), occurs in novel discrete frequency bands, and exhibits nonlinear limitcycle- like behavior. The measured frequencies scale with the magnetic field strength and electron density as expected from the whistler dispersion relation. The modes are stabilized with increasing magnetic field, which is consistent with wave-particle resonance mechanisms. The mode amplitudes show intermittent time variations correlated with changes in the electron cyclotron emission thatmore » follow predator-prey cycles. These can be interpreted as wave-induced pitch angle scattering of moderate energy runaways. The tokamak runaway-whistler mechanisms have parallels to whistler phenomena in ionospheric plasmas. The observations also open new directions for the modeling and active control of runaway electrons in tokamaks.« less

Design and performance of the spin asymmetries of the nucleon experiment

DOE PAGES

Maxwell, J. D.; Armstrong, W. R.; Choi, S.; ...

2018-03-01

The Spin Asymmetries of the Nucleon Experiment (SANE) performed inclusive, double-polarized electron scattering measurements of the proton at the Continuous Electron Beam Facility at Jefferson Lab. A novel detector array observed scattered electrons of four-momentum transfer 2.5 < Q 2 < 6.5 GeV 2 and Bjorken scaling 0.3 < x < 0.8 from initial beam energies of 4.7 and 5.9 GeV. Employing a polarized proton target which could be rotated with respect to the incident electron beam, both parallel and near perpendicular spin asymmetries were measured, allowing model-independent access to transverse polarization observables A 1, A 2, g 1, gmore » 2 and moment d 2 of the proton. This article summarizes the operation and performance of the polarized target, polarized electron beam, and novel detector systems used during the course of the experiment, and describes analysis techniques utilized to access the physics observables of interest.« less

First Direct Observation of Runaway-Electron-Driven Whistler Waves in Tokamaks.

PubMed

Spong, D A; Heidbrink, W W; Paz-Soldan, C; Du, X D; Thome, K E; Van Zeeland, M A; Collins, C; Lvovskiy, A; Moyer, R A; Austin, M E; Brennan, D P; Liu, C; Jaeger, E F; Lau, C

2018-04-13

DIII-D experiments at low density (n_{e}∼10^{19}  m^{-3}) have directly measured whistler waves in the 100-200 MHz range excited by multi-MeV runaway electrons. Whistler activity is correlated with runaway intensity (hard x-ray emission level), occurs in novel discrete frequency bands, and exhibits nonlinear limit-cycle-like behavior. The measured frequencies scale with the magnetic field strength and electron density as expected from the whistler dispersion relation. The modes are stabilized with increasing magnetic field, which is consistent with wave-particle resonance mechanisms. The mode amplitudes show intermittent time variations correlated with changes in the electron cyclotron emission that follow predator-prey cycles. These can be interpreted as wave-induced pitch angle scattering of moderate energy runaways. The tokamak runaway-whistler mechanisms have parallels to whistler phenomena in ionospheric plasmas. The observations also open new directions for the modeling and active control of runaway electrons in tokamaks.

Excitation of Plasma Waves in Aurora by Electron Beams

NASA Technical Reports Server (NTRS)

daSilva, C. E.; Vinas, A. F.; deAssis, A. S.; deAzevedo, C. A.

1996-01-01

In this paper, we study numerically the excitation of plasma waves by electron beams, in the auroral region above 2000 km of altitude. We have solved the fully kinetic dispersion relation, using numerical method and found the real frequency and the growth rate of the plasma wave modes. We have examined the instability properties of low-frequency waves such as the Electromagnetic Ion Cyclotron (EMIC) wave as well as Lower-Hybrid (LH) wave in the range of high-frequency. In all cases, the source of free energy are electron beams propagating parallel to the geomagnetic field. We present some features of the growth rate modes, when the cold plasma parameters are changed, such as background electrons and ions species (H(+) and O(+)) temperature, density or the electron beam density and/or drift velocity. These results can be used in a test-particle simulation code, to investigate the ion acceleration and their implication in the auroral acceleration processes, by wave-particle interaction.

First Direct Observation of Runaway-Electron-Driven Whistler Waves in Tokamaks

DOE Office of Scientific and Technical Information (OSTI.GOV)

Spong, D. A.; Heidbrink, W. W.; Paz-Soldan, C.

DIII-D experiments at low density (n e ~10 19 m -3) have directly measured whistler waves in the 100– 200 MHz range excited by multi-MeV runaway electrons. Whistler activity is correlated with runaway intensity (hard x-ray emission level), occurs in novel discrete frequency bands, and exhibits nonlinear limitcycle- like behavior. The measured frequencies scale with the magnetic field strength and electron density as expected from the whistler dispersion relation. The modes are stabilized with increasing magnetic field, which is consistent with wave-particle resonance mechanisms. The mode amplitudes show intermittent time variations correlated with changes in the electron cyclotron emission thatmore » follow predator-prey cycles. These can be interpreted as wave-induced pitch angle scattering of moderate energy runaways. The tokamak runaway-whistler mechanisms have parallels to whistler phenomena in ionospheric plasmas. The observations also open new directions for the modeling and active control of runaway electrons in tokamaks.« less

On the parameter dependence of the whistler anisotropy instability

NASA Astrophysics Data System (ADS)

An, Xin; Yue, Chao; Bortnik, Jacob; Decyk, Viktor; Li, Wen; Thorne, Richard M.

2017-02-01

The evolution of the whistler anisotropy instability relevant to whistler-mode chorus waves in the Earth's inner magnetosphere is studied using kinetic simulations and is compared with satellite observations. The electron distribution is constrained by the whistler anisotropy instability to a marginal stability state and presents an upper bound of electron anisotropy, which agrees with satellite observations. The electron beta β∥e separates whistler waves into two groups: (i) quasi-parallel whistler waves for β∥e≳0.02 and (ii) oblique whistler waves close to the resonance cone for β∥e≲0.02. Landau damping is important in the saturation and relaxation stage of the oblique whistler wave growth. The saturated magnetic field energy of whistler waves roughly scales with the electron beta β∥e2, shown in both simulations and satellite observations. These results suggest the critical role of electron beta β∥e in determining the whistler wave properties in the inner magnetosphere.

Ion temperature effects on magnetotail Alfvén wave propagation and electron energization: ION TEMPERATURE EFFECTS ON ALFVÉN WAVES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Damiano, P. A.; Johnson, J. R.; Chaston, C. C.

2015-07-01

A new 2-D self-consistent hybrid gyrofluid-kinetic electron model in dipolar coordinates is presented and used to simulate dispersive-scale Alfvén wave pulse propagation from the equator to the ionosphere along an L = 10 magnetic field line. The model is an extension of the hybrid MHD-kinetic electron model that incorporates ion Larmor radius corrections via the kinetic fluid model of Cheng and Johnson (1999). It is found that consideration of a realistic ion to electron temperature ratio decreases the propagation time of the wave from the plasma sheet to the ionosphere by several seconds relative to a ρi=0 case (which alsomore » implies shorter timing for a substorm onset signal) and leads to significant dispersion of wave energy perpendicular to the ambient magnetic field. Additionally, ion temperature effects reduce the parallel current and electron energization all along the field line for the same magnitude perpendicular electric field perturbation.« less

Miniaturized pulsed CO2 laser with sealed electron source

NASA Astrophysics Data System (ADS)

Bychkov, Y. I.; Orlovskiy, V. M.; Osipov, V. V.; Poteryayev, A. G.

1984-04-01

A new miniature electron beam-controlled CO2 laser (the MIG-3) contains an electron accelerator, gas cell and DC supply in one large unit (0.22 x 0,16 x 0.7 m) and the accelerator power supply and laser control panel in a second smaller unit. The overall weight of the instrument in 30 kg. The electron beam is controlled by four vacuum diodes in parallel; a 180 KV pulse is fed to the vacuum diode inputs from a "NORA" series-produced X-ray source (the MIRA-3D) also is used). The total electron beam current from all diodes was 600 A following the foil with a half-height width of 10 ns. The lasing medium is CO2:N2 - 1:1 at 4.5 atm. The maximum stimulated emission pulse energy was 1 J with an efficiency of 8% when the pressure was 4 atm. With a pulse repetition rate of 4 Hz, the average power consumption of the unit was 100 W.

Design and performance of the spin asymmetries of the nucleon experiment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Maxwell, J. D.; Armstrong, W. R.; Choi, S.

The Spin Asymmetries of the Nucleon Experiment (SANE) performed inclusive, double-polarized electron scattering measurements of the proton at the Continuous Electron Beam Facility at Jefferson Lab. A novel detector array observed scattered electrons of four-momentum transfer 2.5 < Q 2 < 6.5 GeV 2 and Bjorken scaling 0.3 < x < 0.8 from initial beam energies of 4.7 and 5.9 GeV. Employing a polarized proton target which could be rotated with respect to the incident electron beam, both parallel and near perpendicular spin asymmetries were measured, allowing model-independent access to transverse polarization observables A 1, A 2, g 1, gmore » 2 and moment d 2 of the proton. This article summarizes the operation and performance of the polarized target, polarized electron beam, and novel detector systems used during the course of the experiment, and describes analysis techniques utilized to access the physics observables of interest.« less

Development of a new EMP code at LANL

NASA Astrophysics Data System (ADS)

Colman, J. J.; Roussel-Dupré, R. A.; Symbalisty, E. M.; Triplett, L. A.; Travis, B. J.

2006-05-01

A new code for modeling the generation of an electromagnetic pulse (EMP) by a nuclear explosion in the atmosphere is being developed. The source of the EMP is the Compton current produced by the prompt radiation (γ-rays, X-rays, and neutrons) of the detonation. As a first step in building a multi- dimensional EMP code we have written three kinetic codes, Plume, Swarm, and Rad. Plume models the transport of energetic electrons in air. The Plume code solves the relativistic Fokker-Planck equation over a specified energy range that can include ~ 3 keV to 50 MeV and computes the resulting electron distribution function at each cell in a two dimensional spatial grid. The energetic electrons are allowed to transport, scatter, and experience Coulombic drag. Swarm models the transport of lower energy electrons in air, spanning 0.005 eV to 30 keV. The swarm code performs a full 2-D solution to the Boltzmann equation for electrons in the presence of an applied electric field. Over this energy range the relevant processes to be tracked are elastic scattering, three body attachment, two body attachment, rotational excitation, vibrational excitation, electronic excitation, and ionization. All of these occur due to collisions between the electrons and neutral bodies in air. The Rad code solves the full radiation transfer equation in the energy range of 1 keV to 100 MeV. It includes effects of photo-absorption, Compton scattering, and pair-production. All of these codes employ a spherical coordinate system in momentum space and a cylindrical coordinate system in configuration space. The "z" axis of the momentum and configuration spaces is assumed to be parallel and we are currently also assuming complete spatial symmetry around the "z" axis. Benchmarking for each of these codes will be discussed as well as the way forward towards an integrated modern EMP code.

Renormalization of effective interactions in a negative charge transfer insulator

NASA Astrophysics Data System (ADS)

Seth, Priyanka; Peil, Oleg E.; Pourovskii, Leonid; Betzinger, Markus; Friedrich, Christoph; Parcollet, Olivier; Biermann, Silke; Aryasetiawan, Ferdi; Georges, Antoine

2017-11-01

We compute from first principles the effective interaction parameters appropriate for a low-energy description of the rare-earth nickelate LuNiO3 involving the partially occupied eg states only. The calculation uses the constrained random-phase approximation and reveals that the effective on-site Coulomb repulsion is strongly reduced by screening effects involving the oxygen-p and nickel-t2 g states. The long-range component of the effective low-energy interaction is also found to be sizable. As a result, the effective on-site interaction between parallel-spin electrons is reduced down to a small negative value. This validates effective low-energy theories of these materials that were proposed earlier. Electronic structure methods combined with dynamical mean-field theory are used to construct and solve an appropriate low-energy model and explore its phase diagram as a function of the on-site repulsion and Hund's coupling. For the calculated values of these effective interactions, we find that in agreement with experiments, LuNiO3 is a metal without disproportionation of the eg occupancy when considered in its orthorhombic structure, while the monoclinic phase is a disproportionated insulator.

Control of runaway electron energy using externally injected whistler waves

NASA Astrophysics Data System (ADS)

Guo, Zehua; McDevitt, Christopher J.; Tang, Xian-Zhu

2018-03-01

One way of mitigating runaway damage of the plasma-facing components in a tokamak fusion reactor is by limiting the runaway electron energy under a few MeV, while not necessarily reducing the runaway current appreciably. Here, we describe a physics mechanism by which such momentum space engineering of the runaway distribution can be facilitated by externally injected high-frequency electromagnetic waves such as whistler waves. The drastic impact that wave-induced scattering can have on the runaway energy distribution is fundamentally the result of its ability to control the runaway vortex in the momentum space. The runaway vortex, which is a local circulation of runaways in momentum space, is the outcome of the competition between Coulomb collisions, synchrotron radiation damping, and runaway acceleration by the parallel electric field. By introducing a wave that resonantly interacts with runaways in a particular range of energies which is mildly relativistic, the enhanced scattering would reshape the vortex by cutting off the part that is highly relativistic. The efficiency of resonant scattering accentuates the requirement that the wave amplitude can be small so the power requirement from external wave injection is practical for the mitigation scheme.

The Strongest 40 keV Electron Acceleration By ICME-driven Shocks At 1 AU

NASA Astrophysics Data System (ADS)

Yang, L.; Wang, L.; Li, G.; Wimmer-Schweingruber, R. F.; He, J.; Tu, C. Y.; Bale, S. D.

2017-12-01

Here we present a comprehensive case study of the in situ electron acceleration at the two ICME-driven shocks observed by WIND/3DP on February 11, 2000 and July 22, 2004. For the 11 February 2000 shock (the 22 July 2004 shock), the shocked electrons in the downstream show significant flux enhancements over the ambient solar wind electrons at energies up to 40 keV (66 keV) with a 6.0 times (1.9 times) ehancment at 40 keV, the strongest among all the quasi-perpendicular (quasi-parallel) ICME-driven shocks observed by the WIND spacecraft at 1 AU from 1995 through 2014. We find that in both shocks, the shocked electron fluxes at 0.5-40 keV fit well to a double power-law spectrum, J ˜ E-β, bending up at ˜2 keV. In the downstream, these shocked electrons show stronger fluxes in the anti-sunward direction, but their enhancement over the ambient fluxes peaks near 90° pitch angle (PA). For the 11 February 2000 shock, the electron spectral index, β, appears to not vary with the electron PA, while for the 22 July 2004 shock, β roughly decreases from the anti-sunward PA direction to the sunward PA direction. All of these spectral indexes are strongly larger than the theoretical prediction of diffusive shock acceleration. At energies above (below) 2 keV, however, the shocked electron β is similar to the solar wind superhalo (halo) electrons observed at quiet times. These results suggest that the electron acceleration at the ICME-driven shocks at 1 AU may favor the shock drift acceleration, and the superthermal electrons accelerated by the interplanetary shocks may contribute to the formation of the halo and superhalo electron populations in the solar wind.

Highly Flexible Self-Assembled V2O5 Cathodes Enabled by Conducting Diblock Copolymers

NASA Astrophysics Data System (ADS)

An, Hyosung; Mike, Jared; Smith, Kendall; Swank, Lisa; Lin, Yen-Hao; Pesek, Stacy; Verduzco, Rafael; Lutkenhaus, Jodie

Structural energy storage materials combining load-bearing mechanical properties and high energy storage performance are desired for applications in wearable devices or flexible displays. Vanadium pentoxide (V2O5) is a promising cathode material for possible use in flexible battery electrodes, but it remains limited by low Li+ diffusion coefficient and electronic conductivity, severe volumetric changes upon cycling, and limited mechanical flexibility. Here, we demonstrate a route to address these challenges by blending a diblock copolymer bearing electron- and ion-conducting blocks, poly(3-hexylthiophene)-block-poly(ethyleneoxide) (P3HT- b-PEO), with V2O5 to form a mechanically flexible, electro-mechanically stable hybrid electrode. V2O5 layers were arranged parallel in brick-and-mortar-like fashion held together by the P3HT- b-PEO binder. This unique structure significantly enhances mechanical flexibility, toughness and cyclability without sacrificing capacity. Electrodes comprised of 10 wt% polymer have unusually high toughness (293 kJ/m3) and specific energy (530 Wh/kg), both higher than reduced graphene oxide paper electrodes.

Dissociative double-photoionization of butadiene in the 25-45 eV energy range using 3-D multi-coincidence ion momentum imaging spectrometry

DOE Office of Scientific and Technical Information (OSTI.GOV)

Oghbaie, Shabnam; Gisselbrecht, Mathieu; Laksman, Joakim

Dissociative double-photoionization of butadiene in the 25-45 eV energy range has been studied with tunable synchrotron radiation using full three-dimensional ion momentum imaging. Using ab initio calculations, the electronic states of the molecular dication below 33 eV are identified. The results of the measurement and calculation show that double ionization from π orbitals selectively triggers twisting about the terminal or central C–C bonds. We show that this conformational rearrangement depends upon the dication electronic state, which effectively acts as a gateway for the dissociation reaction pathway. For photon energies above 33 eV, three-body dissociation channels where neutral H-atom evaporation precedesmore » C–C charge-separation in the dication species appear in the correlation map. The fragment angular distributions support a model where the dication species is initially aligned with the molecular backbone parallel to the polarization vector of the light, indicating a high probability for double-ionization to the “gateway states” for molecules with this orientation.« less

Validation of Ground-based Optical Estimates of Auroral Electron Precipitation Energy Deposition

NASA Astrophysics Data System (ADS)

Hampton, D. L.; Grubbs, G. A., II; Conde, M.; Lynch, K. A.; Michell, R.; Zettergren, M. D.; Samara, M.; Ahrns, M. J.

2017-12-01

One of the major energy inputs into the high latitude ionosphere and mesosphere is auroral electron precipitation. Not only does the kinetic energy get deposited, the ensuing ionization in the E and F-region ionosphere modulates parallel and horizontal currents that can dissipate in the form of Joule heating. Global models to simulate these interactions typically use electron precipitation models that produce a poor representation of the spatial and temporal complexity of auroral activity as observed from the ground. This is largely due to these precipitation models being based on averages of multiple satellite overpasses separated by periods much longer than typical auroral feature durations. With the development of regional and continental observing networks (e.g. THEMIS ASI), the possibility of ground-based optical observations producing quantitative estimates of energy deposition with temporal and spatial scales comparable to those known to be exhibited in auroral activity become a real possibility. Like empirical precipitation models based on satellite overpasses such optics-based estimates are subject to assumptions and uncertainties, and therefore require validation. Three recent sounding rocket missions offer such an opportunity. The MICA (2012), GREECE (2014) and Isinglass (2017) missions involved detailed ground based observations of auroral arcs simultaneously with extensive on-board instrumentation. These have afforded an opportunity to examine the results of three optical methods of determining auroral electron energy flux, namely 1) ratio of auroral emissions, 2) green line temperature vs. emission altitude, and 3) parametric estimates using white-light images. We present comparisons from all three methods for all three missions and summarize the temporal and spatial scales and coverage over which each is valid.

Atypical Particle Heating at a Supercritical Interplanetary Shock

NASA Technical Reports Server (NTRS)

Wilson, Lynn B., III

2010-01-01

We present the first observations at an interplanetary shock of large amplitude (> 100 mV/m pk-pk) solitary waves and large amplitude (approx.30 mV/m pk-pk) waves exhibiting characteristics consistent with electron Bernstein waves. The Bernstein-like waves show enhanced power at integer and half-integer harmonics of the cyclotron frequency with a broadened power spectrum at higher frequencies, consistent with the electron cyclotron drift instability. The Bernstein-like waves are obliquely polarized with respect to the magnetic field but parallel to the shock normal direction. Strong particle heating is observed in both the electrons and ions. The observed heating and waveforms are likely due to instabilities driven by the free energy provided by reflected ions at this supercritical interplanetary shock. These results offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind.

Predictive design and interpretation of colliding pulse injected laser wakefield experiments

NASA Astrophysics Data System (ADS)

Cormier-Michel, Estelle; Ranjbar, Vahid H.; Cowan, Ben M.; Bruhwiler, David L.; Geddes, Cameron G. R.; Chen, Min; Ribera, Benjamin; Esarey, Eric; Schroeder, Carl B.; Leemans, Wim P.

2010-11-01

The use of colliding laser pulses to control the injection of plasma electrons into the plasma wake of a laser plasma accelerator is a promising approach to obtaining stable, tunable electron bunches with reduced emittance and energy spread. Colliding Pulse Injection (CPI) experiments are being performed by groups around the world. We will present recent particle-in-cell simulations, using the parallel VORPAL framework, of CPI for physical parameters relevant to ongoing experiments of the LOASIS program at LBNL. We evaluate the effect of laser and plasma tuning, on the trapped electron bunch and perform parameter scans in order to optimize the quality of the bunch. Impact of non-ideal effects such as imperfect laser modes and laser self focusing are also evaluated. Simulation data are validated against current experimental results, and are used to design future experiments.

Performance of alumina-supported Pt catalysts in an electron-beam-sustained CO2 laser amplifier

NASA Technical Reports Server (NTRS)

Cunningham, D. L.; Jones, P. L.; Miyake, C. I.; Moody, S. E.

1990-01-01

The performance of an alumina-supported Pt catalyst system used to maintain the gas purity in an electron-beam-sustained (636) isotope CO2 laser amplifier has been tested. The system characteristics using the two-zone, parallel flow reactor were determined for both continuous- and end-of-day reactor operation using on-line mass spectrometric sampling. The laser amplifier was run with an energy loading of typically 110 J-l/atm and an electron-beam current of 4 mA/sq cm. With these conditions and a pulse repetition frequency of 10 Hz for up to 10,000 shots, increases on the order of 100 ppm O2 were observed with the purifier on and 150 ppm with it off. The 1/e time recovery time was found to be approximately 75 minutes.

The distinct character of anisotropy and intermittency in inertial and kinetic range solar wind plasma turbulence

NASA Astrophysics Data System (ADS)

Kiyani, Khurom; Chapman, Sandra; Osman, Kareem; Sahraoui, Fouad; Hnat, Bogdan

2014-05-01

The anisotropic nature of the scaling properties of solar wind magnetic turbulence fluctuations is investigated scale by scale using high cadence in situ magnetic field measurements from the Cluster, ACE and STEREO spacecraft missions in both fast and slow quiet solar wind conditions. The data span five decades in scales from the inertial range to the electron Larmor radius. We find a clear transition in scaling behaviour between the inertial and kinetic range of scales, which provides a direct, quantitative constraint on the physical processes that mediate the cascade of energy through these scales. In the inertial (magnetohydrodynamic) range the statistical nature of turbulent fluctuations are known to be anisotropic, both in the vector components of the magnetic field fluctuations (variance anisotropy) and in the spatial scales of these fluctuations (wavevector or k-anisotropy). We show for the first time that, when measuring parallel to the local magnetic field direction, the full statistical signature of the magnetic and Elsasser field fluctuations is that of a non-Gaussian globally scale-invariant process. This is distinct from the classic multi-exponent statistics observed when the local magnetic field is perpendicular to the flow direction. These observations suggest the weakness, or absence, of a parallel magnetofluid turbulence energy cascade. In contrast to the inertial range, there is a successive increase toward isotropy between parallel and transverse power at scales below the ion Larmor radius, with isotropy being achieved at the electron Larmor radius. Computing higher-order statistics, we show that the full statistical signature of both parallel, and perpendicular fluctuations at scales below the ion Larmor radius are that of an isotropic globally scale-invariant non-Gaussian process. Lastly, we perform a survey of multiple intervals of quiet solar wind sampled under different plasma conditions (fast, slow wind; plasma beta etc.) and find that the above results on the scaling transition between inertial and kinetic range scales are qualitatively robust, and that quantitatively, there is a spread in the values of the scaling exponents.

Unusual characteristics of electromagnetic waves excited by cometary newborn ions with large perpendicular energies

NASA Technical Reports Server (NTRS)

Brinca, A. L.; Tsurutani, B. T.

1987-01-01

The characteristics of electromagnetic waves excited by cometary newborn ions with large perpendicular energies are examined using a model of solar wind permeated by dilute drifting ring distributions of electrons and oxygen ions with finite thermal spreads. The model has parameters compatible with the ICE observations at the Giacobini-Zinner comet. It is shown that cometary newborn ions with large perpendicular energies can excite a wave mode with rest frame frequencies in the order of the heavy ion cyclotron frequency, Omega(i), and unusual propagation characteristics at small obliquity angles. For parallel propagation, the mode is left-hand circularly polarized, might be unstable in a frequency range containing Omega(i), and moves in the direction of the newborn ion drift along the static magnetic field.

Multiplexed high resolution soft x-ray RIXS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chuang, Y.-D.; Voronov, D.; Warwick, T.

2016-07-27

High-resolution Resonance Inelastic X-ray Scattering (RIXS) is a technique that allows us to probe the electronic excitations of complex materials with unprecedented precision. However, the RIXS process has a low cross section, compounded by the fact that the optical spectrometers used to analyze the scattered photons can only collect a small solid angle and overall have a small efficiency. Here we present a method to significantly increase the throughput of RIXS systems, by energy multiplexing, so that a complete RIXS map of scattered intensity versus photon energy in and photon energy out can be recorded simultaneously{sup 1}. This parallel acquisitionmore » scheme should provide a gain in throughput of over 100.. A system based on this principle, QERLIN, is under construction at the Advanced Light Source (ALS).« less

Time-dependent current into and through multilevel parallel quantum dots in a photon cavity

NASA Astrophysics Data System (ADS)

Gudmundsson, Vidar; Abdullah, Nzar Rauf; Sitek, Anna; Goan, Hsi-Sheng; Tang, Chi-Shung; Manolescu, Andrei

2017-05-01

We analyze theoretically the charging current into, and the transport current through, a nanoscale two-dimensional electron system with two parallel quantum dots embedded in a short wire placed in a photon cavity. A plunger gate is used to place specific many-body states of the interacting system in the bias window defined by the external leads. We show how the transport phenomena active in the many-level complex central system strongly depend on the gate voltage. We identify a resonant transport through the central system as the two spin components of the one-electron ground state are in the bias window. This resonant transport through the lowest energy electron states seems to a large extent independent of the detuned photon field when judged from the transport current. This could be expected in the small bias regime, but an observation of the occupancy of the states of the system reveals that this picture is not entirely true. The current does not reflect slower photon-active internal transitions bringing the system into the steady state. The number of initially present photons determines when the system reaches the real steady state. With two-electron states in the bias window we observe a more complex situation with intermediate radiative and nonradiative relaxation channels leading to a steady state with a weak nonresonant current caused by inelastic tunneling through the two-electron ground state of the system. The presence of the radiative channels makes this phenomena dependent on the number of photons initially in the cavity.

FAST/Polar Conjunction Study of Field-Aligned Auroral Acceleration and Corresponding Magnetotail Drivers

NASA Technical Reports Server (NTRS)

Schriver, D.; Ashour-Abdalla, M.; Strangeway, R. J.; Richard, R. L.; Klezting, C.; Dotan, Y.; Wygant, J.

2003-01-01

The discrete aurora results when energized electrons bombard the Earth's atmosphere at high latitudes. This paper examines the physical processes that can cause field-aligned acceleration of plasma particles in the auroral region. A data and theoretical study has been carried out to examine the acceleration mechanisms that operate in the auroral zone and to identi@ the magnetospheric drivers of these acceleration mechanisms. The observations used in the study were collected by the Fast Auroral Snapshot (FAST) and Polar satellites when the two satellites were in approximate magnetic conjunction in the auroral region. During these events FAST was in the middle of the auroral zone and Polar was above the auroral zone in the near-Earth plasma sheet. Polar data were used to determine the conditions in the magnetotail at the time field-aligned acceleration was measured by FAST in the auroral zone. For each of the magnetotail drivers identified in the data study, the physics of field-aligned acceleration in the auroral region was examined using existing theoretical efforts and/or a long-system particle in cell simulation to model the magnetically connected region between the two satellites. Results from the study indicate that there are three main drivers of auroral acceleration: (1) field-aligned currents that lead to quasistatic parallel potential drops (parallel electric fields), (2) earthward flow of high-energy plasma beams from the magnetotail into the auroral zone that lead to quasistatic parallel potential drops, and (3) large-amplitude Alfven waves that propagate into the auroral region from the magnetotail. The events examined thus far confm the previously established invariant latitudinal dependence of the drivers and show a strong dependence on magnetic activity. Alfven waves tend to occur primarily at the poleward edge of the auroral region during more magnetically active times and are correlated with intense electron precipitation. At lower latitudes away from the poleward edge of the auroral zone is the primary field-aligned current region which results in the classical field- aligned acceleration associated with the auroral zone (electrons earthward and ion beams tailward). During times of high magnetic activity, high-energy ion beams originating from the magnetotail are observed within, and overlapping, the regions of primary and return field-aligned current. Along the field lines where the high-energy magnetotail ion beams are located, field-aligned acceleration can occur in the auroral zone leading to precipitating electrons and upwelling ionospheric ion beams. Field-aligned currents are present during both quiet and active times, while the Alfven waves and magnetotail ion beams were observed only during more magnetically active events.

Higher-order finite-difference formulation of periodic Orbital-free Density Functional Theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ghosh, Swarnava; Suryanarayana, Phanish, E-mail: phanish.suryanarayana@ce.gatech.edu

2016-02-15

We present a real-space formulation and higher-order finite-difference implementation of periodic Orbital-free Density Functional Theory (OF-DFT). Specifically, utilizing a local reformulation of the electrostatic and kernel terms, we develop a generalized framework for performing OF-DFT simulations with different variants of the electronic kinetic energy. In particular, we propose a self-consistent field (SCF) type fixed-point method for calculations involving linear-response kinetic energy functionals. In this framework, evaluation of both the electronic ground-state and forces on the nuclei are amenable to computations that scale linearly with the number of atoms. We develop a parallel implementation of this formulation using the finite-difference discretization.more » We demonstrate that higher-order finite-differences can achieve relatively large convergence rates with respect to mesh-size in both the energies and forces. Additionally, we establish that the fixed-point iteration converges rapidly, and that it can be further accelerated using extrapolation techniques like Anderson's mixing. We validate the accuracy of the results by comparing the energies and forces with plane-wave methods for selected examples, including the vacancy formation energy in Aluminum. Overall, the suitability of the proposed formulation for scalable high performance computing makes it an attractive choice for large-scale OF-DFT calculations consisting of thousands of atoms.« less

Diverse strategies of O2 usage for preventing photo-oxidative damage under CO2 limitation during algal photosynthesis.

PubMed

Shimakawa, Ginga; Matsuda, Yusuke; Nakajima, Kensuke; Tamoi, Masahiro; Shigeoka, Shigeru; Miyake, Chikahiro

2017-01-20

Photosynthesis produces chemical energy from photon energy in the photosynthetic electron transport and assimilates CO 2 using the chemical energy. Thus, CO 2 limitation causes an accumulation of excess energy, resulting in reactive oxygen species (ROS) which can cause oxidative damage to cells. O 2 can be used as an alternative energy sink when oxygenic phototrophs are exposed to high light. Here, we examined the responses to CO 2 limitation and O 2 dependency of two secondary algae, Euglena gracilis and Phaeodactylum tricornutum. In E. gracilis, approximately half of the relative electron transport rate (ETR) of CO 2 -saturated photosynthesis was maintained and was uncoupled from photosynthesis under CO 2 limitation. The ETR showed biphasic dependencies on O 2 at high and low O 2 concentrations. Conversely, in P. tricornutum, most relative ETR decreased in parallel with the photosynthetic O 2 evolution rate in response to CO 2 limitation. Instead, non-photochemical quenching was strongly activated under CO 2 limitation in P. tricornutum. The results indicate that these secondary algae adopt different strategies to acclimatize to CO 2 limitation, and that both strategies differ from those utilized by cyanobacteria and green algae. We summarize the diversity of strategies for prevention of photo-oxidative damage under CO 2 limitation in cyanobacterial and algal photosynthesis.

Dissociative Electron Attachment in the condensed phase: sample morphology and bio-molecules

NASA Astrophysics Data System (ADS)

Bass, A. D.

2001-10-01

Recent electron impact experiments on condensed plasmid DNA have shown low energy electrons to be remarkably effective in causing damage and reveal that electron-scattering phenomena, such as transient anion formation and their decay via dissociative electron attachment, play a central role in this process. Such experiments may prompt a revision of our understanding of the mutagenic effects of radiation and have significant implications for both radiotherapy and radio-protection. These results can be better understood by investigating electron scattering with the various functional constituents of DNA in condensed environments. Recent work, to be presented here, has focused on electron attachment processes in condensed DNA bases and sugar-like analogues of the DNA backbone, as evidenced by the desorption of fragment anions. Despite this progress, a complete understanding of these processes requires parallel study of simpler `model' systems, which allow the characteristic condensed-phase phenomena modulating electron-scattering to be identified. Factors affecting anion formation and DEA can been classed as either intrinsic (affecting the properties of the resonance) or extrinsic (modifying the energy of electrons before attachment and/or the reactions of fragments, post-dissociation). In this talk we will present new results in which the extrinsic factors of porosity and phase of a sample are probed via the desorption of anionic fragments from either the pure film or from probe molecules condensed upon its surface. We show that anion desorption and hence our ability to observe DEA process, is highly sensitive to sample morphology and phase, a property which can be exploited to study the morphology of the film itself.

Synthesis of parallel and antiparallel core-shell triangular nanoparticles

NASA Astrophysics Data System (ADS)

Bhattacharjee, Gourab; Satpati, Biswarup

2018-04-01

Core-shell triangular nanoparticles were synthesized by seed mediated growth. Using triangular gold (Au) nanoparticle as template, we have grown silver (Ag) shellto get core-shell nanoparticle. Here by changing the chemistry we have grown two types of core-shell structures where core and shell is having same symmetry and also having opposite symmetry. Both core and core-shell nanoparticles were characterized using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) to know the crystal structure and composition of these synthesized core-shell nanoparticles. From diffraction pattern analysis and energy filtered TEM (EFTEM) we have confirmed the crystal facet in core is responsible for such two dimensional growth of core-shell nanostructures.

CCD sensors in synchrotron X-ray detectors

NASA Astrophysics Data System (ADS)

Strauss, M. G.; Naday, I.; Sherman, I. S.; Kraimer, M. R.; Westbrook, E. M.; Zaluzec, N. J.

1988-04-01

The intense photon flux from advanced synchrotron light sources, such as the 7-GeV synchrotron being designed at Argonne, require integrating-type detectors. Charge-coupled devices (CCDs) are well suited as synchrotron X-ray detectors. When irradiated indirectly via a phosphor followed by reducing optics, diffraction patterns of 100 cm 2 can be imaged on a 2 cm 2 CCD. With a conversion efficiency of ˜ 1 CCD electron/X-ray photon, a peak saturation capacity of > 10 6 X-rays can be obtained. A programmable CCD controller operating at a clock frequency of 20 MHz has been developed. The readout rate is 5 × 10 6 pixels/s and the shift rate in the parallel registers is 10 6 lines/s. The test detector was evaluated in two experiments. In protein crystallography diffraction patterns have been obtained from a lysozyme crystal using a conventional rotating anode X-ray generator. Based on these results we expect to obtain at a synchrotron diffraction images at a rate of ˜ 1 frame/s or a complete 3-dimensional data set from a single crystal in ˜ 2 min. In electron energy-loss spectroscopy (EELS), the CCD was used in a parallel detection mode which is similar to the mode array detectors are used in dispersive EXAFS. With a beam current corresponding to 3 × 10 9 electron/s on the detector, a series of 64 spectra were recorded on the CCD in a continuous sequence without interruption due to readout. The frame-to-frame pixel signal fluctuations had σ = 0.4% from which DQE = 0.4 was obtained, where the detector conversion efficiency was 2.6 CCD electrons/X-ray photon. These multiple frame series also showed the time-resolved modulation of the electron microscope optics by stray magnetic fields.

Electronic Switch Arrays for Managing Microbattery Arrays

NASA Technical Reports Server (NTRS)

Mojarradi, Mohammad; Alahmad, Mahmoud; Sukumar, Vinesh; Zghoul, Fadi; Buck, Kevin; Hess, Herbert; Li, Harry; Cox, David

2008-01-01

Integrated circuits have been invented for managing the charging and discharging of such advanced miniature energy-storage devices as planar arrays of microscopic energy-storage elements [typically, microscopic electrochemical cells (microbatteries) or microcapacitors]. The architecture of these circuits enables implementation of the following energy-management options: dynamic configuration of the elements of an array into a series or parallel combination of banks (subarrarys), each array comprising a series of parallel combination of elements; direct addressing of individual banks for charging/or discharging; and, disconnection of defective elements and corresponding reconfiguration of the rest of the array to utilize the remaining functional elements to obtain the desited voltage and current performance. An integrated circuit according to the invention consists partly of a planar array of field-effect transistors that function as switches for routing electric power among the energy-storage elements, the power source, and the load. To connect the energy-storage elements to the power source for charging, a specific subset of switches is closed; to connect the energy-storage elements to the load for discharging, a different specific set of switches is closed. Also included in the integrated circuit is circuitry for monitoring and controlling charging and discharging. The control and monitoring circuitry, the switching transistors, and interconnecting metal lines are laid out on the integrated-circuit chip in a pattern that registers with the array of energy-storage elements. There is a design option to either (1) fabricate the energy-storage elements in the corresponding locations on, and as an integral part of, this integrated circuit; or (2) following a flip-chip approach, fabricate the array of energy-storage elements on a separate integrated-circuit chip and then align and bond the two chips together.

Adsorption and dissociation of molecular oxygen on α-Pu (0 2 0) surface: A density functional study

NASA Astrophysics Data System (ADS)

Wang, Jianguang; Ray, Asok K.

2011-09-01

Molecular and dissociative oxygen adsorptions on the α-Pu (0 2 0) surface have been systematically studied using the full-potential linearized augmented-plane-wave plus local orbitals (FP-LAPW+lo) basis method and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Chemisorption energies have been optimized for the distance of the admolecule from the Pu surface and the bond length of O-O atoms for four adsorption sites and three approaches of O 2 admolecule to the (0 2 0) surface. Chemisorption energies have been calculated at the scalar relativistic level with no spin-orbit coupling (NSOC) and at the fully relativistic level with spin-orbit coupling (SOC). Dissociative adsorptions are found at the two horizontal approaches (O 2 is parallel to the surface and perpendicular/parallel to a lattice vector). Hor2 (O 2 is parallel to the surface and perpendicular to a lattice vector) approach at the one-fold top site is the most stable adsorption site, with chemisorption energies of 8.048 and 8.415 eV for the NSOC and SOC cases, respectively, and an OO separation of 3.70 Å. Molecular adsorption occurs at the Vert (O 2 is vertical to the surface) approach of each adsorption site. The calculated work functions and net spin magnetic moments, respectively, increase and decrease in all cases upon chemisorption compared to the clean surface. The partial charges inside the muffin-tins, the difference charge density distributions, and the local density of states have been used to investigate the Pu-admolecule electronic structures and bonding mechanisms.

An analytic formula for the relativistic incoherent Thomson backscattering spectrum for a drifting bi-Maxwellian plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Naito, O.

2015-08-15

An analytic formula has been derived for the relativistic incoherent Thomson backscattering spectrum for a drifting anisotropic plasma when the scattering vector is parallel to the drifting direction. The shape of the scattering spectrum is insensitive to the electron temperature perpendicular to the scattering vector, but its amplitude may be modulated. As a result, while the measured temperature correctly represents the electron distribution parallel to the scattering vector, the electron density may be underestimated when the perpendicular temperature is higher than the parallel temperature. Since the scattering spectrum in shorter wavelengths is greatly enhanced by the existence of drift, themore » diagnostics might be used to measure local electron current density in fusion plasmas.« less

Multiple resonant railgun power supply

DOEpatents

Honig, E.M.; Nunnally, W.C.

1985-06-19

A multiple repetitive resonant railgun power supply provides energy for repetitively propelling projectiles from a pair of parallel rails. A plurality of serially connected paired parallel rails are powered by similar power supplies. Each supply comprises an energy storage capacitor, a storage inductor to form a resonant circuit with the energy storage capacitor and a magnetic switch to transfer energy between the resonant circuit and the pair of parallel rails for the propelling of projectiles. The multiple serial operation permits relatively small energy components to deliver overall relatively large amounts of energy to the projectiles being propelled.

Multiple resonant railgun power supply

DOEpatents

Honig, Emanuel M.; Nunnally, William C.

1988-01-01

A multiple repetitive resonant railgun power supply provides energy for repetitively propelling projectiles from a pair of parallel rails. A plurality of serially connected paired parallel rails are powered by similar power supplies. Each supply comprises an energy storage capacitor, a storage inductor to form a resonant circuit with the energy storage capacitor and a magnetic switch to transfer energy between the resonant circuit and the pair of parallel rails for the propelling of projectiles. The multiple serial operation permits relatively small energy components to deliver overall relatively large amounts of energy to the projectiles being propelled.

Toward real-time Monte Carlo simulation using a commercial cloud computing infrastructure.

PubMed

Wang, Henry; Ma, Yunzhi; Pratx, Guillem; Xing, Lei

2011-09-07

Monte Carlo (MC) methods are the gold standard for modeling photon and electron transport in a heterogeneous medium; however, their computational cost prohibits their routine use in the clinic. Cloud computing, wherein computing resources are allocated on-demand from a third party, is a new approach for high performance computing and is implemented to perform ultra-fast MC calculation in radiation therapy. We deployed the EGS5 MC package in a commercial cloud environment. Launched from a single local computer with Internet access, a Python script allocates a remote virtual cluster. A handshaking protocol designates master and worker nodes. The EGS5 binaries and the simulation data are initially loaded onto the master node. The simulation is then distributed among independent worker nodes via the message passing interface, and the results aggregated on the local computer for display and data analysis. The described approach is evaluated for pencil beams and broad beams of high-energy electrons and photons. The output of cloud-based MC simulation is identical to that produced by single-threaded implementation. For 1 million electrons, a simulation that takes 2.58 h on a local computer can be executed in 3.3 min on the cloud with 100 nodes, a 47× speed-up. Simulation time scales inversely with the number of parallel nodes. The parallelization overhead is also negligible for large simulations. Cloud computing represents one of the most important recent advances in supercomputing technology and provides a promising platform for substantially improved MC simulation. In addition to the significant speed up, cloud computing builds a layer of abstraction for high performance parallel computing, which may change the way dose calculations are performed and radiation treatment plans are completed.

Electron heating in a Monte Carlo model of a high Mach number, supercritical, collisionless shock

NASA Technical Reports Server (NTRS)

Ellison, Donald C.; Jones, Frank C.

1987-01-01

Preliminary work in the investigation of electron injection and acceleration at parallel shocks is presented. A simple model of electron heating that is derived from a unified shock model which includes the effects of an electrostatic potential jump is described. The unified shock model provides a kinetic description of the injection and acceleration of ions and a fluid description of electron heating at high Mach number, supercritical, and parallel shocks.

Searching for globally optimal functional forms for interatomic potentials using genetic programming with parallel tempering.

PubMed

Slepoy, A; Peters, M D; Thompson, A P

2007-11-30

Molecular dynamics and other molecular simulation methods rely on a potential energy function, based only on the relative coordinates of the atomic nuclei. Such a function, called a force field, approximately represents the electronic structure interactions of a condensed matter system. Developing such approximate functions and fitting their parameters remains an arduous, time-consuming process, relying on expert physical intuition. To address this problem, a functional programming methodology was developed that may enable automated discovery of entirely new force-field functional forms, while simultaneously fitting parameter values. The method uses a combination of genetic programming, Metropolis Monte Carlo importance sampling and parallel tempering, to efficiently search a large space of candidate functional forms and parameters. The methodology was tested using a nontrivial problem with a well-defined globally optimal solution: a small set of atomic configurations was generated and the energy of each configuration was calculated using the Lennard-Jones pair potential. Starting with a population of random functions, our fully automated, massively parallel implementation of the method reproducibly discovered the original Lennard-Jones pair potential by searching for several hours on 100 processors, sampling only a minuscule portion of the total search space. This result indicates that, with further improvement, the method may be suitable for unsupervised development of more accurate force fields with completely new functional forms. Copyright (c) 2007 Wiley Periodicals, Inc.

AMITIS: A 3D GPU-Based Hybrid-PIC Model for Space and Plasma Physics

NASA Astrophysics Data System (ADS)

Fatemi, Shahab; Poppe, Andrew R.; Delory, Gregory T.; Farrell, William M.

2017-05-01

We have developed, for the first time, an advanced modeling infrastructure in space simulations (AMITIS) with an embedded three-dimensional self-consistent grid-based hybrid model of plasma (kinetic ions and fluid electrons) that runs entirely on graphics processing units (GPUs). The model uses NVIDIA GPUs and their associated parallel computing platform, CUDA, developed for general purpose processing on GPUs. The model uses a single CPU-GPU pair, where the CPU transfers data between the system and GPU memory, executes CUDA kernels, and writes simulation outputs on the disk. All computations, including moving particles, calculating macroscopic properties of particles on a grid, and solving hybrid model equations are processed on a single GPU. We explain various computing kernels within AMITIS and compare their performance with an already existing well-tested hybrid model of plasma that runs in parallel using multi-CPU platforms. We show that AMITIS runs ∼10 times faster than the parallel CPU-based hybrid model. We also introduce an implicit solver for computation of Faraday’s Equation, resulting in an explicit-implicit scheme for the hybrid model equation. We show that the proposed scheme is stable and accurate. We examine the AMITIS energy conservation and show that the energy is conserved with an error < 0.2% after 500,000 timesteps, even when a very low number of particles per cell is used.

Alfvén Waves and the Aurora (Hannes Alfvén Medal Lecture)

NASA Astrophysics Data System (ADS)

Lysak, Robert

2015-04-01

The most compelling visual evidence of plasma processes in the magnetosphere of Earth as well as the other magnetized planets is the aurora. Over 40 years of research have indicated that the aurora is a consequence of the acceleration of charged particles toward the neutral atmosphere, where the excitation of neutral atoms and their subsequent relaxation to the ground state produces the auroral light. Much of this acceleration can be described by acceleration in a quasi-static electric field parallel to the geomagnetic field, producing nearly monoenergetic beams of electrons. While a variety of quasi-static models to describe such parallel electric fields have been developed, the dynamics of how these fields evolve is still an open question. Satellite measurements have indicated that a primary source of energy to support these fields is the Poynting flux associated with shear Alfvén waves propagating along auroral field lines. These Alfvén waves are generated in the magnetosphere and reflect from the ionosphere. On closed field lines, Alfvén waves bouncing between conjugate ionospheres produce field line resonances that have be observed both in space and by ground magnetometers. However, some auroral emissions do not follow this scenario. In these cases, the accelerated electrons are observed to have a broad energy spectrum, rather than a monoenergetic peak. Such a spectrum is suggestive of a time-dependent acceleration process that operates on a time scale of a few seconds, comparable to the electron transit time across the acceleration region. While field line resonances have a time scale on the order of minutes, waves with periods of a few seconds can be produced by partial reflections in the Ionospheric Alfvén Resonator, a resonant cavity formed by the rapid decrease of the plasma density and increase of the Alfvén speed above the ionosphere. In order to develop a parallel electric field that can accelerate auroral particles, these Alfvén waves must develop small spatial scales, where MHD theory breaks down. In this regime, the waves are called kinetic Alfvén waves. These small scales can be produced most simply be phase mixing, although ionospheric feedback and nonlinear effects may also be important. Since kinetic Alfvén waves require perpendicular wavelengths the order of a few kilometers, this model also provides a natural explanation of the narrow scales of discrete auroral arcs. These interactions between magnetosphere and ionosphere and the development of parallel electric fields have been described by means of numerical simulations that serve to illustrate these complex processes.

Parallel closure theory for toroidally confined plasmas

NASA Astrophysics Data System (ADS)

Ji, Jeong-Young; Held, Eric D.

2017-10-01

We solve a system of general moment equations to obtain parallel closures for electrons and ions in an axisymmetric toroidal magnetic field. Magnetic field gradient terms are kept and treated using the Fourier series method. Assuming lowest order density (pressure) and temperature to be flux labels, the parallel heat flow, friction, and viscosity are expressed in terms of radial gradients of the lowest-order temperature and pressure, parallel gradients of temperature and parallel flow, and the relative electron-ion parallel flow velocity. Convergence of closure quantities is demonstrated as the number of moments and Fourier modes are increased. Properties of the moment equations in the collisionless limit are also discussed. Combining closures with fluid equations parallel mass flow and electric current are also obtained. Work in collaboration with the PSI Center and supported by the U.S. DOE under Grant Nos. DE-SC0014033, DE-SC0016256, and DE-FG02-04ER54746.

Pitch angle distributions of electrons at dipolarization sites during geomagnetic activity: THEMIS observations

NASA Astrophysics Data System (ADS)

Wang, Kaiti; Lin, Ching-Huei; Wang, Lu-Yin; Hada, Tohru; Nishimura, Yukitoshi; Turner, Drew L.; Angelopoulos, Vassilis

2014-12-01

Changes in pitch angle distributions of electrons with energies from a few eV to 1 MeV at dipolarization sites in Earth's magnetotail are investigated statistically to determine the extent to which adiabatic acceleration may contribute to these changes. Forty-two dipolarization events from 2008 and 2009 observed by Time History of Events and Macroscale Interactions during Substorms probes covering the inner plasma sheet from 8 RE to 12 RE during geomagnetic activity identified by the AL index are analyzed. The number of observed events with cigar-type distributions (peaks at 0° and 180°) decreases sharply below 1 keV after dipolarization because in many of these events, electron distributions became more isotropized. From above 1 keV to a few tens of keV, however, the observed number of cigar-type events increases after dipolarization and the number of isotropic events decreases. These changes can be related to the ineffectiveness of Fermi acceleration below 1 keV (at those energies, dipolarization time becomes comparable to electron bounce time). Model-calculated pitch angle distributions after dipolarization with the effect of betatron and Fermi acceleration tested indicate that these adiabatic acceleration mechanisms can explain the observed patterns of event number changes over a large range of energies for cigar events and isotropic events. Other factors still need to be considered to assess the observed increase in cigar events around 2 keV. Indeed, preferential directional increase/loss of electron fluxes, which may contribute to the formation of cigar events, was observed. Nonadiabatic processes to accelerate electrons in a parallel direction may also be important for future study.

Electrostatic focal spot correction for x-ray tubes operating in strong magnetic fields.

PubMed

Lillaney, Prasheel; Shin, Mihye; Hinshaw, Waldo; Fahrig, Rebecca

2014-11-01

A close proximity hybrid x-ray/magnetic resonance (XMR) imaging system offers several critical advantages over current XMR system installations that have large separation distances (∼5 m) between the imaging fields of view. The two imaging systems can be placed in close proximity to each other if an x-ray tube can be designed to be immune to the magnetic fringe fields outside of the MR bore. One of the major obstacles to robust x-ray tube design is correcting for the effects of the MR fringe field on the x-ray tube focal spot. Any fringe field component orthogonal to the x-ray tube electric field leads to electron drift altering the path of the electron trajectories. The method proposed in this study to correct for the electron drift utilizes an external electric field in the direction of the drift. The electric field is created using two electrodes that are positioned adjacent to the cathode. These electrodes are biased with positive and negative potential differences relative to the cathode. The design of the focusing cup assembly is constrained primarily by the strength of the MR fringe field and high voltage standoff distances between the anode, cathode, and the bias electrodes. From these constraints, a focusing cup design suitable for the close proximity XMR system geometry is derived, and a finite element model of this focusing cup geometry is simulated to demonstrate efficacy. A Monte Carlo simulation is performed to determine any effects of the modified focusing cup design on the output x-ray energy spectrum. An orthogonal fringe field magnitude of 65 mT can be compensated for using bias voltages of +15 and -20 kV. These bias voltages are not sufficient to completely correct for larger orthogonal field magnitudes. Using active shielding coils in combination with the bias electrodes provides complete correction at an orthogonal field magnitude of 88.1 mT. Introducing small fields (<10 mT) parallel to the x-ray tube electric field in addition to the orthogonal field does not affect the electrostatic correction technique. However, rotation of the x-ray tube by 30° toward the MR bore increases the parallel magnetic field magnitude (∼72 mT). The presence of this larger parallel field along with the orthogonal field leads to incomplete correction. Monte Carlo simulations demonstrate that the mean energy of the x-ray spectrum is not noticeably affected by the electrostatic correction, but the output flux is reduced by 7.5%. The maximum orthogonal magnetic field magnitude that can be compensated for using the proposed design is 65 mT. Larger orthogonal field magnitudes cannot be completely compensated for because a pure electrostatic approach is limited by the dielectric strength of the vacuum inside the x-ray tube insert. The electrostatic approach also suffers from limitations when there are strong magnetic fields in both the orthogonal and parallel directions because the electrons prefer to stay aligned with the parallel magnetic field. These challenging field conditions can be addressed by using a hybrid correction approach that utilizes both active shielding coils and biasing electrodes.

Electrostatic focal spot correction for x-ray tubes operating in strong magnetic fields

PubMed Central

Lillaney, Prasheel; Shin, Mihye; Hinshaw, Waldo; Fahrig, Rebecca

2014-01-01

Purpose: A close proximity hybrid x-ray/magnetic resonance (XMR) imaging system offers several critical advantages over current XMR system installations that have large separation distances (∼5 m) between the imaging fields of view. The two imaging systems can be placed in close proximity to each other if an x-ray tube can be designed to be immune to the magnetic fringe fields outside of the MR bore. One of the major obstacles to robust x-ray tube design is correcting for the effects of the MR fringe field on the x-ray tube focal spot. Any fringe field component orthogonal to the x-ray tube electric field leads to electron drift altering the path of the electron trajectories. Methods: The method proposed in this study to correct for the electron drift utilizes an external electric field in the direction of the drift. The electric field is created using two electrodes that are positioned adjacent to the cathode. These electrodes are biased with positive and negative potential differences relative to the cathode. The design of the focusing cup assembly is constrained primarily by the strength of the MR fringe field and high voltage standoff distances between the anode, cathode, and the bias electrodes. From these constraints, a focusing cup design suitable for the close proximity XMR system geometry is derived, and a finite element model of this focusing cup geometry is simulated to demonstrate efficacy. A Monte Carlo simulation is performed to determine any effects of the modified focusing cup design on the output x-ray energy spectrum. Results: An orthogonal fringe field magnitude of 65 mT can be compensated for using bias voltages of +15 and −20 kV. These bias voltages are not sufficient to completely correct for larger orthogonal field magnitudes. Using active shielding coils in combination with the bias electrodes provides complete correction at an orthogonal field magnitude of 88.1 mT. Introducing small fields (<10 mT) parallel to the x-ray tube electric field in addition to the orthogonal field does not affect the electrostatic correction technique. However, rotation of the x-ray tube by 30° toward the MR bore increases the parallel magnetic field magnitude (∼72 mT). The presence of this larger parallel field along with the orthogonal field leads to incomplete correction. Monte Carlo simulations demonstrate that the mean energy of the x-ray spectrum is not noticeably affected by the electrostatic correction, but the output flux is reduced by 7.5%. Conclusions: The maximum orthogonal magnetic field magnitude that can be compensated for using the proposed design is 65 mT. Larger orthogonal field magnitudes cannot be completely compensated for because a pure electrostatic approach is limited by the dielectric strength of the vacuum inside the x-ray tube insert. The electrostatic approach also suffers from limitations when there are strong magnetic fields in both the orthogonal and parallel directions because the electrons prefer to stay aligned with the parallel magnetic field. These challenging field conditions can be addressed by using a hybrid correction approach that utilizes both active shielding coils and biasing electrodes. PMID:25370658

Electron acceleration in the Solar corona - 3D PiC code simulations of guide field reconnection

NASA Astrophysics Data System (ADS)

Alejandro Munoz Sepulveda, Patricio

2017-04-01

The efficient electron acceleration in the solar corona detected by means of hard X-ray emission is still not well understood. Magnetic reconnection through current sheets is one of the proposed production mechanisms of non-thermal electrons in solar flares. Previous works in this direction were based mostly on test particle calculations or 2D fully-kinetic PiC simulations. We have now studied the consequences of self-generated current-aligned instabilities on the electron acceleration mechanisms by 3D magnetic reconnection. For this sake, we carried out 3D Particle-in-Cell (PiC) code numerical simulations of force free reconnecting current sheets, appropriate for the description of the solar coronal plasmas. We find an efficient electron energization, evidenced by the formation of a non-thermal power-law tail with a hard spectral index smaller than -2 in the electron energy distribution function. We discuss and compare the influence of the parallel electric field versus the curvature and gradient drifts in the guiding-center approximation on the overall acceleration, and their dependence on different plasma parameters.

Study of the effective point of measurement for ion chambers in electron beams by Monte Carlo simulation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, L. L. W.; Rogers, D. W. O.

In current dosimetry protocols for electron beams, for plane-parallel chambers, the effective point of measurement is at the front face of the cavity, and, for cylindrical chambers, it is at a point shifted 0.5r upstream from the cavity center. In this study, Monte Carlo simulations are employed to study the issue of effective point of measurement for both plane-parallel chambers and cylindrical thimble chambers in electron beams. It is found that there are two ways of determining the position of the effective point of measurement: One is to match the calculated depth-ionization curve obtained from a modeled chamber to amore » calculated depth-dose curve; the other is to match the electron fluence spectrum in the chamber cavity to that in the phantom. For plane-parallel chambers, the effective point of measurement determined by the first method is generally not at the front face of the chamber cavity, which is obtained by the second method, but shifted downstream toward the cavity center by an amount that could be larger than one-half a millimeter. This should not be ignored when measuring depth-dose curves in electron beams. For cylindrical chambers, these two methods also give different positions of the effective point of measurement: The first gives a shift of 0.5r, which is in agreement with measurements for high-energy beams and is the same as the value currently used in major dosimetry protocols; the latter gives a shift of 0.8r, which is closer to the value predicted by a theoretical calculation assuming no-scatter conditions. The results also show that the shift of 0.8r is more appropriate if the cylindrical chamber is to be considered as a Spencer-Attix cavity. In electron beams, since the water/air stopping-power ratio changes with depth in a water phantom, the difference of the two shifts (0.3r) will lead to an incorrect evaluation of the water/air stopping-power ratio at the point of measurement, thus resulting in a systematic error in determining the absorbed dose by cylindrical chambers. It is suggested that a shift of 0.8r be used for electron beam calibrations with cylindrical chambers and a shift of 0.4r-0.5r be used for depth-dose measurements.« less

Study of the effective point of measurement for ion chambers in electron beams by Monte Carlo simulation.

PubMed

Wang, L L W; Rogers, D W O

2009-06-01

In current dosimetry protocols for electron beams, for plane-parallel chambers, the effective point of measurement is at the front face of the cavity, and, for cylindrical chambers, it is at a point shifted 0.5r upstream from the cavity center. In this study, Monte Carlo simulations are employed to study the issue of effective point of measurement for both plane-parallel chambers and cylindrical thimble chambers in electron beams. It is found that there are two ways of determining the position of the effective point of measurement: One is to match the calculated depth-ionization curve obtained from a modeled chamber to a calculated depth-dose curve; the other is to match the electron fluence spectrum in the chamber cavity to that in the phantom. For plane-parallel chambers, the effective point of measurement determined by the first method is generally not at the front face of the chamber cavity, which is obtained by the second method, but shifted downstream toward the cavity center by an amount that could be larger than one-half a millimeter. This should not be ignored when measuring depth-dose curves in electron beams. For cylindrical chambers, these two methods also give different positions of the effective point of measurement: The first gives a shift of 0.5r, which is in agreement with measurements for high-energy beams and is the same as the value currently used in major dosimetry protocols; the latter gives a shift of 0.8r, which is closer to the value predicted by a theoretical calculation assuming no-scatter conditions. The results also show that the shift of 0.8r is more appropriate if the cylindrical chamber is to be considered as a Spencer-Attix cavity. In electron beams, since the water/air stopping-power ratio changes with depth in a water phantom, the difference of the two shifts (0.3r) will lead to an incorrect evaluation of the water/air stopping-power ratio at the point of measurement, thus resulting in a systematic error in determining the absorbed dose by cylindrical chambers. It is suggested that a shift of 0.8r be used for electron beam calibrations with cylindrical chambers and a shift of 0.4r-0.5r be used for depth-dose measurements.

Staircase Quantum Dots Configuration in Nanowires for Optimized Thermoelectric Power

PubMed Central

Li, Lijie; Jiang, Jian-Hua

2016-01-01

The performance of thermoelectric energy harvesters can be improved by nanostructures that exploit inelastic transport processes. One prototype is the three-terminal hopping thermoelectric device where electron hopping between quantum-dots are driven by hot phonons. Such three-terminal hopping thermoelectric devices have potential in achieving high efficiency or power via inelastic transport and without relying on heavy-elements or toxic compounds. We show in this work how output power of the device can be optimized via tuning the number and energy configuration of the quantum-dots embedded in parallel nanowires. We find that the staircase energy configuration with constant energy-step can improve the power factor over a serial connection of a single pair of quantum-dots. Moreover, for a fixed energy-step, there is an optimal length for the nanowire. Similarly for a fixed number of quantum-dots there is an optimal energy-step for the output power. Our results are important for future developments of high-performance nanostructured thermoelectric devices. PMID:27550093

Efficient time-dependent density functional theory approximations for hybrid density functionals: analytical gradients and parallelization.

PubMed

Petrenko, Taras; Kossmann, Simone; Neese, Frank

2011-02-07

In this paper, we present the implementation of efficient approximations to time-dependent density functional theory (TDDFT) within the Tamm-Dancoff approximation (TDA) for hybrid density functionals. For the calculation of the TDDFT/TDA excitation energies and analytical gradients, we combine the resolution of identity (RI-J) algorithm for the computation of the Coulomb terms and the recently introduced "chain of spheres exchange" (COSX) algorithm for the calculation of the exchange terms. It is shown that for extended basis sets, the RIJCOSX approximation leads to speedups of up to 2 orders of magnitude compared to traditional methods, as demonstrated for hydrocarbon chains. The accuracy of the adiabatic transition energies, excited state structures, and vibrational frequencies is assessed on a set of 27 excited states for 25 molecules with the configuration interaction singles and hybrid TDDFT/TDA methods using various basis sets. Compared to the canonical values, the typical error in transition energies is of the order of 0.01 eV. Similar to the ground-state results, excited state equilibrium geometries differ by less than 0.3 pm in the bond distances and 0.5° in the bond angles from the canonical values. The typical error in the calculated excited state normal coordinate displacements is of the order of 0.01, and relative error in the calculated excited state vibrational frequencies is less than 1%. The errors introduced by the RIJCOSX approximation are, thus, insignificant compared to the errors related to the approximate nature of the TDDFT methods and basis set truncation. For TDDFT/TDA energy and gradient calculations on Ag-TB2-helicate (156 atoms, 2732 basis functions), it is demonstrated that the COSX algorithm parallelizes almost perfectly (speedup ~26-29 for 30 processors). The exchange-correlation terms also parallelize well (speedup ~27-29 for 30 processors). The solution of the Z-vector equations shows a speedup of ~24 on 30 processors. The parallelization efficiency for the Coulomb terms can be somewhat smaller (speedup ~15-25 for 30 processors), but their contribution to the total calculation time is small. Thus, the parallel program completes a Becke3-Lee-Yang-Parr energy and gradient calculation on the Ag-TB2-helicate in less than 4 h on 30 processors. We also present the necessary extension of the Lagrangian formalism, which enables the calculation of the TDDFT excited state properties in the frozen-core approximation. The algorithms described in this work are implemented into the ORCA electronic structure system.

Transport in a toroidally confined pure electron plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Crooks, S.M.; ONeil, T.M.

1996-07-01

O{close_quote}Neil and Smith [T.M. O{close_quote}Neil and R.A. Smith, Phys. Plasmas {bold 1}, 8 (1994)] have argued that a pure electron plasma can be confined stably in a toroidal magnetic field configuration. This paper shows that the toroidal curvature of the magnetic field of necessity causes slow cross-field transport. The transport mechanism is similar to magnetic pumping and may be understood by considering a single flux tube of plasma. As the flux tube of plasma undergoes poloidal {ital E}{bold {times}}{ital B} drift rotation about the center of the plasma, the length of the flux tube and the magnetic field strength withinmore » the flux tube oscillate, and this produces corresponding oscillations in {ital T}{sub {parallel}} and {ital T}{sub {perpendicular}}. The collisional relaxation of {ital T}{sub {parallel}} toward {ital T}{sub {perpendicular}} produces a slow dissipation of electrostatic energy into heat and a consequent expansion (cross-field transport) of the plasma. In the limit where the cross section of the plasma is nearly circular the radial particle flux is given by {Gamma}{sub {ital r}}=1/2{nu}{sub {perpendicular},{parallel}}{ital T}({ital r}/{rho}{sub 0}){sup 2}{ital n}/({minus}{ital e}{partial_derivative}{Phi}/{partial_derivative}{ital r}), where {nu}{sub {perpendicular},{parallel}} is the collisional equipartition rate, {rho}{sub 0} is the major radius at the center of the plasma, and {ital r} is the minor radius measured from the center of the plasma. The transport flux is first calculated using this simple physical picture and then is calculated by solving the drift-kinetic Boltzmann equation. This latter calculation is not limited to a plasma with a circular cross section. {copyright} {ital 1996 American Institute of Physics.}« less

In situ electron microscope study of the phase transformation, structure and growth of thin Te1-xSex films

NASA Astrophysics Data System (ADS)

Vermaak, J. S.; Raubenheimer, D.

1988-01-01

An in-situ electron microscope technique was utilized to observe directly the amorphous-to-crystalline phase transformation, the isothermal growth rates, as well as the orientation and structure of the recrystallized films for the Te1-xSex alloy system for x=0.2, 0.3 and 0.4. Activation energies of E=0.91, 0.93 and 0.96 eV and crystallization temperatures of Tc=-14, 81.5 and 85°C for the three alloys, respectively, were found. In all three cases the crystallization process originated from single crystalline nuclei with a hexagonal structure and with the c-axis in general parallel to the substrate surface.

Slow dynamics in glasses: A comparison between theory and experiment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Phillips, J. C.

Minimalist theories of complex systems are broadly of two kinds: mean field and axiomatic. So far, all theories of complex properties absent from simple systems and intrinsic to glasses are axiomatic. Stretched Exponential Relaxation (SER) is the prototypical complex temporal property of glasses, discovered by Kohlrausch 150 years ago, and now observed almost universally in microscopically homogeneous, complex nonequilibrium materials, including luminescent electronic Coulomb glasses. A critical comparison of alternative axiomatic theories with both numerical simulations and experiments strongly favors channeled dynamical trap models over static percolative or energy landscape models. The topics discussed cover those reported since the author'smore » review article in 1996, with an emphasis on parallels between channel bifurcation in electronic and molecular relaxation.« less

Electron acceleration in a secondary magnetic island formed during magnetic reconnection with a guide field

NASA Astrophysics Data System (ADS)

Wang, Huanyu; Lu, Quanming; Huang, Can; Wang, Shui

2017-05-01

Secondary magnetic islands may be generated in the vicinity of an X line during magnetic reconnection. In this paper, by performing two-dimensional (2-D) particle-in-cell simulations, we investigate the role of a secondary magnetic island in electron acceleration during magnetic reconnection with a guide field. The electron motions are found to be adiabatic, and we analyze the contributions of the parallel electric field and Fermi and betatron mechanisms to electron acceleration in the secondary island during the evolution of magnetic reconnection. When the secondary island is formed, electrons are accelerated by the parallel electric field due to the existence of the reconnection electric field in the electron current sheet. Electrons can be accelerated by both the parallel electric field and Fermi mechanism when the secondary island begins to merge with the primary magnetic island, which is formed simultaneously with the appearance of X lines. With the increase in the guide field, the contributions of the Fermi mechanism to electron acceleration become less and less important. When the guide field is sufficiently large, the contribution of the Fermi mechanism is almost negligible.

Dissipation and particle energization in moderate to low beta turbulent plasma via PIC simulations

DOE PAGES

Makwana, Kirit; Li, Hui; Guo, Fan; ...

2017-05-30

Here, we simulate decaying turbulence in electron-positron pair plasmas using a fully-kinetic particle-in-cell (PIC) code. We run two simulations with moderate-to-low plasma β (the ratio of thermal pressure to magnetic pressure). The energy decay rate is found to be similar in both cases. The perpendicular wave-number spectrum of magnetic energy shows a slope betweenmore » $${k}_{\\perp }^{-1.3}$$ and $${k}_{\\perp }^{-1.1}$$, where the perpendicular (⊥) and parallel (∥) directions are defined with respect to the magnetic field. The particle kinetic energy distribution function shows the formation of a non-thermal feature in the case of lower plasma β, with a slope close to E-1. The correlation between thin turbulent current sheets and Ohmic heating by the dot product of electric field (E) and current density (J) is investigated. By heating the parallel E∥ centerdot J∥ term dominates the perpendicular E⊥ centerdot J⊥ term. Regions of strong E∥ centerdot J∥ are spatially well-correlated with regions of intense current sheets, which also appear correlated with regions of strong E∥ in the low β simulation, suggesting an important role of magnetic reconnection in the dissipation of low β plasma turbulence.« less

The Acceleration of Thermal Protons and Minor Ions at a Quasi-Parallel Interplanetary Shock

NASA Astrophysics Data System (ADS)

Giacalone, J.; Lario, D.; Lepri, S. T.

2017-12-01

We compare the results from self-consistent hybrid simulations (kinetic ions, massless fluid electrons) and spacecraft observations of a strong, quasi-parallel interplanetary shock that crossed the Advanced Composition Explorer (ACE) on DOY 94, 2001. In our simulations, the un-shocked plasma-frame ion distributions are Maxwellian. Our simulations include protons and minor ions (alphas, 3He++, and C5+). The interplanetary shock crossed both the ACE and the Wind spacecraft, and was associated with significant increases in the flux of > 50 keV/nuc ions. Our simulation uses parameters (ion densities, magnetic field strength, Mach number, etc.) consistent with those observed. Acceleration of the ions by the shock, in a manner similar to that expected from diffusive shock acceleration theory, leads to a high-energy tail in the distribution of the post-shock plasma for all ions we considered. The simulated distributions are directly compared to those observed by ACE/SWICS, EPAM, and ULEIS, and Wind/STICS and 3DP, covering the energy range from below the thermal peak to the suprathermal tail. We conclude from our study that the solar wind is the most significant source of the high-energy ions for this event. Our results have important implications for the physics of the so-called `injection problem', which will be discussed.

Dissipation and particle energization in moderate to low beta turbulent plasma via PIC simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Makwana, Kirit; Li, Hui; Guo, Fan

Here, we simulate decaying turbulence in electron-positron pair plasmas using a fully-kinetic particle-in-cell (PIC) code. We run two simulations with moderate-to-low plasma β (the ratio of thermal pressure to magnetic pressure). The energy decay rate is found to be similar in both cases. The perpendicular wave-number spectrum of magnetic energy shows a slope betweenmore » $${k}_{\\perp }^{-1.3}$$ and $${k}_{\\perp }^{-1.1}$$, where the perpendicular (⊥) and parallel (∥) directions are defined with respect to the magnetic field. The particle kinetic energy distribution function shows the formation of a non-thermal feature in the case of lower plasma β, with a slope close to E-1. The correlation between thin turbulent current sheets and Ohmic heating by the dot product of electric field (E) and current density (J) is investigated. By heating the parallel E∥ centerdot J∥ term dominates the perpendicular E⊥ centerdot J⊥ term. Regions of strong E∥ centerdot J∥ are spatially well-correlated with regions of intense current sheets, which also appear correlated with regions of strong E∥ in the low β simulation, suggesting an important role of magnetic reconnection in the dissipation of low β plasma turbulence.« less

Characteristics of Surface Sterilization using ECR Plasma

NASA Astrophysics Data System (ADS)

Yonesu, Akira; Hara, Kazufumi; Nishikawa, Tatsuya; Hayashi, Nobuya

2015-09-01

Plasma sterilization techniques have superior characteristics such as a short treatment times, non-toxicity and low thermal damages on the sterilized materials. In plasma sterilization, microorganisms can be sterilized by active radicals, energetic charged particles, and vacuum UV radiation. The influence of each factor depends on the plasma operating parameters. Microwave discharges under the electron cyclotron resonance (ECR) condition produce higher electron temperature and density plasma as compared with other plasma generation techniques. In the present study, characteristics of surface sterilization using ECR plasma have been investigated.The experiment was performed in the vacuum chamber which contains a magnet holder. A pair of rectangular Sm-Co permanent magnets is aligned parallel to each other within the magnet holder. The region of the magnetic field for ECR exists near the magnet holder surface. When the microwave is introduced into the vacuum chamber, a ECR plasma is produced around surface of the magnet holder. High energy electrons and oxygen radicals were observed at ECR zone by electric probe method and optical spectroscopic method. Biological indicators (B.I.) having spore of 106 was sterilized in 2min for oxygen discharge. The temperature of the B.I. installation position was about 55°. The sterilization was achieved by the effect of oxygen radicals and high energy electrons.

Electronics Book II.

ERIC Educational Resources Information Center

Johnson, Dennis; And Others

This manual, the second of three curriculum guides for an electronics course, is intended for use in a program combining vocational English as a second language (VESL) with bilingual vocational education. Ten units cover the electrical team, Ohm's law, Watt's law, series resistive circuits, parallel resistive circuits, series parallel circuits,…

High quality ultrafast transmission electron microscopy using resonant microwave cavities.

PubMed

Verhoeven, W; van Rens, J F M; Kieft, E R; Mutsaers, P H A; Luiten, O J

2018-05-01

Ultrashort, low-emittance electron pulses can be created at a high repetition rate by using a TM 110 deflection cavity to sweep a continuous beam across an aperture. These pulses can be used for time-resolved electron microscopy with atomic spatial and temporal resolution at relatively large average currents. In order to demonstrate this, a cavity has been inserted in a transmission electron microscope, and picosecond pulses have been created. No significant increase of either emittance or energy spread has been measured for these pulses. At a peak current of 814 ± 2 pA, the root-mean-square transverse normalized emittance of the electron pulses is ɛ n,x =(2.7±0.1)·10 -12  m rad in the direction parallel to the streak of the cavity, and ɛ n,y =(2.5±0.1)·10 -12  m rad in the perpendicular direction for pulses with a pulse length of 1.1-1.3 ps. Under the same conditions, the emittance of the continuous beam is ɛ n,x =ɛ n,y =(2.5±0.1)·10 -12  m rad. Furthermore, for both the pulsed and the continuous beam a full width at half maximum energy spread of 0.95 ± 0.05 eV has been measured. Copyright © 2018 Elsevier B.V. All rights reserved.

Influence of the normal modes on the plasma uniformity in large scale CCP reactors

NASA Astrophysics Data System (ADS)

Eremin, Denis; Brinkmann, Ralf Peter; Mussenbrock, Thomas; Lane, Barton; Matsukuma, Masaaki; Ventzek, Peter

2016-09-01

Large scale capacitively coupled plasmas (CCP) driven by sources with high frequency components often exhibit phenomena which are absent in relatively well understood small scale CCPs driven at low frequencies. Of particular interest are such phenomena which affect discharge parameters of direct relevance to the plasma processing applications. One of such parameters is plasma uniformity. By using a self-consistent 2d3v Particle-in-cell/Monte-Carlo (PIC/MCC) code parallelized on GPU we have been able to show that uniformity of the plasma generated is influenced predominantly by two factors, the ionization pattern caused by high-energy electrons and the average temperature of low-energy plasma electrons. The heating mechanisms for these two groups of electrons appear to be different leading to different transversal (radial) profiles of the corresponding factors, which is well captured by the kinetic PIC/MCC code. We find that the heating mechanisms are intrinsically connected with excitation of normal modes inherent to a plasma-filled CCP reactor. In this work we study the wave nature of these phenomena, such as their excitation, propagation, and interaction with electrons. Supported by SFB-TR 87 project of the German Research Foundation and by the ``Experimental and numerical analysis of very high frequency capacitively coupled plasma discharges'' mutual research project between RUB and Tokyo Electron Ltd.

Monoatomic and dimer Mn adsorption on the Au(111) surface from first principles

NASA Astrophysics Data System (ADS)

Muñoz, Francisco; Romero, Aldo H.; Mejía-López, Jose; Morán-López, J. L.

2011-05-01

A theoretical study based on the density functional theory of the adsorption of Mn monomers and dimers on a Au-(111) surface is presented. As necessary preliminary steps, the bulk and clean surface electronic structure are calculated, which agree well with previous reports. Then, the electronic structure of the Mn adatom, chemisorbed on four different surface geometries, is analyzed. It is found that the most stable geometry is when the Mn atom is chemisorbed on threefold coordinated sites. Using this geometry for a single adatom a second Mn atom is chemisorbed and the most stable dimer geometrical structure is calculated. The lowest-energy configuration corresponds to the molecule lying parallel to the surface, adsorbed on two topological equivalent threefold coordinated sites. It is also found that the lowest-energy magnetic configuration corresponds to the antiferromagnetic arrangement with individual magnetic moments of 4.64μB. Finally, it is concluded that the dimer is not stable and should fragment at the surface.

Capacitively coupled hydrogen plasmas sustained by tailored voltage waveforms: excitation dynamics and ion flux asymmetry

DOE PAGES

Bruneau, B.; Diomede, P.; Economou, D. J.; ...

2016-06-08

Parallel plate capacitively coupled plasmas in hydrogen at relatively high pressure (~1 Torr) are excited with tailored voltage waveforms containing up to five frequencies. Predictions of a hybrid model combining a particle-in-cell simulation with Monte Carlo collisions and a fluid model are compared to phase resolved optical emission spectroscopy measurements, yielding information on the dynamics of the excitation rate in these discharges. When the discharge is excited with amplitude asymmetric waveforms, the discharge becomes electrically asymmetric, with different ion energies at each of the two electrodes. Unexpectedly, large differences in themore » $$\\text{H}_{2}^{+}$$ fluxes to each of the two electrodes are caused by the different $$\\text{H}_{3}^{+}$$ energies. When the discharge is excited with slope asymmetric waveforms, only weak electrical asymmetry of the discharge is observed. In this case, electron power absorption due to fast sheath expansion at one electrode is balanced by electron power absorption at the opposite electrode due to a strong electric field reversal.« less

Ion resonances and ELF wave production by an electron beam injected into the ionosphere - ECHO 6

NASA Astrophysics Data System (ADS)

Winckler, J. R.; Steffen, J. E.; Malcolm, P. R.; Erickson, K. N.; Abe, Y.; Swanson, R. L.

1984-09-01

Two effects observed with electron antennas ejected from a sounding rocket launched into the ionosphere in March 1983 carrying electron beam guns are discussed. The sensor packages were ejected and travelled parallel to the vehicle trajectory. Electric potentials were measured between the single probes and a plasma diagnostic package while the gun injected electrons into the ionosphere in perpendicular and parallel 1 kHz directions. Signal pulses over the dc-1250 kHz range were detected. A kHz gun frequency caused a signal that decreased by two orders of magnitude between 45-90 m from the beam field line. However, the signal was detectable at 1 mV/m at 120 m, supporting earlier data that indicated that pulsed electron beams can cause ELF waves in space. Beam injection parallel to the magnetic field produced an 840 Hz resonance that could be quenched by activation of a transverse beam.

Particle signatures of magnetic topology at the magnetopause: AMPTE/CCE observations

NASA Technical Reports Server (NTRS)

Fuselier, S. A.; Anderson, B. J.; Onsager, T. G.

1995-01-01

Electron distributions at energies above 50 eV have been found to be a sensitive indicator of magnetic topology for magnetopause crossings of the AMPTE/CCE spacecraft. Progressing from the magnetosheath to the magnetosphere two abrupt transitions occur. First, the magnetosheath electron population directed either parallel or antiparallel to the magnetic field is replaced by a streaming, heated magnetosheath electron population. The other half of the distribution is unchanged. The region with unidirectional, heated magnetosheath electrons is identified as the magnetosheath boundary layer (MSBL). Second, the unheated magnetosheath electron population is replaced by a heated population nearly identical to the population encountered in the MSBL, resulting in a symmetric counterstreaming distribution. The region populated by the bidirectional heated magnetosheath electrons is identified as the low-latitude boundary layer (LLBL). The MSBL and LLBL identified by the electron transitions are the same as the regions identified using ion composition measurements. The magnetosheath-MSBL transition reflects a change in magnetic topology from a solar wind field line to one that threads the magnetopause, and the existence of a magnetosheath-MSBL transition implies that the magnetopause is open. When the current layer is easily identified, the MSBL-LLBL transition coincides with the magnetopause current layer, indicating that the magnetosheath electrons are heated in the current layer. Both magnetosheath-MSBL and MSBL-LLBL transitions are observed for low as well as high magnetic shears. Moreover, the transitions are particularly clear for low shear implying that magnetic topology boundaries are sharp even when abrupt changes in the field and other plasma parameters are absent. Furthermore, for low magnetic shear, solar wind ions with low parallel drift speeds make up the majority of the LLBL population indicating that the magnetosheath plasma has convected directly across the magnetosheath plasma has converted directly across the magnetopause. These observations are consistent with quasi-steady, high-latitude reconnection and indicate that the signatures of this reconnection geometry are commonly present in the subpolar region.

First-principles theoretical investigation of monoatomic and dimer Mn adsorption on noble metal (111) surfaces

NASA Astrophysics Data System (ADS)

Muñoz, Francisco; Romero, Aldo H.; Mejía-López, Jose; Morán-López, J. L.

2012-03-01

A theoretical investigation of the adsorption of Mn single atoms and dimers on the (111) surface of Cu, Ag, and Au, within the framework of the density functional theory, is presented. First, the bulk and the clean (111) surface electronic structures are calculated, with results that agree well with previous reports. To understand the adatom-substrate interaction, also the electronic characteristics of the free Mn dimer are determined. Then, the electronic structure of the Mn adatom, chemisorbed on four different surface geometries, is analyzed for the three noble metals. It is found that the most stable geometry, in all three cases, Cu, Ag, and Au, occurs when the Mn atom is chemisorbed on threefold coordinated sites. For the dimer, the lowest-energy configuration corresponds to the molecule lying parallel to the surface. In the three noble metals, the geometry corresponds to both atoms chemisorbed in threefold coordinated sites, but with different local symmetry. It is also found that the magnetic configuration with the lowest energy corresponds to the antiferromagnetic arrangement of Mn atoms, with individual magnetic moments close to 5μB. The ferromagnetic and antiferromagnetic solutions, in the case of a Ag substrate, are close in energy. It is also found that in this case the Mn2 molecule is chemisorbed with very similar energy on various geometries. To study the dynamical motion of the dimer components, we calculated the potential energy barriers for the Mn motion in the various surfaces. In contrast to Cu and Au, this leads to the conclusion that on Ag the Mn dimer moves relatively freely.

Electromagnetic radiation and nonlinear energy flow in an electron beam-plasma system

NASA Technical Reports Server (NTRS)

Whelan, D. A.; Stenzel, R. L.

1985-01-01

It is shown that the unstable electron-plasma waves of a beam-plasma system can generate electromagnetic radiation in a uniform plasma. The generation mechanism is a scattering of the unstable electron plasma waves off ion-acoustic waves, producing electromagnetic waves whose frequency is near the local plasma frequency. The wave vector and frequency matching conditions of the three-wave mode coupling are experimentally verified. The electromagnetic radiation is observed to be polarized with the electric field parallel to the beam direction, and its source region is shown to be localized to the unstable plasma wave region. The frequency spectrum shows negligible intensity near the second harmonic of the plasma frequency. These results suggest that the observed electromagnetic radiation of type III solar bursts may be generated near the local plasma frequency and observed downstream where the wave frequency is near the harmonic of the plasma frequency.

Observations at mercury encounter by the plasma science experiment on mariner 10.

PubMed

Ogilvie, K W; Scudder, J D; Hartle, R E; Siscoe, G L; Bridge, H S; Lazarus, A J; Asbridge, J R; Bame, S J; Yeates, C M

1974-07-12

A fully developed bow shock and magnetosheath were observed near Mercury, providing unambiguous evidence for a strong interaction between Mercury and the solar wind. Inside the sheath there is a distinct region analogous to the magnetosphere or magnetotail of Earth, populated by electrons with lower density and higher temperature than the electrons observed in the solar wind or magnetosheath. At the time of encounter, conditions were such that a perpendicular shock was observed on the inbound leg and a parallel shock was observed on the outbound leg of the trajectory, and energetic plasma electron events were detected upstream from the outbound shock crossing. The interaction is most likely not atmospheric, but the data clearly indicate that the obstacle to solar wind flow is magnetic, either intrinsic or induced. The particle fluxes and energy spectra showed large variations while the spacecraft was inside the magnetosphere, and these variations could be either spatial or temporal.

Characterization of BN rich layer on ammonia treated Nextel{trademark}312 fibers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Khasgiwale, N.R.; Butler, E.P.; Tsakalakos, L.

A BN rich layer grown on Nextel{trademark}312 fibers by appropriate ammonia treatments was evaluated using various complimentary techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM)/Parallel Electron Energy Loss Spectroscopy (PEELS in TEM). Three different ammonia treatments were studied. Ammonia treatment resulted in crystallization of the Nextel{trademark}312 fiber. The BN rich surface layer formed due to ammonia treatment was clearly detected in XPS and PEELS both before and after oxidation. The layer thickness was estimated to be between 5--10 nm. The layer was stable after oxidation treatment at 600 C formore » 100 hours. High resolution TEM observations of the fiber surface revealed a variable BN rich layer thickness. Patches of turbostratic BN were observed under certain conditions, however mostly the layer appeared to be amorphous.« less

Large-Amplitude Electrostatic Waves Observed at a Supercritical Interplanetary Shock

NASA Technical Reports Server (NTRS)

Wilson, L. B., III; Cattell, C. A.; Kellogg, P. J.; Goetz, K.; Kersten, K.; Kasper, J. C.; Szabo, A.; Wilber, M.

2010-01-01

We present the first observations at an interplanetary shock of large-amplitude (> 100 mV/m pk-pk) solitary waves and large-amplitude (approx.30 mV/m pk-pk) waves exhibiting characteristics consistent with electron Bernstein waves. The Bernstein-like waves show enhanced power at integer and half-integer harmonics of the cyclotron frequency with a broadened power spectrum at higher frequencies, consistent with the electron cyclotron drift instability. The Bernstein-like waves are obliquely polarized with respect to the magnetic field but parallel to the shock normal direction. Strong particle heating is observed in both the electrons and ions. The observed heating and waveforms are likely due to instabilities driven by the free energy provided by reflected ions at this supercritical interplanetary shock. These results offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind.

Roos and NACP-02 ion chamber perturbations and water-air stopping-power ratios for clinical electron beams for energies from 4 to 22 MeV

NASA Astrophysics Data System (ADS)

Bailey, M.; Shipley, D. R.; Manning, J. W.

2015-02-01

Empirical fits are developed for depth-compensated wall- and cavity-replacement perturbations in the PTW Roos 34001 and IBA / Scanditronix NACP-02 parallel-plate ionisation chambers, for electron beam qualities from 4 to 22 MeV for depths up to approximately 1.1 × R50,D. These are based on calculations using the Monte Carlo radiation transport code EGSnrc and its user codes with a full simulation of the linac treatment head modelled using BEAMnrc. These fits are used with calculated restricted stopping-power ratios between air and water to match measured depth-dose distributions in water from an Elekta Synergy clinical linear accelerator at the UK National Physical Laboratory. Results compare well with those from recent publications and from the IPEM 2003 electron beam radiotherapy Code of Practice.

Design of transparent conductors and periodic two-dimensional electron gases without doping

NASA Astrophysics Data System (ADS)

Zhang, Xiuwen; Zhang, Lijun; Zunger, Alex; Perkins, John; Materials by Design Team; John D. Perkins Collaboration

The functionality of transparency plus conductivity plays an important role in renewable energy and information technologies, including applications such as solar cells, touch-screen sensors, and flat panel display. However, materials with such seemingly contraindicated properties are difficult to come by. The traditional strategy for designing bulk transparent conductors (TCs) starts from a wide-gap insulator and finds ways to make it conductive by extensive doping. We propose a different strategy for TC design--starting with a metallic conductor and designing transparency by control of intrinsic interband transitions and intraband plasmonic frequency. We identified specific design principles for prototypical intrinsic TC classes and searched computationally for materials that satisfy them. The electron gases in the 3D intrinsic TCs demonstrate intriguing properties, such as periodic 2D electron gas regions with very high carrier density. We will discuss a more extended search of these functionalities, in parallel with stability and growability calculations

Langmuir waveforms at interplanetary shocks: STEREO statistical analysis

NASA Astrophysics Data System (ADS)

Briand, C.

2016-12-01

Wave-particle interactions and particle acceleration are the two main processes allowing energy dissipation at non collisional shocks. Ion acceleration has been deeply studied for many years, also for their central role in the shock front reformation. Electron dynamics is also important in the shock dynamics through the instabilities they can generate which may impact the ion dynamics.Particle measurements can be efficiently completed by wave measurements to determine the characteristics of the electron beams and study the turbulence of the medium. Electric waveforms obtained from the S/WAVES instrument of the STEREO mission between 2007 to 2014 are analyzed. Thus, clear signature of Langmuir waves are observed on 41 interplanetary shocks. These data enable a statistical analysis and to deduce some characteristics of the electron dynamics on different shocks sources (SIR or ICME) and types (quasi-perpendicular or quasi-parallel). The conversion process between electrostatic to electromagnetic waves has also been tested in several cases.

Equilibrium structure of the plasma sheet boundary layer-lobe interface

NASA Technical Reports Server (NTRS)

Romero, H.; Ganguli, G.; Palmadesso, P.; Dusenbery, P. B.

1990-01-01

Observations are presented which show that plasma parameters vary on a scale length smaller than the ion gyroradius at the interface between the plasma sheet boundary layer and the lobe. The Vlasov equation is used to investigate the properties of such a boundary layer. The existence, at the interface, of a density gradient whose scale length is smaller than the ion gyroradius implies that an electrostatic potential is established in order to maintain quasi-neutrality. Strongly sheared (scale lengths smaller than the ion gyroradius) perpendicular and parallel (to the ambient magnetic field) electron flows develop whose peak velocities are on the order of the electron thermal speed and which carry a net current. The free energy of the sheared flows can give rise to a broadband spectrum of electrostatic instabilities starting near the electron plasma frequency and extending below the lower hybrid frequency.

New two-dimensional boron nitride allotropes with attractive electronic and optical properties

NASA Astrophysics Data System (ADS)

Shahrokhi, Masoud; Mortazavi, Bohayra; Berdiyorov, Golibjon R.

2017-03-01

Using first principles calculations, structural, electronic and optical properties of five new 2D boron nitride (BN) allotropes have been studied. The results exhibit that the cohesive energy for all these five new allotrope is positive such as all these systems are stable; therefore, it is possible to synthesize these structures in experiments. It is found that the band gap of all new 2D BN allotropes is smaller than the h-BN sheet. In our calculations the dielectric tensor is derived within the random phase approximation (RPA). Specifically, the dielectric function, refraction index and the loss function, of the 2D BN allotropes are calculated for both parallel and perpendicular electric field polarizations. The results show that the optical spectra are anisotropic along these two polarizations. The results obtained from our calculations are beneficial to practical applications of these 2D BN allotropes in optoelectronics and electronics.

Cosmic rays, gamma rays and synchrotron radiation from the Galaxy

DOE PAGES

Orlando, Elena

2012-07-30

Galactic cosmic rays (CR), interstellar gamma-ray emission and synchrotron radiation are related topics. CR electrons propagate in the Galaxy and interact with the interstellar medium, producing inverse-Compton emission measured in gamma rays and synchrotron emission measured in radio. I present an overview of the latest results with Fermi/LAT on the gamma-ray diffuse emission induced by CR nuclei and electrons. Then I focus on the recent complementary studies of the synchrotron emission in the light of the latest gamma-ray results. Relevant observables include spectral indices and their variations, using surveys over a wide range of radio frequencies. As a result, thismore » paper emphasizes the importance of using the parallel study of gamma rays and synchrotron radiation in order to constrain the low-energy interstellar CR electron spectrum, models of propagation of CRs, and magnetic fields.« less

Magnetospectroscopy of symmetric and anti-symmetric states in double quantum wells

NASA Astrophysics Data System (ADS)

Marchewka, M.; Sheregii, E. M.; Tralle, I.; Ploch, D.; Tomaka, G.; Furdak, M.; Kolek, A.; Stadler, A.; Mleczko, K.; Zak, D.; Strupinski, W.; Jasik, A.; Jakiela, R.

2008-02-01

The experimental results obtained for magnetotransport in the InGaAs/InAlAs double quantum well (DQW) structures of two different shapes of wells are reported. A beating effect occurring in the Shubnikov-de Haas (SdH) oscillations was observed for both types of structures at low temperatures in the parallel transport when the magnetic field was perpendicular to the layers. An approach for the calculation of the Landau level energies for DQW structures was developed and then applied to the analysis and interpretation of the experimental data related to the beating effect. We also argue that in order to account for the observed magnetotransport phenomena (SdH and integer quantum Hall effect), one should introduce two different quasi-Fermi levels characterizing two electron subsystems regarding the symmetry properties of their states, symmetric and anti-symmetric ones, which are not mixed by electron-electron interaction.

Comparative study of cross-field and field-aligned electron beams in active experiments. [in upper atmosphere

NASA Technical Reports Server (NTRS)

Winglee, R. M.; Pritchett, P. L.

1988-01-01

Beam-plasma interactions associated with the cross-field and field-aligned injection of electron beams from spacecraft were investigated using a two-dimensional (three velocity component) electrostatic particle simulations. It is shown that the beam properties and plasma response can be characterized well by the ratio between the stagnation time and the plasma response time, which depends on the ratio of the ambient plasma density to the beam density, the beam width, the beam energy, and the spacecraft length. It was found that the beams injected across the field lines tend to lose their coherence after about one or two gyrations due to space-charge oscillations induced by the beam, irrespective of the spacecraft charging. These oscillations scatter the beam electrons into a hollow cylinder of a radius equal to a beam electron gyroradius and thickness of the order of two beam Debye lengths. Parallel injected beams are subjected to similar oscillations, which cause the beam to expand to fill a solid cylinder of a comparable thickness.

Simple method for generating adjustable trains of picosecond electron bunches

NASA Astrophysics Data System (ADS)

Muggli, P.; Allen, B.; Yakimenko, V. E.; Park, J.; Babzien, M.; Kusche, K. P.; Kimura, W. D.

2010-05-01

A simple, passive method for producing an adjustable train of picosecond electron bunches is demonstrated. The key component of this method is an electron beam mask consisting of an array of parallel wires that selectively spoils the beam emittance. This mask is positioned in a high magnetic dispersion, low beta-function region of the beam line. The incoming electron beam striking the mask has a time/energy correlation that corresponds to a time/position correlation at the mask location. The mask pattern is transformed into a time pattern or train of bunches when the dispersion is brought back to zero downstream of the mask. Results are presented of a proof-of-principle experiment demonstrating this novel technique that was performed at the Brookhaven National Laboratory Accelerator Test Facility. This technique allows for easy tailoring of the bunch train for a particular application, including varying the bunch width and spacing, and enabling the generation of a trailing witness bunch.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matsutani, Takaomi; Taya, Masaki; Ikuta, Takashi

A parallel image detection system using an annular pupil for electron optics were developed to realize an increase in the depth of focus, aberration-free imaging and separation of amplitude and phase images under scanning transmission electron microscopy (STEM). Apertures for annular pupils able to suppress high-energy electron scattering were developed using a focused ion beam (FIB) technique. The annular apertures were designed with outer diameter of oe 40 {mu}m and inner diameter of oe32 {mu}m. A taper angle varying from 20 deg. to 1 deg. was applied to the slits of the annular apertures to suppress the influence of high-energymore » electron scattering. Each azimuth angle image on scintillator was detected by a multi-anode photomultiplier tube assembly through 40 optical fibers bundled in a ring shape. To focus the image appearing on the scintillator on optical fibers, an optical lens relay system attached with CCD camera was developed. The system enables the taking of 40 images simultaneously from different scattered directions.« less

Electromagnetic dissipation during asymmetric reconnection with a moderate guide field

NASA Astrophysics Data System (ADS)

Genestreti, Kevin; Burch, James; Cassak, Paul; Torbert, Roy; Phan, Tai; Ergun, Robert; Giles, Barbara; Russell, Chris; Wang, Shan; Akhavan-Tafti, Mojtaba; Varsani, Ali

2017-04-01

We calculate the work done on the plasma by the electromagnetic (EM) field, ⃗Jṡ⃗E', and analyze the related electron currents and electric fields, focusing on a single asymmetric guide field electron diffusion region (EDR) event observed by MMS on 8 December 2015. For this event, each of the four MMS spacecraft observed dissipation of EM energy at the in-plane magnetic null point, though large-scale generation/dissipation was observed inconsistently on the magnetospheric side of the boundary. The current at the null was carried by a beam-like population of magnetosheath electrons traveling anti-parallel to the guide field, whereas the current on the Earthward side of the boundary was carried by crescent-shaped electron distributions. We also analyze the terms in Ohm's law, finding a large residual electric field throughout the EDR, inertial and pressure divergence fields at the null, and pressure divergence fields at the magnetosphere-side EDR. Our analysis of the terms in Ohm's law suggests that the EDR had significant three-dimensional structure.

Side-band mutual interactions in the magnetosphere

NASA Technical Reports Server (NTRS)

Chang, D. C. D.; Helliwell, R. A.; Bell, T. F.

1980-01-01

Sideband mutual interactions between VLF waves in the magnetosphere are investigated. Results of an experimental program involving the generation of sidebands by means of frequency shift keying are presented which indicate that the energetic electrons in the magnetosphere can interact only with sidebands generated by signals with short modulation periods. Using the value of the memory time during which electrons interact with the waves implied by the above result, it is estimated that the length of the electron interaction region in the magnetosphere is between 4000 and 2000 km. Sideband interactions are found to be similar to those between constant-frequency signals, exhibiting suppression and energy coupling. Results from a second sideband transmitting program show that for most cases the coherence bandwidth of sidebands is about 50 Hz. Sideband mutual interactions are then explained by the overlap of the ranges of the parallel velocity of the electrons which the sidebands organize, and the wave intensity in the interaction region is estimated to be 2.5-10 milli-gamma, in agreement with satellite measurements.

The influence of the self-consistent mode structure on the Coriolis pinch effect

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peeters, A. G.; Camenen, Y.; Casson, F. J.

This paper discusses the effect of the mode structure on the Coriolis pinch effect [A. G. Peeters, C. Angioni, and D. Strintzi, Phys. Rev. Lett. 98, 265003 (2007)]. It is shown that the Coriolis drift effect can be compensated for by a finite parallel wave vector, resulting in a reduced momentum pinch velocity. Gyrokinetic simulations in full toroidal geometry reveal that parallel dynamics effectively removes the Coriolis pinch for the case of adiabatic electrons, while the compensation due to the parallel dynamics is incomplete for the case of kinetic electrons, resulting in a finite pinch velocity. The finite flux inmore » the case of kinetic electrons is interpreted to be related to the electron trapping, which prevents a strong asymmetry in the electrostatic potential with respect to the low field side position. The physics picture developed here leads to the discovery and explanation of two unexpected effects: First the pinch velocity scales with the trapped particle fraction (root of the inverse aspect ratio), and second there is no strong collisionality dependence. The latter is related to the role of the trapped electrons, which retain some symmetry in the eigenmode, but play no role in the perturbed parallel velocity.« less

Unique Flexibility in Energy Metabolism Allows Mycobacteria to Combat Starvation and Hypoxia

PubMed Central

Berney, Michael; Cook, Gregory M.

2010-01-01

Mycobacteria are a group of obligate aerobes that require oxygen for growth, but paradoxically have the ability to survive and metabolize under hypoxia. The mechanisms responsible for this metabolic plasticity are unknown. Here, we report on the adaptation of Mycobacterium smegmatis to slow growth rate and hypoxia using carbon-limited continuous culture. When M. smegmatis is switched from a 4.6 h to a 69 h doubling time at a constant oxygen saturation of 50%, the cells respond through the down regulation of respiratory chain components and the F1Fo-ATP synthase, consistent with the cells lower demand for energy at a reduced growth rate. This was paralleled by an up regulation of molecular machinery that allowed more efficient energy generation (i.e. Complex I) and the use of alternative electron donors (e.g. hydrogenases and primary dehydrogenases) to maintain the flow of reducing equivalents to the electron transport chain during conditions of severe energy limitation. A hydrogenase mutant showed a 40% reduction in growth yield highlighting the importance of this enzyme in adaptation to low energy supply. Slow growing cells at 50% oxygen saturation subjected to hypoxia (0.6% oxygen saturation) responded by switching on oxygen scavenging cytochrome bd, proton-translocating cytochrome bc1-aa3 supercomplex, another putative hydrogenase, and by substituting NAD+-dependent enzymes with ferredoxin-dependent enzymes thus highlighting a new pattern of mycobacterial adaptation to hypoxia. The expression of ferredoxins and a hydrogenase provides a potential conduit for disposing of and transferring electrons in the absence of exogenous electron acceptors. The use of ferredoxin-dependent enzymes would allow the cell to maintain a high carbon flux through its central carbon metabolism independent of the NAD+/NADH ratio. These data demonstrate the remarkable metabolic plasticity of the mycobacterial cell and provide a new framework for understanding their ability to survive under low energy conditions and hypoxia. PMID:20062806

New Result for the β-decay Asymmetry Parameter A0 from the UCNA Experiment

NASA Astrophysics Data System (ADS)

Brown, M. A.-P.; UCNA Collaboration

2017-09-01

The UCNA Experiment at the Ultracold Neutron facility at LANL uses polarized ultracold neutrons (UCN) to determine the neutron β-decay asymmetry parameter A0, the angular correlation between the neutron spin and the decay electron's momentum. A0 further determines λ =gA /gV , which, when combined with the neutron lifetime, permits extraction of the CKM matrix element Vud solely from neutron decay. In the UCNA experiment, UCN are produced in a pulsed, spallation driven solid deuterium source, polarized using a 7 T magnetic field, and transported through an Adiabatic Fast Passage (AFP) spin flipper prior to storage within a 1 T solenoidal spectrometer housing electron detectors at each end. The spin-flipper allows one to form a super-ratio of decay rates for neutron spins aligned parallel and anti-parallel to the 1 T magnetic field, eliminating to first order errors due to variations in the decay rate and detector efficiencies. Leading systematics and analysis techniques from the most recent analysis of data collected from 2011-2013 will be presented. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Award Number DE-SC-0014622.

Mixed Carrier Conduction in Modulation-doped Field Effect Transistors

NASA Technical Reports Server (NTRS)

Schacham, S. E.; Haugland, E. J.; Mena, R. A.; Alterovitz, S. A.

1995-01-01

The contribution of more than one carrier to the conductivity in modulation-doped field effect transistors (MODFET) affects the resultant mobility and complicates the characterization of these devices. Mixed conduction arises from the population of several subbands in the two-dimensional electron gas (2DEG), as well as the presence of a parallel path outside the 2DEG. We characterized GaAs/AlGaAs MODFET structures with both delta and continuous doping in the barrier. Based on simultaneous Hall and conductivity analysis we conclude that the parallel conduction is taking place in the AlGaAs barrier, as indicated by the carrier freezeout and activation energy. Thus, simple Hall analysis of these structures may lead to erroneous conclusions, particularly for real-life device structures. The distribution of the 2D electrons between the various confined subbands depends on the doping profile. While for a continuously doped barrier the Shubnikov-de Haas analysis shows superposition of two frequencies for concentrations below 10(exp 12) cm(exp -2), for a delta doped structure the superposition is absent even at 50% larger concentrations. This result is confirmed by self-consistent analysis, which indicates that the concentration of the second subband hardly increases.

Koopmans' theorem in the Hartree-Fock method. General formulation

NASA Astrophysics Data System (ADS)

Plakhutin, Boris N.

2018-03-01

This work presents a general formulation of Koopmans' theorem (KT) in the Hartree-Fock (HF) method which is applicable to molecular and atomic systems with arbitrary orbital occupancies and total electronic spin including orbitally degenerate (OD) systems. The new formulation is based on the full set of variational conditions imposed upon the HF orbitals by the variational principle for the total energy and the conditions imposed by KT on the orbitals of an ionized electronic shell [B. N. Plakhutin and E. R. Davidson, J. Chem. Phys. 140, 014102 (2014)]. Based on these conditions, a general form of the restricted open-shell HF method is developed, whose eigenvalues (orbital energies) obey KT for the whole energy spectrum. Particular attention is paid to the treatment of OD systems, for which the new method gives a number of unexpected results. For example, the present method gives four different orbital energies for the triply degenerate atomic level 2p in the second row atoms B to F. Based on both KT conditions and a parallel treatment of atoms B to F within a limited configuration interaction approach, we prove that these four orbital energies, each of which is triply degenerate, are related via KT to the energies of different spin-dependent ionization and electron attachment processes (2p)N → (2p ) N ±1. A discussion is also presented of specific limitations of the validity of KT in the HF method which arise in OD systems. The practical applicability of the theory is verified by comparing KT estimates of the ionization potentials I2s and I2p for the second row open-shell atoms Li to F with the relevant experimental data.

Electronic, Optical and Thermoelectric Properties of 2H-CuAlO2: A First Principles Study

NASA Astrophysics Data System (ADS)

Bhamu, K. C.; Khenata, R.; Khan, Saleem Ayaz; Singh, Mangej; Priolkar, K. R.

2016-01-01

The electronic and optical properties of 2H-CuAlO2, including energy bands, density of states (DOS), optical dielectric behaviour, refractive index, absorption coefficient and optical conductivity, have been investigated within the framework of a full-potential linearized augmented plane wave scheme using different potentials. The direct and indirect band gaps for CuAlO2, computed using the Becke-Johnson potential, are estimated at 3.53 eV and 2.48 eV, respectively, which are in better agreement with the experimentally reported band gaps than those previously computed. The origin of energy bands is elucidated in terms of DOS, while the behaviour of the imaginary part of the dielectric constant is explained in terms of electronic transitions from valence bands to conduction bands. The computed value of the refractive index is 2.25 (1.94) for light perpendicular (parallel) to the c axis, in concordance with the available values. The overall shape of the spectral distribution for absorption coefficient and optical conductivity is also in accord with the reported data. The investigated thermoelectric properties indicate that CuAlO2 is a p-type semiconductor showing high effectiveness at low temperatures.

Recent advances on polyoxometalate-based molecular and composite materials.

PubMed

Song, Yu-Fei; Tsunashima, Ryo

2012-11-21

Polyoxometalates (POMs) are a subset of metal oxides with unique physical and chemical properties, which can be reliably modified through various techniques and methods to develop sophisticated materials and devices. In parallel with the large number of new crystal structures reported in the literature, the application of these POMs towards multifunctional materials has attracted considerable attention. This critical review summarizes recent progress on POM-based molecular and composite materials, and particularly highlights the emerging areas that are closely related to surface, electronic, energy, environment, life science, etc. (171 references).

Surface dose measurements for highly oblique electron beams.

PubMed

Ostwald, P M; Kron, T

1996-08-01

Clinical applications of electrons may involve oblique incidence of beams, and although dose variations for angles up to 60 degrees from normal incidence are well documented, no results are available for highly oblique beams. Surface dose measurements in highly oblique beams were made using parallel-plate ion chambers and both standard LiF:Mg, Ti and carbon-loaded LiF Thermoluminescent Dosimeters (TLD). Obliquity factors (OBF) or surface dose at an oblique angle divided by the surface dose at perpendicular incidence, were obtained for electron energies between 4 and 20 MeV. Measurements were performed on a flat solid water phantom without a collimator at 100 cm SSD. Comparisons were also made to collimated beams. The OBFs of surface doses plotted against the angle of incidence increased to a maximum dose followed by a rapid dropoff in dose. The increase in OBF was more rapid for higher energies. The maximum OBF occurred at larger angles for higher-energy beams and ranged from 73 degrees for 4 MeV to 84 degrees for 20 MeV. At the dose maximum, OBFs were between 130% and 160% of direct beam doses, yielding surface doses of up to 150% of Dmax for the 20 MeV beam. At 2 mm depth the dose ratio was found to increase initially with angle and then decrease as Dmax moved closer to the surface. A higher maximum dose was measured at 2 mm depth than at the surface. A comparison of ion chamber types showed that a chamber with a small electrode spacing and large guard ring is required for oblique dose measurement. A semiempirical equation was used to model the dose increase at the surface with different energy electron beams.

Atomic precision etch using a low-electron temperature plasma

NASA Astrophysics Data System (ADS)

Dorf, L.; Wang, J.-C.; Rauf, S.; Zhang, Y.; Agarwal, A.; Kenney, J.; Ramaswamy, K.; Collins, K.

2016-03-01

Sub-nm precision is increasingly being required of many critical plasma etching processes in the semiconductor industry. Accurate control over ion energy and ion/radical composition is needed during plasma processing to meet these stringent requirements. Described in this work is a new plasma etch system which has been designed with the requirements of atomic precision plasma processing in mind. In this system, an electron sheet beam parallel to the substrate surface produces a plasma with an order of magnitude lower electron temperature Te (~ 0.3 eV) and ion energy Ei (< 3 eV without applied bias) compared to conventional radio-frequency (RF) plasma technologies. Electron beam plasmas are characterized by higher ion-to-radical fraction compared to RF plasmas, so a separate radical source is used to provide accurate control over relative ion and radical concentrations. Another important element in this plasma system is low frequency RF bias capability which allows control of ion energy in the 2-50 eV range. Presented in this work are the results of etching of a variety of materials and structures performed in this system. In addition to high selectivity and low controllable etch rate, an important requirement of atomic precision etch processes is no (or minimal) damage to the remaining material surface. It has traditionally not been possible to avoid damage in RF plasma processing systems, even during atomic layer etch. The experiments for Si etch in Cl2 based plasmas in the aforementioned etch system show that damage can be minimized if the ion energy is kept below 10 eV. Layer-by-layer etch of Si is also demonstrated in this etch system using electrical and gas pulsing.

Cyclotron Acceleration of Relativistic Electrons through Landau Resonance with Obliquely Propagating Whistler Mode Chorus Emissions

NASA Astrophysics Data System (ADS)

Omura, Y.; Hsieh, Y. K.; Foster, J. C.; Erickson, P. J.; Kletzing, C.; Baker, D. N.

2017-12-01

A recent test particle simulation of obliquely propagating whistler mode wave-particle interaction [Hsieh and Omura, 2017] shows that the perpendicular wave electric field can play a significant role in trapping and accelerating relativistic electrons through Landau resonance. A further theoretical and numerical investigation verifies that there occurs nonlinear wave trapping of relativistic electrons by the nonlinear Lorentz force of the perpendicular wave magnetic field. An electron moving with a parallel velocity equal to the parallel phase velocity of an obliquely propagating wave basically see a stationary wave phase. Since the electron position is displaced from its gyrocenter by a distance ρ*sin(φ), where ρ is the gyroradius and φ is the gyrophase, the wave phase is modulated with the gyromotion, and the stationary wave fields as seen by the electron are expanded as series of Bessel functions Jn with phase variations n*φ. The J1 components of the wave electric and magnetic fields rotate in the right-hand direction with the gyrofrequency, and they can be in resonance with the electron undergoing the gyromotion, resulting in effective electron acceleration and pitch angle scattering. We have performed a subpacket analysis of chorus waveforms observed by the Van Allen Probes [Foster et al., 2017], and calculated the energy gain by the cyclotron acceleration through Landau resonance. We compare the efficiencies of accelerations by cyclotron and Landau resonances in typical events of rapid electron acceleration observed by the Van Allen Probes.References:[1] Hsieh, Y.-K., and Y. Omura (2017), Nonlinear dynamics of electrons interacting with oblique whistler mode chorus in the magnetosphere, J. Geophys. Res. Space Physics, 122, 675-694, doi:10.1002/2016JA023255.[2] Foster, J. C., P. J. Erickson, Y. Omura, D. N. Baker, C. A. Kletzing, and S. G. Claudepierre (2017), Van Allen Probes observations of prompt MeV radiation belt electron acceleration in nonlinear interactions with VLF chorus, J. Geophys. Res. Space Physics, 122, 324-339, doi:10.1002/2016JA023429.

Electron heating and acceleration during magnetic reconnection

NASA Astrophysics Data System (ADS)

Dahlin, Joel

2017-10-01

Magnetic reconnection is thought to be an important driver of energetic particles in a variety of astrophysical phenomena such as solar flares and magnetospheric storms. However, the observed fraction of energy imparted to a nonthermal component can vary widely in different regimes. We use kinetic particle-in-cell (PIC) simulations to demonstrate the important role of the non-reversing (guide) field in controlling the efficiency of electron acceleration in collisionless reconnection. In reconnection where the guide field is smaller than the reconnecting component, the dominant electron accelerator is a Fermi-type mechanism that preferentially energizes the most energetic particles. In strong guide field reconnection, the field-line contraction that drives the Fermi mechanism becomes weak. Instead, parallel electric fields are primarily responsible for driving electron heating but are ineffective in driving the energetic component of the spectrum. Three-dimensional simulations reveal that the stochastic magnetic field that develops during 3D guide field reconnection plays a vital role in particle acceleration and transport. The reconnection outflows that drive Fermi acceleration also expel accelerating particles from energization regions. In 2D reconnection, electrons are trapped in island cores and acceleration ceases, whereas in 3D the stochastic magnetic field enables energetic electrons to leak out of islands and freely sample regions of energy release. A finite guide field is required to break initial 2D symmetry and facilitate escape from island structures. We show that reconnection with a guide field comparable to the reconnecting field generates the greatest number of energetic electrons, a regime where both (a) the Fermi mechanism is an efficient driver and (b) energetic electrons may freely access acceleration sites. These results have important implications for electron acceleration in solar flares and reconnection-driven dissipation in turbulence.

Three-dimensional kinetic simulations of whistler turbulence in solar wind on parallel supercomputers

NASA Astrophysics Data System (ADS)

Chang, Ouliang

The objective of this dissertation is to study the physics of whistler turbulence evolution and its role in energy transport and dissipation in the solar wind plasmas through computational and theoretical investigations. This dissertation presents the first fully three-dimensional (3D) particle-in-cell (PIC) simulations of whistler turbulence forward cascade in a homogeneous, collisionless plasma with a uniform background magnetic field B o, and the first 3D PIC simulation of whistler turbulence with both forward and inverse cascades. Such computationally demanding research is made possible through the use of massively parallel, high performance electromagnetic PIC simulations on state-of-the-art supercomputers. Simulations are carried out to study characteristic properties of whistler turbulence under variable solar wind fluctuation amplitude (epsilon e) and electron beta (betae), relative contributions to energy dissipation and electron heating in whistler turbulence from the quasilinear scenario and the intermittency scenario, and whistler turbulence preferential cascading direction and wavevector anisotropy. The 3D simulations of whistler turbulence exhibit a forward cascade of fluctuations into broadband, anisotropic, turbulent spectrum at shorter wavelengths with wavevectors preferentially quasi-perpendicular to B o. The overall electron heating yields T ∥ > T⊥ for all epsilone and betae values, indicating the primary linear wave-particle interaction is Landau damping. But linear wave-particle interactions play a minor role in shaping the wavevector spectrum, whereas nonlinear wave-wave interactions are overall stronger and faster processes, and ultimately determine the wavevector anisotropy. Simulated magnetic energy spectra as function of wavenumber show a spectral break to steeper slopes, which scales as k⊥lambda e ≃ 1 independent of betae values, where lambdae is electron inertial length, qualitatively similar to solar wind observations. Specific spectral indices from simulated wavevector energy spectra do not match the frequency spectral indices from observations due to the inapplicability of Taylor's hypothesis. In contrast, the direct comparison of simulated frequency energy spectra and solar wind observations shows a closer similarity. Electron density fluctuations power spectra also exhibit a close similarity to solar wind observations and MHD predications, both qualitatively and quantitatively. Linear damping represents an intermediate fraction of the total dissipation of whistler turbulence over a wide range of betae and epsilone. The relative importance of linear damping by comparison to nonlinear dissipation increases with increasing beta e but decreases with increasing epsilone. Correlation coefficient calculations imply that the nonlinear dissipation processes in our simulation are primarily associated with dissipation in regions of intermittent current sheet structures. The simulation results suggest that whistler fluctuations could be the substantial constituent of solar wind turbulence at higher frequencies and short wavelengths, and support the magnetosonic-whistler interpretation of the quasilinear scenario. An even larger scale 3D whistler turbulence simulation exhibits both a forward cascade to shorter wavelengths with wavevectors preferentially k⊥ > k∥, and an inverse cascade to longer wavelengths with wavevectors k ≳ k⊥. The inverse cascade process is primarily driven by the nonlinear wave-wave interaction. It is shown that the energy inverse cascade rate is similar to the energy forward cascade rate at early times although the overall energy in the two cascades is very different. The presence of inverse cascade process does not affect qualitative conclusions established from the whistler turbulence forward cascade simulations.

A Statistical Analysis of Langmuir Wave-Electron Correlations Observed by the CHARM II Auroral Sounding Rocket

NASA Astrophysics Data System (ADS)

Dombrowski, M. P.; Labelle, J. W.; Kletzing, C.; Bounds, S. R.; Kaeppler, S. R.

2014-12-01

Langmuir-mode electron plasma waves are frequently observed by spacecraft in active plasma environments such as the ionosphere. Ionospheric Langmuir waves may be excited by the bump-on-tail instability generated by impinging beams of electrons traveling parallel to the background magnetic field (B). The Correlation of High-frequencies and Auroral Roar Measurement (CHARM II) sounding rocket was launched into a substorm at 9:49 UT on 17 February 2010, from the Poker Flat Research Range in Alaska. The primary instruments included the University of Iowa Wave-Particle Correlator (WPC), the Dartmouth High-Frequency Experiment (HFE), several charged particle detectors, low-frequency wave instruments, and a magnetometer. The HFE is a receiver system which effectively yields continuous (100% duty cycle) electric-field waveform measurements from 100 kHz to 5 MHz, and which had its detection axis aligned nominally parallel to B. The HFE output was fed on-payload to the WPC, which uses a phase-locked loop to track the incoming wave frequency with the most power, then sorting incoming electrons at eight energy levels into sixteen wave-phase bins. CHARM II encountered several regions of strong Langmuir wave activity throughout its 15-minute flight, and the WPC showed wave-lock and statistically significant particle correlation distributions during several time periods. We show results of an in-depth analysis of the CHARM II WPC data for the entire flight, including statistical analysis of correlations which show evidence of direct interaction with the Langmuir waves, indicating (at various times) trapping of particles and both driving and damping of Langmuir waves by particles. In particular, the sign of the gradient in particle flux appears to correlate with the phase relation between the electrons and the wave field, with possible implications for the wave physics.

Optical and Terahertz Measurements of Spintronic Materials

NASA Astrophysics Data System (ADS)

Bas, Derek A.

Terahertz time-domain spectroscopy (THz-TDS) is a versatile method to determine lattice, electronic charge and spin dynamics. This dissertation employs THz-TDS to study the spin and charge dynamics in topological insulator and antiferromagnetic systems. Observing time-domain effects on the scale of picoseconds gives unprecedented control over optoelectronic properties. Methods and challenges of THz generation, detection, and transmission are outlined. The wealth of light-matter interactions present in all nonlinear optical experiments are discussed, including primarily optical rectification, shift currents, and injection currents. Each of these gives valuable insight into the carrier dynamics of a material type. Conventional electronics can be improved in their speed and efficiency by taking advantage of an additional degree of freedom- electron spin. Therefore, we consider material types which exhibit great potential to replace common electronic materials while simultaneously employing electron spin for information storage or transport. Antiferromagnets show a type of spin-order that has the ability to store bits without unwanted interactions between neighboring particles. In antiferromagnetic MnF2 which has a Neel temperature of TN = 67 K, THz-TDS is performed on one-magnon and two-magnon resonances in the 0.1-2.3 THz range while varying the temperature from 6 to 295 K. The behavior of the one-magnon resonance is modeled by modified molecular field theory with an additional coupling term j set as a free parameter to fit the data. The resulting best fit value j = 1.1 provides the first experimental evidence indicating that neighboring spins in MnF 2 are only weakly coupled, closely approximating mean-field theory. Time-of-flight analysis was performed on the transmitted THz pulses to measure the temperature-dependent THz refractive index, which was modeled by phonon energy in the T > TN regime and magnetic energy in the T < TN regime. In the range T < 10 K, measured data deviates from this theory, and can be modeled by internal energy from hyperfine interactions, providing the first direct observation of hyperfine interactions in THz spectroscopy. Topological insulators exhibit the ability to transport spin-polarized currents along their surfaces with high mobilities. Phase-related pulses at photon energies 0.8 and 1.6 eV are used to simultaneously inject shift and injection currents into thin-films of the prototypical topological insulator Bi2Se3, and the foundation is laid out for an extensive study of the novel carrier properties in topological surface states. A method of symmetry analysis based on the crystal lattice is developed for isolation and individual study of the surfaceonly shift currents, which are threefold symmetric with equal components parallel and perpendicular to the pump polarization, and bulk/surface injection currents, which are isotropic parallel to the pump polarization and vanishing perpendicular to it. Pump energies can be tuned through the Dirac point, a capability which holds promise for the search of smoking gun evidence for the novel topological insulator surface state behavior that has been theorized.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meisner, L. L., E-mail: llm@ispms.tsc.ru; Meisner, S. N., E-mail: msn@ispms.tsc.ru; National Research Tomsk State University, Tomsk, 634050

This work comprises a study of the influence of the pulse number of low-energy high-current electron beam (LEHCEB) exposure on the value and character of distribution of residual elastic stresses, texturing effects and the relationship between structural-phase states and physical and mechanical properties of the modified surface layers of TiNi alloy. LEHCEB processing of the surface of TiNi samples was carried out using a RITM-SP [3] installation. Energy density of electron beam was constant at E{sub s} = 3.9 ± 0.5 J/cm{sup 2}; pulse duration was 2.8 ± 0.3 μs. The number of pulses in the series was changeable, (n =more » 2–128). It was shown that as the result of multiple LEHCEB processing of TiNi samples, hierarchically organized multilayer structure is formed in the surface layer. The residual stress field of planar type is formed in the modified surface layer as following: in the direction of the normal to the surface the strain component ε{sub ⊥} < 0 (compressing strain), and in a direction parallel to the surface, the strain component ε{sub ||} > 0 (tensile deformation). Texturing effects and the level of residual stresses after LEHCEB processing of TiNi samples with equal energy density of electron beam (∼3.8 J/cm{sup 2}) depend on the number of pulses and increase with the rise of n > 10.« less

Representing and computing regular languages on massively parallel networks

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miller, M.I.; O'Sullivan, J.A.; Boysam, B.

1991-01-01

This paper proposes a general method for incorporating rule-based constraints corresponding to regular languages into stochastic inference problems, thereby allowing for a unified representation of stochastic and syntactic pattern constraints. The authors' approach first established the formal connection of rules to Chomsky grammars, and generalizes the original work of Shannon on the encoding of rule-based channel sequences to Markov chains of maximum entropy. This maximum entropy probabilistic view leads to Gibb's representations with potentials which have their number of minima growing at precisely the exponential rate that the language of deterministically constrained sequences grow. These representations are coupled to stochasticmore » diffusion algorithms, which sample the language-constrained sequences by visiting the energy minima according to the underlying Gibbs' probability law. The coupling to stochastic search methods yields the all-important practical result that fully parallel stochastic cellular automata may be derived to generate samples from the rule-based constraint sets. The production rules and neighborhood state structure of the language of sequences directly determines the necessary connection structures of the required parallel computing surface. Representations of this type have been mapped to the DAP-510 massively-parallel processor consisting of 1024 mesh-connected bit-serial processing elements for performing automated segmentation of electron-micrograph images.« less

Spatial Control of Laser Wakefield Accelerated Electron Beams

NASA Astrophysics Data System (ADS)

Maksimchuk, A.; Behm, K.; Zhao, T.; Joglekar, A. S.; Hussein, A.; Nees, J.; Thomas, A. G. R.; Krushelnick, K.; Elle, J.; Lucero, A.; Samarin, G. M.; Sarry, G.; Warwick, J.

2017-10-01

The laser wakefield experiments to study and control spatial properties of electron beams were performed using HERCULES laser at the University of Michigan at power of 100 TW. In the first experiment multi-electron beam generation was demonstrated using co-propagating, parallel laser beams with a π-phase shift mirror and showing that interaction between the wakefields can cause injection to occur for plasma and laser parameters in which a single wakefield displays no significant injection. In the second experiment a magnetic triplet quadrupole system was used to refocus and stabilize electron beams at the distance of 60 cm from the interaction region. This produced a 10-fold increase in remote gamma-ray activation of 63Cu using a lead converter. In the third experiment measurements of un-trapped electrons with high transverse momentum produce a 500 mrad (FWHM) ring. This ring is formed by electrons that receive a forward momentum boost by traversing behind the bubble and its size is inversely proportional to the plasma density. The characterization of divergence and charge of this electron ring may reveal information about the wakefield structure and trapping potential. Supported by U.S. Department of Energy and the National Nuclear Security Administration and Air Force Office of Scientific Research.

GRAVITATIONAL MODEL OF HIGH-ENERGY PARTICLES IN A COLLIMATED JET

DOE Office of Scientific and Technical Information (OSTI.GOV)

De Freitas Pacheco, J. A.; Gariel, J.; Marcilhacy, G.

2012-11-10

Observations suggest that relativistic particles play a fundamental role in the dynamics of jets emerging from active galactic nuclei as well as in their interaction with the intracluster medium. However, no general consensus exists concerning the acceleration mechanism of those high-energy particles. A gravitational acceleration mechanism is proposed here in which particles leaving precise regions within the ergosphere of a rotating supermassive black hole (BH) produce a highly collimated flow. These particles follow unbound geodesics which are asymptotically parallel to the spin axis of the BH and are characterized by the energy E, the Carter constant Q, and zero angularmore » momentum of the component L{sub z} . If environmental effects are neglected, the present model predicts the presence of electrons with energies around 9.4 GeV at distances of about 140 kpc from the ergosphere. The present mechanism can also accelerate protons up to the highest energies observed in cosmic rays by the present experiments.« less

Density functional theory study of hydrogen atom abstraction from a series of para-substituted phenols: why is the Hammett σ(p)+ constant able to represent radical reaction rates?

PubMed

Yoshida, Tatsusada; Hirozumi, Koji; Harada, Masataka; Hitaoka, Seiji; Chuman, Hiroshi

2011-06-03

The rate of hydrogen atom abstraction from phenolic compounds by a radical is known to be often linear with the Hammett substitution constant σ(+), defined using the S(N)1 solvolysis rates of substituted cumyl chlorides. Nevertheless, a physicochemical reason for the above "empirical fact" has not been fully revealed. The transition states of complexes between the 2,2-diphenyl-1-picrylhydrazyl radical (dpph·) and a series of para-substituted phenols were determined by DFT (Density Functional Theory) calculations, and then the activation energy as well as the homolytic bond dissociation energy of the O-H bond and charge distribution in the transition state were calculated. The heterolytic bond dissociation energy of the C-Cl bond and charge distribution in the corresponding para-substituted cumyl chlorides were calculated in parallel. Excellent correlations among σ(+), charge distribution, and activation and bond dissociation energies revealed quantitatively that there is a strong similarity between the two reactions, showing that the electron-deficiency of the π-electron system conjugated with a substituent plays a crucial role in determining rates of the two reactions. The results provide a new insight into and physicochemical understanding of σ(+) in the hydrogen abstraction from substituted phenols by a radical.

Marginal instability threshold condition of the aperiodic ordinary mode in equal-mass plasmas

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vafin, S.; Schlickeiser, R.; Yoon, P. H.

The purely growing ordinary (O) mode instability for counter-streaming bi-Maxwellian plasma particle distribution functions has recently received renewed attention due to its importance for the solar wind plasma. Here, the analytical marginal instability condition is derived for magnetized plasmas consisting of equal-mass charged particles, distributed in counter-streams with equal temperatures. The equal-mass composition assumption enormously facilitates the theoretical analysis due to the equality of the values of the electron and positron (positive and negative ion) plasma and gyrofrequencies. The existence of a new instability domain of the O-mode at small plasma beta values is confirmed, when the parallel counter-stream freemore » energy exceeds the perpendicular bi-Maxwellian free energy.« less

Status of Beam Line Detectors for the BigRIPS Fragment Separator at RIKEN RI Beam Factory: Issues on High Rates and Resolution

NASA Astrophysics Data System (ADS)

Sato, Yuki; Fukuda, Naoki; Takeda, Hiroyuki; Kameda, Daisuke; Suzuki, Hiroshi; Shimizu, Yohei; Ahn, DeukSoon; Murai, Daichi; Inabe, Naohito; Shimaoka, Takehiro; Tsubota, Masakatsu; Kaneko, Junichi H.; Chayahara, Akiyoshi; Umezawa, Hitoshi; Shikata, Shinichi; Kumagai, Hidekazu; Murakami, Hiroyuki; Sato, Hiromi; Yoshida, Koichi; Kubo, Toshiyuki

A multiple sampling ionization chamber (MUSIC) and parallel-plate avalanche counters (PPACs) were installed within the superconducting in-flight separator, named BigRIPS, at the RIKEN Nishina Center for particle identification of RI beams. The MUSIC detector showed negligible charge collection inefficiency from recombination of electrons and ions, up to a 99-kcps incidence rate for high-energy heavy ions. For the PPAC detectors, the electrical discharge durability for incident heavy ions was improved by changing the electrode material. Finally, we designed a single crystal diamond detector, which is under development for TOF measurements of high-energy heavy ions, that has a very fast response time (pulse width <1 ns).

Whistler turbulence heating of electrons and ions: Three-dimensional particle-in-cell simuations

DOE PAGES

Gary, S. Peter; Hughes, R. Scott; Wang, Joseph

2016-01-14

In this study, the decay of whistler turbulence in a collisionless, homogeneous, magnetized plasma is studied using three-dimensional particle-in-cell simulations. The simulations are initialized with a narrowband, relatively isotropic distribution of long wavelength whistler modes. A first ensemble of simulations at electron betamore » $${\\beta }_{{\\rm{e}}}$$ = 0.25 and ion-to-electron mass ratio $${m}_{{\\rm{i}}}$$/$${m}_{{\\rm{e}}}$$ = 400 is carried out on a domain cube of dimension $$L{\\omega }_{\\mathrm{pi}}$$/c = 5.12 where $${\\omega }_{\\mathrm{pi}}$$ is the ion plasma frequency. The simulations begin with a range of dimensionless fluctuating field energy densities, $${\\epsilon }_{{\\rm{o}}}$$, and follow the fluctuations as they cascade to broadband, anisotropic turbulence which dissipates at shorter wavelengths, heating both electrons and ions. The electron heating is stronger and preferentially parallel/antiparallel to the background magnetic field $${{\\boldsymbol{B}}}_{{\\rm{o}}};$$ the ion energy gain is weaker and is preferentially in directions perpendicular to $${{\\boldsymbol{B}}}_{{\\rm{o}}}$$. The important new results here are that, over 0.01 < $${\\epsilon }_{{\\rm{o}}}$$ < 0.25, the maximum rate of electron heating scales approximately as $${\\epsilon }_{{\\rm{o}}}$$, and the maximum rate of ion heating scales approximately as $${\\epsilon }_{{\\rm{o}}}^{1.5}$$. A second ensemble of simulations at $${\\epsilon }_{{\\rm{o}}}$$ = 0.10 and $${\\beta }_{{\\rm{e}}}$$ = 0.25 shows that, over 25 < $${m}_{{\\rm{i}}}$$/$${m}_{{\\rm{e}}}\\;$$< 1836, the ratio of the maximum ion heating rate to the maximum electron heating rate scales approximately as $${m}_{{\\rm{e}}}$$/$${m}_{{\\rm{i}}}$$.« less

Self-consistent quasi-static parallel electric field associated with substorm growth phase

NASA Astrophysics Data System (ADS)

Le Contel, O.; Pellat, R.; Roux, A.

2000-06-01

A new approach is proposed to calculate the self-consistent parallel electric field associated with the response of a plasma to quasi-static electromagnetic perturbations (ωωd<ω and ωd>ω (ωd being the bounce averaged magnetic drift frequency equal to kyvd, where ky is the wave number in the y direction and vd the bounce averaged magnetic drift velocity). The first regime (ωd<ω) corresponds to small particle energy and/or small ky, while the second regime (ωd>ω) is adapted to large energies and/or large ky. In particular, in the limit ωd<ω and |vd|<|uy|, where uy is the diamagnetic velocity proportional to the pressure gradient, we find a parallel electric field proportional to the pressure gradient and directed toward the ionosphere in the dusk sector and toward the equator in the dawn sector. This parallel electric field corresponds to a potential drop of a few hundred volts that can accelerate electrons and produce a differential drift between electrons and ions.

Parallel image-acquisition in continuous-wave electron paramagnetic resonance imaging with a surface coil array: Proof-of-concept experiments

NASA Astrophysics Data System (ADS)

Enomoto, Ayano; Hirata, Hiroshi

2014-02-01

This article describes a feasibility study of parallel image-acquisition using a two-channel surface coil array in continuous-wave electron paramagnetic resonance (CW-EPR) imaging. Parallel EPR imaging was performed by multiplexing of EPR detection in the frequency domain. The parallel acquisition system consists of two surface coil resonators and radiofrequency (RF) bridges for EPR detection. To demonstrate the feasibility of this method of parallel image-acquisition with a surface coil array, three-dimensional EPR imaging was carried out using a tube phantom. Technical issues in the multiplexing method of EPR detection were also clarified. We found that degradation in the signal-to-noise ratio due to the interference of RF carriers is a key problem to be solved.

Ion beam neutralization using three-dimensional electron confinement by surface modification of magnetic poles

NASA Astrophysics Data System (ADS)

Nicolaescu, Dan; Sakai, Shigeki; Gotoh, Yasuhito; Ishikawa, Junzo

2011-07-01

Advanced implantation systems used for semiconductor processing require transportation of quasi-parallel ion beams, which have low energy (11B+, 31P+,75As+, Eion=200-1000 eV). Divergence of the ion beam due to space charge effects can be compensated through injection of electrons into different regions of the ion beam. The present study shows that electron confinement takes place in regions of strong magnetic field such as collimator magnet provided with surface mirror magnetic fields and that divergence of the ion beam passing through such regions is largely reduced. Modeling results have been obtained using Opera3D/Tosca/Scala. Electrons may be provided by collision between ions and residual gas molecules or may be injected by field emitter arrays. The size of surface magnets is chosen such as not to disturb ion beam collimation, making the approach compatible with ion beam systems. Surface magnets may form thin magnetic layers with thickness h=0.5 mm or less. Conditions for spacing of surface magnet arrays for optimal electron confinement are outlined.

Progress on complementary patterning using plasmon-excited electron beamlets (Conference Presentation)

NASA Astrophysics Data System (ADS)

Du, Zhidong; Chen, Chen; Pan, Liang

2017-04-01

Maskless lithography using parallel electron beamlets is a promising solution for next generation scalable maskless nanolithography. Researchers have focused on this goal but have been unable to find a robust technology to generate and control high-quality electron beamlets with satisfactory brightness and uniformity. In this work, we will aim to address this challenge by developing a revolutionary surface-plasmon-enhanced-photoemission (SPEP) technology to generate massively-parallel electron beamlets for maskless nanolithography. The new technology is built upon our recent breakthroughs in plasmonic lenses, which will be used to excite and focus surface plasmons to generate massively-parallel electron beamlets through photoemission. Specifically, the proposed SPEP device consists of an array of plasmonic lens and electrostatic micro-lens pairs, each pair independently producing an electron beamlet. During lithography, a spatial optical modulator will dynamically project light onto individual plasmonic lenses to control the switching and brightness of electron beamlets. The photons incident onto each plasmonic lens are concentrated into a diffraction-unlimited spot as localized surface plasmons to excite the local electrons to near their vacuum levels. Meanwhile, the electrostatic micro-lens extracts the excited electrons to form a focused beamlet, which can be rastered across a wafer to perform lithography. Studies showed that surface plasmons can enhance the photoemission by orders of magnitudes. This SPEP technology can scale up the maskless lithography process to write at wafers per hour. In this talk, we will report the mechanism of the strong electron-photon couplings and the locally enhanced photoexcitation, design of a SPEP device, overview of our proof-of-concept study, and demonstrated parallel lithography of 20-50 nm features.

Accelerating atomistic calculations of quantum energy eigenstates on graphic cards

NASA Astrophysics Data System (ADS)

Rodrigues, Walter; Pecchia, A.; Lopez, M.; Auf der Maur, M.; Di Carlo, A.

2014-10-01

Electronic properties of nanoscale materials require the calculation of eigenvalues and eigenvectors of large matrices. This bottleneck can be overcome by parallel computing techniques or the introduction of faster algorithms. In this paper we report a custom implementation of the Lanczos algorithm with simple restart, optimized for graphical processing units (GPUs). The whole algorithm has been developed using CUDA and runs entirely on the GPU, with a specialized implementation that spares memory and reduces at most machine-to-device data transfers. Furthermore parallel distribution over several GPUs has been attained using the standard message passing interface (MPI). Benchmark calculations performed on a GaN/AlGaN wurtzite quantum dot with up to 600,000 atoms are presented. The empirical tight-binding (ETB) model with an sp3d5s∗+spin-orbit parametrization has been used to build the system Hamiltonian (H).

Laser-induced extreme magnetic field in nanorod targets

NASA Astrophysics Data System (ADS)

Lécz, Zsolt; Andreev, Alexander

2018-03-01

The application of nano-structured target surfaces in laser-solid interaction has attracted significant attention in the last few years. Their ability to absorb significantly more laser energy promises a possible route for advancing the currently established laser ion acceleration concepts. However, it is crucial to have a better understanding of field evolution and electron dynamics during laser-matter interactions before the employment of such exotic targets. This paper focuses on the magnetic field generation in nano-forest targets consisting of parallel nanorods grown on plane surfaces. A general scaling law for the self-generated quasi-static magnetic field amplitude is given and it is shown that amplitudes up to 1 MT field are achievable with current technology. Analytical results are supported by three-dimensional particle-in-cell simulations. Non-parallel arrangements of nanorods has also been considered which result in the generation of donut-shaped azimuthal magnetic fields in a larger volume.

Laser-driven collimated tens-GeV monoenergetic protons from mass-limited target plus preformed channel

NASA Astrophysics Data System (ADS)

Zheng, F. L.; Wu, S. Z.; Wu, H. C.; Zhou, C. T.; Cai, H. B.; Yu, M. Y.; Tajima, T.; Yan, X. Q.; He, X. T.

2013-01-01

Proton acceleration by ultra-intense laser pulse irradiating a target with cross-section smaller than the laser spot size and connected to a parabolic density channel is investigated. The target splits the laser into two parallel propagating parts, which snowplow the back-side plasma electrons along their paths, creating two adjacent parallel wakes and an intense return current in the gap between them. The radiation-pressure pre-accelerated target protons trapped in the wake fields now undergo acceleration as well as collimation by the quasistatic wake electrostatic and magnetic fields. Particle-in-cell simulations show that stable long-distance acceleration can be realized, and a 30 fs monoenergetic ion beam of >10 GeV peak energy and <2° divergence can be produced by a circularly polarized laser pulse at an intensity of about 1022 W/cm2.

Dark/visible parallel universes and Big Bang nucleosynthesis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bertulani, C. A.; Frederico, T.; Fuqua, J.

We develop a model for visible matter-dark matter interaction based on the exchange of a massive gray boson called herein the Mulato. Our model hinges on the assumption that all known particles in the visible matter have their counterparts in the dark matter. We postulate six families of particles five of which are dark. This leads to the unavoidable postulation of six parallel worlds, the visible one and five invisible worlds. A close study of big bang nucleosynthesis (BBN), baryon asymmetries, cosmic microwave background (CMB) bounds, galaxy dynamics, together with the Standard Model assumptions, help us to set a limitmore » on the mass and width of the new gauge boson. Modification of the statistics underlying the kinetic energy distribution of particles during the BBN is also discussed. The changes in reaction rates during the BBN due to a departure from the Debye-Hueckel electron screening model is also investigated.« less

AlGaAs growth by OMCVD using an excimer laser

NASA Technical Reports Server (NTRS)

Warner, Joseph D.; Wilt, David M.; Pouch, John J.; Aron, Paul R.

1986-01-01

AlGaAs has been grown on GaAs by laser assisted OMCVD using an excimer laser, wavelength 193 nm, and a Cambridge OMCVD reactor. Films were grown at temperatures of 450 and 500 C with the laser beam parallel to the surface and impinging onto the surface at 15 deg from parallel. The samples were heated by RF coils while the laser beam was perpendicular to the gas flow. Typical gas flow parameters are 12 slm of H2, 15 sccm of Ga(CH3)3, 13 sccm of Al(CH3)3, and a pressure of 250 mbar. The initial energy density of the beam at the surface was 40 mJ/sq cm, the pulse rate was 20 pps, and the growth time was 7 min. The films were analyzed by Auger electron spectroscopy for the aluminum concentration and by TEM for the surface morphology.

Photochemistry on ultrathin metal films: Strongly enhanced cross sections for NO2 on Ag /Si(100)

NASA Astrophysics Data System (ADS)

Wesenberg, Claudia; Autzen, Olaf; Hasselbrink, Eckart

2006-12-01

The surface photochemistry of NO2 on ultrathin Ag(111) films (5-60nm ) on Si(100) substrates has been studied. NO2, forming N2O4 on the surface, dissociates to release NO and NO2 into the gas phase with translational energies exceeding the equivalent of the sample temperature. An increase of the photodesorption cross section is observed for 266nm light when the film thickness is decreased below 30nm despite the fact that the optical absorptivity decreases. For 4.4nm film thickness this increase is about threefold. The data are consistent with a similar effect for 355nm light. The reduced film thickness has no significant influence on the average translation energy of the desorbing molecules or the branching into the different channels. The increased photodesorption cross section is interpreted to result from photon absorption in the Si substrate producing electrons with no or little momenta parallel to the surface at energies where this is not allowed in Ag. It is suggested that these electrons penetrate through the Ag film despite the gap in the surface projected band structure.

Time-resolved spectroscopic studies of photosynthetic reaction centers and tetrapyrrole chromophores for biomedical and solar-energy applications

NASA Astrophysics Data System (ADS)

Kee, Hooi Ling

2008-10-01

The photophysical properties of diverse tetrapyrrole chromophores as well as energy and electron transfer processes in tetrapyrrole dyads are investigated using static and time-resolved (femtoseconds to seconds) absorption and fluorescence spectroscopy. The goal of these studies is to elucidate the molecular design principals necessary to construct chromophores with the specific and tunable properties that will enhance applications in optical molecular imaging, photodynamic therapy, and solar-energy conversion. The kinetic properties of the transient intermediate P+H B- involving the bacteriopheophytin molecule HB on the normally inactive (B) cofactor branch of the bacterial photosynthetic reaction center are examined in Rhodobacter capsulatus mutants. Using nanosecond flash photolysis and F(L181)Y/Y(M208)F/L(M212)H mutant, the decay pathways and yields of P+HB- were measured, giving an overall yield of 13% for B-side charge separation P* → P+HB- → P+ QB- in this mutant. The goal of these studies is to understand the fundamental differences in the rates, yields, and mechanisms of charge separation and charge recombination along the two parallel electron-transport chains in the bacterial reaction center.

TU-AB-BRC-07: Efficiency of An IAEA Phase-Space Source for a Low Energy X-Ray Tube Using Egs++

DOE Office of Scientific and Technical Information (OSTI.GOV)

Watson, PGF; Renaud, MA; Seuntjens, J

Purpose: To extend the capability of the EGSnrc C++ class library (egs++) to write and read IAEA phase-space files as a particle source, and to assess the relative efficiency gain in dose calculation using an IAEA phase-space source for modelling a miniature low energy x-ray source. Methods: We created a new ausgab object to score particles exiting a user-defined geometry and write them to an IAEA phase-space file. A new particle source was created to read from IAEA phase-space data. With these tools, a phase-space file was generated for particles exiting a miniature 50 kVp x-ray tube (The INTRABEAM System,more » Carl Zeiss). The phase-space source was validated by comparing calculated PDDs with a full electron source simulation of the INTRABEAM. The dose calculation efficiency gain of the phase-space source was determined relative to the full simulation. The efficiency gain as a function of i) depth in water, and ii) job parallelization was investigated. Results: The phase-space and electron source PDDs were found to agree to 0.5% RMS, comparable to statistical uncertainties. The use of a phase-space source for the INTRABEAM led to a relative efficiency gain of greater than 20 over the full electron source simulation, with an increase of up to a factor of 196. The efficiency gain was found to decrease with depth in water, due to the influence of scattering. Job parallelization (across 2 to 256 cores) was not found to have any detrimental effect on efficiency gain. Conclusion: A set of tools has been developed for writing and reading IAEA phase-space files, which can be used with any egs++ user code. For simulation of a low energy x-ray tube, the use of a phase-space source was found to increase the relative dose calculation efficiency by factor of up to 196. The authors acknowledge partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant No. 432290).« less

The Goddard Space Flight Center Program to develop parallel image processing systems

NASA Technical Reports Server (NTRS)

Schaefer, D. H.

1972-01-01

Parallel image processing which is defined as image processing where all points of an image are operated upon simultaneously is discussed. Coherent optical, noncoherent optical, and electronic methods are considered parallel image processing techniques.

High frequency RF waves

NASA Astrophysics Data System (ADS)

Horton, William; Brookman, M.; Goniche, M.; Peysson, Y.; Ekedahl, A.

2017-10-01

ECH and LHCD- are scattered by the density and magnetic field turbulence from drift waves as measured in and Tore Supra-WEST, EAST and DIII-D. Ray equations give the spreading from plasma refraction from the antenna through the core plasma until and change the parallel phase velocity evolves to where RF waves are absorbed by the electrons. Extensive LH ray tracing and absorption has been reported using the coupled CP3O ray tracing and LUKE electron phase space density code with collisionless electron-wave resonant absorption. In theory and simulations are shown for the ray propagation with the resulting electron distributions along with the predicted X ray distribution that compared to the measured X-ray spectrum. Lower-hybrid is essential for steady-state operation in tokamaks with control of the high-energy electrons intrinsic to tokamaks confinement and heating. The record steady tokamak plasma is Tore Supra a steady 6 minute steady state plasma with 1 Gigajoule energy passing through the plasma. WEST is repeating the experiments with ITER shaped separatrix and divertor chamber and EAST achieved comparable long-pulse plasmas. Results are presented from an IFS-3D spectral code with a pair of inside-outside LHCD antennas and a figure-8 magnetic separatrix are presented. Scattering of the slow wave into the fast wave wave is explored showing the RF scattering from drift wave dne and dB increases the core penetration may account the measured broad X-ray spectrum. Work supported by the DoE through Grants to the Institute for Fusion Studies [DE-FG02-04ER54742], ARLUT and General Atomics, San Diego, California, USA and the IRFM at Cadarache by the Comissariat Energie Atomique, France.

Monte Carlo calculations of electron beam quality conversion factors for several ion chamber types.

PubMed

Muir, B R; Rogers, D W O

2014-11-01

To provide a comprehensive investigation of electron beam reference dosimetry using Monte Carlo simulations of the response of 10 plane-parallel and 18 cylindrical ion chamber types. Specific emphasis is placed on the determination of the optimal shift of the chambers' effective point of measurement (EPOM) and beam quality conversion factors. The EGSnrc system is used for calculations of the absorbed dose to gas in ion chamber models and the absorbed dose to water as a function of depth in a water phantom on which cobalt-60 and several electron beam source models are incident. The optimal EPOM shifts of the ion chambers are determined by comparing calculations of R50 converted from I50 (calculated using ion chamber simulations in phantom) to R50 calculated using simulations of the absorbed dose to water vs depth in water. Beam quality conversion factors are determined as the calculated ratio of the absorbed dose to water to the absorbed dose to air in the ion chamber at the reference depth in a cobalt-60 beam to that in electron beams. For most plane-parallel chambers, the optimal EPOM shift is inside of the active cavity but different from the shift determined with water-equivalent scaling of the front window of the chamber. These optimal shifts for plane-parallel chambers also reduce the scatter of beam quality conversion factors, kQ, as a function of R50. The optimal shift of cylindrical chambers is found to be less than the 0.5 rcav recommended by current dosimetry protocols. In most cases, the values of the optimal shift are close to 0.3 rcav. Values of kecal are calculated and compared to those from the TG-51 protocol and differences are explained using accurate individual correction factors for a subset of ion chambers investigated. High-precision fits to beam quality conversion factors normalized to unity in a beam with R50 = 7.5 cm (kQ (')) are provided. These factors avoid the use of gradient correction factors as used in the TG-51 protocol although a chamber dependent optimal shift in the EPOM is required when using plane-parallel chambers while no shift is needed with cylindrical chambers. The sensitivity of these results to parameters used to model the ion chambers is discussed and the uncertainty related to the practical use of these results is evaluated. These results will prove useful as electron beam reference dosimetry protocols are being updated. The analysis of this work indicates that cylindrical ion chambers may be appropriate for use in low-energy electron beams but measurements are required to characterize their use in these beams.

Atomistic absorption spectra and non-adiabatic dynamics of the LH2 complex with a GPU-accelerated ab initio exciton model

NASA Astrophysics Data System (ADS)

Glowacki, David

Recently, we outlined an efficient multi-tiered parallel excitonic framework that utilizes time dependent density functional theory (TDDFT) to calculate ground/excited state energies and gradients of large supramolecular complexes in atomistic detail. In this paper, we apply our ab initioexciton framework to the 27 coupled bacteriocholorophyll-a chromophores which make up the LH2 complex, using it to compute linear absorption spectra and short-time, on-the-fly nonadiabatic surface-hopping (SH) dynamics of electronically excited LH2. Our ab initio exciton model includes two key parameters whose values are determined by fitting to experiment: d, which is added to the diagonal elements, corrects for the error in TDDFT vertical excitation energies on a single chromophore; and e, which occurs on the off-diagonal matrix elements, describes the average dielectric screening of the inter-chromophore transition-dipole coupling. Using snapshots obtained from equilibrium molecular dynamics simulations (MD) of LH2, best-fit values of both d and e were obtained by fitting to the thermally broadened experimental absorption spectrum within the Frank-Condon approximation, providing a linear absorption spectrum that agrees reasonably well with the experimental observations. We follow the nonadiabatic dynamics using surface hopping to construct time-resolved visualizations of the EET dynamics in the sub-picosecond regime following photoexcitation. This provides some qualitative insight into the excitonic energy transfer (EET) that results from atomically resolved vibrational fluctuations of the chromophores. The dynamical picture that emerges is one of rapidly fluctuating eigenstates that are delocalized over multiple chromophores and undergo frequent crossing on a femtosecond timescale as a result of the underlying chromophore vibrational dynamics. The eigenstate fluctuations arise from disorder in both the diagonal chromophore site energies and the off-diagonal inter-chromophore couplings. The scalability of our excitonic computational framework across massively parallel architectures opens up the possibility of addressing a wide range of questions, including how specific dynamical motions impact both the pathways and efficiency of electronic energy-transfer within large supramolecular systems.

SU-F-T-69: Correction Model of NIPAM Gel and Presage for Electron and Proton PDD Measurement

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, C; Lin, C; Tu, P

Purpose: The current standard equipment for proton PDD measurement is multilayer-parallel-ion-chamber. Disadvantage of multilayer-parallel-ion-chamber is expensive and complexity manipulation. NIPAM-gel and Presage are options for PDD measurement. Due to different stopping power, the result of NIPAM-gel and Presage need to be corrected. This study aims to create a correction model for NIPAM-gel and Presage PDD measurement. Methods: Standard water based PDD profiles of electron 6MeV, 12MeV, and proton 90MeV were acquired. Electron PDD profile after 1cm thickness of NIPAM-gel added on the top of water was measured. Electron PDD profile with extra 1cm thickness of solid water, PTW RW3, wasmore » measured. The distance shift among standard PDD, NIPAM-gel PDD, and solid water PDD at R50% was compared and water equivalent thickness correction factor (WET) was calculated. Similar process was repeated. WETs for electron with Presage, proton with NIPAM-gel, and proton with Presage were calculated. PDD profiles of electron and proton with NIPAM-gel and Presage columns were corrected with each WET. The corrected profiles were compared with standard profiles. Results: WET for electron 12MeV with NIPAM-gel was 1.135, and 1.034 for electron 12Mev with Presage. After correction, PDD profile matched to the standard profile at the fall-off range well. The difference at R50% was 0.26mm shallower and 0.39mm deeper. The same WET was used to correct electron 6MeV profile. Energy independence of electron WET was observed. The difference at R50% was 0.17mm deeper for NIPAM-gel and 0.54mm deeper for Presage. WET for proton 90MeV with NIPAM-gel was 1.056. The difference at R50% was 0.37 deeper. Quenching effect at Bragg peak was revealed. The underestimated dose percentage at Bragg peak was 27%. Conclusion: This correction model can be used to modify PDD profile with depth error within 1mm. With this correction model, NIPAM-gel and Presage can be practical at PDD profile measurement.« less

Parallel processing implementation for the coupled transport of photons and electrons using OpenMP

NASA Astrophysics Data System (ADS)

Doerner, Edgardo

2016-05-01

In this work the use of OpenMP to implement the parallel processing of the Monte Carlo (MC) simulation of the coupled transport for photons and electrons is presented. This implementation was carried out using a modified EGSnrc platform which enables the use of the Microsoft Visual Studio 2013 (VS2013) environment, together with the developing tools available in the Intel Parallel Studio XE 2015 (XE2015). The performance study of this new implementation was carried out in a desktop PC with a multi-core CPU, taking as a reference the performance of the original platform. The results were satisfactory, both in terms of scalability as parallelization efficiency.

A Tractable Estimate for the Dissipation Range Onset Wavenumber Throughout the Heliosphere

NASA Astrophysics Data System (ADS)

Engelbrecht, N. Eugene; Strauss, R. Du Toit

2018-04-01

The modulation of low-energy electrons in the heliosphere is extremely sensitive to the behavior of the dissipation range slab turbulence. The present study derives approximate expressions for the wavenumber at which the dissipation range on the slab turbulence power spectrum commences, by assuming that this onset occurs when dispersive waves propagating parallel to the background magnetic field gyroresonate with thermal plasma particles. This assumption yields results in reasonable agreement with existing spacecraft observations. These expressions are functions of the solar wind proton and electron temperatures, which are here modeled throughout the region where the solar wind is supersonic using a two-component turbulence transport model. The results so acquired are compared with extrapolations of existing models for the dissipation range onset wavenumber, and conclusions are drawn therefrom.

High-pressure, high-temperature synthesis of superhard boron suboxide

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hubert, H.; Garvie, L.A.J.; Leinenweber, K.

A multianvil device was used to investigate the formation of B{sub x}O phases produced in the 2 to 10 GPa pressure range with temperatures between 1,000 and 1,800 C. Amorphous and crystalline B and BP were oxidized using B{sub 2}O{sub 3} and CrO{sub 3}. Using powder X-ray diffraction and parallel electron energy-loss spectroscopy (PEELS), the authors were unable to detect graphitic or diamond-structured B{sub 2}O, reported in previous studies. The refractory boride B{sub 6}O, which has the {alpha}-rhombohedral boron structure, is the dominant suboxide in the P and T range of the investigation. PEELS with a transmission electron microscope wasmore » used to characterize the boron oxides.« less

VLF Wave Properties During Geomagnetic Storms

NASA Astrophysics Data System (ADS)

Blancarte, J.; Artemyev, A.; Mozer, F.; Agapitov, O. V.

2017-12-01

Whistler-mode chorus is important for the global dynamics of the inner magnetosphere electron population due to its ability to scatter and accelerate electrons of a wide energy range in the outer radiation belt. The parameters of these VLF emissions change dynamically during geomagnetic storms. Presented is an analysis of four years of Van Allen probe data, utilizing electric and magnetic field in the VLF range focused on the dynamics of chorus wave properties during the enhancement of geomagnetic activity. It is found that VLF emissions respond to geomagnetic storms in more complicated ways than just by affecting the waves' amplitude growth or depletion. Oblique wave amplitudes grow together with parallel waves during periods of intermediate geomagnetic activity, while the occurrence rate of oblique waves decreases during larger geomagnetic storms.

Radiation reaction and pitch-angle changes for a charge undergoing synchrotron losses

NASA Astrophysics Data System (ADS)

Singal, Ashok K.

2016-05-01

In the derivation of synchrotron radiation formulae, it has been assumed that the pitch angle of a charge remains constant during the radiation process. However, from the radiation reaction formula, while the component of the velocity vector perpendicular to the magnetic field reduces in magnitude due to radiative losses, the parallel component does not undergo any change during radiation. Therefore, there is a change in the ratio of the two components, implying a change in the pitch angle. We derive the exact formula for the change in energy of radiating electrons by taking into account the change of the pitch angle due to radiative losses. From this, we derive the characteristic decay time of synchrotron electrons over which they turn from highly relativistic into mildly relativistic ones.

Weak antilocalization in Cd3As2 thin films

PubMed Central

Zhao, Bo; Cheng, Peihong; Pan, Haiyang; Zhang, Shuai; Wang, Baigeng; Wang, Guanghou; Xiu, Faxian; Song, Fengqi

2016-01-01

Recently, it has been theoretically predicted that Cd3As2 is a three dimensional Dirac material, a new topological phase discovered after topological insulators, which exhibits a linear energy dispersion in the bulk with massless Dirac fermions. Here, we report on the low-temperature magnetoresistance measurements on a ~50 nm-thick Cd3As2 film. The weak antilocalization under perpendicular magnetic field is discussed based on the two-dimensional Hikami-Larkin-Nagaoka (HLN) theory. The electron-electron interaction is addressed as the source of the dephasing based on the temperature-dependent scaling behavior. The weak antilocalization can be also observed while the magnetic field is parallel to the electric field due to the strong interaction between the different conductance channels in this quasi-two-dimensional film. PMID:26935029

Weak antilocalization in Cd3As2 thin films.

PubMed

Zhao, Bo; Cheng, Peihong; Pan, Haiyang; Zhang, Shuai; Wang, Baigeng; Wang, Guanghou; Xiu, Faxian; Song, Fengqi

2016-03-03

Recently, it has been theoretically predicted that Cd3As2 is a three dimensional Dirac material, a new topological phase discovered after topological insulators, which exhibits a linear energy dispersion in the bulk with massless Dirac fermions. Here, we report on the low-temperature magnetoresistance measurements on a ~50 nm-thick Cd3As2 film. The weak antilocalization under perpendicular magnetic field is discussed based on the two-dimensional Hikami-Larkin-Nagaoka (HLN) theory. The electron-electron interaction is addressed as the source of the dephasing based on the temperature-dependent scaling behavior. The weak antilocalization can be also observed while the magnetic field is parallel to the electric field due to the strong interaction between the different conductance channels in this quasi-two-dimensional film.

Current correlations for the transport of interacting electrons through parallel quantum dots in a photon cavity

NASA Astrophysics Data System (ADS)

Gudmundsson, Vidar; Abdullah, Nzar Rauf; Sitek, Anna; Goan, Hsi-Sheng; Tang, Chi-Shung; Manolescu, Andrei

2018-06-01

We calculate the current correlations for the steady-state electron transport through multi-level parallel quantum dots embedded in a short quantum wire, that is placed in a non-perfect photon cavity. We account for the electron-electron Coulomb interaction, and the para- and diamagnetic electron-photon interactions with a stepwise scheme of configuration interactions and truncation of the many-body Fock spaces. In the spectral density of the temporal current-current correlations we identify all the transitions, radiative and non-radiative, active in the system in order to maintain the steady state. We observe strong signs of two types of Rabi oscillations.

6 x 6-cm fully depleted pn-junction CCD for high-resolution spectroscopy in the 0.1- to 15-keV photon energy range

NASA Astrophysics Data System (ADS)

von Zanthier, Christoph; Holl, Peter; Kemmer, Josef; Lechner, Peter; Maier, B.; Soltau, Heike; Stoetter, R.; Braeuninger, Heinrich W.; Dennerl, Konrad; Haberl, Frank; Hartmann, R.; Hartner, Gisela D.; Hippmann, H.; Kastelic, E.; Kink, W.; Krause, N.; Meidinger, Norbert; Metzner, G.; Pfeffermann, Elmar; Popp, M.; Reppin, Claus; Stoetter, Diana; Strueder, Lothar; Truemper, Joachim; Weber, U.; Carathanassis, D.; Engelhard, S.; Gebhart, Th.; Hauff, D.; Lutz, G.; Richter, R. H.; Seitz, H.; Solc, P.; Bihler, Edgar; Boettcher, H.; Kendziorra, Eckhard; Kraemer, J.; Pflueger, Bernhard; Staubert, Ruediger

1998-04-01

The concept and performance of the fully depleted pn- junction CCD system, developed for the European XMM- and the German ABRIXAS-satellite missions for soft x-ray imaging and spectroscopy in the 0.1 keV to 15 keV photon range, is presented. The 58 mm X 60 mm large pn-CCD array uses pn- junctions for registers and for the backside instead of MOS registers. This concept naturally allows to fully deplete the detector volume to make it an efficient detector to photons with energies up to 15 keV. For high detection efficiency in the soft x-ray region down to 100 eV, an ultrathin pn-CCD backside deadlayer has been realized. Each pn-CCD-channel is equipped with an on-chip JFET amplifier which, in combination with the CAMEX-amplifier and multiplexing chip, facilitates parallel readout with a pixel read rate of 3 MHz and an electronic noise floor of ENC < e-. With the complete parallel readout, very fast pn-CCD readout modi can be implemented in the system which allow for high resolution photon spectroscopy of even the brightest x-ray sources in the sky.

Self-Powered Active Sensor with Concentric Topography of Piezoelectric Fibers

NASA Astrophysics Data System (ADS)

Fuh, Yiin Kuen; Huang, Zih Ming; Wang, Bo Sheng; Li, Shan Chien

2017-01-01

In this study, we demonstrated a flexible and self-powered sensor based on piezoelectric fibers in the diameter range of nano- and micro-scales. Our work is distinctively different from previous electrospinning research; we fabricated this apparatus precisely via near-field electrospinning which has a spectacular performance to harvest mechanical deformation in arbitrary direction and a novel concentrically circular topography. There are many piezoelectric devices based on electrospinning polymeric fibers. However, the fibers were mostly patterned in parallel lines and they could be actuated in limited direction only. To overcome this predicament, we re-arranged the parallel alignment into concentric circle pattern which made it possible to collect the mechanical energy whenever the deformation is along same axis or not. Despite the change of topography, the output voltage and current could still reach to 5 V and 400 nA, respectively, despite the mechanical deformation was from different direction. This new arbitrarily directional piezoelectric generator with concentrically circular topography (PGCT) allowed the piezoelectric device to harvest more mechanical energy than the one-directional alignment fiber-based devices, and this PGCT could perform even better output which promised more versatile and efficient using as a wearable electronics or sensor.

Self-Powered Active Sensor with Concentric Topography of Piezoelectric Fibers.

PubMed

Fuh, Yiin Kuen; Huang, Zih Ming; Wang, Bo Sheng; Li, Shan Chien

2017-12-01

In this study, we demonstrated a flexible and self-powered sensor based on piezoelectric fibers in the diameter range of nano- and micro-scales. Our work is distinctively different from previous electrospinning research; we fabricated this apparatus precisely via near-field electrospinning which has a spectacular performance to harvest mechanical deformation in arbitrary direction and a novel concentrically circular topography. There are many piezoelectric devices based on electrospinning polymeric fibers. However, the fibers were mostly patterned in parallel lines and they could be actuated in limited direction only. To overcome this predicament, we re-arranged the parallel alignment into concentric circle pattern which made it possible to collect the mechanical energy whenever the deformation is along same axis or not. Despite the change of topography, the output voltage and current could still reach to 5 V and 400 nA, respectively, despite the mechanical deformation was from different direction. This new arbitrarily directional piezoelectric generator with concentrically circular topography (PGCT) allowed the piezoelectric device to harvest more mechanical energy than the one-directional alignment fiber-based devices, and this PGCT could perform even better output which promised more versatile and efficient using as a wearable electronics or sensor.

Stereodynamics of the photodissociation of nitromethane at 193 nm: unravelling the dissociation mechanism.

PubMed

Rodríguez, J D; González, M G; Rubio-Lago, L; Bañares, L; Samartzis, P C; Kitsopoulos, T N

2013-08-29

The photodissociation of nitromethane at 193 nm is reviewed in terms of new stereodynamical information provided by the measurement of the first four Dixon's bipolar moments, β0(2)(20), β0(0)(22), β0(2)(02), and β0(2)(22), using slice imaging. The measured speed-dependent β0(2)(20) (directly related with the spatial anisotropy parameter β) indicates that after one-photon absorption to the S3(2 (1)A″) state by an allowed perpendicular transition, two reaction pathways can compete with similar probability, a direct dissociation process yielding ground-state CH3 and NO2(1 (2)A2) radicals and a indirect dissociation through conical intersections in which NO2 radicals are formed in lower-lying electronic states. A particularly important result from our measurements is that the low recoil energy part of the methyl fragment translational energy distribution presents a contribution with parallel character, irrespective of the experimental conditions employed, that we attribute to parent cluster dissociation. Moreover, the positive values found for the β0(0)(22) bipolar moment indicates some propensity for the fragment's recoil velocity and angular momentum vectors to be parallel.

Implementation of the DPM Monte Carlo code on a parallel architecture for treatment planning applications.

PubMed

Tyagi, Neelam; Bose, Abhijit; Chetty, Indrin J

2004-09-01

We have parallelized the Dose Planning Method (DPM), a Monte Carlo code optimized for radiotherapy class problems, on distributed-memory processor architectures using the Message Passing Interface (MPI). Parallelization has been investigated on a variety of parallel computing architectures at the University of Michigan-Center for Advanced Computing, with respect to efficiency and speedup as a function of the number of processors. We have integrated the parallel pseudo random number generator from the Scalable Parallel Pseudo-Random Number Generator (SPRNG) library to run with the parallel DPM. The Intel cluster consisting of 800 MHz Intel Pentium III processor shows an almost linear speedup up to 32 processors for simulating 1 x 10(8) or more particles. The speedup results are nearly linear on an Athlon cluster (up to 24 processors based on availability) which consists of 1.8 GHz+ Advanced Micro Devices (AMD) Athlon processors on increasing the problem size up to 8 x 10(8) histories. For a smaller number of histories (1 x 10(8)) the reduction of efficiency with the Athlon cluster (down to 83.9% with 24 processors) occurs because the processing time required to simulate 1 x 10(8) histories is less than the time associated with interprocessor communication. A similar trend was seen with the Opteron Cluster (consisting of 1400 MHz, 64-bit AMD Opteron processors) on increasing the problem size. Because of the 64-bit architecture Opteron processors are capable of storing and processing instructions at a faster rate and hence are faster as compared to the 32-bit Athlon processors. We have validated our implementation with an in-phantom dose calculation study using a parallel pencil monoenergetic electron beam of 20 MeV energy. The phantom consists of layers of water, lung, bone, aluminum, and titanium. The agreement in the central axis depth dose curves and profiles at different depths shows that the serial and parallel codes are equivalent in accuracy.

Resolving the Mystery of Transport Within Internal Transport Barriers

NASA Astrophysics Data System (ADS)

Staebler, G. M.

2013-10-01

The Trapped Gyro-Landau Fluid (TGLF) quasilinear model, which is calibrated to approximate non-linear gyro-kinetic turbulence simulations, is now able to predict the electron density, electron and ion temperatures and ion toroidal rotation simultaneously for internal transport barrier (ITB) discharges in excellent agreement with data from the DIII-D tokamak. This is a strong validation of gyro-kinetic theory of ITBs, requiring multiple instabilities responsible for transport in different channels at different scales. Inside the ITB, the ion energy transport is observed to be reduced to the neoclassical level which is consistent with the theory of turbulence suppression by E × B velocity shear acting on low wavenumber turbulence. The electron energy transport is observed to be far above the neoclassical level which is consistent with electron energy transport due to high wavenumber electron temperature gradient (ETG) modes. Since the ETG modes do not produce particle and ion momentum transport, and low wavenumber modes are suppressed, these channels are expected to be reduced to the neoclassical level in striking disagreement with experimental measurements. A possible resolution of this conundrum was found in 2005 when gyro-kinetic turbulence simulations showed that the parallel velocity shear driven Kelvin-Helmholtz (KH) mode can arrest the suppression of transport by the shear in the E × B velocity Doppler shift at high toroidal flow shear. The success of TGLF in predicting ITB transport is due to the inclusion of ion gyro-radius scale modes that become dominant at high E × B shear and to recent improvements to TGLF that allow the KH mode to be faithfully modeled. The resolution of this long-standing mystery of the missing particle and momentum transport in an ITB is the result of the steady advances in gyro-kinetic simulations and quasilinear modeling. Supported by the US Department of Energy under DE-FG02-95ER54309.

Diffusion Region's Structure at the Subsolar Magnetopause with MMS Data

NASA Astrophysics Data System (ADS)

Cozzani, G.; Retino, A.; Califano, F.; Alexandrova, A.; Catapano, F.; Fu, H.; Le Contel, O.; Khotyaintsev, Y. V.; Vaivads, A.; Ahmadi, N.; Lindqvist, P. A.; Breuillard, H.; Mirioni, L.; Ergun, R.; Torbert, R. B.; Giles, B. L.; Russell, C. T.; Nakamura, R.; Moore, T. E.; Fuselier, S. A.; Mauk, B.; Burch, J.

2017-12-01

Magnetic reconnection occurs in the magnetosphere in thin current sheets, where a change in the magneticfield topology leads to rapid conversion of magnetic energy into ion and electron energy. To allow for magneticfield reconfiguration, both ions and electrons have to become demagnetized in the ion and electron diffusionregions, respectively. MMS spacecraft observations at inter-spacecraft separation ˜ 10 km (correspondingto ˜ 5 d_e at the magnetopause) allow, for the first time, to make multi-point studies of the structure of theelectron diffusion region (EDR). We present MMS observations on January,27th 2017 of one magnetopausecrossing close to the subsolar point showing several signatures consistent with an EDR encounter nearbya magnetic field minimum. The proximity to the reconnection site is further substantiated by the FirstOrder Taylor Expansion (FOTE) method applied to the magnetic field data. Observations suggest that allspacecraft passed through the EDR. Despite of the small inter-spacecraft separation (7 km), the observationsshow important differences among spacecraft. We focus on the comparison between MMS3 and MMS4 sincethey show the most striking differences. MMS3 measures a stronger parallel electron heating and highercurrent densities than MMS4. Both satellites observe crescent-shaped electron distribution functions on themagnetospheric side but MMS4 observes them over a longer time interval. These observations suggest thatMMS3 is passing closer to the reconnection site than MMS4. The differences between the observations by thetwo spacecraft indicate that the EDR is rather structured over scales of a few electron inertial lengths. Wealso evaluate the Generalized Ohm's law and find that the electric field is mainly balanced by the divergenceof the electron pressure tensor while the electron inertia term is negligible.

Secondary electron emission influenced by oxidation on the aluminum surface: the roles of the chemisorbed oxygen and the oxide layer

NASA Astrophysics Data System (ADS)

Li, Jiangtao; Hoekstra, Bart; Wang, Zhen-Bin; Qiu, Jie; Pu, Yi-Kang

2018-04-01

A relationship between the apparent secondary electron yield ({γ }{{se}}) and the oxygen coverage/oxide layer thickness on an aluminum cathode is obtained in an experiment under a controlled environment. The apparent secondary electron yield ({γ }{{se}}) is deduced from the breakdown voltage between two parallel plate electrodes in a 360 mTorr argon environment using a simple Townsend breakdown model with the assumption that the variation of the apparent secondary electron yield is dominated by the variation of the argon ion induced processes. The oxygen coverage/oxide layer thickness on the aluminum cathode is measured by a semi in situ x-ray photoemission spectroscopy equipment which is directly attached to the discharge chamber. It is found that three phases exist: (1) in the monomonolayer regime, as the oxygen coverage increases from 0 to 0.3, {γ }{{se}} decreases by nearly 40 % , (2) as the oxygen coverage increases from 0.3 to 1, {γ }{{se}} keeps nearly constant, (3) as the oxide layer thickness increases from about 0.3 nm to about 1.1 nm, {γ }{{se}} increases by 150 % . We propose that, in the submonolayer regime, the chemisorbed oxygen on the aluminum surface causes the decrease of {γ }{{se}} by creating a local potential barrier, which reduces the Auger neutralization rate and the energy gained by the Auger electrons. In the multilayer regime, as the oxide layer grows in thickness, there are three proposed mechanisms which cause the increase of {γ }{{se}}: (1) the work function decreases; (2) resonance neutralization and Auger de-excitation may exist. This is served as another channel for secondary electron production; (3) the kinetic energy of Auger electrons is increased on average, leading to a higher probability for electrons to overcome the surface potential barrier.

SU-E-T-98: An Analysis of TG-51 Electron Beam Calibration Correction Factor Uncertainty

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, P; Alvarez, P; Taylor, P

Purpose: To analyze the uncertainty of the TG-51 electron beam calibration correction factors for farmer type ion chambers currently used by institutions visited by IROC Houston. Methods: TG-51 calibration data were collected from 181 institutions visited by IROC Houston physicists for 1174 and 197 distinct electron beams from modern Varian and Elekta accelerators, respectively. Data collected and analyzed included ion chamber make and model, nominal energy, N{sub D,w}, I{sub 50}, R{sub 50}, k’R{sub 50}, d{sub ref}, P{sub gr} and pdd(d{sub ref}). k’R{sub 50} data for parallel plate chambers were excluded from the analysis. Results: Unlike photon beams, electron nominal energymore » is a poor indicator of the actual energy as evidenced by the range of R{sub 50} values for each electron beam energy (6–22MeV). The large range in R{sub 50} values resulted k’R{sub 50} values with a small standard deviation but large range between maximum value used and minimum value (0.001–0.029) used for a specific Varian nominal energy. Varian data showed more variability in k’R{sub 50} values than the Elekta data (0.001–0.014). Using the observed range of R{sub 50} values, the maximum spread in k’R{sub 50} values was determined by IROC Houston and compared to the spread of k’R{sub 50} values used in the community. For Elekta linacs the spreads were equivalent, but for Varian energies of 6 to 16MeV, the community spread was 2 to 6 times larger. Community P{sub gr} values had a much larger range of values for 6 and 9 MeV values than predicted. The range in Varian pdd(d{sub ref} ) used by the community for low energies was large, (1.4–4.9 percent), when it should have been very close to unity. Exradin, PTW Roos and PTW farmer chambers N{sub D,w} values showed the largest spread, ≥11 percent. Conclusion: While the vast majority of electron beam calibration correction factors used are accurate, there is a surprising spread in some of the values used.« less

Monte Carlo simulations and benchmark measurements on the response of TE(TE) and Mg(Ar) ionization chambers in photon, electron and neutron beams

NASA Astrophysics Data System (ADS)

Lin, Yi-Chun; Huang, Tseng-Te; Liu, Yuan-Hao; Chen, Wei-Lin; Chen, Yen-Fu; Wu, Shu-Wei; Nievaart, Sander; Jiang, Shiang-Huei

2015-06-01

The paired ionization chambers (ICs) technique is commonly employed to determine neutron and photon doses in radiology or radiotherapy neutron beams, where neutron dose shows very strong dependence on the accuracy of accompanying high energy photon dose. During the dose derivation, it is an important issue to evaluate the photon and electron response functions of two commercially available ionization chambers, denoted as TE(TE) and Mg(Ar), used in our reactor based epithermal neutron beam. Nowadays, most perturbation corrections for accurate dose determination and many treatment planning systems are based on the Monte Carlo technique. We used general purposed Monte Carlo codes, MCNP5, EGSnrc, FLUKA or GEANT4 for benchmark verifications among them and carefully measured values for a precise estimation of chamber current from absorbed dose rate of cavity gas. Also, energy dependent response functions of two chambers were calculated in a parallel beam with mono-energies from 20 keV to 20 MeV photons and electrons by using the optimal simple spherical and detailed IC models. The measurements were performed in the well-defined (a) four primary M-80, M-100, M120 and M150 X-ray calibration fields, (b) primary 60Co calibration beam, (c) 6 MV and 10 MV photon, (d) 6 MeV and 18 MeV electron LINACs in hospital and (e) BNCT clinical trials neutron beam. For the TE(TE) chamber, all codes were almost identical over the whole photon energy range. In the Mg(Ar) chamber, MCNP5 showed lower response than other codes for photon energy region below 0.1 MeV and presented similar response above 0.2 MeV (agreed within 5% in the simple spherical model). With the increase of electron energy, the response difference between MCNP5 and other codes became larger in both chambers. Compared with the measured currents, MCNP5 had the difference from the measurement data within 5% for the 60Co, 6 MV, 10 MV, 6 MeV and 18 MeV LINACs beams. But for the Mg(Ar) chamber, the derivations reached 7.8-16.5% below 120 kVp X-ray beams. In this study, we were especially interested in BNCT doses where low energy photon contribution is less to ignore, MCNP model is recognized as the most suitable to simulate wide photon-electron and neutron energy distributed responses of the paired ICs. Also, MCNP provides the best prediction of BNCT source adjustment by the detector's neutron and photon responses.

Emission spectra of electron irradiated metal foils

DOE Office of Scientific and Technical Information (OSTI.GOV)

Emerson, L. C.; Arakawa, E. T.; Ritchie, R. H.

1963-08-09

Thesis submitted to Univ. of Tennessee by L. C. Emerson. An experimental investigation of the visible and ultraviolet light emitted when a charged particle moves across a boundary between two media with different dielectric properties was carried out. The spectral distributions of the light from evaporated foils of copper, germanium, silver, tin, and antimony bombarded by a 1.5-microamp beam of electrons were measured as a function of electron energy between 25 and 100 kev. The analysis was carried out with a Seya-Namioka vacuum ultraviolet spectrometer, Glan-Foucault prism polarizer, and a quartz- window photomultiplier. Calibration of the optical system with amore » NBS tungsten filament lamp enabled the intensity measurements to be carried out on an absolute basis. The experimental results for light polarized parallel to the plane containing the photon and the electron were compared with the calculated intensity of transftion radiation, and in general the agreement was found to be excellent. The component of the photon intensity polarized perpendicular to this plane was compared with the calculated intensity of the optical portion of the bremsstrahlung spectrum. The predicted dependence on electron energy, photon wavelength, and foil thickness was observed although the measured yield was higher than that predicted by theory. The possibility of contributions from Cherenkov radiation and plasma radiation is also considered, but it is shown that it would be unlikely for these radiations to be observed under the experimental conditions of this study.« less

Tests with beam setup of the TileCal phase-II upgrade electronics

NASA Astrophysics Data System (ADS)

Reward Hlaluku, Dingane

2017-09-01

The LHC has planned a series of upgrades culminating in the High Luminosity LHC which will have an average luminosity 5-7 times larger than the nominal Run-2 value. The ATLAS Tile calorimeter plans to introduce a new readout architecture by completely replacing the back-end and front-end electronics for the High Luminosity LHC. The photomultiplier signals will be fully digitized and transferred for every bunch crossing to the off-detector Tile PreProcessor. The Tile PreProcessor will further provide preprocessed digital data to the first level of trigger with improved spatial granularity and energy resolution in contrast to the current analog trigger signals. A single super-drawer module commissioned with the phase-II upgrade electronics is to be inserted into the real detector to evaluate and qualify the new readout and trigger concepts in the overall ATLAS data acquisition system. This new super-drawer, so-called hybrid Demonstrator, must provide analog trigger signals for backward compatibility with the current system. This Demonstrator drawer has been inserted into a Tile calorimeter module prototype to evaluate the performance in the lab. In parallel, one more module has been instrumented with two other front-end electronics options based on custom ASICs (QIE and FATALIC) which are under evaluation. These two modules together with three other modules composed of the current system electronics were exposed to different particles and energies in three test-beam campaigns during 2015 and 2016.

TEM preparation methods and influence of radiation damage on the beam sensitive CaCO3 shell of Emiliania huxleyi.

PubMed

Hoffmann, Ramona; Wochnik, Angela S; Betzler, Sophia B; Matich, Sonja; Griesshaber, Erika; Schmahl, Wolfgang W; Scheu, Christina

2014-07-01

The ultrastructure of biologically formed calcium carbonate crystals like the shell of Emiliania huxleyi depends on the environmental conditions such as pH value, temperature and salinity. Therefore, they can be used as indicator for climate changes. However, for this a detailed understanding of their crystal structure and chemical composition is required. High resolution methods like transmission electron microscopy can provide those information on the nanoscale, given that sufficiently thin samples can be prepared. In our study, we developed sample preparation techniques for cross-section and plan-view investigations and studied the sample stability under electron bombardment. In addition to the biological material (Emiliania huxleyi) we also prepared mineralogical samples (Iceland spar) for comparison. High resolution transmission electron microscopy imaging, electron diffraction and electron energy-loss spectroscopy studies revealed that all prepared samples are relatively stable under electron bombardment at an acceleration voltage of 300 kV when using a parallel illumination. Above an accumulated dose of ∼10(5) e/nm2 the material--independent whether its origin is biological or geological--transformed to poly-crystalline calcium oxide. Copyright © 2014 Elsevier Ltd. All rights reserved.

Acceleration of Ions and Electrons by Coronal Shocks

NASA Astrophysics Data System (ADS)

Sandroos, A.

2013-12-01

Diffusive shock acceleration (DSA) of particles at collisionless shock waves driven by coronal mass ejections (CMEs) is the best developed theory for the genesis of gradual solar energetic particle (SEP) events. According to DSA, particles scatter from fluctuations present in the ambient magnetic field, which causes some particles to encounter the shock front repeatedly and to gain energy during each crossing. DSA operating in solar corona is a complex process whose outcome depends on multiple parameters such as shock speed and strength, magnetic geometry, and composition of seed particles. Currently, STEREO and other near-Earth spacecraft are providing valuable multi-point information on how SEP properties, such as composition and energy spectra, vary in longitude. Initial results have shown that longitude distributions of large CME-associated SEP events are much wider than previously thought. These findings have many important consequences on SEP modeling. For example, it is important to extend the present models into two or three spatial coordinates to properly account for the effects of coronal and interplanetary magnetic geometry and the evolution of the CME-driven shock wave on the acceleration and transport of SEPs. We present a new model for the shock acceleration of ions and electrons in the solar corona and discuss implications for particle properties (energy spectra, longitudinal distribution, composition) in the resulting gradual SEP events. We also discuss the possible emission of type II radio waves by the accelerated coronal electrons. In the new model, the ion pitch angle scattering rate is calculated from modeled Alfvén wave power spectra using quasilinear theory. The energy gained by ions in scatterings are self-consistently removed from waves so that total energy (ions+waves) is conserved. New model has been implemented on massively parallel simulation platform Corsair.

Roothaan's approach to solve the Hartree-Fock equations for atoms confined by soft walls: Basis set with correct asymptotic behavior.

PubMed

Rodriguez-Bautista, Mariano; Díaz-García, Cecilia; Navarrete-López, Alejandra M; Vargas, Rubicelia; Garza, Jorge

2015-07-21

In this report, we use a new basis set for Hartree-Fock calculations related to many-electron atoms confined by soft walls. One- and two-electron integrals were programmed in a code based in parallel programming techniques. The results obtained with this proposal for hydrogen and helium atoms were contrasted with other proposals to study just one and two electron confined atoms, where we have reproduced or improved the results previously reported. Usually, an atom enclosed by hard walls has been used as a model to study confinement effects on orbital energies, the main conclusion reached by this model is that orbital energies always go up when the confinement radius is reduced. However, such an observation is not necessarily valid for atoms confined by penetrable walls. The main reason behind this result is that for atoms with large polarizability, like beryllium or potassium, external orbitals are delocalized when the confinement is imposed and consequently, the internal orbitals behave as if they were in an ionized atom. Naturally, the shell structure of these atoms is modified drastically when they are confined. The delocalization was an argument proposed for atoms confined by hard walls, but it was never verified. In this work, the confinement imposed by soft walls allows to analyze the delocalization concept in many-electron atoms.

Studies on the π-π stacking features of imidazole units present in a series of 5-amino-1-alkylimidazole-4-carboxamides

NASA Astrophysics Data System (ADS)

Ray, Sibdas; Das, Aniruddha

2015-06-01

Reaction of 2-ethoxymethyleneamino-2-cyanoacetamide with primary alkyl amines in acetonitrile solvent affords 1-substituted-5-aminoimidazole-4-carboxamides. Single crystal X-ray diffraction studies of these imidazole compounds show that there are both anti-parallel and syn-parallel π-π stackings between two imidazole units in parallel-displaced (PD) conformations and the distance between two π-π stacked imidazole units depends mainly on the anti/ syn-parallel nature and to some extent on the alkyl group attached to N-1 of imidazole; molecules with anti-parallel PD-stacking arrangements of the imidazole units have got vertical π-π stacking distance short enough to impart stabilization whereas the imidazole unit having syn-parallel stacking arrangement have got much larger π-π stacking distances. DFT studies on a pair of anti-parallel imidazole units of such an AICA lead to curves for 'π-π stacking stabilization energy vs. π-π stacking distance' which have got similarity with the 'Morse potential energy diagram for a diatomic molecule' and this affords to find out a minimum π-π stacking distance corresponding to the maximum stacking stabilization energy between the pair of imidazole units. On the other hand, a DFT calculation based curve for 'π-π stacking stabilization energy vs. π-π stacking distance' of a pair of syn-parallel imidazole units is shown to have an exponential nature.

Competition of Perpendicular and Parallel Flows in a Straight Magnetic Field

NASA Astrophysics Data System (ADS)

Li, Jiacong; Diamond, Patrick; Hong, Rongjie; Tynan, George

2017-10-01

In tokamaks, intrinsic rotations in both toroidal and poloidal directions are important for the stability and confinement. Since they compete for energy from background turbulence, the coupling of them is the key to understanding the physics of turbulent state and transport bifurcations, e.g. L-H transition. V⊥ can affect the parallel Reynolds stress via cross phase and energetics, and thus regulates the parallel flow generation. In return, the turbulence driven V∥ plays a role in the mean vorticity flux, influencing the generation of V⊥. Also, competition of intrinsic azimuthal and axial flows is observed in CSDX-a linear plasma device with straight magnetic fields. CSDX is a well diagnosed venue to study the basic physics of turbulence-flow interactions in straight magnetic fields. Here, we study the turbulent energy branching between the turbulence driven parallel flow and perpendicular flow. Specifically, the ratio between parallel and perpendicular Reynolds power decreases when the mean perpendicular flow increases. As the mean parallel flow increases, this ratio first increases and then decreases before the parallel flow shear hits the parallel shear flow instability threshold. We seek to understand the flow states and compare with CSDX experiments. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, under Award Number DE-FG02-04ER54738.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cao, H.

We have measured the scintillation and ionization yield of recoiling nuclei in liquid argon as a function of applied electric field by exposing a dual-phase liquid argon time projection chamber (LAr-TPC) to a low energy pulsed narrow band neutron beam produced at the Notre Dame Institute for Structure and Nuclear Astrophysics. Liquid scintillation counters were arranged to detect and identify neutrons scattered in the TPC and to select the energy of the recoiling nuclei. We also report measurements of the scintillation yields for nuclear recoils with energies from 10.3 to 57.3 keV and for median applied electric fields from 0more » to 970 V/cm. For the ionization yields, we report measurements from 16.9 to 57.3 keV and for electric fields from 96.4 to 486 V/cm. Furthermore, we report the observation of an anticorrelation between scintillation and ionization from nuclear recoils, which is similar to the anticorrelation between scintillation and ionization from electron recoils. Assuming that the energy loss partitions into excitons and ion pairs from 83mKr internal conversion electrons is comparable to that from 207Bi conversion electrons, we obtained the numbers of excitons (N ex) and ion pairs (N i) and their ratio (N ex/N i) produced by nuclear recoils from 16.9 to 57.3 keV. Motivated by arguments suggesting direction sensitivity in LAr-TPC signals due to columnar recombination, a comparison of the light and charge yield of recoils parallel and perpendicular to the applied electric field is presented for the first time.« less

Efficient algorithms for the simulation of non-adiabatic electron transfer in complex molecular systems: application to DNA.

PubMed

Kubař, Tomáš; Elstner, Marcus

2013-04-28

In this work, a fragment-orbital density functional theory-based method is combined with two different non-adiabatic schemes for the propagation of the electronic degrees of freedom. This allows us to perform unbiased simulations of electron transfer processes in complex media, and the computational scheme is applied to the transfer of a hole in solvated DNA. It turns out that the mean-field approach, where the wave function of the hole is driven into a superposition of adiabatic states, leads to over-delocalization of the hole charge. This problem is avoided using a surface hopping scheme, resulting in a smaller rate of hole transfer. The method is highly efficient due to the on-the-fly computation of the coarse-grained DFT Hamiltonian for the nucleobases, which is coupled to the environment using a QM/MM approach. The computational efficiency and partial parallel character of the methodology make it possible to simulate electron transfer in systems of relevant biochemical size on a nanosecond time scale. Since standard non-polarizable force fields are applied in the molecular-mechanics part of the calculation, a simple scaling scheme was introduced into the electrostatic potential in order to simulate the effect of electronic polarization. It is shown that electronic polarization has an important effect on the features of charge transfer. The methodology is applied to two kinds of DNA sequences, illustrating the features of transfer along a flat energy landscape as well as over an energy barrier. The performance and relative merit of the mean-field scheme and the surface hopping for this application are discussed.

BIDIRECTIONAL FLUXES OF NEARLY RELATIVISTIC ELECTRONS DURING THE ONSET OF SOLAR ENERGETIC PARTICLE EVENTS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sun, L. P.; Li, C., E-mail: pmo_sunlp@msn.com

2013-03-10

We report intensity and anisotropy measurements of energetic electrons in the energy range of {approx}27-{approx}500 keV as observed with the Wind and Advanced Composition Explorer (ACE) spacecraft in 2000 June for several solar energetic particle (SEP) events. The solar sources of the SEP events are inferred from observations from the Solar and Heliospheric Observatory spacecraft. All of the events originate from the western limb active regions (ARs), which are well connected by interplanetary magnetic field (IMF) lines linking the Sun to near-Earth space. The observations on board Wind show bimodal pitch angle distributions (PADs), whereas ACE shows PADs with onemore » peak, as is usually observed for impulsive injection of electrons at the Sun. During the time of observations, Wind was located, upstream of the Earth's bow shock in the dawn-noon sector, at distances of {approx}40-{approx}80 R{sub E} from the Earth, and we infer that it was magnetically connected to the quasi-parallel bow shock. Meanwhile, ACE, orbiting the Sun-Earth libration point L1, was not connected to the bow shock. The electron intensity-time profiles and the energy spectra show that the backstreaming electrons observed at Wind are not of magnetospheric origin. The observations suggest rather that the bidirectional electron fluxes are due to reflection or scattering by an obstacle located at a distance of less than {approx}150 R{sub E} in the anti-sunward direction, which is compatible with the obstacle being the Earth's bow shock or magnetosheath.« less