Shen, Weimin.
1992-08-01
Studies of magnetic fluctuation induced particle transport on Reversed Field Pinch plasmas were done on the Madison Symmetric Torus. Plasma current density and current density fluctuations were measured using a multi-coil magnetic probes. The low frequency (f<50 kHz) current density fluctuations are consistent with the global resistive tearing instabilities predicted by 3-D MHD simulations. At frequencies above 50 kHz, the magnetic fluctuations were detected to be localized with a radial correlation length of about 1--2 cm. These modes are locally resonant modes since the measured dominant mode number spectra match the local safety factor q. The net charged particle flux induced by magnetic fluctuations was obtained by measuring the correlation term <{tilde j}{sub {parallel}} {tilde B}{sub r}>. The result of zero net charged particle loss was obtained, meaning the flux is ambipolar. The ambipolarity of low frequency global tearing modes is satisfied through the phase relations determined by tearing instabilities. The ambipolarity of high frequency localized modes could be partially explained by the simple model of Waltz based on the radial average of small scale turbulence.
Non-equilibrium steady states: fluctuations and large deviations of the density and of the current
NASA Astrophysics Data System (ADS)
Derrida, Bernard
2007-07-01
These lecture notes give a short review of methods such as the matrix ansatz, the additivity principle or the macroscopic fluctuation theory, developed recently in the theory of non-equilibrium phenomena. They show how these methods allow us to calculate the fluctuations and large deviations of the density and the current in non-equilibrium steady states of systems like exclusion processes. The properties of these fluctuations and large deviation functions in non-equilibrium steady states (for example, non-Gaussian fluctuations of density or non-convexity of the large deviation function which generalizes the notion of free energy) are compared with those of systems at equilibrium.
Current Fluctuations in One Dimensional Diffusive Systems with a Step Initial Density Profile
NASA Astrophysics Data System (ADS)
Derrida, Bernard; Gerschenfeld, Antoine
2009-12-01
We show how to apply the macroscopic fluctuation theory (MFT) of Bertini, De Sole, Gabrielli, Jona-Lasinio, and Landim to study the current fluctuations of diffusive systems with a step initial condition. We argue that one has to distinguish between two ways of averaging (the annealed and the quenched cases) depending on whether we let the initial condition fluctuate or not. Although the initial condition is not a steady state, the distribution of the current satisfies a symmetry very reminiscent of the fluctuation theorem. We show how the equations of the MFT can be solved in the case of non-interacting particles. The symmetry of these equations can be used to deduce the distribution of the current for several other models, from its knowledge (Derrida and Gerschenfeld in J. Stat. Phys. 136, 1-15, 2009) for the symmetric simple exclusion process. In the range where the integrated current Qt˜sqrt{t} , we show that the non-Gaussian decay exp [- Q {/t 3}/ t] of the distribution of Q t is generic.
Vukovic, M.; Harper, M.; Breun, R.; Wukitch, S.
1995-12-31
Current drive experiments on the Phaedrus-T tokamak performed with a low field side two-strap fast wave antenna at frequencies below {omega}{sub cH} show loop volt drops of up to 30% with strap phasing (0, {pi}/2). RF induced density fluctuations in the plasma core have also been observed with a microwave reflectometer. It is believed that they are caused by kinetic Alfven waves generated by mode conversion of fast waves at the Alfven resonance. Correlation of the observed density fluctuations with the magnitude of the {Delta}V{sub loop} suggest that the {Delta}V{sub loop} is attributable to current drive/heating due to mode converted kinetic Alfven waves. The toroidal cold plasma wave code LION is used to model the Alfven resonance mode conversion surfaces in the experiments while the cylindrical hot plasma kinetic wave code ISMENE is used to model the behavior of kinetic Alfven waves at the Alfven resonance location. Initial results obtained from limited density, magnetic field, antenna phase, and impurity scans show good agreement between the RF induced density fluctuations and the predicted behavior of the kinetic Alfven waves. Detailed comparisons between the density fluctuations and the code predictions are presented.
NASA Astrophysics Data System (ADS)
Vinogradov, A. P.; Burokur, N.; Zouhdi, S.
2009-06-01
It is shown that the space fluctuations of concentration of conducting inclusions might be responsible for the well-known disagreement between theory and experiment at determining microwave losses in metal-dielectric mixture: the theories (percolation theory, effective medium theory, etc.) predict much lower losses than those measured in experiment. It is demonstrated that if the effective skin depth in the regions occupied by the fluctuation is comparable to the mean diameter of these regions we can expect additional losses.
Density Fluctuations in Liquid Water
NASA Astrophysics Data System (ADS)
English, Niall J.; Tse, John S.
2011-01-01
The density distributions and fluctuations in grids of varying size in liquid water at ambient pressure, both above the freezing point and in the supercooled state, are analyzed from the trajectories obtained from large-scale molecular dynamics simulations. It is found that the occurrence of low- and high-density regions (LDL and HDL) is transient and their respective residence times are dependent on the size of the simulated system. The spatial extent of density-density correlation is found to be within 7 Å or less. The temporal existence of LDL and HDL arises as a result of natural density fluctuations of an equilibrium system. The density of bulk water at ambient conditions is homogenous.
Current Fluctuations in Stochastic Lattice Gases
NASA Astrophysics Data System (ADS)
Bertini, L.; de Sole, A.; Gabrielli, D.; Jona-Lasinio, G.; Landim, C.
2005-01-01
We study current fluctuations in lattice gases in the macroscopic limit extending the dynamic approach for density fluctuations developed in previous articles. More precisely, we establish a large deviation theory for the space-time fluctuations of the empirical current which include the previous results. We then estimate the probability of a fluctuation of the average current over a large time interval. It turns out that recent results by Bodineau and Derrida [Phys. Rev. Lett.922004180601] in certain cases underestimate this probability due to the occurrence of dynamical phase transitions.
Origin of cosmological density fluctuations
Carr, B.J.
1984-11-01
The density fluctuations required to explain the large-scale cosmological structure may have arisen spontaneously as a result of a phase transition in the early Universe. There are several ways in which such fluctuations may have ben produced, and they could have a variety of spectra, so one should not necessarily expect all features of the large-scale structure to derive from a simple power law spectrum. Some features may even result from astrophysical amplification mechanisms rather than gravitational instability. 128 references.
Fluctuation of heat current in Josephson junctions
Virtanen, P.; Giazotto, F.
2015-02-15
We discuss the statistics of heat current between two superconductors at different temperatures connected by a generic weak link. As the electronic heat in superconductors is carried by Bogoliubov quasiparticles, the heat transport fluctuations follow the Levitov–Lesovik relation. We identify the energy-dependent quasiparticle transmission probabilities and discuss the resulting probability density and fluctuation relations of the heat current. We consider multichannel junctions, and find that heat transport in diffusive junctions is unique in that its statistics is independent of the phase difference between the superconductors.
Quantum density fluctuations in classical liquids.
Ford, L H; Svaiter, N F
2009-01-23
We discuss the density fluctuations of a fluid due to zero point motion, assuming a linear dispersion relation. We argue that density fluctuations in a fluid can be a useful analog model for better understanding fluctuations in relativistic quantum field theory. We calculate the differential cross section for light scattering by the zero point density fluctuations, and find a result proportional to the fifth power of the light frequency. We give some estimates of the relative magnitude of this effect compared to the scattering by thermal density fluctuations, and find that it can be of the order 13% for liquid neon at optical frequencies. This relative magnitude is proportional to frequency and inversely proportional to temperature. Although the scattering by zero point density fluctuation is small, it may be observable.
Universal bounds on current fluctuations
NASA Astrophysics Data System (ADS)
Pietzonka, Patrick; Barato, Andre C.; Seifert, Udo
2016-05-01
For current fluctuations in nonequilibrium steady states of Markovian processes, we derive four different universal bounds valid beyond the Gaussian regime. Different variants of these bounds apply to either the entropy change or any individual current, e.g., the rate of substrate consumption in a chemical reaction or the electron current in an electronic device. The bounds vary with respect to their degree of universality and tightness. A universal parabolic bound on the generating function of an arbitrary current depends solely on the average entropy production. A second, stronger bound requires knowledge both of the thermodynamic forces that drive the system and of the topology of the network of states. These two bounds are conjectures based on extensive numerics. An exponential bound that depends only on the average entropy production and the average number of transitions per time is rigorously proved. This bound has no obvious relation to the parabolic bound but it is typically tighter further away from equilibrium. An asymptotic bound that depends on the specific transition rates and becomes tight for large fluctuations is also derived. This bound allows for the prediction of the asymptotic growth of the generating function. Even though our results are restricted to networks with a finite number of states, we show that the parabolic bound is also valid for three paradigmatic examples of driven diffusive systems for which the generating function can be calculated using the additivity principle. Our bounds provide a general class of constraints for nonequilibrium systems.
Multicellular density fluctuations in epithelial monolayers
NASA Astrophysics Data System (ADS)
Zehnder, Steven M.; Wiatt, Marina K.; Uruena, Juan M.; Dunn, Alison C.; Sawyer, W. Gregory; Angelini, Thomas E.
2015-09-01
Changes in cell size often accompany multicellular motion in tissue, and cell number density is known to strongly influence collective migration in monolayers. Density fluctuations in other forms of active matter have been explored extensively, but not the potential role of density fluctuations in collective cell migration. Here we investigate collective motion in cell monolayers, focusing on the divergent component of the migration velocity field to probe density fluctuations. We find spatial patterns of diverging and converging cell groups throughout the monolayers, which oscillate in time with a period of approximately 3-4 h. Simultaneous fluorescence measurements of a cytosol dye within the cells show that fluid passes between groups of cells, facilitating these oscillations in cell density. Our findings reveal that cell-cell interactions in monolayers may be mediated by intercellular fluid flow.
Mapping current fluctuations of stochastic pumps to nonequilibrium steady states
NASA Astrophysics Data System (ADS)
Rotskoff, Grant M.
2017-03-01
We show that current fluctuations in a stochastic pump can be robustly mapped to fluctuations in a corresponding time-independent nonequilibrium steady state. We thus refine a recently proposed mapping so that it ensures equivalence of not only the averages, but also optimal representation of fluctuations in currents and density. Our mapping leads to a natural decomposition of the entropy production in stochastic pumps similar to the "housekeeping" heat. As a consequence of the decomposition of entropy production, the current fluctuations in weakly perturbed stochastic pumps are shown to satisfy a universal bound determined by the steady state entropy production.
Origin of density fluctuations in extended inflation
NASA Technical Reports Server (NTRS)
Kolb, Edward W.; Salopek, David S.; Turner, Michael S.
1990-01-01
The density fluctuations (both curvature and isocurvature) that arise due to quantum fluctuations in a simple model of extended inflation based upon the Jordan-Brans-Dicke theory are calculated. Curvature fluctuations arise due to quantum fluctuations in the Brans-Dicke field, in general have a nonscale-invariant spectrum, and can have an amplitude that is cosmologically acceptable and interesting without having to tune any coupling constant to a very small value. The density perturbations that arise due to the inflation field are subdominant. If there are other massless fields in the theory, e.g., an axion or an ilion, then isocurvature fluctuations arise in these fields too. Production of gravitational waves and the massless particles associated with excitations of the Brans-Dicke field are also discussed. Several attempts at more realistic models of extended inflation are also analyzed. The importance of the Einstein conformal frame in calculating curvature fluctuations is emphasized. When viewed in this frame, extended inflation closely resembles slow-rollover inflation with an exponential potential and the usual formula for the amplitude of curvature perturbations applies.
Density fluctuations from strings and galaxy formation
NASA Technical Reports Server (NTRS)
Vilenkin, A.; Shafi, Q.
1983-01-01
The spectra of density fluctuations caused by strings in a universe dominated either by baryons, neutrinos, or axions are presented. Realistic scenarios for galaxy formation seem possible in all three cases. Examples of grand unified theories which lead to strings with the desired mass scales are given.
Turbulence velocimetry of density fluctuation imaging data
NASA Astrophysics Data System (ADS)
McKee, G. R.; Fonck, R. J.; Gupta, D. K.; Schlossberg, D. J.; Shafer, M. W.; Holland, C.; Tynan, G.
2004-10-01
Analysis techniques to measure the time-resolved flow field of turbulence are developed and applied to images of density fluctuations obtained with the beam emission spectroscopy diagnostic system on the DIII-D tokamak. Velocimetry applications include measurement of turbulent particle flux, zonal flows, and the Reynolds stress. The flow field of turbulent eddies exhibits quasisteady poloidal flows as well as high-frequency radial and poloidal motion associated with electrostatic potential fluctuations and strongly nonlinear multifield interactions. The orthogonal dynamic programming technique, developed for fluid-based particle and amorphous shape (smoke) flow analysis, is investigated to measure such turbulence flows. Sensitivity and accuracy are assessed and sample results discussed.
Density fluctuations in vibrated granular materials
Nowak, E.R.; Knight, J.B.; Ben-Naim, E.; Jaeger, H.M.; Nagel, S.R.
1998-02-01
We report systematic measurements of the density of a vibrated granular material as a function of time. Monodisperse spherical beads were confined to a cylindrical container and shaken vertically. Under vibrations, the density of the pile slowly reaches a final steady-state value about which the density fluctuates. We have investigated the frequency dependence and amplitude of these fluctuations as a function of vibration intensity {Gamma}. The spectrum of density fluctuations around the steady state value provides a probe of the internal relaxation dynamics of the system and a link to recent thermodynamic theories for the settling of granular material. In particular, we propose a method to evaluate the compactivity of a powder, first put forth by Edwards and co-workers, that is the analog to temperature for a quasistatic powder. We also propose a stochastic model based on free volume considerations that captures the essential mechanism underlying the slow relaxation. We compare our experimental results with simulations of a one-dimensional model for random adsorption and desorption. {copyright} {ital 1998} {ital The American Physical Society}
Non Equilibrium Current Fluctuations in Stochastic Lattice Gases
NASA Astrophysics Data System (ADS)
Bertini, L.; Sole, A. De; Gabrielli, D.; Jona-Lasinio, G.; Landim, C.
2006-04-01
We study current fluctuations in lattice gases in the macroscopic limit extending the dynamic approach for density fluctuations developed in previous articles. More precisely, we establish a large deviation principle for a space-time fluctuation j of the empirical current with a rate functional I( j). We then estimate the probability of a fluctuation of the average current over a large time interval; this probability can be obtained by solving a variational problem for the functional I. We discuss several possible scenarios, interpreted as dynamical phase transitions, for this variational problem. They actually occur in specific models. We finally discuss the time reversal properties of I and derive a fluctuation relationship akin to the Gallavotti-Cohen theorem for the entropy production.
Coherent Density Fluctuations in the HSX Stellarator
NASA Astrophysics Data System (ADS)
Deng, C. B.; Brower, D. L.; Anderson, D. T.; Anderson, F. S. B.; Likin, K. M.; Smoniewski, J.; Talmadge, J. N.
2015-11-01
A multi-channel interferometer system is used to measure equilibrium density profile and its fluctuations in the HSX stellarator. Low-frequency, coherent density fluctuations are observed in certain quasi-helically symmetric (QHS) plasma conditions and has characteristic frequency of 15kHz. The mode is observed for small displacement of the 1st harmonic O-mode ECRH location inward from the magnetic axis. This mode is also observed on magnetic fluctuation signal, using external coils, which shows n =1. When HSX is operated without quasi-helical symmetry (mirror configuration), a coherent electrostatic mode at 28 kHz is observed. While the coherent mode in QHS plasmas shows ballooning effect, the coherent mode in Mirror plasma exhibits an anti-ballooning characteristic. Mode radial structure can be obtained from inversion of interferometer measurement when the m number is known. Under certain Mirror conditions, the coherent modes display strong bi-coherence on Langmuir probe signals. Detailed characterization of the observed coherent modes will be reported and their identification will be explored. Supported by USDOE grants DE-FG03-01ER54615 and DE-FG02-93ER54222.
Fluctuation dynamics in reconnecting current sheets
NASA Astrophysics Data System (ADS)
von Stechow, Adrian; Grulke, Olaf; Ji, Hantao; Yamada, Masaaki; Klinger, Thomas
2015-11-01
During magnetic reconnection, a highly localized current sheet forms at the boundary between opposed magnetic fields. Its steep perpendicular gradients and fast parallel drifts can give rise to a range of instabilities which can contribute to the overall reconnection dynamics. In two complementary laboratory reconnection experiments, MRX (PPPL, Princeton) and VINETA.II (IPP, Greifswald, Germany), magnetic fluctuations are observed within the current sheet. Despite the large differences in geometries (toroidal vs. linear), plasma parameters (high vs. low beta) and magnetic configuration (low vs. high magnetic guide field), similar broadband fluctuation characteristics are observed in both experiments. These are identified as Whistler-like fluctuations in the lower hybrid frequency range that propagate along the current sheet in the electron drift direction. They are intrinsic to the localized current sheet and largely independent of the slower reconnection dynamics. This contribution characterizes these magnetic fluctuations within the wide parameter range accessible by both experiments. Specifically, the fluctuation spectra and wave dispersion are characterized with respect to the magnetic topology and plasma parameters of the reconnecting current sheet.
Vacuum density fluctuations in extended chaotic inflation
NASA Astrophysics Data System (ADS)
Deruelle, Nathalie; Gundlach, Carsten; Langlois, David
1992-12-01
An inflaton (scalar field) with the potential cσ2n is coupled to gravity within the Jordan-Brans-Dicke theory. The corresponding inflationary model (that is, a flat Friedmann-Robertson-Walker solution with a slowly varying inflaton) is constructed for all values of the coupling β of the inflaton to the dilaton (Brans-Dicke scalar field). The linearized perturbations of the metric, the dilaton, and the inflaton are then quantized within a gauge-invariant formalism. The power spectrum of the vacuum density fluctuations is calculated as a function of c,n, and β. It is the juxtaposition of two powers of the wave number corresponding, respectively, to the contribution of the inflaton and the dilaton. We find the value of β for which the dilaton contribution dominates on observable cosmological scales.
Energy density fluctuations in early universe
Guardo, G. L.; Ruggieri, M.; Greco, V.
2014-05-09
The primordial nucleosinthesys of the element can be influenced by the transitions of phase that take place after the Big Bang, such as the QCD transition. In order to study the effect of this phase transition, in this work we compute the time evolution of thermodynamical quantities of the early universe, focusing on temperature and energy density fluctuations, by solving the relevant equations of motion using as input the lattice QCD equation of state to describe the strongly interacting matter in the early universe plasma. We also study the effect of a primordial strong magnetic field by means of a phenomenological equation of state. Our results show that small inhomogeneities of strongly interacting matter in the early Universe are moderately damped during the crossover.
Discriminating the trapped electron modes contribution in density fluctuation spectra
NASA Astrophysics Data System (ADS)
Arnichand, H.; Sabot, R.; Hacquin, S.; Krämer-Flecken, A.; Bourdelle, C.; Citrin, J.; Garbet, X.; Giacalone, J. C.; Guirlet, R.; Hillesheim, J. C.; Meneses, L.
2015-09-01
Quasi-coherent (QC) modes have been reported for more than 10 years in reflectometry fluctuations spectra in the core region of fusion plasmas. They have characteristics in-between coherent and broadband fluctuations as they oscillate at a marked frequency but have a wide spectrum. This work presents further evidences of the link recently established between QC modes and the trapped electron modes (TEM) instabilities (Arnichand et al 2014 Nucl. Fusion 54 123017). In electron cyclotron resonance heated discharges of Tore Supra, an enhancement of QC modes amplitude is observed in a region where TEM cause impurity transport and turbulence. In JET Ohmic plasmas, QC modes disappear during density ramp-up and current ramp-down. This is reminiscent of Tore Supra and TEXTOR observations during transitions from the linear Ohmic confinement (LOC) to the saturated Ohmic confinement (SOC) regimes. Evidencing TEM activity then becomes experimentally possible via analysis of fluctuation spectra.
Effect of Magnetic Fluctuations on Spin Current
NASA Astrophysics Data System (ADS)
Niimi, Yasuhiro; Wei, Dahai; Otani, YoshiChika
2017-01-01
Spin Hall effect (SHE) and its inverse enable the interconversion between charge current and spin current. It is widely recognized that the SHE occurs in a nonmagnetic material with strong spin-orbit interaction. However, it can be generated even in magnetic materials such as ferromagnets, antiferromagnets, and those mixtures, i.e., spin glass. Here we review the SHEs in two typical magnetic systems. One is the SHE in a weak ferromagnetic metal in the vicinity of the Curie temperature where a nonlinear magnetic susceptibility can be detected. The other is the SHE in a spin glass metal where fluctuations at the magnetic impurity sites can be measured electrically in a very sensitive way. We argue that the spin current could be utilized as a sensor to detect a small magnetic fluctuation.
Penetration and scattering of lower hybrid waves by density fluctuations
Horton, W.; Goniche, M.; Peysson, Y.; Decker, J.; Ekedahl, A.; Litaudon, X.
2014-02-12
Lower Hybrid [LH] ray propagation in toroidal plasma is controlled by a combination of the azimuthal spectrum launched from the antenna, the poloidal variation of the magnetic field, and the scattering of the waves by the density fluctuations. The width of the poloidal and radial RF wave spectrum increases rapidly as the rays penetrate into higher density and scatter from the turbulence. The electron temperature gradient [ETG] spectrum is particularly effective in scattering the LH waves due to its comparable wavelengths and parallel phase velocities. ETG turbulence is also driven by the radial gradient of the electron current density giving rise to an anomalous viscosity spreading the LH-driven plasma currents. The scattered LH spectrum is derived from a Fokker-Planck equation for the distribution of the ray trajectories with a diffusivity proportional to the fluctuations. The LH ray diffusivity is large giving transport in the poloidal and radial wavenumber spectrum in one - or a few passes - of the rays through the core plasma.
Current fluctuations in nonequilibrium diffusive systems: an additivity principle.
Bodineau, T; Derrida, B
2004-05-07
We formulate a simple additivity principle allowing one to calculate the whole distribution of current fluctuations through a large one dimensional system in contact with two reservoirs at unequal densities from the knowledge of its first two cumulants. This distribution (which in general is non-Gaussian) satisfies the Gallavotti-Cohen symmetry and generalizes the one predicted recently for the symmetric simple exclusion process. The additivity principle can be used to study more complex diffusive networks including loops.
Current Fluctuations in Nonequilibrium Diffusive Systems: An Additivity Principle
NASA Astrophysics Data System (ADS)
Bodineau, T.; Derrida, B.
2004-05-01
We formulate a simple additivity principle allowing one to calculate the whole distribution of current fluctuations through a large one dimensional system in contact with two reservoirs at unequal densities from the knowledge of its first two cumulants. This distribution (which in general is non-Gaussian) satisfies the Gallavotti-Cohen symmetry and generalizes the one predicted recently for the symmetric simple exclusion process. The additivity principle can be used to study more complex diffusive networks including loops.
The Transport of Density Fluctuations Throughout the Heliosphere
NASA Technical Reports Server (NTRS)
Zank, G. P.; Jetha, N.; Hu, Q.; Hunana, P.
2012-01-01
The solar wind is recognized as a turbulent magnetofluid, for which the properties of the turbulent velocity and magnetic field fluctuations are often described by the equations of incompressible magnetohydrodynamics (MHD). However, low-frequency density turbulence is also ubiquitous. On the basis of a nearly incompressible formulation of MHD in the expanding inhomogeneous solar wind, we derive the transport equation for the variance of the density fluctuations (Rho(exp 2)). The transport equation shows that density fluctuations behave as a passive scalar in the supersonic solar wind. In the absence of sources of density turbulence, such as within 1AU, the variance (Rho(exp 2)) approximates r(exp -4). In the outer heliosphere beyond 1 AU, the shear between fast and slow streams, the propagation of shocks, and the creation of interstellar pickup ions all act as sources of density turbulence. The model density fluctuation variance evolves with heliocentric distance within approximately 300 AU as (Rho(exp 2)) approximates r(exp -3.3) after which it flattens and then slowly increases. This is precisely the radial profile for the density fluctuation variance observed by Voyager 2. Using a different analysis technique, we confirm the radial profile for Rho(exp 2) of Bellamy, Cairns, & Smith using Voyager 2 data. We conclude that a passive scalar description for density fluctuations in the supersonic solar wind can explain the density fluctuation variance observed in both the inner and the outer heliosphere.
Density-noise power fluctuations in vibrated granular media
NASA Astrophysics Data System (ADS)
Nowak, E. R.; Grushin, A.; Barnum, A. C.; Weissman, M. B.
2001-02-01
The noise power spectra of the fluctuations in density of a vibrated column of granular material are found to be time dependent. Spectral analysis of these noise power fluctuations shows nontrivial frequency dependences. The noise powers at different frequencies are also found to fluctuate in a partially correlated way. In most instances, the slow variations of the noise are strongly correlated over a broad range of frequencies. These results indicate that highly cooperative interactions exist between fluctuators. In contrast, effects of such strongly coupled fluctuators are absent in the one-dimensional parking-lot-model, one of the simplest systems used to provide a model for recent granular compaction experiments.
NASA Astrophysics Data System (ADS)
Ding, Weixing; Lin, Liang; Duff, J. R.; Brower, D. L.; Sarff, J. S.
2014-10-01
In the MST reversed field pinch (RFP), the evolution of core tearing mode nonlinear evolution is partially determined by the electron current density profile along with nonlinear interactions among multiple tearing modes. Density fluctuations driven by intrinsic magnetic perturbations are usually large, approximately 1%, in RFP plasmas. These density fluctuations can modify the current density profile via the kinetic dynamo effect, defined as the correlated product of parallel electron pressure and radial magnetic field fluctuations, which alters the temporal dynamics of tearing modes in MST. A component of the kinetic dynamo originating from the correlated product of density and radial magnetic fluctuations has been measured using a high-speed, low phase noise polarimetry-interferometry diagnostic. Between sawtooth crashes it is found that the measured kinetic dynamo has finite amplitude that generates an anti-dynamo in the plasma core, which would tend to flatten the current density profile. These measurements suggest that density fluctuations passively driven by magnetic fluctuations can actively alter tearing modes via fluctuation-induced current transport. Work supported by US DOE and NSF.
Electron cyclotron emission as a density fluctuation diagnostic
Lynn, A.G.; Phillips, P.E.; Hubbard, A.
2004-10-01
A new technique for measuring density fluctuations using a high-resolution heterodyne electron cyclotron emission (ECE) radiometer has been developed. Although ECE radiometry is typically used for electron temperature measurements, the unique viewing geometry of this system's quasioptical antenna has been found to make the detected emission extremely sensitive to refractive effects under certain conditions. This sensitivity gives the diagnostic the ability to measure very low levels of density fluctuations in the core of Alcator C-Mod tokamak. The refractive effects have been modeled using ray-tracing methods, allowing estimates of the density fluctuation magnitude and spatial localization.
Interplay between density and superconducting quantum critical fluctuations.
Caprara, S; Bergeal, N; Lesueur, J; Grilli, M
2015-10-28
We consider the case of a density-driven metal-superconductor transition in the proximity of an electronic phase separation. In particular, we investigate the interplay between superconducting fluctuations and density fluctuations, which become quantum critical when the electronic phase separation vanishes at zero temperature into a quantum critical point. In this situation, the critical dynamical density fluctuations strongly affect the dynamics of the Cooper-pair fluctuations, which acquire a more singular character with a z = 3 dynamical critical index. This gives rise to a scenario that possibly rules the disappearance of superconductivity when the electron density is reduced by electrostatic gating at the LaAlO3/SrTiO3 interface.
Density fluctuations at high density in the ergodic divertor configuration of Tore Supra
NASA Astrophysics Data System (ADS)
Devynck, P.; Gunn, J.; Ghendrih, Ph.; Garbet, X.; Antar, G.; Beyer, P.; Boucher, C.; Honore, C.; Gervais, F.; Hennequin, P.; Quémeneur, A.; Truc, A.
2001-03-01
The effect of the ergodic divertor on the plasma edge in Tore Supra is to enhance the perpendicular transport through ergodization of the magnetic field lines [Ph. Ghendrih et al., Contrib. Plasma Phys. 32 (3&4) (1992) 179]. Nevertheless, the hot spots observed on the divertor plates during ergodic divertor operation indicate that the cross-field transport driven by the fluctuations is still playing an important role, although measurements by CO 2 laser scattering and reflectometry show a decrease of the turbulence level [J. Payan, X. Garbet, J.H. Chatenet et al., Nucl. Fusion 35 (1995) 1357; P. Beyer, X. Garbet, P. Ghendrih, Phys. Plasmas 5 (12) (1998) 4271]. In order to gain more understanding, fluctuation level and poloidal velocity have been measured with a reciprocating Langmuir probe biased to collect the ion saturation current ( jsat) and with a CO 2 laser scattering diagnostic. Though the relative fluctuation level behaves as previously observed at low density, a new interesting result is that this picture is gradually modified when the density is increased. Both diagnostics observe an increase of δn/ n with density in the ergodic region, which is not the usual behavior observed in limiter configuration. This increase is detected on both sides of the Er inversion radius and is therefore also affecting the plasma bulk. Finally, the confinement time is found to follow an L-mode law at all densities indicating that the ergodic divertor does not change the global confinement properties of the plasma.
The Phase Coherence of Interstellar Density Fluctuations
NASA Astrophysics Data System (ADS)
Burkhart, Blakesley; Lazarian, A.
2016-08-01
Studies of MHD turbulence often investigate the Fourier power spectrum to provide information on the nature of the turbulence cascade. However, the Fourier power spectrum only contains the Fourier amplitudes and rejects all information regarding the Fourier phases. Here, we investigate the utility of two statistical diagnostics for recovering information on Fourier phases in ISM column density maps: the averaged amplitudes of the bispectrum and the phase coherence index (PCI), a new phase technique for the ISM. We create three-dimensional density and two-dimensional column density maps using a set of simulations of isothermal ideal MHD turbulence with a wide range of sonic and Alfvénic Mach numbers. We find that the bispectrum averaged along different angles with respect to either the k 1 or k 2 axis is primarily sensitive to the sonic Mach number while averaging the bispectral amplitudes over different annuli is sensitive to both the sonic and Alfvénic Mach numbers. The PCI of density suggests that the most correlated phases occur in supersonic sub-Alfvénic turbulence and near the shock scale. This suggests that nonlinear interactions with correlated phases are strongest in shock-dominated regions, in agreement with findings from the solar wind. Our results suggest that the phase information contained in the bispectrum and PCI can be used to find the turbulence parameters in column density maps.
NASA Technical Reports Server (NTRS)
Huddleston, D. E.; Woo, R.; Neugebauer, M.
1995-01-01
Density fluctuations with periods 10 minutes to 1 hour have been investigated in ISEE 3 plasma measurements of solar wind flows at l AU. Coronal hole, interstream, plasma sheet, coronal mass ejection, and interaction region flow types are considered. The ISEE 3 results support the interpretation of the large-scale variations in density fluctuations observed by Doppler scintillation measurement techniques inside 0.2 AU. The highest absolute and relative density fluctuations occur ahead of and within the plasma from coronal mass ejections, with the maximum values occurring between the associated interplanetary shocks and the driver gas. For the quasi-stationary solar wind, density and relative density fluctuations are highest around the heliospheric current sheet and lowest in the high-speed coronal flow. Superposed epoch analysis shows that the region of enhanced density fluctuations and its abrupt boundaries observed in the vicinity of the heliospheric current sheet near the Sun persists to l AU, providing further support for the filamentary nature of the extensions of coronal streamers. The results of this study confirm the advantages of using density fluctuations rather than density as a tracer of solar wind flows with differing origins at the Sun and as a detector of propagating interplanetary disturbances.
The power associated with density fluctuations and velocity fluctuations in the solar wind
NASA Technical Reports Server (NTRS)
Intriligator, D. S.
1974-01-01
Direct observations from Pioneer 6 of solar-wind-proton fluctuations have been used to obtain the power spectra associated with solar-wind-proton number density and velocity fluctuations in the frequency range of 0.001 to 0.01 Hz, extending previous analyses by an order of magnitude at the higher frequencies. The slopes of the power spectra associated with the density fluctuations and the velocity fluctuations are similar and are in agreement with the shape of the power spectra found at the lower frequencies. The power spectra indicate that the power-law density spectrum observed at lower frequencies extends to at least 0.01 Hz. This smooth variation in the spectrum at these frequencies is consistent with previous extrapolations of both spacecraft and interplanetary scintillation observations.
Current fluctuations in a two dimensional model of heat conduction
NASA Astrophysics Data System (ADS)
Pérez-Espigares, Carlos; Garrido, Pedro L.; Hurtado, Pablo I.
2011-03-01
In this work we study numerically and analytically current fluctuations in the two-dimensional Kipnis-Marchioro-Presutti (KMP) model of heat conduction. For that purpose, we use a recently introduced algorithm which allows the direct evaluation of large deviations functions. We compare our results with predictions based on the Hydrodynamic Fluctuation Theory (HFT) of Bertini and coworkers, finding very good agreement in a wide interval of current fluctuations. We also verify the existence of a well-defined temperature profile associated to a given current fluctuation which depends exclusively on the magnitude of the current vector, not on its orientation. This confirms the recently introduced Isometric Fluctuation Relation (IFR), which results from the time-reversibility of the dynamics, and includes as a particular instance the Gallavotti-Cohen fluctuation theorem in this context but adds a completely new perspective on the high level of symmetry imposed by timereversibility on the statistics of nonequilibrium fluctuations.
Density Fluctuations of Hard-Sphere Fluids in Narrow Confinement
NASA Astrophysics Data System (ADS)
Nygârd, Kim; Sarman, Sten; Hyltegren, Kristin; Chodankar, Shirish; Perret, Edith; Buitenhuis, Johan; van der Veen, J. Friso; Kjellander, Roland
2016-01-01
Spatial confinement induces microscopic ordering of fluids, which in turn alters many of their dynamic and thermodynamic properties. However, the isothermal compressibility has hitherto been largely overlooked in the literature, despite its obvious connection to the underlying microscopic structure and density fluctuations in confined geometries. Here, we address this issue by probing density profiles and structure factors of hard-sphere fluids in various narrow slits, using x-ray scattering from colloid-filled nanofluidic containers and integral-equation-based statistical mechanics at the level of pair distributions for inhomogeneous fluids. Most importantly, we demonstrate that density fluctuations and isothermal compressibilities in confined fluids can be obtained experimentally from the long-wavelength limit of the structure factor, providing a formally exact and experimentally accessible connection between microscopic structure and macroscopic, thermodynamic properties. Our approach will thus, for example, allow direct experimental verification of theoretically predicted enhanced density fluctuations in liquids near solvophobic interfaces.
MEASUREMENTS OF RAPID DENSITY FLUCTUATIONS IN THE SOLAR WIND
Malaspina, D. M.; Ergun, R. E.; Kellogg, P. J.; Bale, S. D.
2010-03-01
The power spectrum of density fluctuations in the solar wind is inferred by tracking small timescale changes in the electron plasma frequency during periods of strong Langmuir wave activity. STEREO electric field waveform data are used to produce time profiles of plasma density from which the density power spectrum is derived. The power spectra obtained by this method extend the observed frequency range by an order of magnitude while remaining consistent with previous results near a few Hertz. Density power spectral indices are found to be organized by the angle between the local magnetic field and the solar wind direction, indicating significant anisotropy in solar wind high-frequency density turbulence.
Density fluctuation measurements by using the Fraunhofer diffraction method in GAMMA10
NASA Astrophysics Data System (ADS)
Morikawa, Y.; Yoshikawa, M.; Kohagura, J.; Shima, Y.; Hasegawa, Y.; Sakamoto, M.; Imai, T.; Ichimura, M.
2013-12-01
We applied Fraunhofer diffraction (FD) method to GAMMA10 plasma. The FD method can measure the density fluctuation in detail and the wave number of the fluctuation. We successfully obtained the density fluctuation spectra in GAMMA 10. Analyzing the FD method signals of radial fluctuation intensity profile, we can successfully obtain the wave number and the phase velocity of the low frequency density fluctuation.
Measurement of high-frequency, small scale density fluctuations in improved confinement RFP plasmas
NASA Astrophysics Data System (ADS)
Duff, J. R.; Chapman, B. E.; Sarff, J. S.; Carmody, D.; Terry, P. W.; den Hartog, D. J.; Morton, L. A.; Lin, L.; Ding, W. X.; Brower, D. L.; MST Team
2014-10-01
In standard MST RFP plasmas, core transport is governed by magnetic fluctuations associated with global tearing modes. Using pulsed parallel current drive, tearing is significantly reduced and smaller-scale fluctuations are likely important to electron particle and heat transport for these improved confinement plasmas. On MST, an 11-chord FIR laser-based interferometry diagnostic, with ~ 8 cm chord spacing, is used to measure electron density fluctuations with wavenumbers k < 1-2 cm-1. An upgrade underway will allow resolution up to k ~ 15 cm-1. A fast magnetic coil array is employed for magnetic fluctuations. High-frequency (>50 kHz) small-scale (n > 15) density and magnetic fluctuations have been observed in the edge plasma, where density and temperature gradients are largest. These fluctuations are distinct from tearing and have amplitudes that correlate with the density gradient and electron beta. The MST is well suited to explore beta scaling given the large dynamic range (9-26%) found in the device. Correlation of the measured density fluctuations with plasma parameters in high beta plasmas will serve to identify the drive and contribute to validation of gyrokinetic codes. Work supported by DOE and NSF.
Collective motion and density fluctuations in swimming bacteria
NASA Astrophysics Data System (ADS)
Zhang, Hepeng
2011-03-01
The emergence of collective motion such as in fish schools, mammal herds, and insect swarms is a ubiquitous self-organization phenomenon. Such collective behavior plays an important role in a range of problems, such as spreading of deceases in animal or fish groups. Current models have provided a qualitative understanding of collective motion, but progress in quantitative modeling in hindered by the lack of experimental data. Here we examine a model microscopic system, where we are able to measure simultaneously the positions, velocities, and orientations of up to a thousand bacteria in a colony. The motile bacteria form closely-packed dynamic clusters within which they move cooperatively. The number of bacteria in a cluster exhibits a power-law distribution truncated by an exponential tail, and the probability of finding large clusters grows markedly as bacterial density increases. Mobile clusters cause anomalous fluctuations in bacterial density as found in mathematical theories and numerical models. Our results demonstrate that bacteria are an excellent system to study general phenomena of collective motion.
Suppression of Density Fluctuations in a Quantum Degenerate Fermi Gas
Sanner, Christian; Su, Edward J.; Keshet, Aviv; Gommers, Ralf; Shin, Yong-il; Huang Wujie; Ketterle, Wolfgang
2010-07-23
We study density profiles of an ideal Fermi gas and observe Pauli suppression of density fluctuations (atom shot noise) for cold clouds deep in the quantum degenerate regime. Strong suppression is observed for probe volumes containing more than 10 000 atoms. Measuring the level of suppression provides sensitive thermometry at low temperatures. After this method of sensitive noise measurements has been validated with an ideal Fermi gas, it can now be applied to characterize phase transitions in strongly correlated many-body systems.
Kernel current source density method.
Potworowski, Jan; Jakuczun, Wit; Lȩski, Szymon; Wójcik, Daniel
2012-02-01
Local field potentials (LFP), the low-frequency part of extracellular electrical recordings, are a measure of the neural activity reflecting dendritic processing of synaptic inputs to neuronal populations. To localize synaptic dynamics, it is convenient, whenever possible, to estimate the density of transmembrane current sources (CSD) generating the LFP. In this work, we propose a new framework, the kernel current source density method (kCSD), for nonparametric estimation of CSD from LFP recorded from arbitrarily distributed electrodes using kernel methods. We test specific implementations of this framework on model data measured with one-, two-, and three-dimensional multielectrode setups. We compare these methods with the traditional approach through numerical approximation of the Laplacian and with the recently developed inverse current source density methods (iCSD). We show that iCSD is a special case of kCSD. The proposed method opens up new experimental possibilities for CSD analysis from existing or new recordings on arbitrarily distributed electrodes (not necessarily on a grid), which can be obtained in extracellular recordings of single unit activity with multiple electrodes.
Distribution of current fluctuations in a bistable conductor
NASA Astrophysics Data System (ADS)
Singh, S.; Peltonen, J. T.; Khaymovich, I. M.; Koski, J. V.; Flindt, C.; Pekola, J. P.
2016-12-01
We measure the full distribution of current fluctuations in a single-electron transistor with a controllable bistability. The conductance switches randomly between two levels due to the tunneling of single electrons in a separate single-electron box. The electrical fluctuations are detected over a wide range of time scales and excellent agreement with theoretical predictions is found. For long integration times, the distribution of the time-averaged current obeys the large-deviation principle. We formulate and verify a fluctuation relation for the bistable region of the current distribution.
Petek, A.; Dolecek, V.; Vlachy, V.
1997-12-01
Current fluctuations during general corrosion of stainless steel in sulfuric acid were studied experimentally and analyzed using a simple electrochemical model. Stochastic behavior of the metal-electrolyte interface in the model was based upon the assumption that elementary fluctuation sources were related to fluxes of electrons that pass from a metal to electron-acceptor ions in solution. The number of successful electron transfers obeyed a Gaussian distribution, from which the corrosion current density and transfer coefficients could be determined.
Baryon number fluctuations at finite temperature and density
NASA Astrophysics Data System (ADS)
Fu, Wei-jie; Pawlowski, Jan M.; Rennecke, Fabian; Schaefer, Bernd-Jochen
2016-12-01
We investigate baryon number fluctuations for finite temperature and density in two-flavor QCD. This is done within a QCD-improved low-energy effective theory in an extension of the approach put forward by Fu and Pawlowski [Phys. Rev. D 92, 116006 (2015), 10.1103/PhysRevD.92.116006 and Phys. Rev. D 93, 091501 (2016), 10.1103/PhysRevD.93.091501]. In the present work, we aim to improve the predictive power of this approach for large temperatures and, in partitular, large densities, that is, for small collision energies. This is achieved by taking into account the full frequency dependence of the quark dispersion. This ensures the necessary Silver Blaze property of finite density QCD for the first time, which so far was only implemented approximately. Moreover, we show that Polyakov-loop fluctuations have a sizeable impact at large temperatures and density. The results for the kurtosis of baryon number fluctuations are compared to previous effective theory results, lattice results, and recent experimental data from STAR.
Radial evolution of the energy density of solar wind fluctuations
NASA Technical Reports Server (NTRS)
Zank, G. P.; Matthaeus, W. H.; Smith, C. W.
1995-01-01
On the basis of transport theories appropriate to a radially expanding solar wind, we describe new results for the radial evolution of the energy density in solar wind fluctuations at MHD scales. These models include the effects of 'mixing' and driving as well as the possibility of non-isotropic MHD turbulence. Implications of these results for solar wind heating, cosmic ray diffusion and interstellar pick-up ions will also be addressed.
Amplitude of primeval fluctuations from cosmological mass density reconstructions
NASA Technical Reports Server (NTRS)
Seljak, Uros; Bertschinger, Edmund
1994-01-01
We use the POTENT reconstruction of the mass density field in the nearby universe to estimate the amplitude of the density fluctuation power spectrum for various cosmological models. We find that sigma(sub 8) Omega(sub m sup 0.6) = 1.3(sub -0.3 sup +0.4), almost independently of the power spectrum. This value agrees well with the Cosmic Background Explorer (COBE) normalization for the standard cold dark matter model, while alternative models predict an excessive amplitude compared with COBE. Flat, low Omega(sub m) models and tilted models with spectral index n less than 0.8 are particularly discordant.
Measurement of high-frequency density fluctuations in improved confinement RFP plasmas
NASA Astrophysics Data System (ADS)
Duff, J. R.; Chapman, B. E.; Anderson, J. K.; Sarff, J. S.; Lin, L.; Ding, W. X.; Brower, D. L.
2013-10-01
In standard RFP plasmas, transport is dominated by global magnetic tearing modes. For improved-confinement plasmas using inductive current profile control (PPCD), smaller-scale fluctuations at higher frequencies (>50 kHz) may become more important as the global tearing modes are significantly reduced. In particular, drift-wave-like instabilities are theoretically unstable to the higher temperature and density gradients achieved during PPCD discharges. On the MST, an eleven chord Far-Infrared (FIR) laser-based diagnostic system with ~ 8 cm spacing is used to measure electron density fluctuations by interferometry and far-forward collective scattering. The existing diagnostic measures line-integrated density fluctuations within the divergence of the probe beam covering a wavenumber range k-< 1.3 cm-1, corresponding to k-ρs < 1.3 (ρs is the ion-sound Larmor radius). Experimentally, in PPCD plasmas, global tearing modes are reduced while high frequency coherent modes (50 < f < 140 kHz) emerge among broadband fluctuations. Correlations of these modes with sources of free energy, such as temperature and density gradients, will be investigated. Additionally, effects of increased plasma flow from a 1MW tangential NBI on high frequency density fluctuations will also be explored. Work Supported by U.S.D.O.E.
Spontaneous density fluctuations in granular flow and traffic
NASA Astrophysics Data System (ADS)
Herrmann, Hans J.
It is known that spontaneous density waves appear in granular material flowing through pipes or hoppers. A similar phenomenon is known from traffic jams on highways. Using numerical simulations we show that several types of waves exist and find that the density fluctuations follow a power law spectrum. We also investigate one-dimensional traffic models. If positions and velocities are continuous variables the model shows self-organized criticality driven by the slowest car. Lattice gas and lattice Boltzmann models reproduce the experimentally observed effects. Density waves are spontaneously generated when the viscosity has a non-linear dependence on density or shear rate as it is the case in traffic or granular flow.
Compaction and density fluctuations in vibrated granular media
NASA Astrophysics Data System (ADS)
Barnum, A. C. B.; Ozbay, A.; Nowak, E. R.
2002-03-01
We report measurements of the density of a vibrated granular material as a function of time or taps. The material studied consists of monodisperse spherical glass beads confined to a long, thin cylindrical tube. Changes in vibration intensity are used to induce transitions between two steady state densities that depend on the intensity of the vibrations. We find a complex time evolution similar to previous work on the irreversible relaxation from a loose state toward a steady state. In addition, frequency dependent third order moments of the density fluctuations are measured. The data indicate a coupling between large variations in density on one time scale and noise power over a broad range of higher-frequency scales. This work was partly supported by Petroleum Research Foundation under award No. 35861-G5.
Compaction and Density Fluctuations in Vibrated Granular Media
NASA Astrophysics Data System (ADS)
Barnum, A. C. B.; Ozbay, Arif; Nowak, E. R.
We report measurements of the density of a vibrated granular material as a function of time or taps. The material studied consists of monodisperse spherical glass beads confined to a long, thin cylindrical tube. Changes in vibration intensity are used to induce transitions between two steady state densities that depend on the intensity of the vibrations. We find a complex time evolution similar to previous work on the irreversible relaxation from a loose state toward a steady state. In addition, frequency dependent third order moments of the density fluctuations are measured. The data indicate a coupling between large variations in density on one time scale and noise power over a broad range of higher-frequency scales.
Current fluctuations at a phase transition
NASA Astrophysics Data System (ADS)
Gerschenfeld, A.; Derrida, B.
2011-10-01
The ABC model is a simple diffusive one-dimensional non-equilibrium system which exhibits a phase transition. Here we show that the cumulants of the currents of particles through the system become singular near the phase transition. At the transition, they exhibit an anomalous dependence on the system size (an anomalous Fourier's law). An effective theory for the dynamics of the single mode which becomes unstable at the transition allows one to predict this anomalous scaling.
Large-amplitude electron density and Hα fluctuations in the sustained spheromak physics experiment
NASA Astrophysics Data System (ADS)
Wang, Zhehui; Barnes, Cris W.; Wurden, G. A.; Hill, D. N.; Hooper, E. B.; McLean, H. S.; Wood, R. D.; Woodruff, S.
2002-06-01
New types of toroidally rotating fluctuations (toroidal mode numbers n = 1 and n = 2) of line-integrated electron density and Hα emission, with frequencies ranging from 10 to 100 kHz, are observed in the sustained spheromak physics experiment (SSPX). The rotating directions of these fluctuations are the same as the direction determined by E×B, while the E and B directions are determined by the gun voltage and gun magnetic flux polarities, respectively. These results take advantage of one distinctive signature of spheromaks, i.e. it is possible to observe toroidal MHD activity during decay and sustainment at any toroidal angle. A theoretical constraint on line-integrated measurement is proposed and is found to be consistent with experimental observations. Fluctuation analysis in the time and frequency domains indicates that the observed density and Hα fluctuations correlate with magnetic modes. Observation of Hα fluctuations correlating with magnetic fluctuations indicates that, at least in some cases, MHD n = 1 modes are due to the so-called `dough-hook' current paths that connect the coaxial gun to the flux conserver, rather than internal kink instabilities. These results also show that electron density and Hα emission diagnostics complement other tools for spheromak mode study.
Heat Current Fluctuations in Quantum Wires
NASA Astrophysics Data System (ADS)
Bogachek, E. N.; Krive, I. V.; Scherbakov, A. G.; Landman, Uzi
2001-03-01
The noise in the heat current for fermionic and bosonic systems is investigated in the Landauer-Buttiker approach. We show that the thermal (Jonhson-Nyquist) noise in perfect quantum wires does not depend on the statistics of the heat carriers. The nonequilibrium noise produced by the temperature difference between the heat reservoirs (hot and cold leads) is different for bosons and fermions. The contribution of ''fermionic'' nonequilibrium noise to the thermal noise is positive, while that due to ''bosonic'' noise is negative. At all temperatures the nonequilibrium noise P_Δ T is much smaller then the Johnson-Nyquist noise P_JN. Even in the most favorable situations | P_Δ T | /P_JN<= 0.3 for bosons and P_Δ T/P_JN<= 0.1 for fermions. The expressions obtained for bosons are applied for a description of the thermal transport through a Luttinger liquid constriction.
Fluctuation-induced pair density wave in itinerant ferromagnets
NASA Astrophysics Data System (ADS)
Conduit, G. J.; Pedder, C. J.; Green, A. G.
2013-03-01
Magnetic fluctuations near to quantum criticality can have profound effects. They lead to characteristic scaling at high temperature which may ultimately give way to a reconstruction of the phase diagram and the formation of new phases at low temperatures. The ferromagnet UGe2 is unstable to p-wave superconducting order—an effect presaged by the superfluidity in 3He—whereas in CeFePO fluctuations drive the formation of spiral magnetic order. Here we develop a general quantum order-by-disorder description of these systems that encompasses both of these instabilities within a unified framework. This allows us to demonstrate that in fact these instabilities intertwine to form a pair density wave.
Observations of ULF wave related equatorial electrojet and density fluctuations
NASA Astrophysics Data System (ADS)
Yizengaw, E.; Zesta, E.; Moldwin, M.; Damtie, B.; Mebrahtu, A.; Anad, F.; Pfaff, R. F.
2011-12-01
Global magnetospheric Ultra Low Frequency (ULF) pulsations with frequencies in the Pc 4-5 range (f = 1.0 - 8 mHz) have been observed for decades in space and on Earth. ULF pulsations contribute to magnetospheric particle transport and diffusion and play an important role in magnetospheric dynamics. However, only a few studies have been performed on ionospheric observations of ULF wave-related perturbations in the vicinity of the equatorial region. In this paper we report on Pc5 wave related electric field and thus vertical drift velocity oscillations at the equator as observed by ground magnetometers and radar. We show that the magnetometer estimated equatorial ExB drift oscillate with the same frequency as ULF Pc5 waves, creating significant ionospheric density fluctuations. For independent confirmation of the vertical drift velocity fluctuation, we used JULIA 150 km radar drift velocities and found similar fluctuation with the period of 8-10 minutes. We also show ionospheric density fluctuations during the period when we observed ULF wave activities. All these demonstrate that the Pc5 wave can penetrate to the equatorial ionosphere and modulate the equatorial electrodynamics. Finally, in order to detect the ULF activities both on the ground and in space, we use groundbased magnetometer data from African Meridian B-field Education and Research (AMBER) and the South American Meridional B-field Array (SAMBA). From space, we use magnetic field observations from the GOES 12 and the Communication/Navigation Outage and Forecast System (C/NOFS) satellites. Using the WIND spacecraft as the upstream solar wind monitor, we present direct evidence that solar wind number density and ram pressure fluctuations observed far upstream from the terrestrial magnetosphere are the main drivers of ULF wave activity inside the magnetosphere. Finally, we show that the ULF waves in the same frequency range are observed in the magnetosphere by the geosynchronous GOES spacecraft, in the
Density fluctuation spectrum of two-dimensional correlated fermion systems
NASA Astrophysics Data System (ADS)
Kotani, Akihiro; Hirashima, Dai
2012-12-01
Density fluctuation spectrum of two-dimensional fermions that interact with short-range repulsive interaction is calculated with the self-consistent perturbation theory. The spectrum extends beyond the particle-hole continuum band in the noninteracting case because of the multiparticle excitations. At a large wave vector, a peak develops in the spectrum near the lower threshold of the particle-hole continuum. These results are compared with the recent inelastic neutron scattering experiment on two-dimensional 3He adsorbed on graphite.
A molecular Rayleigh scattering setup to measure density fluctuations in thermal boundary layers
NASA Astrophysics Data System (ADS)
Panda, J.
2016-12-01
A Rayleigh scattering-based density fluctuation measurement system was set up inside a low-speed wind tunnel of NASA Ames Research Center. The immediate goal was to study the thermal boundary layer on a heated flat plate. A large number of obstacles had to be overcome to set up the system, such as the removal of dust particles using air filters, the use of photoelectron counting electronics to measure low intensity light, an optical layout to minimize stray light contamination, the reduction in tunnel vibration, and an expanded calibration process to relate photoelectron arrival rate to air density close to the plate surface. To measure spectra of turbulent density fluctuations, a two-PMT cross-correlation system was used to minimize the shot noise floor. To validate the Rayleigh measurements, temperature fluctuations spectra were calculated from density spectra and then compared with temperature spectra measured with a cold-wire probe operated in constant current mode. The spectra from the downstream half of the plate were found to be in good agreement with cold-wire probe, whereas spectra from the leading edge differed. Various lessons learnt are discussed. It is believed that the present effort is the first measurement of density fluctuations spectra in a boundary layer flow.
NASA Astrophysics Data System (ADS)
Lin, L.; Ding, W. X.; Brower, D. L.
2014-11-01
Combined polarimetry-interferometry capability permits simultaneous measurement of line-integrated density and Faraday effect with fast time response (˜1 μs) and high sensitivity. Faraday effect fluctuations with phase shift of order 0.05° associated with global tearing modes are resolved with an uncertainty ˜0.01°. For physics investigations, local density fluctuations are obtained by inverting the line-integrated interferometry data. The local magnetic and current density fluctuations are then reconstructed using a parameterized fit of the polarimetry data. Reconstructed 2D images of density and magnetic field fluctuations in a poloidal cross section exhibit significantly different spatial structure. Combined with their relative phase, the magnetic-fluctuation-induced particle transport flux and its spatial distribution are resolved.
Extreme current fluctuations in lattice gases: Beyond nonequilibrium steady states
NASA Astrophysics Data System (ADS)
Meerson, Baruch; Sasorov, Pavel V.
2014-01-01
We use the macroscopic fluctuation theory (MFT) to study large current fluctuations in nonstationary diffusive lattice gases. We identify two universality classes of these fluctuations, which we call elliptic and hyperbolic. They emerge in the limit when the deterministic mass flux is small compared to the mass flux due to the shot noise. The two classes are determined by the sign of compressibility of effective fluid, obtained by mapping the MFT into an inviscid hydrodynamics. An example of the elliptic class is the symmetric simple exclusion process, where, for some initial conditions, we can solve the effective hydrodynamics exactly. This leads to a super-Gaussian extreme current statistics conjectured by Derrida and Gerschenfeld [J. Stat. Phys. 137, 978 (2009), 10.1007/s10955-009-9830-1] and yields the optimal path of the system. For models of the hyperbolic class, the deterministic mass flux cannot be neglected, leading to a different extreme current statistics.
Extreme current fluctuations in lattice gases: beyond nonequilibrium steady states.
Meerson, Baruch; Sasorov, Pavel V
2014-01-01
We use the macroscopic fluctuation theory (MFT) to study large current fluctuations in nonstationary diffusive lattice gases. We identify two universality classes of these fluctuations, which we call elliptic and hyperbolic. They emerge in the limit when the deterministic mass flux is small compared to the mass flux due to the shot noise. The two classes are determined by the sign of compressibility of effective fluid, obtained by mapping the MFT into an inviscid hydrodynamics. An example of the elliptic class is the symmetric simple exclusion process, where, for some initial conditions, we can solve the effective hydrodynamics exactly. This leads to a super-Gaussian extreme current statistics conjectured by Derrida and Gerschenfeld [J. Stat. Phys. 137, 978 (2009)] and yields the optimal path of the system. For models of the hyperbolic class, the deterministic mass flux cannot be neglected, leading to a different extreme current statistics.
Current fluctuation of electron and hole carriers in multilayer WSe{sub 2} field effect transistors
Ko, Seung-Pil; Shin, Jong Mok; Jang, Ho-Kyun; Jin, Jun Eon; Kim, Gyu-Tae; Kim, Yong Jin; Kim, Young Keun; Shin, Minju
2015-12-14
Two-dimensional materials have outstanding scalability due to their structural and electrical properties for the logic devices. Here, we report the current fluctuation in multilayer WSe{sub 2} field effect transistors (FETs). In order to demonstrate the impact on carrier types, n-type and p-type WSe{sub 2} FETs are fabricated with different work function metals. Each device has similar electrical characteristics except for the threshold voltage. In the low frequency noise analysis, drain current power spectral density (S{sub I}) is inversely proportional to frequency, indicating typical 1/f noise behaviors. The curves of the normalized drain current power spectral density (NS{sub I}) as a function of drain current at the 10 Hz of frequency indicate that our devices follow the carrier number fluctuation with correlated mobility fluctuation model. This means that current fluctuation depends on the trapping-detrapping motion of the charge carriers near the channel interface. No significant difference is observed in the current fluctuation according to the charge carrier type, electrons and holes that occurred in the junction and channel region.
Spatial structure of lemming populations (Dicrostonyx groenlandicus) fluctuating in density.
Ehrich, D; Jorde, P E; Krebs, C J; Kenney, A J; Stacy, J E; Stenseth, N C
2001-02-01
The pattern and scale of the genetic structure of populations provides valuable information for the understanding of the spatial ecology of populations, including the spatial aspects of density fluctuations. In the present paper, the genetic structure of periodically fluctuating lemmings (Dicrostonyx groenlandicus) in the Canadian Arctic was analysed using mitochondrial DNA (mtDNA) control region sequences and four nuclear microsatellite loci. Low genetic variability was found in mtDNA, while microsatellite loci were highly variable in all localities, including localities on isolated small islands. For both genetic markers the genetic differentiation was clear among geographical regions but weaker among localities within regions. Such a pattern implies gene flow within regions. Based on theoretical calculations and population census data from a snap-trapping survey, we argue that the observed genetic variability on small islands and the low level of differentiation among these islands cannot be explained without invoking long distance dispersal of lemmings over the sea ice. Such dispersal is unlikely to occur only during population density peaks.
Yun, G S; Lee, W; Choi, M J; Kim, J B; Park, H K; Domier, C W; Tobias, B; Liang, T; Kong, X; Luhmann, N C; Donné, A J H
2010-10-01
The ECE imaging (ECEI) diagnostic tested on the TEXTOR tokamak revealed the sawtooth reconnection physics in unprecedented detail, including the first observation of high-field-side crash and collective heat transport [H. K. Park, N. C. Luhmann, Jr., A. J. H. Donné et al., Phys. Rev. Lett. 96, 195003 (2006)]. An improved ECEI system capable of visualizing both high- and low-field sides simultaneously with considerably better spatial coverage has been developed for the KSTAR tokamak in order to capture the full picture of core MHD dynamics. Direct 2D imaging of other MHD phenomena such as tearing modes, edge localized modes, and even Alfvén eigenmodes is expected to be feasible. Use of ECE images of the optically thin edge region to recover 2D electron density changes during L/H mode transitions is also envisioned, providing powerful information about the underlying physics. The influence of density fluctuations on optically thin ECE is discussed.
Yun, G. S.; Lee, W.; Choi, M. J.; Kim, J. B.; Park, H. K.; Domier, C. W.; Tobias, B.; Liang, T.; Kong, X.; Luhmann, N. C. Jr.; Donne, A. J. H.
2010-10-15
The ECE imaging (ECEI) diagnostic tested on the TEXTOR tokamak revealed the sawtooth reconnection physics in unprecedented detail, including the first observation of high-field-side crash and collective heat transport [H. K. Park, N. C. Luhmann, Jr., A. J. H. Donneet al., Phys. Rev. Lett. 96, 195003 (2006)]. An improved ECEI system capable of visualizing both high- and low-field sides simultaneously with considerably better spatial coverage has been developed for the KSTAR tokamak in order to capture the full picture of core MHD dynamics. Direct 2D imaging of other MHD phenomena such as tearing modes, edge localized modes, and even Alfven eigenmodes is expected to be feasible. Use of ECE images of the optically thin edge region to recover 2D electron density changes during L/H mode transitions is also envisioned, providing powerful information about the underlying physics. The influence of density fluctuations on optically thin ECE is discussed.
Measurements of Electron Density Profile and Fluctuations on HSX*
NASA Astrophysics Data System (ADS)
Deng, C.; Brower, D. L.; Ding, W. X.; Almagri, A. F.; Anderson, D. T.; Anderson, F. S. B.; Gerhardt, S. P.; Probert, P.; Radder, J.; Talmadge, J. N.
2001-10-01
The 288 GHz interferometer system on the quasi-helical stellarator HSX views the plasma cross section along 9 adjacent chords with 1.5 cm spacing. At this frequency refraction is manageable but requires correction when performing inversions. The interferometer has sensitivity n_edl = 8 x 10^11 cm-2 and frequency response up to 1 MHz. Improved time response permits measurement of high-frequency density fluctuations as well as fast changes to the equilibrium profile. First results from HSX with 2nd harmonic ECH at 28 GHz, using a 5 chord version of the interferometer, indicate that the density profile is quite peaked for both quasi-helically symmetric (QHS) plasmas and those where the quasisymmetry is broken (mirror mode) for ne = 1 x 10^12 cm-3. However, for densities ne = 3 x 10^11 cm-3, the profile for the QHS plasma (high stored energy) is narrower when compared to the mirror mode (low stored energy). Density profile variation with plasma configuration and resonant heating location using the 9 channel interferometer will be described. For high density HSX plasmas, ne > 3 x 10^12 cm-3, coherent oscillations are observed in the line-integrated density traces which are out of phase across the magnetic axis. These m=1 oscillations are observed at frequencies of 1-2 kHz and result in a periodic displacement of the density profile. *Supported by USDOE under grant DE-FG03-01ER-54615, Task III and DE-FG02-93ER54222.
Electron-cyclotron wave scattering by edge density fluctuations in ITER
Tsironis, Christos; Peeters, Arthur G.; Isliker, Heinz; Chatziantonaki, Ioanna; Vlahos, Loukas; Strintzi, Dafni
2009-11-15
The effect of edge turbulence on the electron-cyclotron wave propagation in ITER is investigated with emphasis on wave scattering, beam broadening, and its influence on localized heating and current drive. A wave used for electron-cyclotron current drive (ECCD) must cross the edge of the plasma, where density fluctuations can be large enough to bring on wave scattering. The scattering angle due to the density fluctuations is small, but the beam propagates over a distance of several meters up to the resonance layer and even small angle scattering leads to a deviation of several centimeters at the deposition location. Since the localization of ECCD is crucial for the control of neoclassical tearing modes, this issue is of great importance to the ITER design. The wave scattering process is described on the basis of a Fokker-Planck equation, where the diffusion coefficient is calculated analytically as well as computed numerically using a ray tracing code.
Random paths and current fluctuations in nonequilibrium statistical mechanics
Gaspard, Pierre
2014-07-15
An overview is given of recent advances in nonequilibrium statistical mechanics about the statistics of random paths and current fluctuations. Although statistics is carried out in space for equilibrium statistical mechanics, statistics is considered in time or spacetime for nonequilibrium systems. In this approach, relationships have been established between nonequilibrium properties such as the transport coefficients, the thermodynamic entropy production, or the affinities, and quantities characterizing the microscopic Hamiltonian dynamics and the chaos or fluctuations it may generate. This overview presents results for classical systems in the escape-rate formalism, stochastic processes, and open quantum systems.
Shocks, Rarefaction Waves, and Current Fluctuations for Anharmonic Chains
NASA Astrophysics Data System (ADS)
Mendl, Christian B.; Spohn, Herbert
2017-02-01
The nonequilibrium dynamics of anharmonic chains is studied by imposing an initial domain-wall state, in which the two half lattices are prepared in equilibrium with distinct parameters. We analyse the Riemann problem for the corresponding Euler equations and, in specific cases, compare with molecular dynamics. Additionally, the fluctuations of time-integrated currents are investigated. In analogy with the KPZ equation, their typical fluctuations should be of size t^{1/3} and have a Tracy-Widom GUE distributed amplitude. The proper extension to anharmonic chains is explained and tested through molecular dynamics. Our results are calibrated against the stochastic LeRoux lattice gas.
Shocks, Rarefaction Waves, and Current Fluctuations for Anharmonic Chains
NASA Astrophysics Data System (ADS)
Mendl, Christian B.; Spohn, Herbert
2016-10-01
The nonequilibrium dynamics of anharmonic chains is studied by imposing an initial domain-wall state, in which the two half lattices are prepared in equilibrium with distinct parameters. We analyse the Riemann problem for the corresponding Euler equations and, in specific cases, compare with molecular dynamics. Additionally, the fluctuations of time-integrated currents are investigated. In analogy with the KPZ equation, their typical fluctuations should be of size t^{1/3} and have a Tracy-Widom GUE distributed amplitude. The proper extension to anharmonic chains is explained and tested through molecular dynamics. Our results are calibrated against the stochastic LeRoux lattice gas.
Plasma density fluctuations observed during Space Shuttle Orbiter water releases
NASA Technical Reports Server (NTRS)
Pickett, J. S.; D'Angelo, N.; Kurth, W. S.
1989-01-01
Observations by the Langmuir probe on the Plasma Diagnostics Package flown as part of the Spacelab 2 mission in the summer of 1985 show a strong increase in the level of turbulence near the Shuttle Orbiter during operations in which liquid water is released. The spectrum of the plasma density fluctuations peaks at the lowest frequencies measured (a few Hz) and extends up to a few kHz, near the lower hybrid frequency. Two potential mechanisms for generating the plasma turbulence are suggested which are both based on the production of water ions as a result of charge exchange with the ambient oxygen ions in the ionosphere. The first mechanism proposed is the ion-plasma instability which arises from the drift of the contaminant with respect to the ambient oxygen ions. The other mechanism proposed is the Ott-Farley instability, which is a result of the ring distribution formed by the 'pick-up' water ions.
Collective motion and density fluctuations in bacterial colonies
NASA Astrophysics Data System (ADS)
Zhang, Hepeng; Be'Er, Avraham; Florin, E.-L.; Swinney, Harry L.
2010-03-01
The emergence of collective motion such as in fish schools and swarming bacteria is a ubiquitous self-organization phenomenon. Such collective behavior plays an important role in a range of phenomenon, such as formation and migration of animal or fish groups. To understand the collective motion, tracking of large numbers of individuals is needed, but such measurements have been lacking. Here we examine a microscopic system, where we are able to measure simultaneously the positions, velocities, and orientations of up to a thousand bacteria in a colony. The motile bacteria form closely-packed dynamic clusters within which they move cooperatively. The number of bacteria in a cluster exhibits a power-law distribution truncated by an exponential tail, and the probability of finding large clusters grows markedly as bacterial density increases. Mobile clusters exhibit anomalous fluctuations in bacterial density: the standard deviation (δN) grows with the mean (N) of the number of bacteria as δN˜N^3/4 rather than δN˜N^1/2, as in thermal equilibrium systems.
Stochastic protein production and time-dependent current fluctuations
NASA Astrophysics Data System (ADS)
Gorissen, Mieke; Vanderzande, Carlo
2011-03-01
Translation is the cellular process in which ribosomes make proteins from information encoded on messenger RNA. We model this process using driven lattice gases and take into account the finite lifetime of mRNA. The stochastic properties of the translation process can then be determined from the time-dependent current fluctuations of the lattice gas model. We illustrate our ideas with a totally asymmetric exclusion process with extended objects.
NASA Technical Reports Server (NTRS)
Fletcher, Douglas G.; Mckenzie, R. L.
1992-01-01
Nonintrusive measurements of density, temperature, and their turbulent fluctuation levels were obtained in the boundary layer of an unseeded, Mach 2 wind tunnel flow. The spectroscopic technique that was used to make the measurements is based on the combination of laser-induced oxygen fluorescence and Raman scattering by oxygen and nitrogen from the same laser pulse. Results from this demonstration experiment are compared with previous measurements obtained in the same facility using conventional probes and an earlier spectroscopic technique. Densities and temperatures measured with the current technique agree with the previous surveys to within 3 percent and 2 percent, respectively. The fluctuation amplitudes for both variables agree with the measurements obtained using the earlier spectroscopic technique and show evidence of an unsteady, weak shock wave that perturbs the boundary layer.
Tokamak equilibria with reversed current density.
Martynov, A A; Medvedev, S Yu; Villard, L
2003-08-22
Observations of nearly zero toroidal current in the central region of tokamaks (the "current hole") raises the question of the existence of toroidal equilibria with very low or reversed current in the core. The solutions of the Grad-Shafranov equilibrium equation with hollow toroidal current density profile including negative current density in the plasma center are investigated. Solutions of the corresponding eigenvalue problem provide simple examples of such equilibrium configurations. More realistic equilibria with toroidal current density reversal are computed using a new equilibrium problem formulation and computational algorithm which do not assume nested magnetic surfaces.
Fractal fluctuations and complexity: current debates and future challenges.
Delignieres, Didier; Marmelat, Vivien
2012-01-01
Complexity is perhaps one of the less properly understood concepts, even within the scientific community. Recent theoretical and experimental advances, however, based on the close relationship between the complexity of the system and the presence of 1/f fluctuations in its macroscopic behavior, have opened new domains of investigation, which consider fundamental questions as well as more applied perspectives. These approaches allow a better understanding of how essential macroscopic functions could emerge from complex interactive networks. In this review we present the current state of the theoretical debate about the origins of 1/f fluctuations, with special focus on recent hypotheses that establish a direct link between complexity and fractal fluctuations. Further, we clarify some lines of opposition, especially between idiosyncratic versus nomothetic conceptions, and global versus componential approaches. Finally, we discuss the deep questioning that this approach can generate with regard to current theories of motor control and psychological processes, as well as some future developments which may be evoked, especially in the domain of physical medicine and rehabilitation.
Are metastable, precrystallisation, density-fluctuations a universal phenomena?
Heeley, Ellen L; Poh, C Kit; Li, Wu; Maidens, Anna; Bras, Wim; Dolbnya, Igor P; Gleeson, Anthony J; Terrill, Nicolas J; Fairclough, J Patrick A; Olmsted, Peter D; Ristic, Rile I; Hounslow, Micheal J; Ryan, Anthony J
2003-01-01
In-situ observations of crystallisation in minerals and organic polymers have been made by simultaneous, time-resolved small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) techniques. In isotactic polypropylene slow quiescent crystallisation shows the onset of large scale ordering prior to crystal growth. Rapid crystallisations studied by melt extrusion indicate the development of well resolved oriented SAXS patterns associated with long range order before the development of crystalline peaks in the WAXS region. Block copolymers self-assemble into mesophases in polymer melts above a critical chain length (or above a critical temperature) and this self-assembly process is shown to be susceptible to an incipient crystallisation. Mesophase formation is observed at anomalously high temperatures in ethylene-oxide containing block copolymers below the normal melting point of the polyoxy ethylene chains. Formation of calcium carbonate from aqueous solutions of sodium carbonate and calcium nitrate is observed to be a two-stage process and precipitation proceeds by the production of an amorphous metastable phase. This phase grows until it is volume filling and leads to the formation of the two polymorphs Calcite and Vaterite. These three sets of results suggest pre-nucleation density fluctuations, leading to a metastable phase, play an integral role in all three classes of crystallisation. In due course, this phase undergoes transformation to "normal" crystals.
Electron density fluctuation measurements using a multichannel microwave interferometer in GAMMA 10
Yoshikawa, M.; Shima, Y.; Matsumoto, T.; Nakahara, A.; Yanagi, N.; Itakura, A.; Hojo, H.; Kobayashi, T.; Matama, K.; Tatematsu, Y.; Imai, T.; Kohagura, J.; Hirata, M.; Nakashima, Y.; Cho, T.
2006-10-15
Measurement of fluctuation in plasma is important for studying the improvement in plasma confinement by the formation of the plasma confinement potential. The density fluctuation is observed by microwaves by methods such as interferometry, reflectometry and Fraunhofer diffraction method. We have constructed a new multichannel microwave interferometer to measure the plasma density and fluctuation radial profiles in a single plasma shot. We successfully measured the time-dependent density and line-integrated density fluctuation radial profiles in a single plasma shot using the multichannel microwave interferometer. Thus, we have developed a useful tool for studying the improvement in plasma confinement by the formation of plasma confinement potential.
Enhancing critical current density of cuprate superconductors
Chaudhari, Praveen
2015-06-16
The present invention concerns the enhancement of critical current densities in cuprate superconductors. Such enhancement of critical current densities include using wave function symmetry and restricting movement of Abrikosov (A) vortices, Josephson (J) vortices, or Abrikosov-Josephson (A-J) vortices by using the half integer vortices associated with d-wave symmetry present in the grain boundary.
NASA Technical Reports Server (NTRS)
Mielke, Amy F.; Elam, Kristie A.; Sung, Chih-Jen; Panda, Jayanta
2006-01-01
A molecular Rayleigh scattering technique is developed to measure dynamic gas temperature, velocity, and density in unseeded turbulent flows at sampling rates up to 10 kHz. A high power CW laser beam is focused at a point in a heated air jet plume and Rayleigh scattered light is collected and spectrally resolved. The spectrum of the light, which contains information about the temperature, velocity, and density of the flow, is analyzed using a Fabry-Perot interferometer. The circular interference fringe pattern is divided into four concentric regions and sampled at 1 and 10 kHz using photon counting electronics. Monitoring the relative change in intensity within each region allows for measurement of gas temperature and velocity. Independently monitoring the total scattered light intensity provides a measure of gas density. Power spectral density calculations of temperature, velocity, and density fluctuations, as well as mean and fluctuating quantities are demonstrated for various radial locations in the jet flow at a fixed axial distance from the jet exit plane. Results are compared with constant current anemometry and pitot probe measurements at the same locations.
Maps of current density using density-functional methods
NASA Astrophysics Data System (ADS)
Soncini, A.; Teale, A. M.; Helgaker, T.; de Proft, F.; Tozer, D. J.
2008-08-01
The performance of several density-functional theory (DFT) methods for the calculation of current densities induced by a uniform magnetic field is examined. Calculations are performed using the BLYP and KT3 generalized-gradient approximations, together with the B3LYP hybrid functional. For the latter, both conventional and optimized effective potential (OEP) approaches are used. Results are also determined from coupled-cluster singles-and-doubles (CCSD) electron densities by a DFT constrained search procedure using the approach of Wu and Yang (WY). The current densities are calculated within the CTOCD-DZ2 distributed origin approach. Comparisons are made with results from Hartree-Fock (HF) theory. Several small molecules for which correlation is known to be especially important in the calculation of magnetic response properties are considered-namely, O3, CO, PN, and H2CO. As examples of aromatic and antiaromatic systems, benzene and planarized cyclooctatetraene molecules are considered, with specific attention paid to the ring current phenomenon and its Kohn-Sham orbital origin. Finally, the o-benzyne molecule is considered as a computationally challenging case. The HF and DFT induced current maps show qualitative differences, while among the DFT methods the maps show a similar qualitative structure. To assess quantitative differences in the calculated current densities with different methods, the maximal moduli of the induced current densities are compared and integration of the current densities to yield shielding constants is performed. In general, the maximal modulus is reduced in moving from HF to B3LYP and BLYP, and further reduced in moving to KT3, OEP(B3LYP), and WY(CCSD). The latter three methods offer the most accurate shielding constants in comparison with both experimental and ab initio data and hence the more reliable route to DFT calculation of induced current density in molecules.
Fluctuations of energy density of short-pulse optical radiation in the turbulent atmosphere.
Banakh, V A; Smalikho, I N
2014-09-22
Fluctuations of energy density of short-pulse optical radiation in the turbulent atmosphere have been studied based on numerical solution of the parabolic wave equation for the complex spectral amplitude of the wave field by the split-step method. It has been shown that under conditions of strong optical turbulence, the relative variance of energy density fluctuations of pulsed radiation of femtosecond duration becomes much less than the relative variance of intensity fluctuations of continuous-wave radiation. The spatial structure of fluctuations of the energy density with a decrease of the pulse duration becomes more large-scale and homogeneous. For shorter pulses the maximal value of the probability density distribution of energy density fluctuations tends to the mean value of the energy density.
Spatial density fluctuations and selection effects in galaxy redshift surveys
Labini, Francesco Sylos; Tekhanovich, Daniil; Baryshev, Yurij V. E-mail: d.tekhanovich@spbu.ru
2014-07-01
One of the main problems of observational cosmology is to determine the range in which a reliable measurement of galaxy correlations is possible. This corresponds to determining the shape of the correlation function, its possible evolution with redshift and the size and amplitude of large scale structures. Different selection effects, inevitably entering in any observation, introduce important constraints in the measurement of correlations. In the context of galaxy redshift surveys selection effects can be caused by observational techniques and strategies and by implicit assumptions used in the data analysis. Generally all these effects are taken into account by using pair-counting algorithms to measure two-point correlations. We review these methods stressing that they are based on the a-priori assumption that galaxy distribution is spatially homogeneous inside a given sample. We show that, when this assumption is not satisfied by the data, results of the correlation analysis are affected by finite size effects. In order to quantify these effects, we introduce a new method based on the computation of the gradient of galaxy counts along tiny cylinders. We show, by using artificial homogeneous and inhomogeneous point distributions, that this method identifies redshift dependent selection effects and disentangles them from the presence of large scale density fluctuations. We then apply this new method to several redshift catalogs and we find evidence that galaxy distribution, in those samples where selection effects are small enough, is characterized by power-law correlations with exponent γ=0.9 up to 20 Mpc/h followed by a change of slope that, in the range 20–100 Mpc/h, corresponds to a power-law exponent γ=0.25. Whether a crossover to spatial uniformity occurs at ∼ 100 Mpc/h or larger scales cannot be clarified by the present data.
Current fluctuations in a particle-nonconserving reaction-diffusion process
NASA Astrophysics Data System (ADS)
Torkaman, Pegah; Jafarpour, Farhad H.
2013-01-01
We have considered a one-dimensional coagulation-decoagulation system of classical particles on a finite lattice with reflecting boundaries. It is known that the system undergoes a phase transition from a high-density to a low-density phase. Using a matrix product approach we have obtained an exact expression for the average entropy production rate of the system in the thermodynamic limit. We have also performed a large-deviation analysis for fluctuations of entropy production rate and particle current. It turns out that the characteristics of the kink in the large deviation function can be used to spot the phase transition point. We have found that for very weak driving field (when the system approaches its equilibrium) and also for very strong driving field (when the system is in the low-density phase) the large deviation function for fluctuations of entropy production rate is almost parabolic, while in the high-density phase it prominently deviates from Gaussian behavior. The validity of the Gallavotti-Cohen fluctuation relation for the large deviation function for particle current is also verified.
Current fluctuations in a particle-nonconserving reaction-diffusion process.
Torkaman, Pegah; Jafarpour, Farhad H
2013-01-01
We have considered a one-dimensional coagulation-decoagulation system of classical particles on a finite lattice with reflecting boundaries. It is known that the system undergoes a phase transition from a high-density to a low-density phase. Using a matrix product approach we have obtained an exact expression for the average entropy production rate of the system in the thermodynamic limit. We have also performed a large-deviation analysis for fluctuations of entropy production rate and particle current. It turns out that the characteristics of the kink in the large deviation function can be used to spot the phase transition point. We have found that for very weak driving field (when the system approaches its equilibrium) and also for very strong driving field (when the system is in the low-density phase) the large deviation function for fluctuations of entropy production rate is almost parabolic, while in the high-density phase it prominently deviates from Gaussian behavior. The validity of the Gallavotti-Cohen fluctuation relation for the large deviation function for particle current is also verified.
Jacobson, A.R.
1981-04-01
A laser diagnostic scheme is described which facilitates localization of density fluctuations along the line of sight. The method exploits both the generally observed anisotropy of density fluctuations in low-beta plasmas, as well as the twisting of the magnetic field which occurs across the minor diameter of reversed-field pinches, spheromaks, etc. Both interferometric and schlieren variations are discussed.
Field emission current fluctuations due to lithium adsorbed on the W(111) region
NASA Astrophysics Data System (ADS)
Biernat, T.; Kleint, Ch.; Mȩclewski, R.
1991-04-01
Field emission current fluctuations for lithium adsorbed on the tungsten (111) region were investigated by a probe-hole field emission microscope. The coverage dependence of the noise power as well as spectral density functions W(ƒ) at different temperatures were obtained. The spectral density functions have been analysed in terms of the Timm and van der Ziel concentration fluctuation model. Using Comer's method the surface diffusion energies and prefactors have been determined for submonolayer coverages of lithium. They are strongly coverage dependent. The activation energy varies non-monotonically between 0.41 and 0.53 eV and the prefactor between 2.4 × 10 -4 and 1.3 × 10 -2 cm 2/s in the Li surface concentration interval (0.5-3.7) × 10 14 cm -2. The results are compared with those obtained for the W(111)/K system.
Density fluctuations and dielectric constant of water in low and high density liquid states
NASA Astrophysics Data System (ADS)
Lascaris, Erik; Zhang, Cui; Galli, Giulia A.; Franzese, Giancarlo; Stanley, H. Eugene
2012-02-01
The hypothesis of a liquid-liquid critical point (LLCP) in the phase diagram of water, though first published many years ago, still remains the subject of a heated debate. According to this hypothesis there exists a critical point near T 244 K, and P 215 MPa, located at the end of a coexistence line between a high density liquid (HDL) and a low density liquid state (LDL). The LLCP lies below the homogenous nucleation temperature of water and it has so far remained inaccessible to experiments. We study a model of water exhibiting a liquid-liquid phase transition (that is a liquid interacting through the ST2 potential) and investigate the properties of dipolar fluctuations as a function of density, in the HDL and LDL. We find an interesting correlation between the macroscopic dielectric constants and the densities of the two liquids in the vicinity of the critical point, and we discuss possible implications for measurements close to the region where the LLCP may be located.
The origin of density fluctuations in the 'new inflationary universe'
NASA Technical Reports Server (NTRS)
Turner, M. S.
1983-01-01
Cosmological mysteries which are not explained by the Big Bang hypothesis but may be approached by a revamped inflationary universe model are discussed. Attention is focused on the isotropy, the large-scale homogeneity, small-scale inhomogeneity, the oldness/flatness of the universe, and the baryon asymmetry. The universe is assumed to start in the lowest energy state, be initially dominated by false vacuum energy, enter a de Sitter phase, and then cross a barrier which is followed by the formation of fluctuation regions that lead to structure. The scalar fields (perturbation regions) experience quantum fluctuations which produce spontaneous symmetry breaking on a large scale. The scalar field value would need to be much greater than the expansion rate during the de Sitter epoch. A supersymmetric (flat) potential which satisfies the requirement, yields fluctuations of the right magnitude, and allows inflation to occur is described.
Additivity, density fluctuations, and nonequilibrium thermodynamics for active Brownian particles
NASA Astrophysics Data System (ADS)
Chakraborti, Subhadip; Mishra, Shradha; Pradhan, Punyabrata
2016-05-01
Using an additivity property, we study particle-number fluctuations in a system of interacting self-propelled particles, called active Brownian particles (ABPs), which consists of repulsive disks with random self-propulsion velocities. From a fluctuation-response relation, a direct consequence of additivity, we formulate a thermodynamic theory which captures the previously observed features of nonequilibrium phase transition in the ABPs from a homogeneous fluid phase to an inhomogeneous phase of coexisting gas and liquid. We substantiate the predictions of additivity by analytically calculating the subsystem particle-number distributions in the homogeneous fluid phase away from criticality where analytically obtained distributions are compatible with simulations in the ABPs.
Electrodialysis simulation to achieve optimum current density
NASA Technical Reports Server (NTRS)
Herrmann, Cal C.
1993-01-01
Electrodialysis is used to remove salts from waste or other water streams, to yield a concentrated brine and a substatially deionized product water. During the electrodialysis process, the boundary layer adjacent to the ion selective membrane can become depleted of ions, resulting in severe pH changes sometimes accompanied by precipitation, and power losses, by a process known as water splitting. In order to optimize the applied electric current density, to achieve maximum deionization without exceeding the limiting current at any point along the path, a simulation program has been created to plot ion concentrations and fluxes, and cell current densities and voltages along the electrodialysis path. A means for tapering the current density along the path is recommended.
NASA Astrophysics Data System (ADS)
Snicker, Antti; Guidi, Lorenzo; Kohn, Alf; Maj, Omar; Weber, Hannes; Poli, Emanuele
2016-10-01
We present state-of-the-art computations of propagation and absorption of electron cyclotron waves, retaining the effects of scattering due to density fluctuations. In ITER, injected microwaves are foreseen to suppress NTMs by driving current at the resonant surface(s). The good localization of the absorption profile can be spoiled by beam scattering and impair the NTM control capabilities. A novel tool based on the wave kinetic equation has been developed, which retains diffraction, an integral form of the scattering operator assuming the Born scattering approximation, full tokamak geometry and determination of the power absorption profile. This approach has been implemented in the code WKBeam, which has been benchmarked against the beam-tracing code TORBEAM and the full-wave code IPF-FDMC, in particular to verify usage of the Born approximation for ITER parameters. We show that in ITER the radiation transport is diffusive unlike in existing machines. Using WKBeam we demonstrate through parameter scans that the width of the deposition profile in ITER depends on the assumptions on the fluctuations and beam parameters: the effect can be of the order of 100%. A method to quantify mode-to-mode scattering induced by fluctuations has been developed and first results are presented.
Tarp, Jens M; Angheluta, Luiza; Mathiesen, Joachim; Goldenfeld, Nigel
2014-12-31
Plastic deformation mediated by collective dislocation dynamics is investigated in the two-dimensional phase-field crystal model of sheared single crystals. We find that intermittent fluctuations in the dislocation population number accompany bursts in the plastic strain-rate fluctuations. Dislocation number fluctuations exhibit a power-law spectral density 1/f2 at high frequencies f. The probability distribution of number fluctuations becomes bimodal at low driving rates corresponding to a scenario where low density of defects alternates at irregular times with high populations of defects. We propose a simple stochastic model of dislocation reaction kinetics that is able to capture these statistical properties of the dislocation density fluctuations as a function of shear rate.
DENSITY FLUCTUATIONS UPSTREAM AND DOWNSTREAM OF INTERPLANETARY SHOCKS
Pitňa, A.; Šafránková, J.; Němeček, Z.; Goncharov, O.; Němec, F.; Přech, L.; Chen, C. H. K.; Zastenker, G. N.
2016-03-01
Interplanetary (IP) shocks as typical large-scale disturbances arising from processes such as stream–stream interactions or Interplanetary Coronal Mass Ejection (ICME) launching play a significant role in the energy redistribution, dissipation, particle heating, acceleration, etc. They can change the properties of the turbulent cascade on shorter scales. We focus on changes of the level and spectral properties of ion flux fluctuations upstream and downstream of fast forward oblique shocks. Although the fluctuation level increases by an order of magnitude across the shock, the spectral slope in the magnetohydrodynamic range is conserved. The frequency spectra upstream of IP shocks are the same as those in the solar wind (if not spoiled by foreshock waves). The spectral slopes downstream are roughly proportional to the corresponding slopes upstream, suggesting that the properties of the turbulent cascade are conserved across the shock; thus, the shock does not destroy the shape of the spectrum as turbulence passes through it. Frequency spectra downstream of IP shocks often exhibit “an exponential decay” in the ion kinetic range that was earlier reported at electron scales in the solar wind or at ion scales in the interstellar medium. We suggest that the exponential shape of ion flux spectra in this range is caused by stronger damping of the fluctuations in the downstream region.
Density Fluctuations Upstream and Downstream of Interplanetary Shocks
NASA Astrophysics Data System (ADS)
Pitňa, A.; Šafránková, J.; Němeček, Z.; Goncharov, O.; Němec, F.; Přech, L.; Chen, C. H. K.; Zastenker, G. N.
2016-03-01
Interplanetary (IP) shocks as typical large-scale disturbances arising from processes such as stream-stream interactions or Interplanetary Coronal Mass Ejection (ICME) launching play a significant role in the energy redistribution, dissipation, particle heating, acceleration, etc. They can change the properties of the turbulent cascade on shorter scales. We focus on changes of the level and spectral properties of ion flux fluctuations upstream and downstream of fast forward oblique shocks. Although the fluctuation level increases by an order of magnitude across the shock, the spectral slope in the magnetohydrodynamic range is conserved. The frequency spectra upstream of IP shocks are the same as those in the solar wind (if not spoiled by foreshock waves). The spectral slopes downstream are roughly proportional to the corresponding slopes upstream, suggesting that the properties of the turbulent cascade are conserved across the shock thus, the shock does not destroy the shape of the spectrum as turbulence passes through it. Frequency spectra downstream of IP shocks often exhibit “an exponential decay” in the ion kinetic range that was earlier reported at electron scales in the solar wind or at ion scales in the interstellar medium. We suggest that the exponential shape of ion flux spectra in this range is caused by stronger damping of the fluctuations in the downstream region.
The generation of rapid solar flare hard X-ray and microwave fluctuations in current sheets
NASA Astrophysics Data System (ADS)
Holman, Gordon D.
The generation of rapid fluctuations, or spikes, in hard X-ray and microwave bursts via the disruption of electron heating and acceleration in current sheets is studied. It is found that 20 msec hard X-ray fluctuations can be thermally generated in a current sheet if the resistivity in the sheet is highly anomalous, the plasma density in the emitting region is relatively high, and the volume of the emitting region is greater than that of the current sheet. A specific mechanism for producing the fluctuations, involving heating in the presence of ion acoustic turbulence and a constant driving electric field, and interruption of the heating by a strong two-stream instability, is discussed. Variations upon this mechanism are also discussed. This mechanism also modulates electron acceleration, as required for the microwave spike emission. If the hard X-ray emission at energies less than approx. 1000 keV is nonthermal bremsstrahlung, the coherent modulation of electron acceleration in a large number of current sheets is required.
The generation of rapid solar flare hard X-ray and microwave fluctuations in current sheets
NASA Technical Reports Server (NTRS)
Holman, Gordon D.
1986-01-01
The generation of rapid fluctuations, or spikes, in hard X-ray and microwave bursts via the disruption of electron heating and acceleration in current sheets is studied. It is found that 20 msec hard X-ray fluctuations can be thermally generated in a current sheet if the resistivity in the sheet is highly anomalous, the plasma density in the emitting region is relatively high, and the volume of the emitting region is greater than that of the current sheet. A specific mechanism for producing the fluctuations, involving heating in the presence of ion acoustic turbulence and a constant driving electric field, and interruption of the heating by a strong two-stream instability, is discussed. Variations upon this mechanism are also discussed. This mechanism also modulates electron acceleration, as required for the microwave spike emission. If the hard X-ray emission at energies less than approx. 1000 keV is nonthermal bremsstrahlung, the coherent modulation of electron acceleration in a large number of current sheets is required.
Interactions between electrons, mesoscopic Josephson effect and asymmetric current fluctuations
NASA Astrophysics Data System (ADS)
Huard, B.
2006-07-01
This article discusses three experiments on the properties of electronic transport at the mesoscopic scale. The first one allowed to measure the energy exchange rate between electrons in a metal contaminated by a very weak concentration of magnetic impurities. The role played by magnetic impurities in the Kondo regime on those energy exchanges is quantitatively investigated, and the global measured exchange rate is larger than expected. The second experiment is a measurement of the current-phase relation in a system made of two superconductors linked through a single atom. We thus provide quantitative support for the recent description of the mesoscopic Josephson effect. The last experiment is a measurement of the asymmetry of the current fluctuations in a mesoscopic conductor, using a Josephson junction as a threshold detector. Cet ouvrage décrit trois expériences portant sur les propriétés du transport électronique à l'échelle mésoscopique. La première a permis de mesurer le taux d'échange d'énergie entre électrons dans un métal contenant une très faible concentration d'impuretés magnétiques. Nous avons validé la description quantitative du rôle des impuretés magnétiques dans le régime Kondo sur ces échanges énergétiques et aussi montré que le taux global d'échange est plus fort que prévu. La seconde expérience est une mesure de la relation courant-phase dans un système constitué de deux supraconducteurs couplés par un seul atome. Elle nous a permis de conforter quantitativement la récente description de l'effet Josephson mésoscopique. La dernière expérience est unemesure de l'asymétrie des fluctuations du courant dans un conducteur mésoscopique en utilisant une Jonction Josephson comme détecteur de seuil.
Conti, F; Hille, B; Neumcke, B; Nonner, W; Stämpfli, R
1976-01-01
Single myelinated nerve fibres of Rana esculenta were investigated under voltage clamp conditions at 13 degrees C. Fluctuations of steady-state membrane current were measured during the last 152 msec of 190-225 msec pulses depolarizing the membrane by 8-48 mV. Noise power spectral densities were calculated in the frequency range of 6-6-6757 Hz. 2. External application of 150 nM tetrodotoxin (TTX) and/or 10 mM tetraethylammonium (TEA) ion reduced the current fluctuations. The difference of current noise spectra measured in the presence and absence of TTX (TEA) was not changed by the presence of TEA (TTX) during both measurements, and was taken as the spectrum of the Na (K) current fluctuations. 3. Residual current noise during application of both TTX and TEA was, except for some excess noise at the low and high frequency ends of the spectrum, similar to the noise measured from a passive nerve model and could be understood in terms of Nyquist noise of the known resistances and the amplifier noise. 4. Na current fluctuation spectra were interpreted as the sum N/f+SNa(f) where SNa(F) represents the spectrum expected for a set of equal, independent Na channels with only two conductance states (open or closed) which follow Hodgkin-Huxley kinetics. With values of hinfinity, tauh and minfinity measured from macroscopic Na currents, the measured spectra were fitted well by optimizing N, SNa(0) and taum. Values of taum obtained by this method were in fair agreement with values found from macroscopic currents. 5. The 1/f component of Na current noise was roughly proportional to the square of the steady-state Na current, I2. The mean value of N/I2 was (1-1 +/- 0-3) X 10(-4). 6. The current carried by a single Na channel was calculated from fitted spectra and steady-state Na currents measured simultaneously with the current fluctuations. The single channel conductance gamma normalized to zero absolute membrane potential was calculated. The average gamma from twelve measurements
Silva, Filipe da; Pinto, Martin Campos; Després, Bruno; Heuraux, Stéphane
2015-08-15
This work analyzes the stability of the Yee scheme for non-stationary Maxwell's equations coupled with a linear current model with density fluctuations. We show that the usual procedure may yield unstable scheme for physical situations that correspond to strongly magnetized plasmas in X-mode (TE) polarization. We propose to use first order clustered discretization of the vectorial product that gives back a stable coupling. We validate the schemes on some test cases representative of direct numerical simulations of X-mode in a magnetic fusion plasma including turbulence.
Investigation of zonal flows by using the collective scattering measurement of density fluctuations
NASA Astrophysics Data System (ADS)
Shen, H. G.; Yu, Y.; Lan, T.; Li, Y. D.; Liu, A. D.; Xie, J. L.; Liu, W. D.; Yu, C. X.; Zhang, W. Y.; Ti, A.; Li, J. G.
2015-09-01
The poloidal {{E}r}× {{B}\\text{T}} rotation velocities in the core plasma region are studied using the instantaneous frequency method (IFM) with the density fluctuations measured by the CO2 laser collective scattering diagnostics on the HT-7 tokamak. A coherent mode is observed in the fluctuations of poloidal velocities with the mode frequency from 10 to 20 kHz. It is identified as geodesic acoustic mode (GAM) zonal flow with poloidal symmetry (m = 0) and its mode frequency coinciding with the theoretical expected GAM frequency. The nonlinear interactions are investigated by applying the envelope analysis on the density fluctuations. The results confirm that the envelope modulation in the high frequency density fluctuations only comes from the shearing by GAM. The comparison between IFM and envelope analysis is also discussed.
Modeling of Fluctuating Mass Flux in Variable Density Flows
NASA Technical Reports Server (NTRS)
So, R. M. C.; Mongia, H. C.; Nikjooy, M.
1983-01-01
The approach solves for both Reynolds and Favre averaged quantities and calculates the scalar pdf. Turbulent models used to close the governing equations are formulated to account for complex mixing and variable density effects. In addition, turbulent mass diffusivities are not assumed to be in constant proportion to turbulent momentum diffusivities. The governing equations are solved by a combination of finite-difference technique and Monte-Carlo simulation. Some preliminary results on simple variable density shear flows are presented. The differences between these results and those obtained using conventional models are discussed.
Josephson junctions with alternating critical current density
Mints, R.G.; Kogan, V.G.
1997-04-01
The magnetic-field dependence of the critical current I{sub c}(H) is considered for a short Josephson junction with the critical current density j{sub c} alternating along the tunnel contact. Two model cases, periodic and randomly alternating j{sub c}, are treated in detail. Recent experimental data on I{sub c}(H) for grain-boundary Josephson junctions in YBa{sub 2}Cu{sub 3}O{sub x} are discussed. {copyright} {ital 1997} {ital The American Physical Society}
Rotating frame RF current density imaging.
Scott, G C; Joy, M L; Armstrong, R L; Henkelman, R M
1995-03-01
RF current density imaging (RF-CDI) is a new MRI technique for imaging the Larmor frequency current density parallel to B0 in electrolytic media. To extend the use of RF-CDI to biological tissue for generating conductivity contrast, the sensitivity must be increased and the data requirements reduced. A rotating frame approach, in which a large B1 field is applied simultaneously as a rotary echo with RF current, is proposed to meet these requirements. Rotating frame magnetic fields are encoded in the phase of an MRI image. Trials have now been performed with this sequence in a three-compartment cylindrical phantom containing doped water or mineral oil for detecting displacement, conduction and fringe field currents. In a postmortem rat study, 85.56 MHz RF currents injected by implanted electrodes created tissue dependent contrast because of the electrical properties of tissue. A sensitivity and artifact analysis was also performed. The sensitivity of this method is determined by the maximum RF pulse duration. SAR limits pose an upper bound on this time and B1, whereas the avoidance of phase artifacts imposes a lower bound on B1.
Mobility of electrons in supercritical krypton: Role of density fluctuations
Nishikawa, Masaru; Holroyd, Richard A.; Preses, Jack M.
2007-07-07
Excess electrons were generated in supercritical krypton by means of pulsed x-ray irradiation, and the electron transport phenomena were studied. Electron signals immediately after a 30 ps pulse showed a distinctive feature characteristic of the presence of the Ramsauer-Townsend minimum in the momentum transfer cross section. The dependence of the drift velocity v{sub D} on field strength was found to be concave upward in the low field region and then to go through a maximum with increasing field strength, which is also typical of the presence of a minimum in the scattering cross section at an intermediate field strength. A minimum in the electron mobility was observed at about one-half the critical density. The acoustical phonon scattering model, which successfully explained the mobility change in this density region in supercritical xenon, was again found to account for the mobility in supercritical krypton.
Dynamical Selection of the Primordial Density Fluctuation Amplitude
Lehners, Jean-Luc; Steinhardt, Paul J.
2011-02-25
In inflationary models, the predicted amplitude of primordial density perturbations Q is much larger than the observed value ({approx}10{sup -5}) for natural choices of parameters. To explain the requisite exponential fine-tuning, anthropic selection is often invoked, especially in cases where microphysics is expected to produce a complex energy landscape. By contrast, we find examples of ekpyrotic models based on heterotic M theory for which dynamical selection naturally favors the observed value of Q.
Density fluctuation measurement using motional Stark effect optics in JT-60U
Suzuki, T.; Fujita, T.; Oyama, N.; Isayama, A.; Matsunaga, G.; Oikawa, T.; Asakura, N.; Takechi, M.
2006-10-15
The multichannel motional Stark effect (MSE) diagnostic system in JT-60U has been upgraded to measure density fluctuation profile. A 16-channel fast-sampling digitizer has been added in order to measure photomultiplier-tube signals at measurement frequency of 0.5-1 MHz. The new system works as a MSE and beam emission spectroscopy diagnostic. Spatially resolved electron density fluctuation profile measurement in various operation regimes is presented. In the core plasma, density fluctuation induced by rotation of tearing mode islands was observed. Temporal evolution of the fluctuation frequency agrees with that measured by Mirnov coils (poloidal and toroidal mode numbers: 2 and 1, respectively). The phases of the fluctuations on either side of the q=2 surface are inverted, which is consistent with electron cyclotron emission. These measurements show that the density fluctuation is caused by a rotating magnetic island structure induced by the tearing mode. In the scrape-off layer of a H-mode plasma with edge-localized-mode (ELM), i. e., ELMy H-mode outward propagation of strong intermittent emission corresponding to ELM crash was also observed. The propagation velocity is 0.69-2.2 km/s along the MSE measurement points, the time lag and distance between adjacent channels being 67{+-}35 {mu}s and 70 mm, respectively.
On the phase shift between electric potential and plasma density fluctuations in the edge turbulence
Shchepetov, S. V. Kholnov, Yu. V.; Vasil'kov, D. G.
2013-02-15
In some cases, the phase shift between fluctuations of the electric potential and plasma density helps to identify the instability that governs the turbulent state. In this paper, the basic experimental and theoretical results that denote the possibility (or impossibility) of such identification are briefly discussed. The experimental data based on measurements of the phase shift between the floating potential and ion saturation current fluctuations in the L-2M stellarator-a system with externally imposed magnetic surfaces-are presented (Shchepetov, Kholnov, Fedyanin, et al., Plasma Phys. Controlled Fusion 50, 045001 (2008)). It is shown that the observed phase shift {Omega} varies in a wide range from {pi} to 0, gradually decreasing with deepening inside the plasma. A number of arguments are presented suggesting that {Omega} Almost-Equal-To {pi} can indicate that the process is nonlocal, i.e., oscillations at a given spatial point are driven and mainly determined by the processes localized outside of the observation point. We note that, within the framework of the magnetohydrodynamic theory, plasma was definitely unstable with respect to resistive interchange modes in all cases under study. It is demonstrated experimentally that the widespread notion that the phase shift {Omega} Almost-Equal-To {pi}/2 is characteristic of only resistive interchange modes is hardly universal. The experimental results are analyzed on the basis of analytical estimates.
Steadiness in Dilute Pyroclastic Density Currents
NASA Astrophysics Data System (ADS)
Andrews, B. J.
2015-12-01
Pyroclastic density currents (PDCs) are often unsteady, as evidenced by direct observations of dilute lobes or jets emerging from the fronts of larger currents and by deposits that indicate transient transport and depositional regimes. We used scaled experiments to investigate unsteadiness in dilute PDCs. The experimental currents were run in an 8.5x6.1x2.6 m tank and comprised heated or ambient temperature 20-μm talc powder turbulently suspended in air. Experiments were scaled such that densimetric and thermal Richardson numbers, Froude number, and particle Stokes and settling numbers were dynamically similar to natural dilute PDCs. Although the experiment Reynolds numbers are substantially lower than those of natural PDCs, the experiments are fully turbulent. Experiments were observed with video and high-speed cameras and high-frequency thermocouples. Currents were generated with total eruption durations of 100 s. Unsteadiness in source conditions was produced by interrupting supply for intervals, t, with durations of 1, 2.5, 5, and 10 s in the experimental runs at 35 and 70 s. When t<2.5 s, the currents are indistinguishable from currents with steady supply. In runs with t=2.5-5 s, the individual pulses comprising each current are readily apparent near the source, but decay with distance downstream until the currents appear as single (e.g. steady) flows. In experiments with t=10 s, the 3 pulses comprising each run never merge and the currents remain unsteady. Comparison with the integral turbulent timescale, τ, and current velocity, U, show that unsteadiness is persistent when t>3<τ but currents are steady when t<τ. In currents with 3τ>t>τ, unsteadiness decays such that at a distance of ~4Ut, the currents are again steady. Applied to natural dilute PDCs, our results suggest that currents and their resulting deposits, will only show evidence of unsteadiness if they are disrupted for many seconds and those breaks may "heal" over distances of 100s of meters.
Linear magnetoresistance in n-type silicon due to doping density fluctuations
Porter, Nicholas A.; Marrows, Christopher H.
2012-01-01
We report the observation of a large linear magnetoresistance in the ohmic regime in commonplace commercial n-type silicon wafer with a P dopant density of (1.4±0.1) ×1015 cm–3, and report measurements of it in the temperature range 30–200 K. It arises from the deformation of current paths, which causes a part of the Hall field to be detected at the voltage probes. In short, wide samples we found linear magnetoresistance as large as 4707% in an 8 tesla field at 35 K. Sample geometry effects like these are commonplace in commercial Hall sensors. However, we found that the effect persisted in long, thin samples where the macroscopic current flow should be uniform between the voltage probes: we observed a magnetoresistance of 445% under the same conditions as above. We interpret this result as arising due to spatial fluctuations in the donor density, in the spirit of the Herring model. PMID:22876340
NASA Astrophysics Data System (ADS)
Derrida, Bernard; Gerschenfeld, Antoine
2009-07-01
For the symmetric simple exclusion process on an infinite line, we calculate exactly the fluctuations of the integrated current Q t during time t through the origin when, in the initial condition, the sites are occupied with density ρ a on the negative axis and with density ρ b on the positive axis. All the cumulants of Q t grow like sqrt{t} . In the range where Qt˜ sqrt{t} , the decay exp [- Q {/t 3}/ t] of the distribution of Q t is non-Gaussian. Our results are obtained using the Bethe ansatz and several identities derived recently by Tracy and Widom for exclusion processes on the infinite line.
Shock-associated MHD waves - A model for interstellar density fluctuations
NASA Technical Reports Server (NTRS)
Spangler, Steven R.
1988-01-01
The possibility that the density fluctuations responsible for radio scintillations could be due to ion-beam-generated MHD waves near interstellar shock waves is discussed. This suggestion is inspired by spacecraft observations which reveal these phenomena near shocks in the solar system. The model quite naturally accounts for the scale on which these fluctuations occur; it is dictated by the wavelength of the unstable waves.
Nicolaus, Marion; Tinbergen, Joost M; Ubels, Richard; Both, Christiaan; Dingemanse, Niels J
2016-04-01
Heritable personality variation is subject to fluctuating selection in many animal taxa; a major unresolved question is why this is the case. A parsimonious explanation must involve a general ecological process: a likely candidate is the omnipresent spatiotemporal variation in conspecific density. We tested whether spatiotemporal variation in density within and among nest box plots of great tits (Parus major) predicted variation in selection acting on exploratory behaviour (n = 48 episodes of selection). We found viability selection favouring faster explorers under lower densities but slower explorers under higher densities. Temporal variation in local density represented the primary factor explaining personality-related variation in viability selection. Importantly, birds did not anticipate changes in selection by means of adaptive density-dependent plasticity. This study thereby provides an unprecedented example of the key importance of the interplay between fluctuating selection and lack of adaptive behavioural plasticity in maintaining animal personality variation in the wild.
Maps of the little bangs through energy density and temperature fluctuations
Basu, Sumit Chatterjee, Rupa; Nayak, Tapan K.
2016-01-22
Heavy-ion collisions at ultra-relativistic energies are often referred to as little bangs. We propose for the first time to map the heavy-ion collisions at ultra-relativistic energies, similar to the maps of the cosmic microwave background radiation, using fluctuations of energy density and temperature in small phase space bins. We study the evolution of fluctuations at each stage of the collision using an event-by-event hydrodynamic framework. We demonstrate the feasibility of making fluctuation maps from experimental data and its usefulness in extracting considerable information regarding the early stages of the collision and its evolution.
Effect of Heating on Turbulent Density Fluctuations and Noise Generation From High Speed Jets
NASA Technical Reports Server (NTRS)
Panda, Jayanta; Seasholtz, Richard G.; Elam, Kristie A.; Mielke, Amy F.; Eck, Dennis G.
2004-01-01
Heated jets in a wide range of temperature ratios (TR), and acoustic Mach numbers (Ma) were investigated experimentally using far field microphones and a molecular Rayleigh scattering technique. The latter provided density fluctuations measurements. Two sets of operating conditions were considered: (1) TR was varied between 0.84 and 2.7 while Ma was fixed at 0.9; (2) Ma was varied between 0.6 and 1.48, while TR was fixed at 2.27. The implementation of the molecular Rayleigh scattering technique required dust removal and usage of a hydrogen combustor to avoid soot particles. Time averaged density measurements in the first set of data showed differences in the peripheral density shear layers between the unheated and heated jets. The nozzle exit shear layer showed increased turbulence level with increased plume temperature. Nevertheless, further downstream the density fluctuations spectra are found to be nearly identical for all Mach number and temperature ratio conditions. To determine noise sources a correlation study between plume density fluctuations and far field sound pressure fluctuations was conducted. For all jets the core region beyond the end of the potential flow was found to be the strongest noise source. Except for an isothermal jet, the correlations did not differ significantly with increasing temperature ratio. The isothermal jet created little density fluctuations. Although the far field noise from this jet did not show any exceptional trend, the flow-sound correlations were very low. This indicated that the density fluctuations only acted as a "tracer parameter" for the noise sources.
Density Fluctuation in Asymmetric Nozzle Plumes and Correlation with Far Field Noise
NASA Technical Reports Server (NTRS)
Panda, J.; Zaman, K. B. M. Q.
2001-01-01
A comparative experimental study of air density fluctuations in the unheated plumes of a circular, 4-tabbed-circular, chevron-circular and 10-lobed rectangular nozzles was performed at a fixed Mach number of 0.95 using a recently developed Rayleigh scattering based technique. Subsequently, the flow density fluctuations are cross-correlated with the far field sound pressure fluctuations to determine sources for acoustics emission. The nearly identical noise spectra from the baseline circular and the chevron nozzles are found to be in agreement with the similarity in spreading, turbulence fluctuations, and flow-sound correlations measured in the plumes. The lobed nozzle produced the least low frequency noise, in agreement with the weakest overall density fluctuations and flow-sound correlation. The tabbed nozzle took an intermediate position in the hierarchy of noise generation, intensity of turbulent fluctuation and flow-sound correlation. Some of the features in the 4-tabbed nozzle are found to be explainable in terms of splitting of the jet in a central large core and 4 side jetlets.
Control of density fluctuations in atomistic-continuum simulations of dense liquids
NASA Astrophysics Data System (ADS)
Kotsalis, E. M.; Walther, J. H.; Koumoutsakos, P.
2007-07-01
We present a control algorithm to eliminate spurious density fluctuations associated with the coupling of atomistic and continuum descriptions for dense liquids. A Schwartz domain decomposition algorithm is employed to couple molecular dynamics for the simulation of the atomistic system with a continuum solver for the simulation of the Navier-Stokes equations. The lack of periodic boundary conditions in the molecular dynamics simulations hinders the proper accounting for the virial pressure leading to spurious density fluctuations at the continuum-atomistic interface. An ad hoc boundary force is usually employed to remedy this situation. We propose the calculation of this boundary force using a control algorithm that explicitly cancels the density fluctuations. The results demonstrate that the present approach outperforms state-of-the-art algorithms. The conceptual and algorithmic simplicity of the method makes it suitable for any type of coupling between atomistic and continuum descriptions of dense fluids.
Control of density fluctuations in atomistic-continuum simulations of dense liquids.
Kotsalis, E M; Walther, J H; Koumoutsakos, P
2007-07-01
We present a control algorithm to eliminate spurious density fluctuations associated with the coupling of atomistic and continuum descriptions for dense liquids. A Schwartz domain decomposition algorithm is employed to couple molecular dynamics for the simulation of the atomistic system with a continuum solver for the simulation of the Navier-Stokes equations. The lack of periodic boundary conditions in the molecular dynamics simulations hinders the proper accounting for the virial pressure leading to spurious density fluctuations at the continuum-atomistic interface. An ad hoc boundary force is usually employed to remedy this situation. We propose the calculation of this boundary force using a control algorithm that explicitly cancels the density fluctuations. The results demonstrate that the present approach outperforms state-of-the-art algorithms. The conceptual and algorithmic simplicity of the method makes it suitable for any type of coupling between atomistic and continuum descriptions of dense fluids.
NASA Astrophysics Data System (ADS)
Deng, C. B.; Brower, D. L.
2012-10-01
The interferometer system on the Helically Symmetric eXperiment (HSX) stellarator uses an expanded beam and linear detector array to realize a multichord measurement. Unlike conventional interferometry which determines the plasma phase shift with respect to a reference, directly evaluating the phase between two adjacent chords can be employed to measure the change in plasma phase with impact parameter. This approach provides a measure of the equilibrium density gradient or the density gradient fluctuations and is referred to as differential interferometry. For central chords, measurements are spatially localized due to a geometrical weighting factor and can provide information on core density gradient fluctuations. The measurement requires finite coherence between fluctuations in the two spatially offset chords. This technique is applied on the HSX stellarator to measure both broadband turbulence and coherent modes. Spatial localization is exploited to isolate core turbulence changes associated with change in magnetic configuration or heating location.
Anabitarte, E. . Inst. de Energias Renovables); Hanson, G.R.; Harris, J.H.; Wilgen, J.B.; Bell, J.D.; Dunlap, J.L.; Hidalgo, C.; Thomas, C.E.; Uckan, T. )
1990-01-01
A microwave reflectometer system has been installed and operated on the Advanced Toroidal Facility (ATF) to measure density fluctuations. This system consists of two individual reflectometers that use the same antenna system and operate in the 30- to 40-GHz band. This arrangement allows operation at two frequencies along the same radial chord so that radial coherence measurements are possible. During the initial operating period of the reflectometer, a correlation was observed between a change in the edge density fluctuation spectrum and a transition to improved confinement. Recently, local measurements of the density fluctuation spectra in electron-cyclotron-heated (ECH) plasmas has been shown to agree with Langmuir probe measurements at the edge. Furthermore, structure in the spectra has been observed in some ECH plasmas. 4 refs., 3 figs.
Investigation of Density Fluctuations in Supersonic Free Jets and Correlation with Generated Noise
NASA Technical Reports Server (NTRS)
Panda, J.; Seasholtz, R. G.
2000-01-01
The air density fluctuations in the plumes of fully-expanded, unheated free jets were investigated experimentally using a Rayleigh scattering based technique. The point measuring technique used a continuous wave laser, fiber-optic transmission and photon counting electronics. The radial and centerline profiles of time-averaged density and root-mean-square density fluctuation provided a comparative description of jet growth. To measure density fluctuation spectra a two-Photomultiplier tube technique was used. Crosscorrelation between the two PMT signals significantly reduced electronic shot noise contribution. Turbulent density fluctuations occurring up to a Strouhal number (Sr) of 2.5 were resolved. A remarkable feature of density spectra, obtained from the same locations of jets in 0.5< M<1.5 range, is a constant Strouhal frequency for peak fluctuations. A detailed survey at Mach numbers M = 0.95, 1.4 and 1.8 showed that, in general, distribution of various Strouhal frequency fluctuations remained similar for the three jets. In spite of the similarity in the flow fluctuation the noise characteristics were found to be significantly different. Spark schlieren photographs and near field microphone measurements confirmed that the eddy Mach wave radiation was present in Mach 1.8 jet, and was absent in Mach 0.95 jet. To measure correlation between the flow and the far field sound pressure fluctuations, a microphone was kept at a distance of 50 diameters, 30 deg. to the flow direction, and the laser probe volume was moved from point to point in the flow. The density fluctuations in the peripheral shear layer of Mach 1.8 jet showed significant correlation up to the measurement limit of Sr = 2.5, while for Mach 0.95 jet no correlation was measured. Along the centerline measurable correlation was found from the end of the potential core and at the low frequency range (Sr less than 0.5). Usually the normalized correlation values increased with an increase of the jet Mach
Current loops and fluctuations in the zero-range process on a diamond lattice
NASA Astrophysics Data System (ADS)
Villavicencio-Sanchez, R.; Harris, R. J.; Touchette, H.
2012-07-01
We study the zero-range process on a simple diamond lattice with open boundary conditions and determine the conditions for the existence of loops in the mean current. We also perform a large deviation analysis for fluctuations of partial and total currents and check the validity of the Gallavotti-Cohen fluctuation relation for these quantities. In this context, we show that the fluctuation relation is not satisfied for partial currents between sites even if it is satisfied for the total current flowing between the boundaries. Finally, we extend our methods to study a chain of coupled diamonds and demonstrate co-existence of mean current regimes.
Tsikata, S.; Pisarev, V.; Gresillon, D. M.; Lemoine, N.
2009-03-15
Kinetic models and numerical simulations of E-vectorxB-vector plasma discharges predict microfluctuations at the scales of the electron cyclotron drift radius and the ion plasma frequency. With the help of a specially designed collective scattering device, the first experimental observations of small-scale electron density fluctuations inside the plasma volume are obtained, and observed in the expected ranges of spatial and time scales. The anisotropy, dispersion relations, form factor, amplitude, and spatial distribution of these electron density fluctuations are described and compared to theoretical expectations.
NASA Technical Reports Server (NTRS)
Stainback, P. C.; Johnson, C. B.; Basnett, C. B.
1983-01-01
The heat transfer characteristics of a three-wire hot-wire probe operated with a constant temperature anemometer were investigated in the subsonic compressible flow regime. The sensitivity coefficients, with respect to velocity, density and total temperature, were measured and the results were used to calculate the velocity, density, and total temperature fluctuations in the test section of the Langley 0.3-m Transonic Cryogenic Tunnel (TCT). These results were extended to give estimates for fluctuations due to vorticity, sound, and entropy. In addition, attempts were made to determine the major source of disturbances in the 0.3-m TCT.
Comparison of density fluctuation measurements between O-mode and X-mode reflectometry on Tore Supra
Gerbaud, T.; Clairet, F.; Sabot, R.; Sirinelli, A.; Heuraux, S.; Leclert, G.; Vermare, L.
2006-10-15
Reflectometry is a versatile diagnostic which allows both electronic density profile and density fluctuation measurements. Fast sweep heterodyne technique is particularly suitable for precise measurement of the phase of the reflected signal, which records the story of the wave propagation through the plasma up to the cutoff layer, including the density fluctuations. The present article exhibits a comparison of the density fluctuation radial profile measurements between fast sweep frequency technique, both using O-mode and X-mode polarizations, and fixed frequency technique. The correct agreement between all measurements of the relative values of the density fluctuation profiles reinforces the validity of the approximations used.
Gaudillere, J P; Drevon, J J; Bernoud, J P; Jardinet, F; Euvrard, M
1987-01-01
The development of soybean leaves grown at fluctuating photon flux density between 100 and 1500μM m(-2)s(-1) with a period of 160 sec were compared to leaves developed under continuous light with the same mean photon flux density. Number of epidermal cells and stomata, leaf area and specific leaf weight were not affected by the periodic fluctuation of photon flux density. Chloroplastic pigment concentration and chlorophyll fluorescence reveal some photoinhibitory effects of the high photon flux density phase. Stomatal and internal CO2 conductance and the quantum yield were not affected by the light regime. In contrast ribulose 1.5 bisphosphate carboxylase/oxygenase activity before in vitro activation by CO2 and Mg(++) was stimulated by the periodic illumination whereas the total amount of the enzyme and the internal leaf CO2 conductance remained steady. In conclusion, there was no major difference between leaves of plant grown either under a steady or under a periodic fluctuation of the photon flux density except some photoinhibitory symptoms under fluctuating illumination, and a higher in vivo level of activation of the Rubisco.
Gas density fluctuations in the Perseus Cluster: clumping factor and velocity power spectrum
NASA Astrophysics Data System (ADS)
Zhuravleva, I.; Churazov, E.; Arévalo, P.; Schekochihin, A. A.; Allen, S. W.; Fabian, A. C.; Forman, W. R.; Sanders, J. S.; Simionescu, A.; Sunyaev, R.; Vikhlinin, A.; Werner, N.
2015-07-01
X-ray surface brightness fluctuations in the core of the Perseus Cluster are analysed, using deep observations with the Chandra observatory. The amplitude of gas density fluctuations on different scales is measured in a set of radial annuli. It varies from 7 to 12 per cent on scales of ˜10-30 kpc within radii of 30-220 kpc from the cluster centre. Using a statistical linear relation between the observed amplitude of density fluctuations and predicted velocity, the characteristic velocity of gas motions on each scale is calculated. The typical amplitudes of the velocity outside the central 30 kpc region are 90-140 km s-1 on ˜20-30 kpc scales and 70-100 km s-1 on smaller scales ˜7-10 kpc. The velocity power spectrum (PS) is consistent with cascade of turbulence and its slope is in a broad agreement with the slope for canonical Kolmogorov turbulence. The gas clumping factor estimated from the PS of the density fluctuations is lower than 7-8 per cent for radii ˜30-220 kpc from the centre, leading to a density bias of less than 3-4 per cent in the cluster core. Uncertainties of the analysis are examined and discussed. Future measurements of the gas velocities with the Astro-H, Athena and Smart-X observatories will directly measure the gas density-velocity perturbation relation and further reduce systematic uncertainties in this analysis.
Emergent loop current order from pair density wave superconductivity
NASA Astrophysics Data System (ADS)
Kashyap, Manoj; Melchert, Drew; Agterberg, Daniel
2015-03-01
In addition to charge density wave (CDW) order, there is evidence that the pseudogap phase in the cuprates breaks time reversal symmetry. Here we show that pair density wave (PDW) states give rise to a translational invariant non-superconducting order parameter that breaks time reversal and parity symmetries, but preserves their product. This secondary order parameter has a different origin, but shares the same symmetry properties as a magnetoelectric loop current order that has been proposed earlier in the context of the cuprates to explain the appearance of intra-cell magnetic order. We further show that, due to fluctuations, this secondary loop current order, which represents the breaking of discrete symmetries, can preempt PDW order, which breaks both continuous and discrete symmetries. In such a phase, the emergent loop current order coexists with spatial short range CDW and short range superconducting order. Finally, we propose a PDW phase that accounts for intra-cell magnetic order and the Kerr effect, has CDW order consistent with x-ray scattering and nuclear magnetic resonance observations, and quasi-particle properties consistent with angle resolved photoemission scattering. We acknowledge support from NSF Grant No. DMR-1335215
Emergent loop current order from pair density wave superconductivity
NASA Astrophysics Data System (ADS)
Agterberg, D. F.; Melchert, Drew S.; Kashyap, M. K.
2015-02-01
There is evidence that the pseudogap phase in the cuprates breaks time-reversal symmetry. Here we show that pair density wave (PDW) states give rise to a translational invariant nonsuperconducting order parameter that breaks time-reversal and parity symmetries, but preserves their product. This secondary order parameter has a different origin, but shares the same symmetry properties as a magnetoelectric loop current order that has been proposed earlier in the context of the cuprates to explain the appearance of intracell magnetic order. We further show that, due to fluctuations, this secondary loop current order, which breaks only discrete symmetries, can preempt PDW order, which breaks both continuous and discrete symmetries. In such a phase, the emergent loop current order coexists with spatial short-range superconducting order and possibly short-range charge density wave (CDW) order. Finally, we propose a PDW phase that accounts for intracell magnetic order and the Kerr effect, has CDW order consistent with x-ray scattering and nuclear magnetic resonance observations, and quasiparticle properties consistent with angle-resolved photoemission scattering.
Hurtado, Pablo I; Garrido, Pedro L
2010-04-01
Most systems, when pushed out of equilibrium, respond by building up currents of locally conserved observables. Understanding how microscopic dynamics determines the averages and fluctuations of these currents is one of the main open problems in nonequilibrium statistical physics. The additivity principle is a theoretical proposal that allows to compute the current distribution in many one-dimensional nonequilibrium systems. Using simulations, we validate this conjecture in a simple and general model of energy transport, both in the presence of a temperature gradient and in canonical equilibrium. In particular, we show that the current distribution displays a Gaussian regime for small current fluctuations, as prescribed by the central limit theorem, and non-Gaussian (exponential) tails for large current deviations, obeying in all cases the Gallavotti-Cohen fluctuation theorem. In order to facilitate a given current fluctuation, the system adopts a well-defined temperature profile different from that of the steady state and in accordance with the additivity hypothesis predictions. System statistics during a large current fluctuation is independent of the sign of the current, which implies that the optimal profile (as well as higher-order profiles and spatial correlations) are invariant upon current inversion. We also demonstrate that finite-time joint fluctuations of the current and the profile are well described by the additivity functional. These results suggest the additivity hypothesis as a general and powerful tool to compute current distributions in many nonequilibrium systems.
NASA Astrophysics Data System (ADS)
Hurtado, Pablo I.; Garrido, Pedro L.
2010-04-01
Most systems, when pushed out of equilibrium, respond by building up currents of locally conserved observables. Understanding how microscopic dynamics determines the averages and fluctuations of these currents is one of the main open problems in nonequilibrium statistical physics. The additivity principle is a theoretical proposal that allows to compute the current distribution in many one-dimensional nonequilibrium systems. Using simulations, we validate this conjecture in a simple and general model of energy transport, both in the presence of a temperature gradient and in canonical equilibrium. In particular, we show that the current distribution displays a Gaussian regime for small current fluctuations, as prescribed by the central limit theorem, and non-Gaussian (exponential) tails for large current deviations, obeying in all cases the Gallavotti-Cohen fluctuation theorem. In order to facilitate a given current fluctuation, the system adopts a well-defined temperature profile different from that of the steady state and in accordance with the additivity hypothesis predictions. System statistics during a large current fluctuation is independent of the sign of the current, which implies that the optimal profile (as well as higher-order profiles and spatial correlations) are invariant upon current inversion. We also demonstrate that finite-time joint fluctuations of the current and the profile are well described by the additivity functional. These results suggest the additivity hypothesis as a general and powerful tool to compute current distributions in many nonequilibrium systems.
First in-situ observations of neutral and plasma density fluctuations within a PMSE layer
NASA Technical Reports Server (NTRS)
Lubken, Franz-Josef; Lehmacher, Gerald; Blix, Tom; Hoppe, Ulf-Peter; Thrane, Eivind; Cho, John; Swartz, Wesley
1993-01-01
The NLC-91 rocket and radar campaign provided the first opportunity for high resolution neutral and plasma turbulence measurements with simultaneous observations of PMSE (Polar Mesospheric Summer Echoes). During the flight of the TURBO payload on August 1, 1991, Cornell University Portable Radar Interferometer (CUPRI) and European Incoherent Scattter facility (EISCAT) observed double PMSE layers located at 86 and 88 km altitude, respectively. Strong neutral density fluctuations were observed in the upper layer but not in the lower layer. The fluctuation spectra of the ions and neutrals within the upper layer are consistent with standard turbulence theories. However, we show that there is no neutral turbulence present in the lower layer and that something else must have been operating here to create the plasma fluctuations and hence the radar echoes. Although the in situ measurements of the electron density fluctuations are much stronger in the lower layer, the higher absolute electron density of the upper layer more than compensated for the weaker fluctuations yielding comparable radar echo powers.
First in-situ observations of neutral and plasma density fluctuations within a PMSE layer
NASA Astrophysics Data System (ADS)
Lubken, Franz-Josef; Lehmacher, Gerald; Blix, Tom; Hoppe, Ulf-Peter; Thrane, Eivind; Cho, John; Swartz, Wesley
1993-10-01
The NLC-91 rocket and radar campaign provided the first opportunity for high resolution neutral and plasma turbulence measurements with simultaneous observations of PMSE (Polar Mesospheric Summer Echoes). During the flight of the TURBO payload on August 1, 1991, Cornell University Portable Radar Interferometer (CUPRI) and European Incoherent Scattter facility (EISCAT) observed double PMSE layers located at 86 and 88 km altitude, respectively. Strong neutral density fluctuations were observed in the upper layer but not in the lower layer. The fluctuation spectra of the ions and neutrals within the upper layer are consistent with standard turbulence theories. However, we show that there is no neutral turbulence present in the lower layer and that something else must have been operating here to create the plasma fluctuations and hence the radar echoes. Although the in situ measurements of the electron density fluctuations are much stronger in the lower layer, the higher absolute electron density of the upper layer more than compensated for the weaker fluctuations yielding comparable radar echo powers.
Correlations of the density and of the current in non-equilibrium diffusive systems
NASA Astrophysics Data System (ADS)
Sadhu, Tridib; Derrida, Bernard
2016-11-01
We use fluctuating hydrodynamics to analyze the dynamical properties in the non-equilibrium steady state of a diffusive system coupled with reservoirs. We derive the two-time correlations of the density and of the current in the hydrodynamic limit in terms of the diffusivity and the mobility. Within this hydrodynamic framework we discuss a generalization of the fluctuation dissipation relation in a non-equilibrium steady state where the response function is expressed in terms of the two-time correlations. We compare our results to an exact solution of the symmetric exclusion process. This exact solution also allows one to directly verify the fluctuating hydrodynamics equation.
Density matrix of radiation of a black hole with a fluctuating horizon
NASA Astrophysics Data System (ADS)
Iofa, Mikhail Z.
2016-09-01
The density matrix of Hawking radiation is calculated in the model of a black hole with a fluctuating horizon. Quantum fluctuations smear the classical horizon of a black hole and modify the density matrix of radiation producing the off-diagonal elements. The off-diagonal elements may store information on correlations between the radiation and the black hole. The smeared density matrix was constructed by convolution of the density matrix calculated with the instantaneous horizon with the Gaussian distribution over the instantaneous horizons. The distribution has the extremum at the classical radius of the black hole and the width of order of the Planck length. Calculations were performed in the model of a black hole formed by the thin collapsing shell which follows a trajectory that is a solution of the matching equations connecting the interior and exterior geometries.
Gas density fluctuations in the Perseus Cluster: clumping factor and velocity power spectrum
Zhuravleva, I.; Churazov, E.; Arevalo, P.; Schekochihin, A. A.; Allen, S. W.; Fabian, A. C.; Forman, W. R.; Sanders, J. S.; Simionescu, A.; Sunyaev, R.; Vikhlinin, A.; Werner, N.
2015-05-20
X-ray surface brightness fluctuations in the core of the Perseus Cluster are analysed, using deep observations with the Chandra observatory. The amplitude of gas density fluctuations on different scales is measured in a set of radial annuli. It varies from 7 to 12 per cent on scales of ~10–30 kpc within radii of 30–220 kpc from the cluster centre. Using a statistical linear relation between the observed amplitude of density fluctuations and predicted velocity, the characteristic velocity of gas motions on each scale is calculated. The typical amplitudes of the velocity outside the central 30 kpc region are 90–140 km s^{-1} on ~20–30 kpc scales and 70–100 km s^{-1} on smaller scales ~7–10 kpc. The velocity power spectrum (PS) is consistent with cascade of turbulence and its slope is in a broad agreement with the slope for canonical Kolmogorov turbulence. The gas clumping factor estimated from the PS of the density fluctuations is lower than 7–8 per cent for radii ~30–220 kpc from the centre, leading to a density bias of less than 3–4 per cent in the cluster core. Uncertainties of the analysis are examined and discussed. Future measurements of the gas velocities with the Astro-H, Athena and Smart-X observatories will directly measure the gas density–velocity perturbation relation and further reduce systematic uncertainties in this analysis.
Dynamic Harris current sheet thickness from Cluster current density and plasma measurements
NASA Technical Reports Server (NTRS)
Thompson, S. M.; Kivelson, M. G.; Khurana, K. K.; McPherron, R. L.; Weygand, J. M.; Balogh, A.; Reme, H.; Kistler, L. M.
2005-01-01
We use the first accurate measurements of current densities in the plasma sheet to calculate the half-thickness and position of the current sheet as a function of time. Our technique assumes a Harris current sheet model, which is parameterized by lobe magnetic field B(o), current sheet half-thickness h, and current sheet position z(sub o). Cluster measurements of magnetic field, current density, and plasma pressure are used to infer the three parameters as a function of time. We find that most long timescale (6-12 hours) current sheet crossings observed by Cluster cannot be described by a static Harris current sheet with a single set of parameters B(sub o), h, and z(sub o). Noting the presence of high-frequency fluctuations that appear to be superimposed on lower frequency variations, we average over running 6-min intervals and use the smoothed data to infer the parameters h(t) and z(sub o)(t), constrained by the pressure balance lobe magnetic field B(sub o)(t). Whereas this approach has been used in previous studies, the spatial gnuhen& now provided by the Cluster magnetometers were unavailable or not well constrained in earlier studies. We place the calculated hdf&cknessa in a magnetospheric context by examining the change in thickness with substorm phase for three case study events and 21 events in a superposed epoch analysis. We find that the inferred half-thickness in many cases reflects the nominal changes experienced by the plasma sheet during substorms (i.e., thinning during growth phase, thickening following substorm onset). We conclude with an analysis of the relative contribution of (Delta)B(sub z)/(Delta)X to the cross-tail current density during substorms. We find that (Delta)B(sub z)/(Delta)X can contribute a significant portion of the cross-tail c m n t around substorm onset.
Ramírez-Hernández, Abelardo; Peters, Brandon L; Schneider, Ludwig; Andreev, Marat; Schieber, Jay D; Müller, Marcus; de Pablo, Juan J
2017-01-07
Coarse grained simulation approaches provide powerful tools for the prediction of the equilibrium properties of polymeric systems. Recent efforts have sought to develop coarse-graining strategies capable of predicting the non-equilibrium behavior of entangled polymeric materials. Slip-link and slip-spring models, in particular, have been shown to be capable of reproducing several key aspects of the linear response and rheology of polymer melts. In this work, we extend a previously proposed multi-chain slip-spring model in a way that correctly incorporates the effects of the fluctuating environment in which polymer segments are immersed. The model is used to obtain the equation of state associated with the slip-springs, and the results are compared to those of related numerical approaches and an approximate analytical expression. The model is also used to examine a polymer melt confined into a thin film, where an inhomogeneous distribution of polymer segments is observed, and the corresponding inhomogeneities associated with density fluctuations are reflected on the spatial slip-spring distribution.
Current Density Scaling in Electrochemical Flow Capacitors
Hoyt, NC; Wainright, JS; Savinell, RF
2015-02-18
Electrochemical flow capacitors (EFCs) are a recently developed energy storage technology. One of the principal performance metrics of an EFC is the steady-state electrical current density that it can accept or deliver. Numerical models exist to predict this performance for specific cases, but here we present a study of how the current varies with respect to the applied cell voltage, flow rate, cell dimensions, and slurry properties using scaling laws. The scaling relationships are confirmed by numerical simulations and then subsequently by comparison to results from symmetric cell EFC experiments. This modeling approach permits the delimitation of three distinct operational regimes dependent on the values of two nondimensional combinations of the pertinent variables (specifically, a capacitive Graetz number and a conductivity ratio). Lastly, the models and nondimensional numbers are used to provide design guidance in terms of criteria for proper EFC operation. (C) The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.
Supersymmetry and fluctuation relations for currents in closed networks
Sinitsyn, Nikolai A.; Akimov, Alexei; Chernyak, Vladimir Y.
2011-02-15
We demonstrate supersymmetry in the counting statistics of stochastic particle currents and use it to derive exact nonperturbative relations for the statistics of currents induced by arbitrarily fast time-dependent protocols.
Elucidation of constant current density molecular plating
NASA Astrophysics Data System (ADS)
Vascon, A.; Santi, S.; Isse, A. A.; Reich, T.; Drebert, J.; Christ, H.; Düllmann, Ch. E.; Eberhardt, K.
2012-12-01
The production of thin layers by means of constant current or constant voltage electrolysis in organic media is commonly known as molecular plating. Despite the fact that this method has been applied for decades and is known to be among the most efficient ones for obtaining quantitative deposition, a full elucidation of the molecular plating is still lacking. In order to get a general understanding of the process and hence set the basis for further improvements of the method, constant current density electrolysis experiments were carried out in a mixture of isopropanol and isobutanol containing millimolar amounts of HNO3 together with [Nd(NO3)3·6H2O] used as a model electrolyte. The process was investigated by considering the influence of different parameters, namely the electrolyte concentrations (i.e., Nd(NO3)3·6H2O: 0.11, 0.22, 0.44 mM, and HNO3: 0.3, 0.4 mM), the applied current (i.e., 2 mA and 6 mA), and the surface roughness of the deposition substrates (i.e., a few tens to several hundreds of nm). The response of the process to changes of these parameters was monitored recording cell potential curves, which showed to be strongly influenced by the investigated conditions. The produced layers were characterized using γ-ray spectroscopy for the evaluation of Nd deposition yields, X-ray photoelectron spectroscopy for chemical analysis of the surfaces, and atomic force microscopy for surface roughness evaluation. X-ray photoelectron spectroscopy results clearly indicate that Nd is present only as Nd3+ on the cathodic surface after molecular plating. The results obtained from this characterization and some basic features inferred from the study of the cell potential curves were used to interpret the different behaviours of the deposition processes as a consequence of the applied variables.
Propagation of the lower hybrid wave in a density fluctuating scrape-off layer (SOL)
NASA Astrophysics Data System (ADS)
Madi, M.; Peysson, Y.; Decker, J.; Kabalan, K. Y.
2015-12-01
The perturbation of the lower hybrid wave (LH) power spectrum by fluctuations of the plasma in the vicinity of the antenna is investigated by solving the full wave equation in a slab geometry using COMSOL Multiphysics®. The numerical model whose generality allows to study the effect of various types of fluctuations, including those with short characteristic wavelengths is validated against a coupling code in quiescent regimes. When electron density fluctuations along the toroidal direction are incorporated in the dielectric tensor over a thin perturbed layer in front of the grill, the power spectrum may be strongly modified from the antenna mouth to the plasma separatrix as the LH wave propagates. The diffraction effect by density fluctuations leads to the appearance of multiple satellite lobes with randomly varying positions and the averaged perturbation is found to be maximum for the Fourier components of the fluctuating spectrum in the vicinity of the launched LH wavelength. This highlights that fast toroidal inhomogeneities with short characteristics length scales in front of the grill may change significantly the initial LH power spectrum used in coupled ray-tracing and Fokker-Planck calculations.
Fluctuation-Induced Particle Transport and Density Relaxation in a Stochastic Magnetic Field
NASA Astrophysics Data System (ADS)
Brower, David L.
2009-11-01
Particle transport and density relaxation associated with electromagnetic fluctuations is an unresolved problem of long standing in plasma physics and magnetic fusion research. In toroidal fusion plasmas, magnetic field fluctuations can arise spontaneously from global MHD instabilities, e.g., tearing fluctuations associated with sawtooth oscillations. Resonant magnetic perturbations (RMP) have also been externally imposed to mitigate the effect of edge localized modes (ELMs) by locally enhancing edge transport in Tokamaks. Understanding stochastic-field-driven transport processes is thus not only of basic science interest but possibly critical to ELM control in ITER. We report on the first direct measurement of magnetic fluctuation-induced particle transport in the core of a high-temperature plasma, the MST reversed field pinch. Measurements focus on the sawtooth crash, when the stochastic field resulting from tearing reconnection is strongest, and are accomplished using newly developed, laser-based, differential interferometry and Faraday rotation techniques. The measured electron particle flux, resulting from the correlated product of electron density (δn) and radial magnetic fluctuations (δbr), accounts for density profile relaxation during these magnetic reconnection events. Surprisingly, the electron diffusion is 30 times larger than estimates of ambipolarity-constrained transport in a stochastic magnetic field. A significant ion flux associated with parallel ion flow velocity fluctuations (δvi,//) correlated with δbr appears responsible for transport larger than predictions from the quasi-linear test particle model. These results indicate the need for improved understanding of particle transport in a stochastic magnetic field. Work performed in collaboration with W.X. Ding, W.F. Bergerson, T.F. Yates, UCLA; D.J. Den Hartog, G. Fiksel, S.C. Prager, J.S. Sarff and the MST Group, University of Wisconsin-Madison.
Anode current density distribution in a cusped field thruster
Wu, Huan Liu, Hui Meng, Yingchao; Zhang, Junyou; Yang, Siyu; Hu, Peng; Chen, Pengbo; Yu, Daren
2015-12-15
The cusped field thruster is a new electric propulsion device that is expected to have a non-uniform radial current density at the anode. To further study the anode current density distribution, a multi-annulus anode is designed to directly measure the anode current density for the first time. The anode current density decreases sharply at larger radii; the magnitude of collected current density at the center is far higher compared with the outer annuli. The anode current density non-uniformity does not demonstrate a significant change with varying working conditions.
Anode current density distribution in a cusped field thruster
NASA Astrophysics Data System (ADS)
Wu, Huan; Liu, Hui; Meng, Yingchao; Zhang, Junyou; Yang, Siyu; Hu, Peng; Chen, Pengbo; Yu, Daren
2015-12-01
The cusped field thruster is a new electric propulsion device that is expected to have a non-uniform radial current density at the anode. To further study the anode current density distribution, a multi-annulus anode is designed to directly measure the anode current density for the first time. The anode current density decreases sharply at larger radii; the magnitude of collected current density at the center is far higher compared with the outer annuli. The anode current density non-uniformity does not demonstrate a significant change with varying working conditions.
Telloni, D.; Antonucci, E.; Bruno, R.; D'Amicis, R.; Carbone, V.
2009-11-20
This paper investigates the evolution of the plasma density fluctuations of the fast and slow solar wind from the solar corona into the interplanetary space. The study is performed by comparing the low-frequency spectra and the phase correlation of the proton density oscillations, measured in the inner heliosphere with the Helios 2 in situ instrumentation, with those due to the large-scale density perturbations observed with UVCS/SOHO in the outer corona. We find that the characteristics of density fluctuations of the fast solar wind are maintained in the transition from the outer corona to the inner heliosphere, thus suggesting a coronal imprint for the heliospheric large-scale 1/f {sup 2} noise spectrum. In contrast, a quick dynamical evolution is observed in the slow wind, which, starting from large-scale fluctuations with strong phase correlations in the outer corona, gives rise to a Kolmogorov-like spectrum and an accumulation of density structures at small scales at 0.3 AU. This can be explained in the framework of nearly incompressible turbulence.
Two Point Space-Time Correlation of Density Fluctuations Measured in High Velocity Free Jets
NASA Technical Reports Server (NTRS)
Panda, Jayanta
2006-01-01
Two-point space-time correlations of air density fluctuations in unheated, fully-expanded free jets at Mach numbers M(sub j) = 0.95, 1.4, and 1.8 were measured using a Rayleigh scattering based diagnostic technique. The molecular scattered light from two small probe volumes of 1.03 mm length was measured for a completely non-intrusive means of determining the turbulent density fluctuations. The time series of density fluctuations were analyzed to estimate the integral length scale L in a moving frame of reference and the convective Mach number M(sub c) at different narrow Strouhal frequency (St) bands. It was observed that M(sub c) and the normalized moving frame length scale L*St/D, where D is the jet diameter, increased with Strouhal frequency before leveling off at the highest resolved frequency. Significant differences were observed between data obtained from the lip shear layer and the centerline of the jet. The wave number frequency transform of the correlation data demonstrated progressive increase in the radiative part of turbulence fluctuations with increasing jet Mach number.
The nucleation process and the roles of structure and density fluctuations in supercooled liquid Fe
Li, Rong; Wu, Yongquan Xiao, Junjiang
2014-01-21
We observed homogeneous nucleation process of supercooled liquid Fe by molecular dynamics simulations. Using bond-orientational order parameters together with Voronoi polyhedron method, we characterized local structure, calculated the volume of Voronoi polyhedra of atoms and identified the structure and density fluctuations. We monitored the formation of nucleus and analyzed its inner structure. The birth and growth of the pre-nucleus and nucleus are accompanied with aggregating and disaggregating processes in the time scale of femtosecond. Only the initial solid-like clusters (ISLC), ranging from 1 to 7 atoms, pop up directly from liquid. The relation between the logarithm of number of clusters and the cluster size was found to be linear for ISLCs and was observed to be parabolic for all solid-like clusters (SLC) due to aggregating and disaggregating effects. The nucleus and pre-nuclei mainly consist of body centered cubic (BCC) and hexagonal close packed atoms, while the BCC atoms tend to be located at the surface. Medium-range structure fluctuations induce the birth of ISLCs, benefit the aggregation of embryos and remarkably promote the nucleation. But density fluctuations contribute little to nucleation. The lifetime of most icosahedral-like atoms (ICO) is shorter than 0.7 ps. No obvious relationship was found between structure/density fluctuations and the appearance of ICO atoms.
The nucleation process and the roles of structure and density fluctuations in supercooled liquid Fe
NASA Astrophysics Data System (ADS)
Li, Rong; Wu, Yongquan; Xiao, Junjiang
2014-01-01
We observed homogeneous nucleation process of supercooled liquid Fe by molecular dynamics simulations. Using bond-orientational order parameters together with Voronoi polyhedron method, we characterized local structure, calculated the volume of Voronoi polyhedra of atoms and identified the structure and density fluctuations. We monitored the formation of nucleus and analyzed its inner structure. The birth and growth of the pre-nucleus and nucleus are accompanied with aggregating and disaggregating processes in the time scale of femtosecond. Only the initial solid-like clusters (ISLC), ranging from 1 to 7 atoms, pop up directly from liquid. The relation between the logarithm of number of clusters and the cluster size was found to be linear for ISLCs and was observed to be parabolic for all solid-like clusters (SLC) due to aggregating and disaggregating effects. The nucleus and pre-nuclei mainly consist of body centered cubic (BCC) and hexagonal close packed atoms, while the BCC atoms tend to be located at the surface. Medium-range structure fluctuations induce the birth of ISLCs, benefit the aggregation of embryos and remarkably promote the nucleation. But density fluctuations contribute little to nucleation. The lifetime of most icosahedral-like atoms (ICO) is shorter than 0.7 ps. No obvious relationship was found between structure/density fluctuations and the appearance of ICO atoms.
Characterising density fluctuations in liquid yttria aluminates with small angle x-ray scattering
Greaves, G. Neville; Wilding, Martin C.; Vu Van, Quang; Majerus, Odile; Hennet, Louis
2009-01-29
Small angle x-ray scattering (SAXS) has been measured in the wavevector range 0.01density fluctuations deriving from isothermal compressibility. With decreasing Q a minimum is located close to 0.1 A{sup -1} at the foot of the inter-atomic structure factor, below which SAXS rises, suggesting scatter from longer range fluctuating volumes.
Intermediate-k density and magnetic field fluctuations during inter-ELM pedestal evolution in MAST
NASA Astrophysics Data System (ADS)
Hillesheim, J. C.; Dickinson, D.; Roach, C. M.; Saarelma, S.; Scannell, R.; Kirk, A.; Crocker, N. A.; Peebles, W. A.; Meyer, H.; the MAST Team
2016-01-01
Measurements of local density and magnetic field fluctuations near the pedestal top, conditionally averaged over the edge localized mode (ELM) cycle, have been made in Mega Amp Spherical Tokamak (MAST). A Doppler backscattering (DBS) system installed at MAST was used to measure intermediate-k ≤ft({{k}\\bot}{ρi}≈ 3~\\text{to}~4\\right) density fluctuations at the top of the pedestal. A novel diagnostic technique combining DBS with cross-polarization scattering (CP-DBS) enabled magnetic field fluctuations to also be locally measured at similar wave numbers. Polarization isolation and other effects for CP-DBS are discussed. Both measurements were used in a series of high-β ≤ft({βn}≈ 4.0\\right. -4.5) MAST plasmas with large type-I ELMs with an ˜ 8~\\text{to}~9~\\text{ms} period where microtearing modes (MTMs) had been predicted to be unstable in similar conditions (Dickinson 2012 Phys. Rev. Lett. 108 135002). The measured density fluctuation level increased by a factor of about 4 between 2 and 4 ms after the ELM, which was correlated with the recovery of the density profile while the temperature pedestal height continued to increase slowly. Magnetic field fluctuations showed different temporal behaviors, slowly increasing throughout the ELM cycle as the local β increased. Linear GS2 calculations show both MTM and electron temperature gradient (ETG) modes unstable at similar wave numbers as the measurements (although with more overlap between ETG wave numbers and diagnostic spectral resolution) at the top of the pedestal, along with kinetic ballooning modes are unstable lower in the pedestal (at larger wavelengths). The inferred ratio of fluctuation levels from experiment was ≤ft(δ B/B\\right)/≤ft(δ n/n\\right)≈ 1/20 . The comparable ratios from GS2 were ≤ft(δ B/B\\right)/≤ft(δ n/n\\right)≈ 0.4 for the MTM and ≤ft(δ B/B\\right)/≤ft(δ n/n\\right)≈ 0.02 for the ETG. Both the experimental wave number range and the fluctuation ratio
Simultaneous Microwave Imaging System for Density and Temperature Fluctuation Measurements on TEXTOR
H. Park; E. Mazzucato; T. Munsat; C.W. Domier; M. Johnson; N.C. Luhmann, Jr.; J. Wang; Z. Xia; I.G.J. Classen; A.J.H. Donne; M.J. van de Pol
2004-05-07
Diagnostic systems for fluctuation measurements in plasmas have, of necessity, evolved from simple 1-D systems to multi-dimensional systems due to the complexity of the MHD and turbulence physics of plasmas illustrated by advanced numerical simulations. Using the recent significant advancements in millimeter wave imaging technology, Microwave Imaging Reflectometry (MIR) and Electron Cyclotron Emission Imaging (ECEI), simultaneously measuring density and temperature fluctuations, are developed for TEXTOR. The MIR system was installed on TEXTOR and the first experiment was performed in September, 2003. Subsequent MIR campaigns have yielded poloidally resolved spectra and assessments of poloidal velocity. The new 2-D ECE Imaging system (with a total of 128 channels), installed on TEXTOR in December, 2003, successfully captured a true 2-D images of Te fluctuations of m=1 oscillation (''sawteeth'') near the q {approx} 1 surface for the first time.
Anomalous Cross-Field Current and Fluctuating Equilibrium of Magnetized Plasmas
Rypdal, K.; Garcia, O.E.; Paulsen, J.
1997-09-01
It is shown by simple physical arguments and fluid simulations that electrostatic flute-mode fluctuations can sustain a substantial cross-field current in addition to mass and energy transport. The simulations show that this current determines essential features of the fluctuating plasma equilibrium, and explain qualitatively the experimental equilibria and the coherent flute-mode structures observed in a simple magnetized torus. {copyright} {ital 1997} {ital The American Physical Society}
Test of the Additivity Principle for Current Fluctuations in a Model of Heat Conduction
NASA Astrophysics Data System (ADS)
Hurtado, Pablo I.; Garrido, Pedro L.
2009-06-01
The additivity principle allows to compute the current distribution in many one-dimensional (1D) nonequilibrium systems. Using simulations, we confirm this conjecture in the 1D Kipnis-Marchioro-Presutti model of heat conduction for a wide current interval. The current distribution shows both Gaussian and non-Gaussian regimes, and obeys the Gallavotti-Cohen fluctuation theorem. We verify the existence of a well-defined temperature profile associated to a given current fluctuation. This profile is independent of the sign of the current, and this symmetry extends to higher-order profiles and spatial correlations. We also show that finite-time joint fluctuations of the current and the profile are described by the additivity functional. These results suggest the additivity hypothesis as a general and powerful tool to compute current distributions in many nonequilibrium systems.
Test of the additivity principle for current fluctuations in a model of heat conduction.
Hurtado, Pablo I; Garrido, Pedro L
2009-06-26
The additivity principle allows to compute the current distribution in many one-dimensional (1D) nonequilibrium systems. Using simulations, we confirm this conjecture in the 1D Kipnis-Marchioro-Presutti model of heat conduction for a wide current interval. The current distribution shows both Gaussian and non-Gaussian regimes, and obeys the Gallavotti-Cohen fluctuation theorem. We verify the existence of a well-defined temperature profile associated to a given current fluctuation. This profile is independent of the sign of the current, and this symmetry extends to higher-order profiles and spatial correlations. We also show that finite-time joint fluctuations of the current and the profile are described by the additivity functional. These results suggest the additivity hypothesis as a general and powerful tool to compute current distributions in many nonequilibrium systems.
Particle Image Velocimetry Study of Density Current Fronts
ERIC Educational Resources Information Center
Martin, Juan Ezequiel
2009-01-01
Gravity currents are flows that occur when a horizontal density difference causes fluid to move under the action of gravity; density currents are a particular case, for which the scalar causing the density difference is conserved. Flows with a strong effect of the horizontal density difference, even if only partially driven by it--such as the…
NASA Astrophysics Data System (ADS)
Marinoni, A.; Rost, J. C.; Porkolab, M.; Hubbard, A. E.; Osborne, T. H.; White, A. E.; Whyte, D. G.; Rhodes, T. L.; Davis, E. M.; Ernst, D. R.; Burrell, K. H.
2015-09-01
The I-mode regime, routinely observed on the Alcator C-Mod tokamak, is characterized by an edge energy transport barrier without an accompanying particle barrier and with broadband instabilities, known as weakly coherent modes (WCM), believed to regulate particle transport at the edge. Recent experiments on the DIII-D tokamak exhibit I-mode characteristics in various physical quantities. These DIII-D plasmas evolve over long periods, lasting several energy confinement times, during which the edge electron temperature slowly evolves towards an H-mode-like profile, while maintaining a typical L-mode edge density profile. During these periods, referred to as I-mode phases, the radial electric field at the edge also gradually reaches values typically observed in H-mode. Density fluctuations measured with the phase contrast imaging diagnostic during I-mode phases exhibit three features typically observed in H-mode on DIII-D, although they develop progressively with time and without a sharp transition: the intensity of the fluctuations is reduced; the frequency spectrum is broadened and becomes non-monotonic; two dimensional space-time spectra appear to approach those in H-mode, showing phase velocities of density fluctuations at the edge increasing to about 10 km s-1. However, in DIII-D there is no clear evidence of the WCM. Preliminary linear gyro-kinetic simulations are performed in the pedestal region with the GS2 code and its recently upgraded model collision operator that conserves particles, energy and momentum. The increased bootstrap current and flow shear generated by the temperature pedestal are shown to decrease growth rates, thus possibly generating a feedback mechanism that progressively stabilizes fluctuations.
High current density cathode for electrorefining in molten electrolyte
Li, Shelly X.
2010-06-29
A high current density cathode for electrorefining in a molten electrolyte for the continuous production and collection of loose dendritic or powdery deposits. The high current density cathode eliminates the requirement for mechanical scraping and electrochemical stripping of the deposits from the cathode in an anode/cathode module. The high current density cathode comprises a perforated electrical insulated material coating such that the current density is up to 3 A/cm.sup.2.
Effect of environment fluctuations on a Josephson current
NASA Astrophysics Data System (ADS)
Galaktionov, A. V.
2017-01-01
An influence of an electromagnetic environment on a Josephson current through a tunnel junction is studied with the aid of Ambegaokar-Eckern-Schön effective action. Two types of environment are investigated: one, characterized by a resonant mode, and an ohmic one. The crossover to a Josephson π-junction is possible for both of them. In addition the resonant-mode environment results in an increase of a Josephson current when the ratio of the doubled superconducting gap to the frequency of the mode is close to an integer number.
Logarithmic current fluctuations in nonequilibrium quantum spin chains.
Antal, T; Krapivsky, P L; Rákos, A
2008-12-01
We study zero-temperature quantum spin chains, which are characterized by a nonvanishing current. For the XX model starting from the initial state mid R:cdots, three dots, centered upward arrow upward arrow upward arrow downward arrow downward arrow downward arrowcdots, three dots, centered we derive an exact expression for the variance of the total spin current. We show that asymptotically the variance exhibits an anomalously slow logarithmic growth; we also extract the subleading constant term. We then argue that the logarithmic growth remains valid for the XXZ model in the critical region.
Brookman, M. W. Austin, M. E.; Petty, C. C.
2015-12-10
Theoretical work, computation, and results from TCV [J. Decker “Effect of density fluctuations on ECCD in ITER and TCV,” EPJ Web of Conf. 32, 01016 (2012)] suggest that density fluctuations in the edge region of a tokamak plasma can cause broadening of the ECH deposition profile. In this paper, a GUI tool is presented which is used for analysis of ECH deposition as a first step towards looking for this broadening, which could explain effects seen in previous DIII-D ECH transport studies [K.W. Gentle “Electron energy transport inferences from modulated electron cyclotron heating in DIII-D,” Phys. Plasmas 13, 012311 (2006)]. By applying an FFT to the T{sub e} measurements from the University of Texas’s 40-channel ECE Radiometer, and using a simplified thermal transport equation, the flux surface extent of ECH deposition is determined. The Fourier method analysis is compared with a Break-In-Slope (BIS) analysis and predictions from the ray-tracing code TORAY. Examination of multiple Fourier harmonics and BIS fitting methods allow an estimation of modulated transport coefficients and thereby the true ECH deposition profile. Correlations between edge fluctuations and ECH deposition in legacy data are also explored as a step towards establishing a link between fluctuations and deposition broadening in DIII-D.
NASA Astrophysics Data System (ADS)
Brookman, M. W.; Austin, M. E.; Petty, C. C.
2015-12-01
Theoretical work, computation, and results from TCV [J. Decker "Effect of density fluctuations on ECCD in ITER and TCV," EPJ Web of Conf. 32, 01016 (2012)] suggest that density fluctuations in the edge region of a tokamak plasma can cause broadening of the ECH deposition profile. In this paper, a GUI tool is presented which is used for analysis of ECH deposition as a first step towards looking for this broadening, which could explain effects seen in previous DIII-D ECH transport studies [K.W. Gentle "Electron energy transport inferences from modulated electron cyclotron heating in DIII-D," Phys. Plasmas 13, 012311 (2006)]. By applying an FFT to the Te measurements from the University of Texas's 40-channel ECE Radiometer, and using a simplified thermal transport equation, the flux surface extent of ECH deposition is determined. The Fourier method analysis is compared with a Break-In-Slope (BIS) analysis and predictions from the ray-tracing code TORAY. Examination of multiple Fourier harmonics and BIS fitting methods allow an estimation of modulated transport coefficients and thereby the true ECH deposition profile. Correlations between edge fluctuations and ECH deposition in legacy data are also explored as a step towards establishing a link between fluctuations and deposition broadening in DIII-D.
Current density partitioning in time-dependent current density functional theory
Mosquera, Martín A.; Wasserman, Adam
2014-05-14
We adapt time-dependent current density functional theory to allow for a fragment-based solution of the many-electron problem of molecules in the presence of time-dependent electric and magnetic fields. Regarding a molecule as a set of non-interacting subsystems that individually evolve under the influence of an auxiliary external electromagnetic vector-scalar potential pair, the partition 4-potential, we show that there are one-to-one mappings between this auxiliary potential, a sharply-defined set of fragment current densities, and the total current density of the system. The partition electromagnetic (EM) 4-potential is expressed in terms of the real EM 4-potential of the system and a gluing EM 4-potential that accounts for exchange-correlation effects and mutual interaction forces between fragments that are required to yield the correct electron dynamics. We prove the zero-force theorem for the fragmented system, establish a variational formulation in terms of action functionals, and provide a simple illustration for a charged particle in a ring.
ATP transport through a single mitochondrial channel, VDAC, studied by current fluctuation analysis.
Rostovtseva, T K; Bezrukov, S M
1998-01-01
The "molecular Coulter counter" concept has been used to study transport of ATP molecules through the nanometer-scale aqueous pore of the voltage-dependent mitochondrial ion channel, VDAC. We examine the ATP-induced current fluctuations and the change in average current through a single fully open channel reconstituted into a planar lipid bilayer. At high salt concentration (1 M NaCl), the addition of ATP reduces both solution conductivity and channel conductance, but the effect on the channel is several times stronger and shows saturation behavior even at 50 mM ATP concentration. These results and simple steric considerations indicate pronounced attraction of ATP molecules to VDAC's aqueous pore and permit us to evaluate the effect of a single ATP molecule on channel conductance. ATP addition also generates an excess noise in the ionic current through the channel. Analysis of this excess noise shows that its spectrum is flat in the accessible frequency interval up to several kilohertz. ATP exchange between the pore and the bulk is fast enough not to display any dispersion at these frequencies. By relating the low-frequency spectral density of the noise to the equilibrium diffusion of ATP molecules in the aqueous pore, we calculate a diffusion coefficient D = (1.6-3.3)10(-11) m2/s. This is one order of magnitude smaller than the ATP diffusion coefficient in the bulk, but it agrees with recent results on ATP flux measurements in multichannel membranes using the luciferin/luciferase method. PMID:9591663
Interaction between the lower hybrid wave and density fluctuations in the scrape-off layer
Peysson, Y.; Madi, M.; Kabalan, K.; Decker, J.
2015-12-10
In the present paper, the perturbation of the launched power spectrum of the Lower Hybrid wave at the separatrix by electron density fluctuations in the scrape-off layer is investigated. Considering a slab geometry with magnetic field lines parallel to the toroidal direction, the full wave equation is solved using Comsol Multiphysics® for a fully active multi-junction like LH antenna made of two modules. When electron density fluctuations are incorporated in the dielectric tensor over a thin perturbed layer in front of the grill, it is shown that the power spectrum may be strongly modified from the antenna mouth to the plasma separatrix as the wave propagates. The diffraction effect leads to the appearance of multiple satellite lobes with randomly varying positions, a feature consistent with the recently developed model that has been applied successfully to high density discharges on the Tokamak Tore Supra corresponding to the large spectral gap regime [Decker J. et al. Phys. Plasma 21 (2014) 092504]. The perturbation is found to be maximum for the Fourier components of the fluctuating spectrum in the vicinity of the launched LH wavelength.
Interaction between the lower hybrid wave and density fluctuations in the scrape-off layer
NASA Astrophysics Data System (ADS)
Peysson, Y.; Madi, M.; Decker, J.; Kabalan, K.
2015-12-01
In the present paper, the perturbation of the launched power spectrum of the Lower Hybrid wave at the separatrix by electron density fluctuations in the scrape-off layer is investigated. Considering a slab geometry with magnetic field lines parallel to the toroidal direction, the full wave equation is solved using Comsol Multiphysics® for a fully active multi-junction like LH antenna made of two modules. When electron density fluctuations are incorporated in the dielectric tensor over a thin perturbed layer in front of the grill, it is shown that the power spectrum may be strongly modified from the antenna mouth to the plasma separatrix as the wave propagates. The diffraction effect leads to the appearance of multiple satellite lobes with randomly varying positions, a feature consistent with the recently developed model that has been applied successfully to high density discharges on the Tokamak Tore Supra corresponding to the large spectral gap regime [Decker J. et al. Phys. Plasma 21 (2014) 092504]. The perturbation is found to be maximum for the Fourier components of the fluctuating spectrum in the vicinity of the launched LH wavelength.
NASA Astrophysics Data System (ADS)
Novikov, D. V.; Krasovskiĭ, A. N.; Osmolovskaya, N. A.; Efremov, V. I.
2007-02-01
The specific features of the transformation of a polymer solution into a solid state (film) of an amorphous polymer are investigated using electron microscopy. The correspondence between the characteristics of fractal macromolecular aggregates in a solution and the parameters of the spatial distribution of density fluctuations at the surface of the film is established using a linear atactic poly(styrene) as an example. The correspondence exists under the condition that the packing density of coils does not exceed a critical value at the liquid-solid phase transition point and the polymer concentration in the solution provides the formation of a continuous network of entangled macromolecules.
Nonlinear saturation spectra of electric fields and density fluctuations in drift wave turbulence
NASA Technical Reports Server (NTRS)
Kelley, M. C.
1982-01-01
The detection of drift waves in the nonlinear evolution of a space plasma process driven at long wavelengths is considered, adducing measurements of the electric field and density fluctuation power spectra as evidence. Since the driving mechanism is clearly at long wavelengths, the detection of drift waves suggests that they may play an important role in the transfer of wave energy from long to short wavelengths in a low beta plasma. The saturated spectral density is compared with theoretical results in order to estimate the anomalous diffusion rate. The observed spectral form and amplitude is in excellent agreement with drift wave predictions.
NASA Technical Reports Server (NTRS)
Massey, G. A.; Lemon, C. J.
1984-01-01
A tunable line-narrowed ArF laser can selectively excite several rotation al lines of the Schumann-Runge band system of O2 in air. The resulting ultraviolet fluorescence can be monitored at 90 deg to the laser beam axis, permitting space and time resolved observation of density and temperature fluctuations in turbulence. Experiments and calculations show that + or - 1 K, + or - 1 percent density, 1 cu mm spatial, and 1 microsecond temporal resolution can be achieved simultaneously under some conditions.
A new interferometry-based electron density fluctuation diagnostic on Alcator C-Mod.
Kasten, C P; Irby, J H; Murray, R; White, A E; Pace, D C
2012-10-01
The two-color interferometry diagnostic on the Alcator C-Mod tokamak has been upgraded to measure fluctuations in the electron density and density gradient for turbulence and transport studies. Diagnostic features and capabilities are described. In differential mode, fast phase demodulation electronics detect the relative phase change between ten adjacent, radially-separated (ΔR = 1.2 cm, adjustable), vertical-viewing chords, which allows for measurement of the line-integrated electron density gradient. The system can be configured to detect the absolute phase shift of each chord by comparison to a local oscillator, measuring the line-integrated density. Each chord is sensitive to density fluctuations with k(R) < 20.3 cm(-1) and is digitized at up to 10 MS/s, resolving aspects of ion temperature gradient-driven modes and other long-wavelength turbulence. Data from C-Mod discharges is presented, including observations of the quasi-coherent mode in enhanced D-alpha H-mode plasmas and the weakly coherent mode in I-mode.
A new interferometry-based electron density fluctuation diagnostic on Alcator C-Moda)
NASA Astrophysics Data System (ADS)
Kasten, C. P.; Irby, J. H.; Murray, R.; White, A. E.; Pace, D. C.
2012-10-01
The two-color interferometry diagnostic on the Alcator C-Mod tokamak has been upgraded to measure fluctuations in the electron density and density gradient for turbulence and transport studies. Diagnostic features and capabilities are described. In differential mode, fast phase demodulation electronics detect the relative phase change between ten adjacent, radially-separated (ΔR = 1.2 cm, adjustable), vertical-viewing chords, which allows for measurement of the line-integrated electron density gradient. The system can be configured to detect the absolute phase shift of each chord by comparison to a local oscillator, measuring the line-integrated density. Each chord is sensitive to density fluctuations with kR < 20.3 cm-1 and is digitized at up to 10 MS/s, resolving aspects of ion temperature gradient-driven modes and other long-wavelength turbulence. Data from C-Mod discharges is presented, including observations of the quasi-coherent mode in enhanced D-alpha H-mode plasmas and the weakly coherent mode in I-mode.
Anders, Andre; Oks, Efim
2005-12-22
Current fluctuations of cathodic arcs were recorded withhigh analog bandwidth (up to 1 GHz) and fast digital sampling (up to 5Gsamples/sec). The power spectral density of the arc current wasdetermined by fast Fourier transform clearly showing material dependent,non-linear features in the frequency domain. These features can beassociated with the non-linear impedance of the conducting channelbetween cathode and anode, driven by the explosive nature of electronemission and plasma formation. The characteristic times of less than 100ns can be associated with individual explosive processes, "ectons," andtherefore represent the short-time physical cutoff for the fractal modelof cathodic arcs.
NASA Astrophysics Data System (ADS)
Antonov, A. N.; Gaidarov, M. K.; Sarriguren, P.; Moya de Guerra, E.
2016-07-01
The volume and surface components of the nuclear symmetry energy (NSE) and their ratio are calculated within the coherent density fluctuation model (CDFM). The estimations use the results of the model for the NSE in finite nuclei based on the Brueckner energy-density functional for nuclear matter. In addition, we present results for the NSE and its volume and surface contributions obtained by using the Skyrme energy-density functional. The CDFM weight function is obtained using the proton and neutron densities from the self-consistent HF+BCS method with Skyrme interactions. We present and discuss the values of the volume and surface contributions to the NSE and their ratio obtained for the Ni, Sn, and Pb isotopic chains, studying their isotopic sensitivity. The results are compared with estimations of other approaches which have used available experimental data on binding energies, neutron-skin thicknesses, excitation energies to isobaric analog states (IAS), and also with results of other theoretical methods.
NASA Astrophysics Data System (ADS)
Schunck, M.; Hegmann, M.; Sedlmayr, E.
2007-01-01
We investigate the effects of stochastic density fluctuations on the dust temperatures and the resulting infrared (IR) emission spectra of interstellar clouds as an extension of preceding investigations by Hegmann & Kegel. We consider absorption and scattering by dust grains in spherical clouds which are, on average, homogeneous but have a fluctuating density. The spatial variation of the density is described by means of a Markov process. This clump model introduces two parameters: the correlation length ln and the Gaussian width σn of the density fluctuations. As the intensity Iλ,n inherits the randomness of the density n, the ordinary radiative transfer equation has to be replaced by a generalized transfer equation of Fokker-Planck type. In the first part, we investigate the influence of our model parameters on the radiative transport in the ultraviolet (UV) and use the results to calculate the dust temperature in radiative equilibrium. Afterwards, the IR emission of the dust is modelled for the same set of clump parameters. We find that the presence of clumps decreases the effective extinction and therefore leads to substantial differences in UV illumination and dust temperatures, compared with the homogeneous case. Because of the distribution of dust temperatures, the presence of clumps also affects the IR emission and thus possible observations. In the second part, we use a fit with two blackbody spectra to determine the cloud dust mass from our synthetic IR fluxes. It is shown that in a clumpy environment the overall dust mass is generally underestimated. This effect correlates with the degree of cloud fragmentation.
Bluhm, Christian; Scheu, Stefan; Maraun, Mark
2016-04-01
We investigated the oribatid mite density, community structure and the percentage of parthenogenetic individuals in four different forest types across three regions in Germany in 2008 and once again in 2011. We compared temporal (inter-annual) fluctuations in population densities between sexually and parthenogenetically reproducing species of oribatid mites. We hypothesized that population densities in parthenogenetic oribatid mite species fluctuate more than in sexual ones. Further, we expected species composition and dominance of parthenogenetic species to differ between forest types and regions. Oribatid mite community structure did not differ between years but varied with forest type and region, indicating low species turnover in time. As hypothesized, temporal fluctuations were more pronounced in parthenogenetic as compared to sexual species. The percentage of parthenogenetic individuals was significantly higher in coniferous than in beech forests and significantly higher in Schorfheide-Chorin than in Hainich-Dün and Schwäbische Alb. The results indicate that parthenogenetic species flourish if populations are controlled by density-independent factors and dominate at sites were resources are plentiful and easily available, such as coniferous forests, and in regions with more acidic soils and thick organic layers, such as Schorfheide-Chorin. However, historical factors also may have contributed to the increased dominance of parthenogenetic species in the Schorfheide-Chorin, as this region was more heavily glaciated and this may have favoured parthenogenetic species. Overall, our study supports the hypothesis that parthenogenetic species benefit from the lack of density-dependent population control whereas the opposite is true for sexual species.
FAST TRACK COMMUNICATION: Current fluctuations in stochastic systems with long-range memory
NASA Astrophysics Data System (ADS)
Harris, R. J.; Touchette, H.
2009-08-01
We propose a method to calculate the large deviations of current fluctuations in a class of stochastic particle systems with history-dependent rates. Long-range temporal correlations are seen to alter the speed of the large deviation function in analogy with long-range spatial correlations in equilibrium systems. We give some illuminating examples and discuss the applicability of the Gallavotti-Cohen fluctuation theorem.
Synoptic fluctuation of the Taiwan Warm Current in winter on the East China Sea shelf
NASA Astrophysics Data System (ADS)
Xuan, Jiliang; Huang, Daji; Pohlmann, Thomas; Su, Jian; Mayer, Bernhard; Ding, Ruibin; Zhou, Feng
2017-02-01
The seasonal mean and synoptic fluctuation of the wintertime Taiwan Warm Current (TWC) were investigated using a well-validated finite volume community ocean model. The spatial distribution and dynamics of the synoptic fluctuation were highlighted. The seasonal mean of the wintertime TWC has two branches: an inshore branch between the 30 and 100 m isobaths and an offshore branch between the 100 and 200 m isobaths. The Coriolis term is much larger than the inertia term and is almost balanced by the pressure gradient term in both branches, indicating geostrophic balance of the mean current. Two areas with significant fluctuations of the TWC were identified during wintertime. One of the areas is located to the north of Taiwan with velocities varying in the cross-shore direction. These significant cross-shore fluctuations are driven by barotropic pressure gradients associated with the intrusion of the Taiwan Strait Current (TSC). When a strong TSC intrudes to the north of Taiwan, the isobaric slope tilts downward from south to north, leading to a cross-shore current from the coastal area to the offshore area. When the TSC intrusion is weak, the cross-shore current to the north of Taiwan is directed from offshore to inshore. The other area of significant fluctuation is located in the inshore area between the 30 and 100 m isobaths. The fluctuations are generally strong both in the alongshore and cross-shore directions, in particular at the latitudes 26.5 and 28° N. Wind affects the synoptic fluctuation through episodic events. When the northeasterly monsoon prevails, the southwestward Zhe-Min coastal current dominates the inshore area associated with a deepening of the mixed layer. When the winter monsoon is weakened or the southwesterly wind prevails, the northeastward TWC dominates in the inshore area.
Romanenko, S G; Tokarev, O P; Vasilenko, Iu S
2001-01-01
Intralaryngeal electrostimulation of the laryngeal muscles with fluctuating currents with simultaneous mobilization of the arytenoid cartilage and paralysed vocal cord were used in 42 patients with unilateral laryngeal paralysis. The treatment was combined with phonopedic lessons. The control group consisted of 32 patients receiving standard electrostimulation with diadynamic currents. The effect was evaluated by changes in vocal acoustic parameters and stroboscopic parameters. In patients with paramedian fixation of the vocal cords voice improvement was obtained irrespective of electrostimulation type. For patients with intermedian and lateral fixation of the vocal cords more effective was intralaryngeal electrostimulation with fluctuating currents. A good therapeutic effect was achieved in patients with dislocation of the arytenoid cartilage.
Cao, Qifo; Liu, Yong; Zhao, Hailin; Zhou, Tianfu; Ti, Ang; Hu, Liqun
2016-11-01
A system to simultaneously diagnose the electron temperature and density fluctuations is proposed for Experimental Advanced Superconducting Tokamak device. This system includes a common quasi-optical antenna, a correlation electron cyclotron emission (CECE) system that is used to measure the electron temperature fluctuations and a Doppler backscattering (DBS) system that is used to measure the electron density fluctuations. The frequency range of the proposed CECE system is 108-120 GHz, and this corresponds to a radial coverage of normalized radius ((R - R0)/a, R0 = 1850 mm, a = 450 mm) from 0.2 to 0.67 for the plasma operation with a toroidal magnetic field of 2.26 T. This paper focuses on the design of the quasi-optical antenna and aims at optimizing the poloidal resolution for different frequency bands. An optimum result gives the beam radius for the CECE system of 13-15 mm and this corresponds to a wave number range of kθ < 2.4 cm(-1). The beam radius is 20-30 mm for V band (50-75 GHz) and 15-20 mm for W band (75-110 GHz).
NASA Astrophysics Data System (ADS)
Cao, Qifo; Liu, Yong; Zhao, Hailin; Zhou, Tianfu; Ti, Ang; Hu, Liqun
2016-11-01
A system to simultaneously diagnose the electron temperature and density fluctuations is proposed for Experimental Advanced Superconducting Tokamak device. This system includes a common quasi-optical antenna, a correlation electron cyclotron emission (CECE) system that is used to measure the electron temperature fluctuations and a Doppler backscattering (DBS) system that is used to measure the electron density fluctuations. The frequency range of the proposed CECE system is 108-120 GHz, and this corresponds to a radial coverage of normalized radius ((R - R0)/a, R0 = 1850 mm, a = 450 mm) from 0.2 to 0.67 for the plasma operation with a toroidal magnetic field of 2.26 T. This paper focuses on the design of the quasi-optical antenna and aims at optimizing the poloidal resolution for different frequency bands. An optimum result gives the beam radius for the CECE system of 13-15 mm and this corresponds to a wave number range of kθ < 2.4 cm-1. The beam radius is 20-30 mm for V band (50-75 GHz) and 15-20 mm for W band (75-110 GHz).
Density and magnetic fluctuations at JET: experimental observation and numerical characterization
NASA Astrophysics Data System (ADS)
de Masi, Gianluca; Predebon, Italo; Spagnolo, Silvia; Lupelli, Ivan; Hillesheim, Jon; Meneses, Luis; Maggi, Costanza; Delabie, Ephrem; JET Contributors Team
2016-10-01
Density and magnetic fluctuations have been experimentally observed on JET in the inter ELM phases in low beta discharges.They have been characterized in terms of typical frequency range (60-80 kHz), wavenumber (0.01 <=ky ρi <=0.1), radial localization (pedestal top) and correlation with the relevant kinetic quantities. A linear simulation with gyrokinetic code GENE, matching the experimental edge condition has been performed to gain insight on their possible physical interpretation. ITG modes turn out to be the most unstable modes for 0 <=ky ρi <=1, while microtearing modes (MTMs) are the dominant instabilities for ky ρi <= 0.1.A typical oscillation frequency of about 50-100 kHz is associated to both unstable modes, with opposite propagation direction.Different considerations suggest an interpretation in terms of MTMs for the observed magnetic fluctuations, while density fluctuations appear to be dominated by ITG instabilities. EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK.
Impact of density and environmental factors on population fluctuations in a migratory passerine.
Pasinelli, Gilberto; Schaub, Michael; Häfliger, Guido; Frey, Monika; Jakober, Hans; Müller, Mathis; Stauber, Wolfgang; Tryjanowski, Piotr; Zollinger, Jean-Luc; Jenni, Lukas
2011-01-01
1. Populations of plants and animals typically fluctuate because of the combined effects of density-dependent and density-independent processes. The study of these processes is complicated by the fact that population sizes are typically not known exactly, because population counts are subject to sampling variance. Although the existence of sampling variance is broadly acknowledged, relatively few studies on time-series data have accounted for it, which can result in wrong inferences about population processes. 2. To increase our understanding of population dynamics, we analysed time series from six Central European populations of the migratory red-backed shrike Lanius collurio by simultaneously assessing the strength of density dependence, process and sampling variance. In addition, we evaluated hypotheses predicting effects of factors presumed to operate on the breeding grounds, at stopover sites in eastern Africa during fall and spring migration and in the wintering grounds in southern Africa. We used both simple and state-space formulations of the Gompertz equation to model population size. 3. Across populations and modelling approaches, we found consistent evidence for negative density-dependent population regulation. Further, process variance contributed substantially to variation in population size, while sampling variance did not. Environmental conditions in eastern and southern Africa appear to influence breeding population size, as rainfall in the Sahel during fall migration and in the south African wintering areas were positively related to population size in the following spring in four of six populations. In contrast, environmental conditions in the breeding grounds were not related to population size. 4. Our findings suggest negative density-dependent regulation of red-backed shrike breeding populations and are consistent with the long-standing hypothesis that conditions in the African staging and wintering areas influence population numbers of species
Scaling laws of turbulence and heating of fast solar wind: the role of density fluctuations.
Carbone, V; Marino, R; Sorriso-Valvo, L; Noullez, A; Bruno, R
2009-08-07
Incompressible and isotropic magnetohydrodynamic turbulence in plasmas can be described by an exact relation for the energy flux through the scales. This Yaglom-like scaling law has been recently observed in the solar wind above the solar poles observed by the Ulysses spacecraft, where the turbulence is in an Alfvénic state. An analogous phenomenological scaling law, suitably modified to take into account compressible fluctuations, is observed more frequently in the same data set. Large-scale density fluctuations, despite their low amplitude, thus play a crucial role in the basic scaling properties of turbulence. The turbulent cascade rate in the compressive case can, moreover, supply the energy dissipation needed to account for the local heating of the nonadiabatic solar wind.
Schroeder, Indra
2015-01-01
Abstract A main ingredient for the understanding of structure/function correlates of ion channels is the quantitative description of single-channel gating and conductance. However, a wealth of information provided from fast current fluctuations beyond the temporal resolution of the recording system is often ignored, even though it is close to the time window accessible to molecular dynamics simulations. This kind of current fluctuations provide a special technical challenge, because individual opening/closing or blocking/unblocking events cannot be resolved, and the resulting averaging over undetected events decreases the single-channel current. Here, I briefly summarize the history of fast-current fluctuation analysis and focus on the so-called “beta distributions.” This tool exploits characteristics of current fluctuation-induced excess noise on the current amplitude histograms to reconstruct the true single-channel current and kinetic parameters. A guideline for the analysis and recent applications demonstrate that a construction of theoretical beta distributions by Markov Model simulations offers maximum flexibility as compared to analytical solutions. PMID:26368656
Schroeder, Indra
2015-01-01
A main ingredient for the understanding of structure/function correlates of ion channels is the quantitative description of single-channel gating and conductance. However, a wealth of information provided from fast current fluctuations beyond the temporal resolution of the recording system is often ignored, even though it is close to the time window accessible to molecular dynamics simulations. This kind of current fluctuations provide a special technical challenge, because individual opening/closing or blocking/unblocking events cannot be resolved, and the resulting averaging over undetected events decreases the single-channel current. Here, I briefly summarize the history of fast-current fluctuation analysis and focus on the so-called "beta distributions." This tool exploits characteristics of current fluctuation-induced excess noise on the current amplitude histograms to reconstruct the true single-channel current and kinetic parameters. A guideline for the analysis and recent applications demonstrate that a construction of theoretical beta distributions by Markov Model simulations offers maximum flexibility as compared to analytical solutions.
NASA Technical Reports Server (NTRS)
Woo, R.; Armstrong, J. W.
1979-01-01
Solar wind electron density power spectra in the solar equatorial region are inferred from observations of phase scintillations and spectral broadening made with the Viking, Helios, and Pioneer spacecraft. The heliocentric distance range covered is 2-215 solar radii and for some observations close to the sun the spectra extend to fluctuation frequencies as high as 100 Hz. For heliocentric distances of about 20 solar radii the equivalent spacecraft-measured one-dimensional density spectrum is well modeled by a single power law in the frequency range 0.0001-0.05 Hz. The flattening of the density spectrum within 20 solar radii is presumably associated with energy deposition in the near-sun region and acceleration of the solar wind.
NASA Technical Reports Server (NTRS)
Asenov, Asen; Slavcheva, G.; Brown, A. R.; Davies, J. H.; Saini, S.
2000-01-01
In this paper we present a detailed simulation study of the influence of quantum mechanical effects in the inversion layer on random dopant induced threshold voltage fluctuations and lowering in sub 100 nm MOSFETs. The simulations have been performed using a 3-D implementation of the density gradient (DG) formalism incorporated in our established 3-D atomistic simulation approach. This results in a self-consistent 3-D quantum mechanical picture, which implies not only the vertical inversion layer quantisation but also the lateral confinement effects related to current filamentation in the 'valleys' of the random potential fluctuations. We have shown that the net result of including quantum mechanical effects, while considering statistical dopant fluctuations, is an increase in both threshold voltage fluctuations and lowering. At the same time, the random dopant induced threshold voltage lowering partially compensates for the quantum mechanical threshold voltage shift in aggressively scaled MOSFETs with ultrathin gate oxides.
Theory of small-scale density and electric field fluctuations in the nightside Venus ionosphere
NASA Technical Reports Server (NTRS)
Huba, J. D.
1992-01-01
Recently, it has been reported that small-scale (lambda about 0.1-2 km) density irregularities occur during 100-Hz electric field bursts in the nightside ionosphere of Venus. This paper provides a detailed analysis of the lower-hybrid-drift instability as a mechanism to generate the observed irregularities. A fully electromagnetic theory is developed that is relevant to the finite beta plasma in Venus's ionosphere and includes collisional effects (e.g., electron-ion, electron-neutral, and ion-neutral collisions). The key features of the analysis that favor this instability are the following: (1) it is a flute mode and propagates orthogonal to the ambient magnetic field; (2) it is a relatively short wavelength mode and the Doppler-shifted frequency can be greater than about 100 Hz; (3) it can produce both electric field and density fluctuations, as well as magnetic field fluctuations in a finite beta plasma; and (4) it is most unstable in low-beta plasmas so that it is likely to occur in the low-density, high-magnetic-field ionospheric holes. These features are consistent with observational results.
Current density filaments measured by electrostatic, magnetic and optical diagnostics in RFX-mod
NASA Astrophysics Data System (ADS)
Vianello, N.; Spolaore, M.; Agostini, M.; Antoni, V.; Cavazzana, R.; Martines, E.; Serianni, G.; Scarin, P.; Spada, E.; Zuin, M.
2008-11-01
Edge turbulence is ubiquitous in fusion devices and characterized by the formation of coherent structures which are believed to play a relevant role in driving particle losses. These structures have been observed also in the edge region of the RFX-mod Reversed Field Pinch device. In order to gain insight into their origin and features an original probe system has been used measuring both magnetic and electrostatic fluctuations simultaneously and on the same location with a high time resolution. This insertable probe head allows the direct measurements of several plasma parameters including local vorticity patterns and current density fluctuations. An array of toroidally distributed sensors allows following the structures along the main flow direction. It is found that in the cross-field plane bursts correspond to pressure structures and are related to current density filaments mainly oriented along the magnetic field. These results are compared with those provided by the measured relationship between structures on HeI emitted radiation observed by the Gas Puffing Imaging system and magnetic fluctuations. The presence and features of the current density filaments are assessed at different plasma current regimes.
Communication: Linking the dielectric Debye process in mono-alcohols to density fluctuations
NASA Astrophysics Data System (ADS)
Hecksher, Tina
2016-04-01
This work provides the first direct evidence that the puzzling dielectric Debye process observed in mono-alcohols is coupled to density fluctuations. The results open up for an explanation of the Debye process within the framework of conventional liquid-state theory. The spectral shape of the dynamical bulk modulus of the two studied mono-alcohols, 2-ethyl-1-hexanol and 4-methyl-3-heptanol, is nearly identical to that of their corresponding shear modulus, and thus the supramolecular structures believed to be responsible for the slow dielectric Debye process are manifested in the bulk modulus in the same way as in the shear modulus.
Pradhan, Prabhakar; Damania, Dhwanil; Turzhitsky, Vladimir; Subramanian, Hariharan; Backman, Vadim; Joshi, Hrushikesh M.; Dravid, Vinayak P.; Roy, Hemant K.; Taflove, Allen
2010-12-13
We report a study of the nanoscale mass-density fluctuations of heterogeneous optical dielectric media, including nanomaterials and biological cells, by quantifying their nanoscale light-localization properties. Transmission electron microscope images of the media are used to construct corresponding effective disordered optical lattices. Light-localization properties are studied by the statistical analysis of the inverse participation ratio (IPR) of the localized eigenfunctions of these optical lattices at the nanoscale. We validated IPR analysis using nanomaterials as models of disordered systems fabricated from dielectric nanoparticles. As an example, we then applied such analysis to distinguish between cells with different degrees of aggressive malignancy.
NASA Astrophysics Data System (ADS)
Ding, W. X.; Lin, L.; Duff, J. R.; Brower, D. L.; Sarff, J. S.
2012-10-01
The multichannel polarimeter-interferometer system on the MST reversed-field pinch can be utilized to measure far-forward collective scattering from electron density fluctuations. The collective scattering system has 11 viewing chords with ˜8 cm spacing. The source is a 432 μm (694 GHz) far infrared laser and the scattered power is measured using a heterodyne detection scheme. Collective scattering provides a line-integrated measurement of fluctuations within the divergence of the probe beam covering wavenumber range: k⊥ < 1.3 cm-1, corresponding k⊥ρs < 1.3 (ρs is the ion-sound Larmor radius), the region of primary interest for turbulent fluctuation-induced transport. The perpendicular wavenumber consists of toroidal, poloidal, and radial contributions, which vary with chord position. Coherent modes associated with tearing instabilities and neutral-beam driven fast particles are observed along with broadband turbulence at frequencies up to 500 kHz. Changes in frequency are consistent with a Doppler shift due to parallel plasma flow.
Ding, W. X.; Lin, L.; Brower, D. L.; Duff, J. R.; Sarff, J. S.
2012-10-15
The multichannel polarimeter-interferometer system on the MST reversed-field pinch can be utilized to measure far-forward collective scattering from electron density fluctuations. The collective scattering system has 11 viewing chords with {approx}8 cm spacing. The source is a 432 {mu}m (694 GHz) far infrared laser and the scattered power is measured using a heterodyne detection scheme. Collective scattering provides a line-integrated measurement of fluctuations within the divergence of the probe beam covering wavenumber range: k{sub Up-Tack} < 1.3 cm{sup -1}, corresponding k{sub Up-Tack }{rho}{sub s} < 1.3 ({rho}{sub s} is the ion-sound Larmor radius), the region of primary interest for turbulent fluctuation-induced transport. The perpendicular wavenumber consists of toroidal, poloidal, and radial contributions, which vary with chord position. Coherent modes associated with tearing instabilities and neutral-beam driven fast particles are observed along with broadband turbulence at frequencies up to 500 kHz. Changes in frequency are consistent with a Doppler shift due to parallel plasma flow.
Initial density fluctuation measurements from the NSTX Beam Emission Spectroscopy diagnostic system*
NASA Astrophysics Data System (ADS)
Smith, D. R.; Fonck, R. J.; McKee, G. R.; Schoenbeck, N. L.; Thompson, D.; Uzun-Kaymak, I. U.; Stratton, B. C.
2010-11-01
Density fluctuation measurements on the ion gyroscale have been obtained on NSTX with a newly commissioned beam emission spectroscopy (BES) diagnostic system. The BES system measures red-shifted Dα emission near 660 nm from deuterium neutral beams with high throughput optics and high efficiency detectors. The system presently employs 16 detection channels arranged in radial and poloidal arrays, and an expansion to 32 channels is planned. Radial arrays can measure fluctuations from r/a 0.1 to beyond the last closed flux surface and resolve fluctuations with kρi<=1.5. Initial BES measurements reveal broadband turbulence and coherent modes below 300 kHz for r/a>=0.4. The broadband turbulence appears in high gradient regions and increases at H-L transitions. The frequency characteristics of the coherent modes correlate with Alfvén/energetic particle modes in Mirnov probe measurements, but some coherent modes appear in BES measurements only. *Supported by the U.S. Department of Energy under Contract Nos. DE-FG02-89ER53296, DE-AC02-09CH11466 and DE-SC0001288.
Optical observation of spin-density-wave fluctuations in Ba122 iron-based superconductors
NASA Astrophysics Data System (ADS)
Xu, B.; Dai, Y. M.; Xiao, H.; Shen, B.; Ye, Z. R.; Forget, A.; Colson, D.; Feng, D. L.; Wen, H. H.; Qiu, X. G.; Lobo, R. P. S. M.
2016-08-01
In iron-based superconductors, a spin-density-wave (SDW) magnetic order is suppressed with doping, and unconventional superconductivity appears in close proximity to the SDW instability. The optical response of the SDW order shows clear gap features: substantial suppression in the low-frequency optical conductivity, alongside a spectral weight transfer from low to high frequencies. Here, we study the detailed temperature dependence of the optical response in three different series of the Ba122 system [Ba1 -xKxFe2As2 , Ba (Fe1-xCox) 2As2 , and BaFe2(As1-xPx) 2 ]. Intriguingly, we find that the suppression of the low-frequency optical conductivity and spectral weight transfer appear at a temperature T* much higher than the SDW transition temperature TSDW. Since this behavior has the same optical feature and energy scale as the SDW order, we attribute it to SDW fluctuations. Furthermore, T* is suppressed with doping, closely following the doping dependence of the nematic fluctuations detected by other techniques. These results suggest that the magnetic and nematic orders have an intimate relationship, in favor of the magnetic-fluctuation-driven nematicity scenario in iron-based superconductors.
Characterization of density fluctuations during ELMs in the DIII-D tokamak
NASA Astrophysics Data System (ADS)
Coda, S.; Porkolab, M.; Burrell, K. H.
2001-12-01
Bursts of turbulence associated with ELMs have been studied systematically in DIII-D with a multichannel phase contrast imaging (PCI) diagnostic, which is sensitive to the long poloidal wavelength components of the density fluctuations in the outer edge of the tokamak. A comparison of the temporal dynamics of the turbulence with the signature Dα signal from the divertor has revealed systematic differences between type I and type III ELMs: even though precursor fluctuations are sometimes seen before type I ELMs, the PCI signal level remains high until the peak in the Dα signal; by contrast, in type III ELMs the fluctuation burst precedes the Dα peak by 0.4-0.6 ms. Type I ELMs can generate `echoes', i.e. secondary bursts, in the scrape-off layer. Coherent modes are observed during type III ELMs only. The radial and temporal correlation structures and the spectral properties of the turbulence during the transient ELM phase have been reconstructed by averaging over multiple ELMs, in order to improve the statistical accuracy. ELM turbulence is found to share many properties with L mode turbulence, including the main qualitative features of radial wavenumber and frequency spectra and radial dispersion relations. However, features unique to ELM turbulence are also identified.
NASA Technical Reports Server (NTRS)
Smith, M.
1972-01-01
Fluctuations in electron density and temperature coupled through Ohm's law are studied for an ionizable medium. The nonlinear effects are considered in the limit of a third order quasi-linear treatment. Equations are derived for the amplitude of the fluctuation. Conditions under which a steady state can exist in the presence of the fluctuation are examined and effective transport properties are determined. A comparison is made to previously considered second order theory. The effect of third order terms indicates the possibility of fluctuations existing in regions predicted stable by previous analysis.
NASA Technical Reports Server (NTRS)
Smith, J. M.
1972-01-01
Fluctuations in electron density and temperature coupled through Ohm's law are studied for an ionizable medium. The nonlinear effects are considered in the limit of a third order quasi-linear treatment. Equations are derived for the amplitude of the fluctuation. Conditions under which a steady state can exist in the presence of the fluctuation are examined and effective transport properties are determined. A comparison is made to previously considered second order theory. The effect of third order terms indicates the possibility of fluctuations existing in regions predicted stable by previous analysis.
Stochastic Faraday rotation induced by the electric current fluctuations in nanosystems
NASA Astrophysics Data System (ADS)
Smirnov, D. S.; Glazov, M. M.
2017-01-01
We demonstrate theoretically that in gyrotropic semiconductors and semiconductor nanosystems the Brownian motion of electrons results in temporal fluctuations of the polarization plane of light passing through or reflected from the structure, i.e., in stochastic Faraday or Kerr rotation effects. The theory of the effects is developed for a number of prominent gyrotropic systems such as bulk tellurium, ensembles of chiral carbon nanotubes, and GaAs-based quantum wells of different crystallographic orientations. We show that the power spectrum of these fluctuations in thermal equilibrium is proportional to the a c conductivity of the system. We evaluate contributions resulting from the fluctuations of the electric current, as well as of spin, valley polarization, and the spin current to the noise of the Faraday/Kerr rotation. Hence all-optical measurements of the Faraday and Kerr rotation noise provide an access to the transport properties of the semiconductor systems.
Craig, Darren J.G.
1998-09-01
Two techniques are employed in the Madison Symmetric Torus (MST) to test and control different aspects of fluctuation induced transport in the Reversed Field Pinch (RFP). Auxiliary edge currents are driven along the magnetic field to modify magnetic fluctuations, and the particle and energy transport associated with them. In addition, strong edge flows are produced by plasma biasing. Their effect on electrostatic fluctuations and the associated particle losses is studied. Both techniques are accomplished using miniature insertable plasma sources that are biased negatively to inject electrons. This type of emissive electrode is shown to reliably produce intense, directional current without significant contamination by impurities. The two most important conclusions derived from these studies are that the collective modes resonant at the reversal surface play a role in global plasma confinement, and that these modes can be controlled by modifying the parallel current profile outside of the reversal surface. This confirms predictions based on magnetohydrodynamic (MHD) simulations that auxiliary current drive in the sense to flatten the parallel current profile can be successful in controlling magnetic fluctuations in the RFP. However, these studies expand the group of magnetic modes believed to cause transport in MST and suggest that current profile control efforts need to address both the core resonant magnetic modes and those resonant at the reversal surface. The core resonant modes are not significantly altered in these experiments; however, the distribution and/or amplitude of the injected current is probably not optimal for affecting these modes. Plasma biasing generates strong edge flows with shear and particle confinement likely improves in these discharges. These experiments resemble biased H modes in other magnetic configurations in many ways. The similarities are likely due to the common role of electrostatic fluctuations in edge transport.
Current Density and Plasma Displacement Near Perturbed Rational Surface
A.H. Boozer and N. Pomphrey
2010-10-10
The current density in the vicinity of a rational surface of a force-free magnetic field subjected to an ideal perturbation is shown to be the sum of both a smooth and a delta-function distribution, which give comparable currents. The maximum perturbation to the smooth current density is comparable to a typical equilibrium current density and the width of the layer in which the current flows is shown to be proportional to the perturbation amplitude. In the standard linearized theory, the plasma displacement has an unphysical jump across the rational surface, but the full theory gives a continuous displacement.
Theory of tokamak equilibria with central current density reversal.
Wang, Shaojie
2004-10-08
It is found that, with a model current profile, the Grad-Shafranov equation can be reduced to the Helmholtz equation, which can describe a variety of equilibrium configurations. With the eigenvalue problem solved in the toroidal coordinate system, an analytical solution to the Grad-Shafranov equation is found. It is demonstrated that current reversal equilibrium configurations exist with finite radial gradient of plasma pressure and continuous current density, and that current density reversal is accompanied by pressure gradient reversal.
Measurement of temperature and density fluctuations in turbulence using an ultraviolet laser
NASA Technical Reports Server (NTRS)
Massey, G. A.
1984-01-01
Noninvasive measurement of density and temperature fluctuations in turbulent air flow was examined. The approach used fluorescence of oxygen molecules which are selectively excited by a tunable vacuum ultraviolet laser beam. The strength of the fluorescence signal and its dependence on laser wavelength vary with the density and temperature of the air in the laser beam. Because fluorescence can be detected at 90 degrees from the beam propagation direction, spatial resolution in three dimensions, rather than path-integrated measurements can be achieved. With spatial resolutions of the order of a millimeter and at supersonic air velocities it is necessary to perform each measurement in a time of the order of a microsecond; this is possible by by using laser pulses of ten nanosecond duration. In this method atmospheric O2 is excited by the emission of a tunable ArF excimer laser, and the fluorescence, which spans the 210 to 420 range, is detected by an ultraviolet phototube.
Critical density fluctuations in lipid bilayers detected by fluorescence lifetime heterogeneity.
Ruggiero, A; Hudson, B
1989-01-01
The heterogeneity of the decay of the fluorescence of transparinaric acid in single-component lipid bilayers at temperatures above their gel/liquid crystalline phase transition is shown to be due to the presence of regions of higher local density and higher acyl chain order than the predominant fluid regions. This conclusion is based on selective excitation behavior and the observation of time-resolved fluorescence anisotropies that increase at long times. The fractional amplitude of the long lifetime component of the fluorescence shows a temperature variation that conforms to conventional descriptions of critical behavior. The critical exponent extracted from this variation is 1.1, close to the value of 1.0 that describes ultrasonic data. We therefore conclude that liquid crystalline lipid bilayers exhibit critical behavior with significant density and order fluctuations. This behavior must be taken into account in the interpretation of fluorescence and other spectroscopic measurements of the properties of bilayers. PMID:2765649
Density fluctuation spectrum of solar wind turbulence between ion and electron scales.
Chen, C H K; Salem, C S; Bonnell, J W; Mozer, F S; Bale, S D
2012-07-20
We present a measurement of the spectral index of density fluctuations between ion and electron scales in solar wind turbulence using the EFI instrument on the ARTEMIS spacecraft. The mean spectral index at 1 AU was found to be -2.75±0.06, steeper than predictions for pure whistler or kinetic Alfvén wave turbulence but consistent with previous magnetic field measurements. The steep spectra are also consistent with expectations of increased intermittency or damping of some of the turbulent energy over this range of scales. Neither the spectral index nor the flattening of the density spectra before ion scales were found to depend on the proximity to the pressure anisotropy instability thresholds, suggesting that they are features inherent to the turbulent cascade.
NASA Astrophysics Data System (ADS)
Klatt, Michael A.; Torquato, Salvatore
2016-08-01
In the first paper of this series, we introduced Voronoi correlation functions to characterize the structure of maximally random jammed (MRJ) sphere packings across length scales. In the present paper, we determine a variety of different correlation functions that arise in rigorous expressions for the effective physical properties of MRJ sphere packings and compare them to the corresponding statistical descriptors for overlapping spheres and equilibrium hard-sphere systems. Such structural descriptors arise in rigorous bounds and formulas for effective transport properties, diffusion and reactions constants, elastic moduli, and electromagnetic characteristics. First, we calculate the two-point, surface-void, and surface-surface correlation functions, for which we derive explicit analytical formulas for finite hard-sphere packings. We show analytically how the contact Dirac delta function contribution to the pair correlation function g2(r ) for MRJ packings translates into distinct functional behaviors of these two-point correlation functions that do not arise in the other two models examined here. Then we show how the spectral density distinguishes the MRJ packings from the other disordered systems in that the spectral density vanishes in the limit of infinite wavelengths; i.e., these packings are hyperuniform, which means that density fluctuations on large length scales are anomalously suppressed. Moreover, for all model systems, we study and compute exclusion probabilities and pore size distributions, as well as local density fluctuations. We conjecture that for general disordered hard-sphere packings, a central limit theorem holds for the number of points within an spherical observation window. Our analysis links problems of interest in material science, chemistry, physics, and mathematics. In the third paper of this series, we will evaluate bounds and estimates of a host of different physical properties of the MRJ sphere packings that are based on the
Rayleigh Scattering Diagnostic Used to Measure Velocity and Density Fluctuation Spectra
NASA Technical Reports Server (NTRS)
Seasholtz, Richard G.; Panda, Jayanta; Elam, Kristie A.
2003-01-01
A new, molecular Rayleigh-scattering-based flow diagnostic developed at the NASA Glenn Research Center has been used for the first time to measure the power spectrum of both gas density and radial velocity components in the plumes of high-speed jets. The objective of the work is to develop an unseeded, nonintrusive dynamic measurement technique for studying turbulent flows in NASA test facilities. This technique provides aerothermodynamic data not previously obtainable. It is particularly important for supersonic flows, where hot wire and pitot probes are difficult to use and disturb the flow under study. The effort is part of the nonintrusive instrumentation development program supporting propulsion research at the NASA Glenn Research Center. In particular, this work is measuring fluctuations in flow velocity, density, and temperature for jet noise studies. These data are valuable to researchers studying the correlation of flow fluctuations with far-field noise. One of the main objectives in jet noise research is to identify noise sources in the jet and to determine their contribution to noise generation. The technique is based on analyzing light scattered from molecules within the jet using a Fabry-Perot interferometer operating in a static imaging mode. The PC-based data acquisition system can simultaneously sample velocity and density data at rates to about 100 kHz and can handle up to 10 million data records. We used this system to interrogate three different jet nozzle designs in a Glenn free-jet facility. Each nozzle had a 25.4-mm exit diameter. One was convergent, used for subsonic flow measurements and to produce a screeching underexpanded jet with a fully expanded Mach number of 1.42. The other nozzles (Mach 1.4 and 1.8) were convergent-divergent types. The radial component of velocity and gas density were simultaneously measured in this work.
Pedestal density fluctuation dynamics during the inter-ELM cycle in DIII-D
Yan, Z.; McKee, G. R.; Groebner, R. J.; Snyder, P. B.; Osborne, T. H.; Burrell, K. H.; Beurskens, M. N.
2011-05-15
Detailed 2D measurements of long-wavelength density fluctuations in the pedestal region with beam emission spectroscopy during the period between edge localized modes (ELMs) indicate two distinct bands of fluctuations propagating in opposite poloidal directions in the plasma frame: one lower frequency band (50-150 kHz) advects in the ion-diamagnetic drift direction (ion mode) and a higher frequency band (200-400 kHz) advects in the electron diamagnetic drift direction (electron mode). The ion mode amplitude is modulated with the ELM cycle: it increases rapidly after an ELM and then saturates, similar to the evolution of the pedestal electron pressure and density gradients. The electron mode, in contrast, has no significant time evolution between ELMs. The decorrelation time of the ion mode is <5 {mu}s[{tau}{sub c}(c{sub s}/c{sub s}aa){<=}1], the radial correlation length is of order 10 {rho}{sub i} and has poloidal wave-number k{sub {theta}{rho}i{approx}}0.1, and the mode advects at near the ion diamagnetic velocity in the plasma frame. These spatiotemporal dynamics are qualitatively similar to features predicted for kinetic ballooning modes.
Pedestal density fluctuation dynamics during the inter-ELM cycle in DIII-D a)
NASA Astrophysics Data System (ADS)
Yan, Z.; McKee, G. R.; Groebner, R. J.; Snyder, P. B.; Osborne, T. H.; Beurskens, M. N.; Burrell, K. H.
2011-05-01
Detailed 2D measurements of long-wavelength density fluctuations in the pedestal region with beam emission spectroscopy during the period between edge localized modes (ELMs) indicate two distinct bands of fluctuations propagating in opposite poloidal directions in the plasma frame: one lower frequency band (50-150 kHz) advects in the ion-diamagnetic drift direction (ion mode) and a higher frequency band (200-400 kHz) advects in the electron diamagnetic drift direction (electron mode). The ion mode amplitude is modulated with the ELM cycle: it increases rapidly after an ELM and then saturates, similar to the evolution of the pedestal electron pressure and density gradients. The electron mode, in contrast, has no significant time evolution between ELMs. The decorrelation time of the ion mode is <5 μs [τc(cs/csa a)≤1], the radial correlation length is of order 10 ρi and has poloidal wave-number kθρi~0.1, and the mode advects at near the ion diamagnetic velocity in the plasma frame. These spatiotemporal dynamics are qualitatively similar to features predicted for kinetic ballooning modes.
Random-matrix-theory approach to mesoscopic fluctuations of heat current.
Schmidt, Martin; Kottos, Tsampikos; Shapiro, Boris
2013-08-01
We consider an ensemble of fully connected networks of N oscillators coupled harmonically with random springs and show, using random-matrix-theory considerations, that both the average phonon heat current and its variance are scale invariant and take universal values in the large N limit. These anomalous mesoscopic fluctuations is the hallmark of strong correlations between normal modes.
Effect of the electromagnetic environment on current fluctuations in driven tunnel junctions
NASA Astrophysics Data System (ADS)
Frey, Moritz; Grabert, Hermann
2016-07-01
We examine current fluctuations in tunnel junctions driven by a superposition of a constant and a sinusoidal voltage source. In standard setups, the external voltage is applied to the tunneling element via an impedance providing an electromagnetic environment of the junction. The modes of this environment are excited by the time-dependent voltage and are the source of Johnson-Nyquist noise. We determine the autocorrelation function of the current flowing in the leads of the junction in the weak tunneling limit up to terms of second order in the tunneling Hamiltonian. The driven modes of the electromagnetic environment are treated exactly by means of a unitary transformation introduced recently. Particular emphasis is placed on the spectral function of the current fluctuations. The spectrum is found to comprise three contributions: a term arising from the Johnson-Nyquist noise of the environmental impedance, a part due to the shot noise of the tunneling element, and a third contribution which comes from the cross correlation between fluctuations caused by the electromagnetic environment and fluctuations of the tunneling current. All three parts of the spectral function occur already for devices under dc bias. The spectral function of ac driven tunneling elements can be determined from the result for a dc bias by means of a photoassisted tunneling relation of the Tien-Gordon type. Specific results are given for an Ohmic environment and for a junction driven through a resonator.
Note: A real-time beam current density meter
Liu Junliang; Yu Deyang; Ruan Fangfang; Xue Yingli; Wang Wei
2013-03-15
We have developed a real-time beam current density meter for charged particle beams. It measures the mean current density by collimating a uniform and large diameter primary beam. The suppression of the secondary electrons and the deflection of the beam were simulated, and it was tested with a 105 keV Ar{sup 7+} ion beam.
Study of Density Fluctuations and Particle Transport at the Edge of I-Mode Plasmas
NASA Astrophysics Data System (ADS)
Dominguez, Arturo
The wide range of plasma parameters available on Alcator C-Mod has led to the accessibility of many regimes of operation. Since its commissioning, C-Mod has accessed the Linear ohmic confinement, Saturated ohmic confinement, L-Mode and ELM-free, ELMy and Enhanced Dalpha H-Mode regimes. Recently, another novel regime, the I-Mode, has been identified. I-modes feature the presence of steep H-Mode-like electron and ion temperature gradients at the edge of the plasma with L-Mode-like density profiles. The I-Mode, in contrast to the H-mode, shows very weak degradation of energy confinement with increased input power, and routinely reaches H 98 > 1 while operating at low edge collisionalities ( n*ped ˜0.1), making it a good candidate for reactor relevant tokamaks. Also relevant for reactors, this regime can be sustained in steady state for more than ˜15 energy confinement times without the need for ELMs to regulate particle and impurity confinement. Changes in edge density, temperature and magnetic field fluctuations accompany the L-mode to I-mode transition, with reduction of fluctuations in the 50--150kHz range as well as the appearance of a Weakly Coherent Mode (WCM) in the 200-300kHz range, analogous to the Quasi-Coherent Mode (QCM) characteristic of the Enhanced D alpha H-mode. Previous work has established a connection between the midrange fluctuation suppression and reduction in the effective thermal diffusivity, chi eff, in the pedestal region. The mechanism in I-mode for maintaining sufficient particle transport to avoid impurity accumulation and instabilities has been unclear. The O-mode reflectometry system has been extensively used for the characterization and detection of the I-mode and the WCM, in part, enhanced by upgrades to the system which enabled the broadband detection of density fluctuations at an array of cutoff locations at the edge of the plasma. Using a novel model, the autopower signals of reflectometry channels detecting the density
Universal current fluctuations in the symmetric exclusion process and other diffusive systems
NASA Astrophysics Data System (ADS)
Akkermans, Eric; Bodineau, Thierry; Derrida, Bernard; Shpielberg, Ohad
2013-07-01
Using the macroscopic fluctuation theory of Bertini, De Sole, Gabrielli, Jona-Lasinio, and Landim, one can show that the statistics of the current of the symmetric simple exclusion process (SSEP) connected to two reservoirs on an arbitrary large finite domain in dimension d are the same as in the one-dimensional case. Numerical results on squares support this claim while results on cubes exhibit some discrepancy. We argue that the results of the macroscopic fluctuation theory should be recovered by increasing the size of the contacts. The generalization to other diffusive systems is straightforward.
Cosmological Inflation with Multiple Fields and the Theory of Density Fluctuations
NASA Astrophysics Data System (ADS)
van Tent, B. J. W.
2002-09-01
Inflation is a stage of extremely rapid expansion in the very early universe. It was proposed to solve a number of problems in the standard Big Bang theory. In particular it others an explanation for the origin of structures like (clusters of) galaxies on the one hand (by generating small density fluctuations that act as gravitational seeds), and for the largescale homogeneity of the universe on the other hand (because of the enormous expansion). Inflation is driven by one or more scalar fields with an appropriate potential. In this thesis we develop an analytical formalism to describe the generation of density fluctuations during inflation with multiple scalar fields. We allow these fields to live on a non-trivial (curved) field manifold, as is often the case in high-energy theories. We also treat the evolution of the fluctuations after inflation, until the time of recombination when the cosmic microwave background radiation was formed. Using our formalism observations of the CMBR can then be used to set constraints on the parameters in (multiple-field) inflation models. In more detail this thesis covers the following topics. After introductory chapters on cosmology in general and single-field inflation, the theory of inflation with multiple fields and a general (non-trivial) field metric is derived. In particular we introduce a basis in field space that is induced by the background dynamics and allows a clear distinction between effectively single-field and truly multiple-field effects. The important slow-roll approximation is generalized to the case of multiple fields. Next we derive how scalar and tensor fluctuations are generated from a quantum origin during multiple-field inflation, paying special attention to the transition that occurs when a perturbation mode crosses the Hubble scale. Using some simplifying assumptions the evolution of both adiabatic and isocurvature perturbation modes after inflation is treated. The final results are expressions for the
Individual differences in transcranial electrical stimulation current density
Russell, Michael J; Goodman, Theodore; Pierson, Ronald; Shepherd, Shane; Wang, Qiang; Groshong, Bennett; Wiley, David F
2013-01-01
Transcranial electrical stimulation (TCES) is effective in treating many conditions, but it has not been possible to accurately forecast current density within the complex anatomy of a given subject's head. We sought to predict and verify TCES current densities and determine the variability of these current distributions in patient-specific models based on magnetic resonance imaging (MRI) data. Two experiments were performed. The first experiment estimated conductivity from MRIs and compared the current density results against actual measurements from the scalp surface of 3 subjects. In the second experiment, virtual electrodes were placed on the scalps of 18 subjects to model simulated current densities with 2 mA of virtually applied stimulation. This procedure was repeated for 4 electrode locations. Current densities were then calculated for 75 brain regions. Comparison of modeled and measured external current in experiment 1 yielded a correlation of r = .93. In experiment 2, modeled individual differences were greatest near the electrodes (ten-fold differences were common), but simulated current was found in all regions of the brain. Sites that were distant from the electrodes (e.g. hypothalamus) typically showed two-fold individual differences. MRI-based modeling can effectively predict current densities in individual brains. Significant variation occurs between subjects with the same applied electrode configuration. Individualized MRI-based modeling should be considered in place of the 10-20 system when accurate TCES is needed. PMID:24285948
Sawabe, Kosuke; Imakawa, Masaki; Nakano, Masaki; Yamao, Takeshi; Hotta, Shu; Iwasa, Yoshihiro; Takenobu, Taishi
2012-12-04
Extremely high current densities are realized in single-crystal ambipolar light-emitting transistors using an electron-injection buffer layer and a current-confinement structure via laser etching. Moreover, a linear increase in the luminance was observed at current densities of up to 1 kA cm(-2) , which is an efficiency-preservation improvement of three orders of magnitude over conventional organic light-emitting diodes (OLEDs) at high current densities.
The physicist's companion to current fluctuations: one-dimensional bulk-driven lattice gases
NASA Astrophysics Data System (ADS)
Lazarescu, Alexandre
2015-12-01
One of the main features of statistical systems out of equilibrium is the currents they exhibit in their stationary state: microscopic currents of probability between configurations, which translate into macroscopic currents of mass, charge, etc. Understanding the general behaviour of these currents is an important step towards building a universal framework for non-equilibrium steady states akin to the Gibbs-Boltzmann distribution for equilibrium systems. In this review, we consider one-dimensional bulk-driven particle gases, and in particular the asymmetric simple exclusion process (ASEP) with open boundaries, which is one of the most popular models of one-dimensional transport. We focus, in particular, on the current of particles flowing through the system in its steady state, and on its fluctuations. We show how one can obtain the complete statistics of that current, through its large deviation function, by combining results from various methods: exact calculation of the cumulants of the current, using the integrability of the model; direct diagonalization of a biased process in the limits of very high or low current; hydrodynamic description of the model in the continuous limit using the macroscopic fluctuation theory. We give a pedagogical account of these techniques, starting with a quick introduction to the necessary mathematical tools, as well as a short overview of the existing works relating to the ASEP. We conclude by drawing the complete dynamical phase diagram of the current. We also remark on a few possible generalizations of these results.
On bottom density currents on the continental shelves
NASA Technical Reports Server (NTRS)
Anuchin, V. N.; Gusev, A. M.; Pyrkin, Y. G.; Khapayev, M. M.
1975-01-01
The turbulent characteristics of bottom density currents on the continental shelves and their influence on the vertical profiles of current velocities are studied by considering plane parallel flows of a liquid with one density in a motionless liquid and with lighter density along an inclined plane. The motion of the liquid is a result of gravitational force directed along the parallel plane. Vertical distribution of turbulent stress is determined from a known average velocity profile and is used to obtain the vertical profile of the average current velocity.
Structure and Function of Intra–Annual Density Fluctuations: Mind the Gaps
Battipaglia, Giovanna; Campelo, Filipe; Vieira, Joana; Grabner, Michael; De Micco, Veronica; Nabais, Cristina; Cherubini, Paolo; Carrer, Marco; Bräuning, Achim; Čufar, Katarina; Di Filippo, Alfredo; García-González, Ignacio; Koprowski, Marcin; Klisz, Marcin; Kirdyanov, Alexander V.; Zafirov, Nikolay; de Luis, Martin
2016-01-01
Tree rings are natural archives of climate and environmental information with a yearly resolution. Indeed, wood anatomical, chemical, and other properties of tree rings are a synthesis of several intrinsic and external factors, and their interaction during tree growth. In particular, Intra-Annual Density Fluctuations (IADFs) can be considered as tree-ring anomalies that can be used to better understand tree growth and to reconstruct past climate conditions with intra-annual resolution. However, the ecophysiological processes behind IADF formation, as well as their functional impact, remain unclear. Are IADFs resulting from a prompt adjustment to fluctuations in environmental conditions to avoid stressful conditions and/or to take advantage from favorable conditions? In this paper we discuss: (1) the influence of climatic factors on the formation of IADFs; (2) the occurrence of IADFs in different species and environments; (3) the potential of new approaches to study IADFs and identify their triggering factors. Our final aim is to underscore the advantages offered by network analyses of data and the importance of high-resolution measurements to gain insight into IADFs formation processes and their relations with climatic conditions, including extreme weather events. PMID:27200063
NASA Astrophysics Data System (ADS)
Duff, James; Chapman, Brett; Sarff, John; Ding, Weixing; Brower, David; Lin, Liang
2012-10-01
In standard RFP plasmas, transport is governed by magnetic fluctuations associated with global tearing modes. For improved-confinement plasmas using inductive current profile control (PPCD), smaller-scale fluctuations at higher frequencies might become important for transport, especially drift-wave-like instabilities which may be theoretically unstable for the larger temperature gradients achieved. On the MST-RFP, an 11-chord laser-based diagnostic with ˜8 cm chord spacing is and frequency 694 GHz used to measure electron density fluctuations both interferometrically and by far-forward collective scattering. The existing diagnostic configuration measures the line-integrated fluctuations within the divergence of the probe beam covering a wavenumber range k<1.3 cm-1, corresponding to kρs <1.3 (ρs is the ion-sound Larmor radius). Of particular interest is comparing fluctuations in standard and PPCD plasmas. Relative to standard plasmas, tearing mode and higher frequency broadband fluctuations (up to 600 kHz) are suppressed with PPCD. This suppression in PPCD plasmas corresponds to the improved confinement. A diagnostic upgrade, in progress, will improve sensitivity and cover shorter wavelengths. Work supported by U.S.D.O.E.
X-ray Fluctuation Power Spectral Density Survey of Six Seyfert 1 Galaxies
NASA Astrophysics Data System (ADS)
Markowitz, A.
2002-05-01
By combining low-density RXTE long- and medium-term monitoring with high-density, short-term monitoring from XMM and Chandra long-looks, we have constructed X-ray fluctuation power spectral densities (PSDs) for six Seyfert 1 galaxies. These PSDs cover unprecedented dynamic ranges, continuously spanning up to or beyond 4 orders of magnitude in temporal frequency. The PSDs of four targets show significant flattening towards lower frequencies and bear remarkable similarity to X-ray Binary PSDs, strengthening the argument that similar emission processes occur in both types of compact accreting systems, spanning a factor of ~106-7 in luminosity and putative black hole mass. Assuming a linear mass-timescale relation, the resulting PSD break frequencies imply black hole masses which generally agree with reverberation-mapped mass estimates. If the geometric origin of the variability is close to the X-ray corona, then the physical timescales associated with thermal and acoustic disk variations may be relevant.
Density fluctuation dynamics in a dissipative self-gravitating dilute gas revisited
NASA Astrophysics Data System (ADS)
Méndez, A. R.; García-Perciante, A. L.
2016-11-01
The analysis of the behavior of density fluctuations in a dissipative self gravitating gas in the linear regime is revisited. A factorization for the dispersion relation given by approximate roots is proposed, which is analogous to the one introduced in the case without gravitational field. The threshold for the onset of a gravitational instability, namely Jeans wavenumber, is found to be unaltered by the presence of thermal and viscous dissipation. However, the behavior of damped modes does not correspond to the usual Rayleigh-Brillouin spectrum when the gravitational field is taken into account. Additional to the usual central Rayleigh peak and Brillouin doublet, both corrected due to the presence of the field, non-Lorentizan terms are included in the structure factor. These terms are larger in the presence of the gravitational field and may lead in principle to relevant differences in the general properties of the spectrum. The possible mathematical origin of these modifications is briefly discussed.
Density fluctuations and topological structures in collective surface motion of microswimmers
NASA Astrophysics Data System (ADS)
Gao, Tong; Shelley, Michael
2014-11-01
Active matter that consists of self-propelled particles, such as bacterial suspensions and assays of self-driven biofilaments, can exhibit collective motions with large-scale complex flows and topological defect dynamics. Using a Doi-Onsager kinetic theory, we study suspensions of microswimmers confined to an air/liquid interface, and identify correlations between particle density fluctuations, defect structures, nematic order, and surface flows. When considering a free-standing liquid film where the microswimmers are distributed on the air/liquid interfaces, we capture hydrodynamic coupling of the two active surface, characterized by synchronization of motile disclination defects. We estimate the effective ``penetration distance'' between the two coupled surfaces through a linear stability analysis.
Quenching of the beam-plasma instability by 3-D spectra of large scale density fluctuations
NASA Technical Reports Server (NTRS)
Muschietti, L.; Goldman, M. V.; Newman, D.
1984-01-01
A model is presented to explain the highly variable yet low level of Langmuir waves measured in situ by spacecraft when electron beams associated with Type III solar bursts are passing by; the low level of excited waves allows the propagation of such streams from the Sun to well past 1 AU without catastrophic energy losses. The model is based, first, on the existence of large scale density fluctuations that are able to efficiently diffuse small k beam unstable Langmuir waves in phase space, and, second, on the presence of a significantly isotropic nonthermal tail in the distribution function of the background electron population, which is capable of stabilizing larger k modes. The strength of the model lies in its ability to predict various levels of Langmuir waves depending on the parameters. This feature is consistent with the high variability actually observed in the measurements.
Measuring long wavelength plasma density fluctuations by CO2 laser scattering (abstract)
NASA Astrophysics Data System (ADS)
Evans, D. E.
1985-05-01
. Asymmetric profiles are frequently encountered in far forward scattering experiments in plasma, and they are attributed either to (1) the volume effect, that is, the finite width of a plasma wave, or (2) a pair of counter-propagating waves, such as poloidal waves in a torus met twice by a probe beam traversing a minor diameter. The first explanation rests on the difference between the multiple order scattering of a two-dimensional grating (Raman-Nath) and the single-order scattering of a three-dimensional crystal (Bragg). In a regime intermediate between these extremes, both +1 and -1 orders are present, but of unequal intensity, therefore giving rise to asymmetry in the beam profile. The Fourier optics treatment can be extended to describe a wave of arbitrary interaction length L, and a controlling parameter Q=κ2L/k (κ and k being wave numbers of the plasma wave and the probe radiation, respectively) which is ≪1 for Raman-Nath and ≫1 for Bragg, determines the precise regime that prevails.7 Calculations describing the counter-propagating waves model have been performed and verified experimentally, again using transducer-driven waves in air.8 Profiles based on this model are currently providing best fits to data recently recorded from tokamak plasmas in TOSCA. A preliminary inspection of the results of these measurements reveals, from the orientation of the beam profile pattern, predominantly poloidal waves. Their maximum intensity is near 100 kHz and they fall away towards higher frequencies as ν-2.5. Evidence for coherent gross modes at lower frequencies is also seen. Wave numbers are in the range 1 cm-1<κ⊥<30 cm-1, bracketing the neighborhood where κ⊥ρi˜1. The strength of the relative density fluctuation ñe/n¯e of a few per cent is consistent with diffusion coefficients D⊥˜104 cm2 s-1, and there is evidence for inverse correlation between ñe/n¯e and confinement time τE.
On the Lévy-Nature of Magnetic Field Fluctuations During Magnetospheric Tail Current Disruption
NASA Astrophysics Data System (ADS)
Consolini, G.; Lui, A. T.; Zimbardo, G.
2002-12-01
One of the most relevant phenomena occurring at the substorm onset is the development of a current wedge, which is responsible for the magnetosphere-ionosphere coupling during magnetic substorms. This current wedge is generally associated with the diversion or disruption of the near cross-tail current system [Lui, 1996]. In the last years this near-Earth dipolarization phenomenon has been the subject of several observation, as well as, simulation studies, which suggested a multiscale and a non-MHD nature of the phenomenon [Sitnov et al., 2000; Malova et al., 2000; Zelenyi et al., 2000; Miura, 2000]. Here, using magnetic field data relative to 3 current disruption (CD) events as observed by AMPTEE/CCE spacecraft, we investigate the statistical features of magnetic field fluctuations. In the kinetic domain (i.e. above the ion cyclotron frequency during CD) the distribution function of magnetic field fluctuations shows non Gaussian tails and the probability of return Pt(0) scales as t-α with α !=q 1/2 which is compatible with a Lévy-statistics. Conversely, in the magnetohydrodynamic (MHD) region CD magnetic fluctuations are compatible with a classical Brownian motion. These findings seem to indicate that the near-Earth dipolarization process, associated with CD, is a non-MHD phenomenon, during which fast kinetic processes in collisionless plasmas take place.
NASA Astrophysics Data System (ADS)
Phillips, Nicholas G.; Hu, B. L.
2000-10-01
We present calculations of the variance of fluctuations and of the mean of the energy momentum tensor of a massless scalar field for the Minkowski and Casimir vacua as a function of an intrinsic scale defined by a smeared field or by point separation. We point out that, contrary to prior claims, the ratio of variance to mean-squared being of the order unity is not necessarily a good criterion for measuring the invalidity of semiclassical gravity. For the Casimir topology we obtain expressions for the variance to mean-squared ratio as a function of the intrinsic scale (defined by a smeared field) compared to the extrinsic scale (defined by the separation of the plates, or the periodicity of space). Our results make it possible to identify the spatial extent where negative energy density prevails which could be useful for studying quantum field effects in worm holes and baby universes, and for examining the design feasibility of real-life ``time machines.'' For the Minkowski vacuum we find that the ratio of the variance to the mean-squared, calculated from the coincidence limit, is identical to the value of the Casimir case at the same limit for spatial point separation while identical to the value of a hot flat space result with a temporal point separation. We analyze the origin of divergences in the fluctuations of the energy density and discuss choices in formulating a procedure for their removal, thus raising new questions about the uniqueness and even the very meaning of regularization of the energy momentum tensor for quantum fields in curved or even flat spacetimes when spacetime is viewed as having an extended structure.
NASA Astrophysics Data System (ADS)
Nishizawa, Takashi; Craig, D.; den Hartog, D. J.; Nornberg, M. D.
2016-10-01
Passive impurity spectroscopy is used to study high frequency ( 100 kHz) electron density and ion velocity fluctuations in the edge of MST reversed field pinch plasmas. When tearing modes are suppressed, stochastic transport is greatly reduced and microturbulence is anticipated to become important. Gyrokinetic simulations predict unstable trapped electron modes (TEM) in the edge region of these improved-confinement MST plasmas. Interferometry measurements reveal electron density fluctuations with wavenumbers, propagation direction, and a density-gradient threshold in good agreement with predictions for TEMs. These density fluctuations are also observed as emission fluctuations using a recently upgraded Ion Dynamics Spectrometer (IDS II) through edge passive C +2 measurements. The particle transport associated with TEMs will be evaluated directly by correlating the IDS-measured ion velocity and density fluctuations. The measurement is localized to the C +2 emission shell in the edge of the plasma, which is determined by a coronal charge-state balance model using ADAS. We used a large-throughput spectrometer originally developed for fast CHERS measurements and PMTs for light detection to achieve high time resolution. This work is supported by the US DOE.
Improvement of the noise level of the Split Langmuir Probe - a spatial current density meter
NASA Astrophysics Data System (ADS)
Marusenkov, Andriy; Dudkin, Fedor; Shuvalov, Valentyn
2013-04-01
One of the main tasks at the experimental investigations of the wave processes in space plasma is the determination of the dispersion relations between their wave vector and frequency. The frequency analysis of the magnetic field fluctuations and the electric current density in plasma is very efficient in this case. It had been shown that the simultaneous measurements of the magnetic field orthogonal components and the spatial current density fluctuations can give the wave vector k values for the plane wave spectra, by which a wave field in a plasma reference frame can be represented. The measurements of the magnetic field fluctuations usually are made by a variety of magnetometers using well developed methods. Unfortunately, up to the moment the methods and instruments for the reliable measurements of the space current density are not so good developed as the magnetic ones. There are three independent techniques to study the spatial current density in plasma: the contactless Rogovsky coil, the Faraday cap and the Split Langmuir Probe (SLP). The attempt to compare the different approaches and instruments was carried out during the experiment "Variant" onboard Ukrainian remote sensing satellite SICH-1M launched 2004. The clear advantages of the SLP over other instruments were revealed and proved. Using whistler as a test signal the very good consistency between the magnetic and electric fields and the spatial electric current density was obtained. However, the signal-to-noise ratio of the current density meters has to be further improved. In this report we analyze the sources of the SLP noises and propose the ways to decrease it. The computer simulation of the improved current density meter reveals that the introduced changes have almost no influence on the sensor matching with the space plasma and, as a result, the minor changes of the transformation factor in operation frequency band are expected. The modernized version of the SLP was successfully tested in the
NASA Astrophysics Data System (ADS)
Altaner, Bernhard; Wachtel, Artur; Vollmer, Jürgen
2015-10-01
Unlike macroscopic engines, the molecular machinery of living cells is strongly affected by fluctuations. Stochastic thermodynamics uses Markovian jump processes to model the random transitions between the chemical and configurational states of these biological macromolecules. A recently developed theoretical framework [A. Wachtel, J. Vollmer, and B. Altaner, Phys. Rev. E 92, 042132 (2015), 10.1103/PhysRevE.92.042132] provides a simple algorithm for the determination of macroscopic currents and correlation integrals of arbitrary fluctuating currents. Here we use it to discuss energy conversion and nonequilibrium response in different models for the molecular motor kinesin. Methodologically, our results demonstrate the effectiveness of the algorithm in dealing with parameter-dependent stochastic models. For the concrete biophysical problem our results reveal two interesting features in experimentally accessible parameter regions: the validity of a nonequilibrium Green-Kubo relation at mechanical stalling as well as a negative differential mobility for superstalling forces.
High current density pulsed cathode experiments at SLAC
Koontz, R.; Fant, K.; Vlieks, A.
1990-06-01
A 1.9 microperveance beam diode has been constructed to test high current density cathodes for use in klystrons. Several standard and specially coated dispenser cathodes are being tested. Results of tests to date show average cathode current densities in excess of 25 amps/cm, and maximum electric field gradients of more than 450 kV/cm for pulses of the order of 1{mu}sec. 3 refs., 11 figs.
Alternating-Current Equipment for the Measurement of Fluctuations of Air Speed in Turbulent Flow
NASA Technical Reports Server (NTRS)
Mock, W C , Jr
1937-01-01
Recent electrical and mechanical improvements have been made in the equipment developed at the National Bureau of Standards for measurement of fluctuations of air speed in turbulent flow. Data useful in the design of similar equipment are presented. The design of rectified alternating-current power supplies for such apparatus is treated briefly, and the effect of the power supplies on the performance of the equipment is discussed.
Second-order fluctuation theory and time autocorrelation function for currents
NASA Astrophysics Data System (ADS)
Belousov, Roman; Cohen, E. G. D.
2016-12-01
By using recent developments for the Langevin dynamics of spatially asymmetric systems, we routinely generalize the Onsager-Machlup fluctuation theory of the second order in time. In this form, it becomes applicable to fluctuating variables, including hydrodynamic currents, in equilibrium as well as nonequilibrium steady states. From the solution of the obtained stochastic equations we derive an analytical expression for the time autocorrelation function of a general fluctuating quantity. This theoretical result is then tested in a study of a shear flow by molecular dynamics simulations. The proposed form of the time autocorrelation function yields an excellent fit to our computational data for both equilibrium and nonequilibrium steady states. Unlike the analogous result of the first-order Onsager-Machlup theory, our expression correctly describes the short-time correlations. Its utility is demonstrated in an application of the Green-Kubo formula for the transport coefficient. Curiously, the normalized time autocorrelation function for the shear flow, which only depends on the deterministic part of the fluctuation dynamics, appears independent of the external shear force in the linear nonequilibrium regime.
NASA Astrophysics Data System (ADS)
Parke, Eli
Due to long fast ion confinement times, neutral beam injection (NBI) on the Madison Symmetric Torus (MST) yields large fast ion populations with substantial density gradients. Novel application of the unique high-rep-rate (>10 kHz) Thomson scattering diagnostic on MST has enabled characterization of a newly observed beam-driven instability, and detailed measurement of equilibrium changes caused by the fast ion population. While previous work has focused on high-frequency energetic particle modes (EPMs), recent observations indicate that fast ions drive a bursting instability near the plasma rotation frequency under appropriate conditions. The mode chirps strongly, with a frequency of approximately 7 kHz in the plasma reference frame at peak amplitude. Bursts are correlated with EPM activity and core neutral particle analyzer signals drop by 30% during a burst, suggesting that this mode participates in avalanches of the higher frequency EPMs and drives enhanced fast ion transport. Electron temperature fluctuations correlated with this low-frequency mode exhibit a core-peaked structure with a sensitive dependence on the safety factor q. Although this mode has not yet been positively identified, its characteristics and internal structure are suggestive of an internal kink (fishbone) or beta-induced Alfven eigenmode. In addition to driving EPMs, the large fast ion population also modifies the current profile. An increase in on-axis current density driven by NBI is offset by a reduction in the mid-radius, leading to net-zero current drive. This results in a slight flattening of the safety factor profile, observed by precise measurement of the rational surface locations of the dominant tearing modes; these are identified from the phase flip in correlated electron temperature fluctuations recorded by Thomson scattering. For the core n = 6 rational surface, an inward shift of 1.1 +/- 0.6 cm is observed, with an estimated reduction in q0 of 5%. This technique provides a
NASA Astrophysics Data System (ADS)
Bencze, P.; Illés-Almár, E.; Almár, I.
The study of the high resolution total neutral density measurements of the San Marco V satellite revealed also wave-like fluctuations of the density of amplitude increas- ing with height above a given height. The height at which the amplitude of these fluctuations begins to increase has been found to occur in the height range 300-500 km. Analysis of this phenomenon indicated that this height displays a diurnal varia- tion lower heights occurring by day. On the basis of this findings it is assumed that fluctuations of amplitude increasing with height found in the total neutral density are due to convective instability related to the quasi isothermal state of this part of the thermosphere. Under these conditions an infinitesimal disturbance is enough for the development of instability.
NASA Technical Reports Server (NTRS)
Clukey, Steven J.
1988-01-01
The high speed Dynamic Data Acquisition System (DDAS) is described which provides the capability for the simultaneous measurement of velocity, density, and total temperature fluctuations. The system of hardware and software is described in context of the wind tunnel environment. The DDAS replaces both a recording mechanism and a separate data processing system. The data acquisition and data reduction process has been combined within DDAS. DDAS receives input from hot wires and anemometers, amplifies and filters the signals with computer controlled modules, and converts the analog signals to digital with real-time simultaneous digitization followed by digital recording on disk or tape. Automatic acquisition (either from a computer link to an existing wind tunnel acquisition system, or from data acquisition facilities within DDAS) collects necessary calibration and environment data. The generation of hot wire sensitivities is done in DDAS, as is the application of sensitivities to the hot wire data to generate turbulence quantities. The presentation of the raw and processed data, in terms of root mean square values of velocity, density and temperature, and the processing of the spectral data is accomplished on demand in near-real-time- with DDAS. A comprehensive description of the interface to the DDAS and of the internal mechanisms will be prosented. A summary of operations relevant to the use of the DDAS will be provided.
NASA Astrophysics Data System (ADS)
Zhong, H.; Tan, Y.; Liu, Y. Q.; Xie, H. Q.; Gao, Z.
2016-11-01
A single-channel 3 mm interferometer has been developed for plasma density diagnostics in the Sino-UNIted Spherical Tokamak (SUNIST). The extremely compact microwave interferometer utilizes one corrugated feed horn antenna for both emitting and receiving the microwave. The beam path lies on the equatorial plane so the system would not suffer from beam path deflection problems due to the symmetry of the cross section. A focusing lens group and an oblique vacuum window are carefully designed to boost the signal to noise ratio, which allows this system to show good performance even with the small-diameter central column itself as a reflector, without a concave mirror. The whole system discards the reference leg for maximum compactness, which is particularly suitable for the small-sized tokamak. An auto-correcting algorithm is developed to calculate the phase evolution, and the result displays good phase stability of the whole system. The intermediate frequency is adjustable and can reach its full potential of 2 MHz for best temporal resolution. Multiple measurements during ohmic discharges proved the interferometer's capability to track typical density fluctuations in SUNIST, which enables this system to be utilized in the study of MHD activities.
Zhong, H; Tan, Y; Liu, Y Q; Xie, H Q; Gao, Z
2016-11-01
A single-channel 3 mm interferometer has been developed for plasma density diagnostics in the Sino-UNIted Spherical Tokamak (SUNIST). The extremely compact microwave interferometer utilizes one corrugated feed horn antenna for both emitting and receiving the microwave. The beam path lies on the equatorial plane so the system would not suffer from beam path deflection problems due to the symmetry of the cross section. A focusing lens group and an oblique vacuum window are carefully designed to boost the signal to noise ratio, which allows this system to show good performance even with the small-diameter central column itself as a reflector, without a concave mirror. The whole system discards the reference leg for maximum compactness, which is particularly suitable for the small-sized tokamak. An auto-correcting algorithm is developed to calculate the phase evolution, and the result displays good phase stability of the whole system. The intermediate frequency is adjustable and can reach its full potential of 2 MHz for best temporal resolution. Multiple measurements during ohmic discharges proved the interferometer's capability to track typical density fluctuations in SUNIST, which enables this system to be utilized in the study of MHD activities.
Yeo, Joonhyun
2009-11-01
We study a zero-dimensional version of the fluctuating nonlinear hydrodynamics (FNH) of supercooled liquids originally investigated by Das and Mazenko (DM) [Shankar P. Das and Gene F. Mazenko Phys. Rev. A 34, 2265 (1986)]. The time-dependent density-like and momentum-like variables are introduced with no spatial degrees of freedom in this toy model. The structure of nonlinearities takes the similar form to the original FNH, which allows one to study in a simpler setting the issues raised recently regarding the field theoretical approaches to glass forming liquids. We study the effects of density nonlinearities on the time evolution of correlation and response functions by developing field theoretic formulations in two different ways: first by following the original prescription of DM and then by constructing a dynamical action which possesses a linear time-reversal symmetry as proposed recently. We show explicitly that, at the one-loop order of the perturbation theory, the DM-type field theory does not support a sharp ergodic-nonergodic transition, while the other admits one. The simple nature of the toy model in the DM formulation allows us to develop numerical solutions to a complete set of coupled dynamical equations for the correlation and response functions at the one-loop order.
Temporal evolution of lower hybrid waves in the presence of ponderomotive density fluctuations
Karney, C.F.F.
1980-06-01
The propagation of lower hybrid waves in the presence of ponderomotive density density fluctuations is considered. The problem is treated in two dimensions and, in order to be able to correctly impose the boundary conditions, the waves are allowed to evolve in time. The fields are described by i upsilon/sub tau/ - ..integral.. upsilon/sub xi/d/sub zeta/ + upsilon/sub zeta zeta/ + upsilon//sup 2/ upsilon = 0 where upsilon is proportional to the electric field, tau to time, and zeta and xi measure distances across and along the lower hybrid ray. The behavior of the waves is investigated numerically. If the amplitude of the waves is large enough, the spectrum of the waves broadens and their parallel wavelength becomes shorter. The assumptions made in the formulation preclude the application of these results to the lower hybrid heating experiment on Alcator-A. Nevertheless, there are indications that the physics embodied in this problem are responsible for some of the results of that experiment.
Particle and energy dependence of the statistical fluctuations of an ionization chamber current
NASA Astrophysics Data System (ADS)
Purghel, Lidia; Vaˆlcov, Nicolae
For the purpose of getting more detailed information concerning the processes leading to statistical fluctuations of an ionization chamber current, measurements with various radioactive sources have been done. By using the experimental arrangement described elsewhere [A. Necula et al. Nucl. Instr. and Meth. A 332 (1993) 501] the mean value and the standard deviation of the ionization current for 3H (water vapours), 60Co (sealed source), 85Kr (gas), 204Tl (8 mm diameter disk) and 239Pu (10 mm diameter disk), beta, gamma and alpha sources have been measured. A statistical model explaining the experimental data is proposed.
A high current density DC magnetohydrodynamic (MHD) micropump.
Homsy, Alexandra; Koster, Sander; Eijkel, Jan C T; van den Berg, Albert; Lucklum, F; Verpoorte, E; de Rooij, Nico F
2005-04-01
This paper describes the working principle of a DC magnetohydrodynamic (MHD) micropump that can be operated at high DC current densities (J) in 75-microm-deep microfluidic channels without introducing gas bubbles into the pumping channel. The main design feature for current generation is a micromachined frit-like structure that connects the pumping channel to side reservoirs, where platinum electrodes are located. Current densities up to 4000 A m(-2) could be obtained without noticeable Joule heating in the system. The pump performance was studied as a function of current density and magnetic field intensity, as well as buffer ionic strength and pH. Bead velocities of up to 1 mm s(-1) (0.5 microL min(-1)) were observed in buffered solutions using a 0.4 T NdFeB permanent magnet, at an applied current density of 4000 A m(-2). This pump is intended for transport of electrolyte solutions having a relatively high ionic strength (0.5-1 M) in a DC magnetic field environment. The application of this pump for the study of biological samples in a miniaturized total analysis system (microTAS) with integrated NMR detection is foreseen. In the 7 T NMR environment, a minimum 16-fold increase in volumetric flow rate for a given applied current density is expected.
The impact of geocoronal density on ring current development
NASA Astrophysics Data System (ADS)
Ilie, R.; Skoug, R. M.; Funsten, H. O.; Liemohn, M. W.; Bailey, J. J.; Gruntman, M.
2013-07-01
Long-term ring current decay following a magnetic storm is mainly due to charge exchange collisions of ring current ions with geocoronal neutral atoms forming energetic neutral atoms (ENAs) that leave the ring current system. Therefore, the density distribution of these cold and tenuous neutral hydrogen atoms plays a key role in the ring current recovery. TWINS ENA images provide a direct measurement of these ENA losses and therefore insight into the dynamics of the ring current decay through interactions with the geocorona. To assess the influence of geocoronal neutrals on ring current decay, we compare the predicted ENA emission using five different geocoronal models and the HEIDI ring current model to simulate the July 22, 2009 storm. We show that for high energy H+ (≥100 keV), all geocoronal models predict similar decay rates of the ring current ions. However, for low energy ions (≤100 keV), the decay rate varies significantly depending on the geocoronal density model. Comparison with TWINS ENA images shows that the location of the peak ENA enhancements is highly dependent on the distribution of geocoronal hydrogen density. The ring current topology depends greatly on the hydrogen model used, therefore knowing the H-distribution is very important in understanding how the ring current recovers following a magnetic storm.
Casimir effect for scalar current densities in topologically nontrivial spaces
NASA Astrophysics Data System (ADS)
Bellucci, S.; Saharian, A. A.; Saharyan, N. A.
2015-08-01
We evaluate the Hadamard function and the vacuum expectation value (VEV) of the current density for a charged scalar field, induced by flat boundaries in spacetimes with an arbitrary number of toroidally compactified spatial dimensions. The field operator obeys the Robin conditions on the boundaries and quasiperiodicity conditions with general phases along compact dimensions. In addition, the presence of a constant gauge field is assumed. The latter induces Aharonov-Bohm-type effect on the VEVs. There is a region in the space of the parameters in Robin boundary conditions where the vacuum state becomes unstable. The stability condition depends on the lengths of compact dimensions and is less restrictive than that for background with trivial topology. The vacuum current density is a periodic function of the magnetic flux, enclosed by compact dimensions, with the period equal to the flux quantum. It is explicitly decomposed into the boundary-free and boundary-induced contributions. In sharp contrast to the VEVs of the field squared and the energy-momentum tensor, the current density does not contain surface divergences. Moreover, for Dirichlet condition it vanishes on the boundaries. The normal derivative of the current density on the boundaries vanish for both Dirichlet and Neumann conditions and is nonzero for general Robin conditions. When the separation between the plates is smaller than other length scales, the behavior of the current density is essentially different for non-Neumann and Neumann boundary conditions. In the former case, the total current density in the region between the plates tends to zero. For Neumann boundary condition on both plates, the current density is dominated by the interference part and is inversely proportional to the separation.
Rf Gun with High-Current Density Field Emission Cathode
Jay L. Hirshfield
2005-12-19
High current-density field emission from an array of carbon nanotubes, with field-emission-transistor control, and with secondary electron channel multiplication in a ceramic facing structure, have been combined in a cold cathode for rf guns and diode guns. Electrodynamic and space-charge flow simulations were conducted to specify the cathode configuration and range of emission current density from the field emission cold cathode. Design of this cathode has been made for installation and testing in an existing S-band 2-1/2 cell rf gun. With emission control and modulation, and with current density in the range of 0.1-1 kA/cm2, this cathode could provide performance and long-life not enjoyed by other currently-available cathodes
Estimation of nighttime dip-equatorial E-region current density using measurements and models
NASA Astrophysics Data System (ADS)
Pandey, Kuldeep; Sekar, R.; Anandarao, B. G.; Gupta, S. P.; Chakrabarty, D.
2016-08-01
The existence of the possible ionospheric current during nighttime over low-equatorial latitudes is one of the unresolved issues in ionospheric physics and geomagnetism. A detailed investigation is carried out to estimate the same over Indian longitudes using in situ measurements from Thumba (8.5 ° N, 76.9 ° E), empirical plasma drift model (Fejer et al., 2008) and equatorial electrojet model developed by Anandarao (1976). This investigation reveals that the nighttime E-region current densities vary from ∼0.3 to ∼0.7 A/km2 during pre-midnight to early morning hours on geomagnetically quiet conditions. The nighttime current densities over the dip equator are estimated using three different methods (discussed in methodology section) and are found to be consistent with one another within the uncertainty limits. Altitude structures in the E-region current densities are also noticed which are shown to be associated with altitudinal structures in the electron densities. The horizontal component of the magnetic field induced by these nighttime ionospheric currents is estimated to vary between ∼2 and ∼6 nT during geomagnetically quiet periods. This investigation confirms the existence of nighttime ionospheric current and opens up a possibility of estimating base line value for geomagnetic field fluctuations as observed by ground-based magnetometer.
Magneto-optical imaging of transport current densities in superconductors
Crabtree, G.W.; Welp, U.; Gunter, D.O.; Zhong, W.; Balachandran, U.; Haldar, P.; Sokolowski, R.S.; Vlasko-Vlasov, V.K.; Nikitenko, V.I.
1995-12-31
Direct imaging of the paths of transport currents in superconductors creates many new possibilities for exploring the basic features of vortex pinning mechanisms and for improving the performance of superconducting materials. A technique for imaging the path and magnitude of the transport current density flowing in superconductors is described. Results are given for a 37-filament BSCCO 2223 powder-in-tube wire, showing a highly inhomogeneous current path within the filaments.
White, A. E.; Schmitz, L.; Peebles, W. A.; Rhodes, T. L.; Carter, T. A.; McKee, G. R.; Shafer, M. W.; Staebler, G. M.; Burrell, K. H.; DeBoo, J. C.; Prater, R.
2010-02-15
New measurements show that long-wavelength (k{sub t}hetarho{sub s}<0.5) electron temperature fluctuations can play an important role in determining electron thermal transport in low-confinement mode (L-mode) tokamak plasmas. In neutral beam-heated L-mode tokamak plasmas, electron thermal transport and the amplitude of long-wavelength electron temperature fluctuations both increase in cases where local electron cyclotron heating (ECH) is used to modify the plasma profiles. In contrast, the amplitude of simultaneously measured long-wavelength density fluctuations does not significantly increase. Linear stability analysis indicates that the ratio of the trapped electron mode (TEM) to ion temperature gradient (ITG) mode growth rates increases in the cases with ECH. The increased importance of the TEM drive relative to the ITG mode drive in the cases with ECH may be associated with the increases in electron thermal transport and electron temperature fluctuations.
Ionospheric midlatitude electric current density inferred from multiple magnetic satellites
NASA Astrophysics Data System (ADS)
Shore, R. M.; Whaler, K. A.; Macmillan, S.; Beggan, C.; Olsen, N.; Spain, T.; Aruliah, A.
2013-09-01
A method for inferring zonal electric current density in the mid-to-low latitude F region ionosphere is presented. We describe a method of using near-simultaneous overflights of the Ørsted and CHAMP satellites to define a closed circuit for an application of Ampère's integral law to magnetic data. Zonal current density from sources in only the region between the two satellites is estimated for the first time. Six years of mutually available vector magnetic data allows overlaps spanning the full 24 h range of local time twice. Solutions are computed on an event-by-event basis after correcting for estimates of main and crustal magnetic fields. Current density in the range ±0.1 μA/m2 is resolved, with the distribution of electric current largely matching known features such as the Appleton anomaly. The currents appear unmodulated at times of either high-negative Dst or high F10.7, which has implications for any future efforts to model their effects. We resolve persistent current intensifications between geomagnetic latitudes of 30 and 50° in the postmidnight, predawn sector, a region typically thought to be relatively free of electric currents. The cause of these unexpected intensifications remains an open issue. We compare our results with current density predictions made by the Coupled Thermosphere-Ionosphere-Plasmasphere model, a self-consistent, first-principles, three-dimensional numerical dynamic model of ionospheric composition and temperatures. This independent validation of our current density estimates highlights good agreement in the broad spatiotemporal trends we identify, which increases confidence in our results.
Regional absolute conductivity reconstruction using projected current density in MREIT.
Sajib, Saurav Z K; Kim, Hyung Joong; Kwon, Oh In; Woo, Eung Je
2012-09-21
Magnetic resonance electrical impedance tomography (MREIT) is a non-invasive technique for imaging the internal conductivity distribution in tissue within an MRI scanner, utilizing the magnetic flux density, which is introduced when a current is injected into the tissue from external electrodes. This magnetic flux alters the MRI signal, so that appropriate reconstruction can provide a map of the additional z-component of the magnetic field (B(z)) as well as the internal current density distribution that created it. To extract the internal electrical properties of the subject, including the conductivity and/or the current density distribution, MREIT techniques use the relationship between the external injection current and the z-component of the magnetic flux density B = (B(x), B(y), B(z)). The tissue studied typically contains defective regions, regions with a low MRI signal and/or low MRI signal-to-noise-ratio, due to the low density of nuclear magnetic resonance spins, short T(2) or T*(2) relaxation times, as well as regions with very low electrical conductivity, through which very little current traverses. These defective regions provide noisy B(z) data, which can severely degrade the overall reconstructed conductivity distribution. Injecting two independent currents through surface electrodes, this paper proposes a new direct method to reconstruct a regional absolute isotropic conductivity distribution in a region of interest (ROI) while avoiding the defective regions. First, the proposed method reconstructs the contrast of conductivity using the transversal J-substitution algorithm, which blocks the propagation of severe accumulated noise from the defective region to the ROI. Second, the proposed method reconstructs the regional projected current density using the relationships between the internal current density, which stems from a current injection on the surface, and the measured B(z) data. Combining the contrast conductivity distribution in the entire
High Current Density Scandate Cathodes for Future Vacuum Electronics Applications
2008-05-30
braze alloy . The structure was fired in a furnace at 16500 C for 15 minutes. The resultant structure was sectioned to determine if the scandium flowed...Density Cathodes for Future Vacuum Electronics Applications FA9550-07-C-0063 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION...Current Density Scandate Cathodes for Future Vacuum Electronics Applications USAF/AFRL Contract Number FA9550-07-C-0063 Final Report Calabazas Creek
Target current density effects in al-cluster ion emission
NASA Astrophysics Data System (ADS)
Ray, N.; Rajasekar, P.; Chakraborty, P.; Dey, S. D.
1994-04-01
Target current density effects in secondary emission of positively-charged sputtered clusters from an ion-bombarded polycrystalline aluminium surface have been investigated for various primary Cd+ ion energies. For each cluster type, at medium target current densities (Jp : 0?350 ?A/cm2), apart from the presence of the expected linear term in the I+ s α Jp curves, there is a square law term, depicting that some fraction of the pre-emitted neutral clusters gets ionized in vacuum above the target surface. Thus, a sum of these two terms constitutes the total cluster ion current which shows a non-linear dependence on the target current density. At higher target current densities (350 ?A cm?2?1.05 mA cm?2), in addition to the parabolic effect, a higher order non-linearity comes into play and the net cluster ion current I + s , measured as a function of target current density within the Ip range concerned, can be expressed in terms of our previously proposed semi-empirical formula I+ s = C.Ip + D.I2 p + F.In p , which was earlier found to be applicable in case of single and double charged secondary monomers. A higher power (n > 2) dependence has been observed here for the first time in case of cluster ion emission, suggesting chemical enhancement of secondary clusters. Our present observations seem to support ?direct emission model? even for small metal clusters?a proposition which is in apparent contradiction to the current notion in this field, i.e. vacuum recombination of individual atoms in small cluster formation.
NASA Astrophysics Data System (ADS)
Nariyuki, Y.; Seough, J.
2015-12-01
It is well known that low-frequency Alfven waves are unstable to parametric instabilities, in which these waves are nonlinearly coupled with density fluctuations [e.g, Nariyuki+Hada, JGR, 2007 and references therein]. In solar wind plasmas, low-frequency fluctuations with non-zero cross-helicity are frequently observed [e.g., Bruno+Carbone, Living Rev. Solar Phys. (2013) and references therein]. When the absolute values of normalized cross helicities are close to the unity, the fluctuations may be composed of uni-directionally (anti-sunward) propagating Alfven waves. The derivative nonlinear Schrodinger equation (DNLS) has been known as the mode of modulational instabilities of unidirectional Alfven waves [Mio et al, JPSJ, 1976; Mjolhus, JPP, 1976]. In the DNLS, the density fluctuations are assumed to be the quasi-static state, which is determined according to the ponderomotive force of envelope-modulated Alfven waves. The DNLS was extended to include the obliquely propagating, compressional component of magnetic field by Mjolhus and Wyller (JPP, 1988). The kinetically modified DNLS (KDNLS) has also been discussed by many authors [Rogister, POF, 1971; Mjolhus and Wyller, Phys. Scr, 1986; JPP, 1988; Spangler, POF B, 1989; 1990; Medvedev+Diamond, POP, 1996; Nariyuki et al, POP, 2013]. On the other hand, ion acoustic modes [Hada, 1993], large scale inhomogeneity of plasmas [Buti et al, APJ, 1999; Nariyuki, POP, 2015] and random density fluctuations [Ruderman, POP, 2002] can also affect nonlinear evolution of Alfven waves. At the present time, combined effects of these effects are not fully understood. In this presentation, we discuss two models: one of them is the model including both ion kinetic effects and ion acoustic mode and another is the model including finite thermal effects and random density fluctuations. In the former case, ion kinetic effects on both longitudinal [Nariyuki+Hada, JPSJ, 2007] and transverse modulational instabilities are discussed, while the
Current drive at plasma densities required for thermonuclear reactors.
Cesario, R; Amicucci, L; Cardinali, A; Castaldo, C; Marinucci, M; Panaccione, L; Santini, F; Tudisco, O; Apicella, M L; Calabrò, G; Cianfarani, C; Frigione, D; Galli, A; Mazzitelli, G; Mazzotta, C; Pericoli, V; Schettini, G; Tuccillo, A A
2010-08-10
Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confined in toroidal tokamak devices requires the development of efficient current drive methods. Previous experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing significant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.
Zalloni, Enrica; de Luis, Martin; Campelo, Filipe; Novak, Klemen; De Micco, Veronica; Di Filippo, Alfredo; Vieira, Joana; Nabais, Cristina; Rozas, Vicente; Battipaglia, Giovanna
2016-01-01
Tree rings provide information about the climatic conditions during the growing season by recording them in different anatomical features, such as intra-annual density fluctuations (IADFs). IADFs are intra-annual changes of wood density appearing as latewood-like cells within earlywood, or earlywood-like cells within latewood. The occurrence of IADFs is dependent on the age and size of the tree, and it is triggered by climatic drivers. The variations of IADF frequency of different species and their dependence on climate across a wide geographical range have still to be explored. The objective of this study is to investigate the effect of age, tree-ring width and climate on IADF formation and frequency at a regional scale across the Mediterranean Basin in Pinus halepensis Mill., Pinus pinaster Ait., and Pinus pinea L. The analyzed tree-ring network was composed of P. pinea trees growing at 10 sites (2 in Italy, 4 in Spain, and 4 in Portugal), P. pinaster from 19 sites (2 in Italy, 13 in Spain, and 4 in Portugal), and P. halepensis from 38 sites in Spain. The correlations between IADF frequency and monthly minimum, mean and maximum temperatures, as well as between IADF frequency and total precipitation, were analyzed. A significant negative relationship between IADF frequency and tree-ring age was found for the three Mediterranean pines. Moreover, IADFs were more frequent in wider rings than in narrower ones, although the widest rings showed a reduced IADF frequency. Wet conditions during late summer/early autumn triggered the formation of IADFs in the three species. Our results suggest the existence of a common climatic driver for the formation of IADFs in Mediterranean pines, highlighting the potential use of IADF frequency as a proxy for climate reconstructions with geographical resolution.
Zalloni, Enrica; de Luis, Martin; Campelo, Filipe; Novak, Klemen; De Micco, Veronica; Di Filippo, Alfredo; Vieira, Joana; Nabais, Cristina; Rozas, Vicente; Battipaglia, Giovanna
2016-01-01
Tree rings provide information about the climatic conditions during the growing season by recording them in different anatomical features, such as intra-annual density fluctuations (IADFs). IADFs are intra-annual changes of wood density appearing as latewood-like cells within earlywood, or earlywood-like cells within latewood. The occurrence of IADFs is dependent on the age and size of the tree, and it is triggered by climatic drivers. The variations of IADF frequency of different species and their dependence on climate across a wide geographical range have still to be explored. The objective of this study is to investigate the effect of age, tree-ring width and climate on IADF formation and frequency at a regional scale across the Mediterranean Basin in Pinus halepensis Mill., Pinus pinaster Ait., and Pinus pinea L. The analyzed tree-ring network was composed of P. pinea trees growing at 10 sites (2 in Italy, 4 in Spain, and 4 in Portugal), P. pinaster from 19 sites (2 in Italy, 13 in Spain, and 4 in Portugal), and P. halepensis from 38 sites in Spain. The correlations between IADF frequency and monthly minimum, mean and maximum temperatures, as well as between IADF frequency and total precipitation, were analyzed. A significant negative relationship between IADF frequency and tree-ring age was found for the three Mediterranean pines. Moreover, IADFs were more frequent in wider rings than in narrower ones, although the widest rings showed a reduced IADF frequency. Wet conditions during late summer/early autumn triggered the formation of IADFs in the three species. Our results suggest the existence of a common climatic driver for the formation of IADFs in Mediterranean pines, highlighting the potential use of IADF frequency as a proxy for climate reconstructions with geographical resolution. PMID:27200052
The THz Spectrum of Density Fluctuations of Water: The Viscoelastic Regime
Cunsolo, Alessandro
2015-01-01
Relevant advances in the knowledge of the water dynamics at mesoscopic scales are reviewed, while mainly focusing on the contribution provided by high resolution inelastic X-ray scattering (IXS). In particular it is discussed how the use of IXS has improved our understanding of viscoelastic properties of water at THz frequencies. This specifically involves some solid-like features such as the onset of shear wave propagation, a sound velocity surprisingly similar to the one of ice, and an anomalously low sound absorption coefficient. All these properties can be explained by assuming the coupling of THz density fluctuations with a structural relaxation process connected to the breaking and forming of hydrogen bonds (HBs). This review also includes more recent IXS results demonstrating that, upon approaching supercritical conditions, relaxation phenomena in water gradually lose their structural character becoming essentially collisional in character. Furthermore, GHz spectroscopy results on supercooled water, suggesting the occurrence of a structural arrest, are discussed. An overview of the new opportunities offered by next generation IXS spectrometers finally concludes this review.
NASA Astrophysics Data System (ADS)
Pradhan, Prabhakar; Damania, Dhwanil; Joshi, Hrushikesh M.; Turzhitsky, Vladimir; Subramanian, Hariharan; Roy, Hemant K.; Taflove, Allen; Dravid, Vinayak P.; Backman, Vadim
2011-04-01
Most cancers are curable if they are diagnosed and treated at an early stage. Recent studies suggest that nanoarchitectural changes occur within cells during early carcinogenesis and that such changes precede microscopically evident tissue alterations. It follows that the ability to comprehensively interrogate cell nanoarchitecture (e.g., macromolecular complexes, DNA, RNA, proteins and lipid membranes) could be critical to the diagnosis of early carcinogenesis. We present a study of the nanoscale mass-density fluctuations of biological tissues by quantifying their degree of disorder at the nanoscale. Transmission electron microscopy images of human tissues are used to construct corresponding effective disordered optical lattices. The properties of nanoscale disorder are then studied by statistical analysis of the inverse participation ratio (IPR) of the spatially localized eigenfunctions of these optical lattices at the nanoscale. Our results show an increase in the disorder of human colonic epithelial cells in subjects harboring early stages of colon neoplasia. Furthermore, our findings strongly suggest that increased nanoscale disorder correlates with the degree of tumorigenicity. Therefore, the IPR technique provides a practicable tool for the detection of nanoarchitectural alterations in the earliest stages of carcinogenesis. Potential applications of the technique for early cancer screening and detection are also discussed. Originally submitted for the special focus issue on physical oncology.
NASA Astrophysics Data System (ADS)
Fioretto, D.; Comez, L.; Socino, G.; Verdini, L.; Corezzi, S.; Rolla, P. A.
1999-02-01
Brillouin light scattering is used for studying the spectrum of density fluctuations of the glass-forming epoxy resin diglycidyl ether of bisphenol-A. Spectra at different temperatures ranging from the glassy to the liquid phase are obtained from a direct subtraction of depolarized from polarized spectra. In addition to the structural relaxation, evidence is given of a fast secondary relaxation process, which affects Brillouin spectra also at temperatures lower than that of the glass transition Tg. For the elaboration of isotropic spectra, we exploit the possibility of using the same relaxation function gained from dielectric spectra taken from the same sample. The temperature behavior of the relaxation strength shows the existence of an onset for the structural relaxation, located at a temperature about 93 K higher than Tg, consistent with the results of previous dielectric spectroscopy and depolarized light scattering investigations. The role of secondary relaxations of intramolecular nature in the mode-coupling analysis of real glass formers is also discussed.
COSMOLOGICAL DENSITY FLUCTUATIONS ON 100 Mpc SCALES AND THEIR ISW EFFECT
Papai, Peter; Szapudi, Istvan
2010-12-20
We measure the matter probability distribution function (PDF) via counts in cells in a volume-limited subsample of the Sloan Digital Sky Survey Luminous Red Galaxy Catalog on scales from 30 h {sup -1} Mpc to 150 h {sup -1} Mpc and estimate the linear Integrated Sachs-Wolfe effect produced by supervoids and superclusters in the tail of the PDF. We characterize the PDF by the variance, S{sub 3}, and S{sub 4}, and study in simulations the systematic effects due to finite volume, survey shape, and redshift distortion. We compare our measurement to the prediction of {Lambda}CDM with linear bias and find a good agreement. We use the moments to approximate the tail of the PDF with analytic functions. A simple Gaussian model for the superstructures appears to be consistent with the claim by Granett et al. that density fluctuations on 100 h {sup -1} Mpc scales produce hot and cold spots with {Delta}T {approx} 10 {mu}K on the cosmic microwave background.
2015-01-01
Kirkwood-Buff or Fluctuation Solution Theory can be used to provide experimental pair fluctuations, and/or integrals over the pair distribution functions, from experimental thermodynamic data on liquid mixtures. Here, this type of approach is used to provide triplet and quadruplet fluctuations, and the corresponding integrals over the triplet and quadruplet distribution functions, in a purely thermodynamic manner that avoids the use of structure factors. The approach is then applied to binary mixtures of water + methanol and benzene + methanol over the full composition range under ambient conditions. The observed correlations between the different species vary significantly with composition. The magnitude of the fluctuations and integrals appears to increase as the number of the most polar molecule involved in the fluctuation or integral also increases. A simple physical picture of the fluctuations is provided to help rationalize some of these variations. PMID:25747091
Ploetz, Elizabeth A.; Smith, Paul E.
2015-03-07
Kirkwood-Buff or Fluctuation Solution Theory can be used to provide experimental pair fluctuations, and/or integrals over the pair distribution functions, from experimental thermodynamic data on liquid mixtures. Here, this type of approach is used to provide triplet and quadruplet fluctuations, and the corresponding integrals over the triplet and quadruplet distribution functions, in a purely thermodynamic manner that avoids the use of structure factors. The approach is then applied to binary mixtures of water + methanol and benzene + methanol over the full composition range under ambient conditions. The observed correlations between the different species vary significantly with composition. The magnitude of the fluctuations and integrals appears to increase as the number of the most polar molecule involved in the fluctuation or integral also increases. A simple physical picture of the fluctuations is provided to help rationalize some of these variations.
NASA Technical Reports Server (NTRS)
Lee, M. C.; Kuo, S. P.
1985-01-01
The physical mechanism of thermal filamentation instability of radio waves whose frequencies can be as low as in the VLF band and as high as in the SHF band are investigated. This instability can excite large-scale magnetic and plasma density fluctuations simultaneously in the ionosphere and magnetosphere. Relevant experiments are reviewed in terms of this instability and other mechanisms.
Noise distribution and denoising of current density images.
Beheshti, Mohammadali; Foomany, Farbod H; Magtibay, Karl; Jaffray, David A; Krishnan, Sridhar; Nanthakumar, Kumaraswamy; Umapathy, Karthikeyan
2015-04-01
Current density imaging (CDI) is a magnetic resonance (MR) imaging technique that could be used to study current pathways inside the tissue. The current distribution is measured indirectly as phase changes. The inherent noise in the MR imaging technique degrades the accuracy of phase measurements leading to imprecise current variations. The outcome can be affected significantly, especially at a low signal-to-noise ratio (SNR). We have shown the residual noise distribution of the phase to be Gaussian-like and the noise in CDI images approximated as a Gaussian. This finding matches experimental results. We further investigated this finding by performing comparative analysis with denoising techniques, using two CDI datasets with two different currents (20 and 45 mA). We found that the block-matching and three-dimensional (BM3D) technique outperforms other techniques when applied on current density ([Formula: see text]). The minimum gain in noise power by BM3D applied to [Formula: see text] compared with the next best technique in the analysis was found to be around 2 dB per pixel. We characterize the noise profile in CDI images and provide insights on the performance of different denoising techniques when applied at two different stages of current density reconstruction.
Current Density Measurements of an Annular-Geometry Ion Engine
NASA Technical Reports Server (NTRS)
Shastry, Rohit; Patterson, Michael J.; Herman, Daniel A.; Foster, John E.
2012-01-01
The concept of the annular-geometry ion engine, or AGI-Engine, has been shown to have many potential benefits when scaling electric propulsion technologies to higher power. However, the necessary asymmetric location of the discharge cathode away from thruster centerline could potentially lead to non-uniformities in the discharge not present in conventional geometry ion thrusters. In an effort to characterize the degree of this potential non-uniformity, a number of current density measurements were taken on a breadboard AGI-Engine. Fourteen button probes were used to measure the ion current density of the discharge along a perforated electrode that replaced the ion optics during conditions of simulated beam extraction. Three Faraday probes spaced apart in the vertical direction were also used in a separate test to interrogate the plume of the AGI-Engine during true beam extraction. It was determined that both the discharge and the plume of the AGI-Engine are highly uniform, with variations under most conditions limited to +/-10% of the average current density in the discharge and +/-5% of the average current density in the plume. Beam flatness parameter measured 30 mm from the ion optics ranged from 0.85 - 0.95, and overall uniformity was shown to generally increase with increasing discharge and beam currents. These measurements indicate that the plasma is highly uniform despite the asymmetric location of the discharge cathode.
Current Density Measurements of an Annular-Geometry Ion Engine
NASA Technical Reports Server (NTRS)
Shastry, Rohit; Patterson, Michael J.; Herman, Daniel A.; Foster, John E.
2012-01-01
The concept of the annular-geometry ion engine, or AGI-Engine, has been shown to have many potential benefits when scaling electric propulsion technologies to higher power. However, the necessary asymmetric location of the discharge cathode away from thruster centerline could potentially lead to non-uniformities in the discharge not present in conventional geometry ion thrusters. In an effort to characterize the degree of this potential nonuniformity, a number of current density measurements were taken on a breadboard AGI-Engine. Fourteen button probes were used to measure the ion current density of the discharge along a perforated electrode that replaced the ion optics during conditions of simulated beam extraction. Three Faraday probes spaced apart in the vertical direction were also used in a separate test to interrogate the plume of the AGI-Engine during true beam extraction. It was determined that both the discharge and the plume of the AGI-Engine are highly uniform, with variations under most conditions limited to 10% of the average current density in the discharge and 5% of the average current density in the plume. Beam flatness parameter measured 30 mm from the ion optics ranged from 0.85 0.95, and overall uniformity was shown to generally increase with increasing discharge and beam currents. These measurements indicate that the plasma is highly uniform despite the asymmetric location of the discharge cathode.
Amending the uniformity of ion beam current density profile
NASA Astrophysics Data System (ADS)
Zhou, Xiaowei; Xu, Dequan; Liu, Ying; Xu, Xiangdong; Fu, Shaojun
2008-03-01
The uniformity of ion beam current density profile has been amended by changing the flow of the gas and making a new beam channel. The platform scanned in the horizontal orientation in this experiment, so the horizontal ion beam current distribution had hardly any effect on the etching uniformity and amending the ion beam current density profile in the vertical orientation was sufficient for the purpose of plat etching profile. The ratio of the ion source's working gas inputs has some effect for the uniformity and a ratio of 6.50sccm: 8.00sccm: 9.60sccm of the three gas inputs flow1: flow2: flow3 will lead to a more uniform profile. According to the horizontal distribution and the original vertical ion beam current density distribution measured by Faraday Cup, a new beam channel was made. The uniformity of ion beam current density profile is enhanced from +/-4.31%to +/-1.96% in this experiment.
Single Nanotube Voltammetry: Current Fluctuations Are Due to Physical Motion of the Nanotube
2016-01-01
Nanoimpacts of single palladium-coated carbon nanotubes on a gold substrate are studied to elucidate the origins of the fluctuation in the current–time response of the hydrogen oxidation reaction mediated at its surface. The chronoamperometric and cyclic voltammetric responses from a single nanotube immobilized on the gold surface were compared to analogous data on a carbon substrate to determine the possible influence of substrate material on the nanotube–electrode electrical contact. No significant distinction between the gold and carbon was found, indicating in light of the considerable differences in the substrate materials’ intrinsic electronic structures that it is the nanomotion of a nanotube at the electrode surface which is likely responsible for the observed current modulation. This nanomotion creates a varying contact resistance, to which the noise in the current–time signal of the mediated reaction is attributed. In addition, stochastic ex-situ adsorption of single nanotubes onto the gold electrode followed by careful drying of the electrode surface was found to drastically reduce the current fluctuation, again implying that a contact resistance arising from physical motion of the nanotube at the electrode is responsible for the modulation of current. PMID:27066159
Reprint of : Semiclassical theory of persistent current fluctuations in ballistic chaotic rings
NASA Astrophysics Data System (ADS)
Brouwer, Piet W.; Danon, Jeroen
2016-08-01
The persistent current in a mesoscopic ring has a Gaussian distribution with small non-Gaussian corrections. Here we report a semiclassical calculation of the leading non-Gaussian correction, which is described by the three-point correlation function. The semiclassical approach is applicable to systems in which the electron dynamics is ballistic and chaotic, and includes the dependence on the Ehrenfest time. At small but finite Ehrenfest times, the non-Gaussian fluctuations are enhanced with respect to the limit of zero Ehrenfest time.
Three-dimensional structure of dilute pyroclastic density currents
NASA Astrophysics Data System (ADS)
Andrews, B. J.
2013-12-01
Unconfined experimental density currents dynamically similar to pyroclastic density currents (PDCs) suggest that cross-stream motions of the currents and air entrainment through currents' lateral margins strongly affects PDC behavior. Experiments are conducted within an air-filled tank 8.5 m long by 6.1 m wide by 2.6 m tall. Currents are generated by feeding heated powders down a chute into the tank at controlled rates to form dilute, particle-laden, turbulent gravity currents that are fed for 30 to 600 seconds. Powders include 5 μm aluminum oxide, 25 μm talc, 27 μm walnut, 76 μm glass beads and mixtures thereof. Experiments are scaled such that Froude, densimetric and thermal Richardson, particle Stokes and Settling numbers, and thermal to kinetic energy densities are all in agreement with dilute PDCs; experiments have lower Reynolds numbers that natural currents, but the experiments are fully turbulent, thus the large scale structures should be similar. The experiments are illuminated with 3 orthogonal laser sheets (650, 532, and 450 nm wavelengths) and recorded with an array of HD video cameras and a high speed camera (up to 3000 fps); this system provides synchronous observation of a vertical streamwise and cross-stream planes, and a horizontal plane. Ambient temperature currents tend to spread out radially from the source and have long run out distances, whereas warmer currents tend to focus along narrow sectors and have shorter run outs. In addition, when warm currents lift off to form buoyant plumes, lateral spreading ceases. The behavior of short duration currents are dominated by the current head; as eruption duration increases, current transport direction tends to oscillate back and forth (this is particularly true for ambient temperature currents). Turbulent structures in the horizontal plane show air entrainment and advection downstream. Eddies illuminated by the vertical cross-stream laser sheet often show vigorous mixing along the current margins
Electric fields and current densities under small Florida thunderstorms
NASA Technical Reports Server (NTRS)
Deaver, Lance E.; Krider, E. P.
1991-01-01
Results are presented of measurements of the electric field E and Maxwell current density that were performed simultaneously under and near small Florida thunderstorms. It is shown that the amplitude of JM is of the order of 1 nA/sq cm or less in the absence of precipitation and that there are regular time variations in JM during the intervals between lightning discharges that tend to have the same shapes after different discharges in different storms. It is argued that the major causes of time variations in JM between lightning discharges are currents that flow in the finitely conducting atmosphere in response to the field changes rather than rapid time variations in the strength of cloud current sources. The displacement current densities that are computed from the E records dominate JM except when there is precipitation, when E is large and steady, or when E is unusually noisy.
Foroutan, G. R.; Robinson, P. A.; Sobhanian, S.; Moslehi-Fard, M.; Li, B.; Cairns, I. H.
2007-01-15
Gas-dynamic theory is generalized to incorporate the effects of beam-driven Langmuir waves scattering off ambient density fluctuations, and the consequent effects on the propagation of a cloud of hot electrons in an inhomogeneous plasma. Assuming Langmuir scattering as the limit of nonlinear three-wave interactions with fluctuations that are weak, low-frequency, long-wavelength ion-sound waves, the net effect of scattering is equivalent to effective damping of the Langmuir waves. Under the assumption of self-similarity in the evolution of the beam and Langmuir wave distribution functions, gas-dynamic theory shows that the effects of Langmuir scattering on the beam distribution are equivalent to a perturbation in the injection profile of the beam. Analytical expressions are obtained for the height of the plateau of the beam distribution function, wave spectral number density, total wave and particle energy density, and the beam number density. The main results of gas-dynamic theory are then compared with simulation results from numerical solutions of quasilinear equations. The relaxation of the beam in velocity space is retarded in the presence of density fluctuations and the magnitude of the upper velocity boundary is less than that in the absence of fluctuations. There are four different regimes for the height of the plateau, corresponding to different stages of relaxation of the beam in velocity space. Moreover, Langmuir scattering results in transfer of electrons from moderate velocity to low velocity; this effect produces an enhancement in the beam number density at small distances near the injection site and a corresponding decrease at large distances. There are sharp decreases in the profiles of the beam and total wave energy densities, which are related to dissipation of energy at large phase velocities. Due to a slower velocity space diffusion of the beam distribution in the presence of scattering effects, the spatial width of the beam is reduced while its
Observation of magnetic fluctuations and rapid density decay of magnetospheric plasma in Ring Trap 1
Saitoh, H.; Yoshida, Z.; Morikawa, J.; Yano, Y.; Mikami, H.; Kasaoka, N.; Sakamoto, W.
2012-06-15
The Ring Trap 1 device, a magnetospheric configuration generated by a levitated dipole field magnet, has created high-{beta} (local {beta} {approx} 70%) plasma by using electron cyclotron resonance heating (ECH). When a large population of energetic electrons is generated at low neutral gas pressure operation, high frequency magnetic fluctuations are observed. When the fluctuations are strongly excited, rapid loss of plasma was simultaneously observed especially in a quiet decay phase after the ECH microwave power is turned off. Although the plasma is confined in a strongly inhomogeneous dipole field configuration, the frequency spectra of the fluctuations have sharp frequency peaks, implying spatially localized sources of the fluctuations. The fluctuations are stabilized by decreasing the hot electron component below approximately 40%, realizing stable high-{beta} confinement.
Laboratory study of magnetic reconnection with a density asymmetry across the current sheet.
Yoo, Jongsoo; Yamada, Masaaki; Ji, Hantao; Jara-Almonte, Jonathan; Myers, Clayton E; Chen, Li-Jen
2014-08-29
The effects of a density asymmetry across the current sheet on anti-parallel magnetic reconnection are studied systematically in a laboratory plasma. Despite a significant density ratio of up to 10, the in-plane magnetic field profile is not significantly changed. On the other hand, the out-of-plane Hall magnetic field profile is considerably modified; it is almost bipolar in structure with the density asymmetry, as compared to quadrupolar in structure with the symmetric configuration. Moreover, the ion stagnation point is shifted to the low-density side, and the electrostatic potential profile also becomes asymmetric with a deeper potential well on the low-density side. Nonclassical bulk electron heating together with electromagnetic fluctuations in the lower hybrid frequency range is observed near the low-density-side separatrix. The dependence of the ion outflow and reconnection electric field on the density asymmetry is measured and compared with theoretical expectations. The measured ion outflow speeds are about 40% of the theoretical values.
NASA Technical Reports Server (NTRS)
Panda, Jayanta; Seasholtz, Richard G.
2003-01-01
Noise sources in high-speed jets were identified by directly correlating flow density fluctuation (cause) to far-field sound pressure fluctuation (effect). The experimental study was performed in a nozzle facility at the NASA Glenn Research Center in support of NASA s initiative to reduce the noise emitted by commercial airplanes. Previous efforts to use this correlation method have failed because the tools for measuring jet turbulence were intrusive. In the present experiment, a molecular Rayleigh-scattering technique was used that depended on laser light scattering by gas molecules in air. The technique allowed accurate measurement of air density fluctuations from different points in the plume. The study was conducted in shock-free, unheated jets of Mach numbers 0.95, 1.4, and 1.8. The turbulent motion, as evident from density fluctuation spectra was remarkably similar in all three jets, whereas the noise sources were significantly different. The correlation study was conducted by keeping a microphone at a fixed location (at the peak noise emission angle of 30 to the jet axis and 50 nozzle diameters away) while moving the laser probe volume from point to point in the flow. The following figure shows maps of the nondimensional coherence value measured at different Strouhal frequencies ([frequency diameter]/jet speed) in the supersonic Mach 1.8 and subsonic Mach 0.95 jets. The higher the coherence, the stronger the source was.
NASA Astrophysics Data System (ADS)
Carter, Troy; Martin, Michael; van Compernolle, Bart; Gekelman, Walter; Pribyl, Pat; Vincena, Stephen; Tripathi, Shreekrishna; van Eester, Dirk; Crombe, Kristel
2016-10-01
The LArge Plasma Device (LAPD) at UCLA is a 17 m long, up to 60 cm diameter magnetized plasma column with typical plasma parameters ne 1012 -1013 cm-3, Te 1 - 10 eV, and B 1 kG. A new high-power ( 200 kW) RF system and antenna has been developed for LAPD, enabling the generation of large amplitude fast waves in LAPD. Interaction between the fast waves and density fluctuations is observed, resulting in modulation of the coupled RF power. Two classes of RF-induced density fluctuations are observed. First, a coherent (10 kHz) oscillation is observed spatially near the antenna in response to the initial RF turn-on transient. Second, broadband density fluctuations are enhanced when the RF power is above a threshold a threshold. Strong modulation of the fast wave magnetic fluctuations is observed along with broadening of the primary RF spectral line. Ultimately, high power fast waves will be used for ion heating in LAPD through minority species fundamental heating or second harmonic minority or majority heating. Initial experimental results from heating experiments will be presented along with a discussion of future plans. BaPSF supported by NSF and DOE.
Thermal effects on the electron density fluctuation spectra in NIF plasmas
NASA Astrophysics Data System (ADS)
Rozmus, W.; Chapman, T.; Tzoufras, M.; Berger, R.; Brunner, S.; Divol, L.; Michel, P.; Williams, E.; Glenzer, S.
2012-10-01
The high flux model of ignition-scale hohlraum plasmas includes the strong thermal flux from the region of laser beam overlap at the entrance hole of the hohlraum along the directions of the inner cone beams. We have examined results of this large heat flow at the kinetic level using Fokker-Planck codes, which reproduce the temperature profile and corresponding electron distribution functions on the millimeter scale of NIF plasmas. Using the first harmonic of the electron distribution, we have identified contributions from the energetic, heat carrying electrons and the return current component within the bulk of the distribution function. In hot NIF plasmas, the heat-carrying electrons have energies (20-40 keV) that are close to resonance with Langmuir waves produced by SRS. By calculating the plasma dielectric function using distribution functions extracted from Fokker-Planck simulations, we have found a significant reduction in the linear Landau damping for the Langmuir waves propagating in the direction of heat flow, potentially contributing to the onset of backward SRS. This effect was further examined in Vlasov simulations and by calculations of the electrostatic fluctuation levels.
Path to Efficient Lower Hybrid Current Drive at High Density
NASA Astrophysics Data System (ADS)
Baek, S. G.; Bonoli, P. T.; Brunner, D.; Faust, I.; Labombard, B. L.; Parker, R. R.; Shiraiwa, S.; Wallace, G. M.; Wukitch, S.
2015-11-01
Recovery of lower hybrid current drive (LHCD) efficiency at high density was demonstrated on Alcator C-Mod by modifying the scrape-off layer (SOL) plasma. RF probe measurements around the C-Mod tokamak indicate that the LH wave amplitude at the high field side wall significantly attenuates with plasma density. This is interpreted as enhanced collisional loss due to the increase in the SOL density and width. By taking advantage of the narrower SOL width by doubling plasma current to 1.1 MA, it is found that the LH wave amplitude maintains its strength, and an effective current drive is extended to above 1x10e20 m-3. An order of magnitude increase in non-thermal Bremsstrahlung emission is consistent with ray-tracing results which take into account the change of SOL profiles with current. In the coming campaign, a further investigation on the role of the SOL plasma is planned by raising plasma current above 1.1 MA. This will be aided with newly developed RF magnetic loop antennas mounted on a radially movable probe head. This system is expected to intercept the LH resonance cone on the first pass, allowing us to measure radial profiles of both the wave amplitude and dominant parallel wavenumber in the SOL for the first time. These data will be compared with the GENRAY ray-tracing code. Work supported by USDoE awards DE-FC02-99ER54512.
MEG forward problem formulation using equivalent surface current densities.
von Ellenrieder, Nicolás; Muravchik, Carlos H; Nehorai, Arye
2005-07-01
We present a formulation for the magnetoencephalography (MEG) forward problem with a layered head model. Traditionally the magnetic field is computed based on the electric potential on the interfaces between the layers. We propose to express the effect of the volumetric currents in terms of an equivalent surface current density on each interface, and obtain the magnetic field based on them. The boundary elements method is used to compute the equivalent current density and the magnetic field for a realistic head geometry. We present numerical results showing that the MEG forward problem is solved correctly with this formulation, and compare it with the performance of the traditional formulation. We conclude that the traditional formulation generally performs better, but still the new formulation is useful in certain situations.
Miao, Xinyu; Yin, Longfei; Zhuang, Wei; Luo, Bin; Dang, Anhong; Chen, Jingbiao; Guo, Hong
2011-08-01
We demonstrate an external-cavity laser system using an anti-reflection coated laser diode as gain medium with about 60 nm fluorescence spectrum, and a Rb Faraday anomalous dispersion optical filter (FADOF) as frequency-selecting element with a transmission bandwidth of 1.3 GHz. With 6.4% optical feedback, a single stable longitudinal mode is obtained with a linewidth of 69 kHz. The wavelength of this laser is operating within the center of the highest transmission peak of FADOF over a diode current range from 55 mA to 142 mA and a diode temperature range from 15 °C to 35 °C, thus it is immune to the fluctuations of current and temperature.
Modeling dilute pyroclastic density currents on Earth and Mars
NASA Astrophysics Data System (ADS)
Clarke, A. B.; Brand, B. D.; De'Michieli Vitturi, M.
2013-12-01
The surface of Mars has been shaped extensively by volcanic activity, including explosive eruptions that may have been heavily influenced by water- or ice-magma interaction. However, the dynamics of associated pyroclastic density currents (PDCs) under Martian atmospheric conditions and controls on deposition and runout from such currents are poorly understood. This work combines numerical modeling with terrestrial field measurements to explore the dynamics of dilute PDC dynamics on Earth and Mars, especially as they relate to deposit characteristics. We employ two numerical approaches. Model (1) consists of simulation of axi-symmetric flow and sedimentation from a steady-state, depth-averaged density current. Equations for conservation of mass, momentum, and energy are solved simultaneously, and the effects of atmospheric entrainment, particle sedimentation, basal friction, temperature changes, and variations in current thickness and density are explored. The Rouse number and Brunt-Väisälä frequency are used to estimate the wavelength of internal gravity waves in a density-stratified current, which allows us to predict deposit dune wavelengths. The model predicts realistic runout distances and bedform wavelengths for several well-documented field cases on Earth. The model results also suggest that dilute PDCs on Mars would have runout distances up to three times that of equivalent currents on Earth and would produce longer-wavelength bedforms. In both cases results are heavily dependent on source conditions, grain-size characteristics, and entrainment and friction parameters. Model (2) relaxes several key simplifications, resulting in a fully 3D, multiphase, unsteady model that captures more details of propagation, including density stratification, and depositional processes. Using this more complex approach, we focus on the role of unsteady or pulsatory vent conditions typically associated with phreatomagmatic eruptions. Runout distances from Model (2) agree
Planets in other universes: habitability constraints on density fluctuations and galactic structure
NASA Astrophysics Data System (ADS)
Adams, Fred C.; Coppess, Katherine R.; Bloch, Anthony M.
2015-09-01
Motivated by the possibility that different versions of the laws of physics could be realized within other universes, this paper delineates the galactic structure parameters that allow for habitable planets and revisits constraints on the amplitude Q of the primordial density fluctuations. Previous work indicates that large values of Q lead to galaxies so dense that planetary orbits cannot survive long enough for life to develop. Small values of Q lead to delayed star formation, loosely bound galaxies, and compromised heavy element retention. This work generalizes previous treatments in the following directions: [A] We consider models for the internal structure of the galaxies, including a range of stellar densities, and find the fraction of the resulting galactic real estate that allows for stable, long-lived planetary orbits. [B] For high velocity encounters, we perform a large ensemble of numerical simulations to estimate cross sections for the disruption of planetary orbits due to interactions with passing stars. [C] We consider the background radiation fields produced by the galaxies: if a galaxy is too compact, the night sky seen from a potentially habitable planet can provide more power than the host star. [D] One consequence of intense galactic background radiation fields is that some portion of the galaxy, denoted as the Galactic Habitable Zone, will provide the right flux levels to support habitable planets for essentially any planetary orbit including freely floating bodies (but excluding close-in planets). As the value of Q increases, the fraction of stars in a galaxy that allow for (traditional) habitable planets decreases due to both orbital disruption and the intense background radiation. However, the outer parts of the galaxy always allow for habitable planets, so that the value of Q does not have a well-defined upper limit (due to scattering or radiation constraints). Moreover, some Galactic Habitable Zones are large enough to support more
Transport and sedimentation in unconfined experimental dilute pyroclastic density currents
NASA Astrophysics Data System (ADS)
Ramirez, G.; Andrews, B. J.; Dennen, R. L.
2013-12-01
We present results from experiments conducted in a new facility that permits the study of large, unconfined particle laden density currents that are dynamically similar to natural dilute pyroclastic density currents (PDCs). Experiments were run in a sealed, air-filled tank measuring 8.5 m long by 6.1 m wide by 2.6 m tall. Currents were generated by feeding mixture of heated particles (5 μm aluminum oxide, 25 μm talc, 27 μm walnut shell, 76 μm glass beads) down a chute at controlled rates to produce dilute, turbulent gravity currents. Comparison of experimental currents with natural PDCs shows good agreement between Froude, densimetric and thermal Richardson, and particle Stokes and settling numbers; experimental currents have lower Reynolds numbers than natural PDCs, but are fully turbulent. Currents were illuminated with 3 orthogonal laser sheets (650, 532, and 450 nm wavelengths) and recorded with an array of HD video cameras and a high speed camera (up to 3000 fps). Deposits were mapped using a grid of sedimentation traps. We observe distinct differences between ambient temperature and warm currents: * warm currents have shorter run out distances, narrow map view distributions of currents and deposits, thicken with distance from the source, and lift off to form coignimbrite plumes; * ambient temperature currents typically travel farther, spread out radially, do not thicken greatly with transport distance, and do not form coignimbrite plumes. Long duration currents (600 s compared to 30-100 s) oscillate laterally with time (e.g. transport to the right, then the left, and back); this oscillation happens prior to any interaction with the tank walls. Isopach maps of the deposits show predictable trends in sedimentation versus distance in response to eruption parameters (eruption rate, duration, temperature, and initial current mass), but all sedimentation curves can be fit with 2nd order polynomials (R2>.9). Proximal sedimentation is similar in comparable warm
High dislocation density of tin induced by electric current
Liao, Yi-Han; Liang, Chien-Lung; Lin, Kwang-Lung; Wu, Albert T.
2015-12-15
A dislocation density of as high as 10{sup 17} /m{sup 2} in a tin strip, as revealed by high resolution transmission electron microscope, was induced by current stressing at 6.5 x 10{sup 3} A/ cm{sup 2}. The dislocations exist in terms of dislocation line, dislocation loop, and dislocation aggregates. Electron Backscattered Diffraction images reflect that the high dislocation density induced the formation of low deflection angle subgrains, high deflection angle Widmanstätten grains, and recrystallization. The recrystallization gave rise to grain refining.
NASA Technical Reports Server (NTRS)
Asenov, Asen; Kaya, S.; Davies, J. H.; Saini, S.
2000-01-01
We use the density gradient (DG) simulation approach to study, in 3D, the effect of local oxide thickness fluctuations on the threshold voltage of decanano MOSFETs in a statistical manner. A description of the reconstruction procedure for the random 2D surfaces representing the 'atomistic' Si-SiO2 interface variations is presented. The procedure is based on power spectrum synthesis in the Fourier domain and can include either Gaussian or exponential spectra. The simulations show that threshold voltage variations induced by oxide thickness fluctuation become significant when the gate length of the devices become comparable to the correlation length of the fluctuations. The extent of quantum corrections in the simulations with respect to the classical case and the dependence of threshold variations on the oxide thickness are examined.
NASA Astrophysics Data System (ADS)
Nagayama, Yoshio; Yamaguchi, Soichiro; Tsuchiya, Hayato; Kuwahara, Daisuke; LHD Experimental Team
2016-10-01
Visualization of local electron density fluctuations will be very useful to study the physics of confinement and instabilities in fusion plasma. In the Large Helical Device (LHD), the O-mode microwave imaging reflectometry (O-MIR) has been intensively developed in order to visualize the electron density fluctuations. The frequency is 26 - 34 GHz. This corresponds to the electron density of 0.8 - 1.5 × 1019 m-3. The plasma is illuminated by the Gaussian beam with four frequencies. The imaging optics make a plasma image onto the newly developed 2D (8 × 8) Horn-antenna Millimeter-wave Imaging Device (HMID). In HMID, the signal wave that is accumulated by the horn antenna is transduced to the micro-strip line by using the finline transducer. The signal wave is mixed by the double balanced mixer with the local wave that is delivered by cables. By using O-MIR, electron density fluctuations are measured at the H-mode edge and the ITB layer in LHD. This work is supported by NIFS/NINS under the project of Formation of International Scientific Base and Network, by the NIFS LHD project, by KAKENHI, and by IMS.
Karunaweera, Sadish
2015-01-01
Fluctuation solution theory has provided an alternative view of many liquid mixture properties in terms of particle number fluctuations. The particle number fluctuations can also be related to integrals of the corresponding two body distribution functions between molecular pairs in order to provide a more physical picture of solution behavior and molecule affinities. Here, we extend this type of approach to provide expressions for higher order triplet and quadruplet fluctuations, and thereby integrals over the corresponding distribution functions, all of which can be obtained from available experimental thermodynamic data. The fluctuations and integrals are then determined using the International Association for the Properties of Water and Steam Formulation 1995 (IAPWS-95) equation of state for the liquid phase of pure water. The results indicate small, but significant, deviations from a Gaussian distribution for the molecules in this system. The pressure and temperature dependence of the fluctuations and integrals, as well as the limiting behavior as one approaches both the triple point and the critical point, are also examined. PMID:25637990
Ploetz, Elizabeth A.; Karunaweera, Sadish; Smith, Paul E.
2015-01-28
Fluctuation solution theory has provided an alternative view of many liquid mixture properties in terms of particle number fluctuations. The particle number fluctuations can also be related to integrals of the corresponding two body distribution functions between molecular pairs in order to provide a more physical picture of solution behavior and molecule affinities. Here, we extend this type of approach to provide expressions for higher order triplet and quadruplet fluctuations, and thereby integrals over the corresponding distribution functions, all of which can be obtained from available experimental thermodynamic data. The fluctuations and integrals are then determined using the International Association for the Properties of Water and Steam Formulation 1995 (IAPWS-95) equation of state for the liquid phase of pure water. The results indicate small, but significant, deviations from a Gaussian distribution for the molecules in this system. The pressure and temperature dependence of the fluctuations and integrals, as well as the limiting behavior as one approaches both the triple point and the critical point, are also examined.
Tarrab, Leticia; Garcia, Carlos M.; Cantero, Mariano I.; Oberg, Kevin
2012-01-01
This work presents a systematic analysis quantifying the role of the presence of turbulence fluctuations on uncertainties (random errors) of acoustic Doppler current profiler (ADCP) discharge measurements from moving platforms. Data sets of three-dimensional flow velocities with high temporal and spatial resolution were generated from direct numerical simulation (DNS) of turbulent open channel flow. Dimensionless functions relating parameters quantifying the uncertainty in discharge measurements due to flow turbulence (relative variance and relative maximum random error) to sampling configuration were developed from the DNS simulations and then validated with field-scale discharge measurements. The validated functions were used to evaluate the role of the presence of flow turbulence fluctuations on uncertainties in ADCP discharge measurements. The results of this work indicate that random errors due to the flow turbulence are significant when: (a) a low number of transects is used for a discharge measurement, and (b) measurements are made in shallow rivers using high boat velocity (short time for the boat to cross a flow turbulence structure).
Three-dimensional structure of electron density fluctuations in the Hall thruster plasma: ExB mode
Tsikata, S.; Honore, C.; Gresillon, D. M.; Lemoine, N.
2010-11-15
Collective scattering measurements have been conducted on the plasma of a Hall thruster, in which the electron density fluctuations are fully characterized by the dynamic form factor. The dynamic form factor amplitude distribution has been measured depending on the k-vector spatial and frequency components at different locations. Fluctuations are seen as propagating waves. The largest amplitude mode propagates nearly along the cross-field direction but at a phase velocity that is much smaller than the ExB drift velocity. Refined directional analysis of this largest amplitude mode shows a thin angular emission diagram with a mean direction that is not strictly along the ExB direction but at small angles near it. The deviation is oriented toward the anode in the (E,ExB) plane and toward the exterior of the thruster channel in the (B,ExB) plane. The density fluctuation rate is on the order of 1%. These experimentally determined directional fluctuation characteristics are discussed with regard to the linear kinetic theory model and particle-in-cell simulation results.
NASA Astrophysics Data System (ADS)
Burmistrov, I. S.; Gornyi, I. V.; Mirlin, A. D.
2016-05-01
We develop a theory of the local density of states (LDOS) of disordered superconductors, employing the nonlinear sigma-model formalism and the renormalization-group framework. The theory takes into account the interplay of disorder and interaction couplings in all channels, treating the systems with short-range and Coulomb interactions on equal footing. We explore two-dimensional systems that would be Anderson insulators in the absence of interaction and two- or three-dimensional systems that undergo an Anderson transition in the absence of interaction. We evaluate both the average tunneling density of states and its mesoscopic fluctuations which are related to the LDOS multifractality in normal disordered systems. The obtained average LDOS shows a pronounced depletion around the Fermi energy, both in the metallic phase (i.e., above the superconducting critical temperature Tc) and in the insulating phase near the superconductor-insulator transition (SIT). The fluctuations of the LDOS are found to be particularly strong for the case of short-range interactions, especially, in the regime when Tc is enhanced by Anderson localization. On the other hand, the long-range Coulomb repulsion reduces the mesoscopic LDOS fluctuations. However, also in a model with Coulomb interaction, the fluctuations become strong when the systems approach the SIT.
Evidence for explosive volcanic density currents on certain Martian volcanoes
NASA Technical Reports Server (NTRS)
Reimers, C. E.; Komar, P. D.
1979-01-01
The morphologies of certain of the smaller Martian volcanoes are discussed as possible results of explosive volcanic density currents. An examination of newly-photographed flank and caldera features of the Martian volcanoes Ceraunius Tholus, Uranius Tholus, Uranius Patera and Hecates Tholus, including steep slope angles, Krakatoa-type caldera morphologies, erosional features (radial channels and anastamosing gullies) and constructional features (blanketed flanks and possible lava deltas) reveals their similarity to terrestrial cones and composite volcanoes such as Barcena Volcano. Crater age data from the surface of Martian domes and shields indicates that such explosive activity occurred more frequently early in Martian geologic history, consistent with the view that the volcanic density currents were base surges rather than nuees ardentes, with the melting of permafrost supplying the water required in base surge generation.
Enhancement of Current Density by dc Electric Concentrator
Jiang, Wei Xiang; Luo, Chen Yang; Ma, Hui Feng; Mei, Zhong Lei; Cui, Tie Jun
2012-01-01
We investigate a dc electric concentrator for steady current fields theoretically and experimentally. Based on the transformation electrostatics, we show that the dc concentrator can focus electric currents into the central concentrated region and enhance the electric field and current density. Outside the concentrator, the current lines are distributed as the same as those in a homogeneous conducting material. Hence, such a dc electric concentrator has no impact on other external devices. Using the analogy between electrically conducting materials and resistor networks, we design, fabricate, and test a dc concentrator using the circuit theory. The measured results agree very well with the theoretical predictions and numerical simulations, demonstrating the perfect concentrating performance. PMID:23233875
Thin-film Josephson junctions with alternating critical current density
NASA Astrophysics Data System (ADS)
Moshe, Maayan; Kogan, V. G.; Mints, R. G.
2009-01-01
We study the field dependence of the maximum current Im(H) in narrow edge-type thin-film Josephson junctions with alternating critical current density. Im(H) is evaluated within nonlocal Josephson electrodynamics taking into account the stray fields that affect the difference of the order-parameter phases across the junction and therefore the tunneling currents. We find that the phase difference along the junction is proportional to the applied field, depends on the junction geometry, but is independent of the Josephson critical current density gc , i.e., it is universal. An explicit form for this universal function is derived for small currents through junctions of the width W≪Λ , the Pearl length. The result is used to calculate Im(H) . It is shown that the maxima of Im(H)∝1/H and the zeros of Im(H) are equidistant but only in high fields. We find that the spacing between zeros is proportional to 1/W2 . The general approach is applied to calculate Im(H) for a superconducting quantum interference device with two narrow edge-type junctions. If gc changes sign periodically or randomly, as it does in grain boundaries of high- Tc materials and superconductor-ferromagnet-superconductor heterostructures, Im(H) not only acquires the major side peaks, but due to nonlocality the following peaks decay much slower than in bulk junctions.
NASA Astrophysics Data System (ADS)
Hurtado, Pablo I.; Garrido, Pedro L.
2009-02-01
We study the distribution of the time-integrated current in an exactly solvable toy model of heat conduction, both analytically and numerically. The simplicity of the model allows us to derive the full current large deviation function and the system statistics during a large deviation event. In this way we unveil a relation between system statistics at the end of a large deviation event and for intermediate times. The mid-time statistics is independent of the sign of the current, a reflection of the time-reversal symmetry of microscopic dynamics, while the end-time statistics does depend on the current sign, and also on its microscopic definition. We compare our exact results with simulations based on the direct evaluation of large deviation functions, analyzing the finite-size corrections of this simulation method and deriving detailed bounds for its applicability. We also show how the Gallavotti-Cohen fluctuation theorem can be used to determine the range of validity of simulation results.
Ionic screening effect on low-frequency drain current fluctuations in liquid-gated nanowire FETs.
Lu, Ming-Pei; Vire, Eric; Montès, Laurent
2015-12-11
The ionic screening effect plays an important role in determining the fundamental surface properties within liquid-semiconductor interfaces. In this study, we investigated the characteristics of low-frequency drain current noise in liquid-gated nanowire (NW) field effect transistors (FETs) to obtain physical insight into the effect of ionic screening on low-frequency current fluctuation. When the NW FET was operated close to the gate voltage corresponding to the maximum transconductance, the magnitude of the low-frequency noise for the NW exposed to a low-ionic-strength buffer (0.001 M) was approximately 70% greater than that when exposed to a high-ionic-strength buffer (0.1 M). We propose a noise model, considering the charge coupling efficiency associated with the screening competition between the electrolyte buffer and the NW, to describe the ionic screening effect on the low-frequency drain current noise in liquid-gated NW FET systems. This report not only provides a physical understanding of the ionic screening effect behind the low-frequency current noise in liquid-gated FETs but also offers useful information for developing the technology of NW FETs with liquid-gated architectures for application in bioelectronics, nanosensors, and hybrid nanoelectronics.
NASA Astrophysics Data System (ADS)
Lehmacher, Gerald; Larsen, Miguel; Collins, Richard; Bilen, Sven; Croskey, Charles; Mitchell, John; Luebken, Franz-Josef; Rapp, Markus
In February 2009, a rocket experiment was launched from Alaska entitled: Where is the tur-bopause? Instabilities, generation and development of turbulence in the 100-km region. The salvo of four rockets obtained in situ wind and temperature profiles, neutral and plasma fluctu-ations, and wave and tidal activity from ground based lidar, radar, and other instrumentation. Among the goals are comparisons of turbulent energy dissipation rates measured by spectral analysis and from chemical trail expansion rates. Based on trimethyl aluminum trail diffusion we identified regions of mixing around 90 km, 95 km, and also above 100 km. The lower re-gion coincided with layers of density fluctuations, while the upper region was characterized by strong wind shear and kilometer-size density structures in the lower thermosphere. Rayleigh and sodium lidar observed a dominant 4-hour wave motion in the upper mesosphere.
Magnetic topology and current channels in plasmas with toroidal current density inversions
Ciro, D.; Caldas, I. L.
2013-10-15
The equilibrium magnetic field inside axisymmetric plasmas with inversions on the toroidal current density is considered. Previous works have shown that internal regions with negative current density lead to non-nested magnetic surfaces inside the plasma. Following these results, we derive a general expression relating the positive and negative currents inside the non-nested surfaces. This is done in terms of an anisotropy parameter that is model-independent and is based in very general properties of the magnetic field. We demonstrate that the positive currents in axisymmetric islands screen the negative one in the plasma center by reaching about twice its magnitude. Further, we illustrate these results by developing a family of analytical local solutions for the poloidal magnetic field in a region of interest that contains the inverted current. These local solutions exhibit non-nested magnetic surfaces with a combined current of at least twice the magnitude of the negative one, as prescribed from the topological arguments, and allow to study topological transitions driven by geometrical changes in the current profile. To conclude, we discuss the signatures of internal current density inversions in a confinement device and show that magnetic pitch measurements may be inappropriate to differentiate current reversals and small current holes in plasmas.
Morphology and Density Structure of Post-CME Current Sheets
NASA Technical Reports Server (NTRS)
Vrsnak, B.; Poletto, G.; Vujic, E.; Vourlidas, A.
2009-01-01
Eruption of a coronal mass ejection (CME) is believed to drag and open the coronal magnetic field, presumably leading to the formation of a large-scale current sheet and field relaxation by magnetic reconnection. This paper analyzes the physical characteristics of ray-like coronal features formed in the aftermath of CMEs, to confirm whether interpreting such phenomena in terms of a reconnecting current sheet is consistent with observations. Methods: The study focuses on UVCS/SOHO and LASCO/SOHO measurements of the ray width, density excess, and coronal velocity field as a function of the radial distance. The morphology of the rays implies that they are produced by Petschek-like reconnection in the large-scale current sheet formed in the wake of CME. The hypothesis is supported by the flow pattern, often showing outflows along the ray, and sometimes also inflows into the ray. The inferred inflow velocities range from 3 to 30 km/s, and are consistent with the narrow opening-angle of rays, which add up to a few degrees. The density of rays is an order of magnitude higher than in the ambient corona. The model results are consistent with the observations, revealing that the main cause of the density excess in rays is a transport of the dense plasma from lower to higher heights by the reconnection outflow.
Unsteady density-current equations for highly curved terrain
NASA Technical Reports Server (NTRS)
Sivakumaran, N. S.; Dressler, R. F.
1989-01-01
New nonlinear partial differential equations containing terrain curvature and its rate of change are derived that describe the flow of an atmospheric density current. Unlike the classical hydraulic-type equations for density currents, the new equations are valid for two-dimensional, gradually varied flow over highly curved terrain, hence suitable for computing unsteady (or steady) flows over arbitrary mountain/valley profiles. The model assumes the atmosphere above the density current exerts a known arbitrary variable pressure upon the unknown interface. Later this is specialized to the varying hydrostatic pressure of the atmosphere above. The new equations yield the variable velocity distribution, the interface position, and the pressure distribution that contains a centrifugal component, often significantly larger than its hydrostatic component. These partial differential equations are hyperbolic, and the characteristic equations and characteristic directions are derived. Using these to form a characteristic mesh, a hypothetical unsteady curved-flow problem is calculated, not based upon observed data, merely as an example to illustrate the simplicity of their application to unsteady flows over mountains.
NASA Astrophysics Data System (ADS)
Babst, Flurin; Wright, William; Szejner, Paul; Wells, Leon; Belmecheri, Soumaya; Monson, Russell
2016-04-01
Rapidly rising evaporative demand threatens forests in semi-arid areas around the world, but the timing of stem growth response to drought is often coarsely known. This is partly due to a shortage of sub-annual growth records, particularly outside the Mediterranean region where most intra-annual density fluctuation (IADF) chronologies are based. We anticipate that an automated, cost-effective, and easily implementable method to characterize IADFs could foster more widespread development of sub-annual chronologies. Here, we applied a peak detection algorithm to fine-spatial resolution blue intensity (BI) profiles of Ponderosa pine tree rings from two sites located in neighboring mountain ranges in southern Arizona (~300 m elevation difference). This automated procedure proved reliable to isolate and characterize IADFs, thus offering an efficient and objective alternative to visual identification. Out of seven investigated BI parameters, peak height, width, and area showed satisfactory chronology statistics. We assessed the response of these BI and radial growth parameters to six monthly-resolved climate variables and to the onset date of the North American summer monsoon (NAM). The NAM is an atmospheric mode that provides a clear time marker for the termination of a pre-summer drought period (May-June) causing regular IADFs in trees growing near the dry margin of their distribution range. We observed divergent water limitation at the two sites, despite comparable site characteristics. Radial growth at the lower-elevation site depended mainly on winter precipitation, whereas the higher site relied on spring and monsoon precipitation. The pre-summer drought period indeed promoted IADFs in early ring portions at both sites. Yet, IADFs at the higher site were only formed, if spring was sufficiently humid to assume enough radial growth. Late-position IADFs were caused by a weak monsoon and additionally promoted by favorable conditions towards the end of the growing
Inoue, M; Imanaga, I
1996-11-11
Properties of aluminium fluoride (AIF) complex-activated nonselective cation (NS) channels in guinea-pig chromaffin cells were investigated using the patch clamp technique. As the membrane potential was hyperpolarized from the holding potential of -55 mV, the AIF-induced nonselective cation current (INS) diminished progressively. With hyperpolarizations to -100 mV or more negative potentials, the AIF.INS almost instantaneously disappeared. The apparent unit conductance of AIF INS was estimated to be 3 pS by fluctuation analysis. The open state probability of AIF-activated NS channels became large with a decrease in concentration of free Mg2+ ions inside the cell and was less than 0.5 at 12 microM Mg2+. It is concluded that NS channels in the chromaffin cell apparently differ from those in smooth muscle cells.
NASA Technical Reports Server (NTRS)
Clukey, Steven J.; Jones, Gregory S.; Stainback, P. Calvin
1988-01-01
The use of a high-speed Dynamic Data Acquisition System (DDAS) to measure simultaneously velocity, density, and total temperature fluctuations is described. The DDAS is used to automate the acquisition of hot-wire calibration data. The data acquisition, data handling, and data reporting techiques used by DDAS are described. Sample data are used to compare results obtained with the DDAS with those obtained from the FM tape and post-test digitization method.
Lehners, Jean-Luc; Steinhardt, Paul J.
2008-03-15
We analyze the non-Gaussian density perturbations generated in ekpyrotic/cyclic models based on heterotic M theory. In this picture, two scalar fields produce nearly scale-invariant entropic perturbations during an ekpyrotic phase that are converted into curvature modes after the ekpyrotic phase is complete and just before the big bang. Both intrinsic nonlinearity in the entropy perturbation and the conversion process contribute to non-Gaussianity. The range of the non-Gaussianity parameter f{sub NL} depends on how gradual the conversion process is and the steepness of the scalar field potential during the ekpyrotic phase. Although a wider range is possible, in principle, natural values of the ekpyrotic parameters combined with a gradual conversion process lead to values of -50 < or approx. f{sub NL} < or approx. +200, typically much greater than slow-roll inflation but within the current observational bounds.
Magnetohydrodynamically stable plasma with supercritical current density at the axis
Burdakov, A. V.; Postupaev, V. V. Sudnikov, A. V.
2014-05-15
In this work, an analysis of magnetic perturbations in the GOL-3 experiment is given. In GOL-3, plasma is collectively heated in a multiple-mirror trap by a high-power electron beam. During the beam injection, the beam-plasma interaction maintains a high-level microturbulence. This provides an unusual radial profile of the net current (that consists of the beam current, current of the preliminary discharge, and the return current). The plasma core carries supercritical current density with the safety factor well below unity, but as a whole, the plasma is stable with q(a) ≈ 4. The net plasma current is counter-directed to the beam current; helicities of the magnetic field in the core and at the edge are of different signs. This forms a system with a strong magnetic shear that stabilizes the plasma core in good confinement regimes. We have found that the most pronounced magnetic perturbation is the well-known n = 1, m = 1 mode for both stable and disruptive regimes.
High current density, cryogenically cooled sliding electrical joint development
Murray, H.
1986-09-01
In the past two years, conceptual designs for fusion energy research devices have focussed on compact, high magnetic field configurations. The concept of sliding electrical joints in the large magnets allows a number of technical advantages including enhanced mechanical integrity, remote maintainability, and reduced project cost. The rationale for sliding electrical joints is presented. The conceptual configuration for this generation of experimental devices is highlghted by an approx. 20 T toroidal field magnet with a flat top conductor current of approx. 300 kA and a sliding electrical joint with a gross current density of approx. 0.6 kA/cm/sup 2/. A numerical model was used to map the conductor current distribution as a function of time and position in the conductor. A series of electrical joint arrangements were produced against the system code envelope constraints for a specific version of the Ignition Studies Project (ISP) which is designated as 1025.
Effect of current density on enhanced transformation of naphthalene.
Alshawabkeh, Akram N; Sarahney, Hussam
2005-08-01
The effect of current density on electrochemically enhanced transformation of naphthalene is evaluated. Electrochemical reactors, composed of an anode and a cathode separated by a Nafion membrane, were used to evaluatethe effect of three current densities (1,9, and 18 mA/ L) on the transformation of naphthalene at two concentration levels (13 and 25 mg/L). Transformation rates varied based on the concentration and current density. Almost 88% of the 13 mg/L naphthalene is degraded after 8 h of treatment under 18.2 mA/L. At the same time, more than 90 h was required to degrade the same amount under 9 mA/ L. The results show that most of the naphthalene degradation occurred in the first 4 h under transformation rates of 2.24 and 1.11 mg/L h under applied currents of 18.2 and 9 mA/L, respectively. Increasing the naphthalene concentration to 25 mg/L produced similar results. Under 18.2 mA/L, the redox potential increased significantly at the anolyte in the first 8 h to about 900 mV. After that, the redox potential continued to increase, but at a lower rate, until it reached 1380 mV at the end of processing. Similar behavior is noted for the anolyte pH, which decreased significantly in the first 8 h to less than 2.5 and continued to decrease until it reached a pH value of 1.86 at the end of testing. Naphthalene transformation can be attributed to electrochemically enhanced oxidation at the anolyte by chlorine gas produced by electrolysis.
Stochastic Time-Dependent Current-Density Functional Theory
NASA Astrophysics Data System (ADS)
D'Agosta, Roberto
2008-03-01
Static and dynamical density functional methods have been applied with a certain degree of success to a variety of closed quantum mechanical systems, i.e., systems that can be described via a Hamiltonian dynamics. However, the relevance of open quantum systems - those coupled to external environments, e.g., baths or reservoirs - cannot be overestimated. To investigate open quantum systems with DFT methods we have introduced a new theory, we have named Stochastic Time-Dependent Current Density Functional theory (S-TDCDFT) [1]: starting from a suitable description of the system dynamics via a stochastic Schrödinger equation [2], we have proven that given an initial quantum state and the coupling between the system and the environment, there is a one-to-one correspondence between the ensemble-averaged current density and the external vector potential applied to the system.In this talk, I will introduce the stochastic formalism needed for the description of open quantum systems, discuss in details the theorem of Stochastic TD-CDFT, and provide few examples of its applicability like the dissipative dynamics of excited systems, quantum-measurement theory and other applications relevant to charge and energy transport in nanoscale systems.[1] M. Di Ventra and R. D'Agosta, Physical Review Letters 98, 226403 (2007)[2] N.G. van Kampen, Stochastic processes in Physics and Chemistry, (North Holland, 2001), 2nd ed.
Incorporation Of Air Into The Campanian Ignimbrite Pyroclastic Density Current
NASA Astrophysics Data System (ADS)
Ort, M. H.; Giordano, G.; Zanella, E.; Isaia, R.
2015-12-01
Knowing the temperature of emplacement of an ignimbrite can tell us how much cooling air it incorporated during eruption and transport. Currents that incorporate cool matter (air, water, cold clasts) cool more than those that do not. Lithic fragments record the maximum temperature they reached, up to their maximum unblocking temperature. Studies of large ignimbrites (e.g. Cerro Galan Ignimbrite) emplaced by dense currents show they do not cool very much, with emplacement temperatures often above 580 oC. Smaller currents, such as those from Vesuvius and Colima, lose significant heat in the eruption column, and then lose some, but less, heat as they travel laterally. The amount of atmosphere incorporated by large dilute currents is not known. The ~40 ka Campanian Ignimbrite (CI) erupted from the Campi Flegrei caldera near Naples, Italy, and extends to ~75 km from the caldera. The CI was emplaced from a density-stratified current with a dilute transport system and a denser depositional system that overtopped 1600-m-high ridges, with the depositional system re-forming on the far side. Modeling of dilute currents shows that they can pass over obstacles ~1.5 times their thickness without losing momentum, which implies the CI current was >1 km thick. Much of that dilute current was gas, but how much was atmospheric? Partial thermal demagnetization of lithic clasts allows the identification of the temperature of emplacement. We sampled lithic fragments from the CI in 13 locations from proximal to distal along several azimuths. The current passed over 30-35 km of sea to get to two sites. Partial thermal demagnetization of 10 specimens from each site show that they were heated and deposited above 580 oC, the unblocking temperature of magnetite, implying the temperature of emplacement was at or above this temperature. The CI is poor in lithic clasts (<1% in most places) and evidence of non-magmatic water in the outflow sheet is absent. We suggest the CI current was a large
The current density in quantum electrodynamics in external potentials
Schlemmer, Jan; Zahn, Jochen
2015-08-15
We review different definitions of the current density for quantized fermions in the presence of an external electromagnetic field. Several deficiencies in the popular prescription due to Schwinger and the mode sum formula for static external potentials are pointed out. We argue that Dirac’s method, which is the analog of the Hadamard point-splitting employed in quantum field theory in curved space–times, is conceptually the most satisfactory. As a concrete example, we discuss vacuum polarization and the stress–energy tensor for massless fermions in 1+1 dimension. Also a general formula for the vacuum polarization in static external potentials in 3+1 dimensions is derived.
Advances in time-dependent current-density functional theory
NASA Astrophysics Data System (ADS)
Berger, Arjan
In this work we solve the problem of the gauge dependence of molecular magnetic properties (magnetizabilities, circular dichroism) using time-dependent current-density functional theory [1]. We also present a new functional that accurately describes the optical absorption spectra of insulators, semiconductors and metals [2] N. Raimbault, P.L. de Boeij, P. Romaniello, and J.A. Berger Phys. Rev. Lett. 114, 066404 (2015) J.A. Berger, Phys. Rev. Lett. 115, 137402 (2015) This study has been partially supported through the Grant NEXT No. ANR-10-LABX-0037 in the framework of the Programme des Investissements d'Avenir.
Discretizing Transient Current Densities in the Maxwell Equations
Stowell, M L
2008-11-25
We will briefly discuss a technique for applying transient volumetric current sources in full-wave, time-domain electromagnetic simulations which avoids the need for divergence cleaning. The method involves both 'edge-elements' and 'face-elements' in conjunction with a particle-in-cell scheme to track the charge density. Results from a realistic, 6.7 million element, 3D simulation are shown. While the author may have a finite element bias the technique should be applicable to finite difference methods as well.
Nam, Y. U. Wi, H. M.; Zoletnik, S.; Lampert, M.; Kovácsik, Ákos
2014-11-15
Beam emission spectroscopy (BES) system in Korea Superconducting Tokamak Advanced Research (KSTAR) has recently been upgraded. The background intensity was reduced from 30% to 2% by suppressing the stray lights. This allows acquisition of the relative electron density profiles on the plasma edge without background subtraction from the beam power modulation signals. The KSTAR BES system has its spatial resolution of 1 cm, the temporal resolution of 2 MHz, and a total 32 channel (8 radial × 4 poloidal) avalanche photo diode array. Most measurements were done on the plasma edge, r/a ∼ 0.9, with 8 cm radial measurement width that covers the pedestal range. High speed density profile measurements reveal temporal behaviors of fast transient events, such as the precursors of edge localized modes and the transitions between confinement modes. Low background level also allows analysis of the edge density fluctuation patterns with reduced background fluctuations. Propagation of the density structures can be investigated by comparing the phase delays between the spatially distributed channels.
NASA Astrophysics Data System (ADS)
Happel, T.; Manz, P.; Ryter, F.; Bernert, M.; Dunne, M.; Hennequin, P.; Hetzenecker, A.; Stroth, U.; Conway, G. D.; Guimarais, L.; Honoré, C.; Viezzer, E.; The ASDEX Upgrade Team
2017-01-01
Properties of the I-mode confinement regime on the ASDEX Upgrade tokamak are summarized. A weak dependence of the power threshold for the L-I transition on the toroidal magnetic field strength is found. During improved confinement, the edge radial electric field well deepens. Stability calculations show that the I-mode pedestal is peeling-ballooning stable. Turbulence investigations reveal strongly intermittent density fluctuations linked to the weakly coherent mode in the confined plasma, which become stronger as the confinement quality increases. Across all investigated structure sizes ({{k}\\bot}≈ 5 -12 cm-1, with {{k}\\bot} the perpendicular wavenumber of turbulent density fluctuations), the intermittent turbulence bursts are observed. Comparison with bolometry data shows that they move poloidally toward the X-point and finally end up in the divertor. This might be indicative that they play a role in inhibiting the density profile growth, such that no pedestal is formed in the edge density profile.
The dynamics of pyroclastic density currents on Mars
NASA Astrophysics Data System (ADS)
Brand, B. D.; Clarke, A. B.
2010-12-01
The products of explosive volcanism have long been observed on the surface of Mars, and their corresponding dynamics, associated with phenomenon such as dike propagation, magma fragmentation, and eruption columns under Martian conditions, have been modeled with significant success (e.g., Wilson, L., J. W. Head (1994), Mars- Review and analysis of volcanic eruption theory and relationships to observed landforms, Rev. Geophy, 32, 221-263). However, the dynamics of pyroclastic density currents (PDCs) under Martian conditions is still poorly constrained. Our increasing capability to image the surface at high resolution, both from orbit and from rovers, presents an opportunity for more rigorous deposit observations and descriptions. For example, observations and measurements from Orbiters identify what have been interpreted as extensive aprons of volcanic ash deposits in several volcanic regions, namely those surrounding several southern highland patera, which have been interpreted as the deposits of PDCs. In addition the bedded deposits identified by the Spirit rover at “Home Plate,” an outcrop within the Columbia Hills in Gusev Crater, have been interpreted by many as the deposits of dilute pyroclastic density currents. This demonstrates that the need to understand the role of the Martian atmosphere on flow dynamics and depositional processes is much more important and relevant than it has been in the past. We have developed a quantitative, axi-symmetric model for flow of and sedimentation from a steady-state, vertically uniform dilute density current for application to PDCs on Earth and Mars (following Bursik, M. I., A. W. Woods, 1996, The dynamics and thermodynamics of large ash flows, Bull Volcan, 58, 175-193). The conservation of mass, momentum, and energy are solved simultaneously, and include the effects of atmospheric entrainment, particle sedimentation, basal friction, temperature changes, and variations in current thickness and density. For a given set of
Current Fluctuations in the One-Dimensional Symmetric Exclusion Process with Open Boundaries
NASA Astrophysics Data System (ADS)
Derrida, B.; Douçot, B.; Roche, P.-E.
2004-05-01
We calculate the first four cumulants of the integrated current of the one dimensional symmetric simple exclusion process of $N$ sites with open boundary conditions. For large system size $N$, the generating function of the integrated current depends on the densities $\\rho_a$ and $\\rho_b$ of the two reservoirs and on the fugacity $z$, the parameter conjugated to the integrated current, through a single parameter. Based on our expressions for these first four cumulants, we make a conjecture which leads to a prediction for all the higher cumulants. In the case $\\rho_a=1$ and $\\rho_b=0$, our conjecture gives the same universal distribution as the one obtained by Lee, Levitov and Yakovets for one dimensional quantum conductors in the metallic regime.
NASA Technical Reports Server (NTRS)
Clukey, Steven J.
1991-01-01
The real time Dynamic Data Acquisition and Processing System (DDAPS) is described which provides the capability for the simultaneous measurement of velocity, density, and total temperature fluctuations. The system of hardware and software is described in context of the wind tunnel environment. The DDAPS replaces both a recording mechanism and a separate data processing system. DDAPS receives input from hot wire anemometers. Amplifiers and filters condition the signals with computer controlled modules. The analog signals are simultaneously digitized and digitally recorded on disk. Automatic acquisition collects necessary calibration and environment data. Hot wire sensitivities are generated and applied to the hot wire data to compute fluctuations. The presentation of the raw and processed data is accomplished on demand. The interface to DDAPS is described along with the internal mechanisms of DDAPS. A summary of operations relevant to the use of the DDAPS is also provided.
NASA Astrophysics Data System (ADS)
Rhodes, T. L.; Barada, K.; Peebles, W. A.; Crocker, N. A.
2016-11-01
An upgraded cross-polarization scattering (CPS) system for the simultaneous measurement of internal magnetic fluctuations B ˜ and density fluctuations ñ is presented. The system has eight radial quadrature channels acquired simultaneously with an eight-channel Doppler backscattering system (measures density fluctuations ñ and flows). 3-D ray tracing calculations based on the GENRAY ray tracing code are used to illustrate the scattering and geometric considerations involved in the CPS implementation on DIII-D. A unique quasi-optical design and IF electronics system allow direct comparison of B ˜ and ñ during dynamic or transient plasma events (e.g., Edge Localized Modes or ELMs, L to H-mode transitions, etc.). The system design allows the interesting possibility of both magnetic-density ( B ˜ -ñ) fluctuation and magnetic-temperature ( B ˜ - T ˜ ) fluctuation cross-phase measurements suitable for detailed tests of turbulence simulations.
Critical current density and current distribution in field cooled superconducting disks
NASA Astrophysics Data System (ADS)
Bernstein, Pierre; Noudem, Jacques; Dupont, Louis
2016-07-01
Applications of bulk superconductors concern superconducting motors and generators, the levitation of vehicles, the generation of high magnetic fields with small size cryo-magnets, the shielding of magnetic fields and other applications. For all of them, it is essential to determine the critical current density, and to understand the effect of the shape and size of the bulks on the properties of interest. In this contribution, we show how the combination of levitation force and trapped field measurements allow one to determine the characteristics and the potential performances of superconducting disks using analytical modeling. As examples of applications we detail the effects of the magnetizing field and of the bulk sheet critical current density on the levitation force. An important result of the reported measurements is that in field-cooled samples, the shielding currents possibly do not flow along the whole thickness of the disks.
Turbulent small-scale neutral and ion density fluctuations as measured during MAC/Epsilon
NASA Technical Reports Server (NTRS)
Luebken, F.-J.; Hillert, W.; Vonzahn, U.; Blix, T. A.; Thrane, E. V.
1989-01-01
During the MAC/Epsilon campaign (Fall 1987, from Andoya, Northern Norway, 69 N, 16 E) a total of four altitude profiles of neutral gas number densities and six profiles of ion number densities were measured with high spatial resolution in the height range from 60 to 120 km. First results of these rocket-borne experiments are presented with emphasis on small scale turbulent density variations and related turbulent parameter as structure function constants and energy dissipation rates.
Hydraulic jumps within pyroclastic density currents and their sedimentary record
NASA Astrophysics Data System (ADS)
Douillet, G.; Mueller, S.; Kueppers, U.; Dingwell, D. B.
2013-12-01
This contribution presents a complete and comprehensive formulation of the hydraulic jump phenomenon and reviews sedimentary structures that may be associated with them. Beginning from the general fluid phenomenon, we then focus on examples from pyroclastic density currents in order to infer dynamic parameters on the parent flows. A hydraulic jump is a fluid dynamics phenomenon that corresponds to the sudden increase of the thickness of a flow accompanied by a decrease of its velocity and/or density. A hydraulic jump is the expression of the transition of the flow from two different flow regimes: supercritical to subcritical. This entrains a change in the energy balance between kinetic energy and gravity potential energy. Recently, the terms of 'pneumatic jumps' have been used for similar phenomenon driven within a gas phase, and granular jumps for dense granular flows. It is thought that such strong changes in the flow conditions may leave characteristic structures in the sedimentary record. Indeed, the main variables influencing the sedimentation rate are the flow velocity, particle concentration and turbulence level, all of them strongly affected by a hydraulic jump. Structures deposited by hydraulic/pneumatic jumps have been called cyclic steps and chute and pool structures. Chute and pools represent the record of a single supercritical to subcritical transition, whereas cyclic steps are produced by stable trains of hydraulic jumps and subsequent re-accelerations. Pyroclastic density currents (PDCs) are gas and pyroclasts flows. As such, they can be subjected to granular and pneumatic jumps and their deposit have often been interpreted as containing records of jumps. Steep sided truncations covered by lensoidal layers have been interpreted as the record of internal jumps within density stratified flows. Fines-depleted breccias at breaks in slope are thought to result from the enhanced turbulence at a jump of the entire flow. Sudden increases in thickness of
Cabral-Calderin, Yuranny; Williams, Kathleen A; Opitz, Alexander; Dechent, Peter; Wilke, Melanie
2016-11-01
Transcranial alternating current stimulation (tACS) is a promising tool for modulating brain oscillations. Combining tACS with functional magnetic resonance imaging (fMRI), we recently showed that tACS applied over the occipital cortex did not exert its strongest effect on regions below the electrodes, but mainly on more distant fronto-parietal regions. Theoretically, this effect could be explained by tACS-induced modulation of functional connectivity between directly stimulated areas and more distant but anatomically and functionally connected regions. In the present study, we aimed to characterize the effect of tACS on low frequency fMRI signal fluctuations. We employed simultaneous fMRI-tACS in 20 subjects during resting state (eyes open with central fixation for ~8min). Subjects received tACS at different frequencies (10, 16, 40Hz) and with different electrode montages (Cz-Oz, P5-P6) previously used in behavioral studies. Electric field simulations showed that tACS over Cz-Oz directly stimulates occipital cortex, while tACS over P5-P6 primarily targets parietal cortices. Group-level simulation-based functional connectivity maps for Cz-Oz and P5-P6 resembled the visual and fronto-parietal control resting-state networks, respectively. The effects of tACS were frequency and partly electrode montage dependent. In regions where frequency-dependent effects of tACS were observed, 10 and 40Hz tACS generally induced opposite effects. Most tACS effects on functional connectivity were observed between, as opposed to within, resting-state networks. The left fronto-parietal control network showed the most extensive frequency-dependent modulation in functional connectivity, mainly with occipito-parietal regions, where 10Hz tACS increased and 40Hz tACS decreased correlation values. Taken together, our results show that tACS modulates local spontaneous low frequency fluctuations and their correlations with more distant regions, which should be taken into account when
NASA Astrophysics Data System (ADS)
Vaikuntanathan, Suriyanarayanan; Geissler, Phillip L.
2014-01-01
The physics of air-water interfaces plays a central role in modern theories of the hydrophobic effect. Implementing these theories, however, has been hampered by the difficulty of addressing fluctuations in the shape of such soft interfaces. We show that this challenge is a fundamental consequence of mapping long wavelength density variations onto discrete degrees of freedom. Drawing from studies of surface roughness in lattice models, we account for the resulting nonlinearities simply but accurately. Simulations show that this approach captures complex solvation behaviors quantitatively.
Bardóczi, L; Rhodes, T L; Carter, T A; Bañón Navarro, A; Peebles, W A; Jenko, F; McKee, G
2016-05-27
We report the first observation of localized modulation of turbulent density fluctuations n[over ˜] (via beam emission spectroscopy) by neoclassical tearing modes (NTMs) in the core of the DIII-D tokamak. NTMs are important as they often lead to severe degradation of plasma confinement and disruptions in high-confinement fusion experiments. Magnetic islands associated with NTMs significantly modify the profiles and turbulence drives. In this experiment n[over ˜] was found to be modulated by 14% across the island. Gyrokinetic simulations suggest that n[over ˜] could be dominantly driven by the ion temperature gradient instability.
NASA Astrophysics Data System (ADS)
Woo, K. M.; Yu, S. S.; Barnard, J. J.
2013-06-01
It is well known that the imperfection of pulse power sources that drive the linear induction accelerators can lead to time-varying fluctuation in the accelerating voltages, which in turn leads to longitudinal emittance growth. We show that this source of emittance growth is correctable, even in space-charge dominated beams with significant transients induced by space-charge waves. Two correction methods are proposed, and their efficacy in reducing longitudinal emittance is demonstrated with three-dimensional particle-in-cell simulations.
Ultra-high current density thin-film Si diode
Wang; Qi
2008-04-22
A combination of a thin-film .mu.c-Si and a-Si:H containing diode structure characterized by an ultra-high current density that exceeds 1000 A/cm.sup.2, comprising: a substrate; a bottom metal layer disposed on the substrate; an n-layer of .mu.c-Si deposited the bottom metal layer; an i-layer of .mu.c-Si deposited on the n-layer; a buffer layer of a-Si:H deposited on the i-layer, a p-layer of .mu.c-Si deposited on the buffer layer; and a top metal layer deposited on the p-layer.
Sparse imaging of cortical electrical current densities via wavelet transforms
NASA Astrophysics Data System (ADS)
Liao, Ke; Zhu, Min; Ding, Lei; Valette, Sébastien; Zhang, Wenbo; Dickens, Deanna
2012-11-01
While the cerebral cortex in the human brain is of functional importance, functions defined on this structure are difficult to analyze spatially due to its highly convoluted irregular geometry. This study developed a novel L1-norm regularization method using a newly proposed multi-resolution face-based wavelet method to estimate cortical electrical activities in electroencephalography (EEG) and magnetoencephalography (MEG) inverse problems. The proposed wavelets were developed based on multi-resolution models built from irregular cortical surface meshes, which were realized in this study too. The multi-resolution wavelet analysis was used to seek sparse representation of cortical current densities in transformed domains, which was expected due to the compressibility of wavelets, and evaluated using Monte Carlo simulations. The EEG/MEG inverse problems were solved with the use of the novel L1-norm regularization method exploring the sparseness in the wavelet domain. The inverse solutions obtained from the new method using MEG data were evaluated by Monte Carlo simulations too. The present results indicated that cortical current densities could be efficiently compressed using the proposed face-based wavelet method, which exhibited better performance than the vertex-based wavelet method. In both simulations and auditory experimental data analysis, the proposed L1-norm regularization method showed better source detection accuracy and less estimation errors than other two classic methods, i.e. weighted minimum norm (wMNE) and cortical low-resolution electromagnetic tomography (cLORETA). This study suggests that the L1-norm regularization method with the use of face-based wavelets is a promising tool for studying functional activations of the human brain.
Rayleigh Scattering Diagnostic for Measurement of Velocity and Density Fluctuation Spectra
NASA Astrophysics Data System (ADS)
Seasholtz, Richard G.; Panda, Jayanta; Elam, Kristie A.
2002-04-01
A new molecular Rayleigh scattering based flow diagnostic is used for the first time to measure the power spectrum of gas density and radial velocity component in the plumes of high speed jets. The technique is based on analyzing the Rayleigh scattered light with a Fabry-Perot interferometer used in the static, imaging mode. The PC based data acquisition system is capable of simultaneous sampling of velocity and density at rates to 100 kHz and data record lengths to 10 million. Velocity and density power spectra and velocity-density cross spectra are presented for a subsonic jet, an underexpanded screeching jet, and for Mach 1.4 and Mach 1.8 supersonic jets. Software and hardware interfaces were developed to allow computer control of all aspects of the experiment and data acquisition.
Rayleigh Scattering Diagnostic for Measurement of Velocity and Density Fluctuation Spectra
NASA Technical Reports Server (NTRS)
Seasholtz, Richard G.; Panda, Jayanta; Elam, Kristie A.
2002-01-01
A new molecular Rayleigh scattering based flow diagnostic is used for the first time to measure the power spectrum of gas density and radial velocity component in the plumes of high speed jets. The technique is based on analyzing the Rayleigh scattered light with a Fabry-Perot interferometer used in the static, imaging mode. The PC based data acquisition system is capable of simultaneous sampling of velocity and density at rates to 100 kHz and data record lengths to 10 million. Velocity and density power spectra and velocity-density cross spectra are presented for a subsonic jet, an underexpanded screeching jet, and for Mach 1.4 and Mach 1.8 supersonic jets. Software and hardware interfaces were developed to allow computer control of all aspects of the experiment and data acquisition.
Wall current closure effects on plasma and sheath fluctuations in Hall thrusters
Frias, Winston Smolyakov, Andrei I.; Kaganovich, Igor D.; Raitses, Yevgeny
2014-06-15
The excitation of negative energy, ion sound type modes driven by the E × B drift and the reactive/dissipative response of the wall sheath interface is analyzed for conditions typical in a Hall thruster. Such sheath impedance modes are sensitive to the dielectric properties of the thruster wall material, which therefore may have direct influence (other than via the secondary electron emission) on fluctuations and transport. Our results predict mode frequencies consistent with the frequencies of fluctuations observed experimentally.
Ion diffusion may introduce spurious current sources in Current-Source Density (CSD) analysis.
Halnes, Geir; Mäki-Marttunen, Tuomo; Pettersen, Klas H; Andreassen, Ole A; Einevoll, Gaute T
2017-03-15
Current-source density (CSD) analysis is a well-established method for analyzing recorded ocal field potentials (LFPs), that is, the low-frequency part of extracellular potentials. Standard CSD theory is based on the assumption that all extracellular currents are purely ohmic, and thus neglects the possible impact from ionic diffusion on recorded potentials. However, it has previously been shown that in physiological conditions with large ion-concentration gradients, diffusive currents can evoke slow shifts in extracellular potentials. Using computer simulations, we here show that diffusion-evoked potential shifts can introduce errors in standard CSD analysis, and can lead to prediction of spurious current sources. Further, we show that the diffusion-evoked prediction errors can be removed by using an improved CSD estimator which accounts for concentration-dependent effects.
High current density contacts for photoconductive semiconductor switches
Baca, A.G.; Hjalmarson, H.P.; Loubriel, G.M.; McLaughlin, D.L.; Zutavern, F.J.
1993-08-01
The current densities implied by current filaments in GaAs photoconductive semiconductor switches (PCSS) are in excess of 1 MA/cm{sup 2}. As the lateral switches are tested repeatedly, damage accumulates at the contacts until electrical breakdown occurs across the surface of the insulating region. In order to improve the switch lifetime, the incorporation of n- and p-type ohmic contacts in lateral switches as well as surface geometry modifications have been investigated. By using p-type AuBe ohmic contacts at the anode and n-type AuGe ohmic contacts at the cathode, contact lifetime improvements of 5--10x were observed compared to switches with n-type contacts at both anode and cathode. Failure analysis on samples operated for 1--1,000 shots show that extensive damage still exists for at least one contact on all switches observed and that temperatures approaching 500{degrees}C are can be reached. However, the n-type AuGe cathode is often found to have no damage observable by scanning electron microscopy (SEM). The observed patterns of contact degradation indicate directions for future contact improvements in lateral switches.
Current densities and total contact currents associated with 400 kV power line tasks.
Korpinen, Leena; Kuisti, Harri; Elovaara, Jarmo
2013-12-01
The aim of the study was to analyze all current values from measured periods while performing tasks on 400 kV power lines. Our aim was also to study the average current densities and average total contact currents caused by electric fields in 400 kV power line tasks. Two workers simulated the following tasks: (A) climbing up a portal tower, (B) climbing up a portal transposing tower, (C) working on the cross-arm of a portal tower, (D) climbing up a portal tube tower, (E) climbing up a Tannenbaum tower on the side of the energized circuit with the other circuit unenergized, (F) climbing up a Tannenbaum tower with both circuits energized, and (G) climbing up a Donau tower. The highest average current density in the neck was 2.5 mA/m(2) (calculated internal electric field 31.5-63.0 mV/m), and the highest average of the contact currents was 240.0 µA. All measured values at 400 kV towers were lower than the limit value of 10 mA/m(2) in the first version of Directive 2004/40/EC and the basic restrictions (0.1 and 0.8 V/m) of the International Commission on Non-ionizing Radiation Protection.
Local Density Fluctuations Predict Photoisomerization Quantum Yield of Azobenzene-Modified DNA.
Kingsland, Addie; Samai, Soumyadyuti; Yan, Yunqi; Ginger, David S; Maibaum, Lutz
2016-08-04
Azobenzene incorporated into DNA has a photoisomerization quantum yield that depends on the DNA sequence near the azobenzene attachment site. We use Molecular Dynamics computer simulations to elucidate which physical properties of the modified DNA determine the quantum yield. We show for a wide range of DNA sequences that the photoisomerization quantum yield is strongly correlated with the variance of the number of atoms in close proximity to the outer phenyl ring of the azobenzene group. We infer that quantum yield is controlled by the availability of fluctuations that enable the conformational change. We demonstrate that these simulations can be used as a qualitative predictive tool by calculating the quantum yield for several novel DNA sequences, and confirming these predictions using UV-vis spectroscopy. Our results will be useful for the development of a wide range of applications of photoresponsive DNA nanotechnology.
Control of the current density profile with lower hybrid current drive on PBX-M
Bell, R.E.; Bernabei, S.; Chu, T.K.; Gettelfinger, G.; Greenough, N.; Hatcher, R.; Ignat, D.; Jardin, S.; Kaita, R.; Kaye, S.; Kozub, T.; Kugel, H.; LeBlanc, B.; Okabayashi, M.; Paul, S.; Sauthoff, N.; Sesnic, S.; Sun, Y.; Takahashi, H.; Tighe, W.; Valeo, E.; von Goeler, S.; Blush, L.; Doerner, R.; Schmitz, L.; Tynan, G.; Dunlap, J.; England, A.; Harris, J.; Hirshman, S.; Isler, R.; Lee, D.; Jones, S.; Kesner, J.; Luckhardt, S.; Paoletti, F.; Levinton, F.; Timini, F.
1993-07-01
Lower hybrid current drive (LHCD) is being explored as a means to control the current density profile on PBX-M with the goal of raising the central safety factor q(O) to values of 1.5-2 to facilitate access to a full-volume second stable regime. Initial experiments have been conducted with up to 400 kW of 4.6 GHz LH power in circular and indented plasmas with modest parameters. A tangential-viewing two-dimensional hard x-ray imaging diagnostic has been used to observe the bremsstrahlung emission from the suprathermal electrons generated during LHCD. Hollow hard x-ray images have indicated off-axis localization of the driven current. A serious obstacle to the control of the current density profile with LHCD is the concomitant generation of MHD activity, which can seriously degrade the confinement of suprathermal electrons. By combining neutral beam injection with LHCD, an MHD-free condition has been obtained where q(O) is raised above 1.
Clemente, R. A.; Gilli, M.; Farengo, R.
2008-10-15
Steady state solutions, suitable for field-reversed configurations (FRCs) sustained by rotating magnetic fields (RMFs) are obtained by properly including three-dimensional effects, in the limit of large FRC elongation, and the radial component of Ohm's law. The steady electrostatic potential, necessary to satisfy Ohm's law, is considered to be a surface function. The problem is analyzed at the midplane of the configuration and it is reduced to the solution of two coupled nonlinear differential equations for the real and imaginary parts of the phasor associated to the longitudinal component of the vector potential. Additional constraints are obtained by requesting that the steady radial current density and poloidal magnetic flux vanish at the plasma boundary which is set at the time-averaged separatrix. The results are presented in terms of the degree of synchronism of the electrons with the RMF and compared with those obtained when radial current effects are neglected. Three important differences are observed when compared with the case without radial current density. First, at low penetration of the RMF into the plasma there is a significant increase in the driven azimuthal current. Second, the RMF amplitude necessary to access the high synchronism regime, starting from low synchronism, is larger and the difference appears to increase as the separatrix to classical skin depth ratio increases. Third, the minimum RMF amplitude necessary to sustain almost full synchronism is reduced.
Bastani, Andisheh; Jaberzadeh, Shapour
2013-01-01
Background Novel non-invasive brain stimulation techniques such as transcranial direct current stimulation (tDCS) have been developed in recent years. TDCS-induced corticospinal excitability changes depend on two important factors current intensity and stimulation duration. Despite clinical success with existing tDCS parameters, optimal protocols are still not entirely set. Objective/hypothesis The current study aimed to investigate the effects of four different anodal tDCS (a-tDCS) current densities on corticospinal excitability. Methods Four current intensities of 0.3, 0.7, 1.4 and 2 mA resulting in current densities (CDs) of 0.013, 0.029, 0.058 and 0.083 mA/cm2 were applied on twelve right-handed (mean age 34.5±10.32 yrs) healthy individuals in different sessions at least 48 hours apart. a-tDCS was applied continuously for 10 minute, with constant active and reference electrode sizes of 24 and 35 cm2 respectively. The corticospinal excitability of the extensor carpi radialis muscle (ECR) was measured before and immediately after the intervention and at 10, 20 and 30 minutes thereafter. Results Post hoc comparisons showed significant differences in corticospinal excitability changes for CDs of 0.013 mA/cm2 and 0.029 mA/cm2 (P = 0.003). There were no significant differences between excitability changes for the 0.013 mA/cm2 and 0.058 mA/cm2 (P = 0.080) or 0.013 mA/cm2 and 0.083 mA/cm2 (P = 0.484) conditions. Conclusion This study found that a-tDCS with a current density of 0.013 mA/cm2 induces significantly larger corticospinal excitability changes than CDs of 0.029 mA/cm2. The implication is that might help to avoid applying unwanted amount of current to the cortical areas. PMID:23991076
Lande, Russell; Engen, Steinar; Saether, Bernt-Erik
2009-06-12
The evolution of population dynamics in a stochastic environment is analysed under a general form of density-dependence with genetic variation in r and K, the intrinsic rate of increase and carrying capacity in the average environment, and in sigma(e)(2), the environmental variance of population growth rate. The continuous-time model assumes a large population size and a stationary distribution of environments with no autocorrelation. For a given population density, N, and genotype frequency, p, the expected selection gradient is always towards an increased population growth rate, and the expected fitness of a genotype is its Malthusian fitness in the average environment minus the covariance of its growth rate with that of the population. Long-term evolution maximizes the expected value of the density-dependence function, averaged over the stationary distribution of N. In the theta-logistic model, where density dependence of population growth is a function of N(theta), long-term evolution maximizes E[N(theta)]=[1-sigma(e)(2)/(2r)]K(theta). While sigma(e)(2) is always selected to decrease, r and K are always selected to increase, implying a genetic trade-off among them. By contrast, given the other parameters, theta has an intermediate optimum between 1.781 and 2 corresponding to the limits of high or low stochasticity.
Fluctuation of Density of States for 1d Schrödinger Operators
NASA Astrophysics Data System (ADS)
Nakano, Fumihiko
2017-03-01
We consider the 1d Schrödinger operator with random decaying potential and compute the 2nd term asymptotics of the density of states, which shows substantial differences between the cases α > 1/2, α < 1/2 and α = 1/2.
NASA Astrophysics Data System (ADS)
Shi, Peiwan; Shi, Zhongbing; Chen, Wei; Zhong, Wulyu; Yang, Zengchen; Jiang, Min; Zhang, Boyu; Li, Yonggao; Yu, Liming; Liu, Zetian; Ding, Xuantong
2016-07-01
A multichannel microwave interferometer system has been developed on the HL-2A tokomak. Its working frequency is well designed to avoid the fringe jump effect. Taking the structure of HL-2A into account, its antennas are installed in the horizontal direction, i.e. one launcher in high field side (HFS) and four receivers in low field side (LFS). The fan-shaped measurement area covers those regions where the magnetohydrodynamics (MHD) instabilities are active. The heterodyne technique contributes to its high temporal resolution (1 μs). It is possible for the multichannel system to realize simultaneous measurements of density and its fluctuation. The quadrature phase detection based on the zero-crossing method is introduced to density measurement. With this system, reliable line-averaged densities and density profiles are obtained. The location of the saturated internal kink mode can be figured out from the mode showing different intensities on four channels, and the result agrees well with that measured by electron cyclotron emission imaging (ECEI). supported by the National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB104002, 2013GB107002, 2014GB107001) and National Natural Science Foundation of China (Nos. 11475058, 11475057, 11261140326, 11405049)
Vibration effect on magnetization and critical current density of superconductors
NASA Astrophysics Data System (ADS)
Golovchanskiy, Igor A.; Pan, Alexey V.; George, Jonathan; Wells, Frederick S.; Fedoseev, Sergey A.; Rozenfeld, Anatoly
2016-07-01
In this work the effect of vibrations on critical current density (J c ) of superconductors has been studied. The vibrations are shown to affect J c of all types of superconductors during their measurements, employing a vibrating sample magnetometer (VSM). Increasing vibration frequency (f) and/or amplitude (A) leads to progressive reduction of J c as a function of magnetic field (B a ). The effect of vibrations is substantially stronger in thin films. It leads to development of unexpected kinks on {J}c({B}a) curves. Analysis of magnetization loops and relaxation of magnetization in YBCO films revealed that the vibration effect can be treated as the effective reduction of pinning potential. The asymmetry of the vibration effect in ascending and descending B a is observed, indicating differences in free energy of the corresponding vortex structures. Thermal effects induced by vibrations with large f and A are shown to have rather insignificant influence, while the vibrational vortex dynamics exhibit a strong impact. The irreversibility field ({B}{{irr}}) is shown to be instrumentally defined, and its value depends on VSM settings. In addition, the practical importance of {B}{{irr}} for J c modeling is demonstrated.
High-density matter: current status and future challenges
NASA Astrophysics Data System (ADS)
Stone, J. R.
2015-05-01
There are many fascinating processes in the Universe which we observe in more and more in detail thanks to increasingly sophisticated technology. One of the most interesting phenomena is the life cycle of stars, their birth, evolution and death. If the stars are massive enough, they end their lives in the core-collapse supernova explosion, the one of the most violent events in the Universe. As the result, the densest objects in the Universe, neutron stars and/or black holes are created. Naturally, the physical basis of these events should be understood in line with observation. The current status of our knowledge of processes in the life of stars is far from adequate for their true understanding. We show that although many models have been constructed their detailed ability to describe observations is limited or non-existent. Furthermore the general failure of all models means that we cannot tell which are heading in the right direction. A possible way forward in modeling of high-density matter is outlined, exemplified by the quark-meson-coupling model (QMC). This model has a natural explanation for the saturation of nuclear forces and depends on very few adjustable parameters, strongly constrained by the underlying physics. Latest QMC results for compact objects and finite nuclei are presented.
Yadav, Vivek Kumar; Chandra, Amalendu
2013-06-14
A first principles study of the dynamics of supercritical methanol is carried out by means of ab initio molecular dynamics simulations. In particular, the fluctuation dynamics of hydroxyl stretch frequencies, hydrogen bonds, dangling hydroxyl groups, and orientation of methanol molecules are investigated for three different densities at 523 K. Apart from the dynamical properties, various equilibrium properties of supercritical methanol such as the local density distributions and structural correlations, hydrogen bonding aspects, frequency-structure correlations, and dipole distributions of methanol molecules are also investigated. In addition to the density dependence of various equilibrium and dynamical properties, their dependencies on dispersion interactions are also studied by carrying out additional simulations using a dispersion corrected density functional for all the systems. It is found that the hydrogen bonding between methanol molecules decreases significantly as we move to the supercritical state from the ambient one. The inclusion of dispersion interactions is found to increase the number of hydrogen bonds to some extent. Calculations of the frequency-structure correlation coefficient reveal that a statistical correlation between the hydroxyl stretch frequency and the nearest hydrogen-oxygen distance continues to exist even at supercritical states of methanol, although it is weakened with increase of temperature and decrease of density. In the supercritical state, the frequency time correlation function is found to decay with two time scales: One around or less than 100 fs and the other in the region of 250-700 fs. It is found that, for supercritical methanol, the times scales of vibrational spectral diffusion are determined by an interplay between the dynamics of hydrogen bonds, dangling OD groups, and inertial rotation of methanol molecules and the roles of these various components are found to vary with density of the supercritical solvent. Effects
Abrasion in pyroclastic density currents: Insights from tumbling experiments
NASA Astrophysics Data System (ADS)
Kueppers, Ulrich; Putz, Constanze; Spieler, Oliver; Dingwell, Donald B.
2012-01-01
During granular mass movements of any kind, particles may interact with one another. The degree of interaction is a function of several variables including; grain-size distribution, particle concentration, density stratification and degree of fluidisation. The impact of particle interaction is additionally influenced by the relative speed, impact angle and clast temperature. Thus, both source conditions and transport-related processes are expected to influence the flow dynamics of pyroclastic density currents and their subsequent deposition. Here, we use tumbling experiments to shed light on the susceptibility of porous clasts to abrasion. We investigated the abrasion of unaltered volcanic rocks (5.7-80 vol.% porosity) from Unzen (Japan), Bezymianny (Russia) and Santorini (Greece) volcanoes as well as one synthetic analogue material, an insulating material with the trade name Foamglas® (95 vol.% porosity). Each experiment started with angular fragments generated in a jaw crusher from larger clasts. Two experimental series were performed; on samples with narrow and broader grain-size distributions, respectively. The dry samples were subject to rotational movement at constant speed and ambient temperature in a gum rotational tumbler for durations of 15, 30, 45, 60 and 120 min. The amount of volcanic ash (particles <2 mm) generated was evaluated as a function of experimental duration and sample porosity. We term “abrasion” as the ash fraction generated during the experiments. The observed increase of “abrasion” with increasing sample porosity and experimental duration is initially non-linear but becomes linear for experiments of 30 min duration or longer. For any given sample, abrasion appears to be more effective for coarser samples and larger initial mass. The observed range of ash generated in our experiments is between 1 and 35 wt.%. We find that this amount generally increases with increasing initial clast size or increasing breadth of the initial grain
Fluctuation of the charge density wave in TTF-TCNQ under high pressure
NASA Astrophysics Data System (ADS)
Murata, Keizo; Weng, Yufeng; Seno, Yuki; Rani Tamilselvan, Natarajan; Kobayashi, Kensuke; Arumugam, Sonachalam; Takashima, Yusaku; Yoshino, Harukazu; Kato, Reizo
2009-03-01
Temperature dependence of the resistivity of TTF-TCNQ along the b-(1D)- and a-axes was studied under hydrostatic pressure up to 8 GPa. A striking contrast was seen between the b-(1D)- and a-axes in the power-law dependence of resistivity ρ=ρ0Tα in the metallic region as well as the activation energy in the charge density wave (CDW) insulating state. We note that the careful terminal configuration is essentially important to obtain these properties.
Realizing life-scalable experimental pyroclastic density currents
NASA Astrophysics Data System (ADS)
Cronin, S. J.; Lube, G.; Breard, E.; Jones, J.; Valentine, G.; Freundt, A.; Hort, M. K.; Bursik, M. I.
2013-12-01
Pyroclastic Density Currents (PDCs) - the most deadly threat from volcanoes - are extremely hot, ground-hugging currents of rock fragments and gas that descend slopes at hundreds of kilometers per hour. These hostile flows are impossible to internally measure, thus volcanologists are persistently blocked in efforts to realistically forecast their internal mechanics and hazards. Attempts to fill this gap via laboratory-scale experiments continue to prove difficult, because they usually mismatch the dynamic and kinematic scaling of real-world flows by several orders of magnitude. In a multi-institutional effort, the first large-scale pyroclastic flow generator that can synthesize repeatable hot high-energy gas-particle mixture flows in safety has been commissioned in New Zealand. The final apparatus stands 15 m high, consisting of a tower/elevator system; an instrumented hopper that can hold >6000 kg (or 3.2 m3) of natural volcanic materials, which can be discharged at a range of controlled rates onto an instrumented, variably inclinable (6-25°) glass-sided chute for examining the vertical profiles of PDCs in motion. The use of rhyolitic pyroclastic material from the 1800 AD Taupo Eruption (with its natural grain-size, sorting and shape characteristics) and gas ensures natural coupling between the solids and fluid phases. PDC analogues with runout of >15 meters and flow depths of 1.5-6 meters are created by generating variably heated falling columns of natural volcanic particles (50-1300 kg/s), dispersed and aerated to controlled particle densities between 3 and 60 vol.% at the base of the elevated hopper. The descending columns rapidly generate high-velocity flows (up to 14 m/s) once impacting on the inclined channel, reproducing many features of natural flows, including segregation into dense and dilute regimes, progressive aggradational and en masse deposition of particles and the development of high internal gas-pore-pressures during flow. The PDC starting
determination of current density distribution in an electron beam
NASA Astrophysics Data System (ADS)
Kandel, Yudhishthir Prasad
Electron beams are useful in many applications because they can be focused down to a spot far exceeding the physical limit of focusing visible light or x-rays. Additionally, electron beams are useful in transferring concentrated amounts of energy to a very small well defined region of a target for a fixed duration. This has led to the development of both scanning electron microscopes (SEMs) and electron beam lithography. The goal of this work was to develop a general method that accurately and easily yields the best estimate of the electron current density distribution of a focused electron beam, known as point spread function (PSF). The method developed is fast, easy to use and accurate. Two specific areas of research have been addressed for PSF determination. The first is concerned with the monotonic response of EUV photoresist as a function of electron beam dose. An external metrology is used for mapping the change in thickness that is smaller than the beam spot size. The method developed in this study simultaneously gives the photo-resist thickness change as a function of electron dose and electron beam PSF. A second thrust of this research has been to develop set of PSF characterization approaches that apply to the SEM. Here a knowledge of the PSF offers many benefits including the ability to monitor and optimize SEM performance such as astigmatism control. Perhaps, even more importantly, a knowledge of the PSF combined with a series of well-defined experimental steps has led to the development of new methods for improving the resolution of SEM images through computational means rather than very costly and complex equipment modification.
Density Fluctuation measurement with Upgraded FIR System on the HSX Stellarator
NASA Astrophysics Data System (ADS)
Deng, C. B.; Brower, D. L.; Anderson, D. T.; Anderson, F. S. B.; Likin, K. M.; Talmadge, J. N.
2016-10-01
Going forward, a primary physics goal for HSX is to study configuration optimization for reducing turbulence which requires measurement of turbulence with kyρs up to 1. For characteristic HSX parameters (Te 200 eV at r/a 0.5 where the density gradient peaks), this condition corresponds to kyup to 7 cm-1. To accommodate this goal, the 9-chord HSX interferometer/far-forward scattering system (k<2 cm-1) will be upgraded to measure density turbulence at higher k. The existing source (4 mW, 288 GHz) employing frequency modulation will be replaced with two high power (30 mW each, 320 GHz), solid-state sources with fixed frequency offset 4 MHz. This will permit true heterodyne detection, thereby realizing faster measurement time response, increased bandwidth and reduced noise. High power sources and high sensitivity planar-diode mixers will allow us to reduce the aperture of the receiver optics to a few mm thereby increasing the maximum wavenumber to k 15 cm-1. Reconfiguring the interferometer system into a finite-angle collective scattering arrangement is also planned as it will increase the measured k-spectrum up to 18 cm-1 with some spatial resolution (core or edge). Supported by USDOE Grants DE-FG03-01ER54615 and DE-FG02-93ER54222.
Anisotropic density fluctuations, plasmons, and Friedel oscillations in nodal line semimetal
NASA Astrophysics Data System (ADS)
Rhim, Jun-Won; Kim, Yong Baek
2016-04-01
Motivated by recent experimental efforts on three-dimensional semimetals, we investigate the static and dynamic density response of the nodal line semimetal by computing the polarizability for both undoped and doped cases. The nodal line semimetal in the absence of doping is characterized by a ring-shape zero energy contour in momentum space, which may be considered as a collection of Dirac points. In the doped case, the Fermi surface has a torus shape and two independent processes of the momentum transfer contribute to the singular features of the polarizability even though we only have a single Fermi surface. In the static limit, there exist two independent singularities in the second derivative of the static polarizability. This results in the highly anisotropic Friedel oscillations which show the angle-dependent algebraic power law and the beat phenomena in the oscillatory electron density near a charged impurity. Furthermore, the dynamical polarizability has two singular lines along {\\hslash }ω =γ p and {\\hslash }ω =γ p{sin}η , where η is the angle between the external momentum {p} and the plane where the nodal ring lies. From the dynamical polarizability, we obtain the plasmon modes in the doped case, which show anisotropic dispersions and angle-dependent plasma frequencies. Qualitative differences between the low and high doping regimes are discussed in light of future experiments.
NASA Astrophysics Data System (ADS)
Majumdar, Arun K.; Luna, Carlos E.; Idell, Paul S.
2007-09-01
A new method of reconstructing and predicting an unknown probability density function (PDF) characterizing the statistics of intensity fluctuations of optical beams propagating through atmospheric turbulence is presented in this paper. The method is based on a series expansion of generalized Laguerre polynomials ; the expansion coefficients are expressed in terms of the higher-order intensity moments of intensity statistics. This method generates the PDF from the data moments without any prior knowledge of specific statistics and converges smoothly. The utility of reconstructed PDF relevant to free-space laser communication in terms of calculating the average bit error rate and probability of fading is pointed out. Simulated numerical results are compared with some known non-Gaussian test PDFs: Log-Normal, Rice-Nakagami and Gamma-Gamma distributions and show excellent agreement obtained by the method developed. The accuracy of the reconstructed PDF is also evaluated.
Da Silva-Cadoux, Cécile; Hazemann, Jean-Louis; Testemale, Denis; Proux, Olivier; Rochas, Cyrille
2012-01-28
Synchrotron small angle X-ray scattering measurements on water and alkaline bromine aqueous solutions (XBr, with X = Li, Rb, or Cs) were carried out from ambient to supercritical conditions. The temperature was increased from 300 to 750 K along several isobars between 24 and 35 MPa. The correlation length and the structure factor were extracted from the data following the Ornstein-Zernike formalism. We obtained experimental evidence of the shift of the critical point and isochore and their dependence on the ions concentration (0.33 mol/kg and 1.0 mol/kg). We also observed that the size of the density fluctuations and the structure factor increase with the presence of the ions and that this effect is positively correlated with the atomic number of the cation. These behaviors were compared with ZnBr(2) and NaCl systems from the literature.
NASA Astrophysics Data System (ADS)
Cao, G. M.; Li, Y. D.; Li, Q.; Sun, P. J.; Wu, G. J.; Hu, L. Q.; the EAST Team
2015-08-01
Beta-induced Alfvén eigenmodes (BAEs) during strong tearing modes (TMs) have been frequently observed in fast-electron plasmas of EAST tokamak. The dynamics of the short-scale ({k}\\perp {ρ }s~{1.5-4.3}) density fluctuations during the activity of BAEs with strong TMs has been preliminarily investigated by a tangential CO2 laser collective scattering system. The results suggest the active, but different, response of short-scale density fluctuations to the TMs and BAEs. In the low-frequency (0-10 kHz) part of density fluctuations, there are harmonic oscillations totally corresponding to those of TMs. In the medium-high frequency (10-250 kHz) part of density fluctuations, with the appearance of the BAEs, the medium-high frequency density fluctuations begin to be dominated by several quasi-coherent (QC) modes, and the frequencies of the QC modes seem to be related to the changes of both TMs and BAEs. These results would shed some light on the understanding of the multi-scale interaction physics.
NASA Astrophysics Data System (ADS)
Riquelme-Galván, Mauricio; Robledo, Alberto
2017-02-01
We improve on the description of the relationship that exists between critical clusters in thermal systems and intermittency near the onset of chaos in low-dimensional systems. We make use of the statistical-mechanical language of inhomogeneous systems and of the renormalization group (RG) method in nonlinear dynamics to provide a more accurate, formal, approach to the subject. The description of this remarkable correspondence encompasses, on the one hand, the density functional formalism, where classical and quantum mechanical analogues match the procedure for one-dimensional clusters, and, on the other, the RG fixed-point map of functional compositions that captures the essential dynamical behavior. We provide details of how the above-referred theoretical approaches interrelate and discuss the implications of the correspondence between the high-dimensional (degrees of freedom) phenomenon and low-dimensional dynamics.
High-density lipoprotein cholesterol: current perspective for clinicians.
Whayne, Thomas F
2009-01-01
High-density lipoproteins are regarded as ''good guys'' but not always. Situations involving high-density lipoproteins are discussed and medication results are considered. Clinicians usually consider high-density lipoprotein cholesterol. Nicotinic acid is the best available medication to elevate high-density lipoprotein cholesterol and this appears beneficial for cardiovascular risk. The major problem with nicotinic acid is that many patients do not tolerate the associated flushing. Laropiprant decreases this flushing and has an approval in Europe but not in the United States. The most potent medications for increasing high-density lipoprotein cholesterol are cholesteryl ester transfer protein inhibitors. The initial drug in this class, torcetrapib, was eliminated by excess cardiovascular problems. Two newer cholesteryl ester transfer protein inhibitors, R1658 and anacetrapib, initially appear promising. High-density lipoprotein cholesterol may play an important role in improving cardiovascular risk in the 60% of patients who do not receive cardiovascular mortality/morbidity benefit from low-density lipoproteins reduction by statins.
NASA Technical Reports Server (NTRS)
Knudsen, D. L.; Kabirzadeh, R.; Burchill, J. K.; Pfaff, Robert F.; Wallis, D. D.; Bounds, S. R.; Clemmons, J. H.; Pincon, J.-L.
2012-01-01
The Geoelectrodynamics and Electro-Optical Detection of Electron and SuprathermalIon Currents (GEODESIC) sounding rocket encountered more than 100 filamentary densitycavities associated with enhanced plasma waves at ELF (3 kHz) and VLF (310 kHz)frequencies and at altitudes of 800990 km during an auroral substorm. These cavities weresimilar in size (20 m diameter in most cases) to so-called lower-hybrid cavities (LHCs)observed by previous sounding rockets and satellites; however, in contrast, many of theGEODESIC cavities exhibited up to tenfold enhancements in magnetic wave powerthroughout the VLF band. GEODESIC also observed enhancements of ELF and VLFelectric fields both parallel and perpendicular to the geomagnetic field B0 within cavities,though the VLF E field increases were often not as large proportionally as seen in themagnetic fields. This behavior is opposite to that predicted by previously published theoriesof LHCs based on passive scattering of externally incident auroral hiss. We argue thatthe GEODESIC cavities are active wave generation sites capable of radiating VLF wavesinto the surrounding plasma and producing VLF saucers, with energy supplied by cold,upward flowing electron beams composing the auroral return current. This interpretation issupported by the observation that the most intense waves, both inside and outside cavities,occurred in regions where energetic electron precipitation was largely inhibited orabsent altogether. We suggest that the wave-enhanced cavities encountered by GEODESICwere qualitatively different from those observed by earlier spacecraft because of thefortuitous timing of the GEODESIC launch, which placed the payload at apogee within asubstorm-related return current during its most intense phase, lasting only a few minutes.
Jing, Bin; Liu, Chun-Hong; Ma, Xin; Yan, Hua-Gang; Zhuo, Zhi-Zheng; Zhang, Yu; Wang, Su-Hong; Li, Hai-Yun; Wang, Chuan-Yue
2013-12-02
Medical intervention for major depressive disorder (MDD) can be more appropriately focused through the identification and characterization of neurobiological markers that are specific to the disorder, and this study aims to examine the abnormality in the fractional amplitude of low-frequency fluctuation (fALFF) and the amplitude of low-frequency fluctuation (ALFF) in currently depressed and remitted female MDD patients and to correlate these fluctuations with clinical markers of MDD. Nineteen currently depressed female patients, 19 remitted female patients, as well as 19 age- and education-matched healthy females participated in the resting-state functional magnetic resonance imaging (fMRI) analysis. We compared the fALFF/ALFF maps among the three groups and investigated the correlation between clinical measurements and statistically significant differences in the fALFF/ALFF of various brain regions. Compared with healthy controls, both currently depressed and remitted patients showed increased fALFF/ALFF in the right putamen. Currently depressed MDD patients showed increased fALFF/ALFF in the right ventral median frontal gyrus relative to both the remitted MDD group and the healthy control group. The ALFF of the right precuneus was found to be positively correlated with the number of depressive episodes and the fALFF of the right precuneus to be positively correlated with the disease duration in currently depressed MDD patients. An abnormal fALFF/ALFF in the right ventral median frontal gyrus was found only in currently depressed patients, suggesting that such an anomaly may play a critical role in depressive symptomatology and may be a therapeutic target for MDD. An abnormal fALFF/ALFF in the right putamen is a potential candidate as a trait-related marker of vulnerability to major depression.
X-Ray Fluctuation Power Spectral Densities of Seyfert 1 Galaxies
NASA Technical Reports Server (NTRS)
Markowitz, A.; Edelson, R.; Vaughan, S.; Uttley, P.; George, I. M.; Griffiths, R. E.; Kaspi, S.; Lawrence, A.; McHandy, I.; Nandra, K.
2003-01-01
By combining complementary monitoring observations spanning long, medium and short time scales, we have constructed power spectral densities (PSDs) of six Seyfert 1 galaxies. These PSDs span approx. greater than 4 orders of magnitude in temporal frequency, sampling variations on time scales ranging from tens of minutes to over a year. In at least four cases, the PSD shows a "break," a significant departure from a power law, typically on time scales of order a few days. This is similar to the behavior of Galactic X-ray binaries (XRBs), lower mass compact systems with breaks on time scales of seconds. NGC 3783 shows tentative evidence for a doubly-broken power law, a feature that until now has only been seen in the (much better-defined) PSDs of low-state XRBs. It is also interesting that (when one previously-observed object is added to make a small sample of seven), an apparently significant correlation is seen between the break time scale T and the putative black hole mass M(sub BH), while none is seen between break time scale and luminosity. The data are consistent with the linear relation T = M(sub BH) /10(exp 6.5) solar mass; extrapolation over 6-7 orders of magnitude is in reasonable agreement with XRBs. All of this strengthens the case for a physical similarity between Seyfert 1s and XRBs.
Ikefuji, Hiroyuki; Nomura, Masahiro; Nakaya, Yutaka; Mori, Toshifumi; Kondo, Noriyasu; Ieishi, Kiyoshi; Fujimoto, Sayuri; Ito, Susumu
2007-02-01
A close relationship exists between electric current and the magnetic field. However, electricity and magnetism have different physical characteristics, and magnetocardiography (MCG) may provide information on cardiac current that is difficult to obtain by electrocardiography (ECG). In the present study, we investigated the issue of whether the current density map method, in which cardiac current is estimated from the magnetic gradient, facilitates the visualization of cardiac current undetectable by ECG. The subjects were 50 healthy adults (N group), 40 patients with left ventricular overloading (LVO group), 15 patients with right ventricular overloading (RVO group), 10 patients with an old inferior myocardial infarction (OMI group), and 30 patients with diabetes mellitus (DM group). MCGs were recorded with a second derivative superconducting quantum interference device (SQUID) gradiometer using liquid helium. Isopotential maps and current density maps from unipolar precordial ECG leads and MCGs, respectively, were prepared, and the cardiac electric current was examined. The current density map at the ventricular depolarization phase showed one peak of current density in the N group. However, in the OMI group, the current density map showed multiple peaks of current density areas. In the RVO group, two peaks of current densities were detected at the right superior region and left thoracic region and these two diploles appeared to be from the right and left ventricular derived cardiac currents, respectively. Moreover, there was a significant correlation between the magnitude of the current density from the right ventricle and the systolic pulmonary arterial pressure. The current density map at the ventricular repolarization phase in the N group showed only a single current source. However, abnormal current sources in the current density maps were frequently detected even in patients showing no abnormalities on isopotential maps in the LVO, DM, and OMI groups. The
NASA Astrophysics Data System (ADS)
Gao, Wang; Chen, Yun; Jiang, Qing
2016-12-01
Discriminating between metallic (M ) and semiconducting (S ) single-walled carbon nanotubes (SWNTs) remains a fundamental challenge in the field of nanotechnology. We address this issue by studying the adsorption of the isotropic atoms Xe, Kr, and a highly anisotropic molecule n heptane on M - and S -SWNTs with density functional theory that includes many-body dispersion forces. We find that the distinct polarizabilities of M - and S -SWNTs exhibit significantly different physisorption properties, which are also strongly controlled by the SWNT's diameter, adsorption site, adsorbate coverage, and the adsorbate's anisotropy. These findings stem from the wavelike nature of charge-density fluctuations in SWNTs. Particularly, these results allow us to rationalize the unusual √{3 }×√{3 }R 3 00 phase of Kr atoms on small gap M -SWNTs and the double desorption peak temperatures of n heptane on M -SWNTs in experiments, and also propose the n heptane as an effective sensor for experimentally discriminating M - and S -SWNTs.
Wu, J J; Sun, H J; Gao, Z Y
2008-09-01
Detrended fluctuation analysis (DFA) is a useful tool to measure the long-range power-law correlations in 1f noise. In this paper, we investigate the power-law dynamics behavior of the density fluctuation time series generated by the famous Kerner-Klenov-Wolf cellular automata model in road traffic. Then the complexities of spatiotemporal, average speed, and the average density have been analyzed in detail. By introducing the DFA method, our main observation is that the free flow and wide moving jam phases correspond to the long-range anticorrelations. On the contrary, at the synchronized flow phase, the long-range correlated property is observed.
Melamed, E; Zoldan, J; Galili-Mosberg, R; Ziv, I; Djaldetti, R
1999-01-01
Motor fluctuations after long-term administration of levodopa may be due to central pharmacodynamic mechanisms such as reduced striatal synthesis and storage of dopamine from exogenous levodopa and subsensitization of postsynaptic dopaminergic receptors. Peripheral pharmacokinetic mechanisms may be equally important, particularly in motor fluctuations of the "delayed on" (increased time latencies from dose intake to start-up of clinical benefit) and "no-on" (complete failure of a levodopa dose to exert an "on" response) types. Levodopa itself has a very poor solubility. In addition, there is delayed gastric emptying in many advanced patients. Therefore, an oral dose of levodopa may remain in the stomach for long periods of time before it passes into the duodenum where there is immediate absorption. Consequently, in order to overcome response fluctuations caused by impaired pharmacokinetic mechanisms and to improve its absorption, we recommend that levodopa be taken in multiple small doses, on an empty stomach, preferably crushed and mixed with a lot of liquid. Protein intake should be minimized. Prokinetic drugs such as prepulsid (Cisaprid) could be used to facilitate gastric motility and levodopa transit time. Administration of crushed levodopa through nasoduodenal or gastrojejunostomy tubes may be helpful in certain circumstances. Bypassing the stomach with subcutaneous injections of apomorphine may provide dramatic rescue from difficult "off" situations. Oral and s.c. administration of novel, extremely soluble prodrugs of levodopa, e.g., levodopa ethylester, may offer a new approach to overcome difficulties in levodopa absorption. Addition of dopamine agonists, MAO-B inhibitors, COMT inhibitors and controlled release levodopa preparations may be helpful in prolonging the duration of efficacy of each single levodopa dose. Levodopa, administered orally, usually combined with peripheral dopa decarboxylase inhibitors, continues to be the most widely-used and most
Laboratory Study Of Magnetic Reconnection With A Density Asymmetry Across The Current Sheet
Yoo, Joseph; Yamada, Massaaki; Ji, Hantao; Meyers,, Clayton E.; Jara-Almonte,; Chen, Li-Jen
2014-04-18
The effects of an upstream density asymmetry on magnetic reconnection are studied systematically in a laboratory plasma. Despite a significant upstream density asymmetry of up to 10, the reconnecting magnetic field pro file is not signifi cantly changed. On the other hand, the out-of-plane magnetic field profile is considerably modified; it is almost bipolar in structure with the density asymmetry, as compared to the quadrupolar structure in the symmetric configuration. The in-plane ion flow pattern and the electrostatic potential pro file are also affected by the density asymmetry. Strong bulk electron heating is observed near the low-density-side separatrix together with electromagnetic fluctuations in the lower hybrid frequency range. The dependence of the ion outflow and reconnection electric field on the density asymmetry is measured and compared with theoretical expectations.
Oprisan, Ana; Oprisan, Sorinel A; Bayley, Brittany; Hegseth, John J; Garrabos, Yves; Lecoutre-Chabot, Carole; Beysens, Daniel
2012-12-01
Large density fluctuations were observed by illuminating a cylindrical cell filled with sulfur hexafluoride (SF(6)), very near its liquid-gas critical point (|T-T(c)|< 300 μK) and recorded using a microscope with 3 μm spatial resolution. Using a dynamic structure factor algorithm, we determined from the recorded images the structure factor (SF), which measures the spatial distribution of fluctuations at different moments, and the correlation time of fluctuations. This method authorizes local measurements in contrast to the classical scattering techniques that average fluctuations over the illuminating beam. We found that during the very early stages of phase separation the SF scales with the wave vector q according to the Lorentzian q(-2), which shows that the liquid and vapor domains are just emerging. The critical wave number, which is related to the characteristic length of fluctuations, steadily decreases over time, supporting a sustained increase in the spatial scale of the fluctuating domains. The scaled evolution of the critical wave number obeys the universal evolution for the interconnected domains at high volume fraction with an apparent power law exponent of -0.35 ± 0.02. We also determined the correlation time of the fluctuations and inferred values for thermal diffusivity coefficient very near the critical point, above and below. The values were used to pinpoint the crossing of T(c) within 13 μK.
NASA Astrophysics Data System (ADS)
Hillesheim, J. C.; Crocker, N. A.; Peebles, W. A.; Meyer, H.; Meakins, A.; Field, A. R.; Dunai, D.; Carr, M.; Hawkes, N.; the MAST Team
2015-07-01
The high-k (7≲ {{k}\\bot}{ρi}≲ 11 ) wavenumber spectrum of density fluctuations has been measured for the first time in MAST (Lloyd et al 2003 Nucl. Fusion 43 1665). This was accomplished with the first implementation of Doppler backscattering (DBS) for core measurements in a spherical tokamak. DBS has become a well-established and versatile diagnostic technique for the measurement of intermediate- k ({{k}\\bot}{ρi}˜ 1 , and higher) density fluctuations and flows in magnetically confined fusion experiments. Previous implementations of DBS for core measurements have been in standard, large aspect ratio tokamaks. A novel implementation with two-dimensional (2D) steering was necessary to enable DBS measurements in MAST, where the large variation of the magnetic field pitch angle presents a challenge. We report on the scattering considerations and ray tracing calculations used to optimize the design and present data demonstrating measurement capabilities. Initial results confirm the applicability of the design and implementation approaches, showing the strong dependence of scattering alignment on the toroidal launch angle and demonstrating that DBS is sensitive to the local magnetic field pitch angle. We also present comparisons of DBS plasma velocity measurements with charge exchange recombination and beam emission spectroscopy measurements, which show reasonable agreement over most of the minor radius, but imply large poloidal flows approaching the magnetic axis in a discharge with an internal transport barrier. The 2D steering is shown to enable high-k measurements with DBS, at {{k}\\bot}>20 cm-1 ({{k}\\bot}{ρi}>10 ) for launch frequencies less than 75 GHz; this capability is used to measure the wavenumber spectrum of turbulence and we find \\mid n≤ft({{k}\\bot}\\right){{\\mid}2}\\propto k\\bot-4.7+/- 0.2 for {{k}\\bot}{ρi}≈ 7 -11, which is similar to the expectation for the turbulent kinetic cascade of \\mid n≤ft({{k}\\bot}\\right){{\\mid}2}\\propto
NASA Astrophysics Data System (ADS)
Ding, B. J.; Li, Y. C.; Zhang, L.; Li, M. H.; Wei, W.; Kong, E. H.; Wang, M.; Xu, H. D.; Wang, S. L.; Xu, G. S.; Zhao, L. M.; Hu, H. C.; Jia, H.; Cheng, M.; Yang, Y.; Liu, L.; Zhao, H. L.; Peysson, Y.; Decker, J.; Goniche, M.; Amicucci, L.; Cesario, R.; Tuccillo, A. A.; Baek, S. G.; Parker, R.; Bonoli, P. T.; Paoletti, F.; Yang, C.; Shan, J. F.; Liu, F. K.; Zhao, Y. P.; Gong, X. Z.; Hu, L. Q.; Gao, X.; Wan, B. N.; Li, J. G.; the EAST Team
2015-09-01
Two important issues in achieving lower hybrid current drive (LHCD) high confinement plasma in EAST are to improve lower hybrid wave (LHW)-plasma coupling and to drive the plasma current at a high density. Studies in different configurations with different directions of toroidal magnetic field (Bt) show that the density near the antenna is affected by both the radial electric field induced by plasma without a LHW (Er_plasma) in the scrape off layer (SOL), and the radial electric field induced by LHW power (Er_LH) near the grill. Investigations indicate that Er × Bt in the SOL leads to a different effect of configuration on the LHW-plasma coupling and Er_LH × Bt accounts for the asymmetric density behaviour in the SOL observed in the experiments, where Er is the total radial electric field in the SOL. Modelling of parametric instability (PI), collisional absorption (CA) and scattering from density fluctuations (SDF) in the edge region, performed considering the parameters of high density LHCD experiments in EAST, has shown that these mechanisms could be responsible for the low current drive (CD) efficiency at high density. Radiofrequency probe spectra, useful for documenting PI occurrence, show sidebands whose amplitude in the case of the lithiated vacuum chamber is smaller than in the case of poor lithiation, consistently with growth rates from PI modeling of the respective reference discharges. Since strong lithiation is also expected to diminish the parasitic effect on the LHCD of the remaining possible mechanisms, this appears to be a useful method for improving LHCD efficiency at a high density.
Opening a nodal gap by fluctuating spin-density wave in lightly doped La2 -xSrxCuO4
NASA Astrophysics Data System (ADS)
Kapon, Itzik; Ellis, David S.; Drachuck, Gil; Bazalitski, Galina; Weschke, Eugen; Schierle, Enrico; Strempfer, Jörg; Niedermayer, Christof; Keren, Amit
2017-03-01
We investigate whether the spin or charge degrees of freedom are responsible for the nodal gap in underdoped cuprates by performing inelastic neutron scattering and x-ray diffraction measurements on La2 -xSrxCuO4 , which is on the edge of the antiferromagnetic phase. We found that a fluctuating incommensurate spin-density wave (SDW) with a bottom part of an hourglass dispersion exists even in this magnetic sample. The strongest component of these fluctuations diminishes at the same temperature where the nodal gap opens. X-ray scattering measurements on the same crystal show no signature of a charge-density wave (CDW). Therefore, we suggest that the nodal gap in the electronic band of this cuprate opens due to fluctuating SDW with no contribution from CDW.
Hossain, Md Zahid; Rumyantsev, Sergey L; Shahil, Khan M F; Teweldebrhan, Desalegne; Shur, Michael; Balandin, Alexander A
2011-04-26
We report on the low-frequency current fluctuations and electronic noise in thin-films made of Bi(2)Se(3) topological insulators. The films were prepared via the "graphene-like" mechanical exfoliation and used as the current conducting channels in the four- and two-contact devices. The thickness of the films ranged from ∼50 to 170 nm to avoid hybridization of the top and bottom electron surface states. Analysis of the resistance dependence on the film thickness indicates that the surface contribution to conductance is dominant in our samples. It was established that the current fluctuations have the noise spectrum close to the pure 1/f in the frequency range from 1 Hz to 10 kHz (f is the frequency). The relative noise amplitude S(I)/I(2) for the examined Bi(2)Se(3) films was increasing from ∼5 × 10(-8) to 5 × 10(-6) (1/Hz) as the resistance of the channels varied from ∼10(3) to 10(5) Ω. The obtained noise data is important for understanding electron transport through the surface and volume of topological insulators, and proposed applications of this class of materials. The results may help to develop a new method of noise reduction in electronic devices via the "scattering immune" transport through the surface states.
NASA Astrophysics Data System (ADS)
McCray, J. E.; Downs, W.; Falta, R. W.; Housley, T.
2005-12-01
DNAPL sources of carbon tetrachloride (CT) vapors are of interest at the Radioactive Waste Management Complex (RWMC) at the Idaho National Engineering and Environmental Laboratory (INEEL). The site is underlain by thick fractured basalt that includes sedimentary interbeds, each are a few meters thick. Daily atmospheric pressure fluctuations serve as driving forces for CT vapor transport in the subsurface. Other important transport processes for vapor movement include gas-phase diffusion and density-driven transport. The objective of this research is to investigate the influence and relative importance of these processes on gaseous transport of CT. Gas pressure and vapor concentration measurements were conducted at various depths in two wells. A numerical multiphase flow model (TOUGH2), calibrated to field pressure data, is used to conduct sensitivity analyses to elucidate the importance of the different transport mechanisms. Results show that the basalt is highly permeable to vertical air flow. The pressure dampening occurs mainly in the sedimentary interbeds. Model-calibrated permeability values for the interbeds are similar to those obtained in a study by the U.S. Geological Survey for shallow sediments, and an order of magnitude higher than column-scale values obtained by previous studies conducted by INEEL scientists. The transport simulations indicate that considering the effect of barometric pressure changes is critical to simulating transport of pollutants in the vadose zone above the DNAPL source. Predicted concentrations can be orders of magnitude smaller than actual concentrations if the effect is not considered. Below the DNAPL vapor source, accounting for density and diffusion alone would yield acceptable results provided that a 20% error in concentrations are acceptable, and that simulating concentrations trends (and not actual concentrations) is the primary goal.
A Cutoff in the X-Ray Fluctuation Power Density Spectrum of the Seyfert 1 Galaxy NGC 3516
NASA Technical Reports Server (NTRS)
Edelson, Rick; Nandra, Kirpal
1999-01-01
During 1997 March-July, RXTE observed the bright, strongly variable Seyfert 1 galaxy NGC 3516 once every approx. 12.8 hr for 4.5 months and nearly continuously (with interruptions due to SAA passage but not Earth occultation) for a 4.2 day period in the middle. These were followed by ongoing monitoring once every approx. 4.3 days. These data are used to construct the first well-determined X-ray fluctuation power density spectrum (PDS) of an active galaxy to span more than 4 decades of usable temporal frequency. The PDS shows no signs of any strict or quasi-periodicity, but does show a progressive flattening of the power-low slope from -1.74 at short time scales to -0.73 at longer time scales. This is the clearest observation to date of the long-predicted cutoff in the PDS. The characteristic variability time scale corresponding to this cutoff temporal frequency is approx. 1 month. Although it is unclear how this time scale may be interpreted in terms of a physical size or process, there are several promising candidate models. The PDS appears similar to those seen for Galactic black hole candidates such as Cyg X-1, suggesting that these two classes of objects with very different luminosities and putative black hole masses (differing by more than a factor of 10(exp 5)) may have similar X-ray generation processes and structures.
Battipaglia, Giovanna; DE Micco, Veronica; Brand, Willi A; Saurer, Matthias; Aronne, Giovanna; Linke, Petra; Cherubini, Paolo
2014-02-01
Erica arborea (L) is a widespread Mediterranean species, able to cope with water stress and colonize semiarid environments. The eco-physiological plasticity of this species was evaluated by studying plants growing at two sites with different soil moistures on the island of Elba (Italy), through dendrochronological, wood-anatomical analyses and stable isotopes measurements. Intra-annual density fluctuations (IADFs) were abundant in tree rings, and were identified as the key parameter to understand site-specific plant responses to water stress. Our findings showed that the formation of IADFs is mainly related to the high temperature, precipitation patterns and probably to soil water availability, which differs at the selected study sites. The recorded increase in the (13) C-derived intrinsic water use efficiency at the IADFs level was linked to reduced water loss rather than to increasing C assimilation. The variation in vessel size and the different absolute values of δ(18) O among trees growing at the two study sites underlined possible differences in stomatal control of water loss and possible differences in sources of water uptake. This approach not only helped monitor seasonal environmental differences through tree-ring width, but also added valuable information on E. arborea responses to drought and their ecological implications for Mediterranean vegetation dynamics.
NASA Astrophysics Data System (ADS)
Khrennikov, Andrei
2010-05-01
Prequantum classical statistical field theory (PCSFT) is a new attempt to consider quantum mechanics (QM) as an emergent phenomenon, cf. with De Broglie's "double solution" approach, Bohmian mechanics, stochastic electrodynamics (SED), Nelson's stochastic QM and its generalization by Davidson, `t Hooft's models and their development by Elze. PCSFT is a comeback to a purely wave viewpoint on QM, cf. with early Schrödinger. There is no quantum particles at all, only waves. In particular, photons are simply wave-pulses of the classical electromagnetic field, cf. SED. Moreover, even massive particles are special "prequantum fields": the electron field, the neutron field and so on. PCSFT claims that (soon or later) people will be able to measure components of these fields: components of the "photonic field" (the classical electromagnetic field of low intensity), electronic field, neutronic field and so on. However, at the moment (in this paper) we restrict our efforts to reproduce "simply" predictions of QM in the classical field framework. We will show that correlations of entangled systems can be obtained from fluctuations of the prequantum field. We consider the most general case: in QM the state is given by the density operator.
Projected current density comparison in tDCS block and smooth FE modeling.
Indahlastari, Aprinda; Chauhan, Munish; Sadleir, Rosalind J
2016-08-01
Current density distribution and projected current density calculation following transcranial direct current stimulation (tDCS) forward model in a human head were compared between two modeling pipelines: block and smooth. Block model was directly constructed from MRI voxel resolution and simulated in C. Smooth models underwent a boundary smoothing process by applying recursive Gaussian filters and simulated in COMSOL. Three smoothing levels were added to determine their effects on current density distribution compared to block models. Median current density percentage differences were calculated in anterior superior temporal gyrus (ASTG), hippocampus (HIP), inferior frontal gyrus (IFG), occipital lobes (OCC) and precentral gyrus (PRC) and normalized against a baseline value. A maximum of + 20% difference in median current density was found for three standard electrode montages: F3-RS, T7-T8 and Cz-Oz. Furthermore, median current density percentage differences in each montage target brain structures were found to be within + 7%. Higher levels of smoothing increased median current density percentage differences in T7-T8 and Cz-Oz target structures. However, while demonstrating similar trends in each montage, additional smoothing levels showed no clear relationship between their smoothing effects and calculated median current density in the five cortical structures. Finally, relative L2 error in reconstructed projected current density was found to be 17% and 21% for block and smooth pipelines, respectively. Overall, a block model workflow may be a more attractive alternative for simulating tDCS stimulation because involves a shorter modeling time and independence from commercial modeling platforms.
NASA Technical Reports Server (NTRS)
Israelsson, Ulf E. (Inventor); Strayer, Donald M. (Inventor)
1992-01-01
A contact-less method for determining transport critical current density and flux penetration depth in bulk superconductor material. A compressor having a hollow interior and a plunger for selectively reducing the free space area for distribution of the magnetic flux therein are formed of superconductor material. Analytical relationships, based upon the critical state model, Maxwell's equations and geometrical relationships define transport critical current density and flux penetration depth in terms of the initial trapped magnetic flux density and the ratio between initial and final magnetic flux densities whereby data may be reliably determined by means of the simple test apparatus for evaluating the current density and flux penetration depth.
Gonzalo-Gomez, Alicia; Turiegano, Enrique; León, Yolanda; Molina, Isabel; Torroja, Laura; Canal, Inmaculada
2012-01-01
HCN channels are becoming pharmacological targets mainly in cardiac diseases. But apart from their well-known role in heart pacemaking, these channels are widely expressed in the nervous system where they contribute to the neuron firing pattern. Consequently, abolishing Ih current might have detrimental consequences in a big repertoire of behavioral traits. Several studies in mammals have identified the Ih current as an important determinant of the firing activity of dopaminergic neurons, and recent evidences link alterations in this current to various dopamine-related disorders. We used the model organism Drosophila melanogaster to investigate how lack of Ih current affects dopamine levels and the behavioral consequences in the sleep∶activity pattern. Unlike mammals, in Drosophila there is only one gene encoding HCN channels. We generated a deficiency of the DmIh core gene region and measured, by HPLC, levels of dopamine. Our data demonstrate daily variations of dopamine in wild-type fly heads. Lack of Ih current dramatically alters dopamine pattern, but different mechanisms seem to operate during light and dark conditions. Behaviorally, DmIh mutant flies display alterations in the rest∶activity pattern, and altered circadian rhythms. Our data strongly suggest that Ih current is necessary to prevent dopamine overproduction at dark, while light input allows cycling of dopamine in an Ih current dependent manner. Moreover, lack of Ih current results in behavioral defects that are consistent with altered dopamine levels. PMID:22574167
Current Densities in speed analyzer with different symmetries
Valdeblanquez, E.
2006-12-04
A comparative analysis of the currents in speed analyzer of speeds is made with different symmetries. Three kinds of symmetries are considered; plane, cylindrical and spherical. The analyzers considered are formed by threes electrodes, the selector grid, the discriminator and the collector. The selector grid has a negative potential and for the coulombian effect the space charge is formed with the ionic thermal bath. Using kinetic theory a strongly non linear differential equation is obtained which is solved by numerical calculation.
Peltier effect in multilayered nanopillars under high density charge current
NASA Astrophysics Data System (ADS)
Gravier, L.; Fukushima, A.; Kubota, H.; Yamamoto, A.; Yuasa, S.
2006-12-01
From the basic equations of thermoelectricity, we model the thermal regimes that develop in multilayered nanopillar elements experiencing continuous charge currents. The energy conservation principle was applied to all layer-layer and layer-electrode junctions. The obtained set of equations was solved to derive the temperature of each junction. The contribution of the Peltier effect is included in an effective resistance. This model gives satisfactory fits to experimental data obtained on a series of reference nanopillar elements.
Spatial scales of current speed and phytoplankton biomass fluctuations in lake tahoe.
Powell, T M; Richerson, P J; Dillon, T M; Agee, B A; Dozier, B J; Godden, D A; Myrup, L O
1975-09-26
Spectral analysis of current speed and chlorophyll a measurements in Lake Tahoe, California and Nevada, indicates that considerably more variance exists at longer length scales in chlorophyll than in the current speeds. Increasingly, above scales of approximately 100 meters, chlorophyll does not behave as a simple passive contaminant distributed by turbulence, which indicates that biological processes contribute significantly to the observed variance at these large length scales.
NASA Astrophysics Data System (ADS)
Okada, Hiroki; Utsumi, Yasuhiro
2017-02-01
We theoretically investigate the coefficient of performance (COP) of a mesoscopic thermoelectric refrigerator realized by using a tunnel junction. We analyze the influence of particle and heat current fluctuations on the COP out of the equilibrium regime. We calculate the average COP by using full counting statistics and find that it depends on the measurement time τ. The deviation from the macroscopic COP value can be expressed with the Skellam distribution at all times. This result enables us to improve the Gaussian approximation valid within the linear response regime, which cannot predict the average COP in the limit of τ → 0. We illustrate the time dependence of the average COP and find that in the short-time regime, the average COP possesses a minimum. In order to confirm the physical consistency far from equilibrium, we propose checking the correlation coefficient between the particle and the heat currents in addition to the positivity of the entropy production rate.
Phase dynamics of low critical current density YBCO Josephson junctions
NASA Astrophysics Data System (ADS)
Massarotti, D.; Stornaiuolo, D.; Rotoli, G.; Carillo, F.; Galletti, L.; Longobardi, L.; Beltram, F.; Tafuri, F.
2014-08-01
High critical temperature superconductors (HTS) based devices can have impact in the study of the phase dynamics of Josephson junctions (JJs) thanks to the wide range of junction parameters they offer and to their unconventional properties. Measurements of current-voltage characteristics and of switching current distributions constitute a direct way to classify different regimes of the phase dynamics and of the transport, also in nontrivial case of the moderately damped regime (MDR). MDR is going to be more and more common in JJs with advances in nanopatterning superconductors and synthesizing novel hybrid systems. Distinctive signatures of macroscopic quantum tunneling and of thermal activation in presence of different tunable levels of dissipation have been detected in YBCO grain boundary JJs. Experimental data are supported by Monte Carlo simulations of the phase dynamics, in a wide range of temperatures and dissipation levels. This allows us to quantify dissipation in the MDR and partially reconstruct a phase diagram as guideline for a wide range of moderately damped systems.
NASA Astrophysics Data System (ADS)
Devynck, P.; Ghendrih, P.; Sarazin, Y.
2005-05-01
It is shown that intermittent density bursts observed in the scrape-off layer of Tore Supra [J. Jacquinot, Nucl. Fusion 43, 1583 (2003)] are detected in packs on the probe. In such a pack, typically two to three bursts are separated by time intervals smaller than the mean separation time. The long tails above 50μs observed on the autocorrelation function of the density fluctuations are found to be the temporal correlation between the individual bursts within their pack. Packs of density bursts can be detected in two limiting states of the turbulence: when the coupling between density and potential is strong and large density bursts split during their radial propagation or at the opposite when the coupling is weak so that different density bursts can propagate radially along the potential valleys. The lack of spatial resolution of the diagnostic does not allow to discriminate between the two mechanisms.
Devynck, P.; Ghendrih, P.; Sarazin, Y.
2005-05-15
It is shown that intermittent density bursts observed in the scrape-off layer of Tore Supra [J. Jacquinot, Nucl. Fusion 43, 1583 (2003)] are detected in packs on the probe. In such a pack, typically two to three bursts are separated by time intervals smaller than the mean separation time. The long tails above 50 {mu}s observed on the autocorrelation function of the density fluctuations are found to be the temporal correlation between the individual bursts within their pack. Packs of density bursts can be detected in two limiting states of the turbulence: when the coupling between density and potential is strong and large density bursts split during their radial propagation or at the opposite when the coupling is weak so that different density bursts can propagate radially along the potential valleys. The lack of spatial resolution of the diagnostic does not allow to discriminate between the two mechanisms.
NASA Astrophysics Data System (ADS)
Battipaglia, G.; Brand, W. A.; Linke, P.; Schaefer, I.; Noetzli, M.; Cherubini, P.
2009-04-01
Tree- ring growth and wood density have been used extensively as indicators of climate change, and tree-ring has been commonly applied as a proxy estimate for seasonal integration of temperatures and precipitation with annual resolution (Hughes 2002). While these relationships have been well established in temperate ecosystems (Fritts, 1976; Schweingruber, 1988, Briffa et al., 1998, 2004), in Mediterranean region dendrochronological studies are still scarce (Cherubini et al, 2003). In Mediterranean environment, trees may form intra-annual density fluctuations, also called "false rings" or "double rings" (Tingley 1937; Schulman 1938). They are usually induced by sudden drought events, occurring during the vegetative period, and, allowing intra-annual resolution, they may provide detailed information at a seasonal level, as well as species-specific sensitivity to drought. We investigated the variability of tree- ring width and carbon stable isotopes of a Mediterranean species, Arbutus unedo L., sampled on Elba island, (Tuscany, Italy). The samples were taken at two different sites, one characterized by wet and one by dry conditions. d13C was measured using Laser- Ablation- Combustion -GC-IRMS. Here, we present first results showing the impact of drought on tree growth and on false ring formation at the different sites and we underline the importance of using Laser Ablation to infer drought impact at the intra -annual level. Briffa KR, Schweingruber FH, Jones PD, Osborn TJ, Harris IC, Shiyatov SG, Vaganov EA, Grudd H (1998) Trees tell of past climates: but are they speaking less clearly today? Phil Transact Royal Soc London 353:65-73 Briffa KR, Osborn TJ, Schweingruber FH (2004) Large-scale temperature inferences from tree rings: a review. Glob Panet Change 40:11-26 Cherubini, P., B.L. Gartner, R. Tognetti, O.U. Bräker, W. Schoch & J.L. Innes. 2003. Identification, measurement and interpretation of tree rings in woody species from Mediterranean climates. Biol. Rev
Partially coherent fundamental Gaussian wave generated by a fluctuating planar current source.
Seshadri, S R
2010-06-01
The propagation characteristics of a spatially localized electromagnetic wave produced by a planar current source of different states of spatial coherence are analyzed by the use of a Gaussian Schell-model source. A linearly polarized fundamental electromagnetic Gaussian wave with the electric field perpendicular to the direction of propagation is treated. The effects of the degree of coherence of the source distribution on the radiation intensity distribution and the total radiated power are determined.
Wilbanks, Matt C.; Yuter, S. E.; de Szoeke, S.; Brewer, W. A.; Miller, Matthew A.; Hall, Andrew M.; Burleyson, Casey D.
2015-09-01
Density currents (i.e. cold pools or outflows) beneath marine stratocumulus clouds are characterized using a 30-d data set of ship-based observations obtained during the 2008 Variability of American Monsoon Systems (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) in the southeast Pacific. An objective method identifies 71 density current fronts using an air density criterion and isolates each density current’s core (peak density) and tail (dissipating) zone. Compared to front and core zones, most density current tails exhibited weaker density gradients and wind anomalies elongated about the axis of the mean wind. The mean cloud-level advection relative to the surface layer wind (1.9 m s-1) nearly matches the mean density current propagation speed (1.8 m s-1). The similarity in speeds allows drizzle cells to deposit tails in their wakes. Based on high-resolution scanning Doppler lidar data, prefrontal updrafts had a mean intensity of 0.91 m s-1, reached an average altitude of 800 m, and were often surmounted by low-lying shelf clouds not connected to the overlying stratocumulus cloud. Nearly 90% of density currents were identified when C-band radar estimated 30-km diameter areal average rain rates exceeded 1 mm d-1. Rather than peaking when rain rates are highest overnight, density current occurrence peaks between 0600 and 0800 local solar time when enhanced local drizzle co-occurs with shallow subcloud dry and stable layers. The dry layers may contribute to density current formation by enhancing subcloud evaporation of drizzle. Density currents preferentially occur in regions of open cells but also occur in regions of closed cells.
Flux avalanche in a superconducting film with non-uniform critical current density
NASA Astrophysics Data System (ADS)
Lu, Yurong; Jing, Ze; Yong, Huadong; Zhou, Youhe
2016-10-01
The flux avalanche in type-II superconducting thin film is numerically simulated in this paper. We mainly consider the effect of non-uniform critical current density on the thermomagnetic stability. The nonlinear electromagnetic constitutive relation of the superconductor is adopted. Then, Maxwell's equations and heat diffusion equation are numerically solved by the fast Fourier transform technique. We find that the non-uniform critical current density can remarkably affect the behaviour of the flux avalanche. The external magnetic field ramp rate and the environmental temperature have been taken into account. The results are compared with a film with uniform critical current density. The flux avalanche first appears at the interface where the critical current density is discontinuous. Under the same environmental temperature or magnetic field, the flux avalanche occurs more easily for the film with the non-uniform critical current density. The avalanche structure is a finger-like pattern rather than a dendritic structure at low environmental temperatures.
Tellgren, E I; Teale, A M; Furness, J W; Lange, K K; Ekström, U; Helgaker, T
2014-01-21
We present a novel implementation of Kohn-Sham density-functional theory utilizing London atomic orbitals as basis functions. External magnetic fields are treated non-perturbatively, which enable the study of both magnetic response properties and the effects of strong fields, using either standard density functionals or current-density functionals-the implementation is the first fully self-consistent implementation of the latter for molecules. Pilot applications are presented for the finite-field calculation of molecular magnetizabilities, hypermagnetizabilities, and nuclear magnetic resonance shielding constants, focusing on the impact of current-density functionals on the accuracy of the results. Existing current-density functionals based on the gauge-invariant vorticity are tested and found to be sensitive to numerical details of their implementation. Furthermore, when appropriately regularized, the resulting magnetic properties show no improvement over standard density-functional results. An advantage of the present implementation is the ability to apply density-functional theory to molecules in very strong magnetic fields, where the perturbative approach breaks down. Comparison with high accuracy full-configuration-interaction results show that the inadequacies of current-density approximations are exacerbated with increasing magnetic field strength. Standard density-functionals remain well behaved but fail to deliver high accuracy. The need for improved current-dependent density-functionals, and how they may be tested using the presented implementation, is discussed in light of our findings.
Jiménez, Rolando Placeres; Pupo, Ana Elisa Bergues; Cabrales, Jesús Manuel Bergues; Joa, Javier Antonio González; Cabrales, Luis Enrique Bergues; Nava, Juan José Godina; Aguilera, Andrés Ramírez; Mateus, Miguel Angel O'Farril; Jarque, Manuel Verdecia; Brooks, Soraida Candida Acosta
2011-02-01
Electrotherapy with direct current delivered through implanted electrodes is used for local control of solid tumors in both preclinical and clinical studies. The aim of this research is to develop a solution method for obtaining a three-dimensional analytical expression for potential and electric current density as functions of direct electric current intensity, differences in conductivities between the tumor and the surrounding healthy tissue, and length, number and polarity of electrodes. The influence of these parameters on electric current density in both media is analyzed. The results show that the electric current density in the tumor is higher than that in the surrounding healthy tissue for any value of these parameters. The conclusion is that the solution method presented in this study is of practical interest because it provides, in a few minutes, a convenient way to visualize in 3D the electric current densities generated by a radial electrode array by means of the adequate selection of direct current intensity, length, number, and polarity of electrodes, and the difference in conductivity between the solid tumor and its surrounding healthy tissue.
Complexities of determining the Field-Aligned current density from LEO satellites.
NASA Astrophysics Data System (ADS)
Gjerloev, J. W.; Friel, M. M.; Ohtani, S.; Muhleisen, M.; Gjerloev, A. W.; Martin, P.; Barnes, R. J.
2015-12-01
We show results from a study of the field-aligned currents (FAC) as derived from SWARM magnetic field perturbations. We calculate the FAC density using four different techniques and explain why they provide different results. Theoretical work, simulations and data are used to show that widely used techniques can provide current density estimates with errors of 1000%. These errors can be explained by spatial gradients in the currents and temporal variability of the currents as well as a breakdown of other fundamental assumptions. We apply the techniques to auroral crossings with THEMIS ASI coverage and use the SWARM magnetic field observations to calculate the current density. Finally, we show how to estimate the errors in the current density calculations.
Relationship of dislocation density of silicon to solar cell current loss at low temperature.
NASA Technical Reports Server (NTRS)
Mandelkorn, J.; Baraona, C. R.; Lamneck, J. H., Jr.
1972-01-01
Large decreases in short circuit current of silicon solar cells have been reported to occur as temperature is decreased below -60 C. Experimental results are presented which relate high dislocation density of the silicon bulk material of cells to the large current loss effect. These results reveal a direct relationship between low bulk dislocation density and low current loss at low temperature. Oxygen content does not appear to play a significant role in the low temperature-large current loss effect, since some Czochralski cells did not suffer from this effect whereas some float-zone cells did. Other float-zone silicon cells had only medium current losses at low temperature despite their high bulk dislocation density. It appears that use of low-dislocation-density silicon can eliminate the current loss problem in low temperature cell operation.
Shelukhin, D.A.; Vershkov, V.A.; Razumova, K.A.
2005-12-15
In experiments on off-axis electron-cyclotron resonance heating in the T-10 tokamak, a steep gradient of the electron temperature was observed to form for a short time at a relative radius of {rho} {approx_equal} 0.25 after the heating power was switched off. Small-scale fluctuations of the electron density were studied with the help of correlation reflectometry. It was found that, in a narrow region near {rho} {approx_equal} 0.25, the amplitude of the density fluctuations was two times lower than that in the ohmic heating phase. Quasi-coherent fluctuations were suppressed over a period of time during which the steep temperature gradient existed. Measurements of the poloidal rotation velocity of turbulent fluctuations show that there is no velocity shear after the heating is switched off. An analysis of the linear growth rates of instabilities shows that the ion-temperature-gradient mode is unstable at {rho} {approx_equal} 0.25 throughout the entire discharge phase. The effect observed can be explained by an increase in the distance between the rational surfaces near the radius at which the safety factor is q = 1 due to the temporary flattening of the q profile after the off-axis electron-cyclotron resonance heating is switched off.
NASA Astrophysics Data System (ADS)
He, Lianyi
2016-10-01
We present a standard field theoretical derivation of the dynamic density and spin linear response functions of a dilute superfluid Fermi gas in the BCS-BEC crossover in both three and two dimensions. The derivation of the response functions is based on the elegant functional path integral approach which allows us to calculate the density-density and spin-spin correlation functions by introducing the external sources for the density and the spin density. Since the generating functional cannot be evaluated exactly, we consider two gapless approximations which ensure a gapless collective mode (Goldstone mode) in the superfluid state: the BCS-Leggett mean-field theory and the Gaussian-pair-fluctuation (GPF) theory. In the mean-field theory, our results of the response functions agree with the known results from the random phase approximation. We further consider the pair fluctuation effects and establish a theoretical framework for the dynamic responses within the GPF theory. We show that the GPF response theory naturally recovers three kinds of famous diagrammatic contributions: the Self-Energy contribution, the Aslamazov-Lakin contribution, and the Maki-Thompson contribution. We also show that unlike the equilibrium state, in evaluating the response functions, the linear (first-order) terms in the external sources as well as the induced order parameter perturbations should be treated carefully. In the superfluid state, there is an additional order parameter contribution which ensures that in the static and long wavelength limit, the density response function recovers the result of the compressibility (compressibility sum rule). We expect that the f-sum rule is manifested by the full number equation which includes the contribution from the Gaussian pair fluctuations. The dynamic density and spin response functions in the normal phase (above the superfluid critical temperature) are also derived within the Nozières-Schmitt-Rink (NSR) theory.
Covariance of nonstationary sodium current fluctuations at the node of Ranvier.
Sigworth, F J
1981-01-01
A theory is presented which relates the nonstationary autocovariance (covariance) function to the kinetics of independently-gated ionic channels. The experimental covariance was calculated from ensembles of 256--504 current records elicited from single, voltage-clamped, frog myelinated nerve fibers. Analysis of the covariance shows that the decay of channels from conducting to nonconducting states proceeds more slowly late in a depolarization to near 0 mV, as compared with early in the same depolarization. This behavior is inconsistent with there being only one kinetic state corresponding to the open channel. The behavior can be explained by the existence of multiple kinetic states corresponding to the open channel, or, alternatively, by the existence of multiple, kinetically distinct populations of channels. PMID:6260261
Higher-order mesoscopic fluctuations in quantum wires: Conductance and current cumulants
NASA Astrophysics Data System (ADS)
Stenberg, Markku P. V.; Särkkä, Jani
2006-07-01
We study conductance cumulants ⟪gn⟫ and current cumulants Cj related to heat and electrical transport in coherent mesoscopic quantum wires near the diffusive regime. We consider the asymptotic behavior in the limit where the number of channels and the length of the wire in the units of the mean free path are large but the bare conductance is fixed. A recursion equation unifying the descriptions of the standard and Bogoliubov-de Gennes (BdG) symmetry classes is presented. We give values and come up with a novel scaling form for the higher-order conductance cumulants. In the BdG wires, in the presence of time-reversal symmetry, for the cumulants higher than the second it is found that there may be only contributions which depend nonanalytically on the wire length. This indicates that diagrammatic or semiclassical pictures do not adequately describe higher-order spectral correlations. Moreover, we obtain the weak-localization corrections to Cj with j⩽10 .
Estimation of current density distribution of PAFC by analysis of cell exhaust gas
Kato, S.; Seya, A.; Asano, A.
1996-12-31
To estimate distributions of Current densities, voltages, gas concentrations, etc., in phosphoric acid fuel cell (PAFC) stacks, is very important for getting fuel cells with higher quality. In this work, we leave developed a numerical simulation tool to map out the distribution in a PAFC stack. And especially to Study Current density distribution in the reaction area of the cell, we analyzed gas composition in several positions inside a gas outlet manifold of the PAFC stack. Comparing these measured data with calculated data, the current density distribution in a cell plane calculated by the simulation, was certified.
Method for Determining Local Current Density in 2G HTS Tapes
NASA Astrophysics Data System (ADS)
Bludova, A. I.
Practically important problem is to determine the density and direction of 2G HTS induced currents at each point on the tape in order to examine its local deviations. This problem is resolved indirectly by spatial measurement of generated magnetic field with a scanning Hall sensor at a given height above the tape surface. Current density is subsequently determined by the Biot-Savart law inversion in Fourier domain. Tikhonov regularization is used in order to increase precision. Method is verified with the model current density reconstruction. Optimal calculation parameters and resulting precision are described.
NASA Technical Reports Server (NTRS)
Vanzandt, T. E.; Smith, S. A.; Tsuda, T.; Sato, T.; Fritts, D. C.
1990-01-01
Results are presented from a six-day campaign to observe velocity fluctuations in the lower atmosphere using the MU radar (Fukao et al., 1985) in Shigaraki, Japan in March, 1986. Consideration is given to the azimuthal anisotropy, the frequency spectra, the vertical profiles of energy density, and the momentum flux of the motion field. It is found that all of the observed azimuthal variations are probably caused by a gravity wave field whose parameters vary with time. The results show significant differences between the mean zonal and meridional frequency spectra and different profiles of mean energy density with height for different frequency bands and for zonal and meridional components.
Relationship of dislocation density of silicon to solar cell current loss at low temperature
NASA Technical Reports Server (NTRS)
Mandelkorn, J.; Baraona, C. R.; Lamneck, J. H., Jr.
1972-01-01
Large decreases in short circuit current of silicon solar cells have been reported to occur as temperature is decreased below -60 C. Experimental results are presented which relate high dislocation density of the silicon bulk material of cells to the large current loss effect. Solar cells were made by the same processes from a variety of silicon materials, namely low-dislocation-density, high-dislocation-density float-zone, and Czochralski silicon. All cells were etched in a manner which revealed the dislocation density of the cell bulk silicon. It was found that every cell made from any of the various low-dislocation starting materials obtained from three suppliers still had a low-dislocation bulk after cell processing, and that all such cells belonged to category good. Cells made from float-zone materials showed high dislocation densities in their bulk and either fell into category poor, or had intermediate losses of short-circuit current at low temperature.
The Effect of Current Density on CNx Crystal Grain Growth in Electrochemical Deposition
NASA Astrophysics Data System (ADS)
Yu, Wei-Feng; Cao, Rong-Gen; Tian, Yu; Wang, Jian-Zhong; Ning, Xi-Jing
2011-02-01
The effect of charge current density on the growth of CNx films by electrolysis of a methanol-urea solution is investigated experimentally. It is seen that the C-C3N4 phase grains in the films are about 200-300 nm for a density of 55 mA/cm2 and dendrite growth takes place with grains as large as 7 μm formed when density is about 70 mA/cm2.
Characterizing a December 2005 density current event in the Chicago River, Chicago, Illinois
Garcia, C.M.; Jackson, P.R.; Oberg, K.A.; Johnson, K.K.; Garcia, M.H.
2007-01-01
During the winter months, the Chicago River in Chicago, Illinois is subject to bi-directional flows, and density currents are thought to be responsible for these flow variations. This paper presents detailed field measurements using three acoustic Doppler current profiler instruments and simultaneous water-quality measurements made during December 2005. Observations indicate that the formation of density currents within the Chicago River and density differences are mostly due to salinity differences between the North Branch and the main stem of the Chicago River, whereas temperature difference does not appreciably affect the creation of density currents. Sources of higher water temperature, conductivity, and salinity values should be addressed in future studies. ?? 2007 ASCE.
Yang, Renhuan; Li, Xu; Liu, Jun; He, Bin
2011-01-01
It is of importance to image electrical activity and properties of biological tissues. Recently hybrid imaging modality combing ultrasound scanning and source imaging through the acousto-electric (AE) effect has generated considerable interest. Such modality has the potential to provide high spatial resolution current density imaging by utilizing the pressure induced AE resistivity change confined at the ultrasound focus. In this study, we investigate a novel 3-dimensional (3D) ultrasound current source density imaging (UCSDI) approach using unipolar ultrasound pulses. Utilizing specially designed unipolar ultrasound pulses and by combining AE signals associated to the local resistivity changes at the focusing point, we are able to reconstruct the 3D current density distribution with the boundary voltage measurements obtained while performing a 3D ultrasound scan. We have shown in computer simulation that using the present method, it is feasible to image with high spatial resolution an arbitrary 3D current density distribution in an inhomogeneous conductive media. PMID:21628774
NASA Astrophysics Data System (ADS)
Fattah-Alhosseini, Arash; Khan, Hamid Yazdani
2017-02-01
This work aims at studying the influence of high current densities on the anodization of carbon steel. Anodic protective coatings were prepared on carbon steel at current densities of 100, 125, and 150 A/dm2 followed by a final heat treatment. Coatings microstructures and morphologies were analyzed using X-ray diffraction (XRD) and scanning electron microscope (SEM). The corrosion resistance of the uncoated carbon steel substrate and the anodic coatings were evaluated in 3.5 wt pct NaCl solution through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The results showed that the anodic oxide coatings which were prepared at higher current densities had thicker coatings as a result of a higher anodic forming voltage. Therefore, the anodized coatings showed better anti-corrosion properties compared to those obtained at lower current densities and the base metal.
The increase of the spin-transfer torque threshold current density in coupled vortex domain walls.
Lepadatu, S; Mihai, A P; Claydon, J S; Maccherozzi, F; Dhesi, S S; Kinane, C J; Langridge, S; Marrows, C H
2012-01-18
We have studied the dependence on the domain wall structure of the spin-transfer torque current density threshold for the onset of wall motion in curved, Gd-doped Ni(80)Fe(20) nanowires with no artificial pinning potentials. For single vortex domain walls, for both 10% and 1% Gd-doping concentrations, the threshold current density is inversely proportional to the wire width and significantly lower compared to the threshold current density measured for transverse domain walls. On the other hand for high Gd concentrations and large wire widths, double vortex domain walls are formed which require an increase in the threshold current density compared to single vortex domain walls at the same wire width. We suggest that this is due to the coupling of the vortex cores, which are of opposite chirality, and hence will be acted on by opposing forces arising through the spin-transfer torque effect.
Influence of electropolishing current densities on sulfur generation at niobium surface
NASA Astrophysics Data System (ADS)
Tyagi, P. V.; Nishiwaki, M.; Noguchi, T.; Sawabe, M.; Saeki, T.; Hayano, H.; Kato, S.
2013-11-01
We report the effect of different current densities on sulfur generation at Nb surface in the electropolishing (EP) with aged electrolyte. In this regard, we conducted a series of electropolishing (EP) experiments in aged EP electrolyte with high (≈50 mA/cm2) and low (≈30 mA/cm2) current densities on Nb surfaces. The experiments were carried out both for laboratory coupons and a real Nb single cell cavity with six witness samples located at three typical positions (equator, iris and beam pipe). Sample's surfaces were investigated by XPS (X-ray photoelectron spectroscopy), SEM (scanning electron microscope) and EDX (energy dispersive X-ray spectroscopy). The surface analysis showed that the EP with a high current density produced a huge amount of sulfate/sulfite particles at Nb surface whereas the EP with a low current density was very helpful to mitigate sulfate/sulfite at Nb surface in both the experiments.
You, Chun-Yeol
2014-01-28
We investigate the switching current density reduction of perpendicular magnetic anisotropy spin transfer torque magnetic tunneling junctions using micromagnetic simulations. We find that the switching current density can be reduced with elongated lateral shapes of the magnetic tunnel junctions, and additional reduction can be achieved by using a noncollinear polarizer layer. The reduction is closely related to the details of spin configurations during switching processes with the additional in-plane anisotropy.
NASA Astrophysics Data System (ADS)
Sergis, N.; Krimigis, S. M.; Roelof, E. C.; Mitchell, D. G.; Rymer, A. M.; Arridge, C. S.; Krupp, N.; Thomsen, M. F.; Hamilton, D. C.; McAndrews, H. J.; Coates, A. J.; Wilson, R. J.; Dougherty, M. K.; Young, D. T.
2009-12-01
We report initial results on the distribution of the thermal plasma, energetic particle and magnetic field pressure in the equatorial magnetosphere, as measured by the Magnetospheric Imaging Instrument (MIMI), Cassini Plasma Spectrometer (CAPS) and the flux gate magnetometer (MAG) onboard the Cassini spacecraft, currently orbiting Saturn. Data were obtained during 11 passes from September 2005 to May 2006, when the spacecraft was particularly close (±1 Rs) to the nominal magnetic equator in the range 6 to 15 RS. The radial gradient of the total pressure is compared to the inertial body force in order to determine their relative contribution to the Saturnian ring current, and an average radial profile of the azimuthal current intensity is presented. The results can be summarized as follows: (1) The suprathermal (> 3 keV) pressure contribution to the total particle pressure becomes significant outside 8-9 Rs, exceeding 50% for r between 12 and 15 Rs. (2) The plasma beta (particle pressure/magnetic pressure) remains above 1 outside 8 RS, reaching ~3 to ~10 between 11 and 14 Rs. (3) The comparison between the inertial body force and the radial pressure gradient shows that both terms are close at 9-10 Rs, with the pressure gradient becoming dominant outside of 11 Rs. (4) The azimuthal ring current intensity develops a maximum region between approximately 8 and 12 Rs, reaching values of 100-150 pA/m^2. Outside this region, it drops with radial distance faster than the 1/r dependence that a disk current model would suggest.
NASA Astrophysics Data System (ADS)
Krasnoselskikh, V.; Dudok de Wit, T.; Pincon, J.; Lefeuvre, F.; Korepanov, V.; Deferaudy, H.; Chabassiere, M.; Fergeau, P.; Seran, H.; Schekotov, A.; Woolliscroft, L.; Balikhin, M.; Walker, S.; Prado, J.; Kryuchkov, E.
2006-12-01
We present the results of the first direct in situ measurements of current density variations to ion and electron whistler waves in the low latitude ionosphere onboard the Ukrainian Sych 1M satellite, in the frame of Variant experiment. These measurements were based on measurements from the Rogowski coil sensor of the Variant experiment onboard the Ukrainian Sych 1M satellite. This instrument, developed at LPCE, Orleans, France, consists of a toroidal coil with a high permeability core and is dedicated to measurements of the variations of the current component perpendicular to the torus. A more detailed description of the principle of measurement and instrument operation can be found in [1, 2]. The scientific objective of the experiment was to study current density variations in the different regions of the upper ionosphere. Due to a technical failure of the third stage of the launcher, only a limited amount of short duration measurements could occasionally be performed in different regions of the ionosphere. In addition to this, the electromagnetic cleanliness of the satellite was far from perfect, causing the measurements to be very noisy. Special care had to be taken to denoise the data of all instruments using multiscale techniques before any interpretation could be carried out. We were eventually able to find simultaneous occurrences of whistlers in the electric and magnetic field data and in current density data, in a frequency band ranging from several tens of Hz up to approximately 1 kHz. The upper limit was given by the cut off frequency of the preamplifier (400 Hz) and the lower limit by the high noise level in the ELF frequency band. The comparison of electromagnetic and current density data associated with the whistlers provides proof of the concepts and techniques used and validates the use of the Rogowski coil for current density variations measurements. 1. Krasnoselskikh, V. V., Natanzon, A. M., Reznikov, A. E., Schyokotov, A. Yu., Klimov, S. I
Baude, R.; Gaboriau, F.; Hagelaar, G. J. M.
2013-08-15
In the context of low temperature plasma research, we propose a wall current probe to determine the local charged particle fluxes flowing to the chamber walls. This non-intrusive planar probe consists of an array of electrode elements which can be individually biased and for which the current can be measured separately. We detail the probe properties and present the ability of the diagnostic to be used as a space and time resolved measurement of the ion and electron current density at the chamber walls. This diagnostic will be relevant to study the electron transport in magnetized low-pressure plasmas.
Baude, R; Gaboriau, F; Hagelaar, G J M
2013-08-01
In the context of low temperature plasma research, we propose a wall current probe to determine the local charged particle fluxes flowing to the chamber walls. This non-intrusive planar probe consists of an array of electrode elements which can be individually biased and for which the current can be measured separately. We detail the probe properties and present the ability of the diagnostic to be used as a space and time resolved measurement of the ion and electron current density at the chamber walls. This diagnostic will be relevant to study the electron transport in magnetized low-pressure plasmas.
Interaction of pyroclastic density currents with human settlements: Evidence from ancient Pompeii
NASA Astrophysics Data System (ADS)
Gurioli, Lucia; Pareschi, M. Teresa; Zanella, Elena; Lanza, Roberto; Deluca, Enrico; Bisson, Marina
2005-06-01
Integrating field observations and rock-magnetic measurements, we report how a turbulent pyroclastic density current interacted with and moved through an urban area. The data are from the most energetic, turbulent pyroclastic density current of the A.D. 79 eruption of Vesuvius, Italy, which partially destroyed the Roman city of Pompeii. Our results show that the urban fabric was able to divide the lower portion of the current into several streams that followed the city walls and the intracity roads. Vortices, revealed by upstream particle orientations and decreases in deposit temperature, formed downflow of obstacles or inside cavities. Although these perturbations affected only the lower part of the current and were localized, they could represent, in certain cases, cooler zones within which chances of human survival are increased. Our integrated field data for pyroclastic density current temperature and flow direction, collected for the first time across an urban environment, enable verification of coupled thermodynamic numerical models and their hazard simulation abilities.
NASA Astrophysics Data System (ADS)
Ma, Hui; Geng, Yingsan; Liu, Zhiyuan; Wang, Jianhua; Wang, Zhenxing; Zhang, Zaiqin
2016-09-01
The objective of this paper is to measure the effect of an axial magnetic field (AMF) BAMF and arc current on the anode current density in diffuse vacuum arcs. The experimental geometry included a split anode and a butt-type cathode, both with a diameter of 60 mm. The anode surface was divided into a central area and three symmetrically disposed peripheral annular areas. The central area of the split anode had a diameter of 20 mm. The contact material was CuCr25 (25% Cr). The arc current IARC ranged from 4 to 14 kA (rms) at 50 Hz. The opening velocity was 2.4 m/s. The currents of the four areas on the anode contact were measured using four Rogowski coils situated outside the vacuum chamber. An external uniform AMF BAMF ranging from 0 to 110 mT was applied during the experiment. The observed arc modes were recorded by a high-speed charge-coupled device video camera. The experimental results quantitatively reveal that the current density distribution on the anode surface in the diffuse arc mode was not uniform but concentrated in the central area. The current density in the central anode area at the current peak JPeakArea I decreased with increasing BAMF following a power law. For BAMF of 0-110 mT and IARC of 4-14 kA, JPeakArea I = (2.2 IARC + 0.069 IARC2) BAMF-0.22, where JPeakArea I is in A/mm2, BAMF is in mT, and IARC is in kA. Moreover, the current distribution was uneven in the three peripheral areas.
Prasad, Rachna; Anand, Sneh; Koul, Veena
2011-01-01
Introduction: The effect of DC iontophoresis using low (0.2 mA/cm2) and high current density (0.5 mA/cm2) on transdermal permeation of methotrexate loaded into polyacrylamide hydrogel patch was investigated. Results: Flux of 20.57 ± 1.02 μg/cm2/h and 36.8 ± 2.21 μg/cm2/h was achieved with low and high current density DC iontophoresis, respectively. Attenuated total reflectance-Fourier Transform infrared (ATR-FTIR) spectra and microscopic studies of the treated skin samples supported the permeation results. A greater decrease in the peak height of asymmetric, symmetric C-H stretching vibration and ester peak was noticed with 0.5 mA/cm2 current density as compared to 0.2 mA/cm2 current density samples. Furthermore, an increase in the ratio of amide I and amide II bands from 2.6 to 11 with increase in current density was noticed, thus indicating that hydration levels are associated with iontophoresis and play an important role in increasing the drug permeation. Scanning electron microscopy revealed increase in pore size of the hair follicles. Light microscopy studies of the skin samples treated with low current density DC iontophoresis demonstrated epidermal thinning and focal disruptions, spongiosis and appendageal dilatations. With higher current density, disruption of epidermis in almost half of the sectioned area, loss of appendages and fractured collagen in the dermis was noticed. Moreover, the reversibility studies conducted in vivo on mice revealed that the recovery process had started within 24 h and is complete in 48 h for lower current density treated animals. However, the histological changes associated with 0.5 mA/cm2 current density were not reversible in 48 h and edema, appendageal dilatations along with focal disruption of epidermis persisted. Conclusion: Hence our study suggests that high density current is not well-tolerated by the skin. PMID:23071949
Laser-Based Faraday-Effect Measurement of Magnetic Fluctuations and Fluctuation-Induced Transport
NASA Astrophysics Data System (ADS)
Lin, L.; Brower, D. L.; Ding, W. X.; Sarff, J. S.
2013-10-01
A multichord far-infrared laser-based Faraday-effect polarimetry diagnostic has been well developed on MST. Combined polarimetry-interferometry capability permits simultaneous measurement of internal structure of density and magnetic field with fast time response (~ 4 μs) and low phase noise (< 0 .01°) . With this diagnostic, the impact on toroidal current profile from a tangentially injected neutral beam is directly measured, allowing evaluation of non-inductive current drive. In addition, 0 .05° Faraday-effect fluctuations associated with global tearing modes are resolved with an uncertainty below 0 .01° . For physics investigations, these Faraday-effect fluctuations are complicated by contributions from both density and magnetic fluctuations. In our analysis, the local density fluctuations are obtained by inverting the line-integrated interferometry data after resolving the mode helicity through correlation techniques. The local magnetic fluctuations are then reconstructed using a parameterized fit of the polarimetry data, accounting for both the density and magnetic contributions. For the same mode, density and radial magnetic fluctuations exhibit very different spatial structure. In this process, their relative phase is also determined, thereby allowing the determination of magnetic-fluctuation-induced transport. Work supported by US DoE.
Faraday-Effect Polarimeter-Interferometer System for current density measurement on EAST
NASA Astrophysics Data System (ADS)
Liu, Haiqing; Jie, Yinxian; Ding, Weixing; Brower, David Lyn; Zou, Zhiyong; Qian, Jinping; Li, Weiming; Zeng, Long; Zhang, Shoubiao; Hu, Liqun; Wan, Baonian
2015-11-01
An eleven-channel far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique has been implemented for current density and electron density profile measurements in the EAST tokamak. Both polarimetric and interferometric measurement are obtained in a long pulse (~ 52s) discharge. The electron line-integrated density resolution of POINT is less than 5 × 1016 m-2 (~ 2°), and the Faraday rotation angle rms phase noise is <0.1°. With the high temporal (~ 1 μsec) and phase resolution (<0.1°), density perturbations associated with the sawteeth cycle and tearing mode activities have been observed. It is evident that tearing modes are well correlated to dynamics of equilibrium current profile (or q-profile). Faraday rotation angle shows clear variation with low hybrid current drive while line-integrated density remains little changed, implying the current drive in the core. A Digital Phase Detector with 250 kHz bandwidth provides real-time Faraday rotation angle and density phase shift output, which will be integrated into current profile control system in a long pulse discharge in future. This work is supported by the National Magnetic Confinement Fusion Program of China with contract No. 2012GB101002 and partly supported by the US D.O.E. contract DESC0010469.
Current density imaging using directly measured harmonic Bz data in MREIT.
Park, Chunjae; Kwon, Oh In
2013-01-01
Magnetic resonance electrical impedance tomography (MREIT) measures magnetic flux density signals through the use of a magnetic resonance imaging (MRI) in order to visualize the internal conductivity and/or current density. Understanding the reconstruction procedure for the internal current density, we directly measure the second derivative of Bz data from the measured k-space data, from which we can avoid a tedious phase unwrapping to obtain the phase signal of Bz . We determine optimal weighting factors to combine the derivatives of magnetic flux density data, [Symbol: see text](2) Bz , measured using the multi-echo train. The proposed method reconstructs the internal current density using the relationships between the induced internal current and the measured [Symbol: see text](2) Bz data. Results from a phantom experiment demonstrate that the proposed method reduces the scanning time and provides the internal current density, while suppressing the background field inhomogeneity. To implement the real experiment, we use a phantom with a saline solution including a balloon, which excludes other artifacts by any concentration gradient in the phantom.
A new model for marine density-turbidity currents with criteria for ignition
NASA Astrophysics Data System (ADS)
Salusti, E.
We discuss the hydrodynamic stability properties of a one-dimensional quasi-steady marine current, driven by a density excess caused by low temperature or high salinity, and flowing over a regular slope, taking bottom-erosion phenomena into consideration. The term density-turbidity current is used here for a thermohaline density current, with that density increased by entrained sediment. Thermohaline currents are of fundamental importance with regard to the Earth's climate, and the same must apply to density-turbidity currents. To simplify this complex problem, we schematize the flow as a thin turbulent quasisteady current, with gravitational and frictional forces in approximate equilibrium; the effects of small-scale perturbations, and of interaction with the bottom sediment, are then schematised by assuming a heuristic model of sediment evolution. Indeed, as in recent work by Caserta et al. (1990), we postulate that density variation due to bottom erosion or deposition is a function only of the shear stress exerted on the sea bottom by the current. Using these assumptions, we arrive at a complex nonlinear equation which considers both time and space variability for a realistic two-layer model of these density-turbidity currents. This finally gives a nonlinear heat equation that displays both diffusive behaviour and a peculiar type of time-delayed nonlinear behaviour, a previously-unrecognised effect governed by a criterion which defines explosive perturbations. It is of interest that this criterion is not related to energy considerations, like other criteria discussed in the literature, but is based on hydrodynamic instability considerations. The above model can also be applied to classical turbidity currents, i.e. those in which the interstitial fluid has the same density as the ambient fluid. However, the way in which the initial turbid water is generated is of paramount importance; mechanisms include submarine slumping, underflows from large flooded rivers
An adaptive finite element approach to modelling sediment laden density currents
NASA Astrophysics Data System (ADS)
Parkinson, S.; Hill, J.; Allison, P. A.; Piggott, M. D.
2012-04-01
Modelling sediment-laden density currents at real-world scales is a challenging task. Here we present Fluidity, which uses dynamic adaptive re-meshing to reduce computational costs whilst maintaining sufficient resolution where and when it is required. This allows small-scale processes to be captured in large scale simulations. Density currents, also known as gravity or buoyancy currents, occur wherever two fluids with different densities meet. They can occur at scales of up to hundred kilometres in the ocean when continental shelves collapse. This process releases large quantities of sediment into the ocean which increase the bulk density of the fluid to form a density current. These currents can carry sediment hundreds of kilometres, at speeds of up to a hundred kilometres per hour, over the sea bed. They can be tsunamigenic and they have the potential to cause significant damage to submarine infrastructure, such as submarine telecommunications cables or oil and gas infrastructure. They are also a key process for movement of organic material into the depths of the ocean. Due to this, they play an important role in the global carbon cycle on the Earth, forming a significant component of the stratigraphic record, and their deposits can form useful sources of important hydrocarbons. Modelling large scale sediment laden density currents is a very challenging problem. Particles within the current are suspended by turbulence that occurs at length scales that are several orders of magnitude smaller than the size of the current. Models that resolve the vertical structure of the flow require a very large, highly resolved mesh, and substantial computing power to solve. Here, we verify our adaptive model by comparison with a set of laboratory experiments by Gladstone et al. [1998] on the propagation and sediment deposition of bidisperse gravity currents. Comparisons are also made with fixed mesh solutions, and it is shown that accuracy can be maintained with fewer elements
Mino, H
1993-03-01
A method of estimating the parameters of nonstationary ionic channel current fluctuations (NST-ICF's) in the presence of an additive measurement noise is proposed. The case is considered in which the sample records of NST-ICT's corrupted by the measurement noise are available for estimation, where the experiment can be repeated many times to calculate the statistics of noisy NST-ICF's. The conventional second-order regression model expressed in terms of the mean and variance of noisy NST-ICF's is derived theoretically, assuming that NST-ICF's are binomially distributed. Since the coefficients of the regression model are explicitly related to not only the parameters of NST-ICF's but also the measurement noise component, the parameters of NST-ICF's that are of interest can be estimated without interference from the additive measurement noise by identifying the regression coefficients. Furthermore, the accuracy of the parameter estimates is theoretically evaluated using the error-covariance matrix of the regression coefficients. The validity and effectiveness of the proposed method are demonstrated in a Monte Carlo simulation in which a fundamental kinetic scheme of Na+ channels is treated as a specific example.
Rauhalahti, Markus; Taubert, Stefan; Sundholm, Dage; Liégeois, Vincent
2017-03-08
Magnetically induced current density susceptibilities and ring-current strengths have been calculated for neutral and doubly charged persubstituted benzenes C6X6 and C6X6(2+) with X = F, Cl, Br, I, At, SeH, SeMe, TeH, TeMe, and SbH2. The current densities have been calculated using the gauge-including magnetically induced current (GIMIC) method, which has been interfaced to the Gaussian electronic structure code rendering current density calculations using effective core potentials (ECP) feasible. Relativistic effects on the ring-current strengths have been assessed by employing ECP calculations of the current densities. Comparison of the ring-current strengths obtained in calculations on C6At6 and C6At6(2+) using relativistic and non-relativistic ECPs show that scalar relativistic effects have only a small influence on the ring-current strengths. Comparisons of the ring-current strengths and ring-current profiles show that the C6I6(2+), C6At6(2+), C6(SeH)6(2+), C6(SeMe)6(2+), C6(TeH)6(2+), C6(TeMe)6(2+), and C6(SbH2)6(2+) dications are doubly aromatic sustaining spatially separated ring currents in the carbon ring and in the exterior of the molecule. The C6I6(+) radical cation is also found to be doubly aromatic with a weaker ring current than obtained for the dication.
Incorporation of Ca and P on anodized titanium surface: Effect of high current density.
Laurindo, Carlos A H; Torres, Ricardo D; Mali, Sachin A; Gilbert, Jeremy L; Soares, Paulo
2014-04-01
This study systematically evaluated the surface and corrosion characteristics of commercially pure titanium (grade 2) modified by plasma electrolytic oxidation (PEO) with high current density. The anodization process was carried out galvanostatically (constant current density) using a solution containing calcium glycerophosphate (0.02mol/L) and calcium acetate (0.15mol/L). The current densities applied were 400, 700, 1000 and 1200mA/cm(2) for a period of 15s. Composition, crystalline structure, morphology, roughness, wettability and "in-vitro" bioactivity test in SBF of the anodized layer were evaluated by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, profilometry and contact angle measurements. Corrosion properties were evaluated by open circuit potential, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The results show that the TiO2 oxide layers present an increase of thickness, porosity, roughness, wettability, Ca/P ratio, and bioactivity, with the applied current density up to 1000mA/cm(2). Corrosion resistance also increases with applied current density. It is observed that for 1200mA/cm(2), there is a degradation of the oxide layer. In general, the results suggest that the anodized TiO2 layer with better properties is formed with an applied current of 1000mA/cm(2).
Fliegl, Heike; Jusélius, Jonas; Sundholm, Dage
2016-07-21
Gauge-origin independent current density susceptibility tensors have been computed using the gauge-including magnetically induced current (GIMIC) method. The anisotropy of the magnetically induced current density (ACID) functions constructed from the current density susceptibility tensors are therefore gauge-origin independent. The ability of the gauge-origin independent ACID function to provide quantitative information about the current flow along chemical bonds has been assessed by integrating the cross-section area of the ACID function in the middle of chemical bonds. Analogously, the current strength susceptibility passing a given plane through the molecule is obtained by numerical integration of the current flow parallel to the normal vector of the integration plane. The cross-section area of the ACID function is found to be strongly dependent on the exact location of the integration plane, which is in sheer contrast to the calculated ring-current strength susceptibilities that are practically independent of the chosen position of the integration plane. The gauge-origin independent ACID functions plotted for different isosurface values show that a visual assessment of the current flow and degree of aromaticity depends on the chosen isosurface. The present study shows that ACID functions are not an unambiguous means to estimate the degree of molecular aromaticity according to the magnetic criterion and to determine the current pathway of complex molecular rings.
Turrigiano, G; LeMasson, G; Marder, E
1995-05-01
We study the electrical activity patterns and the expression of conductances in adult stomatogastric ganglion (STG) neurons as a function of time in primary cell culture. When first plated in culture, these neurons had few active properties. After 1 d in culture they produced small action potentials that rapidly inactivated during maintained depolarization. After 2 d in culture they fired large action potentials tonically when depolarized, and their properties resembled very closely the properties of STG neurons pharmacologically isolated in the ganglion. After 3-4 d in culture, however, their electrical properties changed and they fired in bursts when depolarized. We characterized the currents expressed by these neurons in culture. They included two TTX-sensitive sodium currents, a calcium current, a delayed-rectifier-like current, a calcium-dependent potassium current, and two A-type currents. The changes in firing properties with time in culture were accompanied by an increase in inward and decrease in outward current densities. A single-compartment conductance-based model of an STG neuron was constructed by fitting the currents measured in the biological neurons. When the current densities in the model neuron were matched to those measured for the biological neurons in each activity state, the model neuron closely reproduced each state, indicating that the changes in current densities are sufficient to account for the changes in intrinsic properties. These data indicate that STG neurons isolated in culture change their intrinsic electrical properties by selectively adjusting the magnitudes of their ionic conductances.
Simulation study of the critical current density of YBa2Cu3O7 ceramics
NASA Astrophysics Data System (ADS)
Cai, Zhi-Xiong; Welch, David O.
1992-02-01
A two-dimensional Josephson-junction-array model is used to study the effect of grain boundaries on the critical current density of YBa2Cu3O7 superconducting ceramics. The model represents a network of superconducting grains with a distribution of tilt angles θ. Each grain boundary has a critical current density Jc(θ) and normal-state resistance R(θ). The current-voltage characteristics are calculated numerically for different tilt-angle distributions. The scaling law and statistics of extremes, introduced by Duxbury, Beale, and Leath for general breakdown behavior, based on the most critical defect (normal region) in the network, are tested and found to be accurate for the predicted critical-current distribution of random samples. When the applied current is larger than but close to its critical value, there is a periodic V(t) with discrete power spectra. When the applied current gets larger, chaotic behavior appears with nearly continuous power spectra.
NASA Astrophysics Data System (ADS)
Kumar, Avinash; Eckel, Stephen; Jendrzejewski, Fred; Campbell, Gretchen
We study the decay of a persistent, quantized current state in a toroidal geometry. Our experiment involves trapping neutral 23Na atoms in an all optical ``target trap'' shaped potential. This potential consists of a disc surrounded by an annular potential. A current in a superfluid can be sustained only below a critical current. This critical current can be tuned by introducing a density perturbation which depletes the local density. The decay time of a persistent current state can also be controlled by enhancing fluctuations of the system thermally. We study the decay at four different temperatures between 30 nK and 190 nK. For each temperature we record the decay at four different perturbation strengths. We find that increasing the magnitude of the density depletion or the temperature leads to a faster decay, and have seen the decay constant change by over two orders of magnitude. We also studied the size of hysteresis loop between different current states as a function of temperature, allowing us to extract a critical velocity. We find that the discrepancies between the experimentally extracted critical velocity and theoretically calculated critical velocity (using local-density approximation) decreases as the temperature is decreased. Now at University of Heidelberg.
NASA Astrophysics Data System (ADS)
Wang, Xiang
The most promising characteristic of a High Temperature Superconductor (HTS) is its ability to carry larger electrical current at liquid nitrogen boiling temperature and strong applied magnetic field with minimal dissipation. Numerous large scale applications such as HTS transmission cables, HTS magnets and HTS motors have been developed using HTS materials. The major limitation that prevents its wide commercialization is its high cost-to-performance ratio. However, the effort to further improve HTS current carrying capability is jeopardized by a mysterious thickness dependence of the critical current density (Jc) --- Jc monotonically decreases with increasing thickness (t) at 77 K and self-field (SF). This poses a great challenge for both HTS applications and the understanding of vortex dynamics. What further complicates this issue is the complex defect structure in HTS films as well as the creep nature of magnetic vortices at a finite temperature. After a systematic study of the temperature and magnetic field effects on Jc--t, we conclude that Jc--t is most likely the result of a collective pinning effect dictated by a random pinning potential. Besides that, thermal fluctuations also alter Jc--t in a predictable way. Therefore, by either modifying the vortex structure or pinning structure, J c--t can be eliminated. Indeed, a thin film J c has been restored in a HTS/insulator/HTS trilayer while the magnetic coupling is weakened. Moreover, Jc--t has been removed when the random distributed point pins are overpowered by strong linear defects.
Induced fermionic charge and current densities in two-dimensional rings
NASA Astrophysics Data System (ADS)
Bellucci, S.; Saharian, A. A.; Grigoryan, A. Kh.
2016-11-01
For a massive quantum fermionic field, we investigate the vacuum expectation values (VEVs) of the charge and current densities induced by an external magnetic flux in a two-dimensional circular ring. Both the irreducible representations of the Clifford algebra are considered. On the ring edges the bag (infinite mass) boundary conditions are imposed for the field operator. This leads to the Casimir type effect on the vacuum characteristics. The radial current vanishes. The charge and the azimuthal current are decomposed into the boundary-free and boundary-induced contributions. Both these contributions are odd periodic functions of the magnetic flux with the period equal to the flux quantum. An important feature that distinguishes the VEVs of the charge and current densities from the VEV of the energy density is their finiteness on the ring edges. The current density is equal to the charge density for the outer edge and has the opposite sign on the inner edge. The VEVs are peaked near the inner edge and, as functions of the field mass, exhibit quite different features for two inequivalent representations of the Clifford algebra. We show that, unlike the VEVs in the boundary-free geometry, the vacuum charge and the current in the ring are continuous functions of the magnetic flux and vanish for half-odd integer values of the flux in units of the flux quantum. Combining the results for two irreducible representations, we also investigate the induced charge and current in parity and time-reversal symmetric models. The corresponding results are applied to graphene rings with the electronic subsystem described in terms of the effective Dirac theory with the energy gap. If the energy gaps for two valleys of the graphene hexagonal lattice are the same, the charge densities corresponding to the separate valleys cancel each other, whereas the azimuthal current is doubled.
NASA Astrophysics Data System (ADS)
Jug, Giancarlo; Ziegler, Klaus
1997-10-01
We present a calculation for the second moment of the local density of states in a model of a two-dimensional quantum dot array near the quantum Hall transition. The quantum dot array model is a realistic adaptation of the lattice model for the quantum Hall transition in the two-dimensional electron gas in an external magnetic field proposed by Ludwig, Fisher, Shankar, and Grinstein. We make use of a Dirac fermion representation for the Green's functions in the presence of fluctuations for the quantum dot energy levels. A saddle-point approximation yields nonperturbative results for the first and second moments of the local density of states, showing interesting fluctuation behavior near the quantum Hall transition. To our knowledge we discuss here one of the first analytic characterizations of chaotic behavior for a two-dimensional mesoscopic structure. The connection with possible experimental investigations of the local density of states in the quantum dot array structures (by means of NMR Knight-shift or single-electron-tunneling techniques) and our work is also established.
Camparo, James; Fathi, Gilda
2009-05-15
Atomic clocks that fly on global-navigation satellites such as global positioning system (GPS) and Galileo employ light from low-temperature, inductively coupled plasmas (ICPs) for atomic signal generation and detection (i.e., alkali/noble-gas rf-discharge lamps). In this application, the performance of the atomic clock and the capabilities of the navigation system depend sensitively on the stability of the ICP's optical emission. In order to better understand the mechanisms that might lead to instability in these rf-discharge lamps, and hence the satellite atomic clocks, we studied the optical emission from a Rb/Xe ICP as a function of the rf power driving the plasma. Surprisingly, we found that the electron density in the plasma was essentially independent of increases in rf power above its nominal value (i.e., 'rf-power gain') and that the electron temperature was only a slowly varying function of rf-power gain. The primary effect of rf power was to increase the temperature of the neutrals in the plasma, which was manifested by an increase in Rb vapor density. Interestingly, we also found evidence for electron temperature fluctuations (i.e., fluctuations in the plasma's high-energy electron content). The variance of these fluctuations scaled inversely with the plasma's mean electron temperature and was consistent with a simple model that assumed that the total electron density in the discharge was independent of rf power. Taken as a whole, our results indicate that the electrons in alkali/noble-gas ICPs are little affected by slight changes in rf power and that the primary effect of such changes is to heat the plasma's neutral species.
Klisz, Marcin; Koprowski, Marcin; Ukalska, Joanna; Nabais, Cristina
2016-01-01
Intra-annual density fluctuations (IADFs) can imprint environmental conditions within the growing season and most of the research on IADFs has been focused on their climatic signal. However, to our knowledge, the genetic influence on the frequency and type of IADFs has not been evaluated. To understand if the genotype can affect the formation of IADFs we have used a common garden experiment using eight families of Larix decidua established in two neighboring forest stands in northern Poland. Four types of IADFs were identified using X-ray density profiles: latewood-like cells within earlywood (IADF-type E), latewood-like cells in the transition from early- to latewood (IADF type E+), earlywood-like cells within latewood (IADF-type L), and earlywood-like cells in the border zone between the previous and present annual ring (IADF-type L+). The influence of explanatory variables i.e., families, sites, and years on identified density fluctuations was analyzed using generalized estimating equations (GEE). We hypothesized that trees from different families will differ in terms of frequency and type of IADFs because each family will react to precipitation and temperature in a different way, depending on the origin of those trees. The most frequent fluctuation was E+ and L types on both sites. The most important factors in the formation of IADFs were the site and year, the last one reflecting the variable climatic conditions, with no significant effect of the family. However, the relation between the formation of IADFs and selected climate parameters was different between families. Although, our results did not give a significant effect of the genotype on the formation of IADFs, the different sensitivity to climatic parameters among different families indicate that there is a genetic influence. PMID:27242883
Coupling of turbulent and non-turbulent flow regimes within pyroclastic density currents
NASA Astrophysics Data System (ADS)
Breard, Eric C. P.; Lube, Gert; Jones, Jim R.; Dufek, Josef; Cronin, Shane J.; Valentine, Greg A.; Moebis, Anja
2016-10-01
Volcanic eruptions are at their most deadly when pyroclastic density currents sweep across landscapes to devastate everything in their path. The internal dynamics underpinning these hazards cannot be directly observed. Here we present a quantitative view inside pyroclastic density currents by synthesizing their natural flow behaviour in large-scale experiments. The experiments trace flow dynamics from initiation to deposition, and can explain the sequence and evolution of real-world deposits. We show that, inside pyroclastic density currents, the long-hypothesized non-turbulent underflow and fully turbulent ash-cloud regions are linked through a hitherto unrecognized middle zone of intermediate turbulence and concentration. Bounded by abrupt jumps in turbulence, the middle zone couples underflow and ash-cloud regions kinematically. Inside this zone, strong feedback between gas and particle phases leads to the formation of mesoscale turbulence clusters. These extremely fast-settling dendritic structures dictate the internal stratification and evolution of pyroclastic density currents and allow the underflows to grow significantly during runout. Our experiments reveal how the underflow and ash-cloud regions are dynamically related--insights that are relevant to the forecasting of pyroclastic density current behaviour in volcanic hazard models.
Stabilizing Effects of Edge Current Density on Peeling-Ballooning Instability
NASA Astrophysics Data System (ADS)
Zhu, P.; Hegna, C. C.; Sovinec, C. R.
2011-10-01
Resistive MHD computations using the NIMROD code find a strong dependence of low- n edge instabilities on the edge parallel current density distribution. Here n is the toroidal mode number. The low- n edge-localized-modes can be driven unstable by increasing the edge current density across the peeling-ballooning stability boundary. When edge peak current density is sufficiently large, the corresponding safety factor q profile obtains an edge region with zero or reversed magnetic shear, and the low- n edge instabilities are partially or fully stabilized. These results are consistent with previous analytic theory on peeling modes which indicates that zero or reversed magnetic shear can be stabilizing. Nonlinear simulations indicate that the stabilizing effects of edge current density on the low- n peeling-dominant modes through zero and reversed shear can persist throughout the nonlinear exponential growth phase. Near the end of this nonlinear phase, the radial extent of the filament exceeds the pedestal width, and disconnected blob-like substructures start to develop within the filaments. Relative pedestal energy loss from these radially extending filaments can reach 20 - 30 % . Both filament size and pedestal energy loss from the nonlinear low- n peeling-dominant instabilities can be reduced and regulated by the equilibrium edge current density distribution. Supported by Grants DE-FG02-86ER53218 and DE-FC02-08ER54975.
Current densities and total contact currents for 110 and 220 kV power line tasks.
Korpinen, Leena; Kuisti, Harri; Elovaara, Jarmo
2014-10-01
The aim of this study was to analyze all values of electric current from measured periods while performing tasks on 110 and 220 kV power lines. Additionally, the objective was to study the average current densities and average total contact currents caused by electric fields in 110 and 220 kV power line tasks. One worker simulated the following tasks: (A) tested insulation voltage at a 110 kV portal tower, (B) checked the wooden towers for rot at a 110 kV portal tower, (C) tested insulation voltage at a 220 kV portal tower, and (D) checked the wooden towers for rot at a 220 kV portal tower. The highest average current density in the neck was 2.0 mA/m(2) (calculated internal electric field was 19.0-38.0 mV/m), and the highest average contact current was 234 µA. All measured values at 110 and 220 kV towers were lower than the basic restrictions (0.1 and 0.8 V/m) of the International Commission on Non-ionizing Radiation Protection.
Draijer, Matthijs J; Hondebrink, Erwin; Larsson, Marcus; van Leeuwen, Ton G; Steenbergen, Wiendelt
2010-10-11
Scattering fluid flux can be quantified with coherent light, either from the contrast of speckle patterns, or from the moments of the power spectrum of intensity fluctuations. We present a theory connecting these approaches for the general case of mixed static-dynamic patterns of boiling speckles without prior assumptions regarding the particle dynamics. An expression is derived and tested relating the speckle contrast to the intensity power spectrum. Our theory demonstrates that in speckle contrast the concentration of moving particles dominates over the contribution of speed to the particle flux. Our theory provides a basis for comparison of both approaches when used for studying tissue perfusion.
Towards the definition of AMS facies in the deposits of pyroclastic density currents
Ort, M.H.; Newkirk, T.T.; Vilas, J.F.; Vazquez, J.A.; Ort, M.H.; Porreca, Massimiliano; Geissman, J.W.
2014-01-01
Anisotropy of magnetic susceptibility (AMS) provides a statistically robust technique to characterize the fabrics of deposits of pyroclastic density currents (PDCs). AMS fabrics in two types of pyroclastic deposits (small-volume phreatomagmatic currents in the Hopi Buttes volcanic field, Arizona, USA, and large-volume caldera-forming currents, Caviahue Caldera, Neuquén, Argentina) show similar patterns. Near the vent and in areas of high topographical roughness, AMS depositional fabrics are poorly grouped, with weak lineations and foliations. In a densely welded proximal ignimbrite, this fabric is overprinted by a foliation formed as the rock compacted and deformed. Medial deposits have moderate–strong AMS lineations and foliations. The most distal deposits have strong foliations but weak lineations. Based on these facies and existing models for pyroclastic density currents, deposition in the medial areas occurs from the strongly sheared, high-particle-concentration base of a density-stratified current. In proximal areas and where topography mixes this denser base upwards into the current, deposition occurs rapidly from a current with little uniformity to the shear, in which particles fall and collide in a chaotic fashion. Distal deposits are emplaced by a slowing or stalled current so that the dominant particle motion is vertical, leading to weak lineation and strong foliation.
Hirotani, Hiroshi; Yu, Ma; Yamada, Takeshi
2013-01-01
Fluctuation of bacteriophage and Escherichia coli densities in naturally developed riverbed biofilms were investigated for a 1-year period. E. coli ranged from 1,500 to 15,500 most probable number (MPN)/100 mL and from 580 to 18,500 MPN/cm(2) in the main channel in the river water and biofilms, respectively. However, the fluctuations were much greater in the tributary, ranging from 0.8 to 100 MPN/100 mL and from 0.3 to 185 MPN/cm(2) in water and biofilms, respectively. The fluctuations of coliphages were also greater in the tributary than in the main channel. FRNA phage serotyping results indicated no significant differences in the source type of the fecal contamination in the main channel and tributary sampling stations. Significant correlations between phage groups in biofilms and water were found at both main channel and tributary. It was assumed that natural biofilms developed in the streambed captured and retained somatic phages in the biofilms for a certain period of time in the main channel site. At the location receiving constant and heavy contamination, the usage of phage indicators may provide additional information on the presence of viruses. In the small tributary it may be possible to estimate the virus concentration by monitoring the E. coli indicator.
Characterization of thunderstorm induced Maxwell current densities in the middle atmosphere
NASA Technical Reports Server (NTRS)
Baginski, Michael Edward
1989-01-01
Middle atmospheric transient Maxwell current densities generated by lightning induced charge perturbations are investigated via a simulation of Maxwell's equations. A time domain finite element analysis is employed for the simulations. The atmosphere is modeled as a region contained within a right circular cylinder with a height of 110 km and radius of 80 km. A composite conductivity profile based on measured data is used when charge perturbations are centered about the vertical axis at altitudes of 6 and 10 km. The simulations indicate that the temporal structure of the Maxwell current density is relatively insensitive to altitude variation within the region considered. It is also shown that the electric field and Maxwell current density are not generally aligned.
NASA Astrophysics Data System (ADS)
Pfaff, R. F., Jr.; Rowland, D. E.; Klenzing, J.; Freudenreich, H. T.; Martin, S. C.; Abe, T.; Habu, H.; Yamamoto, M. Y.; Watanabe, S.; Yamamoto, M.; Yokoyama, T.; Kakinami, Y.; Yamazaki, Y.; Larsen, M. F.; Hurd, L.; Clemmons, J. H.; Bishop, R. L.; Walterscheid, R. L.; Fish, C. S.; Bullett, T. W.; Mabie, J. J.; Murphy, N.; Angelopoulos, V.; Leinweber, H. K.; Bernal, I.; Chi, P. J.
2015-12-01
To investigate the ion-neutral coupling that creates the global electrical daytime "dynamo" currents in the mid-latitude, lower ionosphere, NASA carried out two multiple sounding rocket experiments from Wallops Island, VA on July 10, 2011 (14:00 UT, 10:00 LT) and July 4, 2013 (14:31 UT, 10:31 LT). The rockets were launched in the presence of well-defined, westward Hall currents observed on the ground with ΔH values of -25 nT and -30 nT, respectively, as well as a well-defined, daytime ionospheric density observed by the VIPIR ionosonde at Wallops. During the 2011 experiment, a narrow, intense sporadic-E layer was observed near 102 km. Each experiment consisted of a pair of rockets launched 15 sec apart. The first rocket of each pair carried instruments to measure DC electric and magnetic fields, as well as the ambient plasma and neutral gases and attained apogees of 158 km and 135 km in the 2011 and 2013 experiments, respectively. The second rocket of each pair carried canisters which released a lithium vapor trail along the upleg to illuminate neutral winds in the upper atmosphere. This daytime vapor trail technology was developed jointly by researchers at JAXA and Clemson University. In the second experiment, the lithium release was clearly visible in cameras with infrared filters operated by US and Japanese researchers in a NASA airplane at 9.6 km altitude. The observed wind profiles reached speeds of 100 m/s with strong shears with respect to altitude and were consistent with an independent derivation of the wind from the ionization gauge sensor suite on the instrumented rocket. The "vapor trail" rockets, which also included a falling sphere, attained apogees of 150 km and 143 km in the 2011 and 2013 experiments, respectively. By measuring the current density, conductivity, DC electric fields, and neutral winds, we solve the dynamo equation as a function of altitude, revealing the different contributions to the lower E-region currents. We find that the DC
Climatology of convective density currents in the southern foothills of the Atlas Mountains
NASA Astrophysics Data System (ADS)
Emmel, C.; Knippertz, P.; Schulz, O.
2010-06-01
Density currents fed by evaporationally cooled air are an important dust storm-generating feature and can constitute a source of moisture in arid regions. Recently, the existence of such systems has been demonstrated for the area between the High Atlas Mountains and the Sahara desert in southern Morocco on the basis of case studies. Here, a climatological analysis is presented that uses data from the dense climate station network of the IMPETUS project (An Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa) for the 5 year period 2002-2006. Objective criteria mainly based upon abrupt changes in wind and dew point temperature are defined to identify possible density current situations. The preselected events are then subjectively evaluated with the help of satellite imagery and surface observations to exclude causes for air mass changes other than moist convective cold pool formation. On average, 11 ± 4 density currents per year are detected with the main season lasting from April to September. Density currents occur mainly in the afternoon and evening due to the diurnal cycle of moist convection. Mean changes at the leading edge are increases in 2 m dew point temperature and wind speed by 5.4°C and 8.2 m s-1, respectively, and a decrease in 2 m air temperature of 2.3°C. The High Atlas and Jebel Saghro are found to be the most important source regions, while only a few systems originate over the Saharan lowlands. Labilization of the atmosphere due to upper-level troughs over northwest Africa and an enhanced moisture content favor density current formation. In addition, detailed case studies representative of different density current types are presented.
Corrosion current density prediction in reinforced concrete by imperialist competitive algorithm.
Sadowski, Lukasz; Nikoo, Mehdi
2014-01-01
This study attempted to predict corrosion current density in concrete using artificial neural networks (ANN) combined with imperialist competitive algorithm (ICA) used to optimize weights of ANN. For that reason, temperature, AC resistivity over the steel bar, AC resistivity remote from the steel bar, and the DC resistivity over the steel bar are considered as input parameters and corrosion current density as output parameter. The ICA-ANN model has been compared with the genetic algorithm to evaluate its accuracy in three phases of training, testing, and prediction. The results showed that the ICA-ANN model enjoys more ability, flexibility, and accuracy.
Postprocessing of 3-D current density reconstruction results with equivalent ellipsoids.
Ziolkowski, Marek; Haueisen, Jens; Leder, Uwe
2002-11-01
A method of postprocessing and visualizing three-dimensional vector fields, such as current density reconstruction results, is presented. This method is based on equivalent ellipsoids fitted to the vector fields. The technique has been tested with simulated data and current density reconstructions based on bioelectromagnetic data obtained from a physical thorax phantom. Three different approaches based on: 1) longest distance; 2) dominant direction; and 3) principal component analysis, for fitting the equivalent ellipsoids are proposed. Multiple foci in vector fields are extracted by multiple ellipsoids which are fitted iteratively. The method enables statistical postprocessing for the sake of comparisons of different source reconstructions algorithms or comparisons of groups of patients or volunteers.
NASA Astrophysics Data System (ADS)
Kriha, Vitezslav
2003-10-01
Non-thermal plasma of atmospheric pressure electrical discharges in flowing air can be used to generation of ozone. We have been observed two modes of discharge burning in a hollow needle to plane electrodes configuration studied in the ozone generation experiments: A low current diffuse mode is characterized by increasing of the ozone production with the discharge current; a high current filamentary mode is disadvantageous for the ozone generation(the ozone production decreases when the discharge current increases). A possible interpretation of this effect is following: The filamentary mode discharge current density is redistributed and high current densities in filaments cores lead to degradation of the ozone generation. Local fields in the discharge can be modified by charged metallic and/or dielectric components (passive modulators) in the discharge space. An interactive numerical model has been developed for this purpose. This model is based on Ferguson's polynomial objects for both the discharge chamber scene modelling and the discharge fields analyzing. This approach allows intuitive modifications of modulators shapes and positions in 3D scene followed by quantitative comparison of the current density distribution with previous configurations.
NASA Astrophysics Data System (ADS)
Lin, L.; Ding, W. X.; Brower, D. L.
2016-12-01
Magnetic fluctuation-induced particle transport is explored in the high-temperature, high-beta interior of the Madison symmetric torus (MST) reversed-field pinch by performing a multifield measurement of the correlated product of magnetic and density fluctuations associated with global resistive tearing modes. Local density fluctuations are obtained by inverting the line-integrated interferometry data after resolving the mode helicity through correlation techniques. The local magnetic and current density fluctuations are then reconstructed using a parameterized fit of Faraday-effect polarimetry measurements. Reconstructed 2D images of density and current density perturbations in a poloidal cross section exhibit significantly different spatial structure. Combined with their relative phase, the magnetic-fluctuation-induced particle transport flux and its spatial distribution are resolved. The convective magnetic fluctuation-induced particle flux profile is measured for both standard and high-performance plasmas in MST with tokamak-like confinement, showing large reduction in the flux during improved confinement.
NASA Astrophysics Data System (ADS)
Garzarella, Anthony; Wu, Dong Ho
2015-03-01
In this presentation, we will describe experiments involving all-dielectric electromagnetic field sensors based on electro-optic (EO) and magneto-optic (MO) crystals. EO sensors measure electric fields through the Pockels Effect, while MO sensors measure magnetic fields through the Faraday Effect. These sensors have been attached to optical fibers and calibrated in a variety of radio frequency, pulsed power and microwaves sources ranging in frequencies from dc to 20 GHz and ranging in power from 10-4 to 106 Watts. In this talk, we will focus on recent experimental measurements of electric and magnetic fields generated by current carrying conductors. Our EO and MO sensor data, when combined, allows us to determine current densities and current distributions within a conductor cross section. Additionally, skin effects, which are a major source of Joule heating and resistive wear, can be characterized in detail.
Yoshida, Koh; Baluja, Shipra; Inaba, Akira; Koga, Yoshikata
2011-06-07
Using a differential pressure perturbation calorimetry developed by us recently [K. Yoshida, S. Baluja, A. Inaba, K. Tozaki, and Y. Koga, "Experimental determination of third derivative of G (III): Differential pressure perturbation calorimetry (II)," J. Solution Chem. (in press)], we experimentally determined the partial molar S-V cross fluctuation density of solute B, (SV)δ(B) , in binary aqueous solutions for B = 1-propanol (1P) and glycerol (Gly). This third derivative of G provides information about the effect of solute B on the S-V cross fluctuation density, (SV)δ, in aqueous solution as the concentration of B varies. Having determined (SV)δ(B) by better than 1% uncertainty, we evaluated for the first time the fourth derivative quantity (SV)δ(B-B) = N(∂(SV)δ(B) ∕∂n(B)) for B = 1P and Gly graphically without resorting to any fitting functions within several percent. This model-free quantity gives information about the acceleration of the effect of solute B on (SV)δ. By comparing fourth derivative quantities, (SV)δ(B-B) , among B = 1P, Gly, and 2-butoxyethanol obtained previously, the distinction of the effect of solute on H(2)O becomes clearer than before when only the third derivative quantities were available.
NASA Astrophysics Data System (ADS)
Yoshida, Koh; Baluja, Shipra; Inaba, Akira; Koga, Yoshikata
2011-06-01
Using a differential pressure perturbation calorimetry developed by us recently [K. Yoshida, S. Baluja, A. Inaba, K. Tozaki, and Y. Koga, "Experimental determination of third derivative of G (III): Differential pressure perturbation calorimetry (II)," J. Solution Chem. (in press)], we experimentally determined the partial molar S-V cross fluctuation density of solute B, SVδB, in binary aqueous solutions for B = 1-propanol (1P) and glycerol (Gly). This third derivative of G provides information about the effect of solute B on the S-V cross fluctuation density, SVδ, in aqueous solution as the concentration of B varies. Having determined SVδB by better than 1% uncertainty, we evaluated for the first time the fourth derivative quantity SVδB-B = N(∂SVδB /∂nB) for B = 1P and Gly graphically without resorting to any fitting functions within several percent. This model-free quantity gives information about the acceleration of the effect of solute B on SVδ. By comparing fourth derivative quantities, SVδB-B, among B = 1P, Gly, and 2-butoxyethanol obtained previously, the distinction of the effect of solute on H2O becomes clearer than before when only the third derivative quantities were available.
Small Barriers Trigger Liftoff of Unconfined Dilute Heated Laboratory Density Currents
NASA Astrophysics Data System (ADS)
Fauria, K.; Andrews, B. J.; Manga, M.
2015-12-01
Dilute pyroclastic density currents (PDCs) are hot, turbulent, particle-laden flows that propagate because they are denser than air. PDCs can traverse tens to hundreds of kilometers and surmount ridges 100s of m tall, yet the effects of complex topography on PDC liftoff and runout distance are uncertain. Here we used scaled laboratory experiments to explore how barriers affect dilute density current dynamics and the occurrence of liftoff. We created dilute density currents by heating and suspending 20 μm diameter talc in air in an 8.5 x 6.1 x 2.6 m tank. We scaled the currents with respect to Froude, densimetric and thermal Richardson, particle Stokes and Settling numbers such that they were dynamically similar to natural PDCs. While currents were fully turbulent, their Reynolds numbers were not as high as those for natural PDCs. We performed the first set of experiments in a laterally unconfined volume, used laser sheets to illuminate the currents, measured bulk sedimentation rates down the current centerlines, and positioned four to twenty-four cm tall ridge-like barriers in the path of the currents. We found that relatively small barriers (~ half the current height) caused PDC liftoff. By comparison, conservation of kinetic and potential energy predicts that incompressible density currents are able to surmount barriers twice their height. Furthermore, we observed increased sedimentation immediately upstream of barriers and conclude that small barriers initiated buoyancy reversal through a combination of increased air entrainment and sedimentation. We conducted a second set of experiments with the same thermal scaling and mass flux rates but where the currents were laterally confined within a 0.6 m wide channel. We found that small barriers also triggered liftoff of confined currents, but that the body of these currents reattached after liftoff. Those results suggest that lateral confinement inhibits buoyancy reversal by limiting the surface area of the current
Chiamvimonvat, N; Kargacin, M E; Clark, R B; Duff, H J
1995-01-01
1. Na+ channel mRNA levels in the heart can be modulated by changes in intracellular Ca2+ ([Ca2+]i). We have investigated whether this regulation of Na+ channel biosynthesis by cytosolic Ca2+ translates into functional Na+ channels that can be detected electrophysiologically. 2. Whole-cell Na+ currents (INa) were recorded using patch-clamp techniques from single ventricular myocytes isolated from neonatal rats and maintained in tissue culture for 24 h. Na+ current density, measured at a membrane potential of -10 mV, was significantly decreased in the cells which were exposed for 24 h to culture medium containing 10 mM of both external Ca2+ and K+ in order to raise [Ca2+]i compared with control cells which were maintained in culture medium containing 2 and 5 mM of Ca2+ and K+, respectively. In contrast, Na+ current density (at -10 mV) was significantly increased in cells exposed for 24 h to 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetraacetoxymethyl ester (BAPTA AM; a cell membrane-permeable Ca2+ chelator) which lowered the average [Ca2+]i compared with control. 3. Changes in current density were not associated with changes in the voltage dependence of activation and inactivation of INa. There were no changes in single-channel conductances. 4. It is concluded that Na+ current density in neonatal rat cardiac myocytes is modulated by [Ca2+]i. The findings suggest that the differences in current density are attributable to a change in Na+ channel numbers rather than to changes in single-channel conductance or gating. These changes are consistent with the previously documented modulation of Na+ channel biosynthesis by cytosolic Ca2+. PMID:7650605
Down-regulation of endogenous KLHL1 decreases voltage-gated calcium current density.
Perissinotti, Paula P; Ethington, Elizabeth G; Cribbs, Leanne; Koob, Michael D; Martin, Jody; Piedras-Rentería, Erika S
2014-05-01
The actin-binding protein Kelch-like 1 (KLHL1) can modulate voltage-gated calcium channels in vitro. KLHL1 interacts with actin and with the pore-forming subunits of Cav2.1 and CaV3.2 calcium channels, resulting in up-regulation of P/Q and T-type current density. Here we tested whether endogenous KLHL1 modulates voltage gated calcium currents in cultured hippocampal neurons by down-regulating the expression of KLHL1 via adenoviral delivery of shRNA targeted against KLHL1 (shKLHL1). Control adenoviruses did not affect any of the neuronal properties measured, yet down-regulation of KLHL1 resulted in HVA current densities ~68% smaller and LVA current densities 44% smaller than uninfected controls, with a concomitant reduction in α(1A) and α(1H) protein levels. Biophysical analysis and western blot experiments suggest Ca(V)3.1 and 3.3 currents are also present in shKLHL1-infected neurons. Synapsin I levels, miniature postsynaptic current frequency, and excitatory and inhibitory synapse number were reduced in KLHL1 knockdown. This study corroborates the physiological role of KLHL1 as a calcium channel modulator and demonstrates a novel, presynaptic role.
Dissipative particle dynamics simulation for the density currents of polymer fluids
NASA Astrophysics Data System (ADS)
Li, Yanggui; Geng, Xingguo; Liu, Zhijun; Liu, Qingsheng; Ouyang, Jie
2016-11-01
In this work, the two-dimensional lock-exchange density currents of polymer fluids are numerically investigated using dissipative particle dynamics (DPD) at the mesoscale particle level. A modified finitely extensible nonlinear elastic (FENE) chain model is chosen to describe the polymer system, which perfectly depicts not only the elastic tension but also the elastic repulsion between the adjacent beads with bond length as the equilibrium length of one segment. Through the model and numerical simulation, we analyze the dynamics behavior of the density currents of polymer fluids. A comparison with its Newtonian counterpart suggests that the interface between two polymer fluids is more smoothed, and the front structure is different from the Newtonian case because the Kelvin-Helmholtz instability and cleft instability are suppressed by the polymer. Besides, we also probe the influences of polymer volume concentration, chain length and extensibility on the density currents. These simulation results show that increasing any of the parameters, concentration, chain length, and extensibility, the inhibiting effect of polymer on the density currents becomes more significant.
NASA Astrophysics Data System (ADS)
Singh, Anand; Sharma, S. P.
2016-10-01
A 2D inversion approach is developed to interpret VLF electromagnetic measurement recorded over variable topography. To depict the variable topography accurately, an octree mesh discretization is incorporated. Subsurface structure is modeled in terms of apparent current density distribution and compared with the inversion results for actual resistivity distribution obtained using numerical techniques. The study demonstrates that the results obtained using both approaches (current density and resistivity distribution) are comparable, but due to analytical expression, current density imaging is faster. The conjugate gradient method is used to reduce the computation time and storage space when solving the matrix equations, resulting in feasible and practical imaging inversion of VLF data. The preconditioned matrix, which is determined by the distances between the blocks and observation points, has an important function in improving the resolution. In case of flat earth, preconditioned conjugate gradient inversion of data results in images that are comparable to those obtained using resistivity inversion. We also test whether topography variation in the order of skin depth is significant to incorporate topography in the modeling. The example of a topographical field VLF data inversion shows the efficacy of the presented approach to appraise the subsurface structure in terms of current density.
Transition from Fowler-Nordheim field emission to space charge limited current density
NASA Astrophysics Data System (ADS)
Feng, Y.; Verboncoeur, J. P.
2006-07-01
The Fowler-Nordheim law gives the current density extracted from a surface under strong fields, by treating the emission of electrons from a metal-vacuum interface in the presence of an electric field normal to the surface as a quantum mechanical tunneling process. Child's law predicts the maximum transmitted current density by considering the space charge effect. When the electric field becomes high enough, the emitted current density will be limited by Child's law. This work analyzes the transition of the transmitted current density from the Fowler-Nordheim law to Child's law space charge limit using a one-dimensional particle-in-cell code. Also studied is the response of the emission model to strong electric fields near the transition point. We find the transition without geometrical effort is smooth and much slower than reported previously [J. P. Barbour, W. W. Dolan, J. K. Trolan, E. E. Martin, and W. P. Dyke, Phys. Rev. 92, 45 (1953)]. We analyze the effects of geometric field enhancement and work function on the transition. Using our previous model for effective field enhancement [Y. Feng and J. P. Verboncoeur, Phys. Plasmas 12, 103301 (2005)], we find the geometric effect dominates, and enhancement β >10 can accelerate the approach to the space charge limit at practical electric field. A damped oscillation near the local plasma frequency is observed in the transient system response.
Transition from Fowler-Nordheim field emission to space charge limited current density
Feng, Y.; Verboncoeur, J. P.
2006-07-15
The Fowler-Nordheim law gives the current density extracted from a surface under strong fields, by treating the emission of electrons from a metal-vacuum interface in the presence of an electric field normal to the surface as a quantum mechanical tunneling process. Child's law predicts the maximum transmitted current density by considering the space charge effect. When the electric field becomes high enough, the emitted current density will be limited by Child's law. This work analyzes the transition of the transmitted current density from the Fowler-Nordheim law to Child's law space charge limit using a one-dimensional particle-in-cell code. Also studied is the response of the emission model to strong electric fields near the transition point. We find the transition without geometrical effort is smooth and much slower than reported previously [J. P. Barbour, W. W. Dolan, J. K. Trolan, E. E. Martin, and W. P. Dyke, Phys. Rev. 92, 45 (1953)]. We analyze the effects of geometric field enhancement and work function on the transition. Using our previous model for effective field enhancement [Y. Feng and J. P. Verboncoeur, Phys. Plasmas 12, 103301 (2005)], we find the geometric effect dominates, and enhancement {beta}>10 can accelerate the approach to the space charge limit at practical electric field. A damped oscillation near the local plasma frequency is observed in the transient system response.
Role of fluid density in shaping eruption currents driven by frontal particle blow-out
NASA Astrophysics Data System (ADS)
Carroll, C. S.; Turnbull, B.; Louge, M. Y.
2012-06-01
We study the role of suspension density in eruption currents, a regime of gravity-driven flow that is sustained by massive, localized blow-out of particles acting as a steady source of heavier fluid injected into a uniform flow at high Reynolds number. Inspired by the potential flow solution of Saffman and Yuen ["Finite-amplitude interfacial waves in the presence of a current," J. Fluid Mech. 123, 459-476 (1982), 10.1017/S0022112082003152], we show that the relative density difference between the two fluids swells the size of the current's head without changing its shape, while inducing a velocity jump at the interface. We test this inviscid theory against inviscid and large-eddy-simulations. We also conduct experiments in a water flume, where a line source of fluorescent brines of various densities is injected in a cross-stream and visualized with a narrow sheet of light. Simulations and experiments reveal that, with isotropic velocity distribution on a finite source, eruption currents expand further and develop interface oscillations, but the inviscid theory still captures relative swelling induced by density. We compare predictions to the static pressure data of McElwaine and Turnbull ["Air pressure data from the Vallee de la Sionne avalanches of 2004," J. Geophys. Res. 110, F03010, doi:, 10.1029/2004JF000237 (2005)] in powder snow avalanches.
Model Predictive Control with Integral Action for Current Density Profile Tracking in NSTX-U
NASA Astrophysics Data System (ADS)
Ilhan, Z. O.; Wehner, W. P.; Schuster, E.; Boyer, M. D.
2016-10-01
Active control of the toroidal current density profile may play a critical role in non-inductively sustained long-pulse, high-beta scenarios in a spherical torus (ST) configuration, which is among the missions of the NSTX-U facility. In this work, a previously developed physics-based control-oriented model is embedded in a feedback control scheme based on a model predictive control (MPC) strategy to track a desired current density profile evolution specified indirectly by a desired rotational transform profile. An integrator is embedded into the standard MPC formulation to reject various modeling uncertainties and external disturbances. Neutral beam powers, electron density, and total plasma current are used as actuators. The proposed MPC strategy incorporates various state and actuator constraints directly into the control design process by solving a constrained optimization problem in real-time to determine the optimal actuator requests. The effectiveness of the proposed controller in regulating the current density profile in NSTX-U is demonstrated in closed-loop nonlinear simulations. Supported by the US DOE under DE-AC02-09CH11466.
The Impact of Cathode Material and Shape on Current Density in an Aluminum Electrolysis Cell
NASA Astrophysics Data System (ADS)
Song, Yang; Peng, Jianping; Di, Yuezhong; Wang, Yaowu; Li, Baokuan; Feng, Naixiang
2016-02-01
A finite element model was developed to determine the impact of cathode material and shape on current density in an aluminum electrolysis cell. For the cathode material, results show that increased electrical resistivity leads to a higher cathode voltage drop; however, the horizontal current is reduced in the metal. The horizontal current magnitude for six different cathode materials in decreasing order is graphitized, semi-graphitized, full graphitic, 50% anthracite (50% artificial graphite), 70% anthracite (30% artificial graphite), 100% anthracite. The modified cathode shapes with an inclined cathode surface, higher collector bar and cylindrical protrusions are intended to improve horizontal current and flow resistance. Compared to a traditional cathode, modified collector bar sizes of 70 mm × 230 mm and 80 mm × 270 mm can reduce horizontal current density component Jx by 10% and 19%, respectively, due to better conductivity of the steel. The horizontal current in the metal decreases with increase of cathode inclination. The peak value of Jx can be approximately reduced by 20% for a 2° change in inclination. Cylindrical protrusions lead to local horizontal current increase on their tops, but the average current is less affected and the molten metal is effectively slowed down.
Faraday-effect polarimeter-interferometer system for current density measurement on EAST
NASA Astrophysics Data System (ADS)
Liu, H. Q.; Jie, Y. X.; Ding, W. X.; Brower, D. L.; Zou, Z. Y.; Li, W. M.; Wang, Z. X.; Qian, J. P.; Yang, Y.; Zeng, L.; Lan, T.; Wei, X. C.; Li, G. S.; Hu, L. Q.; Wan, B. N.
2014-11-01
A multichannel far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique is under development for current density and electron density profile measurements in the EAST tokamak. Novel molybdenum retro-reflectors are mounted in the inside wall for the double-pass optical arrangement. A Digital Phase Detector with 250 kHz bandwidth, which will provide real-time Faraday rotation angle and density phase shift output, have been developed for use on the POINT system. Initial calibration indicates the electron line-integrated density resolution is less than 5 × 1016 m-2 (˜2°), and the Faraday rotation angle rms phase noise is <0.1°.
Faraday-effect polarimeter-interferometer system for current density measurement on EAST.
Liu, H Q; Jie, Y X; Ding, W X; Brower, D L; Zou, Z Y; Li, W M; Wang, Z X; Qian, J P; Yang, Y; Zeng, L; Lan, T; Wei, X C; Li, G S; Hu, L Q; Wan, B N
2014-11-01
A multichannel far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique is under development for current density and electron density profile measurements in the EAST tokamak. Novel molybdenum retro-reflectors are mounted in the inside wall for the double-pass optical arrangement. A Digital Phase Detector with 250 kHz bandwidth, which will provide real-time Faraday rotation angle and density phase shift output, have been developed for use on the POINT system. Initial calibration indicates the electron line-integrated density resolution is less than 5 × 10(16) m(-2) (∼2°), and the Faraday rotation angle rms phase noise is <0.1°.
Faraday-effect polarimeter-interferometer system for current density measurement on EAST
Liu, H. Q.; Jie, Y. X. Zou, Z. Y.; Li, W. M.; Wang, Z. X.; Qian, J. P.; Yang, Y.; Zeng, L.; Wei, X. C.; Hu, L. Q.; Wan, B. N.; Ding, W. X.; Brower, D. L.; Lan, T.; Li, G. S.
2014-11-15
A multichannel far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique is under development for current density and electron density profile measurements in the EAST tokamak. Novel molybdenum retro-reflectors are mounted in the inside wall for the double-pass optical arrangement. A Digital Phase Detector with 250 kHz bandwidth, which will provide real-time Faraday rotation angle and density phase shift output, have been developed for use on the POINT system. Initial calibration indicates the electron line-integrated density resolution is less than 5 × 10{sup 16} m{sup −2} (∼2°), and the Faraday rotation angle rms phase noise is <0.1°.
Current densities and total contact currents during forest clearing tasks under 400 kV power lines
Kuisti, Harri; Elovaara, Jarmo
2016-01-01
The aim of the study was to analyze all values of electric currents from measured periods while performing tasks in forest clearing. The objective was also to choose and analyze measurement cases, where current measurements successfully lasted the entire work period (about 30 min). Two forestry workers volunteered to perform four forest clearing tasks under 400 kV power lines. The sampling frequency of the current measurements was 1 sample/s. The maximum values of the current densities were 1.0–1.2 mA/m2 (calculated internal EFs 5.0–12.0 mV/m), and the average values were 0.2–0.4 mA/m2. The highest contact current was 167.4 μA. All measured values during forest clearing tasks were lower than basic restrictions (0.1 V/m and 0.8 V/m) of the International Commission on Non‐Ionizing Radiation Protection. Bioelectromagnetics. 37:423–428, 2016. © 2016 The Authors. Bioelectromagnetics published by Wiley Periodicals, Inc. PMID:27192179
NASA Astrophysics Data System (ADS)
Bellucci, S.; Bezerra de Mello, E. R.; Bragança, E.; Saharian, A. A.
2016-06-01
We evaluate the fermion condensate and the expectation values of the charge and current densities for a massive fermionic field in (2+1)-dimensional conical spacetime with a magnetic flux located at the cone apex. The consideration is done for both irreducible representations of the Clifford algebra. The expectation values are decomposed into the vacuum expectation values and contributions coming from particles and antiparticles. All these contributions are periodic functions of the magnetic flux with the period equal to the flux quantum. Related to the non-invariance of the model under the parity and time-reversal transformations, the fermion condensate and the charge density have indefinite parity with respect to the change of the signs of the magnetic flux and chemical potential. The expectation value of the radial current density vanishes. The azimuthal current density is the same for both the irreducible representations of the Clifford algebra. It is an odd function of the magnetic flux and an even function of the chemical potential. The behavior of the expectation values in various asymptotic regions of the parameters are discussed in detail. In particular, we show that for points near the cone apex the vacuum parts dominate. For a massless field with zero chemical potential the fermion condensate and charge density vanish. Simple expressions are derived for the part in the total charge induced by the planar angle deficit and magnetic flux. Combining the results for separate irreducible representations, we also consider the fermion condensate, charge and current densities in parity and time-reversal symmetric models. Possible applications to graphitic nanocones are discussed.
Remarks on time-dependent [current]-density functional theory for open quantum systems.
Yuen-Zhou, Joel; Aspuru-Guzik, Alán
2013-08-14
Time-dependent [current]-density functional theory for open quantum systems (OQS) has emerged as a formalism that can incorporate dissipative effects in the dynamics of many-body quantum systems. Here, we review and clarify some formal aspects of these theories that have been recently questioned in the literature. In particular, we provide theoretical support for the following conclusions: (1) contrary to what we and others had stated before, within the master equation framework, there is in fact a one-to-one mapping between vector potentials and current densities for fixed initial state, particle-particle interaction, and memory kernel; (2) regardless of the first conclusion, all of our recently suggested Kohn-Sham (KS) schemes to reproduce the current and particle densities of the original OQS, and in particular, the use of a KS closed driven system, remains formally valid; (3) the Lindblad master equation maintains the positivity of the density matrix regardless of the time-dependence of the Hamiltonian or the dissipation operators; (4) within the stochastic Schrödinger equation picture, a one-to-one mapping from stochastic vector potential to stochastic current density for individual trajectories has not been proven so far, except in the case where the vector potential is the same for every member of the ensemble, in which case, it reduces to the Lindblad master equation picture; (5) master equations may violate certain desired properties of the density matrix, such as positivity, but they remain as one of the most useful constructs to study OQS when the environment is not easily incorporated explicitly in the calculation. The conclusions support our previous work as formally rigorous, offer new insights into it, and provide a common ground to discuss related theories.
Oh, Suk Hoon; Lee, Byung Il; Woo, Eung Je; Lee, Soo Yeol; Cho, Min Hyoung; Kwon, Ohin; Seo, Jin Keun
2003-10-07
Magnetic resonance electrical impedance tomography (MREIT) is to provide cross-sectional images of the conductivity distribution sigma of a subject. While injecting current into the subject, we measure one component Bz of the induced magnetic flux density B = (Bx, By, Bz) using an MRI scanner. Based on the relation between (inverted delta)2 Bz and inverted delta sigma, the harmonic Bz algorithm reconstructs an image of sigma using the measured Bz data from multiple imaging slices. After we obtain sigma, we can reconstruct images of current density distributions for any given current injection method. Following the description of the harmonic Bz algorithm, this paper presents reconstructed conductivity and current density images from computer simulations and phantom experiments using four recessed electrodes injecting six different currents of 26 mA. For experimental results, we used a three-dimensional saline phantom with two polyacrylamide objects inside. We used our 0.3 T (tesla) experimental MRI scanner to measure the induced Bz. Using the harmonic Bz algorithm, we could reconstruct conductivity and current density images with 82 x 82 pixels. The pixel size was 0.6 x 0.6 mm2. The relative L2 errors of the reconstructed images were between 13.8 and 21.5% when the signal-to-noise ratio (SNR) of the corresponding MR magnitude images was about 30. The results suggest that in vitro and in vivo experimental studies with animal subjects are feasible. Further studies are requested to reduce the amount of injection current down to less than 1 mA for human subjects.
Zeng, Zhiping; Chen, Xuanze; Wang, Hening; Huang, Ning; Shan, Chunyan; Zhang, Hao; Teng, Junlin; Xi, Peng
2015-02-10
Previous stochastic localization-based super-resolution techniques are largely limited by the labeling density and the fidelity to the morphology of specimen. We report on an optical super-resolution imaging scheme implementing joint tagging using multiple fluorescent blinking dyes associated with super-resolution optical fluctuation imaging (JT-SOFI), achieving ultra-high labeling density super-resolution imaging. To demonstrate the feasibility of JT-SOFI, quantum dots with different emission spectra were jointly labeled to the tubulin in COS7 cells, creating ultra-high density labeling. After analyzing and combining the fluorescence intermittency images emanating from spectrally resolved quantum dots, the microtubule networks are capable of being investigated with high fidelity and remarkably enhanced contrast at sub-diffraction resolution. The spectral separation also significantly decreased the frame number required for SOFI, enabling fast super-resolution microscopy through simultaneous data acquisition. As the joint-tagging scheme can decrease the labeling density in each spectral channel, thereby bring it closer to single-molecule state, we can faithfully reconstruct the continuous microtubule structure with high resolution through collection of only 100 frames per channel. The improved continuity of the microtubule structure is quantitatively validated with image skeletonization, thus demonstrating the advantage of JT-SOFI over other localization-based super-resolution methods.
Waller, V; Pégourié, B; Giruzzi, G; Huysmans, G T A; Garzotti, L; Géraud, A
2003-11-14
A pellet penetrating the inner region of a tokamak discharge, where the safety factor drops below unity, triggers an instability analogous to a sawtooth crash. Because of the simultaneity of the crash and pellet crossing, the latter is an appropriate probe for investigating the current distribution during reconnection. In this Letter, pellet deflection is used to characterize the associated electron distribution function. The perturbation compatible with the observed trajectory requires a negative current layer on the q=1 magnetic surface between 3 and 12 times the equilibrium current density and an expulsion of high energy electrons from the plasma core.
NASA Astrophysics Data System (ADS)
Yang, J.; Lee, J. W.; Jung, B. K.; Chung, K. J.; Hwang, Y. S.
2014-11-01
An internal magnetic probe using Hall sensors to measure a current density profile directly with perturbation of less than 10% to the plasma current is successfully operated for the first time in Versatile Experiment Spherical Torus (VEST). An appropriate Hall sensor is chosen to produce sufficient signals for VEST magnetic field while maintaining the small size of 10 mm in outer diameter. Temperature around the Hall sensor in a typical VEST plasma is regulated by blown air of 2 bars. First measurement of 60 kA VEST ohmic discharge shows a reasonable agreement with the total plasma current measured by Rogowski coil in VEST.
Yang, J; Lee, J W; Jung, B K; Chung, K J; Hwang, Y S
2014-11-01
An internal magnetic probe using Hall sensors to measure a current density profile directly with perturbation of less than 10% to the plasma current is successfully operated for the first time in Versatile Experiment Spherical Torus (VEST). An appropriate Hall sensor is chosen to produce sufficient signals for VEST magnetic field while maintaining the small size of 10 mm in outer diameter. Temperature around the Hall sensor in a typical VEST plasma is regulated by blown air of 2 bars. First measurement of 60 kA VEST ohmic discharge shows a reasonable agreement with the total plasma current measured by Rogowski coil in VEST.
Nanoscale Imaging of Current Density with a Single-Spin Magnetometer.
Chang, K; Eichler, A; Rhensius, J; Lorenzelli, L; Degen, C L
2017-03-24
Charge transport in nanostructures and thin films is fundamental to many phenomena and processes in science and technology, ranging from quantum effects and electronic correlations in mesoscopic physics, to integrated charge- or spin-based electronic circuits, to photoactive layers in energy research. Direct visualization of the charge flow in such structures is challenging due to their nanometer size and the itinerant nature of currents. In this work, we demonstrate noninvasive magnetic imaging of current density in two-dimensional conductor networks including metallic nanowires and carbon nanotubes. Our sensor is the electronic spin of a diamond nitrogen-vacancy center attached to a scanning tip and operated under ambient conditions. Using a differential measurement technique, we detect DC currents down to a few μA with a current density noise floor of ∼2 × 10(4) A/cm(2). Reconstructed images have a spatial resolution of typically 50 nm, with a best-effort value of 22 nm. Current density imaging offers a new route for studying electronic transport and conductance variations in two-dimensional materials and devices, with many exciting applications in condensed matter physics and materials science.
High temperature and current density induced degradation of multi-layer graphene
Wang, Baoming; Haque, M. A.; Mag-isa, Alexander E.; Kim, Jae-Hyun; Lee, Hak-Joo
2015-10-19
We present evidence of moderate current density, when accompanied with high temperature, promoting migration of foreign atoms on the surface of multi-layer graphene. Our in situ transmission electron microscope experiments show migration of silicon atoms at temperatures above 800 °C and current density around 4.2 × 10{sup 7} A/cm{sup 2}. Originating from the micro-machined silicon structures that clamp the freestanding specimen, the atoms are observed to react with the carbon atoms in the multi-layer graphene to produce silicon carbide at temperatures of 900–1000 °C. In the absence of electrical current, there is no migration of silicon and only pyrolysis of polymeric residue is observed.
Transport, deposition, and liftoff in laboratory density currents composed of hot particles in air
NASA Astrophysics Data System (ADS)
Andrews, B. J.; Manga, M.
2010-12-01
Understanding the dynamics of transport, deposition, and air entrainment in pyroclastic density currents (PDCs) is required for accurate predictions of future current behaviors and interpretations of ancient deposits, but directly observing the interiors of natural PDCs is effectively impossible. We model PDCs with scaled, hot, particle-laden density currents generated in a 6 m long, 0.6 m wide, 1.8 m tall air-filled tank. Comparison of relevant scaling between our experiments and natural PDCs indicates that we are accurately capturing much of the dynamics of dilute PDCs: * Reynolds numbers of our experiments are lower than natural currents, 10^3 compared to 10^6, but still fully turbulent; * Densimetric and Thermal Richardson numbers are of O(1) in both natural and modeled currents; * Stokes and settling numbers for particles in the experiments fall within the expected range for natural PDCs. Conditions within the tank are monitored with temperature and humidity probes. Experiments are illuminated with sheet lighting, and recorded with high-definition video cameras. In general, currents have average velocities of 10-20 cm/s, initial thicknesses of 10-20 cm (although thickness greatly increases as currents entrain and expand air), and run out or lift off distances of 3-5 m. Large Kelvin-Helmholtz type eddies usually form along the top of the current immediately behind the head; these vortices are similar in size to the total current thickness. In currents that lift off, the distal current end typically retreats with time. Preliminary results suggest that lift off distance decreases with increasing thermal Richardson number. Analysis of turbulent structures indicates that the current heads are dominated by large coherent structures with length scales, L, comparable to the current thickness. Within 5-10 L of the current fronts, sequences of similar large eddies often occur. At greater distances behind the current fronts, turbulent structures become smaller and less
Role of head of turbulent 3-D density currents in mixing during slumping regime
NASA Astrophysics Data System (ADS)
Bhaganagar, Kiran
2017-02-01
A fundamental study was conducted to shed light on entrainment and mixing in buoyancy-driven Boussinesq density currents. Large-eddy simulation was performed on lock-exchange (LE) release density currents—an idealized test bed to generate density currents. As dense fluid was released over a sloping surface into an ambient lighter fluid, the dense fluid slumps to the bottom and forms a characteristic head of the current. The dynamics of the head dictated the mixing processes in LE currents. The key contribution of this study is to resolve an ongoing debate on mixing: We demonstrate that substantial mixing occurs in the early stages of evolution in an LE experiment and that entrainment is highly inhomogeneous and unsteady during the slumping regime. Guided by the flow physics, entrainment is calculated using two different but related perspectives. In the first approach, the entrainment parameter (E) is defined as the fraction of ambient fluid displaced by the head that entrains into the current. It is an indicator of the efficiency in which ambient fluid is displaced into the current and it serves as an important metric to compare the entrainment of dense currents over different types of surfaces, e.g., roughness configuration. In the second approach, E measures the net entrainment in the current at an instantaneous time t over the length of the current. Net entrainment coefficient is a metric to compare the effects of flow dynamical conditions, i.e., lock-aspect ratio that dictates the fraction of buoyancy entering the head, and also the effect of the sloping angle. Together, the entrainment coefficient and the net entrainment coefficient provide an insight into the entrainment process. The "active" head of the current acts as an engine that mixes the ambient fluid with the existing dense fluid, the 3-D lobes and clefts on the frontal end of the current causes recirculation of the ambient fluid into the current, and Kelvin-Helmholtz rolls are the mixers that
Spectral broadening of parametric instability in lower hybrid current drive at a high density
NASA Astrophysics Data System (ADS)
Cesario, R.; Amicucci, L.; Cardinali, A.; Castaldo, C.; Marinucci, M.; Napoli, F.; Paoletti, F.; De Arcangelis, D.; Ferrari, M.; Galli, A.; Gallo, G.; Pullara, E.; Schettini, G.; Tuccillo, A. A.
2014-04-01
The important goal of adding to the bootstrap current a more flexible tool, capable of producing and controlling steady-state profiles with a high fraction of non-inductive plasma current, could be reached using the lower hybrid current drive (LHCD) effect. Experiments performed on FTU (Frascati Tokamak Upgrade) demonstrated that LHCD can occur at reactor-graded high plasma density, provided that the parametric instability (PI)-produced broadening of the spectrum launched by the antenna is reduced under proper operating conditions, capable of producing relatively high temperature in the outer region of plasma column. This condition was produced by operations that reduce particle recycling from the vessel walls, and enhance the gas fuelling in the core by means of fast pellet. New results of FTU experiments are presented documenting that the useful effect of temperature at the periphery, which reduces the LH spectral broadening and enhances the LH-induced hard-x ray emission level, occurs in a broader range of plasma parameters than in previous work. Modelling results show that a further tool for helping LHCD at a high density would be provided by electron cyclotron resonant heating of plasma periphery. New information is provided on the modelling, able determining frequencies, growth rates and LH spectral broadening produced by PI, which allowed assessing the new method for enabling LHCD at high densities. Further robustness is provided to theoretical and experimental fundaments of the method for LHCD at a high density.
Saito, Miho
2015-08-01
The basic properties of the near-Earth current sheet from 8 RE to 12 RE were determined based on Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations from 2007 to 2013. Ampere's law was used to estimate the current density when the locations of two spacecraft were suitable for the calculation. A total of 3838 current density observations were obtained to study the vertical profile. For typical solar wind conditions, the current density near (off) the central plane of the current sheet ranged from 1 to 2 nA/m(2) (1 to 8 nA/m(2)). All the high current densities appeared off the central plane of the current sheet, indicating the formation of a bifurcated current sheet structure when the current density increased above 2 nA/m(2). The median profile also showed a bifurcated structure, in which the half thickness was about 3 RE . The distance between the peak of the current density and the central plane of the current sheet was 0.5 to 1 RE . High current densities above 4 nA/m(2) were observed in some cases that occurred preferentially during substorms, but they also occurred in quiet times. In contrast to the commonly accepted picture, these high current densities can form without a high solar wind dynamic pressure. In addition, these high current densities can appear in two magnetic configurations: tail-like and dipolar structures. At least two mechanisms, magnetic flux depletion and new current system formation during the expansion phase, other than plasma sheet compression are responsible for the formation of the bifurcated current sheets.
2015-01-01
Abstract The basic properties of the near‐Earth current sheet from 8 RE to 12 RE were determined based on Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations from 2007 to 2013. Ampere's law was used to estimate the current density when the locations of two spacecraft were suitable for the calculation. A total of 3838 current density observations were obtained to study the vertical profile. For typical solar wind conditions, the current density near (off) the central plane of the current sheet ranged from 1 to 2 nA/m2 (1 to 8 nA/m2). All the high current densities appeared off the central plane of the current sheet, indicating the formation of a bifurcated current sheet structure when the current density increased above 2 nA/m2. The median profile also showed a bifurcated structure, in which the half thickness was about 3 RE. The distance between the peak of the current density and the central plane of the current sheet was 0.5 to 1 RE. High current densities above 4 nA/m2 were observed in some cases that occurred preferentially during substorms, but they also occurred in quiet times. In contrast to the commonly accepted picture, these high current densities can form without a high solar wind dynamic pressure. In addition, these high current densities can appear in two magnetic configurations: tail‐like and dipolar structures. At least two mechanisms, magnetic flux depletion and new current system formation during the expansion phase, other than plasma sheet compression are responsible for the formation of the bifurcated current sheets. PMID:27722039
NASA Astrophysics Data System (ADS)
Loehman, R.; Heinsch, F. A.; Mills, J. N.; Wagoner, K.; Running, S.
2003-12-01
Recent predictive models for hantavirus pulmonary syndrome (HPS) have used remotely sensed spectral reflectance data to characterize risk areas with limited success. We present an alternative method using gross primary production (GPP) from the MODIS sensor to estimate the effects of biomass accumulation on population density of Peromyscus maniculatus (deer mouse), the principal reservoir species for Sin Nombre virus (SNV). The majority of diagnosed HPS cases in North America are attributed to SNV, which is transmitted to humans through inhalation of excretions and secretions from infected rodents. A logistic model framework is used to evaluate MODIS GPP, temperature, and precipitation as predictors of P. maniculatus density at established trapping sites across the western United States. Rodent populations are estimated using monthly minimum number alive (MNA) data for 2000 through 2002. Both local meteorological data from nearby weather stations and 1.25 degree x 1 degree gridded data from the NASA DAO were used in the regression model to determine the spatial sensitivity of the response. MODIS eight-day GPP data (1-km resolution) were acquired and binned to monthly average and monthly sum GPP for 3km x 3km grids surrounding each rodent trapping site. The use of MODIS GPP to forecast HPS risk may result in a marked improvement over past reflectance-based risk area characterizations. The MODIS GPP product provides a vegetation dynamics estimate that is unique to disease models, and targets the fundamental ecological processes responsible for increased rodent density and amplified disease risk.
NASA Technical Reports Server (NTRS)
Gao, L.; Meng, R. L.; Xue, Y. Y.; Hor, P. H.; Chu, C. W.
1991-01-01
Using a recently developed pulsed critical current density (Jc) measuring system, the Jc of the high-Jc melt-textured YBa2Cu3O(7-delta) (Y123) bulk samples has been determined. I-V curves with a voltage resolution of 0.5 microV were obtained, and transport Jc's along the a-b plane as high as 7.2 x 10 to the 4th A/sq cm were extracted. These results are comparable to the values obtained magnetically. On the other hand, transport Jc along the c axis were found to be two orders of magnitude smaller, even though the magnetic Jc along the c axis is only about five times smaller than Jc along the a-b plane. It is suggested that for the high-temperature superconducting materials which are highly anisotropic, caution should be taken when using the nontransport magnetic methods to determine Jc.
Effects of Neutral Density on Energetic Ions Produced Near High-Current Hollow Cathodes
NASA Technical Reports Server (NTRS)
Kameyama, Ikuya
1997-01-01
Energy distributions of ion current from high-current, xenon hollow cathodes, which are essential information to understand erosion phenomena observed in high-power ion thrusters, were obtained using an electrostatic energy analyzer (ESA). The effects of ambient pressure and external flow rate introduced immediately downstream of hollow cathode on ion currents with energies greater than that associated with the cathode-to-anode potential difference were investigated. The results were analyzed to determine the changes in the magnitudes of ion currents to the ESA at various energies. Either increasing the ambient pressure or adding external flow induces an increase in the distribution of ion currents with moderate energies (epsilon less than 25 to 35 eV) and a decrease in the distribution for high energies (epsilon greater than 25 to 35 eV). The magnitude of the current distribution increase in the moderate energy range is greater for a cathode equipped with a toroidal keeper than for one without a keeper, but the distribution in the high energy range does not seem to be affected by a keeper. An MHD model, which has been proposed to describe energetic-ion production mechanism in hollow cathode at high discharge currents, was developed to describe these effects. The results show, however, that this model involves no mechanism by which a significant increase of ion current could occur at any energy. It was found, on the other hand, that the potential-hill model of energetic ion production, which assumes existence of a local maximum of plasma potential, could explain combined increases in the currents of ions with moderate energies and decreases in high energy ions due to increased neutral atom density using a charge-exchange mechanism. The existing, simplified version of the potential-hill model, however, shows poor quantitative agreement with measured ion-current-energy-distribution changes induced by neutral density changes.
Thermal history of pyroclastic density currents and pyroclasts at Tungurahua, Ecuador
NASA Astrophysics Data System (ADS)
Benage, M. C.; Dufek, J.
2014-12-01
The associated hazards and opaqueness of pyroclastic density currents (PDCs) make it impossible for in-situ thermal or concentration measurements within the currents that would provide critical information on the dynamics of PDCs. The entrainment of ambient air into these currents significantly impacts their runout distance and thermal history. The most efficient mechanism to cool a PDC is through the entrainment of colder, denser ambient air through Kelvin-Helmholtz and lobe-and-cleft instabilities, which are dependent on density stratification in the current and topographic-current interactions. The combination of high-resolution multiphase numerical models in concert with field measurements of PDC deposits allows us to better understand the evolving concentration gradients, instabilities, entrainment of air, and temperatures of PDCs. We employ a three-dimensional multiphase Eulerian-Eulerian-Lagrangian (EEL) model, high-resolution topography, and field data to understand the PDCs that traveled down the Juive Grande quebrada during the 2006 eruption of Tungurahua volcano. The multiphase model allows us to examine PDC dynamics such as particle concentrations, velocities, thermal heterogeneities, and ambient air entrainment. As the PDC propagates, the entrainment coefficient decreases due to enhanced density stratification. The interaction of the current with rugged topography increases the entrainment coefficient. We also calculate the temperature of deposition and breadcrust bomb rind thickness for individual pyroclasts. The individual pyroclasts are tracked as Lagrangian particles in the multiphase model and we employ the breadcrust bomb model (Benage et al., 2014) to calculate the deposition temperature and the formation of the non-vesicular to low vesicularity rinds. The model results are compared to paleomagnetic data and field measurements of rind thickness, respectively. This allows the deposited pyroclasts to be natural thermometers that help constrain the
Jackson, P. Ryan; Garcia, Carlos M.; Oberg, Kevin A.; Johnson, Kevin K.; Garcia, Marcelo H.
2008-01-01
Bidirectional flows in a river system can occur under stratified flow conditions and in addition to creating significant errors in discharge estimates, the upstream propagating currents are capable of transporting contaminants and affecting water quality. Detailed field observations of bidirectional flows were made in the Chicago River in Chicago, Illinois in the winter of 2005-06. Using multiple acoustic Doppler current profilers simultaneously with a water-quality profiler, the formation of upstream propagating density currents within the Chicago River both as an underflow and an overflow was observed on three occasions. Density differences driving the flow primarily arise from salinity differences between intersecting branches of the Chicago River, whereas water temperature is secondary in the creation of these currents. Deicing salts appear to be the primary source of salinity in the North Branch of the Chicago River, entering the waterway through direct runoff and effluent from a wastewater-treatment plant in a large metropolitan area primarily served by combined sewers. Water-quality assessments of the Chicago River may underestimate (or overestimate) the impairment of the river because standard water-quality monitoring practices do not account for density-driven underflows (or overflows). Chloride concentrations near the riverbed can significantly exceed concentrations at the river surface during underflows indicating that full-depth parameter profiles are necessary for accurate water-quality assessments in urban environments where application of deicing salt is common.
Irreversibility line in superconductor as line of constant shielding current density
Goemoery, F.; Takacs, S.; Holubar, T.
1997-06-01
The irreversibility of magnetic properties of superconductors is due to the existence of macroscopic shielding currents persisting for some period of time. The same currents offer nearly lossless electricity transport. Thus, the extent of magnetic irreversibility is directly proportional to the current-carrying capacity of a superconductor. Because the current-carrying capacity is an intrinsic property of the material, various experimental techniques should give the same irreversibility line corresponding to the same macroscopic shielding current density. Following this approach, the authors compared the irreversibility lines obtained from AC susceptibility measurement with those determined from quasistatic magnetization loops recorded with the help of a SQUID susceptometer. An additional parameter which has to be comparable is the electrical field characterizing the rate of change of the magnetic field. Fulfilling these conditions of equivalency, the authors found that it is possible to explain the irreversibility lines obtained by various techniques and at different conditions by the same physical model. They demonstrate that for the data, taken within two orders of magnitude for the current density and more than seven orders of magnitude for the electrical field, a consistent picture expressing all the observed features by the same model can be found. Measurements are presented from YBCO samples.
Magnetically filtered Faraday probe for measuring the ion current density profile of a Hall thruster
Rovey, Joshua L.; Walker, Mitchell L.R.; Gallimore, Alec D.; Peterson, Peter Y.
2006-01-15
The ability of a magnetically filtered Faraday probe (MFFP) to obtain the ion current density profile of a Hall thruster is investigated. The MFFP is designed to eliminate the collection of low-energy, charge-exchange (CEX) ions by using a variable magnetic field as an ion filter. In this study, a MFFP, Faraday probe with a reduced acceptance angle (BFP), and nude Faraday probe are used to measure the ion current density profile of a 5 kW Hall thruster operating over the range of 300-500 V and 5-10 mg/s. The probes are evaluated on a xenon propellant Hall thruster in the University of Michigan Large Vacuum Test Facility at operating pressures within the range of 4.4x10{sup -4} Pa Xe (3.3x10{sup -6} Torr Xe) to 1.1x10{sup -3} Pa Xe (8.4x10{sup -6} Torr Xe) in order to study the ability of the Faraday probe designs to filter out CEX ions. Detailed examination of the results shows that the nude probe measures a greater ion current density profile than both the MFFP and BFP over the range of angular positions investigated for each operating condition. The differences between the current density profiles obtained by each probe are attributed to the ion filtering systems employed. Analysis of the results shows that the MFFP, operating at a +5 A solenoid current, provides the best agreement with flight-test data and across operating pressures.
NASA Astrophysics Data System (ADS)
Trevelyan, Alexander; Corwin, Eric
2014-03-01
We explore the response of a model statistical system to strong, non-linear perturbations to its state variables. Specifically, we work with a tunable model of Johnson-Nyquist noise, designed to permit a driving of both the drift and diffusion terms in the associated White Noise Langevin Equation. We achieve a simultaneous measurement of both sides of the Fluctuation Dissipation Theorem (FDT) by driving the circuit with digitally generated white noise and measuring the output. This allows us to calculate a frequency-dependent effective temperature for the driven system, which for an equilibrium system should be set by the energy scale of the input white noise. Comparison of the two sides of FDT-the circuit's transfer function and the power spectral density of the voltage fluctuations-across frequency-space proves non-trivial, and methods are discussed for achieving the most reliable estimate. After comparing the response for a series of functional signals, we find that FDT, measured in this simultaneous fashion, remains intact even while the system is being actively driven out of equilibrium.
NASA Astrophysics Data System (ADS)
Yoshimura, Shinji; Terasaka, Kenichiro; Tanaka, Eiki; Aramaki, Mitsutoshi; Tanaka, Masayoshi Y.
An intermittent behavior of local electron flux in a laboratory ECR plasma is statistically analyzed by means of probability density functions (PDFs). The PDF constructed from a time series of the floating potential signal on a Langmuir probe has a fat tail in the negative value side, which reflects the intermittency of the local electron flux. The PDF of the waiting time, which is defined by the time interval between two successive events, is found to exhibit an exponential distribution, suggesting that the phenomenon is characterized by a stationary Poisson process. The underlying Poisson process is also confirmed by the number of events in given time intervals that is Poisson distributed.
NASA Astrophysics Data System (ADS)
Henríquez, Paula; Donoso, Denise S.; Grez, Audrey A.
2009-11-01
Habitat fragmentation results in new environmental conditions that may stress resident populations. Such stress may be reflected in demographical or morphological changes in the individuals inhabiting those landscapes. This study evaluates the effects of fragmentation of the Maulino forest on population density, sex ratio, body size, and fluctuating asymmetry (FA) of the endemic carabid Ceroglossus chilensis. Individuals of C. chilensis were collected during 2006 in five locations at Los Queules National Reserve (continuous forest), in five forest fragments and in five areas of surrounding pine plantations (matrix). In each location, once a season, 40 pitfall traps (20 in the centre, 20 in the edge), were opened for 72 h. Population density of C. chilensis was higher in the small fragments than in the pine matrix, with intermediate densities in the continuous forest; sex ratio did not differ significantly from 1:1 in the three habitats. Individuals from the centre of fragments were smaller than those from the centre of continuous forest, and FA did not vary significantly among habitats. These results suggest that small forest fragments maintain dense populations of C. chilensis and therefore they must be considered in conservation strategies. Although the decrease of the body size suggests that small remnants should be connected by managing the structure of the surrounding matrix, facilitating the dispersion of this carabid across the landscape and avoiding possible antagonistic interactions inside small fragments.
Hansen, T F; Stenseth, N C; Henttonen, H; Tast, J
1999-02-02
A 3- to 5-year cycle of vole abundances is a characteristic phenomenon in the ecology of northern regions, and their explanation stands as a central theoretical challenge in population ecology. Although many species of voles usually coexist and are in severe competition for food and breeding space, the role of interspecific competition in vole cycles has never been evaluated statistically. After studying community effects on the population dynamics of the gray-sided vole (Clethrionomys rufocanus) in the subarctic birch forest at Kilpisjärvi, Finland, we report statistical results showing that both interspecific and intraspecific effects are important in the direct year-to-year density dependence. However, interspecific effects are not detectable in the 2-year delayed density dependence that is crucial for generating the characteristic cycles. Furthermore, we show that most of the competition takes place during the winter. The results are evaluated against two models of community dynamics. One assumes that the delayed effects are caused by an interaction with a specialist predator, and the other assumes that they are caused by overgrazing food plants. These statistical results show that vole cycles may be generated by a species-specific trophic interaction. The results also suggest that the gray-sided vole may be the focal species in the birch-forest community, as field voles may be in the taiga and as lemmings may be on the tundra.
NASA Astrophysics Data System (ADS)
Gao, Xin-Yi
2016-06-01
Liquids with gas bubbles are commonly seen in medical science, natural science, daily life and engineering. Nonlinear-wave symbolic computation on the (3+1)-dimensional variable-coefficient Kudryashov-Sinelshchikov model for a bubbly liquid is hereby performed. An auto-Bäcklund transformation and with some solitonic solutions are obtained. With respect to the density fluctuation of the bubble-liquid mixture, both the auto-Bäcklund transformation and solitonic solutions depend on the bubble-liquid-viscosity, transverse-perturbation, bubble-liquid-nonlinearity and bubble-liquid-dispersion coefficient functions. We note that some shock waves given by our solutions have been observed by the gas-bubble/liquid-mixture experiments. Effects on a bubbly liquid with respect to the bubble-liquid-viscosity, transverse-perturbation, bubble-liquid-nonlinearity and bubble-liquid-dispersion coefficient functions might be detected by the future gas-bubble/liquid-mixture experiments.
Towards time-dependent current-density-functional theory in the non-linear regime
Escartín, J. M.; Vincendon, M.; Dinh, P. M.; Suraud, E.; Romaniello, P.; Reinhard, P.-G.
2015-02-28
Time-Dependent Density-Functional Theory (TDDFT) is a well-established theoretical approach to describe and understand irradiation processes in clusters and molecules. However, within the so-called adiabatic local density approximation (ALDA) to the exchange-correlation (xc) potential, TDDFT can show insufficiencies, particularly in violently dynamical processes. This is because within ALDA the xc potential is instantaneous and is a local functional of the density, which means that this approximation neglects memory effects and long-range effects. A way to go beyond ALDA is to use Time-Dependent Current-Density-Functional Theory (TDCDFT), in which the basic quantity is the current density rather than the density as in TDDFT. This has been shown to offer an adequate account of dissipation in the linear domain when the Vignale-Kohn (VK) functional is used. Here, we go beyond the linear regime and we explore this formulation in the time domain. In this case, the equations become very involved making the computation out of reach; we hence propose an approximation to the VK functional which allows us to calculate the dynamics in real time and at the same time to keep most of the physics described by the VK functional. We apply this formulation to the calculation of the time-dependent dipole moment of Ca, Mg and Na{sub 2}. Our results show trends similar to what was previously observed in model systems or within linear response. In the non-linear domain, our results show that relaxation times do not decrease with increasing deposited excitation energy, which sets some limitations to the practical use of TDCDFT in such a domain of excitations.
Cluster electric current density measurements within a magnetic flux rope in the plasma sheet
NASA Technical Reports Server (NTRS)
Slavin, J. A.; Lepping, R. P.; Gjerloev, J.; Goldstein, M. L.; Fairfield, D. H.; Acuna, M. H.; Balogh, A.; Dunlop, M.; Kivelson, M. G.; Khurana, K.
2003-01-01
On August 22, 2001 all 4 Cluster spacecraft nearly simultaneously penetrated a magnetic flux rope in the tail. The flux rope encounter took place in the central plasma sheet, Beta(sub i) approx. 1-2, near the leading edge of a bursty bulk flow. The "time-of-flight" of the flux rope across the 4 spacecraft yielded V(sub x) approx. 700 km/s and a diameter of approx.1 R(sub e). The speed at which the flux rope moved over the spacecraft is in close agreement with the Cluster plasma measurements. The magnetic field profiles measured at each spacecraft were first modeled separately using the Lepping-Burlaga force-free flux rope model. The results indicated that the center of the flux rope passed northward (above) s/c 3, but southward (below) of s/c 1, 2 and 4. The peak electric currents along the central axis of the flux rope predicted by these single-s/c models were approx.15-19 nA/sq m. The 4-spacecraft Cluster magnetic field measurements provide a second means to determine the electric current density without any assumption regarding flux rope structure. The current profile determined using the curlometer technique was qualitatively similar to those determined by modeling the individual spacecraft magnetic field observations and yielded a peak current density of 17 nA/m2 near the central axis of the rope. However, the curlometer results also showed that the flux rope was not force-free with the component of the current density perpendicular to the magnetic field exceeding the parallel component over the forward half of the rope, perhaps due to the pressure gradients generated by the collision of the BBF with the inner magnetosphere. Hence, while the single-spacecraft models are very successful in fitting flux rope magnetic field and current variations, they do not provide a stringent test of the force-free condition.
NASA Astrophysics Data System (ADS)
Gurioli, L.; Zanella, E.; Pareschi, M. T.; Lanza, R.
2007-05-01
To assess ways in which the products of explosive eruptions interact with human settlements, we performed volcanological and rock magnetic analyses on the deposits of the A.D. 79 eruption at the Pompeii excavations (Italy). During this eruption the Roman town of Pompeii was covered by 2.5 m of fallout pumice and then partially destroyed by pyroclastic density currents (PDCs). Anisotropy of magnetic susceptibility measurements performed on the fine matrix of the deposits allowed the quantification of the variations in flow direction and emplacement mechanisms of the parental PDCs that entered the town. These results, integrated with volcanological field investigations, revealed that the presence of buildings, still protruding through the fallout deposits, strongly affected the distribution and accumulation of the erupted products. All of the PDCs that entered the town, even the most dilute ones, were density stratified currents in which interaction with the urban fabric occurred in the lower part of the current. The degree of interaction varied mainly as a function of obstacle height and density stratification within the current. For examples, the lower part of the EU4pf current left deposits up to 3 m thick and was able to interact with 2- to 4-m-high obstacles. However, a decrease in thickness and grain size of the deposits across the town indicates that even though the upper portion of the current was able to decouple from the lower portion, enabling it to flow over the town, it was not able to fully restore the sediment supply to the lower portion in order to maintain the deposition observed upon entry into the town.
X-ray diagnostic for current density profiling relativistic electron beams in vacuum and gas
Slaughter, D.; Koppel, L.; Smith, J.
1986-02-15
An x-ray imaging technique has been studied for the purpose of observing the current density profile in a high-current relativistic electron beam (50 MeV, 10 kA). Calculations and measurements of energy spectra and intensities are in good agreement. Results indicate sufficient photon yield for pinhole imaging when the beam deposits a small part of its energy in high-Z gas or a thin high-Z foil. Characteristic L and K x-ray emission is not found not be a reliable technique due to strong L and K shell fluorescence in the presence of intense bremsstrahlung radiation. It is also found that at pressures on the order of one atmosphere, the density of energy deposition in a gas cell is too small to generate sufficient photon yield for time-resolved measurements.
High current density and low turn-on field from aligned Cd(OH)2 nanosheets
NASA Astrophysics Data System (ADS)
Bagal, Vivekanand S.; Patil, Girish P.; Deore, Amol B.; Baviskar, Prashant K.; Suryawanshi, Sachin R.; More, Mahendra A.; Chavan, Padmakar G.
2016-04-01
High current density of 9.8 mA/cm2 was drawn at an applied field of 4.1 V/μm from aligned Cd(OH)2 nanosheets and low turn-on field of 1.4 V/μm was found for the emission current density of 10 μA/cm2. The aligned Cd(OH)2 nanosheets were synthesized by CBD technique on Cadmium foil. To the best of our knowledge this is the first report on the field emission studies of Cd(OH)2 nanosheets. Simple synthesis route coupled with superior field emission properties indicate the possible use of Cd(OH)2 nanosheets for micro/nanoelectronic devices.
NASA Astrophysics Data System (ADS)
Kießig, Birgit; Schäfer, Roland; von Löhneysen, Hilbert
2014-01-01
We report on the application of a feedback-controlled electromigration technique for the formation of nanometre-sized gaps in mesoscopic gold wires and rings. The effect of current density and temperature, linked via Joule heating, on the resulting gap size is investigated. Our experiments include in situ measurements of the evolution of the electrical resistance and of the structure of the device during electromigration. Experimentally, a good thermal coupling to the substrate turned out to be crucial to reach electrode spacings below 10 nm and to avoid overall melting of the nanostructures. This finding is supported by numerical calculations of the current-density and temperature profiles for structure layouts subjected to electromigration. The numerical method can be used for optimizing the layout so as to predetermine the location where electromigration leads to the formation of a gap.
Morphological features of the copper surface layer under sliding with high density electric current
Fadin, V. V.; Aleutdinova, M. I.; Rubtsov, V. Ye.; Aleutdinova, V. A.
2015-10-27
Conductivity and wear intensity of copper under the influence of dry friction and electric current with contact density higher 100 A/cm{sup 2} are presented. It is shown that an increase in hardness and heat outflow from a friction zone leads to the reduction of wear intensity and current contact density increase corresponding to the beginning of catastrophic wear. Structural changes, such as the formation of FeO oxide and α-Fe particles in the copper surface layer, have also been found. It is observed that a worn surface is deformed according to a viscous liquid mechanism. Such singularity is explained in terms of appearance of high-excited atomic states in deforming micro-volumes near contact spots that lead to easy stress relaxation by local plastic shears in the vicinity of stress concentrators. In common this effect allows to achieve high wear resistance.
NASA Astrophysics Data System (ADS)
Li, Yanggui; Geng, Xingguo; Wang, Heping; Zhuang, Xin; Ouyang, Jie
2016-06-01
The frontal instability of lock-exchange density currents is numerically investigated using dissipative particle dynamics (DPD) at the mesoscopic particle level. For modeling two-phase flow, the “color” repulsion model is adopted to describe binary fluids according to Rothman-Keller method. The present DPD simulation can reproduce the flow phenomena of lock-exchange density currents, including the lobe-and-cleft instability that appears at the head, as well as the formation of coherent billow structures at the interface behind the head due to the growth of Kelvin-Helmholtz instability. Furthermore, through the DPD simulation, some small-scale characteristics can be observed, which are difficult to be captured in macroscopic simulation and experiment.
Broadband external cavity tunable quantum dot lasers with low injection current density.
Lv, X Q; Jin, P; Wang, W Y; Wang, Z G
2010-04-26
Broadband grating-coupled external cavity laser, based on InAs/GaAs quantum dots, is achieved. The device has a wavelength tuning range from 1141.6 nm to 1251.7 nm under a low continuous-wave injection current density (458 A/cm(2)). The tunable bandwidth covers consecutively the light emissions from both the ground state and the 1st excited state of quantum dots. The effects of cavity length and antireflection facet coating on device performance are studied. It is shown that antireflection facet coating expands the tuning bandwidth up to ~150 nm, accompanied by an evident increase in threshold current density, which is attributed to the reduced interaction between the light field and the quantum dots in the active region of the device.
Effect of particle entrainment on the runout of pyroclastic density currents
NASA Astrophysics Data System (ADS)
Fauria, Kristen E.; Manga, Michael; Chamberlain, Michael
2016-09-01
Pyroclastic density currents (PDCs) can erode soil and bedrock, yet we currently lack a mechanistic understanding of particle entrainment that can be incorporated into models and used to understand how PDC bulking affects runout. Here we quantify how particle splash, the ejection of particles due to impact by a projectile, entrains particles into dilute PDCs. We use scaled laboratory experiments to measure the mass of sand ejected by impacts of pumice, wood, and nylon spheres. We then derive an expression for particle splash that we validate with our experimental results as well as results from seven other studies. We find that the number of ejected particles scales with the kinetic energy of the impactor and the depth of the crater generated by the impactor. Last, we use a one-dimensional model of a dilute, compressible density current—where runout distance is controlled by air entrainment and particle exchange with the substrate—to examine how particle entrainment by splash affects PDC density and runout. Splash-driven particle entrainment can increase the runout distance of dilute PDCs by an order of magnitude. Furthermore, the temperature of entrained particles greatly affects runout and PDCs that entrain ambient temperature particles runout farther than those that entrain hot particles. Particle entrainment by splash therefore not only increases the runout of dilute PDCs but demonstrates that the temperature and composition of the lower boundary have consequences for PDC density, temperature, runout, hazards and depositional record.
Current density and poloidal magnetic field for toroidal elliptic plasmas with triangularity
Martin, P.; Haines, M.G.; Castro, E.
2005-08-15
Changes in the poloidal magnetic field around a tokamak magnetic surface due to different values of triangularity and ellipticity are analyzed in this paper. The treatment here presented allows the determination of the poloidal magnetic field from knowledge of the toroidal current density. Different profiles of these currents are studied. Improvements in the analytic forms of the magnetic surfaces have also been found. The treatment has been performed using a recent published system of coordinates. Suitable analytic equations have been used for the elliptic magnetic surfaces with triangularity and Shafranov shift.
Magnetic resonance electrical impedance tomography (MREIT): conductivity and current density imaging
NASA Astrophysics Data System (ADS)
Seo, Jin Keun; Kwon, Ohin; Woo, Eung Je
2005-01-01
This paper reviews the latest impedance imaging technique called Magnetic Resonance Electrical Impedance Tomography (MREIT) providing information on electrical conductivity and current density distributions inside an electrically conducting domain such as the human body. The motivation for this research is explained by discussing conductivity changes related with physiological and pathological events, electromagnetic source imaging and electromagnetic stimulations. We briefly summarize the related technique of Electrical Impedance Tomography (EIT) that deals with cross-sectional image reconstructions of conductivity distributions from boundary measurements of current-voltage data. Noting that EIT suffers from the ill-posed nature of the corresponding inverse problem, we introduce MREIT as a new conductivity imaging modality providing images with better spatial resolution and accuracy. MREIT utilizes internal information on the induced magnetic field in addition to the boundary current-voltage measurements to produce three-dimensional images of conductivity and current density distributions. Mathematical theory, algorithms, and experimental methods of current MREIT research are described. With numerous potential applications in mind, future research directions in MREIT are proposed.