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1

A Microkelvin Magnetic Flux Noise Thermometer  

NASA Astrophysics Data System (ADS)

Due to its non-driven nature, noise thermometry intrinsically is the method of choice when minimal heat input during the temperature measurement is required. Our noise thermometer, experimentally characterized for temperatures between 42 K and 0.8 K, is a magnetic Johnson noise thermometer. The noise source is a cold-worked high purity copper cylinder, 5 mm in diameter and 20 mm long. The magnetic flux fluctuations generated by the electrons' Brownian motion is measured inductively by two dc-SQUID magnetometers simultaneously. Cross-correlation of the two channels leads to reduction of parasitic noise by more than one order of magnitude which allows for measuring the tiny noise powers at microkelvin temperatures.

Rothfuß, D.; Reiser, A.; Fleischmann, A.; Enss, C.

2014-06-01

2

Magnetic flux noise in copper oxide superconductors  

SciTech Connect

Magnetic flux noise and flux creep in thin films and single crystals of YBa{sub 2}Cu{sub 3}O{sub 7-x}, Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+x}, Tl{sub 2}Ca{sub 2}Ba{sub 2}Cu{sub 3}O{sub x}, and TlCa{sub 2}Ba{sub 2}Cu{sub 3}O{sub x} are measured with a superconducting quantum interference device (SQUID). The noise power spectrum generally scales as 1/f (f is frequency) from 1 Hz to 1 kHz, increases with temperature, and decreases in higher-quality films. It is proportional to the magnetic field B in which the sample is cooled, at least in the range 0.1 mT < B < 3 mT. A model of thermally activated vortex motion is developed which explains the dependence of the noise on frequency, temperature, current, and applied magnetic field. The pinning potential is idealized as an ensemble of double wells, each with a different activation energy separating the two states. From the noise measurements, this model yields the distribution of pinning energies in the samples, the vortex hopping distance, the number density of mobile vortices, and the restoring force on a vortex at a typical pinning site. The distribution of pinning energies in YBa{sub 2}Cu{sub 3}O{sub 7-x} shows a broad peak below 0.1 eV. The small ambient magnetic field, and the detection of noise even in the absence of a driving force, insure that the measured pinning energies are characteristic of isolated vortices near thermal equilibrium. The observed vortex density in fields much less than 0.1 mT is too large to be explained by the ambient field, suggesting a mechanism intrinsic to the sample which produces trapped vortices.

Ferrari, M.J.

1991-11-01

3

Investigations of magnetic flux leakage and magnetic Barkhausen noise signals from pipeline steel  

NASA Astrophysics Data System (ADS)

Magnetic flux leakage (MFL) is used for in-line inspection of underground oil and gas pipelines. MFL signals are sensitive to the line pressure which should be taken into account when estimating the sizes of defects. MFL signals have been studied from electrochemically milled pits with 50% penetration in pipeline steel under the influence of different axial or circumferential stresses and a range of magnetic flux densities. Magnetic Barkhausen noise (MBN) measurements have been used to study the changes in the direction of the magnetic easy axis of the pipe wall. These help one to understand the stress-dependent MFL results. MBN measurements have also been used to estimate the directional magnetic anisotropies of the steels.

Mandal, K.; Dufour, D.; Krause, T. W.; Atherton, D. L.

1997-03-01

4

Correlation between magnetic flux leakage and magnetic Barkhausen noise: Stress dependence in pipeline steel  

NASA Astrophysics Data System (ADS)

The tensile-stress dependence of angular-dependent magnetic Barkhausen noise (MBN) was investigated on the inner and outer surfaces of four sections of pipeline steel. Stresses up to 330 MPa or 70% of the yield strength of the steel were applied in either the circumferential or axial pipe direction. An effective MBN energy (MBN ENERGY), defined as the time integral of the squared voltage MBN signal, was calculated. The variation of the ratio of the MBN ENERGY in the pipe axis direction to that in the circumferential direction was correlated with the stress-dependent variation of the amplitude of radial magnetic flux leakage (MFL pp) signals measured on the outside of the pipe from simulated corrosion pits (13 mm diameter ball-milled pits) in sections of pipeline steel under axial magnetization that was applied from the outer surface of the pipe. The percentage variation of the MFL pp signal with stress was found to agree within uncertainty, with the MFL pp signal variation with stress from defects placed on the pipe wall far surface, at 1.1-1.2 T pipe wall flux density. The MBN ENERGY ratio is proposed as a measure of the relative anisotropy in line with the magnetizing field to that perpendicular to it. It is this relative anisotropy that defines the degree to which lines of flux may pass around a high reluctance defect by either remaining within the steel or being forced into the air.

Krause, T. W.; Mandal, K.; Hauge, C.; Weyman, P.; Sijgers, B.; Atherton, D. L.

1997-05-01

5

Noise-Produced Patterns in Images Constructed from Magnetic Flux Leakage Data  

E-print Network

Magnetic flux leakage measurements help identify the position, size and shape of corrosion-related defects in steel casings used to protect boreholes drilled into oil and gas reservoirs. Images constructed from magnetic flux leakage data contain patterns related to noise inherent in the method. We investigate the patterns and their scaling properties for the case of delta-correlated input noise, and consider the implications for the method's ability to resolve defects. The analytical evaluation of the noise-produced patterns is made possible by model reduction facilitated by large-scale approximation. With appropriate modification, the approach can be employed to analyze noise-produced patterns in other situations where the data of interest are not measured directly, but are related to the measured data by a complex linear transform involving integrations with respect to spatial coordinates.

Pimenova, Anastasiya V; Levesley, Jeremy; Elkington, Peter; Bacciarelli, Mark

2015-01-01

6

Low excess flux noise in YBa2Cu3O7-x dc SQUIDs cooled in static magnetic fields  

Microsoft Academic Search

We have investigated the effect of device geometry on the excess low frequency 1\\/f noise of thin-film YBCO dc SQUIDs cooled in static magnetic fields. The key factor in eliminating this noise is the reduction of the linewidth of the SQUID loop to a value below the average separation of the flux vortices. The spectral density of the flux noise

E. Dantsker; S. Tanaka; P. A. Nilsson; R. Kleiner; John Clarke

1997-01-01

7

Flux controlled magnetic barkhausen noise measurements on grain oriented electrical steels  

NASA Astrophysics Data System (ADS)

This paper investigates magnetic Barkhausen noise (MBN) in grain-oriented electrical steel using flux-control and its relationship to power losses obtained from standard Epstein tests. The effects of flux, frequency, grain size and texture in five samples of similar thickness with known core loss values were examined. A comparison with a non-oriented sample indicated that the grain oriented Si-Fe has higher magnetic anisotropy and greater MBN signal variability with position, which is associated with its larger grain size. Angular MBN measurements demonstrated a major easy axis along the rolling direction (RD) that was attributed to its Goss texture. An inverse relation between MBN energy and core loss values was observed. The inverse relation was associated with a greater proportion of Barkhausen events with magnetization changes projected out of the plane of the sample. This generates microscopic eddy currents that do not add vectorily to the bulk eddy currents and thereby reduces excess power losses. Post publication note (25 July 2012): Author affiliations were corrected.

Samimi, Arash A.; Krause, Thomas W.; Clapham, Lynann

2012-07-01

8

The study of a racetrack-shaped defect in ferromagnetic steel by magnetic Barkhausen noise and flux leakage measurements  

NASA Astrophysics Data System (ADS)

Magnetic Barkhausen noise (MBN) and magnetic flux leakage (MFL) measurements are useful magnetic nondestructive techniques used for various applications. MBN measurements have been used here to study stress distributions around a racetrack-shaped defect made in the wall of a steel pipe. The study shows a stress concentration factor of ˜3 for a racetrack-shaped defect with 50% wall penetration. MFL signals from the same defect have also been studied under various circumferential hoop stresses. It is observed that the external stress can change the MFL signal by more than 40% and the effect of stress is maximum when the pipe wall flux density is ˜1.2 T.

Mandal, K.; Cramer, Th; Atherton, D. L.

2000-03-01

9

Magnetic flux density measurement in magnetic resonance electrical impedance tomography using a low-noise current source  

Microsoft Academic Search

Current injected into an electrically conducting object induces distributions of magnetic flux density as well as voltage and current density. In magnetic resonance electrical impedance tomography (MREIT), we measure the induced magnetic flux density using an MRI scanner to reconstruct cross-sectional conductivity images of the object. The current injection must be synchronized with a chosen MRI pulse sequence and the

Young Tae Kim; Pil Joong Yoo; Tong In Oh; Eung Je Woo

2011-01-01

10

The effects of defect depth and bending stress on magnetic Barkhausen noise and flux-leakage signals  

NASA Astrophysics Data System (ADS)

Magnetic-flux-leakage (MFL) measurements are used for corrosion inspection of buried oil and gas pipelines. MFL signals are sensitive to the depth of defects, such as corrosion pits, in the pipe wall and to external stresses generated mainly by the line pressure. MFL signal variations have been studied from the inside wall in the presence of various circumferential bending stresses and magnetic flux densities for three outside wall pits of depths 30%, 50% and 80% of the wall thickness. Magnetic Barkhausen noise (MBN) measurements have been used to study the stress-induced changes in the direction of the bulk magnetic easy axis and directional anisotropies of the pipe wall. These help one to understand the stress-dependent MFL results.

Mandal, K.; Corey, A.; Loukas, M. E.; Weyman, P.; Eichenberger, J.; Atherton, D. L.

1997-07-01

11

Rejection of seamless pipe noise in magnetic flux leakage data obtained from gas pipeline inspection  

NASA Astrophysics Data System (ADS)

Natural gas is traditionally transmitted from production facilities to customer locations through a vast pipeline network. A major segment of this network employs seamless pipes. This is especially true for smaller diameter transmission and distribution lines. Manufacturing process associated with the production of seamless pipes contribute to a helical variation in the pipe along the axis. The deformation introduces an artifact in the data obtained from MFL inspection of these pipelines. This seamless pipe noise is usually correlated with signals generated by defects and other elements (joints, tees, etc.) in pipelines, and can therefore, mask their indications in MFL data. This warrants the need for methods to improve signal-to-noise ratio (SNR) in MFL data from seamless pipes. This paper presents a technique for detecting signals in MFL data from seamless pipes. The approach processes the data in various steps. First, a wavelet based denoising technique is applied to reduce the noise due to instrumentation and other sources. An adaptive filtering approach is then applied to reject seamless noise in the data. Since the inspection of pipelines typically generates vast amounts of data, it is imperative that the algorithm be computationally efficient. The processing method has to be robust in that it should be data independent. The approach described in this paper meet these criteria. Results from application of the approach to data from field tests are presented.

Afzal, Muhammad; Udpa, Satish; Udpa, Lalita; Lord, William

2000-05-01

12

Modeling Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

The magnetic configuration hosting prominences can be a large-scale helical magnetic flux rope. As a necessary step towards future prominence formation studies, we report on a stepwise approach to study flux rope formation. We start with summarizing our recent three-dimensional (3D) isothermal magnetohydrodynamic (MHD) simulation where a flux rope is formed, including gas pressure and gravity. This starts from a static corona with a linear force-free bipolar magnetic field, altered by lower boundary vortex flows around the main polarities and converging flows towards the polarity inversion. The latter flows induce magnetic reconnection and this forms successive new helical loops so that a complete flux rope grows and ascends. After stopping the driving flows, the system relaxes to a stable helical magnetic flux rope configuration embedded in an overlying arcade. Starting from this relaxed isothermal endstate, we next perform a thermodynamic MHD simulation with a chromospheric layer inserted at the bottom. As a result of a properly parametrized coronal heating, and due to radiative cooling and anisotropic thermal conduction, the system further relaxes to an equilibrium where the flux rope and the arcade develop a fully realistic thermal structure. This paves the way to future simulations for 3D prominence formation.

Xia, Chun; Keppens, Rony

2014-01-01

13

Solar Magnetic Flux Ropes  

E-print Network

The most probable initial magnetic configuration of a CME is a flux rope consisting of twisted field lines which fill the whole volume of a dark coronal cavity. The flux ropes can be in stable equilibrium in the coronal magnetic field for weeks and even months, but suddenly they loose their stability and erupt with high speed. Their transition to the unstable phase depends on the parameters of the flux rope (i.e., total electric current, twist, mass loading etc.), as well as on the properties of the ambient coronal magnetic field. One of the major governing factors is the vertical gradient of the coronal magnetic field which is estimated as decay index (n). Cold dense prominence material can be collected in the lower parts of the helical flux tubes. Filaments are therefore good tracers of the flux ropes in the corona, which become visible long before the beginning of the eruption. The perspectives of the filament eruptions and following CMEs can be estimated by the comparison of observed filament heights with...

Filippov, Boris; Srivastava, Abhishek K; Uddin, Wahab

2015-01-01

14

Permanent magnet flux-biased magnetic actuator with flux feedback  

Microsoft Academic Search

The invention is a permanent magnet flux-biased magnetic actuator with flux feedback for adjustably suspending an element on a single axis. The magnetic actuator includes a pair of opposing electromagnets and provides bi-directional forces along the single axis to the suspended element. Permanent magnets in flux feedback loops from the opposing electromagnets establish a reference permanent magnet flux-bias to linearize

Nelson J. Groom

1991-01-01

15

Magnetic flux transport by dipolarizing flux bundles  

NASA Astrophysics Data System (ADS)

A dipolarizing flux bundle (DFB) is a small magnetotail flux tube (typically < ~3 RE in XGSM and YGSM) with a significantly more dipolar magnetic field than its background. Dipolarizing flux bundles typically propagate earthward at a high speed from the near-Earth reconnection region. Knowledge of a DFB's flux transport properties leads to better understanding of near-Earth (X = -6 to -30 RE) magnetotail flux transport and thus conversion of magnetic energy to kinetic and thermal plasma energy following magnetic reconnection. We explore DFB properties with a statistical study using data from the Time History of Events and Macroscale Interactions during Substorms mission. To establish the importance of DFB flux transport, we compare it with transport by bursty bulk flows (BBFs) that typically envelop DFBs. Because DFBs coexist with flow bursts inside BBFs, they contribute >65% of BBF flux transport, even though they last only ~30% as long as BBFs. The rate of DFB flux transport increases with proximity to Earth and to the premidnight sector, as well as with geomagnetic activity and distance from the neutral sheet. Under the latter two conditions, the total flux transport by a typical DFB also increases. Dipolarizing flux bundles appear more often during increased geomagnetic activity. Since BBFs have been previously shown to be the major flux transporters in the tail, we conclude that DFBs are the dominant drivers of this transport. The occurrence rate of DFBs as a function of location and geomagnetic activity informs us about processes that shape global convection and energy conversion.

Liu, Jiang; Angelopoulos, V.; Zhou, Xu-Zhi; Runov, A.

2014-02-01

16

Permanent magnet flux-biased magnetic actuator with flux feedback  

NASA Technical Reports Server (NTRS)

The invention is a permanent magnet flux-biased magnetic actuator with flux feedback for adjustably suspending an element on a single axis. The magnetic actuator includes a pair of opposing electromagnets and provides bi-directional forces along the single axis to the suspended element. Permanent magnets in flux feedback loops from the opposing electromagnets establish a reference permanent magnet flux-bias to linearize the force characteristics of the electromagnets to extend the linear range of the actuator without the need for continuous bias currents in the electromagnets.

Groom, Nelson J. (inventor)

1991-01-01

17

4914 IEEE TRANSACTIONS ON MAGNETICS, VOL 32, NO 5, SEPTEMBER 1996 e Duality that Relates Magnetic Noise to Electric Shot Noise  

E-print Network

-noise flux-fluctuations to a magnetic entity similar to the dc current in the electric case. The present work that relates the time-derivative of the magnetic flux to the electric current. The magnetic noise measurements to a magnetic entity similar to the dc current in the electric case. The present work shows both theoretically

Paperno, Eugene

18

Magnetic balltracking: Tracking the photospheric magnetic flux  

NASA Astrophysics Data System (ADS)

Context. One aspect of understanding the dynamics of the quiet Sun is to quantify the evolution of the flux within small-scale magnetic features. These features are routinely observed in the quiet photosphere and were given various names, such as pores, knots, magnetic patches. Aims: This work presents a new algorithm for tracking the evolution of the broad variety of small-scale magnetic features in the photosphere, with a precision equal to the instrumental resolution. Methods: We have developed a new technique to track the evolution of the individual magnetic features from magnetograms, called "magnetic balltracking". It quantifies the flux of the tracked features, and it can track the footpoints of magnetic field lines inferred from magnetic field extrapolation. The algorithm can detect and quantify flux emergence, as well as flux cancellation. Results: The capabilities of magnetic balltracking are demonstrated with the detection and the tracking of two cases of magnetic flux emergence that lead to the brightening of X-ray loops. The maximum emerged flux ranges from 1018 Mx to 1019 Mx (unsigned flux) when the X-ray loops are observed. Movies associated to Figs. 6 and 18 are available in electronic form at http://www.aanda.org

Attie, R.; Innes, D. E.

2015-02-01

19

Flux Noise in SQUIDs due to Hyperfine Interactions of Spins on Metals  

NASA Astrophysics Data System (ADS)

Recent experiments at Stanford and Wisconsin have found evidence for magnetic defects on the surface of elemental metals like aluminum, niobium, and gold, but a much reduced signal on silicon. Fluctuations of these impurities are the source of flux noise in SQUIDs, and a major obstacle to the realization of using superconducting qubits to construct quantum computers. We discuss the possibility that the flux noise arises from hyperfine interactions and ways to test this hypothesis.

Wu, Jiansheng; Yu, Clare

2010-03-01

20

Simulations of Magnetic Flux Emergence  

NASA Astrophysics Data System (ADS)

Magnetic flux emerges from the solar surface on a wide range of scales. We review recent simulations of both large and small scale flux emergence. In our own simulations, we represent the magnetic flux produced by the global dynamo as uniform, untwisted, horizontal field advected into the simulation domain by supergranule scale inflows at the bottom. Our computational domain extends from the temperature minimum (half a megameter above the visible surface) to 20 Mm below the surface, which is 10% of the depth of the convection zone, but contains 2/3 of its scale heights. We investigate how magnetic flux rises through the upper solar convection zone and emerges through the surface. Convective up-flows and magnetic buoyancy bring field toward the surface. Convective down-flows pin down field and prevent its rise. Most of the field gets pumped downward by the convection, but some field rises to the surface. The convective motions both confine the flux concentrations (without the need for twist) and shred them. This process creates a hierarchy of magnetic loops with smaller loops riding "piggy-back", in a serpentine pattern, on larger loops. As a result, magnetic flux emerges in a mixed polarity, "pepper and salt" pattern. The small loops appear as horizontal field over granules with their vertical legs in the bounding intergranular lanes. The fields are quickly swept into the intergranular lanes. As the larger, parent, flux concentrations reach the surface with their legs rooted in the the downflow boundaries of the underlying, supergranule-scale, convective cells near the bottom of the simulation domain, the surface field counter-streams into separate, opposite polarity concentrations, creating pores and spots. The subsurface magnetic field lines of the pores and spots formed by the magneto-convection (without being imposed as an initial condition) are braided, some tightly, some loosely and they connect in complicated ways to the surrounding field at large depths. The pores evolve on the timescale of the underlying supergranules. Thus, long lives of solar active regions imply that they are rooted in larger, more slowly evolving, deeper convective structures. Based on these simulations we summarize the robust properties of emerging magnetic flux.

Stein, Robert; Nordlund, Aake

21

MAGNETIC FLUX BALANCE IN THE HELIOSPHERE  

SciTech Connect

Understanding the evolution of magnetic flux in the heliosphere remains an unresolved issue. The current solar minimum between cycles 23 and 24 is anomalously long, which gives rare insight into the long-term evolution of heliospheric magnetic flux when the coronal mass ejection (CME) rate and the flux emergence rate from CMEs were very low. The precipitous drop of heliospheric magnetic flux to levels lower than have ever been observed directly shows that there may be a persistent loss of open magnetic flux through disconnection, the reconnection between opposite polarity heliospheric magnetic field lines relatively near the Sun (beneath the Alfven point). Here, we develop a model for the levels of magnetic flux in the inner heliosphere balancing new flux injected by CMEs, flux lost through disconnection, and closed flux lost through interchange reconnection near the Sun. This magnetic flux balance is a fundamental property that regulates the plasma and radiation environment of our solar system.

Schwadron, N. A. [Department of Astronomy, Boston University, Boston, MA 02215 (United States); Connick, D. E.; Smith, C. [Physics Department, Space Science Center, University of New Hampshire, Durham, NH (United States)

2010-10-20

22

Force sensor using changes in magnetic flux  

NASA Technical Reports Server (NTRS)

A force sensor includes a magnetostrictive material and a magnetic field generator positioned in proximity thereto. A magnetic field is induced in and surrounding the magnetostrictive material such that lines of magnetic flux pass through the magnetostrictive material. A sensor positioned in the vicinity of the magnetostrictive material measures changes in one of flux angle and flux density when the magnetostrictive material experiences an applied force that is aligned with the lines of magnetic flux.

Pickens, Herman L. (Inventor); Richard, James A. (Inventor)

2012-01-01

23

Topological Quantization of the Magnetic Flux  

Microsoft Academic Search

The quantization of the magnetic flux in superconducting rings is studied in the frame of a topological model of electromagnetism that gives a topological formulation of electric charge quantization. It turns out that the model also embodies a topological mechanism for the quantization of the magnetic flux with the same relation between the fundamental units of magnetic charge and flux

Antonio F. Rańada; José Luis Trueba

2006-01-01

24

Magnetic-flux generation by multistep transfer  

Microsoft Academic Search

to generate large amounts of electromagnetic energy but also to produce in the load a magnetic flux exceeding the initial flux introduced into the compression circuit, Cog., in supplying high-power explosive magnetic generators working in the megajoule range and above. A method exists [1-3] for producing a magnetic flux in an inductive load, which has been implemented in various devices

V. A. Davydov; V. K. Chernyshev

1981-01-01

25

Measurement of Integrated Low Frequency Flux Noise in Superconducting Flux/Phase Qubits  

SciTech Connect

We measured the integrated low frequency flux noise ({approx}1 m{phi}{sub 0}) of an rf SQUID as a flux qubit by fitting the resonant peaks from photon assistant tunneling (PAT). The energy relaxation time Tl between the ground and first excited states in the same potential well, measured directly in time domain, is 3 ns. From these results we identified low frequency flux noise as the dominant source of decoherence. In addition, we found that the measured values of integrated flux noise in three qubits of various sizes differ more than an order of magnitude.

Mao Bo; Qiu Wei; Han Siyuan [Department of Physics and Astronomy, University of Kansas, Lawrence, KS, US, 66044 (United States)

2008-11-07

26

Basal magnetic flux and the local solar dynamo  

NASA Astrophysics Data System (ADS)

The average unsigned magnetic flux density in magnetograms of the quiet Sun is generally dominated by instrumental noise. Due to the entirely different scaling behavior of the noise and the solar magnetic pattern it has been possible to determine the standard deviation of the Gaussian noise distribution and remove the noise contribution from the average unsigned flux density for the whole 15-yr SOHO/MDI data set and for a selection of SDO/HMI magnetograms. There is a very close correlation between the MDI disk-averaged unsigned vertical flux density and the sunspot number, and regression analysis gives a residual level of 2.7 G when the sunspot number is zero. The selected set of HMI magnetograms, which spans the most quiet phase of solar activity, has a lower limit of 3.0 G to the noise-corrected average flux density. These apparently cycle-independent levels may be identified as a basal flux density, which represents an upper limit to the possible flux contribution from a local dynamo, but not evidence for its existence. The 3.0 G HMI level, when scaled to the Hinode spatial resolution, translates to 3.5 G, which means that the much higher average flux densities always found by Hinode in quiet regions do not originate from a local dynamo. The contributions to the average unsigned flux density come almost exclusively from the extended wings of the probability density function, also in the case of HMI magnetograms with only basal-level magnetic flux. These wings represent intermittent magnetic flux. As the global dynamo continually feeds flux into the small scales at a fast rate through turbulent shredding, a hypothetical local dynamo may only be relevant to the Sun if its rate of flux build-up can be competitive. While the global dynamo appears to dominate the magnetic energy spectrum at all the resolved spatial scales, there are indications from the observed Hanle depolarization in atomic lines that the local dynamo may dominate the spectrum at scales of order 1-10 km and below.

Stenflo, J. O.

2012-11-01

27

The distribution of solar magnetic fluxes and the nonlinearity of stellar flux-flux relations  

Microsoft Academic Search

Synoptic maps for the 1975-1984 period are used to determine the time-dependent distribution function of magnetic flux densities in the solar atmosphere. The distribution function depends only on the global level of magnetic activity, and it is used to study how relations between magnetic flux densities and radiative flux densities from different temperature regimes in the outer atmosphere (derived from

C. J. Schrijver; K. L. Harvey; AZ Tucson

1989-01-01

28

One-sided fluxes -- A magnetic curiosity?  

Microsoft Academic Search

It is shown that a previously unknown class of magnetization patterns exists in planar structures which have the unique property that all the flux escapes from one surface with none leaving the other side. A simple case is a constant amplitude rotating vector magnetization where the sense of rotation dictates which surface has no flux. More complicated magnetization patterns are

John C. Mallinson

1973-01-01

29

Self-organization in magnetic flux ropes  

NASA Astrophysics Data System (ADS)

This cross-disciplinary special issue on 'Self-organization in magnetic flux ropes' follows in the footsteps of another collection of manuscripts dedicated to the subject of magnetic flux ropes, a volume on 'Physics of magnetic flux ropes' published in the American Geophysical Union's Geophysical Monograph Series in 1990 [1]. Twenty-four years later, this special issue, composed of invited original contributions highlighting ongoing research on the physics of magnetic flux ropes in astrophysical, space and laboratory plasmas, can be considered an update on our state of understanding of this fundamental constituent of any magnetized plasma. Furthermore, by inviting contributions from research groups focused on the study of the origins and properties of magnetic flux ropes in a variety of different environments, we have attempted to underline both the diversity of and the commonalities among magnetic flux ropes throughout the solar system and, indeed, the universe. So, what is a magnetic flux rope? The answer will undoubtedly depend on whom you ask. A flux rope can be as narrow as a few Larmor radii and as wide as the Sun (see, e.g., the contributions by Heli Hietala et al and by Angelous Vourlidas). As described below by Ward Manchester IV et al , they can stretch from the Sun to the Earth in the form of interplanetary coronal mass ejections. Or, as in the Swarthmore Spheromak Experiment described by David Schaffner et al , they can fit into a meter-long laboratory device tended by college students. They can be helical and line-tied (see, e.g., Walter Gekelman et al or J Sears et al ), or toroidal and periodic (see, e.g., John O'Bryan et al or Philippa Browning et al ). They can form in the low plasma beta environment of the solar corona (Tibor Török et al ), the order unity beta plasmas of the solar wind (Stefan Eriksson et al ) and the plasma pressure dominated stellar convection zones (Nicholas Nelson and Mark Miesch). In this special issue, Setthivoine You describes how canonical helicity can determine the result of reconnection and merging of multiple magnetic flux ropes, John Finn et al focus on diagnosing flux rope reconnection using quasi-separatrix layers, and Stefano Markidis et al investigate how a single flux rope can become unstable and begin to fall apart. With these many examples of different magnetized plasma structures, which can all be called magnetic flux ropes, the question still stands: just what is it that makes a volume of magnetized plasma a magnetic flux rope? There may not be a strict definition of a magnetic flux rope that everyone can agree on. Nonetheless, the ingredient common to all magnetic flux ropes is that the magnetic field lines that thread nearby plasma elements at one location along the flux rope must wind around and not diverge away from each other over a sufficiently long distance to look like a piece of an ordinary rope. In a way, it is similar to turbulence—you know it when you see it. The figures and illustrations included in this special issue provide plenty of examples of observed, measured, modeled and imagined magnetic flux ropes for you, the reader, to develop an appreciation of what different members of our research community mean by a magnetic flux rope. If you have never studied magnetic flux ropes, we hope that this special issue inspires you to look into their many mysteries. If magnetic flux ropes are already an integral part of your research, we hope the diversity of perspectives presented herein refresh your interest in the underlying plasma physics of whichever kind of magnetic flux rope you happen to be working with. References [1] Russell C T, Priest E R and Lee L-C 1990 Physics of Magnetic Flux Ropes AGU Geophysical Monograph Series vol 58 (Washington, DC: American Geophysical Union)

Lukin, Vyacheslav S.

2014-06-01

30

DOES MAGNETIC FLUX SUBMERGE AT FLUX CANCELATION SITES?  

E-print Network

, Laboratory for Astronomy and Solar Physics, Southwest Solar Station, c/o P.O. Box 26732, Tucson, AZ 85718 USADOES MAGNETIC FLUX SUBMERGE AT FLUX CANCELATION SITES? Karen L. Harvey Solar Physics Research Corporation, 4720 Calle Desecada, Tucson, AZ 85718, USA Harrison P. Jones NASA Goddard Space Flight Center

Schrijver, Karel

31

Complex inductance, excess noise, and surface magnetism in dc SQUIDs.  

PubMed

We have characterized the complex inductance of dc SQUIDs cooled to millikelvin temperatures. The SQUID inductance displays a rich, history-dependent structure as a function of temperature, with fluctuations of order 1 fH. At a fixed temperature, the SQUID inductance fluctuates with a 1/f power spectrum; the inductance noise is highly correlated with the conventional 1/f flux noise. The data are interpreted in terms of the reconfiguration of clusters of surface spins, with correlated fluctuations of effective magnetic moments and relaxation times. PMID:19792393

Sendelbach, S; Hover, D; Mück, M; McDermott, R

2009-09-11

32

Metamaterial anisotropic flux concentrators and magnetic arrays  

NASA Astrophysics Data System (ADS)

A metamaterial magnetic flux concentrator is investigated in detail in combination with a Halbach cylinder of infinite length. A general analytical solution to the field is determined and the magnetic figure of merit is determined for a Halbach cylinder with a flux concentrator. It is shown that an ideal flux concentrator will not change the figure of merit of a given magnet design, while the non-ideal will always lower it. The geometric parameters producing maximum figure of merit, i.e., the most efficient devices, are determined. The force and torque between two concentric Halbach cylinders with flux concentrators is determined and the maximum torque is found. Finally, the effect of non-ideal flux concentrators and the practical use of flux concentrators, as well as demagnetization issues, is discussed.

Bjřrk, R.; Smith, A.; Bahl, C. R. H.

2013-08-01

33

Magnetic Flux Emergence Along the Solar Cycle  

NASA Astrophysics Data System (ADS)

Flux emergence plays an important role along the solar cycle. Magnetic flux emergence builds sunspot groups and solar activity. The sunspot groups contribute to the large scale behaviour of the magnetic field over the 11 year cycle and the reversal of the North and South magnetic polarity every 22 years. The leading polarity of sunspot groups is opposite in the North and South hemispheres and reverses for each new solar cycle. However the hemispheric rule shows the conservation of sign of the magnetic helicity with positive and negative magnetic helicity in the South and North hemispheres, respectively. MHD models of emerging flux have been developed over the past twenty years but have not yet succeeded to reproduce solar observations. The emergence of flux occurs through plasma layers of very high gradients of pressure and changing of modes from a large ? to a low ? plasma (<1). With the new armada of high spatial and temporal resolution instruments on the ground and in space, emergence of magnetic flux is observed in tremendous detail and followed during their transit through the upper atmosphere. Signatures of flux emergence in the corona depend on the pre-existing magnetic configuration and on the strength of the emerging flux. We review in this paper new and established models as well as the recent observations.

Schmieder, B.; Archontis, V.; Pariat, E.

2014-12-01

34

Magnetic bearing. [for supplying magnetic fluxes  

NASA Technical Reports Server (NTRS)

A magnetic bearing is described which includes a pair of coaxial, toroidal, and permanent magnets having axially directed poles. Like poles of the permanent magnets are adjacent to each other, whereby the permanent magnets have a tendency to be urged apart along the common axis. An electromagnet is wound coaxially with the permanent magnets in such a manner that the poles are axially directed. Between the poles of each permanent magnet there is a low magnetic reluctance circuit including two series air gaps. Between the poles of the electromagnet a low reluctance path including only one air gap of each of the low magnetic reluctance circuits is provided. The low reluctance path for the electromagnet includes a ring axially translatable relative to the permanent magnets. The ring forms opposite faces of the air gaps in the magnetic circuits for each permanent magnet.

Studer, P. A. (inventor)

1975-01-01

35

FLUX EMERGENCE IN A MAGNETIZED CONVECTION ZONE  

SciTech Connect

We study the influence of a dynamo magnetic field on the buoyant rise and emergence of twisted magnetic flux ropes and their influence on the global external magnetic field. We ran three-dimensional MHD numerical simulations using the ASH code (anelastic spherical harmonics) and analyzed the dynamical evolution of such buoyant flux ropes from the bottom of the convection zone until the post-emergence phases. The global nature of this model can only very crudely and inaccurately represent the local dynamics of the buoyant rise of the implanted magnetic structure, but nonetheless allows us to study the influence of global effects, such as self-consistently generated differential rotation and meridional circulation, and of Coriolis forces. Although motivated by the solar context, this model cannot be thought of as a realistic model of the rise of magnetic structures and their emergence in the Sun, where the local dynamics are completely different. The properties of initial phases of the buoyant rise are determined essentially by the flux-rope's properties and the convective flows and consequently are in good agreement with previous studies. However, the effects of the interaction of the background dynamo field become increasingly strong as the flux ropes evolve. During the buoyant rise across the convection zone, the flux-rope's magnetic field strength scales as B{proportional_to}{rho}{sup {alpha}}, with {alpha} {approx}< 1. An increase of radial velocity, density, and current density is observed to precede flux emergence at all longitudes. The geometry, latitude, and relative orientation of the flux ropes with respect to the background magnetic field influences the resulting rise speeds, zonal flow amplitudes (which develop within the flux ropes), and the corresponding surface signatures. This influences the morphology, duration and amplitude of the surface shearing, and the Poynting flux associated with magnetic flux-rope emergence. The emerged magnetic flux influences the system's global polarity, leading in some cases to a polarity reversal while inhibiting the background dynamo from doing so in others. The emerged magnetic flux is slowly advected poleward while being diffused and assimilated by the background dynamo field.

Pinto, R. F.; Brun, A. S., E-mail: rui.pinto@cea.fr [Laboratoire AIM Paris-Saclay, CEA/Irfu Universite Paris-Diderot CNRS/INSU, F-91191 Gif-sur-Yvette (France)

2013-07-20

36

Modeling the Sun's open magnetic flux  

NASA Astrophysics Data System (ADS)

Context: .The heliospheric magnetic field can be extrapolated from the photospheric field distribution using models based upon various approximations. Such models are required for the reconstruction of the open solar magnetic flux prior to the time of direct measurements on the basis of surface flux transport simulations. Aims: .We evaluate the consistency of extrapolation models with direct measurements of the heliospheric magnetic field. Furthermore, we study whether extrapolations on the basis of a surface flux transport model for the photospheric magnetic field reproduce the temporal evolution of the measured near-Earth magnetic field. Methods: .We use the potential field source surface (PFSS) model and the current sheet source surface (CSSS) model to extrapolate the heliospheric field on the basis of the Wilcox Solar Observatory (WSO) synoptic maps of the solar surface field from 1976-2005. The results are compared with the near-Earth measurements of the radial heliospheric field and its independence of latitude found with Ulysses. Furthermore, we determine extrapolations on the basis of photospheric flux distributions computed with a surface flux transport code, using as input sunspot group areas from the SOON database. Results: .The CSSS model based upon WSO data, with a source surface located at ?10~R? and cusp surface at 1.7~R?, yields the best agreement with the measurements. The flux transport simulations reproduce the observed surface flux together with the open flux if the tilt angle of emerging bipolar magnetic regions is smaller than commonly assumed, but consistent with sunspot observations. Conclusions: .The CSSS model with a source surface in the vicinity of the Alfvénic point of the solar wind leads to a good extrapolation of the heliospheric field from solar surface data. A surface flux transport model based upon sunspot data with consistently calibrated tilt angles reproduces the observed evolution of the solar total open flux.

Schüssler, M.; Baumann, I.

2006-12-01

37

Photospheric processes and magnetic flux tubes  

Microsoft Academic Search

New high-resolution observations reveal that small-scale magnetic flux concentrations have a delicate substructure on a spatial scale of 0.1''. Their basic structure can be interpreted in terms of a magnetic flux sheet or tube that vertically extends through the ambient weak-field or field-free atmosphere with which it is in mechanical equilibrium. A more refined interpretation comes from new three-dimensional magnetohydrodynamic

Oskar Steiner

2007-01-01

38

Magnetic flux generator for balanced membrane loudspeaker  

Microsoft Academic Search

This paper reports the development of a magnetic flux generator with an application in a hearing aid loudspeaker produced in microsystem technology (MST). The technology plans for two different designs for the magnetic flux generator utilizing a softmagnetic substrate or electroplated NiCoFe as core material are presented and the production and characterization of four different mono- and double-layer planar coil

Jörg Rehder; Pirmin Rombach; Ole Hansen

2002-01-01

39

Poloidal magnetic flux control experiments in Rotamak  

Microsoft Academic Search

A new set of equilibrium coils has been installed in cylindrical chamber rotamak to allow for an active control of poloidal magnetic flux in 40-ms plasma discharges. The coils are powered by programmable current source with 1-3 ms response time. The coils allow controlling both the shape of the plasma and the boundary flux magnitude. Without the current in the

Yuri Petrov; Xiaokang Yang; Tian-Sen Huang

2008-01-01

40

Magnetic refrigeration using flux compression in superconductors  

NASA Technical Reports Server (NTRS)

The feasibility of using flux compression in high-temperature superconductors to produce the large time-varying magnetic fields required in a field cycled magnetic refrigerator operating between 20 K and 4 K is presently investigated. This paper describes the refrigerator concept and lists limitations and advantages in comparison with conventional refrigeration techniques. The maximum fields obtainable by flux compression in high-temperature supercoductor materials, as presently prepared, are too low to serve in such a refrigerator. However, reports exist of critical current values that are near usable levels for flux pumps in refrigerator applications.

Israelsson, U. E.; Strayer, D. M.; Jackson, H. W.; Petrac, D.

1990-01-01

41

Ising-Glauber Spin Cluster Model for Temperature-Dependent Magnetization Noise in SQUIDs  

NASA Astrophysics Data System (ADS)

Clusters of interacting two-level-systems, likely due to Farbe+(F+) centers at the metal-insulator interface, are shown to self-consistently lead to 1 /f? magnetization noise [with ? (T )?1 ] in SQUIDs. Model calculations, based on a new method of obtaining correlation functions, explains various puzzling experimental features. It is shown why the inductance noise is inherently temperature dependent while the flux noise is not, despite the same underlying microscopics. Magnetic ordering in these systems, established by three-point correlation functions, explains the observed flux- inductance-noise cross correlations. Since long-range ferromagnetic interactions are shown to lead to a more weakly temperature dependent flux noise when compared to short-range interactions, the time reversal symmetry of the clusters is also not likely broken by the same mechanism which mediates surface ferromagnetism in nanoparticles and thin films of the same insulator materials.

De, Amrit

2014-11-01

42

Topological Quantization of the Magnetic Flux  

Microsoft Academic Search

We discuss the quantum problem of a charged particle in a multiply connected region encircling a magnetic tube, using a theory in which space and internal coordinates are derived from the parameters of a linear group of transformations (group space quantum theory). Based only on symmetry considerations, we show that, the magnetic flux in the tube must be quantized in

Daniel Wisnivesky

2000-01-01

43

Photospheric processes and magnetic flux tubes  

E-print Network

In the first part of these lecture notes, new high-resolution observations of small-scale magnetic flux concentrations are presented and compared to results from new three-dimensional magnetohydrodynamic simulations. Special attention is paid to the physics of faculae and to new three-dimensional radiation magnetohydrodynamic simulations of the integral layers from the top of the convection zone to the mid-chromosphere. The second part is devoted to a few basic properties of magnetic flux tubes, which can be considered to be an abstraction of the more complicated flux concentrations known from observations and numerical simulations. We treat electrical current sheets, the mechanical equilibrium condition at magnetic interfaces, the equations for constructing a magnetohydrostatic flux tube embedded in a gravitationally stratified atmosphere, the condition of radiative equilibrium, and the condition for interchange stability.

O. Steiner

2007-09-01

44

Equivalent Magnetic Noise Limit of Low-Cost GMI Magnetometer  

Microsoft Academic Search

We present a noise analysis of a giant magnetoimpedance (GMI) sensor using a peak detector at the optimal magnetic field working bias point of a sensor wire, by considering internal noise sources (intrinsic GMI device associated noise sources and conditioning electronic noise sources). An expression is obtained for the theoretical expected noise for known electronic design parameters and physical characteristics

Lehui Ding; SÉbastien Saez; Christophe Dolabdjian; Luiz G. C. Melo; Arthur Yelon; David Menard

2009-01-01

45

Magnetic flux reconstruction methods for shaped tokamaks  

SciTech Connect

The use of a variational method permits the Grad-Shafranov (GS) equation to be solved by reducing the problem of solving the 2D non-linear partial differential equation to the problem of minimizing a function of several variables. This high speed algorithm approximately solves the GS equation given a parameterization of the plasma boundary and the current profile (p` and FF` functions). The author treats the current profile parameters as unknowns. The goal is to reconstruct the internal magnetic flux surfaces of a tokamak plasma and the toroidal current density profile from the external magnetic measurements. This is a classic problem of inverse equilibrium determination. The current profile parameters can be evaluated by several different matching procedures. Matching of magnetic flux and field at the probe locations using the Biot-Savart law and magnetic Green`s function provides a robust method of magnetic reconstruction. The matching of poloidal magnetic field on the plasma surface provides a unique method of identifying the plasma current profile. However, the power of this method is greatly compromised by the experimental errors of the magnetic signals. The Casing Principle provides a very fast way to evaluate the plasma contribution to the magnetic signals. It has the potential of being a fast matching method. The performance of this method is hindered by the accuracy of the poloidal magnetic field computed from the equilibrium solver. A flux reconstruction package has been implemented which integrates a vacuum field solver using a filament model for the plasma, a multi-layer perception neural network as an interface, and the volume integration of plasma current density using Green`s functions as a matching method for the current profile parameters. The flux reconstruction package is applied to compare with the ASEQ and EFIT data. The results are promising.

Tsui, Chi-Wa

1993-12-01

46

Stress effects on magnetic Barkhausen noise measurements from magnetized pipeline steel  

NASA Astrophysics Data System (ADS)

The magnetic flux leakage (MFL) technique is used to determine nondestructively the maximum allowable operating pressure of steel pipelines for oil and gas [1,2]. This method involves magnetically saturating the pipe wall and measuring the leakage flux near the pipe wall surface with Hall probes or induction coils. The goal of the nondestructive testing (NDT) industry is to estimate the losses in the pipe wall to better than 5% precision. Pipelines are essentially pressure vessels that operate up to 70% of their yield strength [3]. The magnitude of leakage flux depends on several parameters including the magnetic anisotropy of pipeline steel which is stress dependent [4, 5, 6, 7, 8, 9]. Magnetic Barkhausen noise (MBN), the irreversible motion of 180-degree walls, is sensitive to residual and applied stress in a ferromagnetic material, such as steel. Thus MBN has been proposed as a viable nondestructive evaluation technique for monitoring magnetic anisotropy [10] and inhomogeneity in magnetic materials [11, 12, 13, 14]. MBN occurs at the greatest rate of change in magnetization, essentially where B ˜ 0 T [15]. This is not the same condition under which MFL is performed on pipeline steel when it is almost magnetically saturated. In this work MBN signals from magnetized (˜1.6 T) pipeline steel are acquired and the effects of stress are also studied. A coercive field or pinning model was developed for the MBN anisotropy data acquired from the magnetized but unstressed steel pipe that has proven to yield the magnetic easy axis of the sample prior to magnetization. Finally, both the MBN and MFL techniques were used on a magnetized (1.8 T) and stressed (up to 270 MPa) pipe sample such that differences in the permeability of regions of magnetic inhomogeneity could be compared.

Hsu, Annie

47

Photospheric Magnetic Flux Transport - Supergranules Rule  

NASA Technical Reports Server (NTRS)

Observations of the transport of magnetic flux in the Sun's photosphere show that active region magnetic flux is carried far from its origin by a combination of flows. These flows have previously been identified and modeled as separate axisymmetric processes: differential rotation, meridional flow, and supergranule diffusion. Experiments with a surface convective flow model reveal that the true nature of this transport is advection by the non-axisymmetric cellular flows themselves - supergranules. Magnetic elements are transported to the boundaries of the cells and then follow the evolving boundaries. The convective flows in supergranules have peak velocities near 500 m/s. These flows completely overpower the superimposed 20 m/s meridional flow and 100 m/s differential rotation. The magnetic elements remain pinned at the supergranule boundaries. Experiments with and without the superimposed axisymmetric photospheric flows show that the axisymmetric transport of magnetic flux is controlled by the advection of the cellular pattern by underlying flows representative of deeper layers. The magnetic elements follow the differential rotation and meridional flow associated with the convection cells themselves -- supergranules rule!

Hathaway, David H.; Rightmire-Upton, Lisa

2012-01-01

48

Magnetic Flux Transients during Solar Flares  

NASA Astrophysics Data System (ADS)

Solar flares result from the sudden release of energy stored in the magnetic field of the solar atmosphere, attributed to magnetic reconnection. In this work, we use line-of-sight magnetograms to study the changes in photospheric magnetic field during large solar flares. The magnetograms are derived from observations using NASA's Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory, and have a cadence of 3 minutes at a 0.5 arcsecond spatial resolution. We studied the inferred magnetic flux changes in 11 X-class flares from (2011-2012) and 26 M-class flares (2011). Of the 37 flares, 32 exhibited short-lived (less than 30 minutes) magnetic flux transients (MFTs) during the progress of the flare, similar to those by Maurya et al. (2012). We note that MFTs were co-temporal with GOES X-ray peaks. Flares with rapid rises (impulsive flares) had stronger transients while those with slower rises (gradual flares) had weak or no MFTs. Finally, flares with stronger GOES X-ray peaks (flare class) showed stronger MFTs. We believe that these changes are non-physical because the changes in the magnetic field are transient (the magnetic field returns to the pre-flare state) and coincide with the impulsive phase of the flare. This work supported by the US Airforce Office of Scientific Research and the AFRL/RV Space Scholar Program.

Balasubramaniam, K. S.; Delgado, F.; Hock, R. A.

2013-12-01

49

What is a flux tube? On the magnetic field topology of buoyant flux structures  

E-print Network

What is a flux tube? On the magnetic field topology of buoyant flux structures Fausto Cattaneo study the topology of field lines threading buoyant magnetic flux struc- tures. The magnetic structures of the evolution, and therefore the degree of symmetry, the resulting magnetic structures can have field lines

50

Electric Flux Tube in Magnetic Plasma  

E-print Network

In this paper we study a methodical problem related to the magnetic scenario recently suggested and initiated by the authors \\cite{Liao_ES_mono} to understand the strongly coupled quark-gluon plasma (sQGP): the electric flux tube in monopole plasma. A macroscopic approach, interpolating between Bose condensed (dual superconductor) and classical gas medium is developed first. Then we work out a microscopic approach based on detailed quantum mechanical calculation of the monopole scattering on electric flux tube, evaluating induced currents for all partial waves. As expected, the flux tube looses its stability when particles can penetrate it: we make this condition precise by calculating the critical value for the product of the flux tube size times the particle momentum, above which the flux tube dissolves. Lattice static potentials indicate that flux tubes seem to dissolve at $T>T_{dissolution} \\approx 1.3 T_c$. Using our criterion one gets an estimate of the magnetic density $n\\approx 4.4 \\sim 6.6 fm^{-3}$ at this temperature.

Jinfeng Liao; Edward Shuryak

2007-12-06

51

Synthetic magnetic fluxes on the honeycomb lattice  

NASA Astrophysics Data System (ADS)

We devise experimental schemes that are able to mimic uniform and staggered magnetic fluxes acting on ultracold two-electron atoms, such as ytterbium atoms, propagating in a honeycomb lattice. The atoms are first trapped into two independent state-selective triangular lattices and then further exposed to a suitable configuration of resonant Raman laser beams. These beams induce hops between the two triangular lattices and make atoms move in a honeycomb lattice. Atoms traveling around each unit cell of this honeycomb lattice pick up a nonzero phase. In the uniform case, the artificial magnetic flux sustained by each cell can reach about two flux quanta, thereby realizing a cold-atom analog of the Harper model with its notorious Hofstadter’s butterfly structure. Different condensed-matter phenomena such as the relativistic integer and fractional quantum Hall effects, as observed in graphene samples, could be targeted with this scheme.

Górecka, Agnieszka; Grémaud, Benoît; Miniatura, Christian

2011-08-01

52

Synthetic magnetic fluxes on the honeycomb lattice  

SciTech Connect

We devise experimental schemes that are able to mimic uniform and staggered magnetic fluxes acting on ultracold two-electron atoms, such as ytterbium atoms, propagating in a honeycomb lattice. The atoms are first trapped into two independent state-selective triangular lattices and then further exposed to a suitable configuration of resonant Raman laser beams. These beams induce hops between the two triangular lattices and make atoms move in a honeycomb lattice. Atoms traveling around each unit cell of this honeycomb lattice pick up a nonzero phase. In the uniform case, the artificial magnetic flux sustained by each cell can reach about two flux quanta, thereby realizing a cold-atom analog of the Harper model with its notorious Hofstadter's butterfly structure. Different condensed-matter phenomena such as the relativistic integer and fractional quantum Hall effects, as observed in graphene samples, could be targeted with this scheme.

Gorecka, Agnieszka [Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543 (Singapore); Gremaud, Benoit [Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543 (Singapore); Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 (Singapore); Laboratoire Kastler Brossel, Ecole Normale Superieure, CNRS, UPMC, 4 Place Jussieu, FR-75005 Paris (France); Miniatura, Christian [Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543 (Singapore); Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 (Singapore); Institut Non Lineaire de Nice, UMR 6618, UNS, CNRS, 1361 Route des Lucioles, FR-06560 Valbonne (France); Institute of Advanced Studies, Nanyang Technological university, 60 Nanyang View, Singapore 639673 (Singapore)

2011-08-15

53

Magnetic flux concentrations in a polytropic atmosphere  

NASA Astrophysics Data System (ADS)

Context. Strongly stratified hydromagnetic turbulence has recently been identified as a candidate for explaining the spontaneous formation of magnetic flux concentrations by the negative effective magnetic pressure instability (NEMPI). Much of this work has been done for isothermal layers, in which the density scale height is constant throughout. Aims: We now want to know whether earlier conclusions regarding the size of magnetic structures and their growth rates carry over to the case of polytropic layers, in which the scale height decreases sharply as one approaches the surface. Methods: To allow for a continuous transition from isothermal to polytropic layers, we employ a generalization of the exponential function known as the q-exponential. This implies that the top of the polytropic layer shifts with changing polytropic index such that the scale height is always the same at some reference height. We used both mean-field simulations (MFS) and direct numerical simulations (DNS) of forced stratified turbulence to determine the resulting flux concentrations in polytropic layers. Cases of both horizontal and vertical applied magnetic fields were considered. Results: Magnetic structures begin to form at a depth where the magnetic field strength is a small fraction of the local equipartition field strength with respect to the turbulent kinetic energy. Unlike the isothermal case where stronger fields can give rise to magnetic flux concentrations at larger depths, in the polytropic case the growth rate of NEMPI decreases for structures deeper down. Moreover, the structures that form higher up have a smaller horizontal scale of about four times their local depth. For vertical fields, magnetic structures of super-equipartition strengths are formed, because such fields survive downward advection that causes NEMPI with horizontal magnetic fields to reach premature nonlinear saturation by what is called the "potato-sack" effect. The horizontal cross-section of such structures found in DNS is approximately circular, which is reproduced with MFS of NEMPI using a vertical magnetic field. Conclusions: Results based on isothermal models can be applied locally to polytropic layers. For vertical fields, magnetic flux concentrations of super-equipartition strengths form, which supports suggestions that sunspot formation might be a shallow phenomenon.

Losada, I. R.; Brandenburg, A.; Kleeorin, N.; Rogachevskii, I.

2014-04-01

54

SYNOPTIC MAPPING OF CHROMOSPHERIC MAGNETIC FLUX  

SciTech Connect

We used daily full-disk Ca II 854.2 nm magnetograms from the Synoptic Optical Long Term Investigations of the Sun (SOLIS) facility to study the chromospheric magnetic field from 2006 April through 2009 November. We determined and corrected previously unidentified zero offsets in the SOLIS magnetograms. By tracking the disk passages of stable unipolar regions, the measured net flux densities were found to systematically decrease from the disk center to the limb by a factor of about two. This decrease was modeled using a thin flux tube model with a difference in signal formation height between the center and limb sides. Comparison of photospheric and chromospheric observations shows that their differences are largely due to horizontal spreading of magnetic flux with increasing height. The north polar magnetic field decreased nearly linearly with time during our study period while the south polar field was nearly constant. We used the annual change in the viewing angle of the polar regions to estimate the radial and meridional components of the polar fields and found that the south polar fields were tilted away from the pole. Synoptic maps of the chromospheric radial flux density distribution were used as boundary conditions for extrapolation of the field from the chromosphere into the corona. A comparison of modeled and observed coronal hole boundaries and coronal streamer positions showed better agreement when using the chromospheric rather than the photospheric synoptic maps.

Jin, C. L. [Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 (China); Harvey, J. W.; Pietarila, A., E-mail: cljin@nao.cas.cn, E-mail: jharvey@nso.edu, E-mail: apietarila@nso.edu [National Solar Observatory, Tucson, AZ 85719 (United States)

2013-03-10

55

Concentrated winding axial flux permanent magnet motor with plastic bonded magnets and sintered segmented magnets  

Microsoft Academic Search

Direct drive axial flux permanent magnet (PM) motors are a cost effective and an energy saving choice for industrial use. Open slots make concentrated winding machines a favourable configuration with respect to manufacturing. However, open slots expose rotor surface magnets to large flux pulsations and the losses of sintered magnets may not be neglected. Plastic bonded magnets have very low

Hanne Jussila; Pia Salminen; Asko Parviainen; Janne Nerg; Juha Pyrhönen

2008-01-01

56

Magnetic Flux Compression Experiments Using Plasma Armatures  

NASA Technical Reports Server (NTRS)

Magnetic flux compression reaction chambers offer considerable promise for controlling the plasma flow associated with various micronuclear/chemical pulse propulsion and power schemes, primarily because they avoid thermalization with wall structures and permit multicycle operation modes. The major physical effects of concern are the diffusion of magnetic flux into the rapidly expanding plasma cloud and the development of Rayleigh-Taylor instabilities at the plasma surface, both of which can severely degrade reactor efficiency and lead to plasma-wall impact. A physical parameter of critical importance to these underlying magnetohydrodynamic (MHD) processes is the magnetic Reynolds number (R(sub m), the value of which depends upon the product of plasma electrical conductivity and velocity. Efficient flux compression requires R(sub m) less than 1, and a thorough understanding of MHD phenomena at high magnetic Reynolds numbers is essential to the reliable design and operation of practical reactors. As a means of improving this understanding, a simplified laboratory experiment has been constructed in which the plasma jet ejected from an ablative pulse plasma gun is used to investigate plasma armature interaction with magnetic fields. As a prelude to intensive study, exploratory experiments were carried out to quantify the magnetic Reynolds number characteristics of the plasma jet source. Jet velocity was deduced from time-of-flight measurements using optical probes, and electrical conductivity was measured using an inductive probing technique. Using air at 27-inHg vacuum, measured velocities approached 4.5 km/s and measured conductivities were in the range of 30 to 40 kS/m.

Turner, M. W.; Hawk, C. W.; Litchford, R. J.

2003-01-01

57

Coronal mass ejections and magnetic flux buildup in the heliosphere  

E-print Network

Coronal mass ejections and magnetic flux buildup in the heliosphere M. J. Owens1 and N. U. Crooker1] To test for magnetic flux buildup in the heliosphere from coronal mass ejections (CMEs), we simulate, the ICME flux opening rate is an important factor. Two separate forms for the ICME flux opening rate

California at Berkeley, University of

58

MAGNETIC FLUX CONSERVATION IN THE HELIOSHEATH  

SciTech Connect

Voyager 1(V1) and Voyager 2(V2) have observed heliosheath plasma since 2005 December and 2007 August, respectively. The observed speed profiles are very different at the two spacecrafts. Speeds at V1 decreased to zero in 2010 while the average speed at V2 is a constant 150 km s{sup -1} with the direction rotating tailward. The magnetic flux is expected to be constant in these heliosheath flows. We show that the flux is constant at V2 but decreases by an order of magnitude at V1, even after accounting for divergence of the flows and changes in the solar field. If reconnection were responsible for this decrease, the magnetic field would lose 70% of its free energy to reconnection and the energy density released would be 0.6 eV cm{sup -3}.

Richardson, J. D. [Kavli Center for Astrophysics and Space Science Massachusetts Institute of Technology, Cambridge, MA 02139 (United States); Burlaga, L. F. [NASA Goddard Space Flight Center, Code 673, Greenbelt, MD 20771 (United States); Decker, R. B. [Applied Physics Laboratory, The Johns Hopkins University, Laurel, MD 20723 (United States); Drake, J. F. [Department of Physics and Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742 (United States); Ness, N. F. [Institute for Astrophysics and Computational Sciences, Catholic University of America, Washington, DC 20064 (United States); Opher, M., E-mail: jdr@space.mit.edu, E-mail: lburlagahsp@verizon.net, E-mail: robert.decker@jhuapl.edu, E-mail: drake@umd.edu, E-mail: nfnudel@yahoo.com, E-mail: mopher@bu.edu [Astronomy Department, Boston University, 675 Commonwealth Avenue, Boston, MA 02215 (United States)

2013-01-01

59

Stress effects and magnetic flux leakage induced by defects in pipelines  

NASA Astrophysics Data System (ADS)

The operating pressure in oil and gas pipelines can produce hoop stress as high as 70% of yield stress. Superimposed on this are bending stresses and residual stresses from pipe forming operations. Furthermore defects cause local changes in both the magnitude and direction of pipe wall flux density and stress. The combined stresses affect magnetic properties, such as anisotropy, and hence alter magnetic flux leakage signals from magnetic flux leakage (MFL) inspection tools used to detect and size defects. These factors need to be mitigated and taken into consideration when interpreting MFL signals to size defects. This paper describes research undertaken to elucidate the effects of these complex interactions. Results from magnetic Barkhausen noise, neutron diffraction and flux leakage monitoring of local, bulk surface and "at depth" stresses are presented. Tensor measurements of magnetic properties and stress-induced variations are described and discussed.

Clapham, L.; Atherton, D. L.

2000-05-01

60

Self-similar magnetic structures and magnetic flux “Giant” creep  

Microsoft Academic Search

The penetration of a magnetic flux into a type-II high-T\\u000a c superconductor occupying the half-space x > 0 is considered. At the superconductor surface, the magnetic field amplitude increases in accordance with the law b(0, t) = b\\u000a 0(1 + t)m (in dimensionless coordinates), where m > 0. The velocity of penetration of vortices is determined in the regime of

I. B. Krasnyuk

2007-01-01

61

Self-similar magnetic structures and magnetic flux ``Giant'' creep  

Microsoft Academic Search

The penetration of a magnetic flux into a type-II high-T c superconductor occupying the half-space x > 0 is considered. At the superconductor surface, the magnetic field amplitude increases in accordance with the law b(0, t) = b 0(1 + t)m (in dimensionless coordinates), where m > 0. The velocity of penetration of vortices is determined in the regime of

I. B. Krasnyuk

2007-01-01

62

Surface magnetic Barkhausen noise response to plastic yield of steel  

NASA Astrophysics Data System (ADS)

Magnetic Barkhausen noise (MBN) occurs when a changing magnetic field is applied to a ferromagnetic material, and is due to irreversible motion of domain walls as they overcome local pinning sites. MBN is shown to be an effective tool for investigating magnetic behaviour of deformed steels because of its high sensitivity to strain. A comprehensive study of elastic and plastic deformation influences on surface magnetic Barkhausen noise is presented. MBN signal behaviour displayed in the elastic range is compared to the one observed beyond the macroscopic yield point. A mild steel specimen was deformed plastically by successively loading it in tension up to 13.71% longitudinal strain. While still in the elastic range of deformation, various strain levels displayed an abrupt increase or decrease in a parameter termed 'MBNenergy'. Overall, the MBNenergy increased significantly in the elastic, yet only slightly in the plastic range of deformation. Variations in the angular dependence of the MBNenergy with strain indicated changes in magnetic anisotropy. In this investigated case, an easy axis of magnetization was produced parallel to the applied elastic tensile stress, and became more pronounced with plastic stress. Several mild steel specimens were stress cycled and analyzed after unloading to separate elastic from plastic deformation effects. Plastic strains obtained ranged from 0.02% to 39.69%. The magnetic easy axis revealed during loading became less pronounced with unloading due to the presence of axial compressive residual stresses. Build-up of tensile residual stresses in the transverse direction increased MBNenergy values. The increased number of large voltage pulses indicated slip along planes of maximum shearing stress. Bending of a mild steel specimen to 0.21% longitudinal surface strain confirmed MBN behaviour observed during uniaxial loading. The tensile stress on the outer surface of the specimen produced abrupt changes in MBNenergy before the macroscopic yield point. An overall marked increase in MBN energy values, and the development of an easy axis of magnetization were notable. Cold rolling of nuclear reactor pressure vessel steel specimens to reduction ratios between 0% and 60% caused changes in magnetic anisotropy. Redistribution of internal stresses through reorientation of crystallographic planes destroyed the magnetic easy axis present in the undeformed state. The number of large voltage pulses detected along the rolling direction decreased initially. Residual stress configurations present at higher reduction ratios restored the easy axis, and increased the number of large voltage pulses. Experimental findings of this work have implications for other nondestructive evaluation (NDE) techniques such as magnetic flux leakage (MFL).

Stefanita, Carmen-Gabriela

63

Slip Running Reconnection in Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

Magnetic flux ropes are due to helical currents and form a dense carpet of arches on the surface of the sun. Occasionally one tears loose as a coronal mass ejection and its rope structure can be detected by satellites close to the earth. Current sheets can tear into filaments and these are nothing other than flux ropes. Ropes are not static, they exert mutual ?c{J}×?c{B} forces causing them to twist about each other and eventually merge. Kink instabilities cause them to violently smash into each other and reconnect at the point of contact. We report on experiments on two adjacent ropes done in the large plasma device (LAPD) at UCLA ( ne ˜ 1012, Te ˜ 6 eV, B0z=330G, Brope}\\cong{10G,trep=1 Hz). The currents and magnetic fields form exotic shapes with no ignorable direction and no magnetic nulls. Volumetric space-time data (70,600 spatial locations) show multiple reconnection sites with time-dependent locations. The concept of a quasi-separatrix layer (QSL), a tool to understand and visualize 3D magnetic field lines reconnection without null points is introduced. Three-dimensional measurements of the QSL derived from magnetic field data are presented. Within the QSL field lines that start close to one another rapidly diverge as they pass through one or more reconnection regions. The motion of magnetic field lines are traced as reconnection proceeds and they are observed to slip through the regions of space where the QSL is largest. As the interaction proceeds we double the current in the ropes. This accompanied by intense heating as observed in uv light and plasma flows measured by Mach probes. The interaction of the ropes is clearly seen by vislaulizng magnetic field data , as well as in images from a fast framing camera. Work supported by the Dept. of Energy and The National Science Foundation, done at the Basic Plasma Science Facility at UCLA.Magnetic Field lines (measured) of three flux ropes and the plasma currents associated with them

Gekelman, W. N.; Van Compernolle, B.; Vincena, S. T.; De Hass, T.

2012-12-01

64

SEED BANKS FOR MAGNETIC FLUX COMPRESSION GENERATORS  

SciTech Connect

In recent years the Lawrence Livermore National Laboratory (LLNL) has been conducting experiments that require pulsed high currents to be delivered into inductive loads. The loads fall into two categories (1) pulsed high field magnets and (2) the input stage of Magnetic Flux Compression Generators (MFCG). Three capacitor banks of increasing energy storage and controls sophistication have been designed and constructed to drive these loads. One bank was developed for the magnet driving application (20kV {approx} 30kJ maximum stored energy.) Two banks where constructed as MFCG seed banks (12kV {approx} 43kJ and 26kV {approx} 450kJ). This paper will describe the design of each bank including switching, controls, circuit protection and safety.

Fulkerson, E S

2008-05-14

65

Cogging Torque Analysis of Flux-Switching Permanent Magnet Motor  

Microsoft Academic Search

A novel doubly salient permanent magnet motor named flux switch permanent magnet motor (FSPM) was designed and analyzed. The stator and rotor of FSPM are all salient structures. There are a marked verge domino effect and serious saturation phenomenon. Due to high nonlinearities of FSPM, finite element method (FEM) was used to analyze magnetic field distribution, permanent magnet flux linkage,

Junyou Yang; Yuan Deng; Qiang Ma; Wenzeng Zhao

2010-01-01

66

Magnetic flux in modeled magnetic clouds at 1 AU and some specific comparisons to associated photospheric flux  

NASA Technical Reports Server (NTRS)

In order to better understand the solar origins of magnetic clouds, statistical distributions of the estimated axial magnetic flux of 30 magnetic clouds at 1 AU, separated according to their occurrence during the solar cycle, were obtained and a comparison was made of the magnetic flux of a magnetic cloud to the aggregate flux of apparently associated photospheric magnetic flux tubes, for some specific cases. The 30 magnetic clouds comprise 12 cases from WIND, and the remainder from IMP-8, earlier IMPs, the International Sun-Earth Explorer (ISEE) 3 and HELIOS. The total magnetic flux along the cloud axis was estimated using a constant alpha, cylindrical, force-free flux rope model to determine cloud diameter and axial magentic field strength. The distribution of magentic fluxes for the 30 clouds is shown to be in the form of a skewed Gaussian.

Lepping, R. P.; Szabo, A.; DeForest, C. E.; Thompson, B. J.

1997-01-01

67

A review on equivalent magnetic noise of magnetoelectric laminate sensors  

PubMed Central

Since the turn of the millennium, multi-phase magnetoelectric (ME) composites have been subject to attention and development, and giant ME effects have been found in laminate composites of piezoelectric and magnetostrictive layers. From an application perspective, the practical usefulness of a magnetic sensor is determined not only by the output signal of the sensor in response to an incident magnetic field, but also by the equivalent magnetic noise generated in the absence of such an incident field. Here, a short review of developments in equivalent magnetic noise reduction for ME sensors is presented. This review focuses on internal noise, the analysis of the noise contributions and a summary of noise reduction strategies. Furthermore, external vibration noise is also discussed. The review concludes with an outlook on future possibilities and scientific challenges in the field of ME magnetic sensors. PMID:24421380

Wang, Y. J.; Gao, J. Q.; Li, M. H.; Shen, Y.; Hasanyan, D.; Li, J. F.; Viehland, D.

2014-01-01

68

A review on equivalent magnetic noise of magnetoelectric laminate sensors.  

PubMed

Since the turn of the millennium, multi-phase magnetoelectric (ME) composites have been subject to attention and development, and giant ME effects have been found in laminate composites of piezoelectric and magnetostrictive layers. From an application perspective, the practical usefulness of a magnetic sensor is determined not only by the output signal of the sensor in response to an incident magnetic field, but also by the equivalent magnetic noise generated in the absence of such an incident field. Here, a short review of developments in equivalent magnetic noise reduction for ME sensors is presented. This review focuses on internal noise, the analysis of the noise contributions and a summary of noise reduction strategies. Furthermore, external vibration noise is also discussed. The review concludes with an outlook on future possibilities and scientific challenges in the field of ME magnetic sensors. PMID:24421380

Wang, Y J; Gao, J Q; Li, M H; Shen, Y; Hasanyan, D; Li, J F; Viehland, D

2014-02-28

69

Surface Magnetic Flux Maintenance In Quiet Sun  

E-print Network

We investigate surface processes of magnetic patches, namely merging, splitting, emergence, and cancellation, by using an auto-detection technique. We find that merging and splitting are locally predominant in the surface level, while the frequencies of the other two are less by one or two orders of magnitude. The frequency dependences on flux con- tent of surface processes are further investigated. Based on these observations, we discuss a possible whole picture of the maintenance. Our conclusion is that the photospheric magnetic field structure, especially its power-law nature, is maintained by the processes locally in the surface not by the interactions between different altitudes. We suggest a scenario of the flux maintenance as follows: The splitting and merging play a crucial role for the generation of the power-law distribution, not the emergence nor cancellation do. This power-law distribution results in another power-law one of the cancellation with an idea of the random convective transport. The cancellation and emergence have a common value for the power-law indices in their frequency distributions, which may suggest a "recycle of fluxes by submergence and re-emergence".

Y. Iida

2012-12-27

70

Linear magnetic motor/generator. [to generate electric energy using magnetic flux for spacecraft power supply  

NASA Technical Reports Server (NTRS)

A linear magnetic motor/generator is disclosed which uses magnetic flux to provide mechanical motion or electrical energy. The linear magnetic motor/generator includes an axially movable actuator mechanism. A permament magnet mechanism defines a first magnetic flux path which passes through a first end portion of the actuator mechanism. Another permament magnet mechanism defines a second magnetic flux path which passes through a second end portion of the actuator mechanism. A drive coil defines a third magnetic flux path passing through a third central portion of the actuator mechanism. A drive coil selectively adds magnetic flux to and subtracts magnetic flux from magnetic flux flowing in the first and second magnetic flux path.

Studer, P. A. (inventor)

1982-01-01

71

Vortex Pinning and Flux Noise in High-Tc SQUIDs Studied by Low Temperature Scanning Electron Microscopy  

NASA Astrophysics Data System (ADS)

We investigated the correlation between the low-frequency magnetic flux noise and the spatial distribution of vortices in YBCO dc SQUIDs operated at 77K in variable magnetic field. The investigated SQUIDs consisted either of thin film washers, some of them containing regular arrays of micron sized holes (antidots), and one device had an input coil patterned in a second YBCO layer, integrated on the same chip. The vortex imaging with about 1?m resolution and the noise measurements are performed with the SQUIDs mounted on a liquid nitrogen cryostage of a scanning electron microscope(R. Gerber et al.), Cryogenics 37, 21 (1997). The vortex imaging is based on the electron-beam-induced local displacement ? r of vortices, which is detected as a flux change ?? = ? r (partial?/partial r) in the SQUID( S. Keil, et al.), IEEE Trans. Appl. Supercond. 9, 2961 (1999). Hence, the signal height provides direct information on the coupling strength partial?/partial r. Since partial?/partial r determines the amount of flux noise which a fluctuating vortex induces in the SQUID, we obtain valuable information on possible low-frequency noise sources in the SQUIDs, which can be correlated with the morphology and microstructure of the YBCO film forming the SQUID loop( D. Koelle et al.), Physica C 332, 148 (2000). Vortex images obtained on solid washer SQUIDs cooled in fields up to 60 ?T reveal a strongly disordered vortex lattice with distinct strong pinning sites. Vortex images of antidot washers show both, antidot vortices and interstitial vortices, which e.g. allows direct observation of matching phenomena. Depending on the spatial vortex configuration the shape of the flux noise can change considerably from being 1/f-like up to showing Lorentzian-type noise.

Koelle, Dieter

2001-03-01

72

Quantitative observation of magnetic flux distribution in new magnetic films for future high  

E-print Network

configuration of magnetic nanoparticles, 15 magnetic films, 16 magnetic tunnel junctions, 17 or even magnetiteQuantitative observation of magnetic flux distribution in new magnetic films for future high

Recanati, Catherine

73

Permanent-magnet switched-flux machine  

DOEpatents

A permanent-magnet switched-flux (PMSF) device has a ferromagnetic outer stator mounted to a shaft about a central axis extending axially through the PMSF device. Pluralities of top and bottom stator poles are respectively mounted in first and second circles, radially outwardly in first and second transverse planes extending from first and second sections of the central axis adjacent to an inner surface of the ferromagnetic outer stator. A ferromagnetic inner rotor is coupled to the shaft and has i) a rotor core having a core axis co-axial with the central axis; and ii) first and second discs having respective outer edges with first and second pluralities of permanent magnets (PMs) mounted in first and second circles, radially outwardly from the rotor core axis in the first and second transverse planes. The first and second pluralities of PMs each include PMs of alternating polarity.

Trzynadlowski, Andrzej M.; Qin, Ling

2010-01-12

74

Permanent-magnet switched-flux machine  

DOEpatents

A permanent-magnet switched-flux (PMSF) device has an outer rotor mounted to a shaft about a central axis extending axially through the PMSF device. First and second pluralities of permanent-magnets (PMs) are respectively mounted in first and second circles, radially outwardly in first and second transverse planes extending from first and second sections of the central axis adjacent to an inner surface of the outer rotor. An inner stator is coupled to the shaft and has i) a stator core having a core axis co-axial with the central axis; and ii) first and second pluralities of stator poles mounted in first and second circles, radially outwardly from the stator core axis in the first and second transverse planes. The first and second pluralities of PMs each include PMs of alternating polarity.

Trzynadlowski, Andrzej M.; Qin, Ling

2012-02-21

75

Permanent-magnet switched-flux machine  

DOEpatents

A permanent-magnet switched-flux (PMSF) device has an outer rotor mounted to a shaft about a central axis extending axially through the PMSF device. First and second pluralities of permanent-magnets (PMs) are respectively mounted in first and second circles, radially outwardly in first and second transverse planes extending from first and second sections of the central axis adjacent to an inner surface of the outer rotor. An inner stator is coupled to the shaft and has i) a stator core having a core axis co-axial with the central axis; and ii) first and second pluralities of stator poles mounted in first and second circles, radially outwardly from the stator core axis in the first and second transverse planes. The first and second pluralities of PMs each include PMs of alternating polarity.

Trzynadlowski, Andrzej M.; Qin, Ling

2011-06-14

76

Topology of magnetic flux ropes and formation of fossil flux transfer events and boundary layer plasmas  

NASA Technical Reports Server (NTRS)

A mechanism for the formation of fossil flux transfer events and the low-level boundary layer within the framework of multiple X-line reconnection is proposed. Attention is given to conditions for which the bulk of magnetic flux in a flux rope of finite extent has a simple magnetic topology, where the four possible connections of magnetic field lines are: IMF to MSP, MSP to IMF, IMF to IMF, and MSP to MSP. For a sufficient relative shift of the X lines, magnetic flux may enter a flux rope from the magnetosphere and exit into the magnetosphere. This process leads to the formation of magnetic flux ropes which contain a considerable amount of magnetosheath plasma on closed magnetospheric field lines. This process is discussed as a possible explanation for the formation of fossil flux transfer events in the magnetosphere and the formation of the low-latitude boundary layer.

Lee, L. C.; Ma, Z. W.; Fu, Z. F.; Otto, A.

1993-01-01

77

NONLINEAR THREE-DIMENSIONAL MAGNETOCONVECTION AROUND MAGNETIC FLUX TUBES  

SciTech Connect

Magnetic flux in the solar photosphere forms concentrations from small scales, such as flux elements, to large scales, such as sunspots. This paper presents a study of the decay process of large magnetic flux tubes, such as sunspots, on a supergranular scale. Three-dimensional nonlinear resistive magnetohydrodynamic numerical simulations are performed in a cylindrical domain, initialized with axisymmetric solutions that consist of a well-defined central flux tube and an annular convection cell surrounding it. As the nonlinear convection evolves, the annular cell breaks up into many cells in the azimuthal direction, allowing magnetic flux to slip between cells away from the central flux tube (turbulent erosion). This lowers magnetic pressure in the central tube, and convection grows inside the tube, possibly becoming strong enough to push the tube apart. A remnant of the central flux tube persists with nonsymmetric perturbations caused by the convection surrounding it. Secondary flux concentrations form between convection cells away from the central tube. Tube decay is dependent on the convection around the tube. Convection cells forming inside the tube as time-dependent outflows will remove magnetic flux. (This is most pronounced for small tubes.) Flux is added to the tube when flux caught in the surrounding convection is pushed toward it. The tube persists when convection inside the tube is sufficiently suppressed by the remaining magnetic field. All examples of persistent tubes have the same effective magnetic field strength, consistent with the observation that pores and sunspot umbrae all have roughly the same magnetic field strength.

Botha, G. J. J. [Centre for Fusion, Space and Astrophysics, Physics Department, University of Warwick, Coventry CV4 7AL (United Kingdom); Rucklidge, A. M. [Department of Applied Mathematics, University of Leeds, Leeds LS2 9JT (United Kingdom); Hurlburt, N. E., E-mail: G.J.J.Botha@warwick.ac.uk, E-mail: A.M.Rucklidge@leeds.ac.uk, E-mail: hurlburt@lmsal.com [Lockheed Martin Solar and Astrophysics Laboratory, Organization ADBS Building 252, Palo Alto, CA 94304 (United States)

2011-04-20

78

Noise analysis in fast magnetic resonance electrical impedance tomography (MREIT) based on spoiled multi gradient echo (SPMGE) pulse sequence.  

PubMed

Magnetic resonance electrical impedance tomography (MREIT) is a promising non-invasive method to visualize a static cross-sectional conductivity and/or current density image by injecting low frequency currents. MREIT measures one component of the magnetic flux density caused by the injected current using a magnetic resonance (MR) scanner. For practical in vivo implementations of MREIT, especially for soft biological tissues where the MR signal rapidly decays, it is crucial to develop a technique for optimizing the magnetic flux density signal by the injected current while maintaining spatial-resolution and contrast. We design an MREIT pulse sequence by applying a spoiled multi-gradient-echo pulse sequence (SPMGE) to the injected current nonlinear encoding (ICNE), which fully injects the current at the end of the read-out gradient. The applied ICNE-SPMGE pulse sequence maximizes the duration of injected current almost up to a repetition time by measuring multiple magnetic flux density data. We analyze the noise level of measured magnetic flux density with respect to the pulse width of injection current and T*(2) relaxation time. In due consideration of the ICNE-SPMGE pulse sequence, using a reference information of T*(2) values in a local region of interest by a short pre-scan data, we predict the noise level of magnetic flux density to be measured for arbitrary repetition time TR. Results from phantom experiment demonstrate that the proposed method can predict the noise level of magnetic flux density for an appropriate TR = 40 ms using a reference scan for TR = 75 ms. The predicted noise level was compared with the noise level of directly measured magnetic flux density data. PMID:25082797

Oh, Tong In; Jeong, Woo Chul; Kim, Ji Eun; Sajib, Saurav Z K; Kim, Hyung Joong; Kwon, Oh In; Woo, Eung Je

2014-08-21

79

Noise analysis in fast magnetic resonance electrical impedance tomography (MREIT) based on spoiled multi gradient echo (SPMGE) pulse sequence  

NASA Astrophysics Data System (ADS)

Magnetic resonance electrical impedance tomography (MREIT) is a promising non-invasive method to visualize a static cross-sectional conductivity and/or current density image by injecting low frequency currents. MREIT measures one component of the magnetic flux density caused by the injected current using a magnetic resonance (MR) scanner. For practical in vivo implementations of MREIT, especially for soft biological tissues where the MR signal rapidly decays, it is crucial to develop a technique for optimizing the magnetic flux density signal by the injected current while maintaining spatial-resolution and contrast. We design an MREIT pulse sequence by applying a spoiled multi-gradient-echo pulse sequence (SPMGE) to the injected current nonlinear encoding (ICNE), which fully injects the current at the end of the read-out gradient. The applied ICNE-SPMGE pulse sequence maximizes the duration of injected current almost up to a repetition time by measuring multiple magnetic flux density data. We analyze the noise level of measured magnetic flux density with respect to the pulse width of injection current and T_{2}^{*} relaxation time. In due consideration of the ICNE-SPMGE pulse sequence, using a reference information of T_{2}^{*} values in a local region of interest by a short pre-scan data, we predict the noise level of magnetic flux density to be measured for arbitrary repetition time TR. Results from phantom experiment demonstrate that the proposed method can predict the noise level of magnetic flux density for an appropriate TR = 40 ms using a reference scan for TR = 75 ms. The predicted noise level was compared with the noise level of directly measured magnetic flux density data.

In Oh, Tong; Jeong, Woo Chul; Kim, Ji Eun; Sajib, Saurav Z. K.; Kim, Hyung Joong; In Kwon, Oh; Woo, Eung Je

2014-08-01

80

Noise-cancelling quadrature magnetic position, speed and direction sensor  

DOEpatents

An array of three magnetic sensors in a single package is employed with a single bias magnet for sensing shaft position, speed and direction of a motor in a high magnetic noise environment. Two of the three magnetic sensors are situated in an anti-phase relationship (i.e., 180.degree. out-of-phase) with respect to the relationship between the other of the two sensors and magnetically salient target, and the third magnetic sensor is situated between the anti-phase sensors. The result is quadrature sensing with noise immunity for accurate relative position, speed and direction measurements.

Preston, Mark A. (Niskayuna, NY); King, Robert D. (Schenectady, NY)

1996-01-01

81

Magnetic Flux Concentration and Zonal Flows in Magnetorotational Instability Turbulence  

NASA Astrophysics Data System (ADS)

Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.

Bai, Xue-Ning; Stone, James M.

2014-11-01

82

Magnetic flux array for spontaneous magnetic reconnection experiments A. Kesich, J. Bonde, J. Egedal,a  

E-print Network

Magnetic flux array for spontaneous magnetic reconnection experiments A. Kesich, J. Bonde, J; published online 24 June 2008 Experimental investigation of reconnection in magnetized plasmas relies on accurate characterization of the evolving magnetic fields. In experimental configurations where the plasma

Egedal, Jan

83

Thermal magnetic field noise: Electron optics and decoherence.  

PubMed

Thermal magnetic field noise from magnetic and non-magnetic conductive parts close to the electron beam recently has been identified as a reason for decoherence in high-resolution transmission electron microscopy (TEM). Here, we report about new experimental results from measurements for a layered structure of magnetic and non-magnetic materials. For a simplified version of this setup and other situations we derive semi-analytical models in order to predict the strength, bandwidth and spatial correlation of the noise fields. The results of the simulations are finally compared to previous and new experimental data in a quantitative manner. PMID:25499019

Uhlemann, Stephan; Müller, Heiko; Zach, Joachim; Haider, Max

2015-04-01

84

Surface magnetic Barkhausen noise response to plastic yield of steel  

Microsoft Academic Search

Magnetic Barkhausen noise (MBN) occurs when a changing magnetic field is applied to a ferromagnetic material, and is due to irreversible motion of domain walls as they overcome local pinning sites. MBN is shown to be an effective tool for investigating magnetic behaviour of deformed steels because of its high sensitivity to strain. A comprehensive study of elastic and plastic

Carmen-Gabriela Stefanita

1999-01-01

85

Complexity and Diffusion of Magnetic Flux Surfaces in Anisotropic Turbulence  

NASA Astrophysics Data System (ADS)

The complexity of magnetic flux surfaces is investigated analytically and numerically in static homogeneous magnetic turbulence. Magnetic surfaces are computed to large distances in magnetic fields derived from a reduced magnetohydrodynamic model. The question addressed is whether one can define magnetic surfaces over large distances when turbulence is present. Using a flux surface spectral analysis, we show that magnetic surfaces become complex at small scales, experiencing an exponential thinning that is quantified here. The computation of a flux surface is of either exponential or nondeterministic polynomial complexity, which has the conceptual implication that global identification of magnetic flux surfaces and flux exchange, e.g., in magnetic reconnection, can be intractable in three dimensions. The coarse-grained large-scale magnetic flux experiences diffusive behavior. The link between the diffusion of the coarse-grained flux and field-line random walk is established explicitly through multiple scale analysis. The Kubo number controls both large and small scale limits. These results have consequences for interpreting processes such as magnetic reconnection and field-line diffusion in astrophysical plasmas.

Servidio, S.; Matthaeus, W. H.; Wan, M.; Ruffolo, D.; Rappazzo, A. F.; Oughton, S.

2014-04-01

86

Geotail observations of magnetic flux ropes in the plasma sheet  

Microsoft Academic Search

Examination of Geotail measurements in the near-tail (X > ?30 RE) has revealed the presence of small flux ropes in the plasma sheet. A total of 73 flux rope events were identified in the Geotail magnetic field measurements between November 1998 and April 1999. This corresponds to an estimated occurrence frequency of ?1 flux rope per 5 hours of central

J. A. Slavin; R. P. Lepping; J. Gjerloev; D. H. Fairfield; M. Hesse; C. J. Owen; M. B. Moldwin; T. Nagai; A. Ieda; T. Mukai

2003-01-01

87

Electrical low frequency random telegraph noise in magnetic tunnel junctions  

Microsoft Academic Search

Low frequency random telegraph noise (RTN) of electrical origin is reported in magnetic tunnel junction heads with ultrathin tunnel barriers (<1 nm). The RTN is characterized by abrupt and randomly stepped voltage spikes which modulates 1\\/f noise property in frequency domain. The appearance of electrical RTN is restricted to limited ranges of bias voltage and temperature. Dependence of RTN statistics

Feng Liu; Yunfei Ding; Ravindra Kemshetti; Kenneth Davies; Paul Rana; Sining Mao

2009-01-01

88

Protostellar Accretion Flows Destabilized by Magnetic Flux Redistribution  

NASA Astrophysics Data System (ADS)

Magnetic flux redistribution lies at the heart of the problem of star formation in dense cores of molecular clouds that are magnetized to a realistic level. If all of the magnetic flux of a typical core were to be dragged into the central star, the stellar field strength would be orders of magnitude higher than the observed values. This well-known magnetic flux problem can in principle be resolved through non-ideal MHD effects. Two-dimensional (axisymmetric) calculations have shown that ambipolar diffusion, in particular, can transport magnetic flux outward relative to matter, allowing material to enter the central object without dragging the field lines along. We show through simulations that such axisymmetric protostellar accretion flows are unstable in three dimensions to magnetic interchange instability in the azimuthal direction. The instability is driven by the magnetic flux redistributed from the matter that enters the central object. It typically starts to develop during the transition from the prestellar phase of star formation to the protostellar mass accretion phase. In the latter phase, the magnetic flux is transported outward mainly through advection by strongly magnetized low-density regions that expand against the collapsing inflow. The tussle between the gravity-driven infall and magnetically driven expansion leads to a highly filamentary inner accretion flow that is more disordered than previously envisioned. The efficient outward transport of magnetic flux by advection lowers the field strength at small radii, making the magnetic braking less efficient and the formation of rotationally supported disks easier in principle. However, we find no evidence for such disks in any of our rotating collapse simulations. We conclude that the inner protostellar accretion flow is shaped to a large extent by the flux redistribution-driven magnetic interchange instability. How disks form in such an environment is unclear.

Krasnopolsky, Ruben; Li, Zhi-Yun; Shang, Hsien; Zhao, Bo

2012-09-01

89

Flux Transport and the Sun's Global Magnetic Field  

NASA Technical Reports Server (NTRS)

The Sun s global magnetic field is produced and evolved through the emergence of magnetic flux in active regions and its transport across the solar surface by the axisymmetric differential rotation and meridional flow and the non-axisymmetric convective flows of granulation, supergranulation, and giant cell convection. Maps of the global magnetic field serve as the inner boundary condition for space weather. The photospheric magnetic field and its evolution determine the coronal and solar wind structures through which CMEs must propagate and in which solar energetic particles are accelerated and propagate. Producing magnetic maps which best represent the actual field configuration at any instant requires knowing the magnetic field over the observed hemisphere as well as knowing the flows that transport flux. From our Earth-based vantage point we only observe the front-side hemisphere and each pole is observable for only six months of the year at best. Models for the surface magnetic flux transport can be used to provide updates to the magnetic field configuration in those unseen regions. In this presentation I will describe successes and failures of surface flux transport and present new observations on the structure, the solar cycle variability, and the evolution of the flows involved in magnetic flux transport. I find that supergranules play the dominant role due to their strong flow velocities and long lifetimes. Flux is transported by differential rotation and meridional flow only to the extent that the supergranules participate in those two flows.

Hathaway, David H.

2010-01-01

90

Siphon flows in isolated magnetic flux tubes. II - Adiabatic flows  

NASA Technical Reports Server (NTRS)

This paper extends the study of steady siphon flows in isolated magnetic flux tubes surrounded by field-free gas to the case of adiabatic flows. The basic equations governing steady adiabatic siphon flows in a thin, isolated magnetic flux tube are summarized, and qualitative features of adiabatic flows in elevated, arched flux tubes are discussed. The equations are then cast in nondimensional form and the results of numerical computations of adiabatic siphon flows in arched flux tubes are presented along with comparisons between isothermal and adiabatic flows. The effects of making the interior of the flux tube hotter or colder than the surrounding atmosphere at the upstream footpoint of the arch is considered. In this case, is it found that the adiabatic flows are qualitatively similar to the isothermal flows, with adiabatic cooling producing quantitative differences. Critical flows can produce a bulge point in the rising part of the arch and a concentration of magnetic flux above the bulge point.

Montesinos, Benjamin; Thomas, John H.

1989-01-01

91

Analyses of noise in a permanent magnet wind power generator  

Microsoft Academic Search

? ? ? In this paper the finite element method and Rotating Field Theory are applied to calculate and analyze the origin of induction waves that generate the radial magnetic forces which excite stators vibrations and noise. This methodology is applied in a permanent magnet wind power generator prototype. The calculated results are proven by measurements. The main sources of

C. G. C. Neves

2011-01-01

92

Statistics of magnetic noise in neutron star crusts.  

PubMed

The neutron star crust magnetodynamics is demonstrated to exhibit erratic jumps at the fields corresponding to a sharp change of nuclide magnetic moments induced by quantization effects. Such a noise originates from magnetic avalanches and shows intensity and statistical properties which are favorably compared to the burst activity of soft gamma repeaters. PMID:12059410

Kondratyev, V N

2002-06-01

93

A kinematically distorted flux rope model for magnetic clouds  

Microsoft Academic Search

Constant-? force-free magnetic flux rope models have proven to be a valuable first step toward understanding the global context of in situ observations of magnetic clouds. However, cylindrical symmetry is necessarily assumed when using such models, and it is apparent from both observations and modeling that magnetic clouds have highly noncircular cross sections. A number of approaches have been adopted

M. J. Owens; V. G. Merkin; P. Riley

2006-01-01

94

Vortex Avalanches and Magnetic Flux Fragmentation in Superconductors  

SciTech Connect

We report the results of numerical simulations of nonisothermal dendritic flux penetration in type-II superconductors. We propose a generic mechanism of dynamic branching of a propagating hot spot of a flux flow/normal state triggered by a local heat pulse. The branching occurs when the flux hot spot reflects from inhomogeneities or the boundary on which magnetization currents either vanish, or change direction. The hot spot then undergoes a cascade of successive splittings, giving rise to a dissipative dendritic-type flux structure. This dynamic state eventually cools down, turning into a frozen multifilamentary pattern of magnetization currents.

Aranson, Igor; Gurevich, Alex; Vinokur, Valerii

2001-08-06

95

Signal photon flux and background noise in a coupling electromagnetic detecting system for high-frequency gravitational waves  

SciTech Connect

A coupling system among Gaussian-type microwave photon flux, a static magnetic field, and fractal membranes (or other equivalent microwave lenses) can be used to detect high-frequency gravitational waves (HFGWs) in the microwave band. We study the signal photon flux, background photon flux, and the requisite minimal accumulation time of the signal in the coupling system. Unlike the pure inverse Gertsenshtein effect (G effect) caused by the HFGWs in the gigahertz band, the electromagnetic (EM) detecting scheme proposed by China and the U.S. HFGW groups is based on the composite effect of the synchroresonance effect and the inverse G effect. The key parameter in the scheme is the first-order perturbative photon flux (PPF) and not the second-order PPF; the distinguishable signal is the transverse first-order PPF and not the longitudinal PPF; the photon flux focused by the fractal membranes or other equivalent microwave lenses is not only the transverse first-order PPF but the total transverse photon flux, and these photon fluxes have different signal-to-noise ratios at the different receiving surfaces. Theoretical analysis and numerical estimation show that the requisite minimal accumulation time of the signal at the special receiving surfaces and in the background noise fluctuation would be {approx}10{sup 3}-10{sup 5} seconds for the typical laboratory condition and parameters of h{sub rms}{approx}10{sup -26}-10{sup -30}/{radical}(Hz) at 5 GHz with bandwidth {approx}1 Hz. In addition, we review the inverse G effect in the EM detection of the HFGWs, and it is shown that the EM detecting scheme based only on the pure inverse G effect in the laboratory condition would not be useful to detect HFGWs in the microwave band.

Li Fangyu; Yang Nan; Fang Zhenyun; Wen Hao [Department of Physics, Chongqing University, Chongqing 400044 (China); Baker, Robert M. L. Jr. [GRAWAVE registered LLC, 8123 Tuscany Avenue, Playa del Rey, California 90293 (United States); Stephenson, Gary V. [Seculine Consulting, P.O. Box 925, Redondo Beach, California 90277 (United States)

2009-09-15

96

Localization of metal-induced gap states at the metal-insulator interface: Origin of flux noise in SQUIDs and superconducting qubits  

SciTech Connect

The origin of magnetic flux noise in Superconducting Quantum Interference Devices with a power spectrum scaling as 1/f (f is frequency) has been a puzzle for over 20 years. This noise limits the decoherence time of superconducting qubits. A consensus has emerged that the noise arises from fluctuating spins of localized electrons with an areal density of 5 x 10(17)m(-2). We show that, in the presence of potential disorder at the metal-insulator interface, some of the metal-induced gap states become localized and produce local moments. A modest level of disorder yields the observed areal density.

Choi, SangKook; Lee, Dung-Hai; Louie, Steven G.; Clarke, John

2009-10-10

97

MAGNETIC FLUX PARADIGM FOR RADIO LOUDNESS OF ACTIVE GALACTIC NUCLEI  

SciTech Connect

We argue that the magnetic flux threading the black hole (BH), rather than BH spin or Eddington ratio, is the dominant factor in launching powerful jets and thus determining the radio loudness of active galactic nuclei (AGNs). Most AGNs are radio quiet because the thin accretion disks that feed them are inefficient in depositing magnetic flux close to the BH. Flux accumulation is more likely to occur during a hot accretion (or thick disk) phase, and we argue that radio-loud quasars and strong emission-line radio galaxies occur only when a massive, cold accretion event follows an episode of hot accretion. Such an event might be triggered by the merger of a giant elliptical galaxy with a disk galaxy. This picture supports the idea that flux accumulation can lead to the formation of a so-called magnetically choked accretion flow. The large observed range in radio loudness reflects not only the magnitude of the flux pressed against the BH, but also the decrease in UV flux from the disk, due to its disruption by the ''magnetosphere'' associated with the accumulated flux. While the strongest jets result from the secular accumulation of flux, moderate jet activity can also be triggered by fluctuations in the magnetic flux deposited by turbulent, hot inner regions of otherwise thin accretion disks, or by the dissipation of turbulent fields in accretion disk coronae. These processes could be responsible for jet production in Seyferts and low-luminosity AGNs, as well as jets associated with X-ray binaries.

Sikora, Marek [Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw (Poland); Begelman, Mitchell C., E-mail: sikora@camk.edu.pl, E-mail: mitch@jila.colorado.edu [JILA, University of Colorado and National Institute of Standards and Technology, 440 UCB, Boulder, CO 80309 (United States)

2013-02-20

98

Resistive magnetic flux emergence and formation of solar active regions  

NASA Astrophysics Data System (ADS)

Magnetic flux emergence as the mechanism leading to the formation of magnetized structures in the solar atmosphere plays a key role in the dynamic of the Sun. Observed as a whole, emerging flux regions show clear signs of twisted structure, bearing the magnetic free energy necessary to power active events. The high resolution observations of the recent solar observatories (e.g. Hinode, SDO) have revealed how intermittent the magnetic field appears and how various active events induced by flux emergence are. Magnetic field reconstructions methods show that the topology of the field in interspot regions presents a serpentine structure, i.e. field lines having successive U and ? parts. Associated with the appearance of magnetic polarities, a tremendous number of brief small scale brightening are observed in different photospheric and chromospheric lines, e.g. Ellerman Bombs, along with small scale jet-like structures. These events are believed to be the observational signatures of the multiple magnetic reconnections which enable the magnetic field to emerge further up and magnetically structure the corona above active region. Meanwhile a world-wide effort to numerically model the emergence of magnetic field forming solar active region is been carried on. Using different types of physical paradigm - e.g. idealized magnetohydrodynamic model, advanced treatment of the physical equations, data-driven simulations - these numerical experiments highlight how electric currents can build-up during flux emergence, lead to reconnection and thus explain the formation of the different observed transients.

Pariat, E.; Schmieder, B.; Masson, S.; Aulanier, G.

2012-06-01

99

A correlative study of magnetic flux transfer in the magnetosphere  

NASA Technical Reports Server (NTRS)

The applied magnetic flux of the southward component of the IMF in GSM coordinates (Bz-) which impinges upon the sunward magnetopause is compared to the time integral of the auroral AL index during 56 intervals within a 3-month period in 1969 when interplanetary records from Heos 1 and Explorer 35 were available. The periods of magnetic activity were those for which Bz was greater than 0 and AL was less than 20gamma at the beginning and end of the interval. It was found that for these intervals, the time integral of the AL index was proportional to the applied magnetic flux with a correlation coefficient of 0.94. In addition, the empirical relationships between magnetic flux transfer, applied southward flux, and the time integral of AL arrived at by Holzer and Slavin (1978) on the basis of expansion and contractions of the forward magnetosphere observed with OGO 5 are reexamined and improved.

Holzer, R. E.; Slavin, J. A.

1979-01-01

100

Force-Free Magnetic Flux Ropes in the Solar Corona  

NASA Astrophysics Data System (ADS)

In the course of an ongoing investigation of force-free magnetic fields in the spherical geometry appropriate to the solar corona, we have found solutions that represent magnetic flux ropes. The magnetic energy stored in these ropes and the surrounding field is larger than that which can be stored in simple magnetic arcades with the same boundary conditions, and in some cases exceeds slightly the Aly-Sturrock limit on the energy of a closed force-free magnetic field with all its magnetic flux connected to the coronal base. Flux-rope solutions with the highest energies tend to arise when a strong potential field overlies a region of sheared field containing field-aligned currents. These flux-rope solutions have an unusual topology; instead of a single twisted, disconnected flux system, there are two distinct rope structures. A two-dimensional slice through each rope contains an O-type magnetic neutral point, and the overall solution therefore correspondingly contains two X-type neutral points. We speculate on the relation of this unusual topology to observations of magnetic clouds as interplanetary signatures of coronal mass ejections. This work was supported by NASA grant NAG5-9733 to Middlebury College.

Wolfson, R.

2003-05-01

101

Optimization of the magnetic noise and sensitivity of giant magnetoimpedance sensors  

Microsoft Academic Search

The performance of magnetic field sensors based upon the giant magnetoimpedance (GMI) effect in soft magnetic wires is investigated in the MHz frequency range. The performance of the sensor is based on its sensitivity, voltage noise level and (voltage) noise-to-sensitivity ratio, or intrinsic magnetic noise level. Optimization of the sensitivity and noise response of the sensor through variation of the

L. G. C. Melo; D. Ménard; A. Yelon; L. Ding; S. Saez; C. Dolabdjian

2008-01-01

102

Are Polar Field Magnetic Flux Concentrations Responsible for Missing Interplanetary Flux?  

NASA Astrophysics Data System (ADS)

Magnetohydrodynamic (MHD) simulations are now routinely used to produce models of the solar corona and inner heliosphere for specific time periods. These models typically use magnetic maps of the photospheric magnetic field built up over a solar rotation, available from a number of ground-based and space-based solar observatories. The line-of-sight field at the Sun's poles is poorly observed, and the polar fields in these maps are filled with a variety of interpolation/extrapolation techniques. These models have been found to frequently underestimate the interplanetary magnetic flux (Riley et al., 2012, in press, Stevens et al., 2012, in press) near the minimum part of the cycle unless mitigating correction factors are applied. Hinode SOT observations indicate that strong concentrations of magnetic flux may be present at the poles (Tsuneta et al. 2008). The ADAPT flux evolution model (Arge et al. 2010) also predicts the appearance of such concentrations. In this paper, we explore the possibility that these flux concentrations may account for a significant amount of magnetic flux and alleviate discrepancies in interplanetary magnetic flux predictions. Research supported by AFOSR, NASA, and NSF.

Linker, Jon A.; Downs, C.; Mikic, Z.; Riley, P.; Henney, C. J.; Arge, C. N.

2012-05-01

103

The dynamics of imploding liners in magnetic flux compression experiments  

Microsoft Academic Search

A simple explosive-driven flux compression device is described which generates magnetic fields up to 4 MG. Results from experiments with these devices as well as from other laboratories are analysed from the point of view of energy transfer. It is possible to arrive at realistic estimates of the peak field that can be achieved in any given flux compression experiment

Fritz Herlach; J. E. Kennedy

1973-01-01

104

Spatial Transport of Magnetic Flux Surfaces in Strongly Anisotropic Turbulence  

NASA Astrophysics Data System (ADS)

Magnetic flux surfaces afford familiar descriptions of spatial structure, dynamics, and connectivity of magnetic fields, with particular relevance in contexts such as solar coronal flux tubes, magnetic field connectivity in the interplanetary and interstellar medium, as well as in laboratory plasmas and dynamo problems [1-4]. Typical models assume that field-lines are orderly, and flux tubes remain identifiable over macroscopic distances; however, a previous study has shown that flux tubes shred in the presence of fluctuations, typically losing identity after several correlation scales [5]. Here, the structure of magnetic flux surfaces is numerically investigated in a reduced magnetohydrodynamic (RMHD) model of homogeneous turbulence. Short and long-wavelength behavior is studied statistically by propagating magnetic surfaces along the mean field. At small scales magnetic surfaces become complex, experiencing an exponential thinning. At large scales, instead, the magnetic flux undergoes a diffusive behavior. The link between the diffusion of the coarse-grained flux and field-line random walk is established by means of a multiple scale analysis. Both large and small scales limits are controlled by the Kubo number. These results have consequences for understanding and interpreting processes such as magnetic reconnection and field-line diffusion in plasmas [6]. [1] E. N. Parker, Cosmical Magnetic Fields (Oxford Univ. Press, New York, 1979). [2] J. R. Jokipii and E. N. Parker, Phys. Rev. Lett. 21, 44 (1968). [3] R. Bruno et al., Planet. Space Sci. 49, 1201 (2001). [4] M. N. Rosenbluth et al., Nuclear Fusion 6, 297 (1966). [5] W. H. Matthaeus et al., Phys. Rev. Lett. 75, 2136 (1995). [6] S. Servidio et al., submitted (2013).

Matthaeus, W. H.; Servidio, S.; Wan, M.; Ruffolo, D. J.; Rappazzo, A. F.; Oughton, S.

2013-12-01

105

Sudden flux change studies in high field superconducting accelerator magnets  

SciTech Connect

As part of the High Field Magnet Program at Fermilab many magnets have been tested which utilize multi strand Rutherford type cable made of state-of-the art Nb{sub 3}Sn strands. During these magnet tests we observed sudden flux changes by monitoring coil voltages and the magnetic field close to the magnets. These flux changes might be linked to magnet instabilities. The voltage spike signals were correlated with quench antenna signals, a strong indication that these are magnet phenomena. With a new high resolution voltage spike detection system, we were able to observe the detailed structure of the spikes. Two fundamentally different signal shapes were distinguished, most likely generated by different mechanisms.

Feher, S.; Bordini, B.; Carcagno, R.; Makulski, A.; Orris, D.F.; Pischalnikov, Y.M.; Sylvester, C.; Tartaglia, M.; Tompkins, J.C.; Zlobin, A.V.; /Fermilab

2004-10-01

106

Magnetic flux generation and transport in cool stars  

NASA Astrophysics Data System (ADS)

Context. The Sun and other cool stars harbouring outer convection zones manifest magnetic activity in their atmospheres. The connection between this activity and the properties of a deep-seated dynamo generating the magnetic flux is not well understood. Aims: By employing physical models, we study the spatial and temporal characteristics of the observable surface field for various stellar parameters. Methods: We combine models for magnetic flux generation, buoyancy instability, and transport, which encompass the entire convection zone. The model components are: (i) a thin-layer ?? dynamo at the base of the convection zone; (ii) buoyancy instabilities and the rise of flux tubes through the convection zone in 3D, which provides a physically consistent determination of emergence latitudes and tilt angles; and (iii) horizontal flux transport at the surface. Results: For solar-type stars and rotation periods longer than about 10 days, the latitudinal dynamo waves generated by the deep-seated ?? dynamo are faithfully reflected by the surface distribution of magnetic flux. For rotation periods of the order of two days, however, Coriolis acceleration of rising flux loops leads to surface flux emergence at much higher latitudes than the dynamo waves at the bottom of the convection zone reach. A similar result is found for a K0V star with a rotation period of two days. In the case of a rapidly rotating K1 subgiant, overlapping dynamo waves lead to noisy activity cycles and mixed-polarity fields at high latitudes. Conclusions: The combined model reproduces the basic observed features of the solar cycle. The differences between the latitude distributions of the magnetic field at the bottom of the convection zone and the emerging surface flux grow with increasing rotation rate and convection zone depth, becoming quite substantial for rapidly rotating dwarfs and subgiants. The dynamical evolution of buoyantly rising magnetic flux should be considered as an essential ingredient in stellar dynamo models. 3 movies are only available in electronic form at http://www.aanda.org

I??k, E.; Schmitt, D.; Schüssler, M.

2011-04-01

107

Noise in small magnetic systems—applications to very sensitive magnetoresistive sensors  

NASA Astrophysics Data System (ADS)

Reduction for 1/ f noise (or random telegraph noise) is a crucial issue for small magnetic sensors which is strongly related to structural properties and magnetic configuration. We show how it is possible to eliminate magnetic noise at low frequency in GMR/TMR sensors by a combination of cross anisotropies, window frame shapes and suitably designed magnetoresisitive stack. These sensors are superior to almost all existing field and flux sensors. Results are presented on a mixed sensor, where a superconducting loop acts as a flux-to-field transformer to the GMR sensor. This device is suitable for detection of biomagnetic signals, such as in magnetocardiography or in magnetoencephalography. Measurements on niobium-based and YBCO-based sensors are presented, leading to sensitivity of 30 fT/?Hz at 77 K for small samples. Sensitivity lower than 1 fT/?(Hz) is expected with appropriate design and use of TMR or CMR layers, which makes these a powerful alternative to SQUIDs.

Pannetier, M.; Fermon, C.; Le Goff, G.; Simola, J.; Kerr, E.; Coey, J. M. D.

2005-04-01

108

DETECTION OF MECHANICAL DAMAGE USING THE MAGNETIC FLUX LEAKAGE TECHNIQUE  

Microsoft Academic Search

Since magnetism is strongly stress dependent, Magnetic Flux Leakage (MFL) inspection tools have the potential to locate and characterize mechanical damage in pipelines. However, MFL application to mechanical damage detection faces hurdles which make signal interpretation problematic: 1) the MFL signal is a superposition of geometrical and stress effects, 2) the stress distribution around a mechanically damaged region is very

L. Clapham; V. Babbar; James Byrne

109

On the magnetic flux conservation in the partially ionized plasma  

NASA Astrophysics Data System (ADS)

Ohm, Hall, and ambipolar diffusions in the partially ionized plasma are considered. The statement of Pandey & Wardle (2011,2012) that only Ohmic diffusion is capable of destroying the magnetic flux is not sufficiently correct due to the formal dependence of the magnetic diffusion on a frame of reference.

Tsap, Yuriy

2013-07-01

110

Electromagnetic modelling of permanent magnet axial flux motors and generators  

Microsoft Academic Search

Purpose – To provide a general framework for the electromagnetic analysis of axial flux motors and generators. Design\\/methodology\\/approach – The procedure is based on the solution of Maxwell's equation in a cylindrical frame. All field sources (permanent magnets, windings) are subdivided into filamentary windings. The expansion of the 2D air-gap magnetic field into a Fourier series is computed at every

Fabrizio Marignetti; Gianni Tomassi; James R. Bumby

2006-01-01

111

Magnetic Flux Dynamics in Horizontally Cooled Superconducting Cavities  

E-print Network

Previous studies on magnetic flux expulsion as a function of cooling details have been performed for superconducting niobium cavities with the cavity beam axis placed parallel respect to the helium cooling flow, and findings showed that for sufficient cooling thermogradients all magnetic flux could be expelled and very low residual resistance could be achieved. In this paper we investigate the flux trapping and its impact on radio frequency surface resistance when the resonators are positioned perpendicularly to the helium cooling flow, which is representative of how superconducting radio-frequency (SRF) cavities are cooled in an accelerator. We also extend the studies to different directions of applied magnetic field surrounding the resonator. Results show that in the cavity horizontal configuration there is a different impact of the various field components on the final surface resistance, and that several parameters have to be considered to understand flux dynamics. A newly discovered phenomenon of concent...

Martinello, M; Grassellino, A; Crawford, A C; Melnychuk, O; Romanenko, A; Sergatkov, D A

2015-01-01

112

SIGNATURES OF MAGNETIC RECONNECTION AT BOUNDARIES OF INTERPLANETARY SMALL-SCALE MAGNETIC FLUX ROPES  

SciTech Connect

The interaction between interplanetary small-scale magnetic flux ropes and the magnetic field in the ambient solar wind is an important topic in the understanding of the evolution of magnetic structures in the heliosphere. Through a survey of 125 previously reported small flux ropes from 1995 to 2005, we find that 44 of them reveal clear signatures of Alfvenic fluctuations and thus classify them as Alfven wave trains rather than flux ropes. Signatures of magnetic reconnection, generally including a plasma jet of {approx}30 km s{sup -1} within a magnetic field rotational region, are clearly present at boundaries of about 42% of the flux ropes and 14% of the wave trains. The reconnection exhausts are often observed to show a local increase in the proton temperature, density, and plasma beta. About 66% of the reconnection events at flux rope boundaries are associated with a magnetic field shear angle larger than 90{sup 0} and 73% of them reveal a decrease of 20% or more in the magnetic field magnitude, suggesting a dominance of anti-parallel reconnection at flux rope boundaries. The occurrence rate of magnetic reconnection at flux rope boundaries through the years 1995-2005 is also investigated and we find that it is relatively low around the solar maximum and much higher when approaching solar minima. The average magnetic field depression and shear angle for reconnection events at flux rope boundaries also reveal a similar trend from 1995 to 2005. Our results demonstrate for the first time that boundaries of a substantial fraction of small-scale flux ropes have properties similar to those of magnetic clouds, in the sense that both of them exhibit signatures of magnetic reconnection. The observed reconnection signatures could be related either to the formation of small flux ropes or to the interaction between flux ropes and the interplanetary magnetic fields.

Tian Hui; Yao Shuo; Zong Qiugang; Qi Yu [School of Earth and Space Sciences, Peking University, 100871 Beijing (China); He Jiansen, E-mail: tianhui924@pku.edu.c [Max-Planck-Institut fuer Sonnensystemforschung, 37191 Katlenburg-Lindau (Germany)

2010-09-01

113

The Solar Internetwork. I. Contribution to the Network Magnetic Flux  

NASA Astrophysics Data System (ADS)

The magnetic network (NE) observed on the solar surface harbors a sizable fraction of the total quiet Sun flux. However, its origin and maintenance are not well known. Here we investigate the contribution of internetwork (IN) magnetic fields to the NE flux. IN fields permeate the interior of supergranular cells and show large emergence rates. We use long-duration sequences of magnetograms acquired by Hinode and an automatic feature tracking algorithm to follow the evolution of NE and IN flux elements. We find that 14% of the quiet Sun (QS) flux is in the form of IN fields with little temporal variations. IN elements interact with NE patches and modify the flux budget of the NE either by adding flux (through merging processes) or by removing it (through cancellation events). Mergings appear to be dominant, so the net flux contribution of the IN is positive. The observed rate of flux transfer to the NE is 1.5 × 1024 Mx day-1 over the entire solar surface. Thus, the IN supplies as much flux as is present in the NE in only 9-13 hr. Taking into account that not all the transferred flux is incorporated into the NE, we find that the IN would be able to replace the entire NE flux in approximately 18-24 hr. This renders the IN the most important contributor to the NE, challenging the view that ephemeral regions are the main source of flux in the QS. About 40% of the total IN flux eventually ends up in the NE.

Goši?, M.; Bellot Rubio, L. R.; Orozco Suárez, D.; Katsukawa, Y.; del Toro Iniesta, J. C.

2014-12-01

114

Algebraic reconstruction for 3D magnetic resonance-electrical impedance tomography (MREIT) using one component of magnetic flux density.  

PubMed

Magnetic resonance-electrical impedance tomography (MREIT) algorithms fall into two categories: those utilizing internal current density and those utilizing only one component of measured magnetic flux density. The latter group of algorithms have the advantage that the object does not have to be rotated in the magnetic resonance imaging (MRI) system. A new algorithm which uses only one component of measured magnetic flux density is developed. In this method, the imaging problem is formulated as the solution of a non-linear matrix equation which is solved iteratively to reconstruct resistivity. Numerical simulations are performed to test the algorithm both for noise-free and noisy cases. The uniqueness of the solution is monitored by looking at the singular value behavior of the matrix and it is shown that at least two current injection profiles are necessary. The method is also modified to handle region-of-interest reconstructions. In particular it is shown that, if the image of a certain xy-slice is sought for, then it suffices to measure the z-component of magnetic flux density up to a distance above and below that slice. The method is robust and has good convergence behavior for the simulation phantoms used. PMID:15005322

Ider, Y Ziya; Onart, Serkan

2004-02-01

115

Superpoissonian shot noise in organic magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

Organic molecules have recently revolutionized ways to create new spintronic devices. Despite intense studies, the statistics of tunneling electrons through organic barriers remains unclear. Here, we investigate conductance and shot noise in magnetic tunnel junctions with 3,4,9,10-perylene-teracarboxylic dianhydride (PTCDA) barriers a few nm thick. For junctions in the electron tunneling regime, with magnetoresistance ratios between 10% and 40%, we observe superpoissonian shot noise. The Fano factor exceeds in 1.5-2 times the maximum values reported for magnetic tunnel junctions with inorganic barriers, indicating spin dependent bunching in tunneling. We explain our main findings in terms of a model which includes tunneling through a two level (or multilevel) system, originated from interfacial bonds of the PTCDA molecules. Our results suggest that interfaces play an important role in the control of shot noise when electrons tunnel through organic barriers.

Cascales, Juan Pedro; Hong, Jhen-Yong; Martinez, Isidoro; Lin, Minn-Tsong; Szczepa?ski, Tomasz; Dugaev, Vitalii K.; Barna?, Józef; Aliev, Farkhad G.

2014-12-01

116

Magnetic flux leakage investigation of interacting defects: Stress and geometry effects  

NASA Astrophysics Data System (ADS)

The Magnetic Flux Leakage (MFL) non-destructive technique is the most cost-effective way to monitor corrosion on in-service oil and gas pipelines. The method consists of magnetizing the pipe wall and detecting the leakage flux as a defect indication. The MFL signals are used for locating and sizing metal loss defects in the pipe. The shape and amplitude of the MFL signals do not depend only on defect geometry, but also on tool speed, applied flux density, inspection configuration, magnetic properties, and stress state of the pipe wall. The last two factors cannot be accounted for through calibration measurements. High operating pressures of the pipelines, combined with the local variation of both stress and magnetic properties of the material make the MFL field data difficult to interpret. The main objective of this project was the study of interacting defects, a term referring to two sufficiently close corrosion pits that the area between them is subjected to stress superposition and magnetic flux shielding. This research analyzed the stress-dependent magnetic flux leakage (MFL) signatures from various single and interacting defects, electrochemically milled on steel samples at no applied stress or at an applied tensile stress of 46% of the material yield strength, and magnetized at moderate and high field amplitudes. A complementary technique, Magnetic Barkhausen Noise (MBN), capable of concurrently detecting magnetic properties and local stresses on a microscopic scale, was used in interpreting the stress-affected MFL signals. The MBN method was also employed for macroscopic magnetic characterization of the ferromagnetic samples under uniaxial tensile load. Bulk and local MBN experimental results were compared with existing theoretical and numerical models of stress patterns. The MBN results were used to model the MFL response to stress under low MFL applied field conditions. Under high field MFL conditions, stress effects are small and flux shielding dominates the signal from the interacting defects. An analytical model was developed for the latter case, which relates the normal and tangential MFL components to the defect size and location, respectively. Flux shielding effects were accounted for in the model and good agreement between theoretical and experimental results suggested that this method can be used as an inverse MFL data interpretation technique.

Mandache, Catalin

117

Topology bifurcation of a magnetic flux surface in toroidal plasmas  

NASA Astrophysics Data System (ADS)

The bifurcation of magnetic topology is identified by a significant change in the heat pulse propagation properties in magnetized plasmas. Clear evidence of stochastization of the magnetic surfaces near a rational surface is observed in the core plasma with weak magnetic shear in the Large Helical Device by slowly decreasing the magnetic shear and measured by applying heat pulses driven by modulated electron cyclotron heating (MECH). Three topologies of the magnetic flux surfaces (a nested magnetic island and partial and full stochastization) are identified by the patterns of heat pulse propagation observed in the flat temperature region in the plasma. Slow heat pulse propagation exhibiting a non-monotonically increasing delay time is evidence of a magnetic island, while the fast heat pulse propagation observed in the plasma with medium magnetic shear is evidence of the stochastization of the magnetic surfaces. The region with the fast heat pulse propagation varies with a slight change of magnetic shear. There are two types of stochastization of the magnetic surfaces. In one, the stochastization region is localized near the rational surface (partial stochastization) and in the other, the region is extended to the magnetic axis (full stochastization). The appearance of a stochastic magnetic field is not caused by MHD instability. The significant increase of the ratio of electron thermal diffusivity to ion thermal diffusivity is consistent with that expected by stochastization of the magnetic field.

Ida, K.; Kobayashi, T.; Inagaki, S.; Suzuki, Y.; Sakakibara, S.; Itoh, K.; Tsuchiya, H.; Suzuki, C.; Yoshinuma, M.; Narushima, Y.; Yokoyama, M.; Itoh, S.-I.

2015-01-01

118

Powering jets with small-scale magnetic flux  

NASA Astrophysics Data System (ADS)

The relativistic jets associated with radio galaxies and some X-ray binaries are generally believed to be driven by magnetic fields in the immediate vicinity of a central black hole. The classic Blandford-Znajek process, usually invoked to explain the energy and angular momentum transfer from the black hole to the surrounding plasma, requires a large-scale ordered magnetic field and a significant magnetic flux through the ergosphere; in this scenario the jet power scales with the square of the magnetic flux. In many observed systems the presence of the required large-scale flux is questionable. I will describe an alternative model, in which jets are produced by the accretion of small magnetic loops, whose energy is amplified by the differential rotation of the accretion disk and by the rotating spacetime of the black hole. The build up of energy in these coronal flux systems eventually leads to runaway field line inflation and a temporary Blandford-Znajek phase, magnetic reconnection in thin current layers, and the ejection of bubbles of magnetically dominated plasma. In this scenario counter-rotating accretion flows actually produce jets more naturally: prograde disks power jets only when they create flux systems larger than a critical poloidal scale, whereas there is no minimal scale for retrograde accretion. The mechanism will be illustrated using new general-relativistic simulations of the coronae of rotating black holes, performed in the nearly force-free, or high magnetization, limit of MHD. The dissipation of magnetic energy by coronal reconnection events, as demonstrated in these simulations, is a potential source of the observed coronal high-energy emission from accreting black holes.

Parfrey, Kyle; Giannios, Dimitrios; Beloborodov, Andrei M

2014-08-01

119

Casimir interactions between magnetic flux tubes in a dense lattice  

NASA Astrophysics Data System (ADS)

We use the worldline numerics technique to study a cylindrically symmetric model of magnetic flux tubes in a dense lattice and the nonlocal Casimir forces acting between regions of magnetic flux. Within a superconductor the magnetic field is constrained within magnetic flux tubes and if the background magnetic field is on the order the quantum critical field strength, Bk=m/2 e =4.4 ×1013 Gauss, the magnetic field is likely to vary rapidly on the scales where QED effects are important. In this paper, we construct a cylindrically symmetric toy model of a flux tube lattice in which the nonlocal influence of QED on neighboring flux tubes is taken into account. We compute the effective action densities using the worldline numerics technique. The numerics predict a greater effective energy density in the region of the flux tube, but a smaller energy density in the regions between the flux tubes compared to a locally constant-field approximation. We also compute the interaction energy between a flux tube and its neighbors as the lattice spacing is reduced from infinity. Because our flux tubes exhibit compact support, this energy is entirely nonlocal and predicted to be zero in local approximations such as the derivative expansion. This Casimir-Polder energy can take positive or negative values depending on the distance between the flux tubes, and it may cause the flux tubes in neutron stars to form bunches. In addition to the above results we also discuss two important subtleties of determining the statistical uncertainties within the worldline numerics technique. Firstly, the distributions generated by the worldline ensembles are highly non-Gaussian, and so the standard error in the mean is not a good measure of the statistical uncertainty. Secondly, because the same ensemble of worldlines is used to compute the Wilson loops at different values of T and xcm, the uncertainties associated with each computed value of the integrand are strongly correlated. We recommend a form of jackknife analysis which deals with both of these problems.

Mazur, Dan; Heyl, Jeremy S.

2015-03-01

120

The Evolution of Open Magnetic Flux Driven by Photospheric Dynamics  

NASA Technical Reports Server (NTRS)

The coronal magnetic field is of paramount importance in solar and heliospheric physics. Two profoundly different views of the coronal magnetic field have emerged. In quasi-steady models, the predominant source of open magnetic field is in coronal holes. In contrast, in the interchange model, the open magnetic flux is conserved, and the coronal magnetic field can only respond to the photospheric evolution via interchange reconnection. In this view the open magnetic flux diffuses through the closed, streamer belt fields, and substantial open flux is present in the streamer belt during solar minimum. However, Antiochos and co-workers, in the form of a conjecture, argued that truly isolated open flux cannot exist in a configuration with one heliospheric current sheet (HCS) - it will connect via narrow corridors to the polar coronal hole of the same polarity. This contradicts the requirements of the interchange model. We have performed an MHD simulation of the solar corona up to 20R solar to test both the interchange model and the Antiochos conjecture. We use a synoptic map for Carrington Rotation 1913 as the boundary condition for the model, with two small bipoles introduced into the region where a positive polarity extended coronal hole forms. We introduce flows at the photospheric boundary surface to see if open flux associated with the bipoles can be moved into the closed-field region. Interchange reconnection does occur in response to these motions. However, we find that the open magnetic flux cannot be simply injected into closed-field regions - the flux eventually closes down and disconnected flux is created. Flux either opens or closes, as required, to maintain topologically distinct open and closed field regions, with no indiscriminate mixing of the two. The early evolution conforms to the Antiochos conjecture in that a narrow corridor of open flux connects the portion of the coronal hole that is nearly detached by one of the bipoles. In the later evolution, a detached coronal hole forms, in apparent violation of the Antiochos conjecture. Further investigation reveals that this detached coronal hole is actually linked to the extended coronal hole by a separatrix footprint on the photosphere of zero width. Therefore, the essential idea of the conjecture is preserved, if we modify it to state that coronal holes in the same polarity region are always linked, either by finite width corridors or separatrix footprints. The implications of these results for interchange reconnection and the sources of the slow solar wind are briefly discussed.

Linker, Jon A.; Lionello, Roberto; Mikic, Zoran; Titov, Viacheslav S.; Antiochos, Spiro K.

2010-01-01

121

A generalized flux function for three-dimensional magnetic reconnection  

SciTech Connect

The definition and measurement of magnetic reconnection in three-dimensional magnetic fields with multiple reconnection sites is a challenging problem, particularly in fields lacking null points. We propose a generalization of the familiar two-dimensional concept of a magnetic flux function to the case of a three-dimensional field connecting two planar boundaries. In this initial analysis, we require the normal magnetic field to have the same distribution on both boundaries. Using hyperbolic fixed points of the field line mapping, and their global stable and unstable manifolds, we define a unique flux partition of the magnetic field. This partition is more complicated than the corresponding (well-known) construction in a two-dimensional field, owing to the possibility of heteroclinic points and chaotic magnetic regions. Nevertheless, we show how the partition reconnection rate is readily measured with the generalized flux function. We relate our partition reconnection rate to the common definition of three-dimensional reconnection in terms of integrated parallel electric field. An analytical example demonstrates the theory and shows how the flux partition responds to an isolated reconnection event.

Yeates, A. R.; Hornig, G. [Division of Mathematics, University of Dundee, Dundee DD1 4HN (United Kingdom)

2011-10-15

122

Modeled ground magnetic signatures of flux transfer events  

NASA Technical Reports Server (NTRS)

The magnetic field on the ground due to a small (not greater than 200 km scale size) localized field-aligned current (FAC) system interacting with the ionosphere is calculated in terms of an integral over the ionospheric distribution of FAC. Two different candidate current systems for flux transfer events (FTEs) are considered: (1) a system which has current flowing down the center of a cylindrical flux tube with a return current uniformly distributed along the outside edge; and (2) a system which has upward current on one half of the perimeter of a cylindrical flux tube with downward current on the opposite half. The peak magnetic field on the ground is found to differ by a factor of 2 between the two systems, and the magnetic perturbations are in different directions depending on the observer's position.

Mchenry, Mark A.; Clauer, C. Robert

1987-01-01

123

Tubes of Magnetic Flux and Electric Current in Space Physics  

NASA Astrophysics Data System (ADS)

The singularities of an irrotational magnetic field are lines of electric current. This property derives from the relationship between vector fields and the topology of the underlying three-space and allows for a definition of cosmic field flux tubes and flux ropes as cores (in the sense of the physics of defects) of helical singularities. When applied to force-free flux ropes, and assuming current conservation, an interesting feature is the quantization of the radii, pitches, and helicities. One expects similar quantization effects in the general case. In the special case when the total electric current vanishes, a force-free rope embedded in a medium devoid of magnetic field is nonetheless topologically stable, because it is the core of a singularity of the vector potential. Magnetic merging is also briefly discussed in the same framework.

Kleman, M.; Robbins, J. M.

2014-04-01

124

Determining the axis orientation of cylindrical magnetic flux rope  

NASA Astrophysics Data System (ADS)

We develop a new simple method for inferring the orientation of a magnetic flux rope, which is assumed to be a time-independent cylindrically symmetric structure via the direct single-point analysis of magnetic field structure. The model tests demonstrate that, for the cylindrical flux rope regardless of whether it is force-free or not, the method can consistently yield the axis orientation of the flux rope with higher accuracy and stability than the minimum variance analysis of the magnetic field and the Grad-Shafranov reconstruction technique. Moreover, the radial distance to the axis center and the current density can also be estimated consistently. Application to two actual flux transfer events observed by the four satellites of the Cluster mission demonstrates that the method is more appropriate to be used for the inner part of flux rope, which might be closer to the cylindrical structure, showing good agreement with the results obtained from the optimal Grad-Shafranov reconstruction and the least squares technique of Faraday's law, but fails to produce such agreement for the outer satellite that grazes the flux rope. Therefore, the method must be used with caution.

Rong, Zhaojin; Wan, Weixing; Shen, Chao; Zhang, Tielong; Lui, Anthony; Wang, Yuming; Dunlop, malcolm; Zhang, Yongcun; Zong, Qiugang

2013-04-01

125

The magnetic flux in the quiet sun network  

NASA Technical Reports Server (NTRS)

Direct magnetic measurements are used to confirm that the Ca II K line emission from the quiet sun network does not vary with the 11 year cycle (White and Livingston, 1981). As the K emission intensity is correlated with magnetic field strength, magnetic flux measurements are valid for comparison. Data were taken from the full-disk Mount Wilson daily magnetograms, and the aperture was decreased from 17.5 to 12.5 during the interval 1970 through 1980. Measurements are restricted to the latitude zones centered on + or - 1.7 degrees. The mean total flux in the + or - 15.3 degree zones increased by a factor of 10 between activity minimum and maximum, and the quiet sun flux shows no variation, thus confirming White and Livingston's (1981) result. In order for the amplitude of the quiet sun magnetic flux to remain constant, a balance between increasing and decreasing factors must exist, and the primary factor in this balance is the rate of destruction of the quiet sun flux.

Labonte, B. J.; Howard, R.

1982-01-01

126

Sausage Mode Propagation in a Thick Magnetic Flux Tube  

E-print Network

The aim of this paper is to model the propagation of slow magnetohydrodynamic (MHD) sausage waves in a thick expanding magnetic flux tube in the context of the quiescent (VAL C) solar atmosphere. The propagation of these waves is found to be described by the Klein-Gordon equation. Using the governing MHD equations and the VAL C atmosphere model we study the variation of the cut-off frequency along and across the magnetic tube guiding the waves. Due to the radial variation of the cut-off frequency the flux tubes act as low frequency filters for waves.

Pardi, Anabele-Linda; Marcu, Alexandru; Orza, Beniamin

2013-01-01

127

A study of magnetic flux-leakage signals  

NASA Astrophysics Data System (ADS)

Magnetic flux-leakage (MFL) measurement is the most widely used non-destructive technique for in-service inspection of oil and gas pipelines. The effect of line-pressure-induced hoop stress on MFL signals has been studied for an electrochemically milled pit (50% penetration) in a 9 mm thick steel pipe wall. The existing theoretical models for flux-leakage signals are discussed. Among them the Zatsepin-Shcherbinin and Edwards-Palmer models have been fitted to the axial and radial MFL signals. Both the leakage-flux components under various stresses are scaled to make them stress independent.

Mandal, K.; Atherton, D. L.

1998-11-01

128

Magnetic and Electric Dipole Constraints on Extra Dimensions and Magnetic Fluxes  

E-print Network

The propagation of charged particles and gauge fields in a compact extra dimension contributes to $g-2$ of the charged particles. In addition, a magnetic flux threading this extra dimension generates an electric dipole moment for these particles. We present constraints on the compactification size and on the possible magnetic flux imposed by the comparison of data and theory of the magnetic moment of the muon and from limits on the electric dipole moments of the muon, neutron and electron.

Aaron J. Roy; Myron Bander

2008-05-10

129

Experimentally realizable devices for controlling the motion of magnetic flux quanta in anisotropic superconductors.  

PubMed

A new generation of microscopic ratchet systems is currently being developed for controlling the motion of electrons and fluxons, as well as for particle separation and electrophoresis. Virtually all of these use static spatially asymmetric potential energies to control transport properties. Here we propose completely new types of ratchet-like systems that do not require fixed spatially asymmetric potentials in the samples. As specific examples of this novel general class of ratchets, we propose devices that control the motion of flux quanta in superconductors and could address a central problem in many superconducting devices; namely, the removal of trapped magnetic flux that produces noise. In layered superconductors there are two interpenetrating perpendicular vortex lattices consisting of Josephson vortices (JVs) and pancake vortices (PVs). We show that, owing to the JV-PV mutual interaction and asymmetric driving, the a.c. motion of JVs and/or PVs can provide a net d.c. vortex current. This controllable vortex motion can be used for making pumps, diodes and lenses of quantized magnetic flux. These proposed devices sculpt the microscopic magnetic flux profile by simply modifying the time dependence of the a.c. drive, without the need for samples with static pinning--for example, without lithography or irradiation. PMID:12618807

Savel'ev, Sergey; Nori, Franco

2002-11-01

130

Numerical simulation of reconnection in an emerging magnetic flux region  

NASA Technical Reports Server (NTRS)

Numerical solutions in two dimensions are presented for the resistive MHD equations of an initial boundary value problem, simulating reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. This numerical model displays four main phases, which are interpreted in terms of (1) a slowly evolving quasi-steady phase during which most of the magnetic flux emerges, with approximate equilibrium between magnetic and pressure forces; (2) an impulsive phase in which either the reconnection or continuing emergence of the first phase comes to disrupt the equilibrium, and extensive acceleration occurs as the high pressure region within the emerging region drives the fluid upwards and outwards; (3) a second quasi-steady phase; and (4) a potential-static phase in which continuing reconnection, ohmic dissipation, and fluid transport through boundaries depletes the system of all currents and flows.

Forbes, T. G.; Priest, E. R.

1984-01-01

131

FLUXCAP: A flux-coupled ac/dc magnetizing device  

E-print Network

We report on an instrument for applying ac and dc magnetic fields by capturing the flux from a rotating permanent magnet and projecting it between two adjustable pole pieces. This can be an alternative to standard electromagnets for experiments with small samples or in probe stations in which an applied magnetic field is needed locally, with advantages that include a compact form-factor, very low power requirements and dissipation as well as fast field sweep rates. This flux capture instrument (FLUXCAP) can produce fields from -400 to +400 mT, with field resolution less than 1 mT. It generates static magnetic fields as well as ramped fields, with ramping rates as high as 10 T/s. We demonstrate the use of this apparatus for studying the magnetotransport properties of spin-valve nanopillars, a nanoscale device that exhibits giant magnetoresistance.

Gopman, Daniel B; Kent, Andrew D

2012-01-01

132

A flux-coupled ac/dc magnetizing device  

NASA Astrophysics Data System (ADS)

We report on an instrument for applying ac and dc magnetic fields by capturing the flux from a rotating permanent magnet and projecting it between two adjustable pole pieces. This can be an alternative to standard electromagnets for experiments with small samples or in probe stations in which an applied magnetic field is needed locally, with advantages that include a compact form-factor, very low power requirements and dissipation as well as fast field sweep rates. This flux capture instrument (FLUXCAP) can produce fields from -400 to +400 mT, with field resolution less than 1 mT. It generates static magnetic fields as well as ramped fields, with ramping rates as high as 10 T/s. We demonstrate the use of this apparatus for studying the magnetotransport properties of spin-valve nanopillars, a nanoscale device that exhibits giant magnetoresistance.

Gopman, D. B.; Liu, H.; Kent, A. D.

2013-06-01

133

A magnetic flux leakage NDE system for CANDU feeder pipes  

NASA Astrophysics Data System (ADS)

This work examines the application of different magnetic flux leakage (MFL) inspection concepts to the non destructive evaluation (NDE) of residual (elastic) stresses in CANDURTM reactor feeder pipes. The stress sensitivity of three MFL inspection techniques was examined with flat plate samples, with stress-induced magnetic anisotropy (SMA) demonstrating the greatest stress sensitivity. A prototype SMA testing system was developed to apply magnetic NDE to feeders. The system consists of a flux controller that incorporates feedback from a wire coil and a Hall sensor (FCV2), and a magnetic anisotropy prototype (MAP) probe. The combination of FCV2 and the MAP probe was shown to provide SMA measurements on feeder pipe samples and predict stresses from SMA measurements with a mean accuracy of +/-38MPa.

Mak, Thomas Don

134

1/f noise and susceptibility-magnetization correlation in disordered ferromagnets  

NASA Astrophysics Data System (ADS)

We consider a strongly disordered ferromagnet modeled by Ising spins placed at random in 2D with ferromagnetic interactions decaying exponentially with inter-site distance. Ferromagnetic phase in this model arises due to formation of infinite percolation cluster of strongly interacting spins. Fractal nature of the percolation cluster manifests itself in the dynamics of the system in the vicinity of the percolation transition. Simulating the dynamics with single spin flip Monte Carlo algorithm we observe 1/f power spectra of magnetization noise in a wide temperature range near the transition. Subjected to external AC magnetic field the system shows significant cross-correlation between susceptibility and magnetization in the ferromagnetic phase. This results suggest a possible explanation of the inductance-flux cross-correlation recently observed in SQUIDs [1]. [4pt] [1] S. Sendelbach, D. Hover, M. Muck, and R. McDermott, Phys. Rev. Lett. 103, 117001 (2009)

Kechedzhi, Kostyantyn

2011-03-01

135

Flux noise resulting from vortex avalanches using a simple kinetic model  

SciTech Connect

We have carried out a model calculation of the flux noise produced by vortex avalanches in a type-II superconductor, using a simple kinetic model proposed by Bassler and Paczuski. Over a broad range of frequencies, we find that the flux noise S{sub {Phi}}({omega}) has a power-law dependence on frequency, S{sub {Phi}}({omega}){approximately}{omega}{sup {minus}s}, with s{approximately}1.4 in reasonable agreement with experiment. In addition, for small lattices, the calculated S{sub {Phi}}({omega}) has a high-frequency knee, which is seen in some experiments, and which is due to the finite lattice size. Deviations between calculation and experiment are attributed mostly to uncertainties in the measured critical current densities and pinning strengths of the experimental samples. {copyright} {ital 1999} {ital The American Physical Society}

Mohler, G.; Stroud, D. [Department of Physics, The Ohio State University, Columbus, Ohio 43210 (United States)] [Department of Physics, The Ohio State University, Columbus, Ohio 43210 (United States)

1999-10-01

136

MESSENGER Observations of Magnetic Flux Ropes in Mercury's Plasma Sheet  

NASA Astrophysics Data System (ADS)

MESSENGER orbital observations provide a new opportunity to investigate magnetic reconnection in the cross-tail current sheet of Mercury's magnetotail. Here we use measurements collected by the Magnetometer and Fast Imaging Plasma Spectrometer (FIPS) during 'hot seasons,' when the orbital periapsis is on Mercury's dayside and MESSENGER crosses the plasma sheet at distances of ~1.5 to 3 RM (where RM is Mercury's radius, or 2440 km). These data frequently contain signatures of large-scale magnetic reconnection in the form of plasmoid-type magnetic flux ropes and southward magnetic fields in the post-plasmoid plasma sheet. In the cross-tail current sheet, which separates the north and south lobes of the magnetotail, flux ropes are formed by reconnection at two or more X-lines and are then transported either toward or away from the planet by the Alfvénic flow emanating from the X-lines. Here we present a survey of 49 plasmoid-type flux ropes identified during seven MESSENGER 'hot seasons,' for which minimum variance analysis indicates that the spacecraft passed near the central axis of the structure. The locations of the selected flux ropes range between 1.7 and 2.8 RM down the tail from the center of the planet. With FIPS measurements, we determined an average proton density of 2.55 cm-3 in the adjacent plasma sheet surrounding the flux ropes, implying an Alfvén speed of ~450 km s-1. Under the assumption that the flux ropes are moving at the local Alfvén speed, we used the mean duration of 0.74 ą 0.15 s to calculate a typical diameter of ~0.14 RM, or ~340 km. We have modeled the plasmoids as force-free flux ropes in order to confirm this result. A superposed epoch analysis demonstrates that the magnetic structure of the flux ropes is similar to what is observed at Earth, but the timescales are 40 times faster at Mercury. The results of this flux rope survey indicate that intense magnetic reconnection occurs frequently in the cross-tail current layer of this small but extremely dynamic magnetosphere.

DiBraccio, Gina A.; Slavin, James A.; Imber, Suzanne M.; Gershman, Daniel J.; Raines, Jim M.; Boardsen, Scott A.; Anderson, Brian J.; Korth, Haje; Zurbuchen, Thomas H.; McNutt, Ralph L., Jr.; Solomon, Sean C.

2014-05-01

137

Isolated Warm Flux Tubes in the Inner Magnetosphere: Agents for Magnetic Flux Transport  

NASA Astrophysics Data System (ADS)

A surprising observation during the initial Saturn Insertion Orbit was the discovery of magnetic flux tubes, apparently isolated from similar plasma conditions, with depressed magnitudes in an otherwise quiet region of the E-ring plasma torus. These were not reported on previous missions (the flybys of Pioneer 11 and Voyagers 1 and 2), apparently as subsequent Cassini observations show, because these flux tubes are rare. At this writing they have been detected on only three passes through the E-ring. The tubes sometimes appear to be twisted, as would be appropriate if they were isolated tubes moving relative to other magnetospheric flux tubes. Also the tubes are inclined to neighboring flux tubes when observed away from the equator. The sense of the inclination is such that these tubes are less stretched and more dipolar than their neighbors. This suggests that these tubes are more buoyant than surrounding flux tubes and indeed electron density measurements with the plasma wave subsystem are greatly reduced. Thus these tubes appear to be the counterpart to the jovian flux tubes seen in the Io torus called depleted flux tubes. However, in the jovian case the flux tubes have an increased rather than decreased field strength. At both planets we interpret these tubes as agents for the return of `emptied' magnetic flux tubes, from which the mass loaded on the tubes has been removed, presumably by reconnection in the more distant magnetotail. The scarcity of these tubes may be a function of the infrequency of reconnection events (substorms?) together with their rapid return to the inner magnetosphere once formed.

Russell, C. T.; Leisner, J. S.; Khurana, K. K.; Dougherty, M. K.; Kurth, W. S.

2005-12-01

138

Double dipolar halbach array for rheological measurements on magnetic fluids at variable magnetic flux density B  

NASA Astrophysics Data System (ADS)

A new experimental setup for measuring rheological properties of magnetic fluids at variable magnetic flux density B with increased homogeneity is described. The proposed setup is mounted on a commercial strain-controlled ARES rotational rheometer. The magnetic flux is generated via two concentric Halbach cylindrical arrays made from permanent NdFeB magnets. The use of permanent magnets overcomes some of the disadvantages of electromagnets (e.g., excessive heating of the coils, formation of large radial stray fields, cost of electricity, cooling, etc). The performance of the new setup is tested for a magnetorheological fluid in both steady and oscillatory shear regimes.

Barroso, V. C.; Raich, H.; Blümler, P.; Wilhelm, M.

2009-02-01

139

Role of Magnetic Flux Distribution in Coronal Energy Storage  

NASA Astrophysics Data System (ADS)

Magnetic fields in the solar corona are most likely the dominant source of energy that powers coronal mass ejections (CMEs). Such energy must be above and beyond that of a potential (current-free) magnetic field, and thus the pre-CME coronal magnetic field should contain significant electric currents. Given the diffuse nature of the corona, the coronal magnetic field is likely to be largely force free, implying that electric currents are closely aligned with the field itself. In this work we explore such force-free fields, in the spherical geometry appropriate to the solar corona, with the aim of understanding how the magnetic flux distribution at the coronal base affects the storage of magnetic energy. We find that energy storage is enhanced when a region of strong potential field overlies a nonpotential field whose footpoints are confined to low solar latitudes. Furthermore, those flux distributions consistent with strong overlying potential fields may enable larger energy buildup, when examined in the context of limits imposed by the scalar virial theorem and the Aly-Sturrock theorem. Finally, we demonstrate the existence of force-free fields containing detached flux ropes, with energies that lie above the Aly-Sturrock limit.

Wolfson, Richard

2003-08-01

140

Origin of 1/f noise peaks of YBa{sub 2}Cu{sub 3}O{sub x} films in a magnetic field  

SciTech Connect

The temperature and magnetic field dependence of 1/f noise has been measured in epitaxial YBa{sub 2}Cu{sub 3}O{sub x} films. In a magnetic field, two noise peaks were observed as temperature decreases; one at higher temperature was found to match the thermal fluctuation of the sample resistance and the other near the foot of the transition was found to be magnetic-field dependent. The location of the latter was shifted toward low temperature and the peak height was decreased as a magnetic field increased. In a zero field only one peak from the resistance fluctuation was observed. We show that the field-dependent noises are due to flux motion interacting with the pinning potential. A classical model is used to explain the field-induced noise peaks. We interpret that the temperature dependences of the flux bundle size and the sample resistance are the reasons for the noise peaks, and a rough estimation of the temperature and field dependence of the flux bundle size is presented. Any possible relationship between the noise peaks and the flux-line-lattice phase transition is also discussed.

Kang, W.N.; Kim, D.H.; Park, J.H. [Korea Institute of Science and Technology, Seoul (Korea, Democratic People`s Republic of). Applied Physics Group] [and others

1994-02-01

141

Magnetic coil parameters design of oil casing damage detector based on magnetic flux leakage  

Microsoft Academic Search

The magnetic flux leakage detection method is one of the important oil casing defects detection methods. It is significant to locate the position of the defects and evaluate the degree of the damages. Based on analyzing the magnetic path of the oil casing, a design of the magnetic core parameters that guarantees the detection results can be hardly affected by

Wei Zhang; Yi Bing Shi; Yan Jun Li

2011-01-01

142

Internal structure of a magnetic flux rope from Cluster observations  

Microsoft Academic Search

We investigate a magnetic flux rope (MFR) observed by Cluster in the magnetotail during a substorm on 2001 August 22. The MFR was aligned with its principal axis closely along the dawn-dusk direction and had a small size of ~2 R E with a total current of ~0.8 MA. The four spacecraft traversed the MFR at different distances from its

A. T. Y. Lui; M. W. Dunlop; H. Rčme; L. M. Kistler; G. Gustafsson; Q.-G. Zong

2007-01-01

143

Self-field and magnetic-flux quantum mechanics  

E-print Network

Self-field and quantized magnetic-flux are employed to generate the quantum numbers n, m, and l of atomic physics. Wave-particle duality is shown to be a natural outcome of having a particle and its self-field.

Paul Harris

2005-04-06

144

Thermodynamic state of the magnetic flux compression generator volume  

Microsoft Academic Search

The thermodynamic state of the gas trapped in the volume of helical magnetic flux compression generators was measured using optical emission spectroscopy and fast pressure probes. Three main stages of operation are discussed: (1) the initial stage, which can be represented by a freely expanding armature, that shows fairly low gas temperatures, as low as 2000 K; (2) the intermediate

Andreas Neuber; Thomas Holt; James C. Dickens; Magne Kristiansen

2002-01-01

145

CHARGE TO MAGNETIC FLUX RATIOS Gustavo Gonzlez-Martn  

E-print Network

1 CHARGE TO MAGNETIC FLUX RATIOS Gustavo González-Martín Departamento de Física, Universidad Simón Bolívar, Apartado 89000, Caracas 1080-A, Venezuela. Web page URL http:\\\\prof.usb.ve\\ggonzalm\\ It is shown the theory of representations of the Lorentz group to characterize the electron states in a central potential

González Martín, Gustavo R.

146

Intelligent defect recognition from magnetic flux l eakage inspection  

Microsoft Academic Search

THIS paper presents a new approach for detection of oil-gas pipeline corrosion defects in which pipeline magnetic flux leakage (MFL) detection syst em is adopted. To test the new approach, some artificial defects are simulated mathematicall y on the surface of oil-gas pipes to simulate different defects happening in practice. The interf erence eliminating method is introduced for compensation of

ALI SADR; SAEEDREZA EHTERAM

2008-01-01

147

Permanent magnet axial flux disc generator for small wind turbines  

Microsoft Academic Search

In the paper a permanent magnet axial flux disc generator (AFPMG) with fractional slot pitch winding for small wind energy applications is proposed. Its main features are high specific power, very low torque ripple and a quasi-sinusoidal output voltage. A preliminary design can be carried out by a simplified analytical procedure. Afterwards, an automated sequence of magnetostatic FEM analyses on

Mauro Andriollo; Manuel De Bortoli; Giovanni Martinelli; Augusto Morini; Andrea Tortella

2008-01-01

148

Development of small, tapered stator helical magnetic flux compression generators  

Microsoft Academic Search

For some applications of magnetic flux compression generators, shorter pulse widths, with associated higher voltages and power level, are required. We have developed a family of small, tapered stator generators that begin to address these applications. Specifically, the three versions of these FCGs developed use stator angles of 10°, 12°, and 13° with respect to the generator axis. The armature

Bruce L. Freeman; John C. Boydston; Jim M. Ferguson; Brent A. Lindeburg; Alvin D. Luginbill; Teresa E. Tutt

2003-01-01

149

Type II superconductivity and magnetic flux transport in neutrons stars  

E-print Network

The transition to a type II proton superconductor which is believed to occur in a cooling neutron star is accompanied by changes in the equation of hydrostatic equilibrium and by the formation of proton vortices with quantized magnetic flux. Analysis of the electron Boltzmann equation for this system and of the proton supercurrent distribution formed at the transition leads to the derivation of a simple expression for the transport velocity of magnetic flux in the liquid interior of a neutron star. This shows that flux moves easily as a consequence of the interaction between neutron and proton superfluid vortices during intervals of spin-down or spin-up in binary systems. The differences between the present analysis and those of previous workers are reviewed and an error in the paper of Jones (1991) is corrected.

P. B. Jones

2005-10-13

150

Influence of temperature on stability of trapped flux magnets  

NASA Astrophysics Data System (ADS)

Trapped flux in YBCO plates has an inherent dependence on temperature. The electro-magnetic current density and thermal specific heat are both highly dependent upon the temperature. Modeling and experimental data investigate the nature of a YBCO sample that has a heat pulse forced upon a trapped magnetic field by measuring the change in the magnetic field before and after the energy input. The experimental data suggest interesting trends that are further explained with models. An analytical approach and FEA simulations are performed to discover the physics behind the experimental data. By comparing the data, the temperature dependence on the YBCO sample is further investigated and illustrated.

Pienkos, J. E.; Masson, P. J.; Douine, B.; Leveque, J.; Luongo, C. A.

2010-03-01

151

Materials for efficient high-flux magnetic bearing actuators  

NASA Technical Reports Server (NTRS)

Magnetic bearings have demonstrated the capability for achieving positioning accuracies at the nanometer level in precision motion control stages. This makes possible the positioning of a wafer in six degrees of freedom with the precision necessary for photolithography. To control the position of an object at the nanometer level, a model of the magnetic bearing actuator force-current-airgap relationship must be accurately obtained. Additionally, to reduce thermal effects the design of the actuator should be optimized to achieve maximum power efficiency and flux density. Optimization of the actuator is accomplished by proper pole face sizing and utilizing a magnetic core material which can be magnetized to the highest flux density with low magnetic loss properties. This paper describes the construction of a magnetic bearing calibration fixture designed for experimental measurement of the actuator force characteristics. The results of a material study that review the force properties of nickel-steel, silicon-steel, and cobalt-vanadium-iron, as they apply to magnetic bearing applications are also presented.

Williams, M. E.; Trumper, D. L.

1994-01-01

152

From mean-field hydromagnetics to solar magnetic flux concentrations  

NASA Astrophysics Data System (ADS)

The main idea behind the work presented in this thesis is to investigate if it is possible to find a mechanism that leads to surface magnetic field concentrations and could operate under solar conditions without postulating the presence of magnetic flux tubes rising from the bottom of the convection zone, a commonly used yet physically problematic approach. In this context we study the "negative effective magnetic pressure effect": it was pointed out in earlier work (Kleeorin et al., 1989) that the presence of a weak magnetic field can lead to a reduction of the mean turbulent pressure on large length scales. This reduction is now indeed clearly observed in simulations. As magnetic fluctuations experience an unstable feedback through this effect, it leads, in a stratified medium, to the formation of magnetic structures, first observed numerically in the fifth paper of this thesis. While our setup is relatively simple, one wonders if this instability, as a mechanism able to concentrate magnetic fields in the near surface layers, may play a role in the formation of sunspots, starting from a weak dynamo-generated field throughout the convection zone rather than from strong flux tubes stored at the bottom. A generalization of the studied case is ongoing.

Kemel, Koen

153

Black hole jets without large-scale net magnetic flux  

NASA Astrophysics Data System (ADS)

We propose a scenario for launching relativistic jets from rotating black holes, in which small-scale magnetic flux loops, sustained by disc turbulence, are forced to inflate and open by differential rotation between the black hole and the accretion flow. This mechanism does not require a large-scale net magnetic flux in the accreting plasma. Estimates suggest that the process could operate effectively in many systems, and particularly naturally and efficiently when the accretion flow is retrograde. We present the results of general-relativistic force-free electrodynamic simulations demonstrating the time evolution of the black hole's magnetosphere, the cyclic formation of jets, and the effect of magnetic reconnection. The jets are highly variable on time-scales ˜10-103rg/c, where rg is the black hole's gravitational radius. The reconnecting current sheets observed in the simulations may be responsible for the hard X-ray emission from accreting black holes.

Parfrey, Kyle; Giannios, Dimitrios; Beloborodov, Andrei M.

2015-01-01

154

Resolving Magnetic Flux Patches at the Surface of the Core  

NASA Technical Reports Server (NTRS)

The geomagnetic field at a given epoch can be used to partition the surface of the liquid outer core into a finite number of contiguous regions in which the radial component of the magnetic flux density, B (sub r), is of one sign. These flux patches are instrumental in providing detail to surface fluid flows inferred from the changing geomagnetic field and in evaluating the validity of the frozen-flux approximation on which such inferences rely. Most of the flux patches in models of the modem field are small and enclose little flux compared to the total unsigned flux emanating from the core. To demonstrate that such patches are not required to explain the most spatially complete and accurate data presently available, those from the Magsat mission, I have constructed a smooth core field model that fits the Magsat data but does not possess small flux patches. I conclude that our present knowledge of the geomagnetic field does not allow us to resolve these features reliably at the core-mantle boundary; thus we possess less information about core flow than previously believed.

OBrien, Michael S.

1996-01-01

155

Collapsed, uncollapsed, and hidden magnetic flux on the quiet Sun  

NASA Astrophysics Data System (ADS)

Since the first applications of the Stokes V line ratio in the early 1970s and the Hanle depolarization effect in the early 1980s we have had a dualistic view of quiet-Sun magnetism: intermittent kG flux tubes surrounded by an ocean of turbulent fields with strengths of order 10-100 G. There has been the concern that this dualism could be an artefact of using two mutually almost exclusive diagnostic tools, the Zeeman and Hanle effect. We find however that the Hinode line-ratio data alone, without any reference to the Hanle effect, reveal the existence of two distinct flux populations, representing strong (collapsed) and weak (uncollapsed) flux. The collapsed population is preferentially located in the intergranular lanes, while the uncollapsed population is most visible in the bright cell interiors. From a comparison between the intrinsic field strengths, as derived from the line ratio for the collapsed population, and the corresponding flux densities, we can deduce the size distribution of the flux tubes. The majority of them are found to have sizes in the range 10-70 km. The intrinsic flux tube field strength decreases with diminishing size to become substantially smaller than kG for sizes below about 60 km. Comparison between the average of the unsigned flux density in the Hinode quiet-Sun data set and earlier constraints from the Hanle depolarization effect shows that most of the flux remains invisible at the Hinode resolution scale due to cancellation of the opposite magnetic polarities within the spatial resolution element. We have derived the cancellation function that describes how the visibility of the hidden flux improves with increased spatial resolution. It needs to be extrapolated to extremely small scales before the constraints imposed by the Hanle effect get satified, which suggests that the bulk of the hidden flux resides at scales near the end of the magnetic scale spectrum (of order 10 m). Appendix is only available in electronic form at http://www.aanda.org

Stenflo, J. O.

2011-05-01

156

C 4 fluxes from the sun as a star and the correlation with magnetic flux  

NASA Technical Reports Server (NTRS)

A total of 144 C 4 wavelength 1548 SMM-UVSP spectroheliograms of solar plages were analyzed, some of which are series of exposures of the same region on the same day. Also analyzed were C 4 wavelength 1551 rasters of plages and C 4 1548 rasters of the quiet sun. The sample contains data on 17 different plages, observed on 50 different days. The center-to-limb variations of the active regions show that the optical thickness effects in the C 4 wavelength 1548 line can be neglected in the conversion from intensity to flux density. As expected for the nearly optically thin situation, the C 4 1548 line is twice as bright as the C 4 wavelength 1551 line. The average C 4 wavelength 1548 flux density for a quiet is 2700 erg/cm/s and, with surprisingly little scatter, 18,000 erg/cm/s for plages. The intensity histograms of rasters obtained at disk centers can be separated into characteristic plage and quiet-sun contributions with variable relative filling factors. The disk-averaged flux density in the C 4 doublet and the disk-averaged magnitude of the magnetic flux density are related. The relationship between the C 4 and magnetic flux densities for spatially resolved data is inferred to be almost the same, with only an additional factor of order unity in the constant of proportionality.

Schrijver, C. J.; Linsky, J. L.; Bennett, J.; Brown, A.; Saar, S. H.

1988-01-01

157

Noise-induced bifurcations in magnetization dynamics of uniaxial nanomagnets  

NASA Astrophysics Data System (ADS)

Stochastic magnetization dynamics in uniformly magnetized nanomagnets is considered. The system is assumed to have rotational symmetry as the anisotropy axis, the applied field, and the spin polarization are all aligned along an axis of symmetry. By appropriate integration of the Fokker-Planck equation associated to the problem, the stochastic differential equation governing the evolution of the angle between the magnetization orientation and the symmetry axis is derived. The drift terms present in this equation contain a noise-induced drift term, which, in combination with drift terms of deterministic origin, can be written as the derivative of an effective potential. Superparamagnetic-like transitions are studied in connections with the bifurcations of the effective potential as temperature and excitation conditions are varied.

Serpico, C.; Perna, S.; Bertotti, G.; d'Aquino, M.; Quercia, A.; Mayergoyz, I. D.

2015-05-01

158

Experimental Design of a Magnetic Flux Compression Experiment  

NASA Astrophysics Data System (ADS)

Generation of ultrahigh magnetic fields is an interesting topic of high-energy-density physics, and an essential aspect of Magnetized Target Fusion (MTF). To examine plasma formation from conductors impinged upon by ultrahigh magnetic fields, in a geometry similar to that of the MAGO experiments, an experiment is under design to compress magnetic flux in a toroidal cavity, using the Shiva Star or Atlas generator. An initial toroidal bias magnetic field is provided by a current on a central conductor. The central current is generated by diverting a fraction of the liner current using an innovative inductive current divider, thus avoiding the need for an auxiliary power supply. A 50-mm-radius cylindrical aluminum liner implodes along glide planes with velocity of about 5 km/s. Inward liner motion causes electrical closure of the toroidal chamber, after which flux in the chamber is conserved and compressed, yielding magnetic fields of 2-3 MG. Plasma is generated on the liner and central rod surfaces by Ohmic heating. Diagnostics include B-dot probes, Faraday rotation, radiography, filtered photodiodes, and VUV spectroscopy. Optical access to the chamber is provided through small holes in the walls.

Fuelling, Stephan; Awe, Thomas J.; Bauer, Bruno S.; Goodrich, Tasha; Lindemuth, Irvin R.; Makhin, Volodymyr; Siemon, Richard E.; Atchison, Walter L.; Reinovsky, Robert E.; Salazar, Mike A.; Scudder, David W.; Turchi, Peter J.; Degnan, James H.; Ruden, Edward L.

2007-06-01

159

Weak localization in a distribution of magnetic flux tubes  

SciTech Connect

Thin gates of type-II superconducting materials have been prepared on top of the two-dimensional electron gas in a GaAs/AlGaAs heterostructure. In an applied magnetic field the flux distribution at the electron gas takes the form of flux tubes which are much narrower than an electron phase coherence length. We observe a qualitatively new weak-localization magnetoconductance for small fields proportional to {vert bar}{ital B}{vert bar}, in contrast to the {ital B}{sup 2} homogeneous result and in semiquantitative agreement with the theory of Rammer and Shelankov.

Bending, S.J.; von Klitzing, K.; Ploog, K. (Max-Planck-Institut fuer Festkoerperforschung, Stuttgart (Federal Republic of Germany))

1990-08-20

160

MHD waves on solar magnetic flux tubes - Tutorial review  

NASA Technical Reports Server (NTRS)

Some of the highly simplified models that have been developed for solar magnetic flux tubes, which are intense photospheric-level fields confined by external gas pressure but able to vary rapidly with height, are presently discussed with emphasis on the torsional Alfven mode's propagation, reflection, and non-WKB properties. The 'sausage' and 'kink' modes described by the thin flux-tube approximation are noted. Attention is also given to the surface waves and resonance absorption of X-ray-emitting loops, as well as to the results of recent work on the resonant instabilities that occur in the presence of bulk flows.

Hollweg, Joseph V.

1990-01-01

161

Twisted magnetic flux tubes in the solar wind  

NASA Astrophysics Data System (ADS)

Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surface, where the tubes are twisted owing to photospheric motions. The twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through observing satellite. The twist of isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. The twisted tubes are unstable to kink instability when the twist exceeds a critical value. It is shown that the critical twist angle of the tube with a homogeneous twist is 70 degree, but the angle can be decreased owing to the motion of the tube with regards to the solar wind stream. Tangential velocity discontinuity near the boundaries of individual tubes may also result in the Kelvin-Helmholtz instability. It is shown that the axial magnetic field stabilizes the instability in the case of sub-Alfvenic speeds. But even small twist in the external magnetic field allows the Kelvin-Helmholtz instability to be developed for any speed. Therefore, twisted magnetic flux tubes can be unstable to Kelvin-Helmholtz instability when they move with small speed relative to main solar wind stream. The Kelvin-Helmholtz vortices may significantly contribute into the solar wind turbulence. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement N 313038/STORM and from the Austrian 'Fonds zur Förderung der wissenschaftlichen Forschung' under project P24740-N27.

Zaqarashvili, Teimuraz; Voros, Zoltan; Zhelyazkov, Ivan; Narita, Yasuhito; Bruno, Roberto

2014-05-01

162

Evidence of Magnetic Helicity in Emerging Flux and Associated Flare  

NASA Astrophysics Data System (ADS)

The aim of this paper is to look at the magnetic helicity structure of an emerging active region and show that both emergence and flaring signatures are consistent with a same sign for magnetic helicity. We present a multiwavelength analysis of an M1.6 flare occurring in the NOAA active region 10365 on 27 May 2003, in which a large new bipole emerges in a decaying active region. The diverging flow pattern and the “tongue” shape of the magnetic field in the photosphere with elongated polarities are highly suggestive of the emergence of a twisted flux tube. The orientation of these tongues indicates the emergence of a flux tube with a right-hand twist ( i.e., positive magnetic helicity). The flare signatures in the chromosphere are ribbons observed in H? by the MSDP spectrograph in the Meudon solar tower and in 1600 Ĺ by TRACE. These ribbons have a J shape and are shifted along the inversion line. The pattern of these ribbons suggests that the flare was triggered by magnetic reconnection at coronal heights below a twisted flux tube of positive helicity, corresponding to that of the observed emergence. It is the first time that such a consistency between the signatures of the emerging flux through the photosphere and flare ribbons has been clearly identified in observations. Another type of ribbons observed during the flare at the periphery of the active region by the MSDP and SOHO/EIT is related to the existence of a null point, which is found high in the corona in a potential field extrapolation. We discuss the interpretation of these secondary brightenings in terms of the “breakout” model and in terms of plasma compression/heating within large-scale separatrices.

Chandra, R.; Schmieder, B.; Aulanier, G.; Malherbe, J. M.

2009-08-01

163

Magnetic Flux Conservation in the Heliosheath Including Solar Cycle Variations of Magnetic Field Intensity  

NASA Astrophysics Data System (ADS)

In the heliosheath (HS), Voyager 2 has observed a flow with constant radial velocity and magnetic flux conservation. Voyager 1, however, has observed a decrease in the flow’s radial velocity and an order of magnitude decrease in magnetic flux. We investigate the role of the 11 yr solar cycle variation of the magnetic field strength on the magnetic flux within the HS using a global 3D magnetohydrodynamic model of the heliosphere. We use time and latitude-dependent solar wind velocity and density inferred from Solar and Heliospheric Observatory/SWAN and interplanetary scintillations data and implemented solar cycle variations of the magnetic field derived from 27 day averages of the field magnitude average of the magnetic field at 1 AU from the OMNI database. With the inclusion of the solar cycle time-dependent magnetic field intensity, the model matches the observed intensity of the magnetic field in the HS along both Voyager 1 and 2. This is a significant improvement from the same model without magnetic field solar cycle variations, which was over a factor of two larger. The model accurately predicts the radial velocity observed by Voyager 2; however, the model predicts a flow speed ?100 km s?1 larger than that derived from LECP measurements at Voyager 1. In the model, magnetic flux is conserved along both Voyager trajectories, contrary to observations. This implies that the solar cycle variations in solar wind magnetic field observed at 1 AU does not cause the order of magnitude decrease in magnetic flux observed in the Voyager 1 data.

Michael, A. T.; Opher, M.; Provornikova, E.; Richardson, J. D.; Tóth, G.

2015-04-01

164

Reduction of low frequency magnetic noise by voltage-induced magnetic anisotropy modulation in tunneling magnetoresistance sensors  

NASA Astrophysics Data System (ADS)

We demonstrate the reduction and control of magnetic noise by voltage-induced perpendicular anisotropy modulation in CoFeB/MgO/CoFeB sensors. The noise decreases with the increase of the perpendicular anisotropy energy induced by the bias voltage polarity reversal. The bias reversal between -1 and +1 V results in a reduction of the normalized 1/f magnetic noise parameters by a factor of 7.3 and the thermal magnetic noise by a factor of 6.8. In the state of the highest field sensitivity, the lowest normalized 1/f magnetic noise parameter reaches 6.45 × 10-14 ?m3T. The results indicate that voltage-induced anisotropy modulation can be used to control and suppress magnetization fluctuations in the sensing layer and thus, significantly reduce the magnetic noise.

Wisniowski, P.; D?bek, M.; Skowronski, W.; Stobiecki, T.; Cardoso, S.; Freitas, P. P.

2014-08-01

165

Magnetic Flux Compression Concept for Aerospace Propulsion and Power  

NASA Technical Reports Server (NTRS)

The objective of this research is to investigate system level performance and design issues associated with magnetic flux compression devices for aerospace power generation and propulsion. The proposed concept incorporates the principles of magnetic flux compression for direct conversion of nuclear/chemical detonation energy into electrical power. Specifically a magnetic field is compressed between an expanding detonation driven diamagnetic plasma and a stator structure formed from a high temperature superconductor (HTSC). The expanding plasma cloud is entirely confined by the compressed magnetic field at the expense of internal kinetic energy. Electrical power is inductively extracted, and the detonation products are collimated and expelled through a magnetic nozzle. The long-term development of this highly integrated generator/propulsion system opens up revolutionary NASA Mission scenarios for future interplanetary and interstellar spacecraft. The unique features of this concept with respect to future space travel opportunities are as follows: ability to implement high energy density chemical detonations or ICF microfusion bursts as the impulsive diamagnetic plasma source; high power density system characteristics constrain the size, weight, and cost of the vehicle architecture; provides inductive storage pulse power with a very short pulse rise time; multimegajoule energy bursts/terawatt power bursts; compact pulse power driver for low-impedance dense plasma devices; utilization of low cost HTSC material and casting technology to increase magnetic flux conservation and inductive energy storage; improvement in chemical/nuclear-to-electric energy conversion efficiency and the ability to generate significant levels of thrust with very high specific impulse; potential for developing a small, lightweight, low cost, self-excited integrated propulsion and power system suitable for space stations, planetary bases, and interplanetary and interstellar space travel; potential for attaining specific impulses approaching 10 (exp 6) seconds, which would enable missions to the outer planets within ten years and missions at interstellar distances within fifty years.

Litchford, Ron J.; Robertson, Tony; Hawk, Clark W.; Turner, Matt; Koelfgen, Syri

2000-01-01

166

Evidence of Magnetic Helicity in Emerging Flux and Associated Flare  

E-print Network

The aim of this paper is to look at the magnetic helicity structure of an emerging active region and show that both emergence and flaring signatures are consistent with a same sign for magnetic helicity. We present a multi-wavelength analysis of an M1.6 flare occurring in the active region NOAA 10365 on 27 May, 2003, in which a large new bipole emerges in a decaying active region. The diverging flow pattern and the "tongue" shape of the magnetic field in the photosphere with elongated polarities are highly suggestive of the emergence of a twisted flux tube. The orientation of these tongues indicates the emergence of a flux tube with a right hand twist, i.e. positive magnetic helicity. The flare signatures in the chromosphere are ribbons observed in H-alpha by the MSDP spectrograph in the Meudon solar tower and in 1600 A by TRACE. These ribbons have a `J' shape and are shifted along the inversion line. The pattern of these ribbons suggests that the flare was triggered by magnetic reconnection at coronal height...

Chandra, R; Aulanier, G; Malherbe, J M

2009-01-01

167

Magnetic Flux Ropes from the Sun to 1 AU  

NASA Astrophysics Data System (ADS)

Any practical model of the dynamics of a coronal mass ejection (CME) must conform to available observational constraints from sun and to the earth. We present model/data comparisons near the sun (using coronagraph image data) and in the heliosphere (using in situ measurements) to show that our flux rope model[1-3] provides an accurate physics-based characterization of flux-rope coronal mass ejections over this range. As our understanding increases, we are approaching a point where we can use near-sun data to provide model inputs, with the model output being a prediction of near-earth magnetic fields. These magnetic fields can drive geomagnetic storms, an element of our local ``space weather''. [1] Chen, J. 1996, JGR, 101, 27499 [2] Krall, J. et al., 2000, ApJ, 539, 964 [3] Chen, J., and Krall, J. 2003, JGR, 108, 1410

Krall, Jonathan; Chen, James

2004-11-01

168

Real-Time Data Mining in Magnetic Flux Leakage Detecting in Boiler Pipeline  

Microsoft Academic Search

For boiler in magnetic flux leakage testing data characteristics on the basis of full analysis, Combining the application of industrial control integrated automation needs, proposed the pipeline magnetic flux leakage testing data mining system framework. Through analysis of magnetic flux leakage pipeline inspection data and mines the key data. It could be better to achieve detection and prediction of the

Ke MinYi; Liao Pan; Song XiaoChun

2010-01-01

169

Rough Design of a Double-Stator Axial Flux Permanent Magnet Generator  

E-print Network

, rim-driven concept, permanent magnet machine, axial flux machine, electromagnetic model, thermal modelRough Design of a Double-Stator Axial Flux Permanent Magnet Generator for a Rim-Driven Marine.s@eca-en.fr Abstract--This paper deals with the rough design of a Double-Stator Axial Flux Permanent Magnet Machine

Paris-Sud XI, UniversitĂŠ de

170

Collapsed, uncollapsed, and hidden magnetic flux on the quiet Sun  

Microsoft Academic Search

Since the first applications of the Stokes V line ratio in the early 1970s and the Hanle depolarization effect in the early 1980s we have had a dualistic view of quiet-Sun magnetism: intermittent kG flux tubes surrounded by an ocean of turbulent fields with strengths of order 10-100 G. There has been the concern that this dualism could be an

J. O. Stenflo

2011-01-01

171

Flux Rope Acceleration and Enhanced Magnetic Reconnection Rate  

SciTech Connect

A physical mechanism of flares, in particular for the flare rise phase, has emerged from our 2-1/2-dimensional resistive MHD simulations. The dynamical evolution of current-sheet formation and magnetic reconnection and flux-rope acceleration subject to continuous, slow increase of magnetic shear in the arcade are studied by employing a non-uniform anomalous resistivity in the reconnecting current sheet under gravity. The simulation results directly relate the flux rope's accelerated rising motion with an enhanced magnetic reconnection rate and thus an enhanced reconnection electric field in the current sheet during the flare rise phase. The simulation results provide good quantitative agreements with observations of the acceleration of flux rope, which manifests in the form of SXR ejecta or erupting filament or CMEs, in the low corona. Moreover, for the X-class flare events studied in this paper the peak reconnection electric field is about O(10{sup 2} V/m) or larger, enough to accelerate p articles to over 100 keV in a field-aligned distance of 10 km. Nonthermal electrons thus generated can produce hard X-rays, consistent with impulsive HXR emission observed during the flare rise phase.

C.Z. Cheng; Y. Ren; G.S. Choe; Y.-J. Moon

2003-03-25

172

Magnetic flux transport of decaying active regions and enhanced magnetic network. [of solar supergranulation  

NASA Technical Reports Server (NTRS)

Several series of coordinated observations on decaying active regions and enhanced magnetic network regions on the sun were carried out jointly at Big Bear Solar Observatory and at the Huairou Solar Observing Station of the Bejing Astronomical Observatory in China. The magnetic field evolution in several regions was followed closely for three to seven days. The magnetic flux transport from the remnants of decayed active regions was studied, along with the evolution and lifetime of the magnetic network which defines the boundaries of supergranules. The magnetic flux transport in an enhanced network region was studied in detail and found to be negative. Also briefly described are some properties of moving magnetic features around a sunspot. Results of all of the above studies are presented.

Wang, Haimin; Zirin, Harold; Ai, Guoxiang

1991-01-01

173

Magnetic Flux Circulation During Dawn-Dusk Oriented Interplanetary Magnetic Field  

NASA Technical Reports Server (NTRS)

Magnetic flux circulation is a primary mode of energy transfer from the solar wind into the ionosphere and inner magnetosphere. For southward interplanetary magnetic field (IMF), magnetic flux circulation is described by the Dungey cycle (dayside merging, night side reconnection, and magnetospheric convection), and both the ionosphere and inner magnetosphere receive energy. For dawn-dusk oriented IMF, magnetic flux circulation is not well understood, and the inner magnetosphere does not receive energy. Several models have been suggested for possible reconnection patterns; the general pattern is: dayside merging; reconnection on the dayside or along the dawn/dusk regions; and, return flow on dayside only. These models are consistent with the lack of energy in the inner magnetosphere. We will present evidence that the Dungey cycle does not explain the energy transfer during dawn-dusk oriented IMF. We will also present evidence of how magnetic flux does circulate during dawn-dusk oriented IMF, specifically how the magnetic flux reconnects and circulates back.

Mitchell, E. J.; Lopez, R. E.; Fok, M.-C.; Deng, Y.; Wiltberger, M.; Lyon, J.

2010-01-01

174

Magnetic flux concentrations from dynamo-generated fields  

NASA Astrophysics Data System (ADS)

Context. The mean-field theory of magnetized stellar convection gives rise to two distinct instabilities: the large-scale dynamo instability, operating in the bulk of the convection zone and a negative effective magnetic pressure instability (NEMPI) operating in the strongly stratified surface layers. The latter might be important in connection with magnetic spot formation. However, as follows from theoretical analysis, the growth rate of NEMPI is suppressed with increasing rotation rates. On the other hand, recent direct numerical simulations (DNS) have shown a subsequent increase in the growth rate. Aims: We examine quantitatively whether this increase in the growth rate of NEMPI can be explained by an ?2 mean-field dynamo, and whether both NEMPI and the dynamo instability can operate at the same time. Methods: We use both DNS and mean-field simulations (MFS) to solve the underlying equations numerically either with or without an imposed horizontal field. We use the test-field method to compute relevant dynamo coefficients. Results: DNS show that magnetic flux concentrations are still possible up to rotation rates above which the large-scale dynamo effect produces mean magnetic fields. The resulting DNS growth rates are quantitatively reproduced with MFS. As expected for weak or vanishing rotation, the growth rate of NEMPI increases with increasing gravity, but there is a correction term for strong gravity and large turbulent magnetic diffusivity. Conclusions: Magnetic flux concentrations are still possible for rotation rates above which dynamo action takes over. For the solar rotation rate, the corresponding turbulent turnover time is about 5 h, with dynamo action commencing in the layers beneath.

Jabbari, S.; Brandenburg, A.; Losada, I. R.; Kleeorin, N.; Rogachevskii, I.

2014-08-01

175

Anisotropy study of grain oriented steels with Magnetic Barkhausen Noise  

NASA Astrophysics Data System (ADS)

Grain oriented electrical steels present strong anisotropy, due to a {110} <001> texture (Goss), with [100] direction parallel to rolling direction (RD) and [110] direction parallel to transverse direction (TD). MBN (Magnetic Barkhausen Noise) were employed to measure magnetic properties in several angles towards RD using a 15° step. For 90° to the rolling direction (i.e., TD), the MBN signal changes, decreasing the MBNrms. It is found a connection between initial permeability and MBNrms. The lower initial permeability for the TD is related to a larger contribution of irreversible rotation in the hysteresis. The MBN procedure is non-destructive and provides rapid understanding of the anisotropy of the material, without the use of laborious methods like Epstein frame or toroidal coils.

de Campos, M. F.; Campos, M. A.; Landgraf, F. J. G.; Padovese, L. R.

2011-07-01

176

A dynamo effect due to instability of magnetic flux tubes  

NASA Astrophysics Data System (ADS)

We show that a non-axisymmetric instability of toroidal magnetic flux tubes in a rotating star provides a dynamo effect. The instability occurs in the form of propagating helical waves; their growth in amplitude causes a phase shift between the perturbations of magnetic field and velocity, which leads to an electric field (anti)parallel to the direction of the unperturbed field. Together with differential rotation, this effects is capable of driving a dynamo of the alpha omega-type. In contrast to the conventional alpha-effect in cyclonic convention, this dynamo effect operates in strong (super-equipartition) magnetic fields which resist against distortion by convective flows. We calculate the induced electric field using results from linear stability and a model of the solar convection zone which consistently includes an overshoot layer. We find that for growing magnetic field the dynamo effect occurs first in high latitudes (near the poles) in a region of weak instability which moves towards the equator as the field strength increases further. The dependence of the dynamo effect on the location of the flux tubes (in depth and latitude) and on the rotation rate is discussed.

Ferriz-Mas, A.; Schmitt, D.; Schuessler, M.

1994-09-01

177

Magnetic Flux Compression Concept for Nuclear Pulse Propulsion and Power  

NASA Technical Reports Server (NTRS)

The desire for fast, efficient interplanetary transport requires propulsion systems having short acceleration times and very high specific impulse attributes. Unfortunately, most highly efficient propulsion systems which are within the capabilities of present day technologies are either very heavy or yield very low impulse such that the acceleration time to final velocity is too long to be of lasting interest, One exception, the nuclear thermal thruster, could achieve the desired acceleration but it would require inordinately large mass ratios to reach the range of desired final velocities. An alternative approach, among several competing concepts that are beyond our modern technical capabilities, is a pulsed thermonuclear device utilizing microfusion detonations. In this paper, we examine the feasibility of an innovative magnetic flux compression concept for utilizing microfusion detonations, assuming that such low yield nuclear bursts can be realized in practice. In this concept, a magnetic field is compressed between an expanding detonation driven diamagnetic plasma and a stationary structure formed from a high temperature superconductor (HTSC). In general, we are interested in accomplishing two important functions: (1) collimation of a hot diamagnetic plasma for direct thrust production; and (2) pulse power generation for dense plasma ignition. For the purposes of this research, it is assumed that rnicrofusion detonation technology may become available within a few decades, and that this approach could capitalize on recent advances in inertial confinement fusion ICF) technologies including magnetized target concepts and antimatter initiated nuclear detonations. The charged particle expansion velocity in these detonations can be on the order of 10 (exp 6)- 10 (exp 7) meters per second, and, if effectively collimated by a magnetic nozzle, can yield the Isp and the acceleration levels needed for practical interplanetary spaceflight. The ability to ignite pure fusion micro-bursts with reasonable levels of input energy is an equally challenging scientific problem. It remains to be seen, however, whether an effective ignition driver can be developed which meets the requirements for practical spaceflight application (namely high power density, compactness, low weight, and low cost). In this paper, system level performance and design issues are examined including generator performance, magnetic flux compression processes, magnetic diffusion processes, high temperature superconductor (HTSC) material properties, plasmadynamic processes, detonation plasma expansion processes, magnetohydrodynamic instabilities, magnetic nozzle performance, and thrust production performance. Representative generator performance calculations based on a simplified skin layer formulation are presented as well as the results of exploratory small-scale laboratory experiments on magnetic flux diffusion in HTSC materials. In addition, planned follow-on scientific feasibility experiments are described which utilize high explosive detonations and high energy gas discharges to simulate the plasma conditions associated with thermonuclear micro-detonations.

Litchford, Ronald J.

2000-01-01

178

Acoustic waves in random ensembles of magnetic fluxes  

SciTech Connect

To analyze the observational data and provide the appropriate diagnostic procedure for photospheric manifestation of solar oscillations it is necessary to take into account strong inhomogeneity of solar atmosphere with respect to distribution of magnetic fields. We study the collective phenomena in the propagation of acoustic waves and unsteady wave-packets through quite regions, sunspots and plages, including time-dependent response of these regions to solar oscillations, the energy transfer mechanisms, frequency shift effects and reradiation of the acoustic waves in higher layers of atmosphere. We show that the dynamics of differently magnetized regions, their dispersion properties, and their response to the propagation of acoustic waves are completely different. We describe the effects caused by the specific distribution and randomness of magnetic flux tubes, which can be observed and which can provide the tools for diagnostic goals.

Ryutova, M.P.

1995-10-10

179

The magnetic non-equilibrium of buoyant flux tubes in the solar corona  

NASA Technical Reports Server (NTRS)

The magnetic field in the convection zone and photosphere of the sun exists mostly as concentrated tubes of magnetic flux. It is, therefore, necessary to study the basic properties of magnetic flux tubes to obtain a basis for understanding the behavior of the sun's magnetic field. The present investigation is concerned with the global equilibrium shape of a flux tube in the stratified solar atmosphere. A fundamental property of isolated flux tubes is magnetic buoyancy. Attention is given to flux tubes with external field, and twisted flux tubes. It is shown that the analysis of Parker (1975, 1979) and Spruit (1981) for calculating the equilibrium of a slender flux tube in a stratified atmosphere may be extended to more general situations. The slender tube approximation provides a method of solving the problem of modeling the overall curvature of flux tubes. It is found that for a twisted flux tube, there can be two possible equilibrium values of the height.

Browning, P. K.; Priest, E. R.

1984-01-01

180

The flux-dependent amplitude of broadband noise variability in X-ray binaries and active galaxies  

E-print Network

Standard shot-noise models, which seek to explain the broadband noise variability that characterises the X-ray lightcurves of X-ray binaries and active galaxies, predict that the power spectrum of the X-ray lightcurve is stationary (i.e. constant amplitude and shape) on short time-scales. We show that the broadband noise power spectra of the black hole candidate Cyg X-1 and the accreting millisecond pulsar SAX J1808.4-3658 are intrinsically non-stationary, in that RMS variability scales linearly with flux. Flux-selected power spectra confirm that this effect is due to changes in power-spectral amplitude and not shape. The lightcurves of three Seyfert galaxies are also consistent with a linear relationship between RMS variability and flux, suggesting that it is an intrinsic feature of the broadband noise variability in compact accreting systems over more than 6 decades of central object mass. The RMS variability responds to flux variations on all measured time-scales, raising fundamental difficulties for shot-noise models which seek to explain this result by invoking variations in the shot parameters. We suggest that models should be explored where the longest time-scale variations are fundamental and precede the variations on shorter time-scales. Possible models which can explain the linear RMS-flux relation include the fractal break-up of large coronal flares, or the propagation of fluctuations in mass accretion rate through the accretion disk. The linear relationship between RMS variability and flux in Cyg X-1 and SAX J1808.4-3658 is offset on the flux axis, suggesting the presence of a second, constant-flux component to the lightcurve which contributes 25% of the total flux. The spectrum of this constant component is similar to the total spectrum [abridged].

Philip Uttley; Ian M. McHardy

2001-03-30

181

Magnetic flux compression for high voltage pulse applications  

NASA Astrophysics Data System (ADS)

Helical Magnetic Flux Compression Generators (MFCG) are the most promising energy sources with respect to their current amplification and compactness. However, the main concern is their intrinsic flux loss that limits severely their performance and which is not yet well understood. All flux losses have a differing degree of impact, depending on the generator's volume, current and energy amplification, size of the driven load, and angular frequency of armature-helix contact point. Although several computer models have been developed, none of them truly quantify, the ohmic and intrinsic flux losses. This dissertation describes a novel method that provides a separate calculation of intrinsic flux losses (flux that is left behind in the conductors and lost for compression) and ohmic losses. I also provided a second method that uses a simple flux quantification, making a mathematical connection between the intrinsic flux losses, quantified by the first method, and the intrinsic flux losses observed in the generators. This second method can also be used with the first method to a priori estimate the MFCG performance. Simple MFCG with a single helix produce high output energy only into low inductance loads, thus producing several 100kA of current at a voltage level of less than 10kV. Many pulsed power devices require less current but a considerably higher voltage level. For effectively driving a high inductance load of several muH, a multistage MFCG design has been successfully tested with a total length of 250mm, a helix inner diameter of 51mm, achieving an energy gain of ˜13 into a 3muH load. Typical load parameters of electron beam devices are several 100kV operating voltage with an impedance of a few tens of Ohms. Utilizing a multi-stage FCG as primary source for inductive energy storage with opening switch enables the production of voltages in excess of 100's kV. We built an exploding wire fuse with a length of 140mm and 100mm in diameter (including the storage inductor), conditioned to the MFCG described above. We achieved a voltage of ˜43kV directly across the 3muH inductor, and more than 120kV with the fuse opening switch operating into a ˜10Ohm load.

Hernandez Llambes, Juan Carlos

182

Measurements of EUV Coronal Holes and Open Magnetic Flux  

E-print Network

Coronal holes are regions on the Sun's surface that map the foot-prints of open magnetic field lines. We have developed an automated routine to detect and track boundaries of long-lived coronal holes using full-disk EUV images obtained by SoHO:EIT, SDO:AIA, and STEREO:EUVI. We measure coronal hole areas and magnetic flux in these holes, and compare the measurements with calculations by the PFSS model. It is shown that, from 1996 through 2010, the total area of coronal holes measured with EIT images varies between 5$\\%$ and 17$\\%$ of the total solar surface area, and the total unsigned open flux varies between $2-5 \\times 10^{22}$ Mx. The solar cycle dependence of these measurements are similar to the PFSS results, but the model yields larger hole areas and greater open flux than observed by EIT. The AIA/EUVI measurements from 2010-2013 show coronal hole area coverage of 5-10$\\%$ of the total surface area, with significant contribution from low latitudes, which is under-represented by EIT. AIA/EUVI have measur...

Lowder, Chris; Leamon, Robert; Liu, Yang

2015-01-01

183

Collapse and Fragmentation of Rotating Magnetized Clouds. I. Magnetic Flux - Spin Relation  

E-print Network

We discuss evolution of the magnetic flux density and angular velocity in a molecular cloud core, on the basis of three-dimensional numerical simulations, in which a rotating magnetized cloud fragments and collapses to form a very dense optically thick core of > 5 times 10 ^10 cm^-3 . As the density increases towards the formation of the optically thick core, the magnetic flux density and angular velocity converge towards a single relationship between the two quantities. If the core is magnetically dominated its magnetic flux density approaches 1.5 (n/5 times 10^10 cm^-3)^1/2 mG, while if the core is rotationally dominated the angular velocity approaches 2.57 times 10^-3, (n/5 times 10^10 cm^-3)^1/2 yr^-1, where n is the density of the gas. We also find that the ratio of the angular velocity to the magnetic flux density remains nearly constant until the density exceeds 5 times 10^10 cm^-3. Fragmentation of the very dense core and emergence of outflows from fragments are shown in the subsequent paper.

Masahiro N Machida; Tomoaki Matsumoto; Kohji Tomisaka; Tomoyuki Hanawa

2005-06-19

184

Investigation of the magnetic flux leakage signatures of dents and gouges  

NASA Astrophysics Data System (ADS)

A study of gouges and dents in the context of pipelines has been completed, using the non-destructive evaluation (NDE) techniques of magnetic flux leakage (MFL) and magnetic Barkhausen noise (MBN). The research is part of an ongoing effort by the Applied Magnetics Group (AMG) at Queen's University to improve the interpretation of the MFL signal, which is used extensively by industry for defect detection and evaluation. The gouges were found to have distinctive MFL signatures depending on their orientation relative to the magnetization axis. Features in the MFL signal were identified as superpositions of geometry-related effects and strain or work hardening of the surface material. A qualitative magnetic permeability distribution in the material near a gouge has been proposed. The distribution is expected to vary in magnitude and extent according to the defect severity. The MFL results of the dent studies, on samples made available by Gaz de France (GdF), largely agreed qualitatively with previous research of dents. However, the differences pointed to the need for study of more varied dent shapes; new signal features were observed that suggested tensile residual strain in the dent rim is more prominent than earlier studies and modeling have predicted. Additionally, upgrades made to the MFL scanning system used by the AMG and a novel approach for building computer models are detailed.

Marble, Kris

185

Magnetic flux pileup and plasma depletion in Mercury's subsolar magnetosheath  

NASA Astrophysics Data System (ADS)

from the Fast Imaging Plasma Spectrometer (FIPS) and Magnetometer (MAG) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft during 40 orbits about Mercury are used to characterize the plasma depletion layer just exterior to the planet's dayside magnetopause. A plasma depletion layer forms at Mercury as a result of piled-up magnetic flux that is draped around the magnetosphere. The low average upstream Alfvénic Mach number (MA ~3-5) in the solar wind at Mercury often results in large-scale plasma depletion in the magnetosheath between the subsolar magnetopause and the bow shock. Flux pileup is observed to occur downstream under both quasi-perpendicular and quasi-parallel shock geometries for all orientations of the interplanetary magnetic field (IMF). Furthermore, little to no plasma depletion is seen during some periods with stable northward IMF. The consistently low value of plasma ?, the ratio of plasma pressure to magnetic pressure, at the magnetopause associated with the low average upstream MA is believed to be the cause for the high average reconnection rate at Mercury, reported to be nearly 3 times that observed at Earth. Finally, a characteristic depletion length outward from the subsolar magnetopause of ~300 km is found for Mercury. This value scales among planetary bodies as the average standoff distance of the magnetopause.

Gershman, Daniel J.; Slavin, James A.; Raines, Jim M.; Zurbuchen, Thomas H.; Anderson, Brian J.; Korth, Haje; Baker, Daniel N.; Solomon, Sean C.

2013-11-01

186

Simulation of magnetic flux leakage: Application to tube inspection  

NASA Astrophysics Data System (ADS)

The detection of flaws in steel pipes using Magnetic Flux Leakage (MFL) consists in detecting magnetic flux leaks outside the pipe, either with a magnetic sensor or with an induction coil, while the pipe is rotating. In the Vallourec group, many NDT units use MFL for testing ferromagnetic pipes. In order to improve the performances of flaw detection, CEA LIST and the Vallourec Research Aulnoye (VRA) group are collaborating on MFL modelling. The aim is to be able to perform parametric studies thanks to a fast 3D numerical model dedicated to MFL systems. A simplified 2D geometry has already been derived for the development of first simulation tools. When considering the B-H curve of ferromagnetic materials, the non-linear magnetostatic problem can be solved with the generalized boundary element method (BEMG), which comes to the evaluation of two equivalent scalar potentials: the surface charge density and the volume charge density. When applying the Galerkin method for the discretization of integral equations, the particularity of this numerical model lies in the implementation of high order basis functions for the interpolation of the scalar unknowns. This paper presents some first numerical results for the numerical validation of the semi-analytical model.

Prémel, Denis; Fnaeich, E. A.; Djafa, S.; Pichon, L.; Trillon, A.; Bisiaux, B.

2012-05-01

187

A sensor measuring the Fourier coefficients of the magnetic flux density for pipe crack detection using the magnetic flux leakage method  

Microsoft Academic Search

A simple sensor for magnetic flux leakage methods of detecting cracks on the surfaces of ferromagnetic pipes is proposed. We show that the crack position can be determined by the Fourier coefficients of the leakage magnetic flux density on a circle inside the pipe in the pipe cross-sectional plane. Coils that directly output the Fourier cosine and sine coefficients were

T. Nara; Y. Takanashi; M. Mizuide

2011-01-01

188

Spectral features of background ULF noise during magnetic storms  

NASA Astrophysics Data System (ADS)

The monitoring of tangential components of ULF noise below first Schumann resonance was permanently provided since 2003 up to now at mid-latitude station New Life (56N, 44,7E). The regular structure in ULF spectrograms named SRS -spectral resonance structure could be observed practically every night excepting years of solar activity maximum. The SRS appears in the ULF noise due existing the resonator for Alfven waves in the ionosphere at heights from the bottom up to 600 -1000 km [1]. It was noticed that during magnetically disturbed time the strong anti-correlation between the intensity of SRS events and K index from station on neighbor magnetic meridian [2]. For more detail investigation it was choused several magnetic storms. First one is storm witch has began at November 20, 2003 with maximum Kp=9 (strong), second -at March 19, 2006 with Kp=6+ (moderate) and the third -at November 25, 2008 with Kp=5 (weak). The spectra of magnetic component amplitude and polarization parameter were analyzed. The comparison with ionosond and magnetic field data as well as world TEC maps for mentioned storm periods were made. The dynamics of changing the properties of ULF spectra during the storms were explored. The general regularities of these changes were displayed. One of the prominent feathers appeared at the end of the strong and moderate storms looked like fish shoals. Perhaps this kind of ULF bursts is the same nature as well known pearls pulsation but displayed in broad frequency range 1-6 Hz. May be it could be classified as special spectral structure. The analysis of changing the properties of ULF spectra during the storms together with geo-physical data has shown that the main regularities in changes the SRS caused by changing of the ionospheric parameters above the observation point. But some of ULF spectral features are results of interaction waves and particles in the magnetosphere. 1. Belyaev P.P., Polyakov S.V., Ermakova E.N., Isaev S.V. Solar cycle variations in the iono-spheric Alfven resonator1985-1995, J. Atmos. Solar-Terr. Phys. 2000. V.62, No 4. P.239-248 2. D.S. Kotik, E.N. Ermakova, The anticorrelation between SRS and geomagnetic activity levels at mid latitudes. Abstracts of XXX Annual Seminar "Physics of Auroral phenomena", 28 February 2007, p. 18.

Kotik, Dmitry; Ermakova, Elena; Ryabov, Alexander; Shchecoldin, Aexander

189

Spins on Metals: Noise in SQUIDs and Spin Glasses  

NASA Astrophysics Data System (ADS)

Recent experiments at Stanford and Wisconsin have found evidence for magnetic defects on the surface of elemental metals like aluminum, niobium, and gold. Fluctuations of these impurities are the source of flux noise in SQUIDs. Flux noise is a major obstacle to the realization of using superconducting qubits to construct quantum computers. To see if flux noise can be described by spin glass noise, we have used Monte Carlo simulations of a 3D Ising spin glass to produce noise. We find that the noise is a maximum at the critical temperature. We compare our results to experimental measurements of the susceptibility, as well as the flux and inductance noise measured in SQUIDs.

Chen, Zhi; Yu, Clare

2010-03-01

190

System having unmodulated flux locked loop for measuring magnetic fields  

DOEpatents

A system (10) for measuring magnetic fields, wherein the system (10) comprises an unmodulated or direct-feedback flux locked loop (12) connected by first and second unbalanced RF coaxial transmission lines (16a, 16b) to a superconducting quantum interference device (14). The FLL (12) operates for the most part in a room-temperature or non-cryogenic environment, while the SQUID (14) operates in a cryogenic environment, with the first and second lines (16a, 16b) extending between these two operating environments.

Ganther, Jr., Kenneth R. (Olathe, KS); Snapp, Lowell D. (Blue Springs, MO)

2006-08-15

191

Decoupling Suspension Controller Based on Magnetic Flux Feedback  

PubMed Central

The suspension module control system model has been established based on MIMO (multiple input and multiple output) state feedback linearization. We have completed decoupling between double suspension points, and the new decoupling method has been applied to CMS04 magnetic suspension vehicle in national mid-low-speed maglev experiment field of Tangshan city in China. Double suspension system model is very accurate for investigating stability property of maglev control system. When magnetic flux signal is taken back to the suspension control system, the suspension module's antijamming capacity for resisting suspension load variety has been proved. Also, the external force interference has been enhanced. As a result, the robustness and stability properties of double-electromagnet suspension control system have been enhanced. PMID:23844415

Zhang, Wenqing; Li, Jie; Zhang, Kun; Cui, Peng

2013-01-01

192

Low-noise magnetic sensing for marine munitions characterization  

NASA Astrophysics Data System (ADS)

Because the recovery of underwater munitions is many times more expensive than recovering the same items on dry land, there is a continuing need to advance marine geophysical characterization methods. To efficiently and reliably conduct surveying in marine environments, low-noise geophysical sensors are being configured to operate close to the sea bottom. We describe systems that are deployed from surface vessels via rigid or flexible tow cables or mounted directly to submersible platforms such as unmanned underwater vehicles. Development and testing of a towed configuration has led to a 4 meter wide hydrodynamically stable tow wing with an instrumented top-side assembly mounted on the stern of a surface survey vessel. An integrated positioning system combined with an instrumented cable management system, vessel and wing attitude and wing depth measurements to provide sub-meter positional accuracy in up to 25 meter water depths and within 1 to 2 meters of the seafloor. We present the results of data collected during an instrument validation survey over a series of targets emplaced at measured locations. Performance of the system was validated through analyses of data collected at varying speeds, headings, and height above the seafloor. Implementation of the system during live-site operations has demonstrated its capability to survey hundreds of acres of marine or lacustrine environment. Unique deployment concepts that utilize new miniaturized and very low noise sensors show promise for expanding the applicability of magnetic sensing at marine sites.

Schultz, Gregory; Keranen, Joe; Billings, Stephen; Fonda, Raul; Foley, Jack; Bassani, Chet

2011-06-01

193

Three-dimensional Magnetostriction and Vector Magnetic Properties under Alternating Magnetic Flux Conditions in Arbitrary Direction  

NASA Astrophysics Data System (ADS)

This paper presents measured magnetostriction of electrical steel sheets under alternating magnetic flux conditions. In the measurements, we used a two-dimensional vector magnetic property measurement system, and a three-axial strain gauge. In order to know magnetostrictions in arbitrary direction, the strain tensor was evaluated. In addition, we try to evaluate the magnetostrictions in thickness direction. In this paper, the three-dimensional magnetostriction in non-oriented and grain-oriented silicon steel sheets are compared.

Wakabayashi, Daisuke; Todaka, Takashi; Enokizono, Masato

194

Magnetic Field-line Twist and Length Distributions inside Interplanetary Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

?We report on the detailed and systematic study of field-line twist and length distributions within magnetic flux ropes embedded in Interplanetary Coronal Mass Ejections (ICMEs). The Grad-Shafranov reconstruction method is utilized together with a constant-twist nonlinear force-free (Gold-Hoyle) flux rope model and the commonly known Lundquist (linear force-free) model to reveal the close relation between the field-line twist and length in cylindrical flux ropes, based on in-situ spacecraft magnetic field and plasma measurements. In particular, we utilize energetic electron burst observations at 1 AU together with associated type III radio emissions detected by the Wind spacecraft to provide unique measurements of magnetic field-line lengths within selected ICME events. These direct measurements are compared with flux-rope model calculations to help assess the fidelity of different models and to provide diagnostics of internal structures. We show that our initial analysis of field-line twist indicates clear deviation from the Lundquist model, but better consistency with the Gold-Hoyle model. By using the different flux-rope models, we conclude that the in-situ direct measurements of field-line lengths are consistent with a flux-rope structure with spiral field lines of constant and low twist, largely different from that of the Lundquist model, especially for relatively large-scale flux ropes. We will also discuss the implications of our analysis of flux-rope structures on the origination and evolution processes in their corresponding solar source regions.

Hu, Qiang; Qiu, Jiong; Krucker, Sam

2015-04-01

195

C IV fluxes from the Sun as a star, and the correlation with magnetic flux  

NASA Technical Reports Server (NTRS)

A total of 144 C IV wavelength 1548 Solar Maximum Mission (SMM)-UVSP spectroheliograms of solar plages were analyzed, some of which are series of exposures of the same region on the same day. Also analyzed were the C IV wavelength 1551 rasters of plages and C IV wavelength 1548 rasters of the quiet sun. The sample contained data on 17 different plages, observed on 50 different days. The center-to-limb variations of the active regions show that the optical thickness effects in the C IV wavelength 1548 line can be neglected in the conversion from intensity to flux density. As expected for the nearly optically thin situation, the C IV wavelength 1548 line is twice as bright as the C IV 1551 line. The average C IV wavelength 1548 flux density for a quiet region is 2700 ergs/cm/s and, with surprisingly little scatter, 18,000 erg/cm/s for plages. The intensity histograms of rasters obtained at disk center can be separated into characteristic plage and quiet sun contributions with variable relative filling factors. The relationship between the C IV and magnetic flux densities for spatially resolved data is inferred to be almost the same, with only an additional factor of order unity in the constant of proportionality.

Schrijver, C. J.; Linsky, J. L.; Bennett, J.; Brown, A.; Saar, S. H.

1988-01-01

196

Numerical Modeling of a Magnetic Flux Compression Experiment  

NASA Astrophysics Data System (ADS)

A possible plasma target for Magnetized Target Fusion (MTF) is a stable diffuse z-pinch in a toroidal cavity, like that in MAGO experiments. To examine key phenomena of such MTF systems, a magnetic flux compression experiment with this geometry is under design. The experiment is modeled with 3 codes: a slug model, the 1D Lagrangian RAVEN code, and the 1D or 2D Eulerian Magneto-Hydro-Radiative-Dynamics-Research (MHRDR) MHD simulation. Even without injection of plasma, high- Z wall plasma is generated by eddy-current Ohmic heating from MG fields. A significant fraction of the available liner kinetic energy goes into Ohmic heating and compression of liner and central-core material. Despite these losses, efficiency of liner compression, expressed as compressed magnetic energy relative to liner kinetic energy, can be close to 50%. With initial fluctuations (1%) imposed on the liner and central conductor density, 2D modeling manifests liner intrusions, caused by the m = 0 Rayleigh-Taylor instability during liner deceleration, and central conductor distortions, caused by the m = 0 curvature-driven MHD instability. At many locations, these modes reduce the gap between the liner and the central core by about a factor of two, to of order 1 mm, at the time of peak magnetic field.

Makhin, Volodymyr; Bauer, Bruno S.; Awe, Thomas J.; Fuelling, Stephan; Goodrich, Tasha; Lindemuth, Irvin R.; Siemon, Richard E.; Garanin, Sergei F.

2007-06-01

197

Performance comparison of dual-rotor radial-flux and axial-flux permanent-magnet BLDC machines  

Microsoft Academic Search

A novel machine family-dual-rotor, Radial-Flux, Permanent-Magnet (RFPM) machines-has demonstrated that it can substantially improve machine torque density and efficiency. The objective of this paper is to provide a performance comparison between two major alternatives of this technology: Surface-mounted dual-rotor RFPM machines and Axial-Flux Permanent-Magnet (AFPM) machines. The comparison is accomplished at four power levels ranging from 3 to 50 HP

Ronghai Qu; Metin Aydin; Thomas A. Lipo

2003-01-01

198

The continuum intensity as a function of magnetic field. II. Local magnetic flux and convective flows  

NASA Astrophysics Data System (ADS)

Context. To deepen our understanding of the role of small-scale magnetic fields in active regions (ARs) and in the quiet Sun (QS) on the solar irradiance, it is fundamental to investigate the physical processes underlying their continuum brightness. Previous results showed that magnetic elements in the QS reach larger continuum intensities than in ARs at disk center, but left this difference unexplained. Aims: We use Hinode/SP disk center data to study the influence of the local amount of magnetic flux on the vigour of the convective flows and the continuum intensity contrasts. Methods: The apparent (i.e. averaged over a pixel) longitudinal field strength and line-of-sight (LOS) plasma velocity were retrieved by means of Milne-Eddington inversions (VFISV code). We analyzed a series of boxes taken over AR plages and the QS, to determine how the continuum intensity contrast of magnetic elements, the amplitude of the vertical flows and the box-averaged contrast were affected by the mean longitudinal field strength in the box (which scales with the total unsigned flux in the box). Results: Both the continuum brightness of the magnetic elements and the dispersion of the LOS velocities anti-correlate with the mean longitudinal field strength. This can be attributed to the "magnetic patches" (here defined as areas where the longitudinal field strength is above 100 G) carrying most of the flux in the boxes. There the velocity amplitude and the spatial scale of convection are reduced. Due to this hampered convective transport, these patches appear darker than their surroundings. Consequently, the average brightness of a box decreases as the the patches occupy a larger fraction of it and the amount of embedded flux thereby increases. Conclusions: Our results suggest that as the magnetic flux increases locally (e.g. from weak network to strong plage), the heating of the magnetic elements is reduced by the intermediate of a more suppressed convective energy transport within the larger and stronger magnetic patches. This, together with the known presence of larger magnetic features, could explain the previously found lower contrasts of the brightest magnetic elements in ARs compared to the QS. The inhibition of convection also affects the average continuum brightness of a photospheric region, so that at disk center, an area of photosphere in strong network or plage appears darker than a purely quiet one. This is qualitatively consistent with the predictions of 3D MHD simulations.

Kobel, P.; Solanki, S. K.; Borrero, J. M.

2012-06-01

199

Measuring the Magnetic Flux Density in the CMS Steel Yoke  

E-print Network

The Compact Muon Solenoid (CMS) is a general purpose detector, designed to run at the highest luminosity at the CERN Large Hadron Collider (LHC). Its distinctive features include a 4 T superconducting solenoid with 6-m-diameter by 12.5-m-length free bore, enclosed inside a 10000-ton return yoke made of construction steel. The return yoke consists of five dodecagonal three-layered barrel wheels and four end-cap disks at each end comprised of steel blocks up to 620 mm thick, which serve as the absorber plates of the muon detection system. Accurate characterization of the magnetic field everywhere in the CMS detector is required. To measure the field in and around the steel, a system of 22 flux-loops and 82 3-D Hall sensors is installed on the return yoke blocks. Fast discharges of the solenoid (190 s time-constant) made during the CMS magnet surface commissioning test at the solenoid central fields of 2.64, 3.16, 3.68 and 4.01 T were used to induce voltages in the flux-loops. The voltages are measured on-line a...

Klyukhin, V I; Ball, A; Curé, B; Gaddi, A; Gerwig, H; Hervé, A; Mulders, M; Loveless, R

2012-01-01

200

Estimating total heliospheric magnetic flux from single-point in situ measurements  

Microsoft Academic Search

A fraction of the total photospheric magnetic flux opens to the heliosphere to form the interplanetary magnetic field carried by the solar wind. While this open flux is critical to our understanding of the generation and evolution of the solar magnetic field, direct measurements are generally limited to single-point measurements taken in situ by heliospheric spacecraft. An observed latitude invariance

M. J. Owens; C. N. Arge; N. U. Crooker; N. A. Schwadron; T. S. Horbury

2008-01-01

201

Inertia Wheel on Low-Noise Active Magnetic Suspension  

NASA Astrophysics Data System (ADS)

Magnetic bearings are particularly suited for space applications for a number of reasons: - they are ideally suited for vacuum applications; - the lack of lubrication and wear enhances the reliability and guaranties a long maintenance-free operation - the low drag torque decreases power consumption and reduces the torque exerted on the stator of the machine. - the possibility of insulating actively the spacecraft from the excitation due to unbalance of the rotating system In the case of reaction wheels, a well designed magnetic suspension allows high speed operation with a very low power consumption and vibration level. Conversely, microgravity (and possibly vacuum) operation is an advantage for magnetic bearings. The absence of static forces allows to operate with low current levels, thus reducing electrical noise and allowing to reach even lower vibration levels than in Earth applications of magnetic bearings. Active magnetic bearings (AMB) allow to adapt the working characteristics of the system to the operating needs: it is possible to use the actuators to lock the system during launch (absence of grabbers) and to stiffen the suspension when the spacecraft is accelerated (impulsive phases), while working in conditions optimised for microgravity when this is needed. Magnetic suspension systems designed for microgravity environment cannot be correctly tested on the ground. Testing in ground conditions results in the need of grossly overdesigning the levitation device; furthermore, in some cases ground testing is completely impossible, if not by introducing devices which compensate for the Earth gravitational field. If the compensation for the gravitational force is supplied by the same actuators used for microgravity operation, the actuators and the power amplifiers must be overdesigned and in some cases the suspension can be altogether impossible. They work in conditions which are much different from nominal ones and, above all, it is impossible to reach the precision in force measuring or vibration isolation which are required. Note that the stiffness of a magnetic suspension usually increases when it must compensate for a large static force and the increase of stiffness changes drastically the vibration isolation characteristics. It is also possible to support the rotor using a separate controlled electromagnet, but the latter will introduce disturbances which make impossible to evaluate the performances of the magnetic levitation system. Moreover, the sensitivity of the device to the operating conditions makes testing in conditions so different from the actual ones of very little significance. This is particularly true when accurate force measuring or vibration isolation is required or when low power consumption is one of the design specifications. Finally, if an external electromagnetic device is used for compensating for weight, its presence changes the stiffness of the system, to the point of altering drastically its stability characteristics. Parabolic flight is not a solution for this problem: the duration of low gravity conditions during parabolic flights is too short to perform significant experiments on magnetic suspension systems, particularly if the natural frequency of the suspension is very low as is typical of devices aimed at the isolation from low frequency vibrations. The environment in which parabolic flight testing is performed is also too rough for accurate testing. The availability of the space station changes deeply this situation: magnetic levitation systems built for space application can be tested in conditions which are very close to the operating ones. Although the space station environment is not vibrationally so clean as it would be necessary for some application, it is nevertheless far better than any simulated environment on the ground. The present paper deals with the design and construction of an engineering model of an inertia wheel on AMB. The aim of the project is to test the performance of the inertia wheel, particularly for what the disturbances caused by the rotor on the suppor

Carabelli, S.; Genta, G.; Silvagni, M.; Tonoli, A.

2002-01-01

202

Spatial resolution of SQUID magnetometers and comparison with low noise room temperature magnetic sensors  

NASA Astrophysics Data System (ADS)

Any magnetic sensor placed in a spatially inhomogeneous magnetic field delivers a signal proportional the mean field value taken over an effective area or volume which depends on the type of sensor considered. In the case of the field produced by a magnetic dipole and detected by a square or circular planar sensor, the overall measured spatial resolution ideally depends on the ratio of the mean dipole-sensor distance z0 to the square root of the effective sensor area AE. For Z 0/ A E?1 , the spatial resolution is limited by the size of the sensor, whereas for z 0/ A E?1 the dipole-sensor distance is the predominant factor. To compare various low noise magnetic sensors operating either at low temperature or at room temperature, we have measured their sensitivities and spatial responses to the field produced by a magnetic moment having the form of a tiny circular current loop. The sensors could be moved in all directions with respect to the current loop. The transfer of each sensor to the magnetic dipole field was compared to their response in a homogeneous field so as to deduce their effective area and compare this area to that deduced from independent spatial resolution measurements. We report the experimental results given by four types of sensors namely a dc-SQUID, a Hall effect sensor, a giant magneto-resistive sensor and a flux-gate sensor and discuss them by mean of a “figure of merit” criterion combining their spatial resolution and their sensitivity.

Dolabdjian, C.; Qasimi, A.; Bloyet, D.; Mosser, V.

2002-03-01

203

Micro-magnetic imprinting of high field gradient magnetic flux sources  

NASA Astrophysics Data System (ADS)

We report here on the fabrication of hard magnetic powder based micro-flux sources using micro-patterned hard magnetic films as templates or master structures. The micro-magnetic imprinting (?MI) process is simple and the constituent materials of the final structures, commercial hard magnetic powders and polymer, are inexpensive. The structures may be transparent, and either flexible or rigid, depending on the choice of polymer matrix used. The peak-to-peak intensity of the z-component of the stray magnetic field measured above a test ?MI structure made with spherical NdFeB particles of average particle size 16 ?m is in good agreement with simulated field values (150 mT at 5 ?m). Simulations indicate magnetic field gradients of up to 5 × 105 T/m at the surface of such ?MI structures. The trapping of cells functionalised with superparamagnetic beads by these structures has been demonstrated. The ?MI fabrication technique has much potential for the development of high field gradient magnetic flux sources for applications in biology and beyond.

Dempsey, N. M.; Le Roy, D.; Marelli-Mathevon, H.; Shaw, Gorky; Dias, A.; Kramer, R. B. G.; Viet Cuong, Le; Kustov, M.; Zanini, L. F.; Villard, C.; Hasselbach, K.; Tomba, C.; Dumas-Bouchiat, F.

2014-06-01

204

Flux-Weakening Regime Operation of an Interior Permanent-Magnet Synchronous Motor Drive  

Microsoft Academic Search

The interior permanent magnet (IPM) synchronous motor is compatible with extended-speed-range constant-power operation by means of flux-weakening control. Flux weakening uses stator current components to counter the fixed-amplitude magnetic airgap flux generated by the rotor magnets, performing a role similar to field weakening in a separately excited dc motor. The nature of current regulator saturation caused by the finite inverter

Thomas M. Jahns

1987-01-01

205

Analysis of local projected current density from one component of magnetic flux density in MREIT  

NASA Astrophysics Data System (ADS)

Magnetic resonance electrical impedance tomography is a new modality capable of imaging the static electrical conductivity of an object by measuring Bz data, a component of the magnetic flux density B = (Bx, By, Bz), perturbed by an external injection current. In an imaging area, the current density J induced by the external injection current can be uniquely decomposed into a recoverable component JP and an invisible component from the measured Bz data. In the case of in vivo animal and human imaging experiments, the imaging area frequently includes local defective regions with a low signal-to-noise ratio. As a result, the measured Bz data in the defective regions include serious noise due to rapid T2 decay, a small amount of internal current density and weak MR signals. In this paper, we propose an algorithm to reconstruct a recoverable current density \\mathbf {J}^R_{ {D}} from the measured Bz data in a local region {D} avoiding the defective regions. We estimate the L2-norm of the difference between the induced internal current density J and the locally recovered \\mathbf {J}^R_{ {D}} from the measured Bz data in the local region {D}. The difference only depends on the z-components of J and J0 and the values of Bx and By on the boundary \\partial {D}, where J0 is the background current density by the injected current. Numerical simulations and phantom experiments demonstrate that the proposed method directly reconstructs a local current density avoiding noise effects in defective regions.

Kim, Hyung Joong; Sajib, Saurav Z. K.; Jeong, Woo Chul; Nyoun Kim, Myoung; In Kwon, Oh; Woo, Eung Je

2013-07-01

206

Optimization of the magnetic noise and sensitivity of giant magnetoimpedance sensors  

NASA Astrophysics Data System (ADS)

The performance of magnetic field sensors based upon the giant magnetoimpedance (GMI) effect in soft magnetic wires is investigated in the MHz frequency range. The performance of the sensor is based on its sensitivity, voltage noise level and (voltage) noise-to-sensitivity ratio, or intrinsic magnetic noise level. Optimization of the sensitivity and noise response of the sensor through variation of the sample anisotropy direction and external applied axial field and dc bias current suggest an intrinsic noise level in the fT/?Hz level. Qualitative agreement is obtained between theory and experiment on a CoFeSiB microwire, for the maximum sensitivity and the corresponding noise, as a function of the external field and dc bias current.

Melo, L. G. C.; Ménard, D.; Yelon, A.; Ding, L.; Saez, S.; Dolabdjian, C.

2008-02-01

207

Spectral signature of magnetic flux tubes in sunspot penumbrae  

NASA Astrophysics Data System (ADS)

We study the polarization of spectral lines in the penumbra by integrating the radiative transfer equation of polarized light for a three-dimensional model atmosphere of a sunspot. In this model, the Evershed flow is confined to magnetic flux tubes which are embedded in a static background atmosphere, in accordance with the moving tube model of Schlichenmaier et al. (\\cite{Schlichenmaier1998apjl},b). The gradients and/or discontinuities in the fluid velocity and the magnetic field at the flux tube boundaries give rise to asymmetric Stokes profiles. We concentrate on the Stokes-V profiles and study the net circular polarization (NCP) of two photospheric spectral lines of neutral iron, Fe I 630.25 nm and Fe I 1564.8 nm. The different behavior of these two lines, which are exemplary for atomic spectral lines with a large Landé factor and significantly different wavelength, is based on the difference in excitation potential of the corresponding atomic transitions and the fact that the wavelength dependence of the Doppler shift is linear, while that of the Zeeman splitting is quadratic. We find that the azimuthal variation of the NCP, N(psi, is a predominantly antisymmetric function of psi with respect to the line connecting disk center and spot center (line-of-symmetry) for the infrared line of Fe I 1564.8 nm, while the variation is predominantly symmetric for Fe I 630.25 nm. We show that the antisymmetric variation is caused by anomalous dispersion (Faraday pulsation) and the discontinuity in the azimuthal angle of the magnetic field, which is due to the relative inclination between flux tube and background field. We then compute synthetic NCP maps of a sunspot and compare them with observational results. Finally, the center-to-limb variation of the NCP, N(theta ), of these spectral lines is investigated. We show that the location of the zero-crossing point of N(theta ) on the center side of the line-of-symmetry represents a diagnostic tool to determine the inclination angle of the Evershed flow: A vanishing NCP on the center-side of the line-of-symmetry is an indirect evidence of downflows in the penumbra.

Müller, D. A. N.; Schlichenmaier, R.; Steiner, O.; Stix, M.

2002-10-01

208

Anthropogenic electromagnetic noise disrupts magnetic compass orientation in a migratory bird.  

PubMed

Electromagnetic noise is emitted everywhere humans use electronic devices. For decades, it has been hotly debated whether man-made electric and magnetic fields affect biological processes, including human health. So far, no putative effect of anthropogenic electromagnetic noise at intensities below the guidelines adopted by the World Health Organization has withstood the test of independent replication under truly blinded experimental conditions. No effect has therefore been widely accepted as scientifically proven. Here we show that migratory birds are unable to use their magnetic compass in the presence of urban electromagnetic noise. When European robins, Erithacus rubecula, were exposed to the background electromagnetic noise present in unscreened wooden huts at the University of Oldenburg campus, they could not orient using their magnetic compass. Their magnetic orientation capabilities reappeared in electrically grounded, aluminium-screened huts, which attenuated electromagnetic noise in the frequency range from 50?kHz to 5?MHz by approximately two orders of magnitude. When the grounding was removed or when broadband electromagnetic noise was deliberately generated inside the screened and grounded huts, the birds again lost their magnetic orientation capabilities. The disruptive effect of radiofrequency electromagnetic fields is not confined to a narrow frequency band and birds tested far from sources of electromagnetic noise required no screening to orient with their magnetic compass. These fully double-blinded tests document a reproducible effect of anthropogenic electromagnetic noise on the behaviour of an intact vertebrate. PMID:24805233

Engels, Svenja; Schneider, Nils-Lasse; Lefeldt, Nele; Hein, Christine Maira; Zapka, Manuela; Michalik, Andreas; Elbers, Dana; Kittel, Achim; Hore, P J; Mouritsen, Henrik

2014-05-15

209

Influence of the inductor shape, and the magnetization processes on a trapped magnetic flux in a superconducting  

E-print Network

, such as magnetic bearings [6, 7], and high power density rotating machines [8-12] because it can trap a high valueInfluence of the inductor shape, and the magnetization processes on a trapped magnetic flux.gony@univ-lorraine.fr Abstract. In this paper, we study the form of the inductor for producing a magnetic field

Boyer, Edmond

210

Coupling Superconducting Flux Qubits Using AC Magnetic Flxues  

NASA Astrophysics Data System (ADS)

Exploiting the intrinsic nonlinearity of superconducting Josephson junctions, we propose a scalable circuit with superconducting (SC) qubits which is very similar to the successful one now being used for trapped ions. The SC qubits are coupled to the "vibrational" mode provided by a superconducting LC circuit or its equivalent (e.g., a SQUID). Both single-qubit rotations and qubit-LC-circuit couplings/decouplings can be controlled by varying frequencies of the applied ac magnetic fluxes. The circuit is scalable since the qubit-qubit interactions, mediated by the LC circuit, can be selectively performed, and the information transfer can be realized in a controllable way. We also discuss how to control the couplings between the qubit and the data bus via "dressed" qubit states. This approach can also be reduced to the one used for trapped ions.

Liu, Yu-Xi; Nori, Franco

2009-06-01

211

Results of railgun experiments powered by magnetic flux compression generators  

SciTech Connect

Researchers from LLNL and LANSL initiated a joint railgun research and development program to explore the potential of electromagnetic railguns to accelerate projectiles to hypervelocities. The effort was intended to determine experimentally the limits of railgun operation, to verify calculations of railgun performance, and to establish a data base at megampere currents. The program has led to the selection of a particular magnetic flux compression generator (MFCG) design for a set of initial experiments and to the design of small- and large-square bore railguns to match the expected MFCG power profile. The bore sizes are 12.7 and 50 mm, respectively. The design of the railguns and the diagnostic and data reduction techniques, followed by the results of eight experiments with the two railgun types are presented.

Hawke, R.S.; Brooks, A.L.; Deadrick, F.J.; Scudder, J.K.; Fowler, C.M.; Caird, R.S.; Peterson, D.R.

1980-10-24

212

The cancellation of magnetic flux. II - In a decaying active region. [of sun  

NASA Technical Reports Server (NTRS)

H-alpha filtergrams and videomagnetograms are used to study an active region during its period of decay on August 3-8, 1984; the decay had been initiated by a fragmentation process in which very small knots of magnetic flux separated from larger concentration of flux. The disappearance of magnetic flux was always observed when the small fragments of flux encountered other small fragments or concentrations of flux of opposite polarity. Such 'cancellations' are shared by both polarities of magnetic field, and it is deduced that the disappearance of flux occurred either at or within 5 arcsec of the apparent dividing line between the opposite polarities. All of the 22 flares observed during the decay of this region were initiated around sites where magnetic flux was cancelling or was deduced to be cancelling during the flares. It is hypothesized that cancellation was one of the necessary conditions for flaring in this active region.

Martin, S. F.; Livi, S. H. B.; Wang, J.

1985-01-01

213

Zigzag-shaped AMR magnetic sensors: Transfer characteristics and noise (Invited Paper)  

NASA Astrophysics Data System (ADS)

Microfabricated magnetoresistive zigzag-shaped elements based on the anisotropic magnetoresistance effect were studied for use as magnetic field sensors. Images taken using scanning electron microscopy with polarization analysis show that the magnetization in the devices tends to follow the edges of the device, thereby providing a geometrical (45°) bias that alternates along length of the sensor. It was found that these devices are primarily sensitive to magnetic fields applied along the long axis; a flat response is observed for perpendicular fields. The alternating magnetization bias provides the directionality of the sensor because the angles in adjacent zigzag blocks scissor for fields parallel to the long axis and rotate for perpendicular fields. This results in resistances that either add or cancel, respectively. A single-domain, coherent rotation description provides an estimate for the qualitative behavior of these zigzag structures and indicates the possible role of exchange in the shape of the transfer curves. Noise measurements were also taken on these devices. Thermal resistance noise was the dominant noise source above about 10 kHz. At low frequencies the resistance noise was found to be dominated by a 1/f contribution that depends on the applied magnetic field. The 1/f noise is relatively low and field independent when the element is in a saturated magnetic state and contains a relatively large and field dependent excess contribution when the magnetic field is in the sensitive field range of the element. The 1/f noise level observed in the saturated state depends in a nontrivial way on the quality and processing of the magnetic element, showing a trend for lower normalized noise in elements having higher sensitivity. In the most sensitive elements (magnetoresistance > 1%) the 1/f noise level is comparable to that found in nonmagnetic metals. We attribute the origin of noise to defect motion. In the unsaturated state, the excess noise is found to track the dc resistance susceptibility. For particular values of applied field we also observed large random telegraph signals in the time domain. The telegraph noise was extremely sensitive to the applied field, becoming active and inactive in our measurement bandwidth for changes in field of only a few Oersteds. This behavior indicates a magnetic origin to the excess noise. The variation of the excess noise level with applied dc magnetic field can be explained qualitatively using a model based on thermal excitation of the magnetization direction and/or domain wall hopping between pinning sites.

Jiang, L.; Gokce, Aisha; da Silva, F. C. S.; Nowak, E. R.

2005-05-01

214

Topological Structure of Magnetic Flux Lines Generated by Thermal Convection in a Rotating Spherical Shell  

Microsoft Academic Search

Topological structure and reconnection of magnetic flux lines are investigated by analyzing the numerical solutionof the Boussinesq magneto-hydrodynamic equationsin a rotating spherical shell.Five pairs of Taylor-Proudman vortex columns are generatedand drift westward steadily.Magnetic field is intensified around the tops of these vortex columns.Magnetic flux linesconnect east-west adjacent domains of intense magnetic field,which migrate eastward relative to the vortex columns.We describe

Hideaki Kitauchi

1998-01-01

215

Three-Dimensional Magnetic Field Line Reconnection involving Magnetic Flux Ropes (Invited)  

NASA Astrophysics Data System (ADS)

We report on two experiments in which three dimensional magnetic field line reconnection plays a role. Magnetic field line reconnection is a processes in which the magnetic field energy is converted to particle energy and heating accompanied by changes in the magnetic topology. In the first experiment two magnetic flux ropes are generated from initially adjacent pulsed current channels in a background magnetoplasma in the LAPD device at UCLA. The currents exert mutual jXB forces causing them to twist about each other and merge. The currents are not static but move towards or away from each other in time. In addition the currents are observed to filament after merging. Volumetric space-time data show multiple reconnection sites with time-dependent locations. The quasi-separatrix layer (QSL) is a narrow region between the flux ropes. Two field lines on either side of the QSL will have closely spaced foot-points at on end of the flux ropes, but a very different separation at the other end. Outside the QSL, neighboring field lines do not diverge. The QSL has been measured, for the first time in this experiment [1] and its three dimensional development will be shown in movies made from the data. A system involving the reconnection of three flux ropes will also be presented. Three flux ropes are generated by drawing currents through apertures in a carbon shield located in front of a 10 cm diameter cathode immersed in the background magnetoplasma. The currents are observed to twist about themselves, writhe about each other and thrash about due to kink the kink instability. Multiple reconnection regions (which are three dimensional) and a complex QSL are observed. The magnetic helicity is evaluated from volumetric data in both cases and its rate of change is used to estimate the plasma resistivity. These measurements lead one to suspect that magnetic field line reconnection is not an independent topic, which can be studied in isolation, but part of the phenomena associated with broader subject of 3D waves and current systems in plasmas. [1] E. Lawrence, W. Gekelman, Phys. Rev. Lett., 103, 105002 (2009). This work was done at the Basic Plasma Science Facility at UCLA and funded by NSF and DOE.

Gekelman, W. N.; van Compernolle, B.; Lawrence, E.; Vincena, S. T.

2010-12-01

216

Testing of the Mark 101 magnetic flux compression generator  

SciTech Connect

The Mark 101 explosive flux compression generator is a line-initiated, vacuum/magnetically insulated, helical generator. This device offered some unique challenges in transforming the theoretical design into a testable experiment. The two main reasons for this are that in theory an eight-turn, four-wire Mark 101 possesses a terminal dL/dt of approx.0.5 ..cap omega.. and operates with electric fields which are greater than the threshold for electron field emission. With this in mind, we designed an integral vacuum-jacket-generator configuration with a passive load inductance of less than or equal to0.5 ..mu..H. The generator contained approx.8 ..mu..H of initial inductance. The field emission required the stator to be entirely sealed within the vacuum jacket. The open, helical stator resulted in the presence of non-trivial leakage fields and voltages. To accommodate these fields, the vacuum chamber for the generator was segmented and axially insulated with rings of acrylic, similar to stacked-ring diodes. We made no attempt to break the azimuthal metal surfaces due to the physical difficulty this would incur. Diagnostics included an input current Rogowski loop, a load Rogowski loop, two dB/dt probes in the load, a Faraday fiber-optic current sensor, and two dB/dt probes in the region between the stator winding and the vacuum jacket to measure the leakage azimuthal and axial magnetic fields. The results of explosive tests are presented.

Freeman, B.L.; Fowler, C.M.; King, J.C.; Martinez, A.R.

1986-01-01

217

Simulations of Emerging Magnetic Flux. II. The Formation of Unstable Coronal Flux Ropes and the Initiation of Coronal Mass Ejections  

NASA Astrophysics Data System (ADS)

We present results from three-dimensional magnetohydrodynamic simulations of the emergence of a twisted convection zone flux tube into a pre-existing coronal dipole field. As in previous simulations, following the partial emergence of the sub-surface flux into the corona, a combination of vortical motions and internal magnetic reconnection forms a coronal flux rope. Then, in the simulations presented here, external reconnection between the emerging field and the pre-existing dipole coronal field allows further expansion of the coronal flux rope into the corona. After sufficient expansion, internal reconnection occurs beneath the coronal flux rope axis, and the flux rope erupts up to the top boundary of the simulation domain (~36 Mm above the surface). We find that the presence of a pre-existing field, orientated in a direction to facilitate reconnection with the emerging field, is vital to the fast rise of the coronal flux rope. The simulations shown in this paper are able to self-consistently create many of the surface and coronal signatures used by coronal mass ejection (CME) models. These signatures include surface shearing and rotational motions, quadrupolar geometry above the surface, central sheared arcades reconnecting with oppositely orientated overlying dipole fields, the formation of coronal flux ropes underlying potential coronal field, and internal reconnection which resembles the classical flare reconnection scenario. This suggests that proposed mechanisms for the initiation of a CME, such as "magnetic breakout," are operating during the emergence of new active regions.

Leake, James E.; Linton, Mark G.; Antiochos, Spiro K.

2014-05-01

218

Signal extraction using ensemble empirical mode decomposition and sparsity in pipeline magnetic flux leakage nondestructive evaluation  

NASA Astrophysics Data System (ADS)

The commonly used and cost effective corrosion inspection tools for the evaluation of pipelines utilize the magnetic flux leakage (MFL) technique. The MFL signal is usually contaminated by various noise sources. In this paper, we propose that the pipeline flaw MFL signal is extracted using the ensemble empirical mode decomposition (EEMD) and the sparsity. At first, we introduce the EEMD method. The EEMD defines the true intrinsic mode function (IMF) components as the mean of an ensemble of trials, each consisting of the signal plus a white noise of finite amplitude. Moreover, sparsity selection restriction was defined. Then, The MFL signal is decomposed into several IMFs used for signal reconstruction. Some modes are selected to reconstruct a new signal considering their sparsity. Finally, the comparison is made with the empirical mode decomposition. At the same time, the comparison of the selection restriction between the sparsity and the energy is described. The results show that the EEMD and the sparsity is an efficient technology with the pipeline flaw extraction.

Chen, Liang; Li, Xing; Li, Xun-bo; Huang, Zuo-ying

2009-02-01

219

Signal extraction using ensemble empirical mode decomposition and sparsity in pipeline magnetic flux leakage nondestructive evaluation.  

PubMed

The commonly used and cost effective corrosion inspection tools for the evaluation of pipelines utilize the magnetic flux leakage (MFL) technique. The MFL signal is usually contaminated by various noise sources. In this paper, we propose that the pipeline flaw MFL signal is extracted using the ensemble empirical mode decomposition (EEMD) and the sparsity. At first, we introduce the EEMD method. The EEMD defines the true intrinsic mode function (IMF) components as the mean of an ensemble of trials, each consisting of the signal plus a white noise of finite amplitude. Moreover, sparsity selection restriction was defined. Then, The MFL signal is decomposed into several IMFs used for signal reconstruction. Some modes are selected to reconstruct a new signal considering their sparsity. Finally, the comparison is made with the empirical mode decomposition. At the same time, the comparison of the selection restriction between the sparsity and the energy is described. The results show that the EEMD and the sparsity is an efficient technology with the pipeline flaw extraction. PMID:19256676

Chen, Liang; Li, Xing; Li, Xun-bo; Huang, Zuo-ying

2009-02-01

220

The structure of untwisted magnetic flux tubes. [solar magnetic field distribution  

NASA Technical Reports Server (NTRS)

While most previous investigations have concentrated on slender flux tubes, the present study of the equilibrium structure of an axisymmetric magnetic flux tube, confined by an external pressure that varies along the length of the tube, explores the properties of thick tubes in order to establish the degree to which slender tube theory is valid. It is found that slender flux tube results may in some cases give no indication of thick tube behavior in a nonuniform atmosphere. Depending on boundary conditions applied at the ends of the tube, it may expand or contract upon entering a region of increasing pressure. Rather than expanding indefinitely, the tube surface may form a cusped shape when a point of external pressure on the tube surface falls to equality with the internal pressure. Numerical solutions for an initially uniform tube give smaller expansions than would be expected from slender tube theory.

Browning, P. K.; Priest, E. R.

1982-01-01

221

Enhancement of magnetic flux distribution in a DC superconducting electric motor  

NASA Astrophysics Data System (ADS)

Most motor designs require an air gap between the rotor and stator to enable the armature to rotate freely. The interaction of magnetic flux from rotor and stator within the air gap will provide the thrust for rotational motion. Thus, the understanding of magnetic flux in the vicinity of the air gap is very important to mathematically calculate the magnetic flux generated in the area. In this work, a finite element analysis was employed to study the behavior of the magnetic flux in view of designing a synchronous DC superconducting electric motor. The analysis provides an ideal magnetic flux distribution within the components of the motor. From the flux plot analysis, it indicates that flux losses are mainly in the forms of leakage and fringe effect. The analysis also shows that the flux density is high at the area around the air gap and the rotor. The high flux density will provide a high force area that enables the rotor to rotate. In contrast, the other parts of the motor body do not show high flux density indicating low distribution of flux. Consequently, a bench top model of a DC superconducting motor was developed where by motor with a 2-pole type winding was chosen. Each field coil was designed with a racetrack-shaped double pancake wound using DI-BSCCO Bi-2223 superconducting tapes. The performance and energy efficiency of the superconducting motor was superior when compared to the conventional motor with similar capacity.

Hamid, N. A.; Ewe, L. S.; Chin, K. M.

2013-06-01

222

New models of Jupiter's magnetic field constrained by the Io flux tube footprint  

Microsoft Academic Search

Spherical harmonic models of the planetary magnetic field of Jupiter are obtained from in situ magnetic field measurements and remote observations of the position of the foot of the Io flux tube in Jupiter's ionosphere. The Io flux tube (IFT) footprint locates the ionospheric footprint of field lines traced from Io's orbital radial distance in the equator plane (5.9 Jovian

J. E. P. Connerney; M. H. Acufia; N. F. Ness; T. Satoh

1998-01-01

223

Nonlinear model of flaw detection in steel pipes by magnetic flux leakage  

Microsoft Academic Search

A numerical model of crack detection by magnetic flux leakage that takes into account the saturation of the induction flux density with the applied field is developed. The model contains no free parameters and is validated with detailed measurements of materials properties and magnetic fields reported by Förster. It is then used to discuss the dependence of signals on the

Eduardo Altschuler; Alberto Pignotti

1995-01-01

224

Cutoff-free Propagation of Torsional Alfvén Waves along Thin Magnetic Flux Tubes  

Microsoft Academic Search

Propagation of torsional Alfvén waves along magnetic flux tubes has been extensively studied for many years, but no conclusive results regarding the existence of a cutoff frequency for these waves have been obtained. The main purpose of this paper is to derive new wave equations that describe the propagation of linear torsional Alfvén waves along thin and isothermal magnetic flux

Z. E. Musielak; S. Routh; R. Hammer

2007-01-01

225

Poles position identification of permanent magnet axial flux motor using PIPCRM sensorless method  

Microsoft Academic Search

The paper describes new sensorless method control of an axial flux permanent magnet motor at standstill and at low speed. This sensorless method called PIPCRM (Position Identification by Parallel Current Rate Measurement) was developed to provide sensorless start axial flux permanent magnet motor in demanded direction from standstill. The PIPCRM method is used to detect rotor poles position and then

Janusz Wisniewski; Piotr Jakubowski

2007-01-01

226

Magnetohydrostatic Equilibrium. II. Three-dimensional Multiple Open Magnetic Flux Tubes in the Stratified Solar Atmosphere  

NASA Astrophysics Data System (ADS)

A system of multiple open magnetic flux tubes spanning the solar photosphere and lower corona is modeled analytically, within a realistic stratified atmosphere subject to solar gravity. This extends results for a single magnetic flux tube in magnetohydrostatic equilibrium, described in Gent et al. Self-similar magnetic flux tubes are combined to form magnetic structures, which are consistent with high-resolution observations. The observational evidence supports the existence of strands of open flux tubes and loops persisting in a relatively steady state. Self-similar magnetic flux tubes, for which an analytic solution to the plasma density and pressure distribution is possible, are combined. We calculate the appropriate balancing forces, applying to the equations of momentum and energy conservation to preserve equilibrium. Multiplex flux tube configurations are observed to remain relatively stable for up to a day or more, and it is our aim to apply our model as the background condition for numerical studies of energy transport mechanisms from the solar surface to the corona. We apply magnetic field strength, plasma density, pressure, and temperature distributions consistent with observational and theoretical estimates for the lower solar atmosphere. Although each flux tube is identical in construction apart from the location of the radial axis, combinations can be applied to generate a non-axisymmetric magnetic field with multiple non-uniform flux tubes. This is a considerable step forward in modeling the realistic magnetized three-dimensional equilibria of the solar atmosphere.

Gent, F. A.; Fedun, V.; Erdélyi, R.

2014-07-01

227

Empirical Modeling of Radiative versus Magnetic Flux for the Sun-as-a-Star  

E-print Network

We study the relationship between full-disk solar radiative flux at different wavelengths and average solar photospheric magnetic-flux density, using daily measurements from the Kitt Peak magnetograph and other instruments extending over one or more solar cycles. We use two different statistical methods to determine the underlying nature of these flux-flux relationships. First, we use statistical correlation and regression analysis and show that the relationships are not monotonic for total solar irradiance and for continuum radiation from the photosphere, but are approximately linear for chromospheric and coronal radiation. Second, we use signal theory to examine the flux-flux relationships for a temporal component. We find that a well-defined temporal component exists and accounts for some of the variance in the data. This temporal component arises because active regions with high magnetic field strength evolve, breaking up into small-scale magnetic elements with low field strength, and radiative and magnet...

Preminger, Dora; Chapman, Gary; Martens, Petrus C H; 10.1007/s11207-010-9560-1

2010-01-01

228

Numerical simulations of three-dimensional magnetic swirls in a solar flux-tube  

NASA Astrophysics Data System (ADS)

We aim to numerically study evolution of Alfvén waves that accompany short-lasting swirl events in a solar magnetic flux-tube that can be a simple model of a magnetic pore or a sunspot. With the use of the FLASH code we numerically solve three-dimensional ideal magnetohydrodynamic equations to simulate twists which are implemented at the top of the photosphere in magnetic field lines of the flux-tube. Our numerical results exhibit swirl events and Alfvén waves with associated clockwise and counterclockwise rotation of magnetic lines, with the largest values of vorticity at the bottom of the chromosphere, and a certain amount of energy flux.

Chmielewski, Piotr; Murawski, Krzysztof; Solov'ev, Alexandr A.

2014-07-01

229

Analysis of shot noise suppression in mesoscopic cavities in a magnetic field  

Microsoft Academic Search

We present a numerical investigation of shot noise suppression in mesoscopic\\u000acavities and an intuitive semiclassical explanation of the behavior observed in\\u000athe presence of an orthogonal magnetic field. In particular, we conclude that\\u000athe decrease of shot noise for increasing magnetic field is the result of the\\u000ainterplay between the diameter of classical cyclotron orbits and the width of

P. Marconcini; M. Macucci; G. Iannaccone; B. Pellegrini; G. Marola

2006-01-01

230

A sensor measuring the Fourier coefficients of the magnetic flux density for pipe crack detection using the magnetic flux leakage method  

NASA Astrophysics Data System (ADS)

A simple sensor for magnetic flux leakage methods of detecting cracks on the surfaces of ferromagnetic pipes is proposed. We show that the crack position can be determined by the Fourier coefficients of the leakage magnetic flux density on a circle inside the pipe in the pipe cross-sectional plane. Coils that directly output the Fourier cosine and sine coefficients were made. The experimental results showed that, using only these two coils rather than dozens of magnetic sensors, the center position of a crack on the inside/outside surface of the pipe could be localized.

Nara, T.; Takanashi, Y.; Mizuide, M.

2011-04-01

231

Algebraic reconstruction for 3D magnetic resonance-electrical impedance tomography (MREIT) using one component of magnetic flux density  

Microsoft Academic Search

Magnetic resonance-electrical impedance tomography (MREIT) algorithms fall into two categories: those utilizing internal current density and those utilizing only one component of measured magnetic flux density. The latter group of algorithms have the advantage that the object does not have to be rotated in the magnetic resonance imaging (MRI) system. A new algorithm which uses only one component of measured

Y. Ziya Ider; Serkan Onart

2004-01-01

232

Magnetohydrodynamic waves in a compressible magnetic flux tube with elliptical cross-section  

Microsoft Academic Search

Aims: The propagation of magnetohydrodynamic (MHD) waves in a finite, compressible magnetic flux tube with an elliptical cross-section embedded in a magnetic environment is investigated. Methods: We present the derivation of the general dispersion relation of linear magneto-acoustic wave propagation for a compressible magnetic flux tube with elliptical cross-section in a plasma with finite beta. The wave modes of propagation

R. Erdélyi; R. J. Morton

2009-01-01

233

Polar magnetic flux on the Sun in 1996-2003 from SOHO\\/MDI data  

Microsoft Academic Search

Polarity of the polar magnetic fields on the Sun changes in every 11-year sunspot cycle, affecting the large-scale structure of the corona and heliosphere. Using the SOHO\\/MDI data, the flux of the radial component of the magnetic field is estimated for different latitudinal zones. It is estimated that the total magnetic flux of the Northern polar zone, from 78° to

E. E. Benevolenskaya

2004-01-01

234

Superconducting flux pump for high-temperature superconductor insert coils of NMR magnets  

NASA Astrophysics Data System (ADS)

This paper describes a prototype flux pump recently operated at the MIT Francis Bitter Magnet Laboratory. The results of the prototype flux pump will be used in the development of a full-scale flux pump that will be coupled to a high-temperature superconductor (HTS) insert coil of a high-field NMR magnet. Such an HTS insert is unlikely to operate in persistent mode because of the conductor's low index (n). The flux pump can compensate for field decay in the HTS insert coil and make the insert operate effectively in persistent mode. The flux pump, comprised essentially of a transformer and two switches, all made of superconductor, transfers into the insert coil a fraction of a magnetic energy that is first introduced in the secondary circuit of the transformer by a current supplied to the primary circuit. A prototype flux pump has been designed, fabricated, and operated to demonstrate that a flux pump can indeed supply a small metered current into a load superconducting magnet. A current increment in the range of microamperes has been measured in the magnet after each pumping action. The superconducting prototype flux pump is made of Nb3Sn tape. The pump is placed in a gaseous environment above the liquid helium level to keep its heat dissipation from directly discharged in the liquid; the effluent helium vapor maintains the thermal stability of the flux pump.

Jeong, S.; Lee, H.; Iwasa, Y.

2002-05-01

235

The magnetic, basal, and radiative-equilibrium components in Mount Wilson Ca II H + K fluxes  

SciTech Connect

Mount Wilson Ca II H + K flux measurements of cool dwarf stars are analyzed and compared with stellar Mg II h + k fluxes, variability amplitudes, rotation rates, and solar data. It is concluded that the Mount Wilson Ca II H + K fluxes comprise three principal parts: (1) a photospheric contribution in the line wings, (2) a basal chromospheric component that appears to be unrelated to stellar magnetic activity and is, therefore, possibly nonmagnetic in origin, and (3) a chromospheric component which is associated with magnetically active regions and the (quiet and active) network. The basal chromosphere appears to cover the entire surface of magnetically inactive stars. The basal Ca II H + K flux density for solar-type stars equals the average emission observed in the centers of solar supergranulation cells, where the magnetic flux density is small. 27 refs.

Schrijver, C.J.; Dobson, A.K.; Radick, R.R. (National Solar Observatory, Sunspot, NM (USA); Joint Institute for Laboratory Astrophysics, Boulder, CO (USA))

1989-06-01

236

The magnetic, basal, and radiative-equilibrium components in Mount Wilson Ca II H + K fluxes  

NASA Technical Reports Server (NTRS)

Mount Wilson Ca II H + K flux measurements of cool dwarf stars are analyzed and compared with stellar Mg II h + k fluxes, variability amplitudes, rotation rates, and solar data. It is concluded that the Mount Wilson Ca II H + K fluxes comprise three principal parts: (1) a photospheric contribution in the line wings, (2) a basal chromospheric component that appears to be unrelated to stellar magnetic activity and is, therefore, possibly nonmagnetic in origin, and (3) a chromospheric component which is associated with magnetically active regions and the (quiet and active) network. The basal chromosphere appears to cover the entire surface of magnetically inactive stars. The basal Ca II H + K flux density for solar-type stars equals the average emission observed in the centers of solar supergranulation cells, where the magnetic flux density is small.

Schrijver, C. J.; Dobson, Andrea K.; Radick, Richard R.

1989-01-01

237

Modeling the influence of varying magnetic properties in soft magnetic materials on the hysteresis shape using the flux tube approach  

NASA Astrophysics Data System (ADS)

Magnetic properties can vary significantly inside soft magnetic steel sheets (SMSSs), both due to mechanical stresses and structural changes originating from different manufacturing processes. The integral consideration, i.e. averaging these effects over the SMSS, leads to a strong simplification of the underlying mechanisms. Such simplification is often inadequate when considering the influence of the varying magnetic properties on the hysteresis loop shape and its dynamic behavior. This paper presents a new approach to model irregular hysteresis loops of non-oriented SMSSs using the flux tube approach, where the SMSS is divided into several flux tubes having different magnetic properties. This enables to model non-homogeneous distributions of the magnetic flux and irregular hysteresis loops subject to varying magnetic properties.

Petrun, M.; Steentjes, S.; Hameyer, K.; Dolinar, D.

2015-05-01

238

Low frequency noise peak near magnon emission energy in magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

We report on the low frequency (LF) noise measurements in magnetic tunnel junctions (MTJs) below 4 K and at low bias, where the transport is strongly affected by scattering with magnons emitted by hot tunnelling electrons, as thermal activation of magnons from the environment is suppressed. For both CoFeB/MgO/CoFeB and CoFeB/AlOx/CoFeB MTJs, enhanced LF noise is observed at bias voltage around magnon emission energy, forming a peak in the bias dependence of noise power spectra density, independent of magnetic configurations. The noise peak is much higher and broader for unannealed AlOx-based MTJ, and besides Lorentzian shape noise spectra in the frequency domain, random telegraph noise (RTN) is visible in the time traces. During repeated measurements the noise peak reduces and the RTN becomes difficult to resolve, suggesting defects being annealed. The Lorentzian shape noise spectra can be fitted with bias-dependent activation of RTN, with the attempt frequency in the MHz range, consistent with magnon dynamics. These findings suggest magnon-assisted activation of defects as the origin of the enhanced LF noise.

Liu, Liang; Xiang, Li; Guo, Huiqiang; Wei, Jian; Li, D. L.; Yuan, Z. H.; Feng, J. F.; Han, X. F.; Coey, J. M. D.

2014-12-01

239

Comparison of Magnetic Flux Distribution between a Coronal Hole and a Quiet Region  

E-print Network

Employing Big Bear Solar Observatory (BBSO) deep magnetograms and H${\\alpha}$ images in a quiet region and a coronal hole, observed on September 14 and 16, 2004, respectively, we have explored the magnetic flux emergence, disappearance and distribution in the two regions. The following results are obtained: (1) The evolution of magnetic flux in the quiet region is much faster than that in the coronal hole, as the flux appeared in the form of ephemeral regions in the quiet region is 4.3 times as large as that in the coronal hole, and the flux disappeared in the form of flux cancellation, 2.9 times as fast as in the coronal hole. (2) More magnetic elements with opposite polarities in the quiet region are connected by arch filaments, estimating from magnetograms and H${\\alpha}$ images. (3) We measured the magnetic flux of about 1000 magnetic elements in each observing region. The flux distribution of network and intranetwork (IN) elements is similar in both polarities in the quiet region. For network fields in the coronal hole, the number of negative elements is much more than that of positive elements. However for the IN fields, the number of positive elements is much more than that of negative elements. (4) In the coronal hole, the fraction of negative flux change obviously with different threshold flux density. 73% of the magnetic fields with flux density larger than 2 Gauss is negative polarity, and 95% of the magnetic fields is negative, if we only measure the fields with their flux density larger than 20 Gauss. Our results display that in a coronal hole, stronger fields is occupied by one predominant polarity; however the majority of weaker fields, occupied by the other polarity.

Jun Zhang; Jun Ma; Haimin Wang

2007-05-04

240

Potential Magnetic Field around a Helical Flux-rope Current Structure in the Solar Corona  

E-print Network

We consider the potential magnetic field associated with a helical electric line current flow, idealizing the near-potential coronal field within which a highly localized twisted current structure is embedded. It is found that this field has a significant axial component off the helical magnetic axis where there is no current flow, such that the flux winds around the axis. The helical line current field, in including the effects of flux rope writhe, is therefore more topologically complex than straight line and ring current fields sometimes used in solar flux rope models. The axial flux in magnetic fields around confined current structures may be affected by the writhe of these current structures such that the field twists preferentially with the same handedness as the writhe. This property of fields around confined current structures with writhe may be relevant to classes of coronal magnetic flux rope, including structures observed to have sigmoidal forms in soft X-rays and prominence magnetic fields. For example, ``bald patches'' and the associated heating by Parker current sheet dissipation seem likely. Thus some measurements of flux rope magnetic helicities may derive from external, near-potential fields. The predicted hemispheric preference for positive and negative magnetic helicities is consistent with observational results for prominences and sigmoids and past theoretical results for flux rope internal fields.

G. J. D. Petrie

2007-02-06

241

Improvement of the sensor system in magnetic flux leakage-type nondestructive testing (NDT)  

Microsoft Academic Search

In magnetic flux leakage (MFL)-type nondestructive testing (NDT) system, the sensitivity of MFL sensor depends on the change of the magnetic field, not just on the magnitude of the field. In this paper, the leakage parameter was defined to determine the operating point in saturation curves, and an optimum design method to determine the size of the magnet to maximize

Gwan Soo Park; Eun Sik Park

2002-01-01

242

Dispersal of Magnetic Flux in the Quiet Solar Photosphere  

NASA Astrophysics Data System (ADS)

We study the random walk of magnetic flux concentrations on two sequences of high-resolution magnetograms, observed with the Michelson Doppler Imager on board SOHO. The flux contained in the concentrations ranges from |?|=1018 Mx to |?|=1019 Mx, with an average of |?|=2.5×1018 Mx. Larger concentrations tend to move slower and live longer than smaller ones. On short timescales, the observed mean-square displacements are consistent with a random walk, characterized by a diffusion coefficient D(t<10 ks)=70-90 km2 s-1. On longer timescales, the diffusion coefficient increases to D(t>30 ks)=200-250 km2 s-1, approaching the measurements for a five-day set of Big Bear magnetograms, D~=250 km2 s-1. The transition between the low and large diffusion coefficients is explained with a model and simulations of the motions of test particles, subject to random displacements on both the granular and supergranular scales, simultaneously. In this model, the supergranular flow acts as a negligible drift on short timescale, but dominates the granular diffusion on longer timescales. We also investigate the possibility that concentrations are temporarily confined, as if they were caught in supergranular vertices, that form short-lived, relatively stable environments. The best agreement of model and data is found for step lengths of 0.5 and 8.5 Mm, associated evolution times of 14 minutes and 24 hr, and a confinement time of no more than a few hours. On our longest timescale, DSim(t>105)-->285 km2 s-1, which is the sum of the small- and large-scale diffusion coefficients. Models of random walk diffusion on the solar surface require a larger value: DWang=600+/-200 km2 s-1. One possible explanation for the difference is a bias in our measurements to the longest lived, and therefore slower concentrations in our data sets. Another possibility is the presence of an additional, much larger diffusive scale.

Hagenaar, H. J.; Schrijver, C. J.; Title, A. M.; Shine, R. A.

1999-02-01

243

PLASTIC VERSUS ELASTIC DEFORMATION EFFECTS ON MAGNETIC BARKHAUSEN NOISE IN STEEL  

E-print Network

PLASTIC VERSUS ELASTIC DEFORMATION EFFECTS ON MAGNETIC BARKHAUSEN NOISE IN STEEL C.-G. STEFANITA, D) AbstractÐA study was performed to dierentiate the eects of elastic and plastic deformation on magnetic samples subjected to varying degrees of uniaxial elastic and plastic deformation up to H40% strain

Clapham, Lynann

244

Magnetic Barkhausen noise: the influence of microstructure and deformation in bending  

Microsoft Academic Search

The effect of microstructure and bending deformation on the characteristics of magnetic Barkhausen noise profiles were examined in carbon steel of the type used for wear-resistant engineering components. Microstructures associated with magnetic softness produced the largest profile peaks and the lowest peak positions. Multi-peak profiles were observed in compression in spherodised cementite specimens and to a much smaller extent in

M. Blaow; J. T. Evans; B. A. Shaw

2005-01-01

245

Analysis and Design of a High Power Density Axial Flux Permanent Magnet Linear Synchronous Machine Used for Stirling System  

Microsoft Academic Search

A high power density axial flux permanent magnet linear synchronous machine and the stirling system will be introduced. This machine is a tubular axial flux permanent magnet machine. It comprises two parts: stator and mover. With the 2D finite-element method (FEM), the high power density axial flux permanent magnet linear synchronous machine will be analyzed and designed. To get the

Ping Zheng; Xuhui Gan; Lin Li

2010-01-01

246

Magnetic pinning of flux lattice in superconducting-nanomagnet hybrids  

Microsoft Academic Search

Strong superconducting pinning effects are observed from magnetic landscapes produced by arrays of circular rings with varying magnetic remanent states. The collective and the background pinning of superconducting Nb films is strongly enhanced by the stray magnetic field produced by an array of circular Ni rings magnetized to form “onion” (bidomain) states. On the other hand, when the same rings

D. Perez de Lara; B. G. Ng; R. K. Dumas; E. M. Gonzalez; Kai Liu; C. A. Ross; Ivan K. Schuller; J. L. Vicent

2011-01-01

247

Influence of the Permanent Magnet Magnetization Length on the Performance of a Tubular Transverse Flux Permanent Magnet Linear Machine Used for Electromagnetic Launch  

Microsoft Academic Search

This paper investigates the underlying influence of permanent magnet (PM) magnetization length on the performance of a tubular transverse flux PM linear machine working as the propeller of an electromagnetic launcher, including its main flux linkage, cogging force, and thrust force. The theoretical analysis is brought forward and verified with 3-D numerical computations. It is shown that the cogging force

Jibin Zou; Qian Wang; Yongxiang Xu

2011-01-01

248

Fermionic condensate in a conical space with a circular boundary and magnetic flux  

SciTech Connect

The fermionic condensate is investigated in a (2+1)-dimensional conical spacetime in the presence of a circular boundary and a magnetic flux. It is assumed that on the boundary the fermionic field obeys the MIT bag boundary condition. For irregular modes, we consider a special case of boundary conditions at the cone apex, when the MIT bag boundary condition is imposed at a finite radius, which is then taken to zero. The fermionic condensate is a periodic function of the magnetic flux with the period equal to the flux quantum. For both exterior and interior regions, the fermionic condensate is decomposed into boundary-free and boundary-induced parts. Two integral representations are given for the boundary-free part for arbitrary values of the opening angle of the cone and magnetic flux. At distances from the boundary larger than the Compton wavelength of the fermion particle, the condensate decays exponentially, with the decay rate depending on the opening angle of the cone. If the ratio of the magnetic flux to the flux quantum is not a half-integer number for a massless field the boundary-free part in the fermionic condensate vanishes, whereas the boundary-induced part is negative. For half-integer values of the ratio of the magnetic flux to the flux quantum, the irregular mode gives a nonzero contribution to the fermionic condensate in the boundary-free conical space.

Bellucci, S. [INFN, Laboratori Nazionali di Frascati, Via Enrico Fermi 40, 00044 Frascati (Italy); Bezerra de Mello, E. R. [Departamento de Fisica, Universidade Federal da Paraiba 58.059-970, Caixa Postal 5.008, Joao Pessoa, PB (Brazil); Saharian, A. A. [Department of Physics, Yerevan State University, Alex Manoogian Street, 0025 Yerevan (Armenia)

2011-04-15

249

An Experimental Determination of Static Magnetic Fields Induced Noise in Living Systems  

NASA Astrophysics Data System (ADS)

Living systems are constantly exposed to static magnetic fields (SMFs) from both natural and man-made sources. Exposures vary in dose and duration ranging from geomagnetic (˜50?T) to residential and industrial (˜10s of mT) fields. Efforts to characterize responses to SMFs have yielded conflicting results, showing a dependence on experimental variables used. Here we argue that low to moderate SMF exposure is a sub-threshold perturbation operating below thermal noise, and assays that evaluate statistical characteristics of a single cell may identify responses not consistently found by population averaging approaches. Recent studies of gene expression show that it is a stochastic process capable of producing bursting dynamics. Moreover, theoretical and experimental methods have also been developed to allow quantitative estimates of the associated biophysical parameters. These developments provide a new way to assess responses of living systems to SMFs. In this work, we report on our efforts to use single molecule fluorescence in situ hybridization to assess responses of NIH-3T3 cells to SMF exposure at flux densities ranging from 1 to 440 mT for 48 hours. Results will contribute to determining mechanisms by which SMF exposure influences gene expression.

Brady, Megan; Laramee, Craig

2013-03-01

250

CHARACTERIZATION OF ACOUSTIC NOISE AND MAGNETIC FIELD FLUCTUATIONS IN A 4 T WHOLE-BODY MRI SCANNER  

Microsoft Academic Search

High-field, high-speed magnetic resonance imaging (MRI) can generate high levels of sound within and around the scanner. The process that produces the gradient magnetic field is the primary cause of this noise. With the push to greater background magnetic field strength and gradient field switching speed, in order to improve image quality and resolution, the noise situation is becoming worse.

CHRIS K. MECHEFSKE; Yuhua Wu; Brian Rutt

2002-01-01

251

Magnetic flux conversion and relaxation toward a minimum-energy state in S-1 spheromak plasmas  

SciTech Connect

S-1 Spheromak currents and magnetic fluxes have been measured with Rogowski coils and flux loops external to the plasma. Toroidal plasma currents up to 350 kA and spheromak configuration lifetimes over 1.0 msec have been achieved at moderate power levels. The plasma formation in the S-1 Spheromak device is based on an inductive transfer of poloidal and toroidal magnetic flux from a toroidal ''flux core'' to the plasma. Formation is programmed to guide the configuration into a force-free, minimum-energy Taylor state. Properly detailed programming of the formation process is found not to be essential since plasmas adjust themselves during formation to a final equilibrium near the Taylor state. After formation, if the plasma evolves away from the stable state, then distinct relaxation oscillation events occur which restore the configuration to that stable state. The relaxation process involves reconnection of magnetic field lines, and conversion of poloidal to toroidal magnetic flux (and vice versa) has been observed and documented. The scaling of toroidal plasma current and toroidal magnetic flux in the plasma with externally applied currents is consistent with the establishment of a Taylor state after formation. In addition, the magnetic helicity is proportional to that injected from the flux core, independent of how that helicity is generated.

Janos, A.

1985-09-01

252

LARGE-SCALE MAGNETIC HELICITY FLUXES ESTIMATED FROM MDI MAGNETIC SYNOPTIC CHARTS OVER THE SOLAR CYCLE 23  

SciTech Connect

To investigate the characteristics of large-scale and long-term evolution of magnetic helicity with solar cycles, we use the method of Local Correlation Tracking to estimate the magnetic helicity evolution over solar cycle 23 from 1996 to 2009 using 795 MDI magnetic synoptic charts. The main results are as follows: the hemispheric helicity rule still holds in general, i.e., the large-scale negative (positive) magnetic helicity dominates the northern (southern) hemisphere. However, the large-scale magnetic helicity fluxes show the same sign in both hemispheres around 2001 and 2005. The global, large-scale magnetic helicity flux over the solar disk changes from a negative value at the beginning of solar cycle 23 to a positive value at the end of the cycle, while the net accumulated magnetic helicity is negative in the period between 1996 and 2009.

Yang Shangbin; Zhang Hongqi, E-mail: yangshb@nao.cas.cn [Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing (China)

2012-10-10

253

Plasma signatures in large Martian magnetic flux ropes: MARSIS/ASPERA-3 observations  

NASA Astrophysics Data System (ADS)

Cylindrical structures of highly twisted magnetic field (flux ropes) have been observed at Mars, using measurements by the MAG-ER magnetometer-electron reflectometer onboard Mars Global Surveyor (MGS) and by the MARSIS radar sounder onboard Mars Express (MEX). Signatures of flux ropes are spikes of magnetic field strength and magnetic field rotations. Both small scale flux ropes (diameters of a few tens of km) and large scale flux ropes (diameters of around 100 km) have been found at Mars. We look at times of presumed flux ropes on the dayside of Mars, detected in the local magnetic field strength given by MARSIS. The signatures in MARSIS are magnetic field strength increases (peak strength reaches several tens to hundred nT) for several minutes (size of hundreds of km along the spacecraft track), found outside but near crustal magnetic field regions. Although we cannot determine the presence of a magnetic field rotation because of the lack of a magnetometer onboard MEX, we assume that these magnetic field increases are large flux ropes. There are indeed large flux ropes with similar characteristics which were established by the magnetometer data from MGS, and thought to form by stretching and reconnection of crustal magnetic field by the solar wind. On the other hand, MEX possesses in situ ion measurements, unlike MGS. We will use the ion and electron data from the ASPERA-3 particle instrument onboard MEX in order to characterize the plasma (ionospheric only or mixing with shocked plasma?) inside the flux ropes, which will give hints on their origin.

Diéval, Catherine; Morgan, David; Duru, Firdevs; Gurnett, Donald

2014-05-01

254

Collapsing magnetic instability to solar intermittent flux and anomalous viscosity in accretion disks  

NASA Astrophysics Data System (ADS)

It is shown that the transverse plasmon field is modulationally unstable in the Lyapunov sense, leading to a self-similar collapse of the magnetic flux. Such a collapsing magnetic instability is analyzed in both cases of magneto-hydrodynamics and kinetic plasma physics, with their applications to solar intermittent flux and anomalous viscosity in accretion disks, respectively. In the first case, we find that the equilibrium between the ponderomotive force and the Lorentz force in a current sheet gives rise to a more spatially intermittent collapsing magnetic flux, very similar to a turbulent pattern; as a result, the 0.1 kG flux cells with larger scales emerging at the surface from the solar interior become shredded and the flux is thereby contracted rather quickly to a small scale of the order of 100 km as well as concentrated into a 1-2 kG state. In the second case, based on Vlasov equations and Maxwell equations, the collapsing feature of the self-generated magnetic field from transverse plasmons is investigated on rather small scales of the motion or electric current in accretion disks; as the effects of the intermittent magnetic flux, an anomalous magnetic viscosity and an anomalous resistivity are indicated, with a different magnetic Prandtl number, which is not very sensitive to the temperature T.

Li, X. Q.; Zhang, H.

2002-08-01

255

Asymmetric planar gradiometer for rejection of uniform ambient magnetic noise  

DOEpatents

An asymmetric planar gradiometer for use in making biomagnetic measurements. The gradiometer is formed from a magnetometer which is inductively-coupled to the smaller of two connected loops patterned in a superconducting film which form a flux transformer. The magnetometer is based on a SQUID formed from a high T.sub.c superconducting material. The flux transformer and magnetometer may be formed on separate substrates, allowing the baseline to be increased relative to presently available devices.

Dantsker, Eugene (Torrance, CA); Clarke, John (Berkeley, CA)

2000-01-01

256

Surface flux density distribution characteristics of bulk high- Tc superconductor in external magnetic field  

NASA Astrophysics Data System (ADS)

Various magnetic levitation systems using oxide superconductors are developed as strong pinning forces are obtained in melt-processed bulk. However, the trapped flux of superconductor is moved by flux creep and fluctuating magnetic field. Therefore, to examine the internal condition of superconductor, the authors measure the dynamic surface flux density distribution of YBCO bulk. Flux density measurement system has a structure with the air-core coil and the Hall sensors. Ten Hall sensors are arranged in series. The YBCO bulk, which has 25 mm diameter and 13 mm thickness, is field cooled by liquid nitrogen. After that, magnetic field is changed by the air-core coil. This paper describes about the measured results of flux density distribution of YBCO bulk in the various frequencies of air-core coils currents.

Torii, S.; Yuasa, K.

2004-10-01

257

Noise in the processing and application of magnetic gradients Leon Foks, Kristofer Davis, and Yaoguo Li, Center for Gravity, Electrical, and Magnetics, Colorado School of Mines,  

E-print Network

, and Yaoguo Li, Center for Gravity, Electrical, and Magnetics, Colorado School of Mines, Golden, ColoradoNoise in the processing and application of magnetic gradients Leon Foks, Kristofer Davis SUMMARY The increased use of magnetic gradients brings about the need for reliable noise characterization

258

BaBar technical design report: Chapter 9, Magnet coil and flux return  

SciTech Connect

The BaBar magnet is a thin, 1.5 T superconducting solenoid with a hexagonal flux return. This chapter discusses the physics requirements and performance goals for the magnet, describes key interfaces, and summarizes the projected magnet performance. It also presents the design of the superconducting solenoid, including magnetic design, cold mass design, quench protection and stability, cold mass cooling, cryostat design, and coil assembly and transportation. The cryogenic supply system and instrumentation are described briefly, and the flux return is described.

O`Connor, T.; The BaBar Collaboration

1995-03-01

259

Avalanche dynamics of magnetic flux in a two-dimensional discrete superconductor  

SciTech Connect

The critical state of a two-dimensional discrete superconductor in an external magnetic field is studied. This state is found to be self-organized in the generalized sense, i.e., is a set of metastable states that transform to each other by means of avalanches. An avalanche is characterized by the penetration of a magnetic flux to the system. The sizes of the occurring avalanches, i.e., changes in the magnetic flux, exhibit the power-law distribution. It is also shown that the size of the avalanche occurring in the critical state and the external magnetic field causing its change are statistically independent quantities.

Ginzburg, S. L.; Nakin, A. V.; Savitskaya, N. E. [Russian Academy of Sciences, Petersburg Nuclear Physics Institute (Russian Federation)], E-mail: savitska@thd.pnpi.spb.ru

2006-11-15

260

Morphology and magnetic flux distribution in superparamagnetic, single-crystalline Fe3O4 nanoparticle rings  

NASA Astrophysics Data System (ADS)

This study reports on the correlation between crystal orientation and magnetic flux distribution of Fe3O4 nanoparticles in the form of self-assembled rings. High-resolution transmission electron microscopy demonstrated that the nanoparticles were single-crystalline, highly monodispersed, (25 nm average diameter), and showed no appreciable lattice imperfections such as twins or stacking faults. Electron holography studies of these superparamagnetic nanoparticle rings indicated significant fluctuations in the magnetic flux lines, consistent with variations in the magnetocrystalline anisotropy of the nanoparticles. The observations provide useful information for a deeper understanding of the micromagnetics of ultrasmall nanoparticles, where the magnetic dipolar interaction competes with the magnetic anisotropy.

Takeno, Yumu; Murakami, Yasukazu; Sato, Takeshi; Tanigaki, Toshiaki; Park, Hyun Soon; Shindo, Daisuke; Ferguson, R. Matthew; Krishnan, Kannan M.

2014-11-01

261

Effect of an External Magnetic Flux on Antitumor Antibiotic Neocarzinostatin Yield by Streptomyces carzinostaticus var. F-41  

NASA Astrophysics Data System (ADS)

The yield of the antitumor antibiotic neocarzinostatin (NCS), produced by Streptomyces carzinostaticus var. F-41, was sensitive to an external magnetic flux. When this strain was cultivated at 28°C in a NCS-producing medium under various magnetic flux densities, good NCS yield was observed at below 250 G magnetic flux density during the exponential growth phase as compared with that obtained in the same medium without magnetic flux, but was not observed at more than 500 G. However, no definite effect on the physiological characteristics and carbohydrate utilization of this strain, and primary physicochemical properties of NCS from magnetic flux could be detected.

Kudo, Kozo; Yoshida, Yuko; Yoshimura, Noboru; Ishida, Nakao

1993-11-01

262

Development of an isolated flyback converter employing boundary-mode operation and magnetic flux sensing feedback  

E-print Network

This thesis focuses on the marriage of magnetic-flux-sensing feedback and boundary-mode operation in a flyback converter to create a simple, small, low-cost, isolated, and tightly regulated power supply. Although each ...

Kenia, Mayur V. (Mayur Vasant), 1981-

2004-01-01

263

Experimental Study of Interference Factors and Simulation on Oil-Gas Pipeline Magnetic Flux Leakage Density Signal  

Microsoft Academic Search

Some methods of enhancing oil-gas pipeline magnetic flux leakage (MFL) detection technique are introduced in the paper. Some man-made defects or imperfections on the pipe surface are detected via the axial magnetization inspection vehicle along the pipeline. The magnetic dipole model of corrosion defect is stated and the important interference factors on magnetic flux leakage are analyzed. Finite element method

Jiang Qi

2007-01-01

264

Fast modeling of flux trapping cascaded explosively driven magnetic flux compression generators  

NASA Astrophysics Data System (ADS)

To predict the performance of flux trapping cascaded flux compression generators, a calculation model based on an equivalent circuit is investigated. The system circuit is analyzed according to its operation characteristics in different steps. Flux conservation coefficients are added to the driving terms of circuit differential equations to account for intrinsic flux losses. To calculate the currents in the circuit by solving the circuit equations, a simple zero-dimensional model is used to calculate the time-varying inductance and dc resistance of the generator. Then a fast computer code is programmed based on this calculation model. As an example, a two-staged flux trapping generator is simulated by using this computer code. Good agreements are achieved by comparing the simulation results with the measurements. Furthermore, it is obvious that this fast calculation model can be easily applied to predict performances of other flux trapping cascaded flux compression generators with complex structures such as conical stator or conical armature sections and so on for design purpose.

Wang, Yuwei; Zhang, Jiande; Chen, Dongqun; Cao, Shengguang; Li, Da; Liu, Chebo

2013-01-01

265

Computer model simulation of null-flux magnetic suspension and guidance  

SciTech Connect

This paper discusses the magnetic force computations in a null-flux suspension system using dynamic circuit theory. A computer simulation model that can be used to compute magnetic forces and predict the system performance is developed on the basis of dynamic circuit theory. Numerical examples are presented to demonstrate the application of the model. The performance of the null-flux suspension system is simulated and discussed. 8 refs.

He, Jianliang; Rote, D.M.

1992-06-01

266

Computer model simulation of null-flux magnetic suspension and guidance  

SciTech Connect

This paper discusses the magnetic force computations in a null-flux suspension system using dynamic circuit theory. A computer simulation model that can be used to compute magnetic forces and predict the system performance is developed on the basis of dynamic circuit theory. Numerical examples are presented to demonstrate the application of the model. The performance of the null-flux suspension system is simulated and discussed. 8 refs.

He, Jianliang; Rote, D.M.

1992-01-01

267

Superhigh-vacuum linear asynchronous motor with rolling rotor and magnetic-flux concentrators  

Microsoft Academic Search

The analytical method permitting to calculate the super-vacuum linear induction motor with rolling rotor and magnetic flux\\u000a concentrators has been presented. It is based on analogue approximation of orthotropic mediums. The electromagnetic torque\\u000a is calculated according to differential performances of electromagnetic field obtained by considering the configuration of\\u000a magnetic flux concentrators.

V. I. Polevskii; A. V. Sapsalev

2009-01-01

268

A moving hum filter to suppress rotor noise in high-resolution airborne magnetic data  

USGS Publications Warehouse

A unique filtering approach is developed to eliminate helicopter rotor noise. It is designed to suppress harmonic noise from a rotor that varies slightly in amplitude, phase, and frequency and that contaminates aero-magnetic data. The filter provides a powerful harmonic noise-suppression tool for data acquired with modern large-dynamic-range recording systems. This three-step approach - polynomial fitting, bandpass filtering, and rotor-noise synthesis - significantly reduces rotor noise without altering the spectra of signals of interest. Two steps before hum filtering - polynomial fitting and bandpass filtering - are critical to accurately model the weak rotor noise. During rotor-noise synthesis, amplitude, phase, and frequency are determined. Data are processed segment by segment so that there is no limit on the length of data. The segment length changes dynamically along a line based on modeling results. Modeling the rotor noise is stable and efficient. Real-world data examples demonstrate that this method can suppress rotor noise by more than 95% when implemented in an aeromagnetic data-processing flow. ?? 2005 Society of Exploration Geophysicists. All rights reserved.

Xia, J.; Doll, W.E.; Miller, R.D.; Gamey, T.J.; Emond, A.M.

2005-01-01

269

Defect profile estimation from magnetic flux leakage signal via efficient managing particle swarm optimization.  

PubMed

In this paper, efficient managing particle swarm optimization (EMPSO) for high dimension problem is proposed to estimate defect profile from magnetic flux leakage (MFL) signal. In the proposed EMPSO, in order to strengthen exchange of information among particles, particle pair model was built. For more efficient searching when facing different landscapes of problems, velocity updating scheme including three velocity updating models was also proposed. In addition, for more chances to search optimum solution out, automatic particle selection for re-initialization was implemented. The optimization results of six benchmark functions show EMPSO performs well when optimizing 100-D problems. The defect simulation results demonstrate that the inversing technique based on EMPSO outperforms the one based on self-learning particle swarm optimizer (SLPSO), and the estimated profiles are still close to the desired profiles with the presence of low noise in MFL signal. The results estimated from real MFL signal by EMPSO-based inversing technique also indicate that the algorithm is capable of providing an accurate solution of the defect profile with real signal. Both the simulation results and experiment results show the computing time of the EMPSO-based inversing technique is reduced by 20%-30% than that of the SLPSO-based inversing technique. PMID:24926693

Han, Wenhua; Xu, Jun; Wang, Ping; Tian, Guiyun

2014-01-01

270

Defect Profile Estimation from Magnetic Flux Leakage Signal via Efficient Managing Particle Swarm Optimization  

PubMed Central

In this paper, efficient managing particle swarm optimization (EMPSO) for high dimension problem is proposed to estimate defect profile from magnetic flux leakage (MFL) signal. In the proposed EMPSO, in order to strengthen exchange of information among particles, particle pair model was built. For more efficient searching when facing different landscapes of problems, velocity updating scheme including three velocity updating models was also proposed. In addition, for more chances to search optimum solution out, automatic particle selection for re-initialization was implemented. The optimization results of six benchmark functions show EMPSO performs well when optimizing 100-D problems. The defect simulation results demonstrate that the inversing technique based on EMPSO outperforms the one based on self-learning particle swarm optimizer (SLPSO), and the estimated profiles are still close to the desired profiles with the presence of low noise in MFL signal. The results estimated from real MFL signal by EMPSO-based inversing technique also indicate that the algorithm is capable of providing an accurate solution of the defect profile with real signal. Both the simulation results and experiment results show the computing time of the EMPSO-based inversing technique is reduced by 20%–30% than that of the SLPSO-based inversing technique. PMID:24926693

Han, Wenhua; Xu, Jun; Wang, Ping; Tian, Guiyun

2014-01-01

271

THE DYNAMICS OF MAGNETIC FLUX TUBES IN THE SOLAR CONVECTION ZONE  

E-print Network

National Solar Observatory 950 North Cherry Avenue Tucson, AZ 85719 AND D. W. LONGCOPE AND M. G. LINTONTHE DYNAMICS OF MAGNETIC FLUX TUBES IN THE SOLAR CONVECTION ZONE A Study of Active Region Formation of our recent work on the dynamics of flux tubes in the solar convection zone. We focus on two topics

Fisher, George H.

272

Reconnection of Twisted Magnetic Flux Tubes as a Solar Flare Mechanism  

Microsoft Academic Search

The collision and reconnection of magnetic flux tubes in the solar corona is often proposed as a mechanism for solar flares. We study this process by simulating the collision of pairs of twisted flux tubes with a massively parallel collocation viscoresistive MHD code using up to 256 x 256 x 256 Fourier modes. Our aim is to investigate the energy

M. G. Linton; R. B. Dahlburg; S. K. Antiochos

2001-01-01

273

Suppression of the kink instability for magnetic flux ropes in the chromosphere  

Microsoft Academic Search

Energy storage in chromospheric flux ropes is discussed, in the context of solar flares. The structure is represented by a cylindrically symmetric magnetic field of finite length. The field is assumed to be approximately force-free. The stability of the field to a kink perturbation is investigated. Flux ropes are rooted in dense photospheric material. So the ends of the field

M. A. Raadu

1972-01-01

274

FOC and flux weakening for traction drive with permanent magnet synchronous motor  

Microsoft Academic Search

This paper analyses control algorithm for permanent magnet synchronous traction motor (traction PMSM) usable for light rail vehicle. The algorithm is based on field oriented control. The field oriented control and carrier based pulse width modulation allow simply implementation of flux weakening control algorithm. The flux weakening provides wider speed operation of traction drive with PMSM. Simulations and experimental results

Jiri Simanek; Jaroslav Novak; Ondrej Cerny; Radovan Dolecek

2008-01-01

275

Comparison of Magnetic Flux Distribution between a Coronal Hole and a Quiet Region  

E-print Network

Employing Big Bear Solar Observatory (BBSO) deep magnetograms and H${\\alpha}$ images in a quiet region and a coronal hole, observed on September 14 and 16, 2004, respectively, we have explored the magnetic flux emergence, disappearance and distribution in the two regions. The following results are obtained: (1) The evolution of magnetic flux in the quiet region is much faster than that in the coronal hole, as the flux appeared in the form of ephemeral regions in the quiet region is 4.3 times as large as that in the coronal hole, and the flux disappeared in the form of flux cancellation, 2.9 times as fast as in the coronal hole. (2) More magnetic elements with opposite polarities in the quiet region are connected by arch filaments, estimating from magnetograms and H${\\alpha}$ images. (3) We measured the magnetic flux of about 1000 magnetic elements in each observing region. The flux distribution of network and intranetwork (IN) elements is similar in both polarities in the quiet region. For network fields in t...

Zhang, Jun; Wang, Haimin

2006-01-01

276

Are heliospheric flows magnetic line- or flux-conserving?  

Microsoft Academic Search

This article discusses and tests the validity of the frozen in magnetic field paradigm (or 'ideal magnetohydrodynamics (MHD) constraint') which is usually adopted by many authors dealing with heliospheric physics. To show the problem of using ideal MHD in such a counterflow configuration like the heliosphere, we first recapitulate the basic concepts of freezing-in of magnetic fields, respectively magnetic topology

D. H. Nickeler; M. Karlický

2006-01-01

277

The application of wavelet shrinkage denoising to magnetic Barkhausen noise measurements  

SciTech Connect

The application of Magnetic Barkhausen Noise (MBN) as a non-destructive method of defect detection has proliferated throughout the manufacturing community. Instrument technology and measurement methodology have matured commensurately as applications have moved from the R and D labs to the fully automated manufacturing environment. These new applications present a new set of challenges including a bevy of error sources. A significant obstacle in many industrial applications is a decrease in signal to noise ratio due to (i) environmental EMI and (II) compromises in sensor design for the purposes of automation. The stochastic nature of MBN presents a challenge to any method of noise reduction. An application of wavelet shrinkage denoising is proposed as a method of decreasing extraneous noise in MBN measurements. The method is tested and yields marked improvement on measurements subject to EMI, grounding noise, and even measurements in ideal conditions.

Thomas, James [American Stress Technologies, Inc., 540 Alpha Drive, Pittsburgh, PA 15238 (United States)

2014-02-18

278

Direct control of air gap flux in permanent magnet machines  

DOEpatents

A method and apparatus for field weakening in PM machines uses field weakening coils (35, 44, 45, 71, 72) to produce flux in one or more stators (34, 49, 63, 64), including a flux which counters flux normally produced in air gaps between the stator(s) (34, 49, 63, 64) and the rotor (20, 21, 41, 61) which carries the PM poles. Several modes of operation are introduced depending on the magnitude and polarity of current in the field weakening coils (35, 44, 45, 71, 72). The invention is particularly useful for, but not limited to, the electric vehicle drives and PM generators.

Hsu, John S. (Oak Ridge, TN)

2000-01-01

279

Observation of an evolving magnetic flux rope before and during a solar eruption.  

PubMed

Explosive energy release is a common phenomenon occurring in magnetized plasma systems ranging from laboratories, Earth's magnetosphere, the solar corona and astrophysical environments. Its physical explanation is usually attributed to magnetic reconnection in a thin current sheet. Here we report the important role of magnetic flux rope structure, a volumetric current channel, in producing explosive events. The flux rope is observed as a hot channel before and during a solar eruption from the Atmospheric Imaging Assembly telescope on board the Solar Dynamic Observatory. It initially appears as a twisted and writhed sigmoidal structure with a temperature as high as 10 MK, and then transforms toward a semi-circular shape during a slow-rise phase, which is followed by fast acceleration and onset of a flare. The observations suggest that the instability of the magnetic flux rope triggers the eruption, thus making a major addition to the traditional magnetic-reconnection paradigm. PMID:22434190

Zhang, Jie; Cheng, Xin; Ding, Ming-de

2012-01-01

280

On the role of crystal and stress anisotropy in magnetic Barkhausen noise  

NASA Astrophysics Data System (ADS)

The article describes the micromagnetic behavior of non- and pre-plastically deformed high strength steel samples under applied stress using different magnetic nondestructive methods such as magnetic Barkhausen noise analysis and hysteresis measurements. It was found that the maximum amplitude of Barkhausen noise (MMAX) increases with applied stress up to a certain point and then decreases again (so-called MMAX(?)-curve). Changes of magnetostriction, hysteresis curves and magnetic domain structures have been measured and have been further investigated to find out the reasons with respect to macro- and microscopic material behavior. The results obtained are mainly discussed on the basis of the Villari effect and the relation between applied stress and the Barkhausen noise parameters is described. It is concluded that the interaction between crystal and stress anisotropy is the main reason of the specific MMAX(?)-curve observed.

Sheikh Amiri, Meisam; Thielen, Matthias; Rabung, Madalina; Marx, Michael; Szielasko, Klaus; Boller, Christian

2014-12-01

281

Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind  

NASA Astrophysics Data System (ADS)

Context. Tangential velocity discontinuity near the boundaries of solar wind magnetic flux tubes results in Kelvin-Helmholtz instability, which might contribute to solar wind turbulence. While the axial magnetic field stabilizes the instability, a small twist in the magnetic field may allow sub-Alfvénic motions to be unstable. Aims: We aim to study the Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind with different configurations of the external magnetic field. Methods: We use magnetohydrodynamic equations in cylindrical geometry and derive the dispersion equations governing the dynamics of twisted magnetic flux tubes moving along its axis in the cases of untwisted and twisted external fields. Then, we solve the dispersion equations analytically and numerically and find thresholds for Kelvin-Helmholtz instability in both cases of the external field. Results: Both analytical and numerical solutions show that the Kelvin-Helmholtz instability is suppressed in the twisted tube by the external axial magnetic field for sub-Alfvénic motions. However, even a small twist in the external magnetic field allows the Kelvin-Helmholtz instability to be developed for any sub-Alfvénic motion. The unstable harmonics correspond to vortices with high azimuthal mode numbers that are carried by the flow. Conclusions: Twisted magnetic flux tubes can be unstable to Kelvin-Helmholtz instability when they move with small speed relative to the main solar wind stream, then the Kelvin-Helmholtz vortices may significantly contribute to the solar wind turbulence.

Zaqarashvili, T. V.; Vörös, Z.; Zhelyazkov, I.

2014-01-01

282

Magnetic and noise properties of Nb added Co-Cr perpendicular medium  

NASA Astrophysics Data System (ADS)

Nb addition to Co-Cr perpendicular magnetic recording films has been investigated. Co 74-Cr 22-Nb 4 films exhibited high Hc? values surpassing a Co-Cr binary system and satisfied a guiding scheme for a low-noise medium. It was found that the domain size of the film as well as SQ ? was an important factor determining medium noise and that the domain size should be smaller to realize a lower medium noise and a higher resolution. MFM observation revealed that high-density recording up to 400 kFRPI and 17 Gbit/in 2 could be achieved for the Co 74-Cr 22-Nb 4 system.

Ariake, Jun; Kiya, Takanori; Honda, Naoki; Ouchi, Kazuhiro

1999-03-01

283

22 year cycle in the imbalance of the photospheric magnetic fluxes  

NASA Astrophysics Data System (ADS)

The manifestation of the 22 year solar magnetic cycle in the imbalance of positive and negative photospheric magnetic fluxes is studied. For the analysis we use synoptic maps of the photospheric magnetic field of Kitt Peak Observatory (1976 - 2003) and John Wilcox Observatory in Stanford (1976 - 2012). We consider strong magnetic fields for the heliolatitudes in the interval from +40° to -40°. It is shown that the sign of the imbalance between positive and negative fluxes remains constant during 11 years from one inversion of the Sun’s global magnetic field to the next one and always coincides with the sign of the polar field in the Northern hemisphere. Thus, the imbalance between the magnetic fluxes of different polarities changes according to the 22 year cycle. The sign of the imbalance is determined both by the phase of the solar cycle (before or after the inversion) and by the parity of the solar cycle. The imbalance of positive and negative magnetic fluxes can be observed not only for the strong fields in the sunspot zone. The mean magnetic field of the Sun (Sun as a star), which is determined by the net flux of the background fields, changes according to the same pattern as the imbalance of the strong fields. The regular changes of the imbalance of the photospheric magnetic fields are reflected also in the parameters of heliosphere. We show the connection of the imbalance with the quadrupole component of the photospheric magnetic field and with the imbalance of the interplanetary magnetic field (the difference between the numbers of the days with positive and negative polarities of the interplanetary magnetic field near Earth).

Vernova, Elena; Baranov, Dmitrii; Tyasto, Marta

284

Magnetic field enhancement of generation-recombination and shot noise in organic light emitting diodes  

NASA Astrophysics Data System (ADS)

We have studied the effect of magnetic field on noise in series of 2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene-based organic light emitting diodes with dominant hole injection, dominant electron injection, and balanced electron and hole injection. The noise spectra of the balanced devices revealed the generation-recombination (g-r) noise term, which we associated with bimolecular electron-hole recombination. The presence of the g-r noise term is correlated with the strong organic magnetoresistance (up to 25%) observed in the balanced devices. The noise spectra also have the shot noise contribution with the Fano factor 0.25-0.4. We found that time constant of the g-r term decreases and the magnitude of shot noise increases when magnetic field is applied. This behavior can be consistently explained within the polaron-polaron model of organic magnetoresistance. We have not found any evidence that the magnetoresistance in studied devices is affected by traps.

Djidjou, T. K.; Chen, Ying; Basel, Tek; Shinar, J.; Rogachev, A.

2015-03-01

285

Dependence of the Perpendicular Residual Leakage Magnetic Flux Density on Fatigue Damage in an Austenitic Stainless Steel  

NASA Astrophysics Data System (ADS)

In order to estimate the amount of plane bending fatigue damage in an austenitic stainless steel (SUS304), we were investigating the relationship between plane bending fatigue damage and the perpendicular residual leakage magnetic flux density caused by martensitic structure induced by plane bending fatigue. A specimen such as SUS304 had been excited in a constant external magnetic field perpendicularly to measure dependence of the perpendicular residual leakage magnetic flux density on plane bending fatigue damage accurately. The Z component of the magnetic flux density at 1 mm above a specimen is measured by using a thin-film flux-gate (FG) magnetic sensor. Residual magnetization is caused by partial martensitic structure in an austenitic stainless steel induced by cyclic bending stress. From our experiments, we can evaluate dependence of the perpendicular residual leakage magnetic flux density on plane bending fatigue damage and know the relationship between growth of a crack and the perpendicular residual leakage magnetic flux density.

Oka, M.; Yakushiji, T.; Tsuchida, Y.; Enokizono, M.

2003-03-01

286

Negative differential conductance and super-Poissonian shot noise in single-molecule magnet junctions  

NASA Astrophysics Data System (ADS)

Molecular spintroinic device based on a single-molecule magnet is one of the ultimate goals of semiconductor nanofabrication technologies. It is thus necessary to understand the electron transport properties of a single-molecule magnet junction. Here we study the negative differential conductance and super-Poissonian shot noise properties of electron transport through a single-molecule magnet weakly coupled to two electrodes with either one or both of them being ferromagnetic. We predict that the negative differential conductance and super-Poissonian shot noise, which can be tuned by a gate voltage, depend sensitively on the spin polarization of the source and drain electrodes. In particular, the shot noise in the negative differential conductance region can be enhanced or decreased originating from the different formation mechanisms of negative differential conductance. The effective competition between fast and slow transport channels is responsible for the observed negative differential conductance and super-Poissonian shot noise. In addition, we further discuss the skewness and kurtosis properties of transport current in the super-Poissonian shot noise regions. Our findings suggest a tunable negative differential conductance molecular device, and the predicted properties of high-order current cumulants are very interesting for a better understanding of electron transport through single-molecule magnet junctions.

Xue, Hai-Bin; Liang, Jiu-Qing; Liu, Wu-Ming

2015-03-01

287

Reducing media noise of perpendicular magnetic recording tape for over-50 TB class data cartridge  

NASA Astrophysics Data System (ADS)

To reduce medium noise inherent in a perpendicular magnetic recording tape, which is deposited by facing targets sputtering, we have examined an epitaxial double-layered structure consisting of a soft magnetic underlayer (SUL), bcc-FeCoB/fcc-NiFe/Si/bcc-FeCoB, on a 4.5 ?m para-aromatic polyamide (aramid) film. An epitaxial soft magnetic bilayer reduces broadband noise by 5.6 dB at 337 kilo fluxchanges per inch (kfci) compared to a single SUL medium (Tape S). This is due to the reduction in the low-frequency noise, which originates from the domain walls of the SUL. Improvement of the crystal orientation of the magnetic layer hcp-CoPtCr-SiO2 and the intermediate layer hcp-Ru by inserting a fcc-NiFe spacer in laminated SULs extends the roll-off curve toward high linear density. When each SUL layer of the bilayer was made thinner, from 25 nm (Tape D2) to 10 nm (Tape D1), the medium noise decreased, improving the signal-to-noise ratio by 8.6 dB at 337 kfci compared to Tape S. In a high resolution playback test of Tape D1, an areal density of 45.0 gigabits per square inch (Gb/in.2) was confirmed. Achieved areal density means the capability of an over-50 terabyte (TB) capacity for a typical linear-formatted data cartridge.

Matsunuma, S.; Inoue, T.; Watanabe, T.; Doi, T.; Gomi, S.; Mashiko, Y.; Hirata, K.; Nakagawa, S.

2011-04-01

288

Negative differential conductance and super-Poissonian shot noise in single-molecule magnet junctions  

PubMed Central

Molecular spintroinic device based on a single-molecule magnet is one of the ultimate goals of semiconductor nanofabrication technologies. It is thus necessary to understand the electron transport properties of a single-molecule magnet junction. Here we study the negative differential conductance and super-Poissonian shot noise properties of electron transport through a single-molecule magnet weakly coupled to two electrodes with either one or both of them being ferromagnetic. We predict that the negative differential conductance and super-Poissonian shot noise, which can be tuned by a gate voltage, depend sensitively on the spin polarization of the source and drain electrodes. In particular, the shot noise in the negative differential conductance region can be enhanced or decreased originating from the different formation mechanisms of negative differential conductance. The effective competition between fast and slow transport channels is responsible for the observed negative differential conductance and super-Poissonian shot noise. In addition, we further discuss the skewness and kurtosis properties of transport current in the super-Poissonian shot noise regions. Our findings suggest a tunable negative differential conductance molecular device, and the predicted properties of high-order current cumulants are very interesting for a better understanding of electron transport through single-molecule magnet junctions. PMID:25736094

Xue, Hai-Bin; Liang, Jiu-Qing; Liu, Wu-Ming

2015-01-01

289

Electron heat flux dropouts in the solar wind - Evidence for interplanetary magnetic field reconnection?  

NASA Technical Reports Server (NTRS)

An examination of ISEE-3 data from 1978 reveal 25 electron heat flux dropout events ranging in duration from 20 min to over 11 hours. The heat flux dropouts are found to occur in association with high plasma densities, low plasma velocities, low ion and electron temperatures, and low magnetic field magnitudes. It is suggested that the heat flux dropout intervals may indicate that the spacecraft is sampling plasma regimes which are magnetically disconnected from the sun and instead are connected to the outer heliosphere at both ends.

Mccomas, D. J.; Gosling, J. T.; Phillips, J. L.; Bame, S. J.; Luhmann, J. G.; Smith, E. J.

1989-01-01

290

Radiative Heating and the Buoyant Rise of Magnetic Flux Tubes in the Solar Interior  

E-print Network

Radiative Heating and the Buoyant Rise of Magnetic Flux Tubes in the Solar Interior Y. Fan National tube. In the overshoot region, the radiative heating is found to cause a quasi-static rising the e ect of radiative heating on the evolution of thin magnetic ux tubes in the solar interior

California at Berkeley, University of

291

Stable magnetic levitation of soft ferromagnetic materials by flux pinning of bulk superconductors  

Microsoft Academic Search

We have investigated electromagnetic force characteristics of magnetic levitation systems consisting only of bulk superconductors and an iron rail, which enable a stable levitation even at a standstill without active control by a magnetic gradient in space. A two-dimensional finite element analysis was used to study the fundamental and general characteristics of the levitation of soft ferromagnetic materials by flux

H. Ohsaki; M. Takabatake; E. Masada

1997-01-01

292

4. The Solenoid Magnet and Flux Return 4.1. Overview  

E-print Network

-II Steel Parameters Overall Barrel Length 4050 mm Overall Door Thickness-II magnets. The magnet coil cryostat is mounted inside the hexagonal barrel flux return by four brackets. At the interface between the barrel and the end doors, approximately 60% of the area is occupied by structural

293

4. The Solenoid Magnet and Flux Return 4.1. Overview  

E-print Network

­II Steel Parameters Overall Barrel Length 4050 mm Overall Door Thickness­II magnets. The magnet coil cryostat is mounted inside the hexagonal barrel flux return by four brackets. At the interface between the barrel and the end doors, approximately 60% of the area is occupied by structural

294

Operation of a magnetic-flux switch in a current-limiting device  

Microsoft Academic Search

The requirements for magnetic-flux switches implemented with the aid of controlled superconducting shields and operating in transformer-type current-limiting devices are formulated on the basis of an examination of the equations for the electric and magnetic circuits of a multiwinding transformer. Results are reported for tests of a model current limiter and sample calculations are given.

Sh. I. Lutidze; V. G. Narovlyanskii; I. V. Yakimets

1980-01-01

295

MAGNETAR GIANT FLARES-FLUX ROPE ERUPTIONS IN MULTIPOLAR MAGNETOSPHERIC MAGNETIC FIELDS  

SciTech Connect

We address a primary question regarding the physical mechanism that triggers the energy release and initiates the onset of eruptions in the magnetar magnetosphere. Self-consistent stationary, axisymmetric models of the magnetosphere are constructed based on force-free magnetic field configurations that contain a helically twisted force-free flux rope. Depending on the surface magnetic field polarity, there exist two kinds of magnetic field configurations, inverse and normal. For these two kinds of configurations, variations of the flux rope equilibrium height in response to gradual surface physical processes, such as flux injections and crust motions, are carefully examined. We find that equilibrium curves contain two branches: one represents a stable equilibrium branch, and the other an unstable equilibrium branch. As a result, the evolution of the system shows a catastrophic behavior: when the magnetar surface magnetic field evolves slowly, the height of the flux rope would gradually reach a critical value beyond which stable equilibriums can no longer be maintained. Subsequently, the flux rope would lose equilibrium and the gradual quasi-static evolution of the magnetosphere will be replaced by a fast dynamical evolution. In addition to flux injections, the relative motion of active regions would give rise to the catastrophic behavior and lead to magnetic eruptions as well. We propose that a gradual process could lead to a sudden release of magnetosphere energy on a very short dynamical timescale, without being initiated by a sudden fracture in the crust of the magnetar. Some implications of our model are also discussed.

Yu Cong, E-mail: cyu@ynao.ac.cn [National Astronomical Observatories/Yunnan Astronomical Observatory, Chinese Academy of Sciences, Kunming, 650011 (China)

2012-09-20

296

A Magnetic Flux Leakage and Magnetostrictive Guided Wave Hybrid Transducer for Detecting Bridge Cables  

PubMed Central

Condition assessment of cables has gained considerable attention for the bridge safety. A magnetic flux leakage and magnetostrictive guided wave hybrid transducer is provided to inspect bridge cables. The similarities and differences between the two methods are investigated. The hybrid transducer for bridge cables consists of an aluminum framework, climbing modules, embedded magnetizers and a ribbon coil. The static axial magnetic field provided by the magnetizers meets the needs of the magnetic flux leakage testing and the magnetostrictive guided wave testing. The magnetizers also provide the attraction for the climbing modules. In the magnetic flux leakage testing for the free length of cable, the coil induces the axial leakage magnetic field. In the magnetostrictive guided wave testing for the anchorage zone, the coil provides a pulse high power variational magnetic field for generating guided waves; the coil induces the magnetic field variation for receiving guided waves. The experimental results show that the transducer with the corresponding inspection system could be applied to detect the broken wires in the free length and in the anchorage zone of bridge cables. PMID:22368483

Xu, Jiang; Wu, Xinjun; Cheng, Cheng; Ben, Anran

2012-01-01

297

Dynamical evolution of twisted magnetic flux tubes. I - Equilibrium and linear stability  

NASA Technical Reports Server (NTRS)

The three-dimensional dynamical evolution of twisted magnetic flux tubes is studied using a time-dependent magnetohydrodynamic (MHD) model. The flux tubes are intended to model solar coronal loops, and include the stabilizing effect of photospheric line tying. The model permits the complete evolution of flux tubes to be followed self-consistently, including the formation, equilibrium, linear instability, and nonlinear behavior. Starting from an initial uniform background magnetic field, a twisted flux tube is created by the application of slow, localized photospheric vortex flows. The flux tube evolves quasi-statically through sequences of equilibria with increasing twist, until it becomes linearly unstable to an ideal MHD kink mode. In this paper, the equilibrium properties and the linear stability behavior are discussed. The application of the method to the uniform-twist, Gold-Hoyle field confirms the previous stability threshold for kink instability and provides estimates of the resulting growth rate.

Mikic, Zoran; Schnack, Dalton D.; Van Hoven, Gerard

1990-01-01

298

Mean-field and direct numerical simulations of magnetic flux concentrations from vertical field  

NASA Astrophysics Data System (ADS)

Context. Strongly stratified hydromagnetic turbulence has previously been found to produce magnetic flux concentrations if the domain is large enough compared with the size of turbulent eddies. Mean-field simulations (MFS) using parameterizations of the Reynolds and Maxwell stresses show a large-scale negative effective magnetic pressure instability and have been able to reproduce many aspects of direct numerical simulations (DNS) regarding growth rate, shape of the resulting magnetic structures, and their height as a function of magnetic field strength. Unlike the case of an imposed horizontal field, for a vertical one, magnetic flux concentrations of equipartition strength with the turbulence can be reached, resulting in magnetic spots that are reminiscent of sunspots. Aims: We determine under what conditions magnetic flux concentrations with vertical field occur and what their internal structure is. Methods: We use a combination of MFS, DNS, and implicit large-eddy simulations (ILES) to characterize the resulting magnetic flux concentrations in forced isothermal turbulence with an imposed vertical magnetic field. Results: Using DNS, we confirm earlier results that in the kinematic stage of the large-scale instability the horizontal wavelength of structures is about 10 times the density scale height. At later times, even larger structures are being produced in a fashion similar to inverse spectral transfer in helically driven turbulence. Using ILES, we find that magnetic flux concentrations occur for Mach numbers between 0.1 and 0.7. They occur also for weaker stratification and larger turbulent eddies if the domain is wide enough. Using MFS, the size and aspect ratio of magnetic structures are determined as functions of two input parameters characterizing the parameterization of the effective magnetic pressure. DNS, ILES, and MFS show magnetic flux tubes with mean-field energies comparable to the turbulent kinetic energy. These tubes can reach a length of about eight density scale heights. Despite being ?1% equipartition strength, it is important that their lower part is included within the computational domain to achieve the full strength of the instability. Conclusions: The resulting vertical magnetic flux tubes are being confined by downflows along the tubes and corresponding inflow from the sides, which keep the field concentrated. Application to sunspots remains a viable possibility.

Brandenburg, A.; Gressel, O.; Jabbari, S.; Kleeorin, N.; Rogachevskii, I.

2014-02-01

299

Superconducting flux pump for high-temperature superconductor insert coils of NMR magnets  

NASA Astrophysics Data System (ADS)

This paper describes a prototype flux pump recently operated at the MIT Francis Bitter Magnet Laboratory. The results of the prototype flux pump will be used in the development of a full-scale flux pump that will be coupled to a high-temperature superconductor (HTS) insert coil of a high-field NMR magnet. Such an HTS insert is unlikely to operate in persistent mode because of the conductor's low index (n). The flux pump can compensate for field decay in the HTS insert coil and make the insert operate effectively in persistent mode. The flux pump, comprised essentially of a transformer and two switches, all made of superconductor, transfers into the insert coil a fraction of a magnetic energy that is first introduced in the secondary circuit of the transformer by a current supplied to the primary circuit. A prototype flux pump has been designed, fabricated, and operated to demonstrate that a flux pump can indeed supply a small metered current into a load superconducting magnet. A current increment in the range of microamperes has been measured in the magnet after each pumping action. The superconducting prototype flux pump is made of Nb3Sn tape. The pump is placed in a gaseous environment above the liquid helium level to keep its heat dissipation from directly discharged in the liquid; the effluent helium vapor maintains the thermal stability of the flux pump. [This paper is also published in Advances in Cryogenic Engineering Volume 47A, AIP Conference Proceedings Volume 613, pp. 441-448.

Jeong, S.; Lee, H.; Iwasa, Y.

2002-05-01

300

Solar Coronal Heating and the Magnetic Flux Content of the Network  

NASA Technical Reports Server (NTRS)

We investigate the heating of the quiet corona by measuring the increase of coronal luminosity with the amount of magnetic flux in the underlying network at solar minimum when there were no active regions on the face of the Sun. The coronal luminosity is measured from Fe IX/X-Fe XII pairs of coronal images from SOHO/EIT. The network magnetic flux content is measured from SOHO/MDI magnetograms. We find that the luminosity of the corona in our quiet regions increases roughly in proportion to the square root of the magnetic flux content of the network and roughly in proportion to the length of the perimeter of the network magnetic flux clumps. From (1) this result, (2) other observations of many fine-scale explosive events at the edges of network flux clumps, and (3) a demonstration that it is energetically feasible for the heating of the corona in quiet regions to be driven by explosions of granule-sized sheared-core magnetic bipoles embedded in the edges of network flux clumps, we infer that in quiet regions that are not influenced by active regions the corona is mainly heated by such magnetic activity in the edges of the network flux clumps. Our observational results together with our feasibility analysis allow us to predict that (1) at the edges of the network flux clumps there are many transient sheared-core bipoles of the size and lifetime of granules and having transverse field strengths greater than approximately - 100 G, (2) approximately 30 of these bipoles are present per supergranule, and (3) most spicules are produced by explosions of these bipoles.

Moore, R. L.; Falconer, D. A.; Porter, J. G.; Hathaway, D. H.

2003-01-01

301

Dynamics of local isolated magnetic flux tubes in a fast-rotating stellar atmosphere  

SciTech Connect

Dynamics of magnetic flux tubes in the fast rotating stellar atmosphere is studied. We focus on the effects and signatures of the instability of the flux tube emergence influenced by the Coriolis force. We present the result from a linear stability analysis and discuss its possible signatures in the course of the evolution of G-type and M-type stars. We present a three dimensional magnetohydrodynamical simulation of local isolated magnetic flux tubes under a magnetic buoyancy instability in co-rotating Cartesian coordinates. We find that the combination of the buoyancy instability and the Coriolis effect gives rise to a mechanism, to twist the emerging magnetic flux tube into a helical structure. The tilt angle, east-west asymmetry and magnetic helicity of the Twisted flux tubes in the simulations are studied in detail. The linear and nonlinear analyses provide hints as to what kind of pattern of large spots in young M-type main-sequence stars might be observed. We find that young and old G-type stars may have different distributions of spots while M-type stars may always have low latitudes spots. The size of stellar spots may decrease when a star becomes older, due to the decreasing of magnetic field. A qualitative comparison with solar observations is also presented.

Chou, W.; Tajima, C.T. [Univ. of Texas, Austin, TX (United States). Dept. of Physics; Matsumoto, R. [Chiba Univ. (Japan)]|[ASRC, JAERI, Naka (Japan); Shibata, K. [National Astronomical Observatory, Mitaka (Japan)

1998-01-01

302

Electric Charge and Magnetic Flux on Rotating Black Holes in a Force-Free Magnetosphere  

E-print Network

The electric charge on rotating black holes is calculated to be ~ BJ in the force-free configuration of Ghosh (2000), with a horizon flux of ~ BM^2. This charge is gravitationally weak for B ~ 10^{15} G, so that the Kerr metric applies. Being similar to the electric charge of a magnetar, both electric charge and magnetic flux should be, in sign and order of magnitude, continuous during stellar collapse into a black hole. Extraction of the rotational energy from newly formed black holes may proceed by interaction with the magnetic field. Keywords:black hole physics --magnetic fields

Hyun Kyu Lee; Chul H. Lee; Maurice H. P. M. van Putten

2001-02-13

303

Magnetic flux transport and the sun's dipole moment - New twists to the Babcock-Leighton model  

NASA Technical Reports Server (NTRS)

The mechanisms that give rise to the sun's large-scale poloidal magnetic field are explored in the framework of the Babcock-Leighton (BL) model. It is shown that there are in general two quite distinct contributions to the generation of the 'alpha effect': the first is associated with the axial tilts of the bipolar magnetic regions as they erupt at the surface, while the second arises through the interaction between diffusion and flow as the magnetic flux is dispersed over the surface. The general relationship between flux transport and the BL dynamo is discussed.

Wang, Y.-M.; Sheeley, N. R., Jr.

1991-01-01

304

CONDITIONS FOR TRANSVERSE WAVES PROPAGATION ALONG THIN MAGNETIC FLUX TUBES ON THE SUN  

SciTech Connect

The propagation of kink waves in the thin gravity stratified flux tubes with a generalized magnetic field distribution model is considered in cylindrical geometry. The new kink wave equations for both wave variables are obtained. It is shown that the inclusion of the radial component of an unperturbed tube magnetic field sufficiently transforms the conditions for the propagation of transverse waves. It is demonstrated that, for the models of isothermal and polytropic atmosphere in the tube and its environment, the propagation of kink waves along thin magnetic flux tubes is cutoff-free.

Lopin, Igor [Ussuriisk Astrophysical Observatory, Russian Academy of Sciences, Ussuriisk (Russian Federation); Nagorny, Ivan, E-mail: lopin78@mail.ru [Institute of Automation and Control Processes FEB RAS, Vladivostok (Russian Federation)

2013-09-10

305

Evaluation of magnetic flux distribution from magnetic domains in [Co/Pd] nanowires by magnetic domain scope method using contact-scanning of tunneling magnetoresistive sensor  

SciTech Connect

Current-driven magnetic domain wall motions in magnetic nanowires have attracted great interests for physical studies and engineering applications. The magnetic force microscope (MFM) is widely used for indirect verification of domain locations in nanowires, where relative magnetic force between the local domains and the MFM probe is used for detection. However, there is an occasional problem that the magnetic moments of MFM probe influenced and/or rotated the magnetic states in the low-moment nanowires. To solve this issue, the “magnetic domain scope for wide area with nano-order resolution (nano-MDS)” method has been proposed recently that could detect the magnetic flux distribution from the specimen directly by scanning of tunneling magnetoresistive field sensor. In this study, magnetic domain structure in nanowires was investigated by both MFM and nano-MDS, and the leakage magnetic flux density from the nanowires was measured quantitatively by nano-MDS. Specimen nanowires consisted from [Co (0.3)/Pd (1.2)]{sub 21}/Ru(3) films (units in nm) with perpendicular magnetic anisotropy were fabricated onto Si substrates by dual ion beam sputtering and e-beam lithography. The length and the width of the fabricated nanowires are 20??m and 150?nm. We have succeeded to obtain not only the remanent domain images with the detection of up and down magnetizations as similar as those by MFM but also magnetic flux density distribution from nanowires directly by nano-MDS. The obtained value of maximum leakage magnetic flux by nano-MDS is in good agreement with that of coercivity by magneto-optical Kerr effect microscopy. By changing the protective diamond-like-carbon film thickness on tunneling magnetoresistive sensor, the three-dimensional spatial distribution of leakage magnetic flux could be evaluated.

Okuda, Mitsunobu, E-mail: okuda.m-ky@nhk.or.jp; Miyamoto, Yasuyoshi; Miyashita, Eiichi; Hayashi, Naoto [NHK Science and Technology Research Laboratories, 1-10-11 Kinuta Setagaya, Tokyo 157-8510 (Japan)

2014-05-07

306

Method for Evaluating Shielding Factor of Double Layered Magnetically-Shielded Rooms for Uniform Magnetic Field Using Exciting Coils Placed on One Side  

Microsoft Academic Search

To shield the environmental magnetic noises, magnetically shielded rooms (MSRs) are used for biomagnetic measurements, etc. The sources of environmental noises are placed far away from the MSR, in such way that almost uniform magnetic fluxes are applied to the MSR. Therefore, to specify the shielding performance of MSR, the shielding factor SFż for the uniform magnetic flux density should

Shunya Odawara; Kazuhiro Muramatsu; Shogo Komori; Kiyotaka Kamata; Keita Yamazaki; Takao Yamaguchi; Mitsuru Sakakibara; Toshifumi Shinnoh; Masao Simokawa; Noboru Ishikawa; Takashi Meguro

2010-01-01

307

Equilibrium features and eruptive instabilities in laboratory magnetic flux rope plasmas  

NASA Astrophysics Data System (ADS)

One avenue for connecting laboratory and solar plasma studies is to carry out laboratory plasma experiments that serve as a well-diagnosed model for specific solar phenomena. In this paper, we present the latest results from one such laboratory experiment that is designed to address ideal instabilities that drive flux rope eruptions in the solar corona. The experiment, which utilizes the existing Magnetic Reconnection Experiment (MRX) at Princeton Plasma Physics Laboratory, generates a quasi-statically driven line-tied magnetic flux rope in a solar-relevant potential field arcade. The parameters of the potential field arcade (e.g., its magnitude, orientation, and vertical profile) are systematically scanned in order to study their influence on the evolution and possible eruption of the line-tied flux rope. Each flux rope discharge is diagnosed using a combination of fast visible light cameras and an in situ 2D magnetic probe array that measures all three components of the magnetic field over a large cross-section of the plasma. In this paper, we present the first results obtained from this new 2D magnetic probe array. With regard to the flux rope equilibrium, non-potential features such as the formation of a characteristic sigmoid shape and the generation of core toroidal field within the flux rope are studied in detail. With regard to instabilities, the onset and evolution of two key eruptive instabilities—the kink and torus instabilities—are quantitatively assessed as a function of the potential field arcade parameters and the amount of magnetic energy stored in the flux rope.This research is supported by DoE Contract Number DE-AC02-09CH11466 and by the NSF/DoE Center for Magnetic Self-Organization (CMSO).

Myers, Clayton E; Yamada, Masaaki; Belova, Elena V; Ji, Hantao; Yoo, Jongsoo; Fox, William

2014-06-01

308

RMHD simulations of collision-induced magnetic dissipations in Poynting flux dominated jets  

NASA Astrophysics Data System (ADS)

We perform a 3D relativistic ideal MHD simulation to study the collision between high-? magnetic blobs which contain both poloidal and toroidal magnetic field components, which can mimic the interactions inside a highly variable Poynting flux dominated jet. We discover a significant Poynting flux energy dissipation component in addition to the dissipation due to the evolution of the blob without collision. We identify this additional Poynting flux energy dissipation as being mainly facilitated by the collision driven magnetic reconnections, through both observing the reconnection events in the simulations and quantitative calculations of the energy dissipation rate. Additional resolution and parameter studies show a robust result that our results are nearly independent of the numerical resolution or most physical parameters in the relevant parameter range. Our results give a good support to some theories in astrophysical systems, such as the internal collision-induced magnetic reconnection and turbulence (ICMART) model for GRBs, and some reconnection triggered mini-jet model for AGNs.

Deng, Wei; Li, Hui; Zhang, Bing; Li, Shengtai

2015-01-01

309

THE EFFECT OF FLUX PINNING AND FLUX CREEP ON MAGNETIC MEASUREMENTS OF SINGLE CRYSTAL Y 1 Ba 2 Cu 3 O 7-x  

Microsoft Academic Search

A series of noncontact magnetic measurements on high-quality single crystals of YlBazCu307-, indicate that the superconducting properties are those of a conventional anisotropic superconductor. However, the magnetic properties are profoundly affected by flux pinning. We develop a classical, thermally activated flux flow model which we use to extract Hcl from the decay of the remnant magnetization at low temperature and

T. W. Worthington; Y. Yeshurun; A. P. Malozemoff; R. M. Yandrofski; F. H. Holtzberg; T. R. Dinger

1988-01-01

310

Twist Accumulation and Topology Structure of a Solar Magnetic Flux Rope  

NASA Astrophysics Data System (ADS)

To study the buildup of a magnetic flux rope before a major flare and coronal mass ejection (CME), we compute the magnetic helicity injection, twist accumulation, and topology structure of the three-dimensional (3D) magnetic field, which is derived by the nonlinear force-free field model. The Extreme-ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory observed a series of confined flares without any CME before a major flare with a CME at 23:02 UT on 2005 January 15 in active region NOAA 10720. We derive the vector velocity at eight time points from 18:27 UT to 22:20 UT with the differential affine velocity estimator for vector magnetic fields, which were observed by the Digital Vector Magnetograph at Big Bear Solar Observatory. The injected magnetic helicity is computed with the vector magnetic and velocity fields. The helicity injection rate was (- 16.47 ą 3.52) × 1040 Mx2 hr-1. We find that only about 1.8% of the injected magnetic helicity became the internal helicity of the magnetic flux rope, whose twist increasing rate was -0.18 ą 0.08 Turns hr-1. The quasi-separatrix layers (QSLs) of the 3D magnetic field are computed by evaluating the squashing degree, Q. We find that the flux rope was wrapped by QSLs with large Q values, where the magnetic reconnection induced by the continuously injected magnetic helicity further produced the confined flares. We suggest that the flux rope was built up and heated by the magnetic reconnection in the QSLs.

Guo, Y.; Ding, M. D.; Cheng, X.; Zhao, J.; Pariat, E.

2013-12-01

311

Coronal Open Magnetic Flux - Comparing two models to the IMF at 1 AU  

NASA Astrophysics Data System (ADS)

We present results of two extrapolation techniques for modeling the magnitude of solar coronal open magnetic flux at 1 AU: PFSS (Potential field – source surface) and HCCSSS (Horizontal current – current sheet – source surface). SDO/HMI photospheric magnetic field data from August 2010 through July 2014 are used as input. We compare the modeling results to the interplanetary magnetic field (IMF) data contained in the OMNI database. We discuss temporal variations in magnitude over the rising part of solar cycle 24.

Arden, William; Norton, Aimee A.

2015-04-01

312

Phonon-assisted tunneling and shot noise in double barrier structures in a longitudinal magnetic field  

Microsoft Academic Search

The phonon-assisted resonant tunneling is studied for the double barrier structures in a longitudinal magnetic field. Using the scattering matrix approach with an appropriate one-particle Green's function we are able to calculate the current and the zero frequency shot noise power spectrum in a large range of the magnetic field and to any order of the electron–phonon interaction. Obtained results

T. Anh Pham; V. Hung Nguyen; V. Lien Nguyen

2008-01-01

313

Adaptive cancellation of geomagnetic background noise for magnetic anomaly detection using coherence  

NASA Astrophysics Data System (ADS)

Magnetic anomaly detection (MAD) is an effective method for the detection of ferromagnetic targets against background magnetic fields. Currently, the performance of MAD systems is mainly limited by the background geomagnetic noise. Several techniques have been developed to detect target signatures, such as the synchronous reference subtraction (SRS) method. In this paper, we propose an adaptive coherent noise suppression (ACNS) method. The proposed method is capable of evaluating and detecting weak anomaly signals buried in background geomagnetic noise. Tests with real-world recorded magnetic signals show that the ACNS method can excellently remove the background geomagnetic noise by about 21?dB or more in high background geomagnetic field environments. Additionally, as a general form of the SRS method, the ACNS method offers appreciable advantages over the existing algorithms. Compared to the SRS method, the ACNS algorithm can eliminate the false target signals and represents a noise suppressing capability improvement of 6.4?dB. The positive outcomes in terms of intelligibility make this method a potential candidate for application in MAD systems.

Liu, Dunge; Xu, Xin; Huang, Chao; Zhu, Wanhua; Liu, Xiaojun; Yu, Gang; Fang, Guangyou

2015-01-01

314

Analysis of magnetic resonance imaging acoustic noise generated by a 4.7 T experimental system.  

PubMed

High intensity acoustic noise is an undesirable side-effect in magnetic resonance imaging (MRI) that can cause discomfort and hearing loss in patients and may be an impediment in functional MRI (fMRI) studies of the auditory system. Experimental MRI systems with high magnetic field strengths may generate acoustic noise of higher sound pressure levels (SPLs) than conventional 1.0 and 1.5 T clinical systems. We measured the SPL and spectral content of the acoustic noise generated by the Bruker Biospect 47/40 4.7 T experimental MRI system during scanning sequences commonly used in animal testing. Each sequence generated acoustic noise of high SPL, rapid pulse rates, amplitude-modulated pulse envelopes and multi-peaked spectra. The rapid acquisition with enhancement sequence with a 0.25 mm slice thickness generated SPLs of up to 129 dB peak SPL and 130 dB (A). Fourier analysis of the spectral content of the acoustic noise generated by each MRI sequence showed a wide band of acoustic energy with spectral peaks from 0.2-5 kHz. The intense MRI acoustic impulse noise generated by the 4.7 T system may cause masking of stimuli used in fMRI of the auditory cortex, reduce the hearing acuity of experimental animals and present a risk for unprotected human ears. PMID:11099151

Counter, S A; Olofsson, A; Borg, E; Bjelke, B; Häggström, A; Grahn, H F

2000-09-01

315

Magnetic Flux Ropes from the Sun to 1 AU*  

NASA Astrophysics Data System (ADS)

Any practical model of the dynamics of a coronal mass ejection (CME) and its interplanetary counterpart (ICME) must conform to available observational constraints from sun and to the earth; the upcoming STEREO mission will add significantly to those constraints. We present model/data comparisons for specific CME/ICME events near the sun (using coronagraph image data) and in the heliosphere (using in situ measurements) to show that the flux rope model of Chen and Krall[1-2] provides an accurate physics-based characterization of flux-rope CMEs over this range. We further show that quantitative results, such as the field energy required for eruption, depend on specific aspects of the flux rope geometry, such as the ratio (length/width) of the elliptical shape traced out by the flux-rope axis. It is this geometry that will be determined, for the first time, by STEREO. [1] Chen, J. 1996, JGR, 101, 27499 [2] Krall, J. et al., 2000, ApJ, 539, 964 *Work supported by ONR, NASA and NSF

Krall, J.; Yurchyshyn, V. B.; St. Cyr, O. C.; Chen, J.

2004-12-01

316

Shot noise of charge current in a quantum dot responded by rotating and oscillating magnetic fields  

SciTech Connect

We have investigated the shot noise and Fano factor of the dynamic spin-polarized quantum dot under the perturbations of a rotating magnetic field (RMF), and an oscillating magnetic field (OMF) by employing the non-equilibrium Green's function approach. The shot noise is enhanced from sub-Poissonian to super-Poissonian due to the application of RMF and OMF, and it is controlled sensitively by the tilt angle ? of RMF. The magnitude of shot noise increases as the photon energy ?? of OMF increases, and its valley eventually is reversed to peaks as the photon energy is large enough. Double-peak structure of Fano factor is exhibited as the frequency of OMF increases to cover a large regime. The Zeeman energy ?{sub 0}B{sub 0} acts as an effective gate bias to exhibit resonant behavior, and novel peak emerges associated with the applied OMF.

Zhao, Hong-Kang, E-mail: zhaohonk@yahoo.com; Zou, Wei-Ke [School of Physics, Beijing Institute of Technology, Beijing 100081 (China); Chen, Qiao [Department of Maths and Physics, Hunan Institute of Engineering, Xiangtan 411104 (China)

2014-09-07

317

Leak current estimated from the shot noise in magnetic tunneling junctions  

NASA Astrophysics Data System (ADS)

We performed the shot noise measurement in epitaxial Fe/MgO/Fe-based magnetic tunneling junctions (MTJs) with various MgO thicknesses between 1.1 nm and 1.625 nm. While the Fano factor to characterize the shot noise is very close to 1 in MTJs with MgO barrier thicker than 1.2 nm, the magnetic configuration-dependent reduction of the Fano factor for MTJs with thin MgO barrier was observed, which is mainly due to the existence of leakage current. By using a simple parallel circuit model, we demonstrated that the contribution of the leak current can be sensitively derived from the shot noise.

Tanaka, Takahiro; Arakawa, Tomonori; Maeda, Masahiro; Kobayashi, Kensuke; Nishihara, Yoshitaka; Ono, Teruo; Nozaki, Takayuki; Fukushima, Akio; Yuasa, Shinji

2014-07-01

318

Inertia Wheel on Low-Noise Active Magnetic Suspension  

Microsoft Academic Search

Magnetic bearings are particularly suited for space applications for a number of reasons: - they are ideally suited for vacuum applications; - the lack of lubrication and wear enhances the reliability and guaranties a long maintenance-free operation - the low drag torque decreases power consumption and reduces the torque exerted on the stator of the machine. - the possibility of

S. Carabelli; G. Genta; M. Silvagni; A. Tonoli

2002-01-01

319

The Feynman integrand for the Charged Particle in a Constant Magnetic field as White Noise Distribution  

E-print Network

The concepts of Feynman integrals in white noise analysis are used to realize the Feynman integrand for a charged particle in a constant magnetic field as a Hida distribution. For this purpose we identify the velocity dependent potential as a so called generalized Gauss kernel.

Wolfgang Bock; Martin Grothaus; Sebastian Jung

2012-03-01

320

Characterizing and modeling magnetic flux transport in the sun's photosphere and determining its impact on the sunspot cycle  

NASA Astrophysics Data System (ADS)

The characterization and modeling of magnetic flux transport within the surface layers of the Sun are vital to explaining the sunspot cycle. The Sun's polar fields at solar cycle minimum are the seeds of the next solar cycle: weak polar fields produce weak cycles. Magnetic flux transport is key to the buildup of the polar fields and the subsequent magnetic reversals that are essential to modulating the sunspot cycle. The primary goals of this dissertation are threefold: 1. Make precise measurements of the Sun's axisymmetric flows (i.e., differential rotation and meridional flow). 2. Create a realistic surface flux transport model that reproduces the magnetic field evolution at the surface by incorporating the observed flows. 3. Investigate the role of flux transport in modulating the polar fields, and thereby the solar activity cycle. This work has been done in collaboration with Dr. David H. Hathaway of NASA Marshall Space Flight Center. In Chapter 1, I provide an introduction to the Sun as a star. I begin with a discussion on stellar structure and evolution. I then discuss the techniques and instruments that have been used to study the Sun. I conclude Chapter 1 with a section on magnetic activity cycles on the Sun and in other stars. Magnetic flux on the Sun is transported by supergranular flows and the axisymmetric flows of differential rotation (DR) and meridional flow (MF). In Chapter 2, I introduce these flows. I then show a derivation of the Surface Flux Transport equation starting from Maxwell's equations and Ohm's Law. I conclude this chapter with an introduction to prior Surface Flux Transport models. In Chapter 3, I discuss a cross-correlation technique that we have used on magnetograms (maps of the magnetic field strengths over the surface of the Sun) to characterize the DR and MF and their variations from 1996 to present. Results show that while variability in DR is negligible, the MF varies in two fundamental ways: over the course of a solar cycle and from one cycle to the next. The MF is faster at solar cycle minimum and slower at maximum. Furthermore, the MF speeds that preceded the Solar Cycle 23/24 minimum were ˜ 20% faster than the MF speeds that preceded the prior minimum. This faster MF may have led to weaker polar field strengths and thus the subsequent extended solar minimum and an unusually weak Cycle 24. To understand the impact flux transport on the evolution of the polar fields, I have conducted experiments using a surface magnetic flux transport model, introduced in Chapter 4, that we have developed. This model advects the magnetic flux emerging in active regions (sunspots) using detailed observations of the near-surface flows that transport the magnetic elements. These flows include the axisymmetric differential rotation and meridional flow and the non-axisymmetric cellular convective flows (supergranules), all of which vary in time in the model as indicated by direct observations. At each time step, magnetic maps of the entire Sun are created. These maps are used to create plots of the Sun's axial dipole moment, a measure of the polar field. Also in Chapter 4, I illustrate how this model is used to create a baseline. In this regime, the model assimilates (i.e. continually adds in data weighted by its noise level) magnetic data from magnetograms at all available latitudes. This ensures that it accurately represents the magnetic fields observed on the surface of the Sun. This baseline is used to illustrate the difference in the timing of the polar field reversals based on four different definitions of polar fields. Advantages and disadvantages of each of these definitions are discussed. In Chapter 5, I discuss how the model is used in a predictive or simulation regime. I have tested the predictability of this model using a baseline map as an initial condition, but with daily sunspot area data used to give the sources of new magnetic flux. I found that the strength of the polar fields at cycle minimum and the polar field reversals at cycle maximum can be reliably predicted up to 3 years in adv

Upton, Lisa A.

321

Synchronized and noise-robust audio recordings during realtime magnetic resonance imaging scans.  

PubMed

This letter describes a data acquisition setup for recording, and processing, running speech from a person in a magnetic resonance imaging (MRI) scanner. The main focus is on ensuring synchronicity between image and audio acquisition, and in obtaining good signal to noise ratio to facilitate further speech analysis and modeling. A field-programmable gate array based hardware design for synchronizing the scanner image acquisition to other external data such as audio is described. The audio setup itself features two fiber optical microphones and a noise-canceling filter. Two noise cancellation methods are described including a novel approach using a pulse sequence specific model of the gradient noise of the MRI scanner. The setup is useful for scientific speech production studies. Sample results of speech and singing data acquired and processed using the proposed method are given. PMID:17069275

Bresch, Erik; Nielsen, Jon; Nayak, Krishna; Narayanan, Shrikanth

2006-10-01

322

Noise temperature improvement for magnetic fusion plasma millimeter wave imaging systems  

SciTech Connect

Significant progress has been made in the imaging and visualization of magnetohydrodynamic and microturbulence phenomena in magnetic fusion plasmas [B. Tobias et al., Plasma Fusion Res. 6, 2106042 (2011)]. Of particular importance have been microwave electron cyclotron emission imaging and microwave imaging reflectometry systems for imaging T{sub e} and n{sub e} fluctuations. These instruments have employed heterodyne receiver arrays with Schottky diode mixer elements directly connected to individual antennas. Consequently, the noise temperature has been strongly determined by the conversion loss with typical noise temperatures of ?60?000 K. However, this can be significantly improved by making use of recent advances in Monolithic Microwave Integrated Circuit chip low noise amplifiers to insert a pre-amplifier in front of the Schottky diode mixer element. In a proof-of-principle design at V-Band (50–75 GHz), significant improvement of noise temperature from the current 60?000 K to measured 4000 K has been obtained.

Lai, J.; Domier, C. W.; Luhmann, N. C. [Department of Electrical and Computer Engineering, University of California at Davis, Davis, California 95616 (United States)] [Department of Electrical and Computer Engineering, University of California at Davis, Davis, California 95616 (United States)

2014-03-15

323

On Polar Magnetic Field Reversal and Surface Flux Transport During Solar Cycle 24  

NASA Astrophysics Data System (ADS)

As each solar cycle progresses, remnant magnetic flux from active regions (ARs) migrates poleward to cancel the old-cycle polar field. We describe this polarity reversal process during Cycle 24 using four years (2010.33-2014.33) of line-of-sight magnetic field measurements from the Helioseismic and Magnetic Imager. The total flux associated with ARs reached maximum in the north in 2011, more than two years earlier than the south; the maximum is significantly weaker than Cycle 23. The process of polar field reversal is relatively slow, north-south asymmetric, and episodic. We estimate that the global axial dipole changed sign in 2013 October; the northern and southern polar fields (mean above 60° latitude) reversed in 2012 November and 2014 March, respectively, about 16 months apart. Notably, the poleward surges of flux in each hemisphere alternated in polarity, giving rise to multiple reversals in the north. We show that the surges of the trailing sunspot polarity tend to correspond to normal mean AR tilt, higher total AR flux, or slower mid-latitude near-surface meridional flow, while exceptions occur during low magnetic activity. In particular, the AR flux and the mid-latitude poleward flow speed exhibit a clear anti-correlation. We discuss how these features can be explained in a surface flux transport process that includes a field-dependent converging flow toward the ARs, a characteristic that may contribute to solar cycle variability.

Sun, Xudong; Hoeksema, J. Todd; Liu, Yang; Zhao, Junwei

2015-01-01

324

Thermal-Electromagnetic Analysis of a Fault-Tolerant Dual Star Flux-Switching Permanent Magnet Motor for Critical  

E-print Network

1 Thermal-Electromagnetic Analysis of a Fault-Tolerant Dual Star Flux-Switching Permanent Magnet of a fault-tolerant dual star Flux-Switching Permanent Magnet (FSPM) motor. The analytical results in terms permanent magnet motors have attracted increasing attentions in safety critical applications such as Hybrid

Paris-Sud XI, UniversitĂŠ de

325

Twist and Writhe of the Magnetic Flux in the Super Active Region NOAA 11429  

NASA Astrophysics Data System (ADS)

We used full-disk line-of-sight magnetograms taken by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) to study the variation of coronal magnetic helicity in the Active Region (AR) NOAA 11429, where several GOES M- and X-class flares and coronal mass ejections (CMEs) occurred. The magnetic flux, total magnetic-helicity flux, and helicity accumulation over the period of interest, i.e. 6 to 11 March 2012, were measured and are discussed. We also evaluated the tilt-angle evolution within the standard polarity flux-weighted centroids approach. The AR displays a shearing motion of the magnetic structures along the polarity inversion line, reaching values of about 1.0 km s-1. The variations of magnetic helicity flux and the tilt-angle seem to be time-correlated, and both display three-phase evolutionary patterns. We also found that the flare/CME activity is higher during the first observation phase when the tilt-angle decreases and the negative magnetic helicity is accumulated. The main changes in the accumulated helicity curve are observed only after the onset of the two strongest flare/CME events. After the major event (GOES X5.4 class/CME of 7 March) there was a decrease in the occurrence of flares and CMEs. This phase is marked by a decrease of the flux of magnetic helicity from the convection zone to the corona and a change in the orientation of the tilt of the AR. This behavior suggests that the combination of these two quantities might be important in the description of the magnetic complexity accumulated by an AR during its lifetime.

Elmhamdi, A.; Romano, P.; Kordi, A. S.; Al-trabulsy, H. A.

2014-08-01

326

Magnetic Flux Compression with a Gas Puff Z Pinch  

Microsoft Academic Search

The dynamics of a hollow gas puff z pinch imploding with a trapped, initially uniform, axial magnetic field is studied on the 0.45 MA, 1.2 mu s rise-time U.C. Irvine z pinch. The primary purpose is to ascertain the feasibility of producing short pulsed ultrahigh magnetic fields with this geometry. Data is taken primarily for Ne and Kr plasmas with

Edward Lee Ruden

1988-01-01

327

Improved thrust calculations of active magnetic bearings considering fringing flux  

NASA Astrophysics Data System (ADS)

A methodology for deriving fringing permeance in axisymmetric devices such as active thrust magnetic bearings (ATMBs) is presented. The methodology is used to develop an improved equivalent magnetic circuit (EMC) for ATMBs, which considers the fringing effect. This EMC was used to characterize the force between the housing and mover and the dependence of thrust and inductance on the air gap and input current, respectively. These characteristics were validated by comparison with those obtained by the finite element method and in experiments.

Jang, Seok-Myeong; Kim, Kwan-Ho; Ko, Kyoung-Jin; Choi, Ji-Hwan; Sung, So-Young; Lee, Yong-Bok

2012-04-01

328

Surface flux density distribution characteristics of bulk high- Tc superconductor in external magnetic field  

NASA Astrophysics Data System (ADS)

This paper describes the measured results of the two-dimensional flux density distribution of a YBCO bulk under applied AC magnetic fields with various frequency. Melt-processed oxide superconductors have been developed in order to obtain strong pinning forces. Various electric mechanical systems or magnetic levitation systems use those superconductors. The major problem is that cracks occur because the bulk superconductors are brittle. The bulk may break in magnetizing process after cracks make superconducting state instable. The trapped flux density and the permanent current characteristics of bulk superconductors have been analyzed, so as to examine the magnetizing processes or superconducting states of the bulk. In those studies, the two-dimensional surface flux density distributions of the bulk in static fields are discussed. On the other hand, the distributions in dynamic fields are little discussed. We attempted to examine the states of the bulk in the dynamic fields, and made a unique experimental device which has movable sensors synchronized with AC applied fields. As a result, the two-dimensional distributions in the dynamic fields are acquired by recombining the one-dimensional distributions. The dynamic states of the flux of the bulk and the influences of directions of cracks are observed from the distributions. In addition, a new method for measuring two-dimensional flux density distribution under dynamic magnetic fields is suggested.

Nishikawa, H.; Torii, S.; Yuasa, K.

2005-10-01

329

Vacuum currents induced by a magnetic flux around a cosmic string with finite core  

NASA Astrophysics Data System (ADS)

We evaluate the Hadamard function and the vacuum expectation value of the current density for a massive complex scalar field in the generalized geometry of a straight cosmic string with a finite core enclosing an arbitrary distributed magnetic flux along the string axis. For the interior geometry, a general cylindrically symmetric static metric tensor is used with finite support. In the region outside the core, both the Hadamard function and the current density are decomposed into the idealized zero-thickness cosmic string and core-induced contributions. The only nonzero component corresponds to the azimuthal current. The zero-thickness part of the latter is a periodic function of the magnetic flux inside the core, with the period equal to the quantum flux. As a consequence of the direct interaction of the quantum field with the magnetic field inside the penetrable core, the core-induced contribution, in general, is not a periodic function of the flux. In addition, the vacuum current, in general, is not a monotonic function of the distance from the string and may change the sign. For a general model of the core interior, we also evaluate the magnetic fields generated by the vacuum current. As applications of the general results, we have considered an impenetrable core modeled by Robin boundary condition, a core with the Minkowski-like interior and a core with a constant positive curvature space. Various exactly solvable distributions of the magnetic flux are discussed.

Bezerra de Mello, E. R.; Bezerra, V. B.; Saharian, A. A.; Harutyunyan, H. H.

2015-03-01

330

Catastrophic eruption of magnetic flux rope in the corona and solar wind with and without magnetic reconnection  

E-print Network

It is generally believed that the magnetic free energy accumulated in the corona serves as a main energy source for solar explosions such as coronal mass ejections (CMEs). In the framework of the flux rope catastrophe model for CMEs, the energy may be abruptly released either by an ideal magnetohydrodynamic (MHD) catastrophe, which belongs to a global magnetic topological instability of the system, or by a fast magnetic reconnection across preexisting or rapidly-developing electric current sheets. Both ways of magnetic energy release are thought to be important to CME dynamics. To disentangle their contributions, we construct a flux rope catastrophe model in the corona and solar wind and compare different cases in which we either prohibit or allow magnetic reconnection to take place across rapidly-growing current sheets during the eruption. It is demonstrated that CMEs, even fast ones, can be produced taking the ideal MHD catastrophe as the only process of magnetic energy release. Nevertheless, the eruptive speed can be significantly enhanced after magnetic reconnection sets in. In addition, a smooth transition from slow to fast eruptions is observed when increasing the strength of the background magnetic field, simply because in a stronger field there is more free magnetic energy at the catastrophic point available to be released during an eruption. This suggests that fast and slow CMEs may have an identical driving mechanism.

Y. Chen; Y. Q. Hu; S. J. Sun

2007-05-26

331

Development of a low noise MREIT current source  

Microsoft Academic Search

In MREIT conductivity imaging experiments of animal and human subjects, we should minimize the noise level in measured magnetic flux density data induced by injection currents with low amplitude. Since noise and artifact from an MREIT current source directly affect the quality of the data, a low-noise current source is desirable. In order to be compatible with various MREIT pulse

Young Tae Kim; Pil Joong Yoo; Tong In Oh; Eung Je Woo

2010-01-01

332

ABSORPTION OF p MODES BY THIN MAGNETIC FLUX TUBES  

SciTech Connect

We study the interaction between p modes and the many magnetic fibrils that lace the solar convection zone. In particular, we investigate the resulting absorption of p-mode energy by the fibril magnetic field. Through mechanical buffeting, the p modes excite tube waves on the magnetic fibrils-in the form of longitudinal sausage waves and transverse kink waves. The tube waves propagate up and down the magnetic fibrils and out of the p-mode cavity, thereby removing energy from the incident acoustic waves. We compute the absorption coefficient associated with this damping mechanism and model the absorption that would be observed for magnetic plage. We compare our results to the absorption coefficient that is measured using the local-helioseismic technique of ridge-filtered holography. We find that, depending on the mode order and the photospheric boundary conditions, we can achieve absorption coefficients for simulated plage that exceed 50%. The observed increase of the absorption coefficient as a function of frequency is reproduced for all model parameters.

Jain, Rekha [Applied Mathematics Department, University of Sheffield, Sheffield S3 7RH (United Kingdom); Hindman, Bradley W. [JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0440 (United States); Braun, Doug C.; Birch, Aaron C. [NWRA CoRA Division, Boulder, CO 80301 (United States)], E-mail: R.Jain@sheffield.ac.uk

2009-04-10

333

Magnetic and spontaneous Barkhausen noise techniques used in investigation of a martensitic transformation  

NASA Astrophysics Data System (ADS)

Magnetic Barkhausen noise (MBN) was used to characterize the progress of austenite to martensite phase transformation while cooling steel specimens, using a conventional Barkhausen noise emission setup stimulated by an alternating magnetic field. The phase transformation was also followed by electrical resistivity measurements and by optical and scanning electron microscopy. MBN measurements on a AISI D2 tool steel austenitized at 1473 K and cooled to liquid nitrogen temperature presented a clear change near 225 K during cooling, corresponding to the MS (martensite start) temperature, as confirmed by resistivity measurements. Analysis of the resulting signals suggested a novel experimental technique that measures spontaneous magnetic emission during transformation, in the absence of any external field. Spontaneous magnetic noise emission measurements were registered in situ while cooling an initially austenitic sample in liquid nitrogen, showing that local microstructural changes, corresponding to an avalanche or "burst" phenomena, could be detected. This spontaneous magnetic emission (SME) can thus be considered a new experimental tool for the study of martensite transformations in ferrous alloys, at the same level as acoustic emission.

Capň Sŕnchez, J.; Huallpa, E.; Farina, P.; Padovese, L. R.; Goldenstein, H.

2011-10-01

334

The magnetic topology of the plasmoid flux rope in a MHD simulation of magnetotail reconnection  

SciTech Connect

On the basis of a three-dimensional MHD simulation we discuss the magnetic topology of a plasmoid that forms by a localized reconnection process in a magnetotail configuration including a net dawn-dusk magnetic field component B/sub yN/. As a consequence of b/sub yN/ /ne/ 0 the plasmid gets a helical flux rope structure rather than an isolated island or bubble structure. Initially all field lines of the plasmid flux rope remain connected with the Earth, while at later times a gradually increasing amount of flux tubes becomes separated, connecting to either the distant boundary or to the flank boundaries. In this stage topologically different flux tubes become tangled and wrapped around each other, consistent with predictions on the basis of ad-hoc plasmid models. 10 refs., 8 figs.

Birn, J.; Hesse, M.

1989-01-01

335

The magnetic topology of the plasmoid flux rope in a MHD-simulation of magnetotail reconnection  

NASA Technical Reports Server (NTRS)

On the basis of a 3D MHD simulation, the magnetic topology of a plasmoid that forms by a localized reconnection process in a magnetotail configuration (including a net dawn-dusk magnetic field component B sub y N is discussed. As a consequence of B sub y N not equalling 0, the plasmoid assumes a helical flux rope structure rather than an isolated island or bubble structure. Initially all field lines of the plasmoid flux rope remain connected with the earth, while at later times a gradually increasing amount of flux tubes becomes separated, connecting to either the distant boundary or to the flank boundaries. In this stage, topologically different flux tubes become tangled and wrapped around each other, consistent with predictions on the basis of an ad hoc plasmoid model.

Birn, J.; Hesse, M.

1990-01-01

336

Exact scattering matrix of graphs in magnetic field and quantum noise  

SciTech Connect

We consider arbitrary quantum wire networks modelled by finite, noncompact, connected quantum graphs in the presence of an external magnetic field. We find a general formula for the total scattering matrix of the network in terms of its local scattering properties and its metric structure. This is applied to a quantum ring with N external edges. Connecting the external edges of the ring to heat reservoirs, we study the quantum transport on the graph in ambient magnetic field. We consider two types of dynamics on the ring: the free Schrödinger and the free massless Dirac equations. For each case, a detailed study of the thermal noise is performed analytically. Interestingly enough, in presence of a magnetic field, the standard linear Johnson-Nyquist law for the low temperature behaviour of the thermal noise becomes nonlinear. The precise regime of validity of this effect is discussed and a typical signature of the underlying dynamics is observed.

Caudrelier, Vincent, E-mail: v.caudrelier@city.ac.uk [Department of Mathematical Science, City University London, Northampton Square, London EC1V 0HB (United Kingdom); Mintchev, Mihail, E-mail: mintchev@df.unipi.it [Istituto Nazionale di Fisica Nucleare and Dipartimento di Fisica dell’Universitŕ di Pisa, Largo Pontecorvo 3, 56127 Pisa (Italy); Ragoucy, Eric, E-mail: eric.ragoucy@lapth.cnrs.fr [LAPTh, Laboratoire d’Annecy-le-Vieux de Physique Théorique, CNRS, Université de Savoie, BP 110, 74941 Annecy-le-Vieux Cedex (France)

2014-08-15

337

Exact scattering matrix of graphs in magnetic field and quantum noise  

NASA Astrophysics Data System (ADS)

We consider arbitrary quantum wire networks modelled by finite, noncompact, connected quantum graphs in the presence of an external magnetic field. We find a general formula for the total scattering matrix of the network in terms of its local scattering properties and its metric structure. This is applied to a quantum ring with N external edges. Connecting the external edges of the ring to heat reservoirs, we study the quantum transport on the graph in ambient magnetic field. We consider two types of dynamics on the ring: the free Schrödinger and the free massless Dirac equations. For each case, a detailed study of the thermal noise is performed analytically. Interestingly enough, in presence of a magnetic field, the standard linear Johnson-Nyquist law for the low temperature behaviour of the thermal noise becomes nonlinear. The precise regime of validity of this effect is discussed and a typical signature of the underlying dynamics is observed.

Caudrelier, Vincent; Mintchev, Mihail; Ragoucy, Eric

2014-08-01

338

Performance comparison of three-phase flux reversal permanent magnet motors in BLDC and BLAC operation mode  

NASA Astrophysics Data System (ADS)

The paper presents a comparison of torque capability and flux-weakening performance of three-phase flux reversal permanent magnet motors with surface and inset permanent magnets. Finite element analysis is employed to determine the performance of each motor in BLDC and BLAC operation mode. It is shown that the torque capability and flux-weakening performance of surface or inset permanent magnet configuration is strongly dependent on the stator teeth number/rotor pole number combination.

Štumberger, B.; Štumberger, G.; Hadžiselimovi?, M.; Hamler, A.; Gori?an, V.; Jesenik, M.; Trlep, M.

339

Analysis of electromagnetic performance of flux-switching permanent-magnet Machines by nonlinear adaptive lumped parameter magnetic circuit model  

Microsoft Academic Search

A nonlinear adaptive lumped parameter magnetic circuit model is developed to predict the electromagnetic performance of a flux-switching permanent-magnet machine. It enables the air-gap field distribution, the back-electromotive force (back-EMF) waveform, the winding inductances, and the electromagnetic torque to be calculated. Results from the model are compared with finite-element predictions and validated experimentally. The influence of end effects is also

Z. Q. Zhu; Y. Pang; D. Howe; S. Iwasaki; R. Deodhar; A. Pride

2005-01-01

340

Parallel heat flux and flow acceleration in open field line plasmas with magnetic trapping  

NASA Astrophysics Data System (ADS)

The magnetic field strength modulation in a tokamak scrape-off layer (SOL) provides both flux expansion next to the divertor plates and magnetic trapping in a large portion of the SOL. Previously, we have focused on a flux expander with long mean-free-path, motivated by the high temperature and low density edge anticipated for an absorbing boundary enabled by liquid lithium surfaces. Here, the effects of magnetic trapping and a marginal collisionality on parallel heat flux and parallel flow acceleration are examined. The various transport mechanisms are captured by kinetic simulations in a simple but representative mirror-expander geometry. The observed parallel flow acceleration is interpreted and elucidated with a modified Chew-Goldberger-Low model that retains temperature anisotropy and finite collisionality.

Guo, Zehua; Tang, Xian-Zhu; McDevitt, Chris

2014-10-01

341

An axial-flux permanent-magnet generator for a gearless wind energy system  

Microsoft Academic Search

The paper discuses the development of an axial-flux permanent-magnet generator for a gearless wind energy system which aims to demonstrate the feasibility of integrating wind and photovoltaic energy converters for the generation of electricity and to achieve optimum exploitation of the two energy sources. The merits of an axial-flux generator topology are discussed with reference to the particular requirements of

B. J. Chalmers; E. Spooner

1999-01-01

342

Investigation and Implementation of Control Strategies for Flux-Switching Permanent Magnet Motor Drives  

Microsoft Academic Search

In this paper, stator-flux orientation control strategy for the flux-switching permanent magnet (FSPM) motor under two pulse width modulation (PWM) generation modes, namely current hysteresis and voltage space vector, is investigated and implemented. Firstly, the definition of d- and q-axes for the FSPM motor, which is crucial to the mathematic model and control strategy, is highlighted. The fundamental relationship and

Hongyun Jia; Ming Cheng; Wei Hua; Wei Lu; Xiaofan Fu

2008-01-01

343

NUMERICAL EXPERIMENTS ON THE TWO-STEP EMERGENCE OF TWISTED MAGNETIC FLUX TUBES IN THE SUN  

SciTech Connect

We present the new results of the two-dimensional numerical experiments on the cross-sectional evolution of a twisted magnetic flux tube rising from the deeper solar convection zone (-20,000 km) to the corona through the surface. The initial depth is 10 times deeper than most of the previous calculations focusing on the flux emergence from the uppermost convection zone. We find that the evolution is illustrated by the following two-step process. The initial tube rises due to its buoyancy, subject to aerodynamic drag due to the external flow. Because of the azimuthal component of the magnetic field, the tube maintains its coherency and does not deform to become a vortex roll pair. When the flux tube approaches the photosphere and expands sufficiently, the plasma on the rising tube accumulates to suppress the tube's emergence. Therefore, the flux decelerates and extends horizontally beneath the surface. This new finding owes to our large-scale simulation, which simultaneously calculates the dynamics within the interior as well as above the surface. As the magnetic pressure gradient increases around the surface, magnetic buoyancy instability is triggered locally and, as a result, the flux rises further into the solar corona. We also find that the deceleration occurs at a higher altitude than assumed in our previous experiment using magnetic flux sheets. By conducting parametric studies, we investigate the conditions for the two-step emergence of the rising flux tube: field strength {approx}> 1.5 x 10{sup 4} G and the twist {approx}> 5.0 x 10{sup -4} km{sup -1} at -20,000 km depth.

Toriumi, S.; Yokoyama, T., E-mail: toriumi@eps.s.u-tokyo.ac.jp [Department of Earth and Planetary Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)

2011-07-10

344

Cross-tail magnetic flux ropes as observed by the GEOTAIL spacecraft  

NASA Technical Reports Server (NTRS)

Ten transient magnetic structures in Earth's magnetotail, as observed in GEOTAIL measurements, selected for early 1993 (at (-) X(sub GSM) = 90 - 130 Earth radii), are shown to have helical magnetic field configurations similar to those of interplanetary magnetic clouds at 1 AU but smaller in size by a factor of approximately = 700. Such structures are shown to be well approximated by a comprehensive magnetic force-free flux-rope model. For this limited set of 10 events the rope axes are seen to be typically aligned with the Y(sub GSM) axis and the average diameter of these structures is approximately = 15 Earth radii.

Lepping, R. P.; Fairfield, D. H.; Jones, J.; Frank, L. A.; Paterson, W. R.; Kokubun, S.; Yamamoto, T.

1995-01-01

345

Influence of specimen velocity on the leakage signal in magnetic flux leakage type nondestructive testing  

E-print Network

We investigate the influence of the specimen velocity on the magnetic flux leakage with the aim of selecting the optimum sensor locations. Parametric numerical simulations where the specimen velocity was in the range [0.1-20] m$\\cdot$s$^{-1}$ were carried out. As the specimen velocity is increased, the magnetic field varies from being symmetrical to being asymmetric. For the radial magnetic induction, the peak to peak value moves from the centre of the bridge towards the direction of the specimen movement. For the axial magnetic induction, the specimen velocity influence is dependent on the sensor location and a signal-velocity independent region was discussed.

Zhang, Lintao; Cameron, Ian; Sienz, Johann; Boat, Matthew; Pearson, Neil

2015-01-01

346

Influence of specimen velocity on the leakage signal in magnetic flux leakage type nondestructive testing  

E-print Network

We investigate the influence of the specimen velocity on the magnetic flux leakage with the aim of selecting the optimum sensor locations. Parametric numerical simulations where the specimen velocity was in the range [0.1-20] m$\\cdot$s$^{-1}$ were carried out. As the specimen velocity is increased, the magnetic field varies from being symmetrical to being asymmetric. For the radial magnetic induction, the peak to peak value moves from the centre of the bridge towards the direction of the specimen movement. For the axial magnetic induction, the specimen velocity influence is dependent on the sensor location and a signal-velocity independent region was discussed.

Lintao Zhang; Fawzi Belblidia; Ian Cameron; Johann Sienz; Matthew Boat; Neil Pearson

2014-12-05

347

A neutron diffraction and magnetic Barkhausen noise evaluation of defect-induced stress concentrations  

NASA Astrophysics Data System (ADS)

This thesis studies the effect of altering both the drilling technique (mechanical drilling vs. Electro-Chemical Milling) and the sequence of defect introduction and load application on the defect-induced stress distributions in sections of line pipe steel material. The defect-induced stress concentrations in loaded samples with defects introduced whilst loaded (in-situ) and prior to the application of load (pre-drilled) were examined using both neutron diffraction and Magnetic Barkhausen Noise (MBN). These results indicated the presence of potentially large levels of residual stress particularly in the in-situ sample. This is believed to be a result of plastic deformation being introduced by the mechanical drilling process. Similar studies on ECM defects showed no signs of drilling-induced stresses. Experimental stress distribution results from the in-situ and pre-drilled samples were compared to those predicted using a three-dimensional finite element model solution. The overall level of agreement was found to be best for the pre-drilled sample case. The study also aimed to determine the effectiveness of MBN as a non-destructive method for characterizing line pipe samples. Results of an initial experiment are presented showing the effectiveness of MBN for performing quantitative strain analysis on samples. This was determined by comparing MBN-measured stress concentrations with those predicted by theory and reported in the literature. Magnetic Flux Leakage (MFL) tests were also performed on the samples in order to examine any stress-induced differences in MFL response. It was found that the differences in stress distribution between the in-situ and pre-drilled samples were also reflected in the trend of the MFL signal amplitude and shape as a function of applied stress. The original work in this thesis includes the first definitive test of the effects of the order of load application and defect introduction on the resulting stress distributions around through-wall holes in plate samples. This effect will have implications for the more accurate sizing of defects and the determination of optimum operating pressures for in-service pipelines. In addition, a comprehensive study is included of the various factors effecting MBN signals from ferromagnetic samples, factors which will influence its wide-spread application as an NDE technique.

Sabet-Sharghi, Riaz

348

Disc formation in turbulent cloud cores: is magnetic flux loss necessary to stop the magnetic braking catastrophe or not?  

NASA Astrophysics Data System (ADS)

Recent numerical analysis of Keplerian disc formation in turbulent, magnetized cloud cores by Santos-Lima et al. demonstrated that reconnection diffusion is an efficient process to remove the magnetic flux excess during the buildup of a rotationally supported disc. This process is induced by fast reconnection of the magnetic fields in a turbulent flow. In a similar numerical study, Seifried et al. concluded that reconnection diffusion or any other non-ideal magnetohydrodynamic effects would not be necessary and turbulence shear alone would provide a natural way to build up a rotating disc without requiring magnetic flux loss. Their conclusion was based on the fact that the mean mass-to-flux ratio (?) evaluated over a spherical region with a radius much larger than the disc is nearly constant in their models. In this paper, we compare the two sets of simulations and show that this averaging over large scales can mask significant real increases of ? in the inner regions where the disc is built up. We demonstrate that turbulence-induced reconnection diffusion of the magnetic field happens in the initial stages of the disc formation in the turbulent envelope material that is accreting. Our analysis is suggestive that reconnection diffusion is present in both sets of simulations and provides a simple solution for the `magnetic braking catastrophe' which is discussed in the literature in relation to the formation of protostellar accretion discs.

Santos-Lima, R.; de Gouveia Dal Pino, E. M.; Lazarian, A.

2013-03-01

349

Comparisons of Earthward Poynting flux and the kinetic energy flux of up-flowing transversely heated ions from the Polar spacecraft on cusp magnetic field lines  

NASA Astrophysics Data System (ADS)

This paper presents estimates of the Poynting flux flowing along magnetic field lines in the Earth's cusp region over altitudes from 0.8 Re to 7 Re using measurements during several passes from the Polar spacecraft. The Poynting flux is calculated from measurements of electric fields from the University of California, Berkeley double probe electric field instrument, and from magnetic field measurements from the U.C.L.A. fluxgate magnetometer. The estimates of Poynting flux are of special interest because the high altitude mapping of the cusp magnetic flux tubes may connect to newly reconnected field lines and the low altitude mapping of these field lines is the scene of powerful acceleration processes, most notably transverse heating and outflow of ions. The data show that the Poynting flux is predominantly downward over the frequency range from 1 mHz to 1 Hz . This frequency range includes the Poynting flux due to steady state convection and field-aligned current systems, Alfven waves, and kinetic Alfven waves. Measurement of transversely heated ions over the energy ranges from 10 eV to several keV and their associated ion kinetic energy flux are presented from the University of Iowa Hydra instrument and compared to the values of the downward Poynting flux. Generally the downward Poynting flux exceeds the upward kinetic energy flux of the ions.

Tian, S.; Wygant, J. R.; Cattell, C. A.; Scudder, J. D.; Mozer, F.; Russell, C. T.

2013-12-01

350

Magnetic flux expulsions and secular acceleration pulses at the core surface: is there a link? (Invited)  

NASA Astrophysics Data System (ADS)

Recent observational studies based upon satellite data have shown that magnetic flux is being expelled from the core in several regions of the core surface. This phenomenon is observed below the South Atlantic Anomaly, where at least two reversed flux patches have been growing for several decades, including one under St Helena Island, and below the North polar region, where a small reversed flux patch has emerged in the 1990s, contributing to the acceleration of the North magnetic pole over the same time interval. Secular acceleration pulses are rapid surges in the second order derivative of the radial magnetic field at the core surface. The most recent pulse occurred in 2005 and was at the origin of the 2003 and 2007 geomagnetic jerks, defined as sudden changes in the field second derivative at the Earth’s surface. It was largest under St Helena and Cocos Islands. The simultaneous occurrences in the 2000s of a flux expulsion and an acceleration pulse under the St Helena region are intriguing. Both phenomena were also simultaneously observed under the North polar region in the 1990s. This presentation will (a) briefly review recent evidence in favor of the existence of magnetic flux expulsions and secular acceleration pulses at the core surface, and (b) discuss possible kinematic and dynamical links between both phenomena.

Chulliat, A.

2010-12-01

351

Statistics of magnetic fields and fluxes of massive OB stars and the origin of neutron star magnetic fields  

NASA Astrophysics Data System (ADS)

Based on the newest measurements, statistical properties of rms mean magnetic fields of OB and neutron stars (NSs) were investigated. The magnetic field distribution function f(B) for OB stars was determined and a sharp decrease of f(B) for weak magnetic fields was found. The mean magnetic fluxes F for all massive stars and NSs with measured magnetic fields was estimated, and it was found that log F = 27.7 for OB stars and log F = 24.5 for NSs. To explain the large differences of the fluxes from normal and neutron stars we studied the birth and evolution of isolated neutron stars in the whole volume of our Galaxy with our new code of population synthesis. We started modeling %with our code from the birth of massive OB stars and followed their motion within the spiral arms to the point of supernova explosion. Next we considered the evolution of NS up to the death line with considering the magnetic field decay. We found that a significant magnetic field decay occurs during the first million years of a NS's life. We have estimated the mean time of the Ohmic decay for NS. We modeled the distributions of pulsar periods P, of period derivatives \\dot P, and of pulsar magnetic fields B, and found that they are in a good agreement with those taken from \\cite{ATNF}.

Igoshev, A. P.; Kholtygin, A. F.

2011-12-01

352

The equilibrium structure of thin magnetic flux tubes. II. [in sun and late stars  

NASA Technical Reports Server (NTRS)

The thermal structure of the medium inside thin, vertical magnetic flux tubes embedded in a given external atmosphere is investigated, assuming cylindrical symmetry and a depth-independent plasma beta. The variation with tube radius of the temperature on the tube axis is computed and the temperature on the tube wall is estimated. The temperature variation across the flux tube is found to be due to the depth variation of the intensity and to the density stratification of the atmosphere. Since the temperature difference between the axis and the wall is small in thin flux tubes (of the order of 10 percent), the horizontal temperature gradient may often be neglected and the temperature in a tube of given radius may be described by a single function of depth. Thus, a more detailed numerical treatment of the radiative transfer within thin flux tubes can be substantially simplified by neglecting horizontal temperature differences within the flux tube proper.

Kalkofen, W.; Rosner, R.; Ferrari, A.; Massaglia, S.

1986-01-01

353

Magnetic flux leakage: Experimental and numerical study of the influence of induced currents  

NASA Astrophysics Data System (ADS)

Eddy currents effects are important in many practical applications of the magnetic flux leakage technique. In particular, it is common to have relative velocities between the inspected part and the excitation yoke of several meters per second. The overall situation is very complex, because of nonlinearity, hysteresis, etc. In this work both experiments and computational results are shown, oriented to provide a better understanding of the role played by induced currents in the detection of internal defects in the magnetic flux inspection of steel pipes, and to identify the issues that must be solved for a meaningful comparison between theory and experiment.

Etcheverry, J. I.; Ziella, D. H.; Sánchez, G. A.

2013-01-01

354

Kondo-induced electric polarization modulated by magnetic flux through a triangular triple quantum dot  

NASA Astrophysics Data System (ADS)

The Kondo effect plays an important role in emergence of electric polarization in a triangular triple-quantum-dot system, where one of the three dots is point-contacted with a single lead, and a magnetic flux penetrates through the triangular loop. The Kondo-induced electric polarization exhibits an Aharonov-Bohm type oscillation as a function of the magnetic flux. Our theoretical study shows various oscillation patterns associated with the field-dependent mixing of twofold orbitally degenerate ground states and their sensitivity to the point contact.

Koga, M.; Matsumoto, M.; Kusunose, H.

2015-03-01

355

Experimental investigations of the change with magnetic flux of quantum number in superconducting ring  

E-print Network

The magnetic dependencies of the critical current of aluminum ring with asymmetric link-up of current leads have been measured in order to clear up the essence of the paradoxical absence of the jump of the critical current at the quantum number change revealed before at the measurements of asymmetric superconducting ring. The measurements have shown that the experimental and theoretical dependencies agree in the region of magnetic field corresponding to integer numbers of the flux quantum and disagree at the half of the flux quantum. The jump is not observed as well as in the asymmetric rings.

A. V. Burlakov; V. L. Gurtovoi; A. I. Ilin; A. V. Nikulov; V. A. Tulin

2011-03-16

356

A TORSIONAL ALFVEN WAVE EMBEDDED WITHIN A SMALL MAGNETIC FLUX ROPE IN THE SOLAR WIND  

SciTech Connect

We describe and use novel techniques to analyze a striking and distinct solar wind event observed by two spacecraft. We show that the event is consistent with an interpretation as a torsional Alfven wave embedded within a small, nearly radially aligned, magnetic flux rope of total width {approx}10{sup 6} km. It seems likely that the torsional wave was generated by distortions produced within a pre-existing flux rope that erupted from the Sun. Our examination of many events previously identified as flux ropes in the solar wind indicates that torsional Alfven waves are extremely rare in such events.

Gosling, J. T.; Teh, W.-L.; Eriksson, S., E-mail: jack.gosling@lasp.colorado.ed [Laboratory for Atmospheric and Space Physics, University of Colorado, 1234 Innovation Drive, Boulder, CO 80303 (United States)

2010-08-10

357

Occurrence rates of magnetic activities and flux maintenance in quiet regions  

NASA Astrophysics Data System (ADS)

We investigate occurrence rates of magnetic activities, namely merging, splitting, cancellation, and emergence as functions of flux content in quiet regions by means of feature tracking technique. The structure of magnetic field on the solar surface is important not only because it is the but also because it is actual magneto-convection on the stellar surface which we can observe for the most details. Recent satellite observation reveals scaling nature of magnetic patches on the solar surface. Parnell et al. (2009) found a power-law distribution with an index of -1.85 for flux content in each magnetic patch and Thornton & Parnell (2011) found a power-law distribution with an index of -2.7 for flux content of emerging flux. However, it is not still understood what mechanism makes these power-law nature. We try this issue by developing auto-tracking technique and quantization of patch activities. We use two data sets of line-of-sight magnetograms by Solar Optical Telescope (SOT)/ Narrowband Filter Imager (NFI) on board Hinode satellite. One has the high temporal cadence, 1 minute, and the other has the longest observational period, 140 hours. More than 3000 and 40000 patches are tracked in the data sets, respectively. We found that the occurrence of merging and splitting is larger than those of emergence and cancellation by one-order of magnitude, which means that the frequency distribution of flux content is maintained by the surface activities not by flux supply itself. The frequency dependence of cancellation on flux amount is also investigated. We found that a power-law distribution with an index of -2.48 with an error bar of 0.24, which is same as that of emergence in Thornton & Parnell (2011). In the discussion, we want to suggest flux re-cycling in quiet regions, which can explain these characters. begin{enumerate} Frequency distribution of flux content is formed to a power-law distribution by merging and splitting on the solar surface. Transport of patches of surface convection results in steep power-law distribution of flux content of cancellation. The submerged fluxes though cancellations re-appear to the solar surface, which results in small-scale flux emergences on the solar surface.

Iida, Yusuke

358

Evidence in Magnetic Clouds for Systematic Open Flux Transport on the Sun  

NASA Technical Reports Server (NTRS)

Most magnetic clouds encountered by spacecraft at 1 AU display a mix of unidirectional suprathermal electrons signaling open field lines and counterstreaming electrons signaling loops connected to the Sun at both ends. Assuming the open fields were originally loops that underwent interchange reconnection with open fields at the Sun, we determine the sense of connectedness of the open fields found in 72 of 97 magnetic clouds identified by the Wind spacecraft in order to obtain information on the location and sense of the reconnection and resulting flux transport at the Sun. The true polarity of the open fields in each magnetic cloud was determined from the direction of the suprathermal electron flow relative to the magnetic field direction. Results indicate that the polarity of all open fields within a given magnetic cloud is the same 89% of the time, implying that interchange reconnection at the Sun most often occurs in only one leg of a flux rope loop, thus transporting open flux in a single direction, from a coronal hole near that leg to the foot point of the opposite leg. This pattern is consistent with the view that interchange reconnection in coronal mass ejections systematically transports an amount of open flux sufficient to reverse the polarity of the heliospheric field through the course of the solar cycle. Using the same electron data, we also find that the fields encountered in magnetic clouds are only a third as likely to be locally inverted as not. While one might expect inversions to be equally as common as not in flux rope coils, consideration of the geometry of spacecraft trajectories relative to the modeled magnetic cloud axes leads us to conclude that the result is reasonable.

Crooker, N. U.; Kahler, S. W.; Gosling, J. T.; Lepping, R. P.

2008-01-01

359

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density Gradient, Magnetic Field, and Mach Number  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Liam Krauss

1996-12-12

360

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density, Magnetic Field, and Mach Number  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Liam Krauss

1996-12-12

361

Torsional Alfven Waves in Solar Magnetic Flux Tubes of Axial Symmetry  

E-print Network

Aims: Propagation and energy transfer of torsional Alfv\\'en waves in solar magnetic flux tubes of axial symmetry is studied. Methods: An analytical model of a solar magnetic flux tube of axial symmetry is developed by specifying a magnetic flux and deriving general analytical formulae for the equilibrium mass density and a gas pressure. The main advantage of this model is that it can be easily adopted to any axisymmetric magnetic structure. The model is used to simulate numerically the propagation of nonlinear Alfv\\'en waves in such 2D flux tubes of axial symmetry embedded in the solar atmosphere. The waves are excited by a localized pulse in the azimuthal component of velocity and launched at the top of the solar photosphere, and they propagate through the solar chromosphere, transition region, and into the solar corona. Results: The results of our numerical simulations reveal a complex scenario of twisted magnetic field lines and flows associated with torsional Alfv\\'en waves as well as energy transfer to t...

Murawski, K; Musielak, Z E; Srivastava, A K; Kraskiewicz, J

2015-01-01

362

Investigation of Surface Magnetic Noise by Shallow Spins in Diamond  

NASA Astrophysics Data System (ADS)

We present measurements of spin relaxation times (T1, T1?, T2) on very shallow (?5 nm) nitrogen-vacancy centers in high-purity diamond single crystals. We find a reduction of spin relaxation times up to 30 times compared to bulk values, indicating the presence of ubiquitous magnetic impurities associated with the surface. Our measurements yield a density of 0.01-0.1?B/nm2 and a characteristic correlation time of 0.28(3) ns of surface states, with little variation between samples and chemical surface terminations. A low temperature measurement further confirms that fluctuations are thermally activated. The data support the atomistic picture where impurities are associated with the top carbon layers, and not with terminating surface atoms or adsorbate molecules. The low spin density implies that the presence of a single surface impurity is sufficient to cause spin relaxation of a shallow nitrogen-vacancy center.

Rosskopf, T.; Dussaux, A.; Ohashi, K.; Loretz, M.; Schirhagl, R.; Watanabe, H.; Shikata, S.; Itoh, K. M.; Degen, C. L.

2014-04-01

363

Fabrication and Characterization of Nano-Sized Magnetic Structures and Their Flux-Pinning Effects on Superconducting Thin Films  

E-print Network

This dissertation describes experimental studies of how a spatially alternating magnetic field can effectively pin the magnetic flux in a superconducting thin film (Pb 82 Bi18), thereby enhancing the superconductivity. The spatially alternating...

Lee, Han Gil

2011-02-22

364

White noise response of turbine blades subjected to heat flux and thermal gradient  

SciTech Connect

Design and production of the complex mechanical structures rarely result in an optimal solution. A typical example for this is a turbine blade design. Fatigue failures of the turbine blades is one of the most vexing problems of turbo-machine manufacturers, ever since the steam turbine became the main stay for power generating equipment and the gas turbines are increasingly used in air transport. Turbine blade failures due to fatigue are predominantly vibration related. The dynamic loads on the blading can arise from many different sources such as the high rotational speed, the high operating temperatures, the asymmetric aerofil tapered form of the turbine blade etc. Therefore, vibratory analysis is one of the most important stage in the designing of the turbine blades. In this study, the random response of the turbine blade to white noise excitation has been consistently calculated, including the internal damping mechanisms of the blade. Beside the damping effects, the rotational speed and the linear thermal gradient along the turbine blade are incorporated into the analysis. Pressure difference between the two surfaces of the turbine blades are modelled as white noise excitation along all over the turbine blade. The system dynamic equation of motion are derived and solved by using the combined Finite Element-Modal Analysis Method.

Karadag, V. [Technical Univ. of Istanbul (Turkey). Dept. of Mechanical Engineering; Aba, E. [Turkish Wagon Industry, Adapazari (Turkey). Dept. of Research and Development; Morguel, O.K. [Sakarya Univ., Adapazari (Turkey). Dept. of Mechanical Engineering

1997-07-01

365

Dynamic analysis of a magnetic bearing system with flux control  

NASA Technical Reports Server (NTRS)

Using measured values of two-dimensional forces in a magnetic actuator, equations of motion for an active magnetic bearing are presented. The presence of geometric coupling between coordinate directions causes the equations of motion to be nonlinear. Two methods are used to examine the unbalance response of the system: simulation by direct integration in time; and determination of approximate steady state solutions by harmonic balance. For relatively large values of the derivative control coefficient, the system behaves in an essentially linear manner, but for lower values of this parameter, or for higher values of the coupling coefficient, the response shows a split of amplitudes in the two principal directions. This bifurcation is sensitive to initial conditions. The harmonic balance solution shows that the separation of amplitudes actually corresponds to a change in stability of multiple coexisting solutions.

Knight, Josiah; Walsh, Thomas; Virgin, Lawrence

1994-01-01

366

Geometrical investigation of the kinetic evolution of the magnetic field in a periodic flux rope  

SciTech Connect

Flux ropes are bundles of magnetic field wrapped around an axis. Many laboratory, space, and astrophysics processes can be represented using this idealized concept. Here, a massively parallel 3D kinetic simulation of a periodic flux rope undergoing the kink instability is studied. The focus is on the topology of the magnetic field and its geometric structures. The analysis considers various techniques such as Poincaré maps and the quasi-separatrix layer (QSL). These are used to highlight regions with expansion or compression and changes in the connectivity of magnetic field lines and consequently to outline regions where heating and current may be generated due to magnetic reconnection. The present study is, to our knowledge, the first QSL analysis of a fully kinetic 3D particle in cell simulation and focuses the existing QSL method of analysis to periodic systems.

Restante, A. L.; Lapenta, G. [Afdeling Plasma-astrofysica, Departement Wiskunde, KULeuven, University of Leuven, Leuven (Belgium)] [Afdeling Plasma-astrofysica, Departement Wiskunde, KULeuven, University of Leuven, Leuven (Belgium); Markidis, S. [High Performance Computing and Visualization (HPCViz) Department, KTH Royal Institute of Technology, Stockholm (Sweden)] [High Performance Computing and Visualization (HPCViz) Department, KTH Royal Institute of Technology, Stockholm (Sweden); Intrator, T. [Los Alamos National Laboratory, M.S. E526, Los Alamos, New Mexico 87545 (United States)] [Los Alamos National Laboratory, M.S. E526, Los Alamos, New Mexico 87545 (United States)

2013-08-15

367

Fabrication issues in optimizing YBa{sub 2}Cu{sub 3}O{sub 7-x} flux transformers for low l/f noise  

SciTech Connect

We describe an improved interconnect technology for the fabrication of multiturn flux transformers from YBa{sub 2}Cu{sub 3}O{sub 7-x}-SrTiO{sub 3}-YBa{sub 2}Cu{sub 3}O{sub 7-x} multilayers. The essential improvements arc reductions in the thicknesses of the trilayer films, typically to 100 nm 250 nm and 250 mn respectively, and in the deposition rate, to 0.07 nm/laser pulse. This process yields crossovers in which the critical current density in the upper YBa{sub 2}Cu{sub 3}O{sub 7-x} film at 77K is (2-3) {times} 10{sup 6} A cm{sup {minus}2}. In situ trilayers exhibited 1/f flux noise levels at lHz below the measurement sensitivity of 15{mu}{Phi}{sub 0} Hz{sup {minus}1/2}, where {Phi}{sub 0} is the flux quantum. However, the flux noise of trilayers in which each layer had been patterned was significantly higher. The best flip-chip magnetometer had a white noise of 40 fT Hz{sup {minus}1/2} increasing to 340 fT Hz{sup {minus}1/2} at lHz; the corresponding flux noise levels were 9 {mu}{Phi}{sub 0} Hz{sup {minus}l/2} and 75 {mu}{Phi}{sub 0} Hz{sup {minus}l/2}, respectively.

Ludwig, F.; Dantsker, E.; Nemeth, D.T.; Koelle, D.; Miklich, A.H.; Clarke, J. [Lawrence Berkeley Lab., CA (United States); Knappe, S.; Koch, H. [Physikalisch-Technische Bundesanstalt - Inst. Berlin (Germany); Thomson, R.E. [National Inst. of Standards and Technology, Boulder, CO (United States)

1993-11-01

368

Magneto-Optical Observation of the Magnetic Flux Structure in Superconducting Niobium  

Microsoft Academic Search

The magnetic flux structure in superconducting niobium has been studied using the high specific Faraday rotation in thin films of a mixture of EuS and EuF2 at low temperatures. The magneto-optical detection system had a resolution of about 20 ?m. The specimens were niobium foils 18-?m-thick and vacuum-deposited niobium films. In the foils, after raising the magnetic field and returning

R. P. Huebener; V. A. Rowe; R. T. Kampwirth

1970-01-01

369

Interaction between emerging magnetic flux and the ambient solar coronal field  

Microsoft Academic Search

We study the interaction between emerging magnetic flux and pre-existing coronal field by means of numerical simulations using the magneto-frictional method. By advancing the induction equation, the magneto-frictional method models the coronal magnetic field as a quasi-static sequence of non-linear force- free field configurations evolving in response to photospheric driving. A general feature of the simulations is the spontaneous formation

M. Cheung; M. Derosa

2008-01-01

370

Visualization of processes of the penetration and trapping of magnetic flux in high-temperature superconductors  

Microsoft Academic Search

A novel photographic visualization method using Bi-containing films with YIG on a CaGe-garnet substrate was developed for investigating magnetic-flux penetration into a high-temperature superconductor at arbitrary temperatures up to the Curie point of the magnetic film. This method makes it possible to reveal sample defects and irregularities, and to determine their superconducting parameters (Hc, Tc, etc.). By a proper selection

A. A. Polianskii; V. K. Vlasko-Vlasov; M. V. Indenbom; V. I. Nikitenko

1989-01-01

371

Magnetic flux of progenitor stars sets gamma-ray burst luminosity and variability  

NASA Astrophysics Data System (ADS)

Long-duration gamma-ray bursts (GRBs) are thought to come from the core collapse of Wolf-Rayet stars. Whereas their stellar masses M* have a rather narrow distribution, the population of GRBs is very diverse, with gamma-ray luminosities L? spanning several orders of magnitude. This suggests the existence of a `hidden' stellar variable whose burst-to-burst variation leads to a spread in L?. Whatever this hidden variable is, its variation should not noticeably affect the shape of GRB light curves, which display a constant luminosity (in a time-average sense) followed by a sharp drop at the end of the burst seen with Swift/XRT. We argue that such a hidden variable is progenitor star's large-scale magnetic flux. Shortly after the core collapse, most of stellar magnetic flux accumulates near the black hole (BH) and remains there. The flux extracts BH rotational energy and powers jets of roughly a constant luminosity, Lj. However, once BH mass accretion rate dot{M} falls below ˜Lj/c2, the flux becomes dynamically important and diffuses outwards, with the jet luminosity set by the rapidly declining mass accretion rate, L_j˜ dot{M}c^2. This provides a potential explanation for the sharp end of GRBs and the universal shape of their light curves. During the GRB, gas infall translates spatial variation of stellar magnetic flux into temporal variation of Lj. We make use of the deviations from constancy in Lj to perform stellar magnetic flux `tomography'. Using this method, we infer the presence of magnetized tori in the outer layers of progenitor stars for GRB 920513 and GRB 940210.

Tchekhovskoy, Alexander; Giannios, Dimitrios

2015-02-01

372

Plasma PhysicsUnivMaryland 19-21Apr2004 Intrator 3D flux tubes 1 Magnetic reconnection, merging flux ropes,  

E-print Network

Plasma PhysicsUnivMaryland 19-21Apr2004 Intrator 3D flux tubes 1 Magnetic reconnection, merging flux ropes, 3D effects in RSX T. Intrator P-24 Plasma Physics I. Furno, E. Hemsing, S. Hsu, + many students G.Lapenta, P.Ricci T-15 Plasma Theory Second Workshop on Thin Current Sheets University

Sitnov, Mikhail I.

373

A flux extraction device to measure the magnetic moment of large samples; application to bulk superconductors.  

PubMed

We report the design and construction of a flux extraction device to measure the DC magnetic moment of large samples (i.e., several cm(3)) at cryogenic temperature. The signal is constructed by integrating the electromotive force generated by two coils wound in series-opposition that move around the sample. We show that an octupole expansion of the magnetic vector potential can be used conveniently to treat near-field effects for this geometrical configuration. The resulting expansion is tested for the case of a large, permanently magnetized, type-II superconducting sample. The dimensions of the sensing coils are determined in such a way that the measurement is influenced by the dipole magnetic moment of the sample and not by moments of higher order, within user-determined upper bounds. The device, which is able to measure magnetic moments in excess of 1 A m(2) (1000 emu), is validated by (i) a direct calibration experiment using a small coil driven by a known current and (ii) by comparison with the results of numerical calculations obtained previously using a flux measurement technique. The sensitivity of the device is demonstrated by the measurement of flux-creep relaxation of the magnetization in a large bulk superconductor sample at liquid nitrogen temperature (77 K). PMID:25725888

Egan, R; Philippe, M; Wera, L; Fagnard, J F; Vanderheyden, B; Dennis, A; Shi, Y; Cardwell, D A; Vanderbemden, P

2015-02-01

374

A flux extraction device to measure the magnetic moment of large samples; application to bulk superconductors  

NASA Astrophysics Data System (ADS)

We report the design and construction of a flux extraction device to measure the DC magnetic moment of large samples (i.e., several cm3) at cryogenic temperature. The signal is constructed by integrating the electromotive force generated by two coils wound in series-opposition that move around the sample. We show that an octupole expansion of the magnetic vector potential can be used conveniently to treat near-field effects for this geometrical configuration. The resulting expansion is tested for the case of a large, permanently magnetized, type-II superconducting sample. The dimensions of the sensing coils are determined in such a way that the measurement is influenced by the dipole magnetic moment of the sample and not by moments of higher order, within user-determined upper bounds. The device, which is able to measure magnetic moments in excess of 1 A m2 (1000 emu), is validated by (i) a direct calibration experiment using a small coil driven by a known current and (ii) by comparison with the results of numerical calculations obtained previously using a flux measurement technique. The sensitivity of the device is demonstrated by the measurement of flux-creep relaxation of the magnetization in a large bulk superconductor sample at liquid nitrogen temperature (77 K).

Egan, R.; Philippe, M.; Wera, L.; Fagnard, J. F.; Vanderheyden, B.; Dennis, A.; Shi, Y.; Cardwell, D. A.; Vanderbemden, P.

2015-02-01

375

Theoretical Noise Analysis on a Position-sensitive Metallic Magnetic Calorimeter  

NASA Technical Reports Server (NTRS)

We report on the theoretical noise analysis for a position-sensitive Metallic Magnetic Calorimeter (MMC), consisting of MMC read-out at both ends of a large X-ray absorber. Such devices are under consideration as alternatives to other cryogenic technologies for future X-ray astronomy missions. We use a finite-element model (FEM) to numerically calculate the signal and noise response at the detector outputs and investigate the correlations between the noise measured at each MMC coupled by the absorber. We then calculate, using the optimal filter concept, the theoretical energy and position resolution across the detector and discuss the trade-offs involved in optimizing the detector design for energy resolution, position resolution and count rate. The results show, theoretically, the position-sensitive MMC concept offers impressive spectral and spatial resolving capabilities compared to pixel arrays and similar position-sensitive cryogenic technologies using Transition Edge Sensor (TES) read-out.

Smith, Stephen J.

2007-01-01

376

Noise estimation in single- and multiple-coil magnetic resonance data based on statistical models.  

PubMed

Noise estimation is a challenging task in magnetic resonance imaging (MRI), with applications in quality assessment, filtering or diffusion tensor estimation. Main noise estimators based on the Rician model are revisited and classified in this article, and new useful methods are proposed. Additionally, all the surveyed estimators are extended to the noncentral chi model, which applies to multiple-coil MRI and some important parallel imaging algorithms for accelerated acquisitions. The proposed new noise estimation procedures, based on the distribution of local moments, show better performance in terms of smaller variance and unbiased estimation over a wide range of experiments, with the additional advantage of not needing to explicitly segment the background of the image. PMID:19570640

Aja-Fernández, Santiago; Tristán-Vega, Antonio; Alberola-López, Carlos

2009-12-01

377

Pre-eruptive Coronal Model for a Magnetic Flux-tube  

NASA Astrophysics Data System (ADS)

Aly conjectured that the energy of a closed magnetic-field configuration above the photosphere cannot exceed the energy of the corresponding open-field configuration with the same normal field distribution at the photosphere. This limitation, later proved by Aly for a planar geometry and by Sturrock for a curved geometry, has important consequences for modeling coronal dynamics, especially solar eruptions such as flares and CMEs, that involve the release of magnetic energy. The above analyses were based on the assumption that all field lines are connected to the photosphere. However, one can conceive of a pre-eruption configuration involving magnetic flux that does not thread the photosphere. A toroidal current-carrying wire embedded in a dipolar arcade can have arbitrarily large magnetic energy relative to the potential configuration as its radius goes to zero, but this would be an unphysical model for the solar corona. We inquire into whether there exist force-free field solutions with ``disconnected'' flux and energy greater than the open-field energy. We examine a magnetic-field model comprising a toroidal, force-free flux tube in the equatorial plane, restrained by an overlying arcade with dipole boundary conditions, analyzing the configuration by the generating-function (Grad-Shafranov) method. We solve the equations numerically by an iterative procedure, relaxing the flux-tube and the dipole field separately, alternately allowing their interface to adjust. We confirm that it is indeed possible for the energy of such a closed magnetic-field configuration to exceed the Aly-Sturrock limit. However, it requires such a large amount of twist that the flux-tube would be unstable in a full 3-D MHD treatment. We gratefully acknowledge that this research was supported by NASA grants NAS 8-37334 and NAG 5-9784.

Weber, M. A.; Sturrock, P. A.

2002-05-01

378

Influence of growth and annealing conditions on low-frequency magnetic 1/f noise in MgO magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

Magnetic 1/f noise is compared in magnetic tunnel junctions with electron-beam evaporated and sputtered MgO tunnel barriers in the annealing temperature range 350 - 425 °C. The variation of the magnetic noise parameter (?mag) of the reference layer with annealing temperature mainly reflects the variation of the pinning effect of the exchange-bias layer. A reduction in ?mag with bias is associated with the bias dependence of the tunneling magnetoresistance. The related magnetic losses are parameterized by a phase lag ?, which is nearly independent of bias especially below 100 mV. The similar changes in magnetic noise with annealing temperature and barrier thickness for two types of MgO magnetic tunnel junctions indicate that the barrier layer quality does not affect the magnetic losses in the reference layer.

Feng, Jiafeng; Diao, Zhu; Kurt, Huseyin; Stearrett, Ryan; Singh, A.; Nowak, Edmund R.; Coey, J. M. D.

2012-11-01

379

TWO-STEP EMERGENCE OF THE MAGNETIC FLUX SHEET FROM THE SOLAR CONVECTION ZONE  

SciTech Connect

We perform two-dimensional magnetodydrodynamic simulations of the flux emergence from the solar convection zone to the corona. The flux sheet is initially located moderately deep in the adiabatically stratified convection zone (-20,000 km) and is perturbed to trigger the Parker instability. The flux rises through the solar interior due to the magnetic buoyancy, but suffers a gradual deceleration and a flattening in the middle of the way to the surface since the plasma piled on the emerging loop cannot pass through the convectively stable photosphere. As the magnetic pressure gradient enhances, the flux becomes locally unstable to the Parker instability so that the further evolution to the corona occurs. The second-step nonlinear emergence is well described by the expansion law by Shibata et al. To investigate the condition for this 'two-step emergence' model, we vary the initial field strength and the total flux. When the initial field is too strong, the flux exhibits the emergence to the corona without a deceleration at the surface and reveals an unrealistically strong flux density at each footpoint of the coronal loop, while the flux either fragments within the convection zone or cannot pass through the surface when the initial field is too weak. The condition for the 'two-step emergence' is found to be 10{sup 21}-10{sup 22} Mx with 10{sup 4} G at z = -20,000 km. We present some discussions in connection with recent observations and the results of the thin-flux-tube model.

Toriumi, S.; Yokoyama, T., E-mail: toriumi@eps.s.u-tokyo.ac.j [Department of Earth and Planetary Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033 (Japan)

2010-05-01

380

Investigations of the Role of Magnetic Twist in Flux Rope Emergence  

NASA Astrophysics Data System (ADS)

Simulations of the rise of buoyant magnetic flux ropes through the solar convection zone show that flux ropes break up and cease to be buoyant if their magnetic twist is too weak. Similarly, simulations of the emergence of magnetic flux ropes through the photosphere into the solar atmosphere show that buoyant flux ropes will not emerge if their twist is too weak. Yet observations consistently show that the majority of emerged active regions have twist which is orders of magnitude smaller than the lower limit required by these simulations.We report on a series of numerical investigations which attempt to resolve this contradiction by searching for mechanisms which can reduce the minimum twist required to successfully emerge flux ropes. We then investigate the magnitude and distribution of alpha, the measure of twist most often observed, at the photosphere in these simulations, and compare these to observed magnitudes and distributions of alpha. We discuss the implications of these results for our current understanding of how active regions rise to the solar surface and emerge into the solar atmosphere.This work was supported by the NASA Living with a Star program.

Linton, Mark G.; Leake, James

2015-04-01

381

A Flux Tube Tectonics Model for Solar Coronal Heating Driven by the Magnetic Carpet.  

E-print Network

A Flux Tube Tectonics Model for Solar Coronal Heating Driven by the Magnetic Carpet. Eric R. Priest heating. The dissipation of energy along sharp boundaries we call, by analogy with geophysi- cal plate tectonics, the tectonics model of coronal heating. Similar to the case on Earth, the relative motions

Priest, Eric

382

THE DYNAMICS OF MAGNETIC FLUX TUBES IN THE SOLAR CONVECTION ZONE  

E-print Network

National Solar Observatory 950 North Cherry Avenue Tucson, AZ 85719 AND D. W. LONGCOPE AND M. G. LINTONTHE DYNAMICS OF MAGNETIC FLUX TUBES IN THE SOLAR CONVECTION ZONE A Study of Active Region Formation of our recent work on the dynamics of ux tubes in the solar convection zone. We focus on two topics

California at Berkeley, University of

383

DYNAMIC COUPLING OF CONVECTIVE FLOWS AND MAGNETIC FIELD DURING FLUX EMERGENCE  

SciTech Connect

We simulate the buoyant rise of a magnetic flux rope from the solar convection zone into the corona to better understand the energetic coupling of the solar interior to the corona. The magnetohydrodynamic model addresses the physics of radiative cooling, coronal heating, and ionization, which allow us to produce a more realistic model of the solar atmosphere. The simulation illustrates the process by which magnetic flux emerges at the photosphere and coalesces to form two large concentrations of opposite polarities. We find that the large-scale convective motion in the convection zone is critical to form and maintain sunspots, while the horizontal converging flows in the near-surface layer prevent the concentrated polarities from separating. The footpoints of the sunspots in the convection zone exhibit a coherent rotation motion, resulting in the increasing helicity of the coronal field. Here, the local configuration of the convection causes the convergence of opposite polarities of magnetic flux with a shearing flow along the polarity inversion line. During the rising of the flux rope, the magnetic energy is first injected through the photosphere by the emergence, followed by energy transport by horizontal flows, after which the energy is subducted back to the convection zone by the submerging flows.

Fang Fang; Manchester IV, Ward; Van der Holst, Bart [Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109 (United States); Abbett, William P. [Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)

2012-01-20

384

Analysis and Performance Evaluation of Radial Flux Air-Cored Permanent Magnet Machines with Concentrated Coils  

Microsoft Academic Search

In this paper two different concentrated coil configurations for radial flux air-cored permanent magnet (RFAPM) machines are analysed and evaluated against a RFAPM machine that utilises overlapping coils. The comparative analysis of the different machines are done analytically. The analytical results are compared with finite element analysis results of simplified linearised equivalent models for the different coil configurations. The finite

P. J. Randewijk; M. J. Kamper; R.-J. Wang

2007-01-01

385

A FLUX-TUBE TECTONICS MODEL FOR SOLAR CORONAL HEATING DRIVEN BY THE MAGNETIC CARPET  

E-print Network

. The dissipation of energy along sharp boundaries we call, by analogy with geophysical plate tectonicsA FLUX-TUBE TECTONICS MODEL FOR SOLAR CORONAL HEATING DRIVEN BY THE MAGNETIC CARPET Eric R. Priest, the tectonics model of coronal heating. Simi- lar to the case on Earth, the relative motions of the photospheric

Priest, Eric

386

American Institute of Aeronautics and Astronautics A Flux-Pinned Magnet-Superconductor Pair for Close-  

E-print Network

the use of system identification techniques to characterize the modal damping and stiffness. Our results and potentially useful nonzero stiffnesses at larger (over 3 cm) separations, with significant damping. We find between the facing surfaces of a superconducting plate and permanent magnet D = flux pinning damping (6

Peck, Mason A.

387

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Liam Krauss

1996-12-12

388

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density Gradient and Velocities  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Liam Krauss

1996-12-12

389

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density Gradient  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Liam Krauss

1996-12-12

390

Magnetic flux expulsion in powerful superbubble explosions and the alpha-Omega dynamo  

Microsoft Academic Search

The possibility of magnetic flux expulsion from the Galaxy in superbubble (SB) explosions, important for the alpha-Omega dynamo, is considered. Special emphasis is put on investigation of the downsliding of the matter from the top of the shell formed by the SB explosion, which is able to influence the kinematics of the shell. It is shown that either Galactic gravity

R. R. Rafikov; R. M. Kulsrud

2000-01-01

391

Magnetic Flux Expulsion in the Powerful Superbubble Explosions and the ?- Dynamo  

Microsoft Academic Search

The possibility of the magnetic flux expulsion from the Galaxy in the superbubble (SB) explosions, important for the ?- dynamo, is considered. Special emphasis is put on the investigation of the downsliding of the matter from the top of the shell formed by the SB explosion which is able to influence the kinematics of the shell. It is shown that

R. R. Rafikov; R. M. Kulsrud

392

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. , XXXX, DOI:10.1029/, Magnetic Flux Emergence in the Sun  

E-print Network

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. , XXXX, DOI:10.1029/, Magnetic Flux Emergence in the Sun V­dimensional evolution of solar eruptions as they leave the Sun and move into the interplanetary space. One of the most important processes, responsible for many dynamical phenomena ob­ served in the Sun, is the emergence

Sengun, Mehmet Haluk

393

Radiation-induced transport of magnetic flux along a superconducting sheet  

Microsoft Academic Search

A switching phenomenon based on the formation of a spot of normal ; conductivity by shining a ray of light upon a superconducting film is discussed. ; Possible applications of this switching process as a modulntor for d-c amplifler ; circuits, as a radiation switch to recouple transformer coils, and for the ; transport of magnetic flux trapped in such

J. F. Marchand; J. Volger

1962-01-01

394

A Power-law Distribution of Solar Magnetic Fields Over More Than Five Decades in Flux  

E-print Network

A Power-law Distribution of Solar Magnetic Fields Over More Than Five Decades in Flux C. E. Parnell. Welsch University of California, Berkeley Space Sciences Laboratory, 7 Gauss Way, CA 94720 welsch, MD 20771 #12;­ 2 ­ ABSTRACT Solar flares, coronal mass ejections, and indeed phenomena on all scales

Parnell, Clare E.

395

Temperature Evolution of a Magnetic Flux Rope in a Failed Solar Eruption  

NASA Astrophysics Data System (ADS)

In this paper, we report for the first time the detailed temperature evolution process of the magnetic flux rope in a failed solar eruption. Occurring on 2013 January 05, the flux rope was impulsively accelerated to a speed of ~400 km s-1 in the first minute, then decelerated and came to a complete stop in two minutes. The failed eruption resulted in a large-size high-lying (~100 Mm above the surface), high-temperature "fire ball" sitting in the corona for more than two hours. The time evolution of the thermal structure of the flux rope was revealed through the differential emission measure analysis technique, which produced temperature maps using observations of the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. The average temperature of the flux rope steadily increased from ~5 MK to ~10 MK during the first nine minutes of the evolution, which was much longer than the rise time (about three minutes) of the associated soft X-ray flare. We suggest that the flux rope is heated by the energy release of the continuing magnetic reconnection, different from the heating of the low-lying flare loops, which is mainly produced by the chromospheric plasma evaporation. The loop arcade overlying the flux rope was pushed up by ~10 Mm during the attempted eruption. The pattern of the velocity variation of the loop arcade strongly suggests that the failure of the eruption was caused by the strapping effect of the overlying loop arcade.

Song, H. Q.; Zhang, J.; Cheng, X.; Chen, Y.; Liu, R.; Wang, Y. M.; Li, B.

2014-03-01

396

The characteristics of trapped magnetic flux inside bulk HTS in the Mixed-? levitation system  

NASA Astrophysics Data System (ADS)

In this paper we propose a new experimental method to investigate the principle of ‘Mixed-?’ levitation systems. To confirm our claim, we measured the attractive force and the variation of flux passing through both, the HTS sample and the face surface of iron yoke, when the yoke is approached/retreated to the surface of field-cooled HTS sample. It is revealed that when the diameter of the yoke is less than the diameter of the HTS, in the small gap by reducing the gap, the flux which passes through the face surface of the yoke and consequently the attractive force will decrease. Therefore, stable levitation is achievable. However, when the diameter of yoke is equal to the diameter of the HTS, the magnetic flux will not decrease for a reducing gap and stable levitation is unfeasible. Briefly, stable levitation is achievable when the yoke diameter is less than the HTS’s diameter. Additionally, as the yoke is approached to the HTS the flux variation of the HTS in 77 K is negligible compare to the flux variation of the HTS in the room temperature. Therefore, in superconductivity state the pinned fluxes in the HTS samples remain approximately constant and the HTS acts as a ‘magnetic isolator’. This specification can be used to simulate the behavior of field-cooled HTS by the FEM software.

Ghodsi, M.; Ueno, T.; Teshima, H.; Hirano, H.; Higuchi, T.

2006-10-01

397

Fabrication of high density magnetic structures and observation of flux closure states in cobalt rings  

NASA Astrophysics Data System (ADS)

For the implementation of magnetic materials in high density data storage, an optimization of the geometry and size of small magnetic elements is of importance. Generally for a given material the geometry determines which kind of magnetization configuration may be present, while the size and lateral extension control the balance between the demagnetization fields and exchange fields and thus the transformation of one magnetization configuration into another. In this work (S.P. Li et al. Phys. Rev. Lett. 86, 1102 (2001)), we explore the magnetization patterns in cobalt rings with diameter between 300 and 800 nm and thickness between 10 and 50 nm. To fabricate efficiently the submicron rings, both high resolution electron beam lithography and tri-layer nanoimprint techniques have been used, followed by lift-off of a cobalt thin film. Two complementary experimental techniques, magneto-optic Kerr-effect (MOKE) hysteresis loops and Magnetic force microscopy (MFM) have been used for characterization. By means of MOKE we show that two types of magnetic states exist in zero field, defining a state diagram as a function of diameter and thickness. MFM imaging reveals that these states correspond to flux closure and single-domain states. For all dimensions investigated the flux closure state is the stable magnetization configuration. However with increasing diameter and decreasing thickness a metastable single-domain state can be obtained in the reversal process in an in-plane applied field. The results are compared to vortex formation in Co dots with comparable diameters and thicknesses. While for the rings the metastable single domain states could be easily switched to the flux-closure state by the perturbation produced by an MFM tip, this was not the case for the dots. For these on the contrary the single-domain state remained stable during the whole magnetization reversal process that was observed to occur by coherent rotation. The experimental results are in good agreement with micromagnetic simulations.

Chen, Yong

2002-03-01

398

Magnetic Flux Expulsion in the Powerful Superbubble Explosions and the Alpha-Omega Dynamo  

E-print Network

The possibility of the magnetic flux expulsion from the Galaxy in the superbubble (SB) explosions, important for the Alpha-Omega dynamo, is considered. Special emphasis is put on the investigation of the downsliding of the matter from the top of the shell formed by the SB explosion which is able to influence the kinematics of the shell. It is shown that either Galactic gravity or the development of the Rayleigh-Taylor instabilities in the shell limit the SB expansion, thus, making impossible magnetic flux expulsion. The effect of the cosmic rays in the shell on the sliding is considered and it is shown that it is negligible compared to Galactic gravity. Thus, the question of possible mechanism of flux expulsion in the Alpha-Omega dynamo remains open.

R. R. Rafikov; R. M. Kulsrud

2000-04-06

399

Single ion as a shot-noise-limited magnetic-field-gradient probe  

SciTech Connect

It is expected that ion-trap quantum computing can be made scalable through protocols that make use of transport of ion qubits between subregions within the ion trap. In this scenario, any magnetic field inhomogeneity the ion experiences during the transport may lead to dephasing and loss of fidelity. Here we demonstrate how to measure, and compensate for, magnetic field gradients inside a segmented ion trap, by transporting a single ion over variable distances. We attain a relative magnetic field sensitivity of {Delta}B/B{sub 0{approx}}5x10{sup -7} over a test distance of 140 {mu}m, which can be extended to the mm range, still with sub-{mu}m resolution. A fast experimental sequence is presented, facilitating its use as a magnetic-field-gradient calibration routine, and it is demonstrated that the main limitation is the quantum shot noise.

Walther, A.; Poschinger, U.; Ziesel, F.; Hettrich, M.; Wiens, A.; Welzel, J.; Schmidt-Kaler, F. [Institut fuer Quantenphysik, Universitaet Mainz, Staudingerweg 7, DE-55128 Mainz (Germany)

2011-06-15

400

Thermal noise of mechanical oscillators in steady states with a heat flux.  

PubMed

We present an experimental investigation of the statistical properties of the position fluctuations of low-loss oscillators in nonequilibrium steady states. The oscillators are coupled to a heat bath, and a nonequilibrium steady state is produced by flowing a constant heat flux, setting a temperature difference across the oscillators. We investigated the distribution of the measurements of the square of the oscillator position and searched for signs of changes with respect to the equilibrium case. We found that, after normalization by the mean value, the second, third, and fourth standardized statistical moments are not modified by the underlying thermodynamic state. This differs from the behavior of the absolute, i.e., not normalized, second moment, which is strongly affected by temperature gradients and heat fluxes. We illustrate this with a numerical experiment in which we study via molecular dynamics the fluctuations of the length of a one-dimensional chain of identical particles interacting via anharmonic interparticle potentials, with the extremes thermostated at different temperatures: we use the variance of the length in correspondence to its first elastic mode of resonance to define an effective temperature which we observe to depart from the thermodynamic one in the nonequilibrium states. We investigate the effect of changing the interparticle potential and show that the qualitative behavior of the nonequilibrium excess is unchanged. Our numerical results are consistent with the chain length being Gaussian distributed in the nonequilibrium states. Our experimental investigation reveals that the position variance is the only, and crucially easily accessible, observable for distinguishing equilibrium from nonequilibrium steady states. The consequences of this fact for the design of interferometric gravitational wave detectors are discussed. PMID:25314407

Conti, Livia; Lazzaro, Claudia; Karapetyan, Gagik; Bonaldi, Michele; Pegoraro, Matteo; Thakur, Ram-Krishna; De Gregorio, Paolo; Rondoni, Lamberto

2014-09-01

401

Characterization of Dual-Phase Steels Using Magnetic Barkhausen Noise Technique  

Microsoft Academic Search

The aim of this work is to nondestructively characterize the dual phase steels using the Magnetic Barkhausen Noise (MBN) method.\\u000a By quenching of AISI 8620 steel specimens having two different starting microstructures, from various intercritical annealing\\u000a temperatures (ICAT) in the ferrite-austenite region, the microstructures consisting of different volume fractions of martensite\\u000a with morphological variations have been obtained. The microstructures were

M. Kaplan; C. H. Gür; M. Erdogan

2007-01-01

402

Convective radial energy flux due to resonant magnetic perturbations and magnetic curvature at the tokamak plasma edge  

SciTech Connect

With the resonant magnetic perturbations (RMPs) consolidating as an important tool to control the transport barrier relaxation, the mechanism on how they work is still a subject to be clearly understood. In this work, we investigate the equilibrium states in the presence of RMPs for a reduced MHD model using 3D electromagnetic fluid numerical code with a single harmonic RMP (single magnetic island chain) and multiple harmonics RMPs in cylindrical and toroidal geometry. Two different equilibrium states were found in the presence of the RMPs with different characteristics for each of the geometries used. For the cylindrical geometry in the presence of a single RMP, the equilibrium state is characterized by a strong convective radial thermal flux and the generation of a mean poloidal velocity shear. In contrast, for toroidal geometry, the thermal flux is dominated by the magnetic flutter. For multiple RMPs, the high amplitude of the convective flux and poloidal rotation are basically the same in cylindrical geometry, but in toroidal geometry the convective thermal flux and the poloidal rotation appear only with the islands overlapping of the linear coupling between neighbouring poloidal wavenumbers m, m – 1, and m?+?1.

Marcus, F. A., E-mail: albertus@if.usp.br [Institute of Physics at University of Săo Paulo, C.P. 66318, 05315-970 Săo Paulo, S.P. (Brazil); Aix-Marseille Université, CNRS, PIIM UMR 7345, 13397 Marseille Cedex 20 (France); Beyer, P.; Fuhr, G.; Monnier, A.; Benkadda, S. [Aix-Marseille Université, CNRS, PIIM UMR 7345, 13397 Marseille Cedex 20 (France)

2014-08-15

403

A very low noise preamplifier for extremely low frequency magnetic antenna  

NASA Astrophysics Data System (ADS)

Besides the electrode-pair antenna, the magnetic antenna is also used for the extremely low frequency (ELF) submarine communication. To receive the weak ELF signals, the structure of a small sized magnetic antenna determines its specific electrical characteristics. The ELF magnetic antenna shows high internal resistance, alternating-current impedance, and a resonance frequency near the operating bandwidth. In accordance with the electrical characteristics of ELF magnetic antenna, a low noise preamplifier and frequency compensation circuit were designed and realized. The preamplifier is a three-stage negative feedback circuit, which is composed of parallel JFET, common-emitter amplifier with a Darlington structure and a common-collector amplifier in push-pull connection. And a frequency compensation circuit is cascaded to compensate the characteristic in low frequency range. In the operating bandwidth f = 30-200 Hz, the circuit has a gain of 39.4 dB. The equivalent input noise is 1.97 nV/?Hz and the frequency response keeps flat in operating bandwidth. The proposed preamplifier of the ELF magnetic antenna performs well in receiving ELF signals.

Shimin, Feng; Suihua, Zhou; Zhiyi, Chen

2013-07-01

404

Controlled reversal of coupled Néel walls in flux-closure magnetic trilayer elements  

NASA Astrophysics Data System (ADS)

We report the detailed field-induced transformation of coupled Néel walls in micron-sized trilayer elliptical elements for novel domain-wall-based device applications. Using in situ Lorentz transmission electron microscopy and micromagnetic simulation, we demonstrate that the magnetostatically coupled composite wall structure can be switched controllably without affecting the overall flux-closure domain configuration via separate translation of vortex cores in the two magnetic layers. The top and bottom Néel walls either trap or expel each other depending on the relative orientation of their magnetization directions, leading to the interesting domain switching behavior observed during magnetization reversal.

Huang, L.; Zhu, Y.

2009-11-01

405

Feasibility study on 3 axis magnetic sensor for flux leakage method  

NASA Astrophysics Data System (ADS)

Most of NDT system by magnetic field sensing has employed coil or semiconductor as sensor which has one axis sensitivity. Recent development of semiconductor technology can makes a chip that enable us to measure 3 axis magnetic field in a 1mm square. This vector information is expected to show a new insight in NDT testing. This presentation will show a basic experimental feasibility study for application of magnetic sensor to flux leakage and eddy current testing method by using a sensing system with the chip.

Sasamoto, Akira

2014-04-01

406

Numerical computation for a new way to reduce vibration and noise due to magnetostriction and magnetic forces of transformer cores  

NASA Astrophysics Data System (ADS)

Magnetostriction (MS) caused by the global magnetization of limbs and yokes and magnetic forces are the undisputed causes of the vibration and noise in power transformer cores. This paper presents a novel way to reduce the vibration and noise, in which nanocrystalline soft magnetic composite (NSMC) material with high permeability is used to fill the step-lap joint gaps of the power transformer magnetic cores. In order to numerically predict the effectiveness of the proposed method, a 3-D magneto-mechanical strong coupled model including MS and magnetic anisotropy of steel sheet was founded. Then, the numerical model was applied to analyze the step-lap joint region of the corner of magnetic cores. The analysis results illustrated that the deformation and noise of core with NSMC are lower than with the traditional epoxy damping material. Moreover, the validity of the proposed new way was verified by the simplified step-lap joint cores, which were achieved based on Epstein Frames.

Zhu, Lihua; Yang, Qingxin; Yan, Rongge; Li, Yongjian; Zhang, Xian; Yan, Weili; Zhu, Jianguo

2013-05-01

407

3D Magnetic Flux Measurement in Joint Region of a Model Core Stacked with Grain-Oriented Electrical Steel  

NASA Astrophysics Data System (ADS)

Grain-oriented electrical steels show a high degree of anisotropy that favors magnetization in the rolling direction. Magnetization also occurs in other directions in the joint regions that are characterized by interlaminar flux. For direct measurement of such flux behavior, flat sensor elements that do not create additional air gaps have been developed. The present study carried out on a model core. To determine the local flux around the joint rejoin, thin Cu film elements were vapor-deposited on both sides of individual laminates. Normal flux components were detected by frame coil arrangements, and in-plane components were determined by films with tip contacts through the coating. The obtained results showed that in the joint regions, the incoming in-plane flux is converted to interlaminar flux in the overlap region, until a high degree of global saturation is reached; at this stage the flux through the air gaps between the laminates leads to more homogeneous magnetization.

Yamaguchi, Hiroi; Pfützner, Helmut; Ishida, Masayoshi

408

Return of the Near 160 Day Periodicity in the Photospheric Magnetic Flux during Solar Cycle 23  

NASA Astrophysics Data System (ADS)

A periodicity between 152 and 158 days was discovered in the daily number of high-energy solar flares detected by the Solar Maximum Mission (SMM) and the Geosynchronous Operational Environmental Satellites (GOES) around the maximum of solar cycle 21. In a previous work we pointed out that this periodicity had been time-coincident with a periodic emergence of magnetic flux in the form of strong magnetic fields, which suggested a causal relationship between both periodicities. Using the Mount Wilson Sunspot Index, evidence is presented for the return of the periodicity in the strong photospheric magnetic flux during the current solar cycle. The periodicity has reappeared around the solar activity maximum with a frequency similar to that of solar cycle 21, but contrary to what happened during cycle 21, it is completely absent in energetic flares. A tentative explanation for this feature is that in the current solar cycle, part of the periodic emergence of magnetic flux has taken place away from already developed sunspot groups and so has not contributed to enhance their magnetic complexity, which has prevented the triggering of periodic energetic flares.

Ballester, J. L.; Oliver, R.; Carbonell, M.

2004-11-01

409

Vortex zero modes, large flux limit and Ambjřrn-Nielsen-Olesen magnetic instabilities  

NASA Astrophysics Data System (ADS)

In the large flux limit vortices become flux tubes with almost constant magnetic field in the interior region. This occurs in the case of non-Abelian vortices as well, and the study of such configurations allows us to reveal a close relationship between vortex zero modes and the gyromagnetic instabilities of vector bosons in a strong background magnetic field discovered by Nielsen, Olesen and Ambjřrn. The BPS vortices are exactly at the onset of this instability, and the dimension of their moduli space is precisely reproduced in this way. We present a unifying picture in which, through the study of the linear spectrum of scalars, fermions and W bosons in the magnetic field background, the expected number of translational, orientational, fermionic as well as semilocal zero modes is correctly reproduced in all cases.

Bolognesi, Stefano; Chatterjee, Chandrasekhar; Gudnason, Sven Bjarke; Konishi, Kenichi

2014-10-01

410

Anomalous electron trapping by magnetic flux tubes and electric current vortices  

E-print Network

We consider an electron with an anomalous magnetic moment, g>2, confined to a plane and interacting with a nonhomogeneous magnetic field B, and investigate the corresponding Pauli Hamiltonian. We prove a lower bound on the number of bound states for the case when B is of a compact support and the related flux is $N+\\epsilon, \\epsilon\\in(0,1]$. In particular, there are at least N+1 bound states if B does not change sign. We also consider the situation where the magnetic field is due to a localized rotationally symmetric electric current vortex in the plane. In this case the flux is zero; there is a pair of bound states for a weak coupling, and higher orbital-momentum "spin-down" states appearing as the current strength increases.

F. Bentosela; P. Exner; V. A. Zagrebnov

1998-11-26

411

Use of magnetic flux leakage (MFL) for the inspection of pipelines and storage tanks  

NASA Astrophysics Data System (ADS)

Vetco Pipeline Services, Inc. (VPSI) has been using the magnetic flux leakage (MFL) technique to inspect operating pipelines and above ground storage tanks for many years. This technique involves magnetically saturating the material under inspection and scanning the surface of the material for MFL fields caused by wall thinning defects. Recently, VPSI has adapted this technology for the inspection of in-service distribution pipelines. This new inspection system couples the MFL techniques used for pipeline inspection to a novel delivery system for the specific task of gathering real time data on the condition and integrity of distribution pipelines. The focus of this paper is to review this application of the magnetic flux leakage technique to describe the novel delivery systems developed and to detail the enabling technologies that now allow real time capture and analysis of inspection data.

Porter, Patrick C.

1995-05-01

412

Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere.  

PubMed

The structure of Earth's magnetosphere is poorly understood when the interplanetary magnetic field is northward. Under this condition, uncharacteristically energetic plasma is observed in the magnetotail lobes, which is not expected in the textbook model of the magnetosphere. Using satellite observations, we show that these lobe plasma signatures occur on high-latitude magnetic field lines that have been closed by the fundamental plasma process of magnetic reconnection. Previously, it has been suggested that closed flux can become trapped in the lobe and that this plasma-trapping process could explain another poorly understood phenomenon: the presence of auroras at extremely high latitudes, called transpolar arcs. Observations of the aurora at the same time as the lobe plasma signatures reveal the presence of a transpolar arc. The excellent correspondence between the transpolar arc and the trapped closed flux at high altitudes provides very strong evidence of the trapping mechanism as the cause of transpolar arcs. PMID:25525244

Fear, R C; Milan, S E; Maggiolo, R; Fazakerley, A N; Dandouras, I; Mende, S B

2014-12-19

413

A high flux of ultra-cold chromium atoms in a magnetic guide  

NASA Astrophysics Data System (ADS)

We report the observation of a very high flux of ultra-cold bosonic chromium atoms in a magnetic guide. The beam is created by operating a magneto-optical trap/moving optical molasses within the magnetic field of the guide. A relative detuning between two pairs of the cooling lasers cools the atoms into a frame moving along the axes of the guide. When the atoms are cooled into a moving frame with a velocity of 6 m s-1 we observe a maximum of the flux of 6 × 109 atoms s-1. For these parameters the transversal temperature of the atoms after a 25 fold increase of the confining magnetic potential is about 1.2 mK. The longitudinal temperature is 400 ľK.

Griesmaier, Axel; Aghajani-Talesh, Anoush; Falkenau, Markus; Sebastian, Jimmy; Greiner, Alexander; Pfau, Tilman

2009-07-01

414

Methods of reducing audible noise caused by magnetic components in variable-frequency-controlled switch-mode converters  

Microsoft Academic Search

Methods and a circuit implementation of audible noise reduction in switch-mode converters with variable switching frequency are presented. The audible noise caused by magnetic components is reduced by controlling the switching frequency so that it stays above the audible range as the load decreases. This is accomplished by decreasing the peak value of the main switch current pulses in discrete

Laszlo Huber; Milan M. Jovanovic

2011-01-01

415

Acoustic Noise of Switched Reluctance and Permanent Magnet Motors: A Comparison in the Context of Electric Brakes  

Microsoft Academic Search

It is held as fact that Permanent Magnet (PM) brushless motors are less noisy than Switched Reluctance (SR) motors, and many methods have been published to reduce the noise of SR drives. This paper provides a systematic experimental comparison of both drives, and of proposed noise- mitigation strategies, in the context of a specific mass-market application: Automotive Electro Mechanical Brakes

Avoki M. Omekanda; Suresh Gopalakrishnan; Harald Klode

2007-01-01

416

Development of a low-cost double rotor axial flux motor with soft magnetic composite and ferrite permanent magnet materials  

NASA Astrophysics Data System (ADS)

This paper proposes a low-cost double rotor axial flux motor (DRAFM) with low cost soft magnetic composite (SMC) core and ferrite permanent magnets (PMs). The topology and operating principle of DRAFM and design considerations for best use of magnetic materials are presented. A 905 W 4800 rpm DRAFM is designed for replacing the high cost NdFeB permanent magnet synchronous motor (PMSM) in a refrigerator compressor. By using the finite element method, the electromagnetic parameters and performance of the DRAFM operated under the field oriented control scheme are calculated. Through the analysis, it is shown that that the SMC and ferrite PM materials can be good candidates for low-cost electric motor applications.

Liu, Chengcheng; Zhu, Jianguo; Wang, Youhua; Guo, Youguang; Lei, Gang; Liu, Xiaojing

2015-05-01

417

Tidal disruption and magnetic flux capture: powering a jet from a quiescent black hole  

NASA Astrophysics Data System (ADS)

The transient Swift J1644+57 is believed to have been produced by an unlucky star wandering too close to a supermassive black hole (BH) leading to a tidal disruption event. This unusual flare displayed highly super-Eddington X-ray emission which likely originated in a relativistic, collimated jet. This presents challenges to modern accretion and jet theory as upper limits of prior BH activity, which we obtain from the radio afterglow of this event, imply that both the pre-disruption BH and stellar magnetic fluxes fall many orders of magnitude short of what is required to power the observed X-ray luminosity. We argue that a pre-existing, `fossil' accretion disc can contain a sufficient reservoir of magnetic flux and that the stellar debris stream is capable of dragging this flux into the BH. To demonstrate this, we perform local, 3D magnetohydrodynamic simulations of the disc-stream interaction and demonstrate that the interface between the two is unstable to mixing. This mixing entrains a sufficient amount of fossil disc magnetic flux into the infalling stellar debris to power the jet. We argue that the interaction with the fossil disc can have a pronounced effect on the structure and dynamics of mass fallback and likely the resulting transient. Finally, we describe possible ramifications of these interactions on unresolved problems in tidal disruption dynamics, in particular, the efficiency of debris circularization, and effects of the disruption on the pre-existing BH system.

Kelley, Luke Zoltan; Tchekhovskoy, Alexander; Narayan, Ramesh

2014-12-01

418

Equilibrium structure of solar magnetic flux tubes: Energy transport with multistream radiative transfer  

NASA Technical Reports Server (NTRS)

We examine the equilibrium structure of vertical intense magnetic flux tubes on the Sun. Assuming cylindrical geometry, we solve the magnetohydrostatic equations in the thin flux-tube approximation, allowing for energy transport by radiation and convection. The radiative transfer equation is solved in the six-stream approximation, assuming gray opacity and local thermodynamic equilibrium. This constitutes a significant improvement over a previous study, in which the transfer was solved using the multidimensional generalization of the Eddington approximation. Convection in the flux tube is treated using mixing-length theory, with an additional parameter alpha, characterizing the suppression of convective energy transport in the tube by the strong magnetic field. The equations are solved using the method of partial linearization. We present results for tubes with different values of the magnetic field strength and radius at a fixed depth in the atmosphere. In general, we find that, at equal geometric heights, the temperature on the tube axis, compared to the ambient medium, is higher in the photosphere and lower in the convection zone, with the dif