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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.  

National Technical Information Service (NTIS)

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 sup...

M. J. Ferrari

1991-01-01

3

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

4

Control of flux in magnetic circuits for Barkhausen noise measurements  

Microsoft Academic Search

The consistency of magnetic Barkhausen noise (MBN) measurements under applied sinusoidal magnetic field control and sinusoidal magnetic circuit flux control was investigated under variable circuit permeability conditions. A U-core electromagnet was used to provide the alternating magnetic excitation. The magnetic circuit permeability was changed by varying excitation magnet lift-off and by using samples with known magnetic anisotropy. By controlling the

Steven White; Thomas Krause; Lynann Clapham

2007-01-01

5

Investigating pipeline steel inhomogeneity with Magnetic Barkhausen Noise and Magnetic Flux Leakage  

Microsoft Academic Search

An understanding of pipeline steel characteristics under conditions of high stress and background magnetization is critical for interpretation of the Magnetic Flux Leakage signals obtained under similar conditions. The Barkhausen effect is due to abrupt and irreversible changes in the domain magnetization of a ferromagnetic material when it is subjected to a varying magnetic field. Magnetic Barkhausen Noise (MBN) is

A. Hsu; L. Clapham; D. L. Atherton

2000-01-01

6

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

7

Investigating pipeline steel inhomogeneity with Magnetic Barkhausen Noise and Magnetic Flux Leakage  

NASA Astrophysics Data System (ADS)

An understanding of pipeline steel characteristics under conditions of high stress and background magnetization is critical for interpretation of the Magnetic Flux Leakage signals obtained under similar conditions. The Barkhausen effect is due to abrupt and irreversible changes in the domain magnetization of a ferromagnetic material when it is subjected to a varying magnetic field. Magnetic Barkhausen Noise (MBN) is proposed as a viable NDE technique for the characterization of pipeline steel samples in conjunction with other widely used NDE methods. A quantity-``MBN Energy''-is defined, which allows the detection of sample features such as pipeline welds, bulk magnetic easy axes and manufacturing stresses indirectly. This work examines the analysis of the MBN signal for the retrieval of information from samples under such extreme circumstances. .

Hsu, A.; Clapham, L.; Atherton, D. L.

2000-05-01

8

Magnetic flux fluctuations due to eddy currents and thermal noise in metallic disks  

Microsoft Academic Search

We derive expressions for the magnetic flux in a circular loop due to eddy currents and thermal noise in coaxial metallic disks. The eddy currents are induced by an applied field that changes sinusoidally in time. We give expressions for the eddy current noise when the frequency of the applied field is very low as well as when it is

S. Uzunbajakau; A. P. Rijpma; J. Dolfsma; H. J. G. Krooshoop; H. J. M. ter Brake; M. J. Peters; H. Rogalla

2003-01-01

9

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

10

Suppression of magnetic flux noise in YBa sub 2 Cu sub 3 O sub 7-x flux transformers  

SciTech Connect

We have constructed hybrid magnetometers by coupling a flux transformer of YBa{sub 2}Cu{sub 3}O{sub 7-x} to a Nb-PbIn SQUID. The low frequency (1/f) noise is dominated by fluctuations in the supercurrent circulating in the transformer, driven by vortex motion. The application of a small static magnetic field induces a supercurrent that reversibly suppresses the noise, in quantitative agreement with a model of thermally activated vortex hopping between symmetrical pairs of pinning sites. A persistent current can be used to reduce the low frequency noise power of high-{Tc} flux transformers by at least one order of magnitude. 11 refs., 4 figs., 1 tab.

Ferrari, M.; Wellstood, F.C.; Kingston, J.J.; Johnson, M.; Clarke, J.

1991-08-01

11

Low-noise flux-gate magnetic-field sensors using ring- and rod-core geometries  

Microsoft Academic Search

We have fabricated ring-core and single-domain rod-core flux-gate magnetic field sensors with 1\\/f noise levels at 1 Hz of 1.4 pT\\/Hz and 3.5 pT\\/Hz, respectively. These noise sensitivities were achieved by applying an electrical current through the core of the flux gate to magnetically bias the magnetic rotation of the core perpendicular to the easy-axis direction. We also found that

R. H. Koch; J. R. Rozen

2001-01-01

12

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

Microsoft Academic Search

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 (MBNENERGY), defined as the time integral of the squared

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

1997-01-01

13

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

Microsoft Academic Search

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

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

1997-01-01

14

Minimizing 1\\/f Noise in Magnetic Sensors Using MEMS Flux Concentrators  

Microsoft Academic Search

The 1\\/f noise of new types of magnetoresistance sensors based on GMR and MTJ limits their sensitivities at low frequencies. Our approach for dealing with this problem is to shift the operating frequency to higher frequencies where the 1\\/f noise is much lower. The shift is accomplished by placing flux concentrators on MEMS flaps. Springs connecting the flaps are used

Alan Edelstein; Greg Fischer; Michael Pedersen; William Bernard; Shu-Fan Cheng; Edmund Nowak

2006-01-01

15

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

16

Use of magnetic Barkhausen noise and magnetic flux leakage signals for analysis of defects in pipeline steel  

SciTech Connect

Oil and gas pipelines are normally buried. They are inspected, while they are in service, by pumping tools through the lines. Inspection tools based on magnetic flux leakage (MFL) measurements are the most economical method used for detecting metal loss, such as corrosion pits. Here the authors report on studies of the effects of hoop and axial tensile stresses on magnetic flux leakage (MFL) signals from 50% penetration electrochemically milled pits eroded in line pipe steel. It is observed that stress can change the MFL signal by more than 50%, depending on the magnetization of the pipe wall. The studies were performed on pits created in both the absence and presence of 330-MPa hoop or axial tensile stress. The MFL results obtained in the two cases show detectable differences if the applied stress is high enough to create plastic deformation in the regions of stress concentration near the pits. The effect of stress applied during pit erosion is less than that when the same stress is applied during the subsequent measurements. Magnetic Barkhausen noise (MBN) measurements have been used to study the stress concentrations around electrochemically milled and mechanically drilled defects and have shown that significant additional stress and plastic deformation can be introduced during mechanical drilling. The MBN results are used to assist interpretation of the stress-dependent MFL results.

Mandal, K. [S.N. Bose National Center, Calcutta (India)] [S.N. Bose National Center, Calcutta (India); Dufour, D. [Pipetronix Ltd., Concord, Ontario (Canada)] [Pipetronix Ltd., Concord, Ontario (Canada); Atherton, D.L. [Queen`s Univ., Kingston, Ontario (Canada). Dept. of Physics] [Queen`s Univ., Kingston, Ontario (Canada). Dept. of Physics

1999-05-01

17

Low-noise flux-gate magnetic-field sensors using ring- and rod-core geometries  

NASA Astrophysics Data System (ADS)

We have fabricated ring-core and single-domain rod-core flux-gate magnetic field sensors with 1/f noise levels at 1 Hz of 1.4 pT/Hz and 3.5 pT/Hz, respectively. These noise sensitivities were achieved by applying an electrical current through the core of the flux gate to magnetically bias the magnetic rotation of the core perpendicular to the easy-axis direction. We also found that in the rod-core sensor, the spatial correlation lengths of the magnetic fluctuations were 25 and 40 mm with and without the biasing current. The cross-power spectrum magnitude at 1 Hz was less than 200 fT/Hz.

Koch, R. H.; Rozen, J. R.

2001-03-01

18

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

19

Suppression of magnetic flux noise in YBa{sub 2}Cu{sub 3}O{sub 7-x} flux transformers  

SciTech Connect

We have constructed hybrid magnetometers by coupling a flux transformer of YBa{sub 2}Cu{sub 3}O{sub 7-x} to a Nb-PbIn SQUID. The low frequency (1/f) noise is dominated by fluctuations in the supercurrent circulating in the transformer, driven by vortex motion. The application of a small static magnetic field induces a supercurrent that reversibly suppresses the noise, in quantitative agreement with a model of thermally activated vortex hopping between symmetrical pairs of pinning sites. A persistent current can be used to reduce the low frequency noise power of high-{Tc} flux transformers by at least one order of magnitude. 11 refs., 4 figs., 1 tab.

Ferrari, M.; Wellstood, F.C.; Kingston, J.J.; Johnson, M.; Clarke, J.

1991-08-01

20

Use of magnetic Barkhausen noise and magnetic flux leakage signals for analysis of defects in pipeline steel  

Microsoft Academic Search

We report on studies of the effects of hoop and axial tensile stresses on magnetic flux leakage (MFL) signals from 50% penetration electrochemically milled pits eroded in line pipe steel. It is observed that stress can change the MFL signal by more than 50%, depending on the magnetization of the pipe wall. The studies were performed on pits created in

K. Mandal; D. Dufour; D. L. Atherton

1999-01-01

21

Gamma-magnetic normalization - new effect to reduce flux-gate magnetometer noise level  

NASA Astrophysics Data System (ADS)

It is the author's opinion confirmed by numerous experiments, that the FGM noise level (NL) is determined not by Barkhausen jumps during the core remagnetization from positive to negative state as the majority of designers believe, but by non-repeatability of the magnetic domains transition from negative to positive states and back. This shows the way how to reduce the magnetic noise: to manufacture the magnetic material with a structure which will create conditions for magnetic domain walls to glide easily and uniformly when changing their orientation leading to minimal efforts at cyclic remagnetization. Ideally, such a material may be represented as a solid "liquid" with freely floating uniform magnetic domains without walls friction. To reduce the specific NL of the materials, several post-melting processing technologies were developed. A set of experiments made by many investigators has shown that the best results gives the magnetic materials annealing in vacuum or in any inert gas applying by this during all annealing time the alternative magnetic field, imitating core excitation field during FGM operation. If to accept the "solid liquid" model, this mechanism of NL decreasing has clear physical explanation: permanent re-magnetization of domains leads to the structural improvements favorable namely for the homogenization of transitions, rise of temperature gives necessary energy for the impurities liquidation. Probably, M. Acuna was the first who reported that during FGM operation in space its NL is decreasing with time and attributed this to the relaxation of mechanical stresses in the core material in weightlessness conditions [1]. We studied in details the conditions in which the core material is in space: weightlessness, vacuum and radiation. Mechanical stresses relaxation hypothesis was rejected because the internal forces in any solid body are much stronger as those to which a gravity force might have influence. Also the tests of FGM sensor in vacuum chamber both in operation and switched off conditions showed any influence of this parameter on NL. And radiation influence, the dose of which was selected approximately equal to yearly dose at near-Earth orbit (~10 krad), revealed interesting facts: the magnetic material itself and the FGM sensor in non-operation state showed any dependence on radiation, whereas this dose applied to the operating FGM sensor lead to the marked NL reducing. This new effect was named "gamma-magnetic normalization" and also can be explained by the accepted model: the mechanism is the same as above, but, because ?-quantum have much greater energy as thermal one, more "rigid" impurities are eliminated at ?-radiation action. The experimental results confirming this hypothesis are presented in the report. This work was partially supported by NASU Contract 1-71/11/1531. [1] Acuna M M, 2002. Space based magnetometers. Rev. Sci. Instr. 73(11) pp. 3717-36.

Korepanov, V.

2012-04-01

22

Investigation of weldments in Victoria-class submarine pressure-hull using magnetic flux leakage and Barkhausen noise  

NASA Astrophysics Data System (ADS)

Evaluation of the stress state within submarine hulls can contribute to risk assessments, which provide assurance that in-service induced stresses will not adversely affect the service life of the naval structure. The purpose of this study was to evaluate the feasibility of using magnetic NDE techniques for identification of stresses associated with weldments in two original pressure hulls of Canada's Victoria class submarines. Magnetic Flux Leakage (MFL) and flux-controlled Barkhausen Noise measurements were investigated for identification of patch boundaries and welds in two sections of Victoria-class submarine-hull steel. While MFL showed clear demarcation of weld boundaries, Barkhausen measurements did not provide sufficiently clear response to identify these features in submarine hull samples. For a better understanding of Barkhausen response, uniaxial tensile stress was investigated on separate samples of submarine steel. A nonlinear dependence of Barkhausen response was observed, with a weaker sensitivity to tensile stresses below 200 MPa. This behavior, combined with the presence of substantial surface compressive stresses, was used to explain the observed insensitivity of Barkhausen measurements to the presence of welds.

Samimi, A. A.; Babbar, V.; Krause, T. W.; Clapham, L.

2014-02-01

23

Signal-to-noise improvement of bio-magnetic signals using a flux-gate probe and real time signal processing  

NASA Astrophysics Data System (ADS)

We have recorded the magnetic signal generated by the human cardiac muscle with the help of a very sensitive flux-gate sensor. First, we have verified the feasibility of such detection inside a magnetically shielded room, then repeated this same experiment within our laboratory and in a low shielded environment. Results have shown the easy operation of a flux-gate sensor for this purpose and the efficiency of fast numerical processing in a low shielded environment. This leads in both cases to a voltage signal-to-noise ratio of about 5 within a 100 Hz bandwidth, which corresponds to the actual limit of the flux-gate probe.

Dolabdjian, C.; Saez, S.; Reyes Toledo, A.; Robbes, D.

1998-10-01

24

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

25

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

Microsoft Academic Search

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

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

1997-01-01

26

Theory of Flux-Flow Noise in Superconductors.  

National Technical Information Service (NTIS)

Voltage noise in magnetic flux-containing type-II superconductors whose linear dimensions are large by comparison with the field-dependent penetration depth lambda/sub B/ is attributable to the motion of singly-quantized flux vortices. The instantaneous v...

J. R. Clem

1978-01-01

27

Magnetic flux transport by dipolarizing flux bundles  

NASA Astrophysics Data System (ADS)

Dipolarizing flux bundles (DFBs) are traveling high-magnetic-field flux tubes in the magnetotail. Due to their high magnetic field and fast motion, DFBs are expected to be important transporters of magnetic flux from the near-earth reconnection site to the inner magnetosphere. Dipolarizing flux bundles are usually embedded in bursty bulk flows (BBFs) which have been shown to be the dominant contributor of tail flux transport. To establish the importance of DFBs in flux transport, we compare the flux transport done by the DFBs embedded in BBFs and that done by the BBFs using a statistical study. We found that DFBs not only are high-field regions of the BBFs but also correlate with the flow bursts inside the BBFs. As a result, the DFBs contribute ~70% of the BBF flux transport, although they take only ~30% of the BBF time. Considering the BBF contribution in tail flux transport, DFBs appear only 3% of the time but accomplish 40-70% of the tail flux transport--they are the major flux transporter of the tail. We also found that DFBs has higher flux transport rate when observed closer to Earth, in the tail's pre-midnight sector, farther away from the neutral sheet, and during substorm time; the high rate in the latter two conditions lead to more flux transport. In addition, DFBs appear more often during substorm time. As DFBs are the major flux transporter of the tail, these DFB flux transport characteristics may shape the entire magnetotail convection.

Liu, J.; Angelopoulos, V.; Zhou, X.

2013-12-01

28

Uncertainties in Solar Synoptic Magnetic Flux Maps  

NASA Astrophysics Data System (ADS)

Magnetic flux synoptic charts are critical for a reliable modeling of the corona and heliosphere. Until now, however, these charts were provided without uncertainty estimates. The uncertainties are due to instrumental noise in the measurements and to the spatial variance of the magnetic flux distribution that contributes to each bin in the synoptic chart. We describe here a simple method to compute synoptic magnetic flux maps and their corresponding magnetic flux spatial variance charts that can be used to estimate the uncertainty in the results of coronal models. We have tested this approach by computing a potential-field source-surface model of the coronal field for a Monte Carlo simulation of Carrington synoptic magnetic flux maps generated from the variance map. We show that these uncertainties affect both the locations of source-surface neutral lines and the distributions of coronal holes in the models.

Bertello, L.; Pevtsov, A. A.; Petrie, G. J. D.; Keys, D.

2014-07-01

29

Novel behavior of magnetic flux lines in type II superconductors  

Microsoft Academic Search

In this thesis we present several studies in the properties of magnetic flux lines in type II superconductors. 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 SFw

Gregory Allan Mohler

2001-01-01

30

Flux Compression Magnetic Nozzle  

NASA Technical Reports Server (NTRS)

In pulsed fusion propulsion schemes in which the fusion energy creates a radially expanding plasma, a magnetic nozzle is required to redirect the radially diverging flow of the expanding fusion plasma into a rearward axial flow, thereby producing a forward axial impulse to the vehicle. In a highly electrically conducting plasma, the presence of a magnetic field B in the plasma creates a pressure B(exp 2)/2(mu) in the plasma, the magnetic pressure. A gradient in the magnetic pressure can be used to decelerate the plasma traveling in the direction of increasing magnetic field, or to accelerate a plasma from rest in the direction of decreasing magnetic pressure. In principle, ignoring dissipative processes, it is possible to design magnetic configurations to produce an 'elastic' deflection of a plasma beam. In particular, it is conceivable that, by an appropriate arrangement of a set of coils, a good approximation to a parabolic 'magnetic mirror' may be formed, such that a beam of charged particles emanating from the focal point of the parabolic mirror would be reflected by the mirror to travel axially away from the mirror. The degree to which this may be accomplished depends on the degree of control one has over the flux surface of the magnetic field, which changes as a result of its interaction with a moving plasma.

Thio, Y. C. Francis; Schafer, Charles (Technical Monitor)

2001-01-01

31

Signal-to-noise improvement of bio-magnetic signals using a flux-gate probe and real time signal processing  

Microsoft Academic Search

We have recorded the magnetic signal generated by the human cardiac muscle with the help of a very sensitive flux-gate sensor. First, we have verified the feasibility of such detection inside a magnetically shielded room, then repeated this same experiment within our laboratory and in a low shielded environment. Results have shown the easy operation of a flux-gate sensor for

C. Dolabdjian; S. Saez; A. Reyes Toledo; D. Robbes

1998-01-01

32

The photospheric magnetic flux budget  

Microsoft Academic Search

The ensemble of bipolar regions and the magnetic network both contain a substantial and strongly variable part of the photospheric magnetic flux at any phase in the solar cycle. The time-dependent distribution of the magnetic flux over and within these components reflects the action of the dynamo operating in the solar interior. We perform a quantitative comparison of the flux

C. J. Schrijver; K. L. Harvey

1994-01-01

33

Mean square flux noise in SQUIDs and qubits: numerical calculations  

NASA Astrophysics Data System (ADS)

The performance of SQUIDs and superconducting qubits based on magnetic flux is degraded by the presence of magnetic flux noise with a spectral density scaling approximately inversely with frequency. It is generally accepted that the noise arises from the random reversal of spins on the surface of the superconductors. We introduce a numerical method of calculating the mean square flux noise \\langle {\\Phi }_{{}}^{2}\\rangle from independently fluctuating spins on the surface of thin-film loops of arbitrary geometry. By reciprocity, \\langle {\\Phi }_{{}}^{2}\\rangle is proportional to , where B(r) is the magnetic field generated by a circulating current around the loop and r varies over the loop surface. By discretizing the loop nonuniformly, we efficiently and accurately compute the current distribution and resulting magnetic field, which may vary rapidly across the loop. We use this method to compute \\langle {\\Phi }_{{}}^{2}\\rangle in a number of scenarios in which we systematically vary physical parameters of the loop. We compare our simulations to an earlier analytic result predicting that \\langle {\\Phi }_{{}}^{2}\\rangle \\propto R/W in the limit where the loop radius R is much greater than the linewidth W. We further show that the previously neglected contribution of edge spins to \\langle {\\Phi }_{{}}^{2}\\rangle is significant—even dominant—in narrow-linewidth loops.

Anton, S. M.; Sognnaes, I. A. B.; Birenbaum, J. S.; O'Kelley, S. R.; Fourie, C. J.; Clarke, John

2013-07-01

34

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

35

Permanent Magnet Flux-Biased Magnetic Actuator with Flux Feedback.  

National Technical Information Service (NTIS)

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 singl...

N. J. Groom

1989-01-01

36

Physics of magnetic flux ropes  

NASA Astrophysics Data System (ADS)

The present work encompasses papers on the structure, waves, and instabilities of magnetic flux ropes (MFRs), photospheric flux tubes (PFTs), the structure and heating of coronal loops, solar prominences, coronal mass ejections and magnetic clouds, flux ropes in planetary ionospheres, the magnetopause, magnetospheric field-aligned currents and flux tubes, and the magnetotail. Attention is given to the equilibrium of MFRs, resistive instability, magnetic reconnection and turbulence in current sheets, dynamical effects and energy transport in intense flux tubes, waves in solar PFTs, twisted flux ropes in the solar corona, an electrodynamical model of solar flares, filament cooling and condensation in a sheared magnetic field, the magnetopause, the generation of twisted MFRs during magnetic reconnection, ionospheric flux ropes above the South Pole, substorms and MFR structures, evidence for flux ropes in the earth magnetotail, and MFRs in 3D MHD simulations.

Russell, C. T.; Priest, E. R.; Lee, L. C.

37

Vortex Fluctuations in High- Tc Films: Flux Noise Spectrum and Complex Impedance  

NASA Astrophysics Data System (ADS)

The flux noise spectrum and complex impedance for a 500 Ĺ thick YBCO film are measured and compared with predictions for two-dimensional vortex fluctuations. It is verified that the complex impedance and the flux noise spectra are proportional to each other, that the logarithm of the flux noise spectra for different temperatures has a common tangent with slope ~-1, and that the amplitude of the noise decreases as d-3, where d is the height above the film at which the magnetic flux is measured. A crossover from normal to anomalous vortex diffusion is indicated by the measurements and is discussed in terms of a two-dimensional decoupling.

Festin, Örjan; Svedlindh, Peter; Kim, Beom Jun; Minnhagen, Petter; Chakalov, Radoslav; Ivanov, Zdravko

1999-12-01

38

The photospheric magnetic flux budget  

NASA Technical Reports Server (NTRS)

The ensemble of bipolar regions and the magnetic network both contain a substantial and strongly variable part of the photospheric magnetic flux at any phase in the solar cycle. The time-dependent distribution of the magnetic flux over and within these components reflects the action of the dynamo operating in the solar interior. We perform a quantitative comparison of the flux emerging in the ensemble of magnetic bipoles with the observed flux content of the solar photosphere. We discuss the photospheric flux budget in terms of flux appearance and disappearance, and argue that a nonlinear dependence exists between the flux present in the photosphere and the rate of flux appearance and disappearance. In this context, we discuss the problem of making quantitative statements about dynamos in cool stars other than the Sun.

Schrijver, C. J.; Harvey, K. L.

1994-01-01

39

The photospheric magnetic flux budget  

Microsoft Academic Search

The ensemble of bipolar regions and the magnetic network both contain a substantial and strongly variable part of the photospheric\\u000a magnetic flux at any phase in the solar cycle. The time-dependent distribution of the magnetic flux over and within these\\u000a components reflects the action of the dynamo operating in the solar interior. We perform a quantitative comparison of the\\u000a flux

C. J. Schrijver; K. L. Harvey

1994-01-01

40

Noise properties of high-Tc superconducting flux transformers fabricated using chemical-mechanical polishing  

NASA Astrophysics Data System (ADS)

Reproducible high-temperature superconducting multilayer flux transformers were fabricated using chemical mechanical polishing. The measured magnetic field noise of the flip-chip magnetometer based on one such flux transformer with a 9 × 9 mm2 pickup loop coupled to a bicrystal dc SQUID was 15 fT/Hz1/2 above 2 kHz. We present an investigation of excess 1/f noise observed at low frequencies and its relationship with the microstructure of the interlayer connections within the flux transformer. The developed high-Tc SQUID magnetometers may be advantageous in ultra-low field magnetic resonance imaging and, with improved low frequency noise, magnetoencephalography applications.

Chukharkin, M.; Kalabukhov, A.; Schneiderman, J. F.; Öisjöen, F.; Snigirev, O.; Lai, Z.; Winkler, D.

2012-07-01

41

Fundamental limits to magnetic-field sensitivity of flux-gate magnetic-field sensors  

Microsoft Academic Search

In this letter we determine the theoretical limit of the magnetic-field sensitivity of the flux-gate magnetometer. In order to do so, we have developed a model for the white noise of a flux gate based on the fundamental dynamics of the magnetic material forming the flux-gate core. Solving this model, we predict that the white noise of a physically realizable

R. H. Koch; J. G. Deak; G. Grinstein

1999-01-01

42

Magnetic noise in structured hard magnets.  

PubMed

The anomalous Hall effect of wires patterned from (Co{90}Fe{10}/Pt){n} multilayers, with 10magnetization process in a small volume of maze domains. Time-independent 1/f noise appears in samples with a quality factor Q<1 at points on the hysteresis loop where the magnetization reverses continuously. The magnetic noise is associated with reversible excursions of segments of a domain wall approximately 100 nm long. Barkhausen jumps are observed close to either the switching field or the saturation field where the noise power spectrum varies as 1/f{1.7}, and its magnitude decays with time. PMID:20366735

Diao, Zhu; Nowak, E R; Feng, Gen; Coey, J M D

2010-01-29

43

Magnetic Flux Transfer by the Solar Wind  

Microsoft Academic Search

We calculated the change of the magnetic flux through the surface encircled by the Earth's orbit. This change is associated with the magnetic flux transfer by the solar wind flow and exhibits clear 22-year periodicity. The magnetic flux transferred by the solar wind is of the same order of magnitude as the flux of the main solar magnetic field through

P. L. Israelevich; A. I. Ershkovich; T. I. Gombosi

2001-01-01

44

Fundamental limits to magnetic-field sensitivity of flux-gate magnetic-field sensors  

NASA Astrophysics Data System (ADS)

In this letter we determine the theoretical limit of the magnetic-field sensitivity of the flux-gate magnetometer. In order to do so, we have developed a model for the white noise of a flux gate based on the fundamental dynamics of the magnetic material forming the flux-gate core. Solving this model, we predict that the white noise of a physically realizable flux gate with a volume of 2×10-8 m3 is less than 100 fT/Hz. The white noise varies with the lossy susceptibility of the core and inversely with the volume. We also compare the measured white noise of a thin-film flux gate with the predictions of our model and find that the measured and predicted noise agree reasonably well.

Koch, R. H.; Deak, J. G.; Grinstein, G.

1999-12-01

45

Noise Measurement in Magnetic Tunnel Junctions  

NASA Astrophysics Data System (ADS)

An investigation has been carried out on the low frequency noise in magnetic tunnel junctions in various magnetic states. The noise measurements show similar 1/f spectra in either the parallel or antiparallel magnetic state. The transition region of applied magnetic field, switching from parallel to antiparallel and vice versa, indicates a 1/f^2 spectrum along with an increase in the low frequency noise. This 1/f^2 spectrum and noise increase is found to be an artifact resulting from the magnetic after effect which occurs in the magnetic switching regions. Removing the effects of the magnetic after effect, only 1/f spectra are measured. The shot noise at higher frequency where the 1/f noise is negligible is also found in the tunnel junctions. Though it appears to be linear with the applied current, the shot noise observed can be largely enhanced.

Guo, Feng; Dahlberg, Dan

2010-03-01

46

Helioseismic Detection of Emerging Magnetic Flux  

NASA Astrophysics Data System (ADS)

Investigating the properties of magnetic flux emergence is one of the most important problems of solar physics. In this study we present a newly developed deep-focus time-distance measurement scheme which is able to detect strong emerging flux events in the deep solar interior, before the flux becomes visible on the surface. We discuss in detail the differences between our method and previous methods, and demonstrate step-by-step how the signal-to-noise (S/N) ratio is increased. The method is based on detection of perturbations in acoustic phase travel times determined from cross-covariances of solar oscillations observed on the surface. We detect strong acoustic travel-time reductions of an order of 12 - 16 seconds at a depth of 42 - 75 Mm. These acoustic anomalies are detected 1 - 2 days before high peaks in the photospheric magnetic flux rate implying that the average emerging speed is 0.3 - 0.6 km s-1. The results of this work contribute to our understanding of solar magnetism and benefit space weather forecasting.

Ilonidis, S.; Zhao, J.; Kosovichev, A. G.

2012-09-01

47

DISCONNECTING OPEN SOLAR MAGNETIC FLUX  

SciTech Connect

Disconnection of open magnetic flux by reconnection is required to balance the injection of open flux by coronal mass ejections and other eruptive events. Making use of recent advances in heliospheric background subtraction, we have imaged many abrupt disconnection events. These events produce dense plasma clouds whose distinctive shape can now be traced from the corona across the inner solar system via heliospheric imaging. The morphology of each initial event is characteristic of magnetic reconnection across a current sheet, and the newly disconnected flux takes the form of a 'U-'shaped loop that moves outward, accreting coronal and solar wind material. We analyzed one such event on 2008 December 18 as it formed and accelerated at 20 m s{sup -2} to 320 km s{sup -1}, thereafter expanding self-similarly until it exited our field of view 1.2 AU from the Sun. From acceleration and photometric mass estimates we derive the coronal magnetic field strength to be 8 {mu}T, 6 R{sub Sun} above the photosphere, and the entrained flux to be 1.6 Multiplication-Sign 10{sup 11} Wb (1.6 Multiplication-Sign 10{sup 19} Mx). We model the feature's propagation by balancing inferred magnetic tension force against accretion drag. This model is consistent with the feature's behavior and accepted solar wind parameters. By counting events over a 36 day window, we estimate a global event rate of 1 day{sup -1} and a global solar minimum unsigned flux disconnection rate of 6 Multiplication-Sign 10{sup 13} Wb yr{sup -1} (6 Multiplication-Sign 10{sup 21} Mx yr{sup -1}) by this mechanism. That rate corresponds to {approx} - 0.2 nT yr{sup -1} change in the radial heliospheric field at 1 AU, indicating that the mechanism is important to the heliospheric flux balance.

DeForest, C. E.; Howard, T. A.; McComas, D. J. [Southwest Research Institute, 1050 Walnut Street Suite 300, Boulder, CO 80302 (United States)

2012-01-20

48

Interactions between magnetic flux ropes  

NASA Astrophysics Data System (ADS)

Magnetic flux ropes are structures, which consist of magnetic field lines with pitch varying with radius across the rope, a result of currents flowing along the field. They exist in abundance in the solar corona and have been observed by satellites near the earth. They often exist in pairs or triplets. Flux ropes have been produced and the interaction and reconnection between two ropes has been studied in detail in the LAPD device at UCLA. In this experiment the space-time behavior of three kink unstable flux ropes in a background magnetoplasma are studied. The ropes twist about each other and themselves as well as collide with one another. The ropes are line tied on one end and are free to move at the other end, where they are observed to rotate around the background magnetic field. The quasi-separatrix layers (regions in which magnetic field line reconnection occurs and neighboring field lines diverge) associated with the ropes are derived from the 3D data and their motion and topology is studied. LIF measurements indicate significant ion heating in the ropes. Swept Langmuir probe measurements indicate large density and temperature changes. These give rise to large amplitude drift waves.

Gekelman, Walter; van Compernolle, Bart

2011-11-01

49

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

50

Chaos in Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

Magnetic Flux Ropes Immersed in a uniform magnetoplasma are observed to twist about themselves, writhe about each other and rotate about a central axis. They are kink unstable and smash into one another as they move. Each collision results in magnetic field line generation and the generation of a quasi-seperatrix layer. Three dimensional magnetic field lines are computed by conditionally averaging the data using correlation techniques. When the currents associated with the ropes are large,this is possible for only a number of rotation cycles as the field line motion becomes chaotic. The permutation entropy1 can be calculated from the the time series of the magnetic field data (this is also done with flows) and used to calculate the positions of the data on a Jensen Shannon complexity map2. The power spectra of much of the magnetic and flow data is exponential and Lorentzian structures in the time domain are embedded in them. The location of data on this map indicates if the magnetic fields are stochastic, or fall into regions of minimal or maximal complexity. The complexity is a function of space and time. The complexity map, and analysis will be explained in the course of the talk. Other types of chaotic dynamical models such as the Lorentz or Gissinger process also fall on the map and can give a clue to the nature of the flux rope turbulence. The ropes fall in the region of the C-H plane where chaotic systems lie. 1 C. Bandt, B. Pompe, Phys. Rev. Lett., 88,174102 (2007) 2 O. Russo et al., Phys. Rev. Lett., 99, 154102 (2007), J. Maggs, G.Morales, “Permutation Entropy analysis of temperature fluctuations from a basic electron heat transport experiment”,submitted PPCF (2013)

Gekelman, Walter; DeHaas, T.; Van Compernolle, B.; Vincena, S.

2013-07-01

51

Solar Photospheric Magnetic Flux Tubes: Observations  

Microsoft Academic Search

The magnetic field on the solar surface (SOLAR MAGNETIC FIELD) is highly filamentary, with a considerable fraction being in the form of flux tubes (roughly speaking bundles of concentrated field lines). Small-scale flux tubes have diameters below approximately 400 km. They appear bright and are the basic magnetic features underlying the magnetic network and active region plages (SOLAR CHROMOSPHER...

S. Solanki

2000-01-01

52

Solar Photospheric Magnetic Flux Tubes: Observations  

NASA Astrophysics Data System (ADS)

The magnetic field on the solar surface (SOLAR MAGNETIC FIELD) is highly filamentary, with a considerable fraction being in the form of flux tubes (roughly speaking bundles of concentrated field lines). Small-scale flux tubes have diameters below approximately 400 km. They appear bright and are the basic magnetic features underlying the magnetic network and active region plages (SOLAR CHROMOSPHER...

Solanki, S.; Murdin, P.

2000-11-01

53

Flux emergence, flux imbalance, magnetic free energy and solar flares  

NASA Astrophysics Data System (ADS)

Emergence of complex magnetic flux in the solar active regions lead to several observational effects such as a change in sunspot area and flux embalance in photospheric magnetograms. The flux emergence also results in twisted magnetic field lines that add to free energy content. The magnetic field configuration of these active regions relax to near potential-field configuration after energy release through solar flares and coronal mass ejections. In this paper, we study the relation of flare productivity of active regions with their evolution of magnetic flux emergence, flux imbalance and free energy content. We use the sunspot area and number for flux emergence study as they contain most of the concentrated magnetic flux in the active region. The magnetic flux imbalance and the free energy are estimated using the HMI/SDO magnetograms and Virial theorem method. We find that the active regions that undergo large changes in sunspot area are most flare productive. The active regions become flary when the free energy content exceeds 50% of the total energy. Although, the flary active regions show magnetic flux imbalance, it is hard to predict flare activity based on this parameter alone.

Choudhary, Debi Prasad; Gosain, Sanjay; Gopalswamy, Nat; Manoharan, P. K.; Chandra, R.; Uddin, W.; Srivastava, A. K.; Yashiro, S.; Joshi, N. C.; Kayshap, P.; Dwivedi, V. C.; Mahalakshmi, K.; Elamathi, E.; Norris, Max; Awasthi, A. K.; Jain, R.

2013-10-01

54

Acoustic noise during functional magnetic resonance imaginga)  

PubMed Central

Functional magnetic resonance imaging (fMRI) enables sites of brain activation to be localized in human subjects. For studies of the auditory system, acoustic noise generated during fMRI can interfere with assessments of this activation by introducing uncontrolled extraneous sounds. As a first step toward reducing the noise during fMRI, this paper describes the temporal and spectral characteristics of the noise present under typical fMRI study conditions for two imagers with different static magnetic field strengths. Peak noise levels were 123 and 138 dB re 20 ?Pa in a 1.5-tesla (T) and a 3-T imager, respectively. The noise spectrum (calculated over a 10-ms window coinciding with the highest-amplitude noise) showed a prominent maximum at 1 kHz for the 1.5-T imager (115 dB SPL) and at 1.4 kHz for the 3-T imager (131 dB SPL). The frequency content and timing of the most intense noise components indicated that the noise was primarily attributable to the readout gradients in the imaging pulse sequence. The noise persisted above background levels for 300-500 ms after gradient activity ceased, indicating that resonating structures in the imager or noise reverberating in the imager room were also factors. The gradient noise waveform was highly repeatable. In addition, the coolant pump for the imager’s permanent magnet and the room air handling system were sources of ongoing noise lower in both level and frequency than gradient coil noise. Knowledge of the sources and characteristics of the noise enabled the examination of general approaches to noise control that could be applied to reduce the unwanted noise during fMRI sessions.

Ravicz, Michael E.; Melcher, Jennifer R.; Kiang, Nelson Y.-S.

2007-01-01

55

Chaos in magnetic flux ropes  

NASA Astrophysics Data System (ADS)

Magnetic flux ropes immersed in a uniform magnetoplasma are observed to twist about themselves, writhe about each other and rotate about a central axis. They are kink unstable and smash into one another as they move. Each collision results in magnetic field line reconnection and the generation of a quasi-separatrix layer. Three-dimensional magnetic field lines are computed by conditionally averaging the data using correlation techniques. Conditional averaging is possible for only a number of rotation cycles as the field line motion becomes chaotic. The permutation entropy can be calculated from the time series of the magnetic field data (this is also done with flows) and is used to calculate the positions of the data on a Jensen–Shannon complexity map. The location of data on this map indicates if the magnetic fields are stochastic, or fall into regions of minimal or maximal complexity. The complexity is a function of space and time. The Lyapunov and Hurst exponents are calculated and the complexity and permutation entropy of the flows and field components are shown throughout the volume.

Gekelman, Walter; Van Compernolle, Bart; DeHaas, Tim; Vincena, Stephen

2014-06-01

56

Chaos in Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

Magnetic Flux Ropes Immersed in a uniform magnetoplasma are observed to twist about themselves, writhe about each other and rotate about a central axis. They are kink unstable and smash into one another as they move. Full three dimensional magnetic field and flows are measured at thousands of time steps. Each collision results in magnetic field line generation and the generation of a quasi-seperatrix layer and induced electric fields. Three dimensional magnetic field lines are computed by conditionally averaging the data using correlation techniques. The permutation entropy1 ,which is related to the Lyapunov exponent, can be calculated from the the time series of the magnetic field data (this is also done with flows) and used to calculate the positions of the data on a Jensen Shannon complexity map2. The location of data on this map indicates if the magnetic fields are stochastic, or fall into regions of minimal or maximal complexity. The complexity is a function of space and time. The complexity map, and analysis will be explained in the course of the talk. Other types of chaotic dynamical models such as the Lorentz, Gissinger and Henon process also fall on the map and can give a clue to the nature of the flux rope turbulence. The ropes fall in the region of the C-H plane where chaotic systems lie. The entropy and complexity change in space and time which reflects the change and possibly type of chaos associated with the ropes. The maps give insight as to the type of chaos (deterministic chaos, fractional diffusion , Levi flights..) and underlying dynamical process. The power spectra of much of the magnetic and flow data is exponential and Lorentzian structures in the time domain are embedded in them. Other quantities such as the Hurst exponent are evaluated for both magnetic fields and plasma flow. Work Supported by a UC-LANL Lab fund and the Basic Plasma Science Facility which is funded by DOE and NSF. 1) C. Bandt, B. Pompe, Phys. Rev. Lett., 88,174102 (2007) 2) O. Russo et al., Phys. Rev. Lett., 99, 154102 (2007), J. Maggs, G.Morales, 55, 085015 (2013)

Gekelman, W. N.; DeHaas, T.; Van Compernolle, B.

2013-12-01

57

Magnetostrictive offset and noise in flux gate magnetometers  

Microsoft Academic Search

Longitudinal magnetostriction of the core material is found to be the principal source of offset and noise in present state-of-the-art flux gate magnetometers. Magnetostrictive offset is identified with even harmonics of the steady-state induced EMF resulting from the periodic elongation (or contraction) of the core material. Magnetostrictive noise is identified with random fluctuations of the magnetostrictive offset caused by frictional

M. Weiner

1969-01-01

58

Magnetic Properties of Photospheric Regions with Very Low Magnetic Flux  

NASA Astrophysics Data System (ADS)

The magnetic properties of the quiet Sun are investigated using a novel inversion code, FATIMA, based on the Principal Component Analysis of the observed Stokes profiles. The stability and relatively low noise sensitivity of this inversion procedure allows for the systematic inversion of large data sets with a very weak polarization signal. Its application to quiet-Sun observations of network and internetwork regions reveals that a significant fraction of the quiet-Sun contains kilogauss fields (usually with very small filling factors) and confirms that the pixels with weak polarization account for most of the magnetic flux. Mixed polarities in the resolution element are also found to occur more likely as the polarization weakens.

Socas-Navarro, H.; Sánchez Almeida, J.

2002-02-01

59

Noise and stability of magnetic sensors  

Microsoft Academic Search

High-TC SQUID sensors having 0.4 pT\\/?Hz at 1 Hz noise at liquid nitrogen temperature are already superior to fluxgate sensors (2.5 pT\\/?Hz at 1 Hz, 50 pT p-p). Ac-flipped permalloy magnetoresistors have offset of 10 NT and 15 nT p-p noise. The noise properties of semiconductor, induction and other types of magnetic field sensors are discussed.

P. Ripka

1996-01-01

60

Flux noise in SQUIDs: calculations of geometrical dependence  

NASA Astrophysics Data System (ADS)

Low frequency (1/f) magnetic flux noise in SQUIDs is understood to arise from the random reversal of electron spins localized at the surface of the superconducting film. Analytical results^1 that assume independent electron spins predict that the spectral density at 1 Hz scales with the washer geometry approximately as R/W in the limit R/W1. Here, R is the outer radius and W is the linewidth. We present numerical calculations that reproduce the analytical result in the appropriate limit and extend these results to arbitrary values of R/W. In addition, a logarithmic dependence on W, evident when R/W is fixed and W is varied, is reproduced and discussed. The contribution of spins at the edge of the film is also computed. We compare the predicted geometrical scaling to our recent measurements of several SQUIDs with varying geometries. Our calculation that is valid for all values of R/W enables us to investigate a possible breakdown of the independent spin model in our experimental data. ^1R. C. Bialczak et al., Phys. Rev. Lett. 99, 187006 (2007).

O'Kelley, Sean; Pepper, Keenan; Anton, Steven; Birenbaum, Jeffrey; Clarke, John

2012-02-01

61

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

62

Solar Photospheric Magnetic Flux Tubes: Theory  

Microsoft Academic Search

The magnetic field in the photospheric layers of the Sun is found to occur not in a homogeneous form but in discrete concentrations of intense field. The most obvious form of magnetic flux (magnetic field strength times surface area occupied by the field) is seen in SUNSPOTS but it turns out that much smaller arrangements of magnetic field are to

B. Roberts

2000-01-01

63

Regulation of the interplanetary magnetic flux  

SciTech Connect

In this study we use a recently developed technique for measuring the 2-D magnetic flux in the ecliptic plane to examine (1) the long term variation of the magnetic flux in interplanetary space and (2) the apparent rate at which coronal mass ejections (CMEs) may be opening new flux from the Sun. Since there is a substantial variation ({approximately}50%) of the flux in the ecliptic plane over the solar cycle, we conclude that there must be some means whereby new flux can be opened from the Sun and previously open magnetic flux can be closed off. We briefly describe recently discovered coronal disconnections events which could serve to close off previously open magnetic flux. CMEs appear to retain at least partial magnetic connection to the Sun and hence open new flux, while disconnections appear to be likely signatures of the process that returns closed flux to the Sun; the combination of these processes could regulate the amount of open magnetic flux in interplanetary space. 6 refs., 3 figs.

McComas, D.J.; Gosling, J.T.; Phillips, J.L.

1991-01-01

64

Eddy Current Measurement of Magnetic Flux Density.  

National Technical Information Service (NTIS)

The report presents the results of an effort to improve the reliability of magnetic particle inspection of large or irregular-shaped parts by assuring optimum magnetic flux densities in the areas of interest during inspection. An assessment of flux densit...

I. R. Kraska R. G. Prusinski

1972-01-01

65

Analysis of Vibration and Acoustic Noise in Permanent Magnet Motors.  

NASA Astrophysics Data System (ADS)

The drive motor is a frequent source of vibration and acoustic noise in many precision spindle motors. One of the electromagnetic sources of vibration in permanent magnet motors is the torque ripple, consisting of the reluctance torque and electromagnetic torque fluctuation. This type of vibration is becoming more serious with the advent of new high-grade magnets with increased flux density. Acoustic noise of electromagnetic origin is difficult to predict and its exact mechanism is unclear. The mechanism of noise generation should be revealed to design a quieter motor which is the modern customer's demand. For motor operation at low speeds and loads, torque ripple due to the reluctance torque is often a source of vibration and control difficulty. The reluctance torque in a motor was calculated from the flux density by a finite element method and the Maxwell stress method. Effects of design parameters, such as stator slot width, permanent slot width, airgap length and magnetization direction, were investigated. Magnet pole shaping, by gradually decreasing the magnet thickness toward edges, yields a sinusoidal shape of the reluctance torque with reduced harmonics, thus reducing the vibration. This dissertation also presents two motor design techniques: stator tooth notching and rotor pole skewing with magnet pole shaping, and the effect of each method on the output torque. The analysis shows that the reluctance torque can be nearly eliminated by the suggested designs, with minimal sacrifice of the output torque. In permanent magnet DC motors, the most popular design type is the trapezoidal back electro-motive force (BEMF), for switched DC controllers. It is demonstrated that the output torque profile of one phase energized is qualitatively equivalent to the BEMF profile for motors with reduced reluctance torque. It implies that design of BEMF profile is possible by magnetic modeling of a motor, without expensive and time-consuming experiments for different designs. The effect of various design parameters on the output torque and torque ripple are discussed. Design parameters include winding patterns, magnetization direction, magnet arc length, number of segments in poles and magnet pole shaping. New designs of trapezoidal BEMF motors are proposed to reduce the electromagnetic torque ripple. Magnet stepping and magnet edge shaping with reduced arc length, significantly reduce torque ripple, with minimal sacrifice of the maximum output torque. Acoustic noise of electromagnetic origin is investigated using a magnetic frame which emulates a DC motor. The driving electromagnetic force is calculated using finite element analysis and the resulting vibration and acoustic noise is measured. Acoustic noise of purely electromagnetic origin was also tested with a DC brushless motor to confirm the results of the magnetic frame. The mechanism of noise generation in a DC motor is a quasi-static response of a stator not only at the fundamental frequency but also at higher harmonic frequencies of alternating switched DC, which is a current characteristic of a DC motor. Noise generation is significantly aggravated when some of those harmonics are close to the resonant frequencies of the stator. Therefore, acoustic noise is highly dependent upon the excitation current shape, as higher harmonics may match with resonant frequencies of the stator.

Hwang, Sangmoon

66

Magnetizer design of circumferential magnetic flux leakage inspection in pipeline  

Microsoft Academic Search

In order to reasonably design magnetizer of circumferential magnetic flux leakage inspection in pipeline, the magnetizer structure that has the circular geometry of quadrupole separation is proposed which meets engineering design requirements with the analysison and comparison of the finite element simulation. After the application of magnetic circuit theory, the influence of the geometric size of permanent magnets on MFL

Zhang Guoguang

2010-01-01

67

Magnetic flux concentration methods for magnetic energy harvesting module  

NASA Astrophysics Data System (ADS)

This paper presents magnetic flux concentration methods for magnetic energy harvesting module. The purpose of this study is to harvest 1 mW energy with a Brooks coil 2 cm in diameter from environmental magnetic field at 60 Hz. Because the harvesting power is proportional to the square of the magnetic flux density, we consider the use of a magnetic flux concentration coil and a magnetic core. The magnetic flux concentration coil consists of an air­core Brooks coil and a resonant capacitor. When a uniform magnetic field crossed the coil, the magnetic flux distribution around the coil was changed. It is found that the magnetic field in an area is concentrated larger than 20 times compared with the uniform magnetic field. Compared with the air­core coil, our designed magnetic core makes the harvested energy ten­fold. According to ICNIRP2010 guideline, the acceptable level of magnetic field is 0.2 mT in the frequency range between 25 Hz and 400 Hz. Without the two magnetic flux concentration methods, the corresponding energy is limited to 1 ľW. In contrast, our experimental results successfully demonstrate energy harvesting of 1 mW from a magnetic field of 0.03 mT at 60 Hz.

Tashiro, Kunihisa; Hattori, Gen-ya; Wakiwaka, Hiroyuki

2013-01-01

68

On the power spectrum of magnetization noise  

NASA Astrophysics Data System (ADS)

Understanding the power spectrum of the magnetization noise is a long standing problem. While the earlier work considered superposition of 'elementary' jumps, without reference to the underlying physics, recent approaches relate the properties of the noise with the critical dynamics of domain walls. In particular, a new derivation of the power spectrum exponent has been proposed for the random-field Ising model. We apply this approach to the experimental data, showing its validity and limitations.

Durin, G.; Zapperi, S.

2002-04-01

69

Sensor noise in low-level flux-gate magnetometers  

Microsoft Academic Search

The noise mechanism in a well-designed magnetometer is shown to be due to small-scale magnetic moments which are probably small volumes of the core material that are not oriented by the drive field. Design developments include the sense coil radius and the development of a drive waveform which yields considerably improved performance over the commonly used sine wave drive.

DONALD C. SCOUTEN

1972-01-01

70

Sensor noise in low-level flux-gate magnetometers.  

NASA Technical Reports Server (NTRS)

The noise mechanism in a well-designed magnetometer is shown to be due to small-scale magnetic moments which are probably small volumes of the core material that are not oriented by the drive field. Design developments include the sense coil radius and the development of a drive waveform which yields considerably improved performance over the commonly used sine wave drive.

Scouten, D. C.

1972-01-01

71

HELICAL LENGTHS OF MAGNETIC CLOUDS FROM THE MAGNETIC FLUX CONSERVATION  

SciTech Connect

We estimate axial lengths of helical parts in magnetic clouds (MCs) at 1 AU from the magnetic flux (magnetic helicity) conservation between solar active regions (ARs) and MCs with the event list of Leamon et al. Namely, considering poloidal magnetic flux (PHI{sub P}) conservation between MCs and ARs, we estimated L{sub h} in MCs, where L{sub h} is the axial length of an MC where poloidal magnetic flux and magnetic twist exist. It is found that L{sub h} is 0.01-1.25 AU in the MCs. If the cylinder flux rope picture is assumed, this result leads to a possible new picture of the cylinder model whose helical structure (namely, poloidal magnetic flux) localizes in a part of a MC.

Yamamoto, Tetsuya T. [Solar-Terrestrial Environment Laboratory, Nagoya University, Chikusa, Nagoya, 464-8601 (Japan); Kataoka, R. [Interactive Research Center of Science, Tokyo Institute of Technology, Meguro, 152-8550 (Japan); Inoue, S., E-mail: tyamamot@stelab.nagoya-u.ac.j [National Institute of Information and Communications Technology, Koganei, 184-8795 (Japan)

2010-02-10

72

Flux Noise Probed with Real Time Qubit Tomography in a Josephson Phase Qubit  

NASA Astrophysics Data System (ADS)

We measure the dependence of qubit phase coherence and flux noise on inductor loop geometry. While wider inductor traces change neither the flux noise power spectrum nor the qubit dephasing time, increased inductance leads to a simultaneous increase in both. Using our new tomographic protocol for measuring low frequency flux noise, we make a direct comparison between the flux noise spectrum and qubit phase decay, finding agreement within 10% of theory.

Sank, Daniel; Barends, R.; Bialczak, Radoslaw C.; Chen, Yu; Kelly, J.; Lenander, M.; Lucero, E.; Mariantoni, Matteo; Megrant, A.; Neeley, M.; O'Malley, P. J. J.; Vainsencher, A.; Wang, H.; Wenner, J.; White, T. C.; Yamamoto, T.; Yin, Yi; Cleland, A. N.; Martinis, John M.

2012-08-01

73

Photospheric processes and magnetic flux tubes  

NASA Astrophysics Data System (ADS)

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 simulations that are capable of reproducing the corrugated shape of magnetic flux concentrations and their signature in the visible continuum. Faculae are another manifestation of small-scale magnetic flux concentrations. It is shown that the characteristic asymmetric shape of the contrast profile of faculae is an effect of radiative transfer across the rarefied atmosphere of the magnetic flux concentration. Also discussed are three-dimensional radiation magnetohydrodynamic simulations of the integral layers from the top of the convection zone to the mid-chromosphere. They show a highly dynamic chromospheric magnetic field, marked by rapidly moving filaments of stronger than average magnetic field that form in the compression zone downstream and along propagating shock fronts. The simulations confirm the picture of flux concentrations that strongly expand through the photosphere into a more homogeneous, space filling chromospheric field. Future directions in the simulation of small-scale magnetic fields are indicated with a few examples from recent reports. The second part of these lecture notes is devoted to a few basic properties of magnetic flux tubes that can be considered to be an abstraction of the more complicated flux concentrations known from observations and numerical simulations. By analytical means we will find that an electrical current flows in a sheet at the surface of a flux-tube for which location we also derive the mechanical equilibrium condition. The equations for constructing a magnetohydrostatic flux tube embedded in a gravitationally stratified atmosphere are derived. It is shown that the expansion of a flux tube with height sensibly depends on the difference in the thermal structure between the atmosphere of the flux tube and the surrounding atmosphere. Furthermore, we will find that radiative equilibrium produces a smaller temperature gradient within the flux tube compared to that in the surrounding atmosphere. The condition for interchange stability is derived and it is shown that small-scale magnetic flux concentrations are liable to the interchange instability.

Steiner, Oskar

2007-07-01

74

Magnetic Flux Expulsion in Star Formation  

NASA Astrophysics Data System (ADS)

Stars form in dense cores of magnetized molecular clouds. If the magnetic flux threading the cores is dragged into the stars, the stellar field would be orders of magnitude stronger than observed. This well-known "magnetic flux problem" demands that most of the core magnetic flux be decoupled from the matter that enters the star. We carry out the first exploration of what happens to the decoupled magnetic flux in three dimensions, using a magnetohydrodynamic (MHD) version of the ENZO adaptive mesh refinement code. The field-matter decoupling is achieved through a sink particle treatment, which is needed to follow the protostellar accretion phase of star formation. We find that the accumulation of the decoupled flux near the accreting protostar leads to a magnetic pressure buildup. The high pressure is released anisotropically along the path of least resistance. It drives a low-density expanding region in which the decoupled magnetic flux is expelled. This decoupling-enabled magnetic structure has never been seen before in three-dimensional MHD simulations of star formation. It generates a strong asymmetry in the protostellar accretion flow, potentially giving a kick to the star. In the presence of an initial core rotation, the structure presents an obstacle to the formation of a rotationally supported disk, in addition to magnetic braking, by acting as a rigid magnetic wall that prevents the rotating gas from completing a full orbit around the central object. We conclude that the decoupled magnetic flux from the stellar matter can strongly affect the protostellar collapse dynamics.

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

2011-11-01

75

Circulating currents in magnetic flux tubes  

NASA Astrophysics Data System (ADS)

There are several phenomena which require the application of a force in the axial direction of a magnetic flux tube, for example: solar plasma loops, where support of the plasma mass against gravity is required within those sections of the flux tube that have vertical components, the observed upward acceleration of the plasma in flux tubes forming the legs of quiescent prominences, and the acceleration of the fast solar wind. In this paper, we show that the origin of such axial forces is the magnetic pressure of an azimuthal magnetic field B? generated by currents flowing in circuits passing inside and outside the flux tube, and that the source of the EMF driving this circulating current is the difference between the temperature gradients inside and outside the flux tube, which generates a thermoelectric potential.

Ashbourn, J. M. A.; Woods, L. C.

2006-09-01

76

Investigation of the magnetic flux leakage signatures of dents and gouges  

Microsoft Academic Search

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

Kris Marble

2009-01-01

77

Simulations of dependence of low frequency flux noise on SQUID geometry  

NASA Astrophysics Data System (ADS)

It is generally accepted that the 1/f magnetic flux noise observed in dc SQUIDs and superconducting qubits originates in the random flipping of a uniform distribution of electron spins localized at the superconductor-insulator interface. Computer simulations and analytical calculations based on this model confirm the experimental result that the noise power at 1 Hz varies only slowly with SQUID dimensions. In particular, analytical calculations for a circular loop with radius R much greater than the loop linewidth W predict that the noise power scales as R/W. We present numerical computations that are valid for arbitrary geometry, including that of the square washer SQUID for which W approaches R. Making use of the reciprocity theorem, we solve the London equations numerically to find the current distribution in the superconductor, evaluate the Biot-Savart integral to find the corresponding magnetic field at any point and integrate over all spins, including those at the edges of the films, to find the total flux noise. We compare our results with our recent experimental measurements.

Anton, Steven; Pepper, Keenan; Clarke, John; Sognnaes, Ida

2011-03-01

78

Vector Magnetic Field in Emerging Flux Regions  

NASA Astrophysics Data System (ADS)

A crucial phase in magnetic flux emergence is the rise of magnetic flux tubes through the solar photosphere, which represents a severe transition between the very different environments of the solar interior and corona. Multi-wavelength observations with Flare Genesis, TRACE, SoHO, and more recently with the vector magnetographs at THEMIS and Hida (DST) led to the following conclusions. The fragmented magnetic field in the emergence region - with dipped field lines or bald patches - is directly related with Ellerman bombs, arch filament systems, and overlying coronal loops. Measurements of vector magnetic fields have given evidence that undulating "serpentine" fields are present while magnetic flux tubes cross the photosphere. See the sketch below, and for more detail see Pariat et al. (2004, 2007); Watanabe et al. (2008):

Schmieder, B.; Pariat, E.

79

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

80

Solar cycle variations of heliospheric magnetic flux  

Microsoft Academic Search

We extend previous studies of BR, the radial component of the heliospheric magnetic field and a measure of heliospheric magnetic flux, to include the maximum in cycle 23 so that most of 4 cycles are covered. Solar rotation averages reveal that BR systematically increases from a minimum value at sunspot minimum to a maximum value during the declining phase. This

Xiaoyan Zhou; Edward J. Smith

2009-01-01

81

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

82

Dipole Modeling of Magnetic Flux Leakage  

Microsoft Academic Search

In this paper, we present an analytical model to represent the 3-D magnetic flux leakage (MFL) field due to the occurrence of a surface-breaking defect in a ferromagnetic specimen. This situation is frequently encountered in the nondestructive evaluation (NDE) of energy pipelines using the MFL technique. The model is derived from first principles, and utilizes the concept of dipolar magnetic

Sushant M. Dutta; Fathi H. Ghorbel; Roderic K. Stanley

2009-01-01

83

Magnetic Flux Quantization of the Landau Problem  

NASA Astrophysics Data System (ADS)

Landau problem has a very important application in modern physics, in which two-dimensional electron gas system and quantum Hall effect are outstanding. In this paper, first we review the solution of the Pauli equation, then using the single electron wave function, we calculate moving area expectations of the ideal 2-dimensional electron gas system and the per unit area's degeneracy of the electron gas system. As a result, how to calculate the magnetic flux of the electron gas system is given. It shows that the magnetic flux of 2-dimensional electron gas system in magnetic field is quantized, and magnetic flux quantization results from the quantization of the moving area expectations of electron gas system.

Wang, Jianhua; Li, Kang; Long, Shuming; Yuan, Yi

2014-04-01

84

Losses in Magnetic Flux Compression Generators: Part 2, Radiation Losses.  

National Technical Information Service (NTIS)

This is the second monograph devoted to the analysis of flux losses in explosive driven magnetic flux compression generators. In the first monograph, flux losses from magnetic field penetration into conductor walls was studied by conventional diffusion th...

C. M. Fowler

1988-01-01

85

Solar Intranetwork Magnetic Elements: Bipolar Flux Appearance  

NASA Astrophysics Data System (ADS)

This study aims to quantify characteristic features of the bipolar flux appearance of solar intranetwork (IN) magnetic elements. To attack this problem, we use the Narrowband Filter Imager (NFI) magnetograms from the Solar Optical Telescope (SOT) on board Hinode; these data are from quiet and enhanced network areas. Cluster emergence of mixed polarities and IN ephemeral regions (ERs) are the most conspicuous forms of bipolar flux appearance within the network. Each of the clusters is characterized by a few well-developed ERs that are partially or fully coaligned in magnetic axis orientation. On average, the sampled IN ERs have a total maximum unsigned flux of several 1017 Mx, a separation of 3 - 4 arcsec, and a lifetime of 10 - 15 minutes. The smallest IN ERs have a maximum unsigned flux of several 1016 Mx, separations of less than 1 arcsec, and lifetimes as short as 5 minutes. Most IN ERs exhibit a rotation of their magnetic axis of more than 10 degrees during flux emergence. Peculiar flux appearance, e.g., bipole shrinkage followed by growth or the reverse, is not unusual. A few examples show repeated shrinkage-growth or growth-shrinkage, like magnetic floats in the dynamic photosphere. The observed bipolar behavior seems to carry rich information on magnetoconvection in the subphotospheric layer.

Wang, Jingxiu; Zhou, Guiping; Jin, Chunlan; Li, Hui

2012-06-01

86

Magnetic flux density in the heliosphere  

NASA Astrophysics Data System (ADS)

Recently it was shown that the evaluation of the open magnetic flux in the heliosphere is strongly affected by the fluctuations of the field. A new method of data analysis has been developed which effectively reduces the bias introduced by the fluctuations (Erd?s, G., & Balogh, A., Ap. J. 753:130, 2012). As a result, the magnetic flux density has been shown to be highly uniform in the heliosphere with respect to latitude and longitude. Also, a regular R-2 form decay by heliospheric distance of the flux density was established. In the present paper these results are further investigated by analysing magnetic field measurements by spacecraft in various heliospheric locations. The main data sources are from observations by ACE, STEREO A and B, and Ulysses; these observations cover a large range of latitude and longitude in location and almost two complete solar cycles in time. Special focus is devoted to the search for any possible longitudinal variations of the magnetic flux density in the ecliptic, in particular as related to CIRs and CMEs and the way these affect the observed relationship between the open solar magnetic field and the heliospheric flux density.

Balogh, André; Erdös, Géza

2013-04-01

87

Crossed Flux Tubes Magnetic Reconnection Experiment  

NASA Astrophysics Data System (ADS)

The dynamics of arched, plasma-filled flux tubes have been studied in experiments at Caltech. These flux tubes expand, undergo kink instabilities, magnetically reconnect, and are subject to magnetohydrodynamic forces. An upgraded experiment will arrange for two of these flux tubes to cross over each other. It is expected then that the flux tubes will undergo magnetic reconnection at the crossover point, forming one long flux tube and one short flux tube. This reconnection should also result in a half-twist in the flux tubes at the crossover point, which will propagate along each tube as Alfv'en waves. The control circuitry requires two independent floating high energy capacitor power supplies to power the plasma loops, which will be put in series when the plasma loops reconnect. Coordinating these two power supplies requires the building of new systems for controlling plasma generation. Unlike with previous designs, all timing functions are contained on a single printed circuit board, allowing the design to be easily replicated for use with each independent capacitor involved. The control circuit sequencing has been tested successfully in generating a single flux tube. The plasma gun is currently under construction, with its installation pending completion of prior experiments.

Tobin, Zachary; Bellan, Paul

2012-10-01

88

The noise level in magnetic stimulation.  

PubMed

The noise generated by stimulating coils may jeopardize the hearing of the patients as well as the hearing of the examiner. To evaluate the potential risk caused by the impulse noise of stimulating coils, we examined the A-weighted peak sound pressure levels from five different types of magnetic stimulator coils. At a distance of 10 cm, with 100% stimulation intensity, the coils with Dantec and Magstim stimulators created maximum peak sound pressure levels of 110 dB. Correspondingly, Cadwell MES-10 created maximum peak sound pressure levels of 132 dB. The decrease in the peak levels followed the distance rule quite closely. At a distance of 40 cm, the decrease in peak level was on average 14 dB (range -1-(+)1 dB). Based on American Conference of Governmental Industrial Hygienists (ACGIH) threshold limits of impact noise, the permitted maximum daily number of magnetic stimuli would be 1000 to 10,000. The permitted number of daily stimuli may be difficult to exceed in clinical practice. We consider the risk as small for the patients that are being examined and the operator using magnetic stimulation. The potential risk can be further diminished by even very light weighted hearing protectors providing proper attenuation to the coil impulses. PMID:8975992

Starck, J; Rimpiläinen, I; Pyykkö, I; Esko, T

1996-01-01

89

Dependence of low frequency flux noise on SQUID-washer dimensions  

NASA Astrophysics Data System (ADS)

The 1/f spectral density of low frequency magnetic flux noise at 1 Hz in dc SQUIDs and qubits varies slowly with the dimensions of the superconducting loop, in reasonably good agreement with predictions. Previous measurements on SQUIDs fabricated from a variety of superconductors under different conditions and in a variety of geometries, however, showed that the slope of the power spectrum varied considerably. We report flux noise measurements on six resistively-shunted dc SQUIDs fabricated simultaneously on a single Si chip using a Nb-trilayer process. The noise spectra of all six devices were measured using a SQUID in a single cool-down of our dilution refrigerator. The linewidths of the SQUID loops were varied systematically by a factor of more than 30. The variation in noise power at 1 Hz was small compared with the variation in line width, while the slope varied significantly, from approximately -0.5 to -1. Furthermore, for a given SQUID, the slope depended on temperature.

Birenbaum, J.; Anton, S. M.; Fefferman, A. D.; O'Kelley, S. R.; Clarke, J.; Cho, H.-M.; Hilton, G. C.; Irwin, K. D.; Wellstood, F. C.

2011-03-01

90

Magnetic noises generated by plants in microgravity: effect of external magnetic fields.  

NASA Astrophysics Data System (ADS)

Magnetic Noises Generated by Plants in Microgravity, Effect of External Magnetic Field. N. Bogatina(1), N. Sheykina(1), E. Kordyum(2). (1) B.Verkin Institute for Low Temperature Physics&Engineering of National Academy of Sciences of Ukraine, Kharkov, Ukraine (2) N.Cholodny Institute of Botany of National Academy of Sciences of Ukraine, Kiev, Ukraine. e-mail:bogatina@ilt.kharkov.ua/Fax: 38-0572-322370. The object of the work is to determine the correlation between the magnetic noises generated by plants and accelaration value during their development in microgravity. The amplitude of magnetic noise spectral density will be mesured. Its dependence on the magnitude of gravity accelaration and magnitudes of static and alternating magnetic fields is supposed to reflect the mechanisms of gravi- and magnitopercrption. We shall try to answer the guestion whether the same mechanism or different mechanisms are responsible for gravi- and magnitopercrption. The gravitropic reaction of plants is needed to be studied simultaneously with observation of magnetic noises. The measurements in microgravity have to be fulfilled by a flux-gate magnetometer connected with Philips spectroanalyser There are two directions of experiments. The first one is experiments that will be carried out on Earth under the conditions of weakening of gravitational force. The preliminary parameters of the required magnetic fields have to be determined in these experiments. The second one is the same experiment in the space. These experiments are supposed to allow us to define the mechanisms of a gravitropic reaction of plants because any motion of ions ( indolil-acetic ions, for instance) is accompanied by an increase of magnetic noise. The experimental results obtained under onGround conditions by using the superconducting technique wwithout weakening the weight force are disscussed. .

Bogatina, N.; Sheykina, N.; Kordyum, E.

91

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

92

Magnetic Flux Leakage: a Benchmark Problem  

NASA Astrophysics Data System (ADS)

The magnetic flux leaked by artificial notches machined by EDM is measured for two different rectangular steel plates. The measurements were performed for different field intensities, different liftoffs, and both sides, simultaneously recording the three components of the magnetic field. Attention was paid to the accurate measurement of the liftoff, and to make the magnetic history of the material as predictable as possible. This was achieved by measuring for decreasing magnetic excitations, starting from saturation. The descending branch of the major loop is measured and reported, to allow for a detailed comparison against numerical experiments.

Etcheverry, J. I.; Sánchez, G. A.; Bonadeo, N.

2011-06-01

93

Flux Pinning and Flux Flow Studies in Superconductors Using Flux Flow Noise Techniques. Final Technical Report, April 1, 1977-March 31, 1984.  

National Technical Information Service (NTIS)

Flux flow and flux pinning have been studied in several alloys characterized by a variety of shapes of volume pinning force curves. Annealing is used to modify these curves and to study the accompanying changes in the flux flow noise. A model of flux moti...

W. C. H. Joiner

1985-01-01

94

Optimization of multiply acquired magnetic flux density Bz using ICNE-Multiecho train in MREIT  

Microsoft Academic Search

The aim of magnetic resonance electrical impedance tomography (MREIT) is to visualize the electrical properties, conductivity or current density of an object by injection of current. Recently, the prolonged data acquisition time when using the injected current nonlinear encoding (ICNE) method has been advantageous for measurement of magnetic flux density data, Bz, for MREIT in the signal-to-noise ratio (SNR). However,

Hyun Soo Nam; Oh In Kwon

2010-01-01

95

Negative energy waves in magnetic flux tubes  

Microsoft Academic Search

The properties of oscillation of a magnetic flux tube in the presence of a shear flow of matter along its axis were studied. The presence of flows is shown to cause a number of new effects: the appearance of negative energy waves, reversal of the sign of radiative damping, the development of explosive instability at the nonlinear stage, and the

M. P. Ryutova

1988-01-01

96

Highly Sensitive Magnetic Flux Superconducting Detector.  

National Technical Information Service (NTIS)

The preliminary results on creation of a highly sensitive superconducting detector of magnetic flux with a toroidal sensor are presented in the paper. The real sensitivity of the detector is about 4x10 exp -11 gxcm exp 2 / sqrt Hz. The peculiarities of ba...

E. N. Ardashev G. I. Selivanov A. G. Kholodenko

1975-01-01

97

Negative Energy Waves in Magnetic Flux Tubes.  

National Technical Information Service (NTIS)

The properties of oscillation of a magnetic flux tube in the presence of a shear flow of matter along its axis have been studied. The presence of flows is shown to cause a number of new effects: the appearance of negative energy waves, reversal of the sig...

M. P. Ryutova

1988-01-01

98

On the Orientation of Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

We have examined 2(1)/(2)D static magnetic flux rope configurations (? /? t ? 0, ? /? z ? 0, but Bz!= 0, z being the flux-rope axis) to determine their axis orientations from single-spacecraft data taken as the flux rope convects past the spacecraft. In a proper frame of reference, (x,y,z), the spacecraft trajectory projected onto the transverse (x-y) plane is a straight line along x across the flux rope structure. The distance from the center of the flux rope to the spacecraft trajectory is defined as the impact parameter. We have developed a multi-step, composite magnetic variance analysis method (the HS method) to accurately determine the axis of flux rope structures having right-left symmetry such as the Lundquist model of an axially symmetric force-free configuration. We will present the results of our analysis for right-left symmetric flux rope models of circular, elliptical and non-elliptical cross-sections (in the x-y plane) for comparison with results from Lepping's method [Lepping et al., 1990], in which the intermediate variance direction, obtained from magnetic variance analysis on unit normalized magnetic vectors (i.e., B), is used as the approximation for the true axis of the Lundquist model. Under certain conditions, our method yields the exact axis orientation, while the result from Lepping's method shows increasing angular error with increasing impact parameter. More general cases of non-symmetric, static flux-rope structures can also be analyzed, since they (as well as the symmetric cases) are governed by the Grad-Shafranov equation, ? t2A=-? 0dPt/dA, where the magnetic potential A is used to describe transverse magnetic field, Bx=? A/? y and By=-? A/? x, and Pt(A)=p(A)+Bz2(A)/2?0. For these asymmetric cases, the determination of the optimal axis orientation is accomplished by searching for the minimum in the fitting residue of Pt(A) subject to certain constraints. This method does not give a unique answer for structures that are right-left symmetric. Examples are presented to illustrate the procedures, to highlight pitfalls, and to show estimates.

Hu, Q.; Sonnerup, B. U.

2001-05-01

99

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

100

Photospheric Signatures of Incoherent Magnetic Flux Emergence  

NASA Astrophysics Data System (ADS)

Much work is being carried out to calculate magnetic energy flux through the photosphere using observational data. The theoretical underpinning of these calculations is the determination of the Poynting flux S = (c/4\\pi)E x B in terms of observed quantities. The standard approach is to use the ideal MHD equation, E + v x B, which leads to S = (1/4 \\pi)B^2 v. B and v are determined using vector magnetogram and Doppler data, respectively. These remote sensing techniques, however, do not directly measure electric or magnetic fields. The formulation of the calculations currently carried out make a number of unrealistic fundamental assumptions that render them irrelevant to photospheric signatures. In particular, the calculations do not include the physical properties of the plasma motion or magnetic field dynamics in the photosphere. One basic implicit assumption is that the magnetic field is coherent and well resolved. The calculated results cannot account for contributions from incoherent, unresolved magnetic field features that emerge and overestimate the observable effects of emerging magnetic energy. The second key assumption is the neglect of the rapid increase in the optical transparency of the atmosphere above the photosphere. We construct a model incorporating the incoherence length scale L_i relative to the instrumental pixel size L_p and incoherence time scale, tau_i. We construct model line profiles relating detectable magnetic energy flux to the degree of incoherence in space and time. It is shown that unless all the magnetic energy appears as coherent features throughout an observed region, the above assumptions fail and the Doppler velocity v is uncorrelated with the speed at which magnetic energy B2/8? is transported through the photosphere. This is traced to the fact that the equation of motion for the photospheric plasmas is not taken into account and the ideal MHD equation does not constrain v_parallel.

Chen, J.; Kunkel, V.

2009-12-01

101

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

102

Wavelet-based Rician noise removal for magnetic resonance imaging  

Microsoft Academic Search

It is well known that magnetic resonance magnitude image data obey a Rician distribution. Unlike additive Gaussian noise, Rician “noise” is signal-dependent, and separating signal from noise is a difficult task. Rician noise is especially problematic in low signal-to-noise ratio (SNR) regimes where it not only causes random fluctuations, but also introduces a signal-dependent bias to the data that reduces

Robert D. Nowak

1999-01-01

103

Solar Intranetwork Magnetic Elements: Flux Distributions  

NASA Astrophysics Data System (ADS)

The current study aims at quantifying the flux distributions of solar intranetwork (IN) magnetic field based on the data taken in four quiet and two enhanced network areas with the Narrow-band Filter Imager of the Solar Optical Telescope on board the Hinode satellite. More than 14000 IN elements and 3000 NT elements were visually identified. They exhibit a flux distribution function with a peak at 1 - 3×1016 Mx (maxwell) and 2 - 3×1017 Mx, respectively. We found that the IN elements contribute approximately to 52 % of the total flux and an average flux density of 12.4 gauss of the quiet region at any given time. By taking the lifetime of IN elements of about 3 min (Zhou et al., Solar Phys. 267, 63, 2010) into account, the IN fields are estimated to have total contributions to the solar magnetic flux up to 3.8×1026 Mx per day. No fundamental distinction can be identified in IN fields between the quiet and enhanced network areas.

Zhou, Guiping; Wang, Jingxiu; Jin, Chunlan

2013-04-01

104

Magnetic merging in colliding flux tubes  

NASA Technical Reports Server (NTRS)

We develop an analytical theory of reconnection between colliding, twisted magnetic flux tubes. Our analysis is restricted to direct collisions between parallel tubes and is based on the collision dynamics worked out by Bogdan (1984). We show that there is a range of collision velocities for which neutral point reconnection of the Parker-Sweet type can occur, and a smaller range for which reconnection leads to coalescence. Mean velocities within the solar convection zone are probably significantly greater than the upper limit for coalescence. This suggests that the majority of flux tube collisions do not result in merging, unless the frictional coupling of the tubes to the background flow is extremely strong.

Zweibel, Ellen G.; Rhoads, James E.

1995-01-01

105

Comparison of the acoustic noise of a flux switching and a switched reluctance drive  

Microsoft Academic Search

This paper describes the results of a comparison between the acoustic noise produced by a two-phase switched reluctance drive and a flux switching motor and drive. For the comparison, two external rotor machines were constructed from identical mechanical parts, and the same lamination stacks. Test results show that there is over 2 dB less acoustic noise from the flux switching

C. Pollock; M. Brackley

2001-01-01

106

Role of Geometry on the Color of Flux Noise in dc SQUIDs.  

National Technical Information Service (NTIS)

We examine the behavior of low frequency flux noise measured in ten Nb-NbOx-PbIn dc SQUIDs in the temperature range 1 K to 20 mK. As previously reported, the flux noise power spectrum Sphi(f) typically scales with frequency as A/f(exp alpha). Remarkably, ...

C. Urbina F. C. Wellstood J. Clarke

2011-01-01

107

Characteristics of soft magnetic composite material under rotating magnetic fluxes  

NASA Astrophysics Data System (ADS)

This paper reports the measurement of magnetic properties of the soft magnetic composite material SOMALOY TM 500 in a square sample under different patterns of flux density with 2D magnetic excitations. The test system, principle of measurement, magnetic power loss calculation, and methods of correction for misalignment of H surface sensing coils are presented. The experimental results show that although nominally isotropic, the SOMALOY TM 500 sample exhibits some anisotropy. The results are useful in the design and performance analysis of rotating electrical machines.

Zhong, J. J.; Guo, Y. G.; Zhu, J. G.; Lin, Z. W.

2006-04-01

108

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

109

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

110

FEA of pipeline magnetic flux leakage NDE  

Microsoft Academic Search

The magnetic flux leakage (MFL) technique is a viable approach to the non-destructive evaluation of pipelines. In this paper, we introduce two-dimensional (2D) finite element analysis (FEA) to estimate the defect profile from the pipeline MFL inspection. The 2D defect characterization would be measured by the axial and the radial components of the MFL. The signals can show the features

Xing Li; Liang Chen; Xiaohong Zeng

2009-01-01

111

Magnetic Sensor Operation Onboard a UUV: Magnetic Noise Investigation using a Total-Field Gradiometer.  

National Technical Information Service (NTIS)

To operate a magnetic sensor on board an Unmanned Underwater Vehicle (UUV), it is necessary to provide a means for compensating magnetic noise from a variety of sources. In previous applications involving passively towed platforms, the noise arises from t...

D. J. Overway J. T. Bono W. M. Wynn

2003-01-01

112

Magnetic-Flux-Compensated Voltage Divider  

NASA Technical Reports Server (NTRS)

A magnetic-flux-compensated voltage-divider circuit has been proposed for use in measuring the true potential across a component that is exposed to large, rapidly varying electric currents like those produced by lightning strikes. An example of such a component is a lightning arrester, which is typically exposed to currents of the order of tens of kiloamperes, having rise times of the order of hundreds of nanoseconds. Traditional voltage-divider circuits are not designed for magnetic-flux-compensation: They contain uncompensated loops having areas large enough that the transient magnetic fluxes associated with large transient currents induce spurious voltages large enough to distort voltage-divider outputs significantly. A drawing of the proposed circuit was not available at the time of receipt of information for this article. What is known from a summary textual description is that the proposed circuit would contain a total of four voltage dividers: There would be two mixed dividers in parallel with each other and with the component of interest (e.g., a lightning arrester), plus two mixed dividers in parallel with each other and in series with the component of interest in the same plane. The electrical and geometric configuration would provide compensation for induced voltages, including those attributable to asymmetry in the volumetric density of the lightning or other transient current, canceling out the spurious voltages and measuring the true voltage across the component.

Mata, Carlos T.

2005-01-01

113

Losses in magnetic flux compression generators. Part 2: Radiation losses  

Microsoft Academic Search

An analysis of flux losses in explosive driven magnetic flux compression generators is given. Flux loss by radiation from the outer conductor walls is treated. Flux leakage rates through walls of finite thickness are first obtained by diffusion theory. It is shown, for normal wall thicknesses, that flux leakage is determined essentially by the wall conductance, defined as the product

C. M. Fowler

1988-01-01

114

Signal processing of magnetic flux leakage surface flaw inspect in pipeline steel  

Microsoft Academic Search

The magnetic flux leakage (MFL) method has established itself as the most widely used inline inspection technique for the\\u000a evaluation of pipelines. The MFL signal is usually contaminated by various noise sources This paper presents the empirical\\u000a mode decomposition (EMD) for enhancing signal-to-noise ratio. Experiments are conducted on the pipeline steel samples with\\u000a different depth artificial defects. The MFL signal

Liang Chen; Xunbo Li; Guangxu Qin; Qiang Lu

2008-01-01

115

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

116

Ultrarelativistic Magnetic Monopole Flux Constraints from RICE  

NASA Astrophysics Data System (ADS)

The Radio Ice Cherenkov Experiment (RICE) is a radio array buried in the Antarctic ice. A Monte Carlo simulation has been developed to model the behavior of an ultrarelativistic magnetic monopole propagating through the ice and RICE's response. With this, we've determined RICE's effective volume for detecting these particles as a function of monopole mass, charge, and gamma-value. As previously reported, RICE failed to detect an in-ice high-energy Cherenkov radiation signal consistent with a relativistic magnetic monopole over roughly two years of livetime. From this null result and the effective volume calculation, we can extrapolate upper bounds on ultrarelativistic magnetic monopole flux. Provisional values are presented.

Hogan, Daniel

2006-04-01

117

Exploring ISEE-3 magnetic cloud polarities with electron heat fluxes  

SciTech Connect

We have used solar wind electron heat fluxes to determine the magnetic polarities of the interplanetary magnetic fields (IMF) during the ISEE-3 observations in 1978{endash}1982. That period included 14 magnetic clouds (MCs) identified by Zhang and Burlaga. The MCs have been modeled as single magnetic flux ropes, and it is generally assumed that they are magnetically closed structures with each end of the flux rope connected to the Sun. The flux rope model is valid only if the magnetic polarity of each MC does not change during the passage of ISEE-3 through the MC. We test this model with the heat flux data, using the dominant heat flux in bidirectional electron heat fluxes to determine the MC polarities. The polarity changes within at least 2, and possibly 6, of the 14 MCs, meaning that those MCs can not fit the model of a single flux rope. {copyright} {ital 1999 American Institute of Physics.}

Kahler, S.W. [Air Force Research Laboratory, 29 Randolph Rd, Hanscom AFB, Massachusetts 01731 (United States); Crooker, N.U. [Center for Space Physics, Boston University, 725 Commonwealth Ave., Boston, Massachusetts 02215 (United States); Gosling, J.T. [Los Alamos National Laboratory, MS D 466, Los Alamos, New Mexico 87545 (United States)

1999-06-01

118

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

119

Vector Magnetic Property Measurement in Magnetic Steel Sheets under High Magnetic Flux Density Conditions  

NASA Astrophysics Data System (ADS)

In order to develop higher torque, lower weight and smaller sized magnetic actuators for driving electrical vehicles and robot arms, designing under high magnetic field and high exciting voltage is most effective. It is therefore very important to understand vector magnetic properties in electrical steel sheets under high magnetic flux density conditions. The vector magnetic properties mean the relationship between the magnetic flux density vector and the magnetic field strength vector and it is very difficult to measure the each component under high magnetic flux density conditions because of the magnetic saturation and the magnetic anisotropy. In this paper, accurate measurement techniques of the vector magnetic properties over 1.5T and measured vector magnetic properties are reported and discussed.

Maeda, Yoshitaka; Todaka, Takashi; Enokizono, Masato

120

Geometry and temperature dependence of low frequency flux noise in dc SQUIDs  

NASA Astrophysics Data System (ADS)

Measurements of low frequency magnetic flux noise in dc SQUIDs demonstrate a spectral density S?(f)=A^2/f^? in which the magnitude A scales only weakly with the washer geometry and typically 0.5flux noise measurements on several dc SQUIDs fabricated simultaneously on a single chip using a Nb trilayer process. Multiple SQUIDs were measured within a single cool-down. The SQUID geometries were divided into two categories: fixed W=500 nm with 3

Birenbaum, Jeffrey; Anton, S. M.; O'Kelley, S. R.; Bolkhovsky, V.; Braje, D. A.; Fitch, G.; Neeley, M.; Oliver, W. D.; Wellstood, F. C.; Clarke, John

2012-02-01

121

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

Microsoft Academic Search

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

Liang Chen; Xing Li; Xun-Bo Li; Zuo-Ying Huang

2009-01-01

122

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

123

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

124

The flux-rope-fibre theory of solar magnetic fields  

Microsoft Academic Search

The flux-rope theory of solar magnetic fields and their effects is reviewed. The traditional theory of emerging magnetic flux is discussed, emphasizing kinematic considerations, dynamic effects, and various difficulties with the traditional concepts. Evidence in favor of the flux-rope-fiber model of solar magnetic fields is examined, including observations of magnetic structures in growing active regions, sunspots and spot groups, fine

J. H. Piddington

1978-01-01

125

Magnetic flux array for spontaneous magnetic reconnection experiments  

SciTech Connect

Experimental investigation of reconnection in magnetized plasmas relies on accurate characterization of the evolving magnetic fields. In experimental configurations where the plasma dynamics are reproducible, magnetic data can be collected in multiple discharges and combined to provide spatially resolved profiles of the plasma dynamics. However, in experiments on spontaneous magnetic reconnection recently undertaken at the Versatile Toroidal Facility at MIT, the reconnection process is not reproducible and all information on the plasma must be collected in a single discharge. This paper describes a newly developed magnetic flux array which directly measures the toroidal component of the magnetic vector potential, A{sub {phi}}. From the measured A{sub {phi}}, the magnetic field geometry, current density, and reconnection rate are readily obtained, facilitating studies of the three-dimensional dynamics of spontaneous magnetic reconnection. The novel design of the probe array allows for accurate characterization of profiles of A{sub {phi}} at multiple toroidal angles using a relatively small number of signal channels and with minimal disturbance of the plasma.

Kesich, A.; Bonde, J.; Egedal, J.; Fox, W.; Goodwin, R.; Katz, N.; Le, A. [Massachusetts Institute of Technology, Plasma Science and Fusion Center, Cambridge, Massachusetts 02139 (United States)

2008-06-15

126

Improvement of the superconducting magnetic levitation system for the determination of the magnetic flux quantum  

Microsoft Academic Search

The authors describe an improvement of the preliminary superconducting magnetic levitation system in progress for the absolute determination of the magnetic flux quantum. This improvement includes the development of the flux-up method to determine the flux in terms of the Josephson voltage. The improvement is essential for the determination of the magnetic flux quantum as well as of the coil

T. Endo; Y. Sakamoto; F. Shiota; K. Nakayama; Y. Nezu; M. Kikuzawa; K. Hara

1989-01-01

127

Small scale magnetic flux-averaged magnetohydrodynamics  

SciTech Connect

By relaxing exact magnetic flux conservation below a scale [lambda] a system of flux-averaged magnetohydrodynamic equations are derived from Hamilton's principle with modified constraints. An energy principle can be derived from the linearized averaged system because the total system energy is conserved. This energy principle is employed to treat the resistive tearing instability and the exact growth rate is recovered when [lambda] is identified with the resistive skin depth. A necessary and sufficient stability criteria of the tearing instability with line tying at the ends for solar coronal loops is also obtained. The method is extended to both spatial and temporal averaging in Hamilton's principle. The resulting system of equations not only allows flux reconnection but introduces irreversibility for appropriate choice of the averaging function. Except for boundary contributions which are modified by the time averaging process total energy and momentum are conserved over times much longer than the averaging time [tau] but not for less than [tau]. These modified boundary contributions correspond to the existence, also, of damped waves and shock waves in this theory. Time and space averaging is applied to electron magnetohydrodynamics and in one-dimensional geometry predicts solitons and shocks in different limits.

Pfirsch, D. (Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, D-8046 Garching (Germany)); Sudan, R.N. (Laboratory of Plasma Studies, Cornell University, Ithaca, New York 14853 (United States))

1994-08-01

128

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

129

Chara cterization of Wire Rope Defects from Magnetic Flux Leakage Signals  

Microsoft Academic Search

When using magnetic flux leakage to inspect wire rope, fwo fypes of signals are produced for analysis, nurn.l-y, Lo=calized Fault (LF) and Loss of Magnetic Cross-section Area (LMA)' However, these signals contain a lot of other noises which makes the defects difficult to perfectly analyze. Therefore, this paper rs written to characterize and analyze signals of the wire rope defects

C. Jomdecha; A. PrateePasen; W. Methong

2003-01-01

130

Comparison of the acoustic noise of a flux-switching and a switched reluctance drive  

Microsoft Academic Search

This paper describes the results of a comparison between the acoustic noise produced by a two-phase switched reluctance drive and a flux-switching motor and drive. For the comparison two external rotor machines were constructed from identical mechanical parts, and the same lamination stacks. Test results show that there is over 2 dB less acoustic noise from the flux-switching topology relative

Charles Pollock; Mark Brackley

2003-01-01

131

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

132

Complementary finite element magnetodynamic formulations with enforced magnetic fluxes  

Microsoft Academic Search

Two complementary 3D finite element formulations, with either the magnetic field or the magnetic vector potential as unknowns, are developed to deal with the modeling of eddy currents in electrical steel laminations. The magnetic flux through the flux gates of the conducting region is imposed via the boundary terms of the weak formulations, in a natural way thanks to the

P. Dular; J. Gyselinck; F. Henrotte; W. Legros; J. Melkebeek

1999-01-01

133

Force-Free Magnetic Flux Ropes in the Solar Corona  

Microsoft Academic Search

Magnetic flux ropes offer a venue for storage of magnetic energy in the solar corona, energy that can contribute to eruptive events such as coronal mass ejections (CMEs). When a flux rope encircles the Sun, the associated magnetic field can store more energy than is needed to open the field fully---one of three energy-demanding tasks required for a CME. This

R. Wolfson

2004-01-01

134

Novel behavior of magnetic flux lines in type II superconductors  

NASA Astrophysics Data System (ADS)

In this thesis we present several studies in the properties of magnetic flux lines in type II superconductors. 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 SFw has a power-law dependence on frequency, SFw ˜ w-s , with s ˜ 1.4 in reasonable agreement with experiment. In addition, for small lattices, the calculated SFw has a high-frequency knee, which is seen in some experiments, and which is due to the finite lattice size. We have analyzed the Lawrence-Doniach free energy in a tilted magnetic field within the lowest Landau level (LLL) approximation for the case of a highly anisotropic high temperature superconductor. The free energy maps onto that of a strictly c-axis field, but with a reduced interlayer coupling. We use this result to calculate the tilt modulus C44 of a vortex lattice and vortex liquid. The vortex contribution to C44 can be expressed in terms of the squared c-axis Josephson plasmon frequency w2pl . We find that the transverse component of the field has very little effect on the position of the melting curve. We present a simple numerical model for the IV characteristics of a highly anisotropic high temperature superconductor in different geometries. An array of grains coupled together by Josephson junctions is used, with a triangular structure in the planes normal to an applied magnetic field and a square structure otherwise. Overdamped junctions are used to describe the CuO2 planes, while underdamped junctions are used to describe the interplanar coupling. Each grain has a capacitive shunt to ground. We measure the depinning current strength, decoupling current strength, and the critical coupling value in the "flux-transformer geometry." We also examine voltage branches in the I--V hysteresis curve for c-axis transport. Finally, we have used a simple cubic 3D XY model to simulate magnetic fields applied to a type II superconductor, tilted with respect to a cubic axis. Our model produces a "crossing lattices" state, wherein two distinct flux line lattices cross each other at right angles. We observe that the flux lines effectively form a "staircase" pattern in order to accommodate the crossing lattices structure. We also measure the helicity modulus as a function of temperature along the direction of each flux line lattice. We find that both lattices melt simultaneously, and that a small transverse field only slightly reduces the melting temperature. Similar behavior is found when this same model is treated within a simple mean field model.

Mohler, Gregory Allan

135

Experiences and results from an automated noise surveillance system of the Fast Flux Test Facility  

SciTech Connect

This paper describes recent accomplishments in the collection and interpretation of noise data from the Fast Flux Test Facility (FFTF). Automated data collection has provided data for a variety of reactor conditions and phenomena from several fuel cycles. This automated method has aided the interpretation of neutron noise from control rod vibrations and structure vibrations.

Thie, J.A.; Mullens, J.A.; Campbell, L.R.

1986-01-01

136

Theory of Flux-Flow Voltage Noise in Type-II Superconductors.  

National Technical Information Service (NTIS)

A theory for the flux-flow voltage noise in a type-II superconductor in the mixed state is developed. The interactions between the vortices are taken into account via a wavevector-dependent interaction matrix. Applied to Johnson noise in a film without fl...

P. Shuk-Yu Li

1981-01-01

137

On-line noise monitoring at the Fast Flux Test Facility  

SciTech Connect

An automated noise surveillance and diagnostics system (ANSDS) is being demonstrated at the Fast Flux Test Facility (FFTF). Three low-level, in-vessel fission chambers (LLFMs), three ex-vessel compensated ion chambers (CICs), and two accelerometers on the mechanism of one advanced absorber (ADVAB) control rod were monitored with an automated noise surveillance and diagnostic system (ANSDS) in late 1983.

Mullens, J.A.; Thie, J.A.; Campbell, L.R.

1984-01-01

138

Simulation of magnetic flux leakage (MFL) analysis using FEMM software  

Microsoft Academic Search

Magnetic flux leakage (MFL) is a method used in oil and gas piping systems which has potential in medical gas piping inspection, cleaning and gauging tasks without interrupting the pipeline transportation process. MFL is a magnetic method of nondestructive testing that applies a magnet to magnetize the steel. At areas where there is corrosion or missing metal, the magnetic field

Zulkarnay Zakaria; M. S. B. Mansor; A. H. Jahidin; M. S. Z. Azlan; Ruzairi Abdul Rahim

2010-01-01

139

Flux-Weakening Capability of Flux-Switching Permanent Magnet Motor  

Microsoft Academic Search

A 12\\/10 flux-switching permanent magnet motor (FSPM) is described. Qualitative analysis of magnetic circuit indicates that a connection between stator segments is benefit to motor operation. From the basic principle of flux-weakening speed-extension operation, weaken magnetic factor and speed-extension multiple are calculated. The analysis results show that this kind of motor has higher flux-weakening capability than that of other general

Junyou Yang; Qiang Ma; Yuan Deng; Yongheng Liu

2010-01-01

140

Asymmetry of Magnetic Helicity Flux in Emerging Bipolar Active Regions  

NASA Astrophysics Data System (ADS)

We apply differential affine velocity estimator (DAVE) to the Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) 12-min line-of-sight magnetograms, and separately calculate the injected magnetic helicity for the leading and the following polarities of nine emerging bipolar active regions (ARs). Comparing magnetic helicity flux of the leading polarity with the following polarity, we find that six ARs studied in this paper have the following polarity that injected more magnetic helicity flux than that of the leading polarity. We also measure the mean area of each polarity in all the nine ARs, and find that the compact polarity tend to possess more magnetic helicity flux than the fragmented one. Our results confirm the previous studies on asymmetry of magnetic helicity that emerging bipolar ARs have a polarity preference in injecting magnetic helicity. Based on the changes of unsigned magnetic flux, we divide the emergence process into two evolutionary stages: (1) an increasing stage before the peak flux and (2) a constant or decreasing stage after the peak flux. Obvious changes on magnetic helicity flux can be seen during transition from one stage to another. Seven ARs have one or both polarity that changed the sign of magnetic helicity flux. Additionally, the prevailing polarity of the two ARs, which injects more magnetic helicity, changes form the following polarity to the leading one.

Yang, Dan; Jiang, Yunchun; Yang, Jiayan; Bi, Yi; Yang, Bo

2014-06-01

141

Giant Flux Ropes Observed in the Magnetized Ionosphere at Venus  

NASA Astrophysics Data System (ADS)

The Venus ionospheric response to solar and solar wind variations is most evident in its magnetic field properties. Early Pioneer Venus observations during the solar maximum revealed that the Venus ionosphere exhibits two magnetic states depending on the solar wind dynamic pressure conditions: magnetized ionosphere with large-scale horizontal magnetic field; or unmagnetized ionosphere with numerous small-scale thin structures, so-called flux ropes. Here we report yet another magnetic state of Venus' ionosphere, giant flux ropes in the magnetized ionosphere, using Venus Express magnetic field measurements during solar minimum. These giant flux ropes all have strong core fields and diameters of hundreds of kilometers, which is about the vertical dimension of the ionosphere. This finding represents the first observation of these giant flux ropes at Venus. The cause of these giant flux ropes remains unknown and speculative.

Zhang, Tielong; baumjohann, Wolfgang; Teh, Wai-Leong; Nakamura, Rumi; Russell, Christopher; Luhmann, Janet; Glassmeier, Karl-Heinz; Dubinin, Edik; Wei, Yong; Du, Aimin; Balikhin, Michael

2013-04-01

142

Giant flux ropes observed in the magnetized ionosphere at Venus  

NASA Astrophysics Data System (ADS)

The Venus ionospheric response to solar and solar wind variations is most evident in its magnetic field properties. Early Pioneer Venus observations during the solar maximum revealed that the Venus ionosphere exhibits two magnetic states depending on the solar wind dynamic pressure conditions: magnetized ionosphere with large-scale horizontal magnetic field; or unmagnetized ionosphere with numerous small-scale thin structures, so-called flux ropes. Here we report yet another magnetic state of Venus' ionosphere, giant flux ropes in the magnetized ionosphere, using Venus Express magnetic field measurements during solar minimum. These giant flux ropes all have strong core fields and diameters of hundreds of kilometers, which is about the vertical dimension of the ionosphere. This finding represents the first observation of these giant flux ropes at Venus. The cause of these giant flux ropes remains unknown and speculative.

Zhang, T. L.; Baumjohann, W.; Teh, W. L.; Nakamura, R.; Russell, C. T.; Luhmann, J. G.; Glassmeier, K. H.; Dubinin, E.; Wei, H. Y.; Du, A. M.; Lu, Q. M.; Wang, S.; Balikhin, M.

2012-12-01

143

Signal and noise estimation from magnetic resonance images  

Microsoft Academic Search

This thesis deals with the estimation of noise and signal from Magnetic Resonance (MR) images with a special reference to magnitude MR images. Furthermore, the estimation and improvement of the image signal-to-noise ratio (SNR) and an application to 3D segmentation is discussed. In order to process experimental data in a scientifically justified manner, knowledge of the underlying probability density function

Jan Sijbers

1998-01-01

144

Magnetic noise in single-machine rotating frequency converters  

NASA Astrophysics Data System (ADS)

The magnetic noise in rotating frequency converters with two magnetic fields in a common structure of an induction machine, each field having a different number of pole pairs corresponding to the frequency conversion ratio, is evaluated on the basis of harmonic analysis of the resultant magnetic induction in the air gap and the radial forces it generates. The rotor in small and medium-size machines is regarded as a solid cylinder so that the hollow stator cylinder remains the principal source of noise. Constraints on the numbers of stator slots and rotors slots are established so as to ensure minimum higher-frequency tooth noise and vibration, while structural requirements are stipulated for minimizing lower-frequency noise and vibration and avoiding resonances at any frequency. Typical design calculations for a three-phase motor-generator machine operating at 1:3 frequency converter are shown, including the amplitudes of design-controlled vibrations of magnetic origin.

Popov, V. I.

1985-01-01

145

Photospheric supergranular flows and magnetic flux emergence  

NASA Astrophysics Data System (ADS)

A recent study carried out on high-sensitivity SUNRISE/IMAX data has reported about areas of limited flux emergence in the quiet Sun. By exploiting an independent and longer (four hours) data set acquired by Hinode/SOT, we investigate these regions in more detail by analysing their spatial distribution and relation with the supergranular flow. Our findings, while confirming these calm areas, also show that the emergence rate of small magnetic elements is largely suppressed at the locations where the divergence of the supergranular plasma flows is positive. This means that the dead-calm areas previously reported in literature are not randomly distributed across the solar photosphere, but are linked to the supergranular cells themselves. These results are discussed in the framework of the recent literature.

Stangalini, M.

2014-01-01

146

Shot noise in magnetic tunnel junctions: evidence for sequential tunneling.  

PubMed

We report the experimental observation of sub-Poissonian shot noise in single magnetic tunnel junctions, indicating the importance of tunneling via impurity levels inside the tunnel barrier. For junctions with weak zero-bias anomaly in conductance, the Fano factor (normalized shot noise) depends on the magnetic configuration being enhanced for antiparallel alignment of the ferromagnetic electrodes. We propose a model of sequential tunneling through nonmagnetic and paramagnetic impurity levels inside the tunnel barrier to qualitatively explain the observations. PMID:17280445

Guerrero, R; Aliev, F G; Tserkovnyak, Y; Santos, T S; Moodera, J S

2006-12-31

147

Relationships of a growing magnetic flux region to flares  

NASA Technical Reports Server (NTRS)

The evolution of flare sites at the boundaries of major new and growing magnetic flux regions within complexes of active regions has been analyzed using H-alpha images. A spectrum of possible relationships of growing flux regions to flares is described. An 'intimate' interaction between old and new flux and flare sites occurs at the boundaries of their regions. Forced or 'intimidated' interaction involves new flux pushing older, lower flux density fields toward a neighboring old polarity inversion line, followed by the occurrence of a flare. In 'influential' interaction, magnetic lines of force over an old polarity inversion line reconnect to new emerging flux, and a flare occurs when the magnetic field overlying the filament becomes too weak to prevent its eruption. 'Inconsequential' interaction occurs when a new flux region is too small or has the wrong orientation for creating flare conditions. 'Incidental' interaction involves a flare occurring without any significant relationship to new flux regions.

Martin, S. F.; Bentley, R. D.; Schadee, A.; Antalova, A.; Kucera, A.; Dezso, L.; Gesztelyi, L.; Harvey, K. L.; Jones, H.; Livi, S. H. B.

1984-01-01

148

Magnetic flux ropes at the high-latitude magnetopause  

NASA Technical Reports Server (NTRS)

We examine the consequences of magnetic reconnection at the high-latitude magnetopause using a three-dimensional global magnetohydrodynamic simulation of the solar wind interaction with the Earth's magnetosphere. Magnetic field lines from the simulation reveal the formation of magnetic flux ropes during periods with northward interplanetary magnetic field. These flux ropes result from multiple reconnection processes between the lobes field lines and draped magnetosheath field lines that are convected around the flank of the magnetosphere. The flux ropes identified in the simulation are consistent with features observed in the magnetic field measured by Hawkeye-1 during some high-latitude magnetopause crossings.

Berchem, Jean; Raeder, Joachim; Ashour-Abdalla, Maha

1995-01-01

149

Magnetic flux ropes in 3-dimensional MHD simulations  

NASA Technical Reports Server (NTRS)

The interaction of the solar wind and the earth's magnetosphere is presently simulated by a 3D, time-dependent, global MHD method in order to model the magnetopause and magnetotail generation of magnetic flux ropes. It is noted that strongly twisted and localized magnetic flux tubes simular to magnetic flux ropes appear at the subpolar magnetopause when the IMF has a large azimuthal component, as well as a southward component. Plasmoids are generated in the magnetotail after the formation of a near-earth magnetic neutral line; the magnetic field lines have a helical structure that is connected from dawn to dusk.

Ogino, Tatsuki; Walker, Raymond J.; Ashour-Abdalla, Maha

1990-01-01

150

Effects of a remanent magnetization on the sensing signals in magnetic flux leakage type NDT  

Microsoft Academic Search

Sensitive detection of the defect signals in NDT system is critical. Because of a strong magnetic field in magnetic flux type NDT system, the object itself is magnetized and shows a hysteresis characteristics during the sensing, which might cause a distortion of the defect signals. In this research, the magnetization characteristics of a pipeline in the magnetic flux leakage type

KangSeo; Jae Man Han; Gwan Soo Park

2007-01-01

151

Flux qubit noise spectroscopy using Rabi oscillations under strong driving conditions  

NASA Astrophysics Data System (ADS)

We infer the high-frequency flux noise spectrum in a superconducting flux qubit by studying the decay of Rabi oscillations under strong driving conditions. The large anharmonicity of the qubit and its strong inductive coupling to a microwave line enabled high-amplitude driving without causing significant additional decoherence. Rabi frequencies up to 1.7 GHz were achieved, approaching the qubit's level splitting of 4.8 GHz, a regime where the rotating-wave approximation breaks down as a model for the driven dynamics. The spectral density of flux noise observed in the wide frequency range decreases with increasing frequency up to 300 MHz, where the spectral density is not very far from the extrapolation of the 1/f spectrum obtained from the free-induction-decay measurements. We discuss a possible origin of the flux noise due to surface electron spins.

Yoshihara, Fumiki; Nakamura, Yasunobu; Yan, Fei; Gustavsson, Simon; Bylander, Jonas; Oliver, William D.; Tsai, Jaw-Shen

2014-01-01

152

Modeling of Flux Switching Permanent Magnet Machines With Fourier Analysis  

Microsoft Academic Search

For applications demanding a high torque density and high speed capability, the flux switching permanent magnet machine is an excellent candidate. However, the double salient structure and nonlinear behavior increases the challenge to model the magnetic field distribution and torque output. To date, only the magnetic equivalent circuit (MEC) is employed to model the magnetic field in an analytical manner.

Bart L. J. Gysen; Esin Ilhan; Koen J. Meessen; Johannes J. H. Paulides; Elena A. Lomonova

2010-01-01

153

Force-Free Magnetic Flux Ropes in the Solar Corona  

Microsoft Academic Search

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

R. Wolfson

2003-01-01

154

Infrared lines as probes of solar magnetic features. XII. Magnetic flux tubes: evidence of convective collapse?  

Microsoft Academic Search

The magnetic field in the solar photosphere is mainly composed of magnetic flux tubes. Their formation is not well understood, largely due to an absence of observational tests of theoretical predictions. Here we use infrared polarimetric data to test and confirm the prediction that whereas the field strength of large flux tubes is almost independent of their magnetic flux, small

S. K. Solanki; D. Zufferey; H. Lin; I. Rueedi; J. R. Kuhn

1996-01-01

155

Calculation of flux-gate pickups for determining magnetic course  

Microsoft Academic Search

Induction sensors of the magnetic course of moving objects are usually based on differential flux-gate pickups with longitudinal excitation. This paper describes the scattering field of the core of the flux-gate pickup as a magnetic dipole field. The effect of this field on the synphase and quadrature components of the pickup signal is evaluated for cases of metallic and nonmetallic

V. V. Meleshko

1977-01-01

156

Adaptive noise cancelling of multichannel magnetic resonance sounding signals  

NASA Astrophysics Data System (ADS)

Adaptive noise cancelling of multichannel magnetic resonance sounding (MRS) signals is investigated. An analysis of the noise sources affecting MRS signals show that the applicability of adaptive noise cancelling is primarily limited to cancel powerline harmonics. The problems of handling spikes in MRS signals are discussed and an efficient algorithm for spike detection is presented. The optimum parameters for multichannel adaptive noise cancelling are identified through simulations with synthetic signals added to noise-only recordings from an MRS instrument. We discuss the design and the efficiency of different stacking methods. The results from multichannel adaptive noise cancelling are compared to time-domain multichannel Wiener filtering. Our results show that within the experimental uncertainty the two methods give identical results.

Dalgaard, E.; Auken, E.; Larsen, J. J.

2012-10-01

157

MAGNETIC BARKHAUSEN NOISE INDICATORS OF CRACKS IN STEEL  

Microsoft Academic Search

High resolution magnetic Barkhausen noise (MBN) measurements were performed on cracks that were present in a section of 390 mm diameter and 29 mm thick steel pipe. The MBN indicators considered were the MBN energy signal, the number of events, and the variation of pulse height distributions (PHD) in the crack vicinity. Results were compared with magnetic particle inspection and

THOMAS W. KRAUSE; D. L. ATHERTON; S. P. SULLIVAN

1997-01-01

158

Magnetic flux emergence and associated dynamic phenomena in the Sun.  

PubMed

We present a review of the process of magnetic flux emergence in the Sun. We focus on observations and numerical experiments that explore the dynamical rise of magnetic fields from the solar interior to the corona. We describe the response of the highly stratified solar atmosphere on flux emergence and, consequently, we present a comprehensive picture of the coupling between solar dynamic events and flux emergence. We discuss potential applications of this process in other astrophysical environments. PMID:22665894

Archontis, Vasilis

2012-07-13

159

PROTOSTELLAR ACCRETION FLOWS DESTABILIZED BY MAGNETIC FLUX REDISTRIBUTION  

SciTech Connect

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; Shang, Hsien [Academia Sinica, Institute of Astronomy and Astrophysics, Taipei, Taiwan (China); Li Zhiyun; Zhao Bo [University of Virginia, Astronomy Department, Charlottesville (United States)

2012-09-20

160

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

161

Control of noise in magnetic multilayers by spin torque  

NASA Astrophysics Data System (ADS)

In this work we show that the stability of magnetic nanostructures can be enhanced by time-dependent spin momentum transfer. Building reliable magnetic devices at smaller scales need to address the issue of thermal noise. Using two commonly studied magnetic systems with multiple stable states at zero temperature as examples, we show that periodic spin torques can enhance the stability of the system and hence suppress the noise due to interwell transitions. In the case of weak periodic spin torques, stochastic resonance which is usually associated with ac magnetic fields is also manifested for non-conservative torques. In more complex systems with a relatively low energy barrier, it is shown that high frequency spin torques can inhibit interwell transitions and in effect suppress the telegraph noise due to the switching between neigboring states.

Rebei, Adnan

2012-02-01

162

Magnetic field analysis of a novel flux switching transverse flux permanent magnet wind generator with 3-D FEM  

Microsoft Academic Search

Transverse flux permanent magnet machine (TFPMM) offers a higher power density than the conventional radial and axial ones. Based on the principle of TFPMM, a novel flux switching transverse flux PM generator (FS-TFPMG) with a unique structure is presented for low speed wind power applications in this paper. The stator space utilization can be improved by arranging more stator cores

Jianhu Yan; Heyun Lin; Yunkai Huang; Hengchuan Liu; Z. Q. Zhu

2009-01-01

163

The calculation of cutoffs of magnetic flux in misaligned, helical, explosive-driven magnetic flux-compression generators  

Microsoft Academic Search

The numerical method is proposed for the calculation of magnetic flux losses due to the emergence of cutoffs in case the alignment of the armature and stator in helical, explosive-driven, magnetic flux-compression generators is disturbed. Comparison is made of the experimental and theoretical results.

Mironychev

1995-01-01

164

Investigation of Low-Frequency Excess Flux Noise in DC SQUIDs at mK Temperatures  

Microsoft Academic Search

The excess low-frequency flux noise in dc super- conducting quantum interference devices (SQUIDs) operated at ultra-low temperatures was studied. A large number of single SQUIDs as well as SQUID arrays from 16 wafers fabricated over a period of six years were characterized at 4.2 K and 320 mK. Considering the large spread in the low-frequency noise at 4.2 K, there

Dietmar Drung; Jörn Beyer; Jan-Hendrik Storm; Margret Peters; Thomas Schurig

2011-01-01

165

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

166

Enhancement of magnetic flux leakage images of defects in carbon steel using Eigen vector based approach  

Microsoft Academic Search

Magnetic flux leakage images of defects are affected by incoherent noise due to non-linear permeability, surface roughness, stresses and lift-off variations. In this paper, an approach that utilizes the energy packing property of the Eigen values is used to process the tangential component images of machined notches in carbon steel plates. These images are obtained by scanning Giant-magneto resistive (GMR)

B. P. C. Rao; S. Thirunavukkarasu; K. Krishna Nand; T. Jayakumar; P. Kalyanasundaram; Baldev Raj

2008-01-01

167

Shot noise in magnetic tunnel junctions from first principles  

NASA Astrophysics Data System (ADS)

We compute the shot noise in ballistic and disordered Fe|MgO|Fe tunnel junctions by a wave-function-matching method. For tunnel barriers with ?5 atomic layers we find a suppression of the Fano factor as a function of the magnetic configuration. In the antiparallel configuration the shot noise is full up to a threshold bias that indicates the onset of resonant tunneling. We find excellent agreement with recent experiments when interface disorder is taken into account.

Liu, Kai; Xia, Ke; Bauer, Gerrit E. W.

2012-07-01

168

Multiresolution Analysis and Prediction of Solar Magnetic Flux  

Microsoft Academic Search

Synoptic maps of the solar magnetic field provide an important visualization of the global transport and evolution of the large-scale magnetic flux. The solar dynamo picture is dependent on both the spatial and time resolution. It is therefore interesting to study the solar magnetic activity for many resolutions at the same time. A multi-resolution analysis gives us the possibility to

Magnus Wik

2008-01-01

169

Regression Models for Identifying Noise Sources in Magnetic Resonance Images  

PubMed Central

Stochastic noise, susceptibility artifacts, magnetic field and radiofrequency inhomogeneities, and other noise components in magnetic resonance images (MRIs) can introduce serious bias into any measurements made with those images. We formally introduce three regression models including a Rician regression model and two associated normal models to characterize stochastic noise in various magnetic resonance imaging modalities, including diffusion-weighted imaging (DWI) and functional MRI (fMRI). Estimation algorithms are introduced to maximize the likelihood function of the three regression models. We also develop a diagnostic procedure for systematically exploring MR images to identify noise components other than simple stochastic noise, and to detect discrepancies between the fitted regression models and MRI data. The diagnostic procedure includes goodness-of-fit statistics, measures of influence, and tools for graphical display. The goodness-of-fit statistics can assess the key assumptions of the three regression models, whereas measures of influence can isolate outliers caused by certain noise components, including motion artifacts. The tools for graphical display permit graphical visualization of the values for the goodness-of-fit statistic and influence measures. Finally, we conduct simulation studies to evaluate performance of these methods, and we analyze a real dataset to illustrate how our diagnostic procedure localizes subtle image artifacts by detecting intravoxel variability that is not captured by the regression models.

Zhu, Hongtu; Li, Yimei; Ibrahim, Joseph G.; Shi, Xiaoyan; An, Hongyu; Chen, Yashen; Gao, Wei; Lin, Weili; Rowe, Daniel B.; Peterson, Bradley S.

2009-01-01

170

Amplitude and phase noise of magnetic tunnel junction oscillators  

NASA Astrophysics Data System (ADS)

The microwave emission linewidth of spin transfer torque nano-oscillators is closely related to their phase and amplitude noise that can be extracted from the magnetoresistive voltage signal V(t) using single shot time domain techniques. Here we report on phase and amplitude noise studies for MgO based magnetic tunnel junction oscillators. The analysis of the power spectral densities allows one to separate the linear and nonlinear contributions to the phase noise, the nonlinear contribution being due to the coupling between phase and amplitude. The coupling strength as well as the amplitude relaxation rate can be directly extracted.

Quinsat, M.; Gusakova, D.; Sierra, J. F.; Michel, J. P.; Houssameddine, D.; Delaet, B.; Cyrille, M.-C.; Ebels, U.; Dieny, B.; Buda-Prejbeanu, L. D.; Katine, J. A.; Mauri, D.; Zeltser, A.; Prigent, M.; Nallatamby, J.-C.; Sommet, R.

2010-11-01

171

Shot noise in magnetic double-barrier tunnel junctions  

NASA Astrophysics Data System (ADS)

We calculate shot noise and the corresponding Fano factors in magnetic double-barrier tunnel junctions. Two situations are analyzed: (i) the central metallic layer is nonmagnetic while the external ones are ferromagnetic, and (ii) all of the metallic layers are ferromagnetic. In the latter case, the number of various magnetic configurations of the junctions is larger, which improves the functionality of such devices. The corresponding shot noise and Fano factor are shown to depend on the magnetic configuration of the junctions. The effect of spin relaxation in the central layer is also taken into account. The theoretical results are compared with experimental data on the shot noise in Fe/MgO/Fe/MgO/Fe structures.

Szczepa?ski, T.; Dugaev, V. K.; Barna?, J.; Cascales, J. P.; Aliev, F. G.

2013-04-01

172

Band-Edge Noise Spectroscopy of a Magnetic Tunnel Junction  

NASA Astrophysics Data System (ADS)

We propose a conceptually new way to gather information on the electron bands of buried metal-(semiconductor-)insulator interfaces. The bias dependence of low frequency noise in Fe1-xVx/MgO/Fe(0magnetic electrodes. The change in magnitude of these noise anomalies with the magnetic state allows evaluating the degree of spin mixing between the spin polarized bands at the ferromagnet-insulator interface. Our results are in qualitative agreement with numerical calculations.

Aliev, Farkhad G.; Cascales, Juan Pedro; Hallal, Ali; Chshiev, Mairbek; Andrieu, Stephane

2014-05-01

173

Magnetic Flux Paradigm for Radio Loudness of Active Galactic Nuclei  

NASA Astrophysics Data System (ADS)

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; Begelman, Mitchell C.

2013-02-01

174

Characteristics of Ionospheric Magnetic Flux Ropes on Venus  

NASA Astrophysics Data System (ADS)

Because Venus has no intrinsic magnetic field, its atmosphere is more directly exposed to the solar wind than an atmosphere of a magnetized planet such as Earth. One observed consequence of this solar wind-atmosphere interaction is the presence of magnetic flux ropes, approximately cylindrical structures consisting of twisted magnetic field lines. The central region of a flux rope contains current that can transport charged particles and may therefore aid in atmospheric escape from Venus. Flux ropes in the ionosphere are observed more often during solar maximum periods, when increased photoionization creates an ionospheric thermal pressure sufficient to exclude the solar wind magnetic field. Despite the discovery of flux ropes more than 30 years ago and the availability of new observations since the arrival of Venus Express (VEX) in 2006, the formation mechanism for ionospheric flux ropes is still unresolved. We present the results of a manual survey of magnetic field data from the VEX magnetometer (MAG) for magnetic flux ropes, which present as localized peaks in magnetic field strength with field rotations consistent with flux rope geometry. We survey data from evenly spaced month-long time intervals from 2006 - 2012 to examine the effects of different stages of the solar cycle on flux rope properties such as location, half-length, and orientation relative to the planet. We present trends in the properties of observed flux ropes, how they are affected by the solar wind, and how they compare to previous results. This research is supported by a NASA Venus Express Supporting Investigator grant.

Steichen, Elise; McEnulty, T. R.; Molaverdikhani, K.; Brain, D.; Zhang, T.

2013-10-01

175

A novel high temperature superconducting magnetic flux pump for MRI magnets  

NASA Astrophysics Data System (ADS)

This paper presents a kind of minitype magnetic flux pump made of high temperature superconductor. This kind of novel high temperature superconducting (HTS) flux pump has not any mechanical revolving parts or thermal switches. The excitation current of copper coils in magnetic pole system is controlled by a singlechip. The structure design and operational principle have been described. The operating performance of the new model magnetic flux pump has been preliminarily tested. The experiments show that the maximum pumping current is approximately 200 A for Bi2223 flux pump and 80 A for MgB 2 flux pump operating at 20 K. By comparison, it is discovered that the operating temperature range is wider, the ripple is smaller and the pumping frequency is higher in Bi2223 flux pump than those in MgB 2 flux pump. These results indicate that the newly developed Bi2223 magnetic flux pump may efficiently compensate the magnetic field decay in HTS magnet and make the magnet operate in persistent current mode, this point is significant to the magnetic resonance imaging (MRI) magnets. This new flux pump is under construction presently. It is expected that the Bi2223 flux pump would be applied to the superconducting MRI magnets by further optimizing structure and improving working process.

Bai, Zhiming; Yan, Guo; Wu, Chunli; Ding, Shufang; Chen, Chuan

2010-10-01

176

Geometry and temperature dependence of low-frequency flux noise in dc SQUIDs  

NASA Astrophysics Data System (ADS)

Measurements on dc SQUIDs reveal a flux noise spectral density S?(f) = A^2/(f/1 Hz)^?. An analytic model assuming non-interacting spins localized at the surface of the SQUID loop predicts that the mean square noise scales as R/W---the radius and width of the loop, respectively. However, there are no established theories for the scaling of ? with geometry or the dependences of A and ? on temperature T. To test the predicted geometric scaling of this model experimentally, we measured flux noise in ten SQUIDs with systematically varying geometries. We find that, at fixed T, A^2 scales approximately as R. From the measured values of A and ?, we estimate the mean square flux noise, which does not scale with R. As T is lowered, ? increases significantly and in such a way that the spectra ``pivot'' about an approximately fixed frequency. This phenomenon implies that the mean square noise is temperature-dependent, an effect not predicted by the analytic model. We discuss our attempts to reconcile these discrepancies by considering the locking together of spins to form clusters.

Anton, S. M.; Birenbaum, J. S.; O'Kelley, S. R.; Golubev, D. S.; Hilton, G. C.; Cho, H.-M.; Irwin, K. D.; Bolkhovsky, V.; Braje, D. A.; Fitch, G.; Neeley, M.; Johnson, R. C.; Oliver, W. D.; Wellstood, F. C.; Clarke, John

2013-03-01

177

Acoustic emission from magnetic flux tubes in the solar network  

NASA Astrophysics Data System (ADS)

We present the results of three-dimensional numerical simulations to investigate the excitation of waves in the magnetic network of the Sun due to footpoint motions of a magnetic flux tube. We consider motions that typically mimic granular buffeting and vortex flows and implement them as driving motions at the base of the flux tube. The driving motions generates various MHD modes within the flux tube and acoustic waves in the ambient medium. The response of the upper atmosphere to the underlying photospheric motion and the role of the flux tube in channeling the waves is investigated. We compute the acoustic energy flux in the various wave modes across different boundary layers defined by the plasma and magnetic field parameters and examine the observational implications for chromospheric and coronal heating.

Vigeesh, G.; Hasan, S. S.

2013-06-01

178

Hybrid-Excited Flux-Switching Permanent-Magnet Machines With Iron Flux Bridges  

Microsoft Academic Search

Hybrid-excited machines utilize the synergies of permanent magnet (PM) and wound field machines. Flux-switching PM machines have emerged as an attractive machine type due to the fact that the excitation sources are located in the stator allowing a simple robust rotor whilst providing high torque density. This paper proposes topologies of hybrid-excited flux-switching PM machines incorporating iron flux bridges to

Richard L. Owen; Z. Q. Zhu; Geraint W. Jewell

2010-01-01

179

Testing of the Mark 101 Magnetic Flux Compression Generator.  

National Technical Information Service (NTIS)

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...

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

1986-01-01

180

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

181

Flux Displacers for Pulse Generation with Superconducting Multipole Magnets.  

National Technical Information Service (NTIS)

Short, steep pulses of high electrical energy are required to drive systems such as pulsed transmitters, light sources and plasma discharges. A low impedance stationary coil will generate such pulses when the magnetic flux threading the coil undergoes lar...

R. J. Thome, G. K. Gaule

1969-01-01

182

Localization of Metal-Induced Gap States at the Metal-Insulator Interface: Origin of Flux Noise in SQUIDs and Superconducting Qubits  

NASA Astrophysics Data System (ADS)

The origin of magnetic flux noise in dc Superconducting Quantum Interference Devices (SQUIDs) with a power spectrum scaling as 1/f (f is frequency) has been a puzzle for over 25 years. This noise limits both the low frequency performance of SQUIDs and the decoherence time of flux-sensitive superconducting qubits, making scaling-up for quantum computing problematic. Recent calculations and experiments indicate that the noise is generated by electrons that randomly reverse their spin directions. Their areal density of ˜ 5 x 10^17 m-2 is relatively insensitive to the nature of the superconductor and substrate. Here, we propose that the local magnetic moments originate in metal-induced gap states (MIGSs) localized by potential disorder at the metal-insulator interface. MIGSs are particularly sensitive to such disorder, so that the localized states have a Coulomb repulsion sufficiently large to make them singly occupied. Our calculations demonstrate that a modest level of disorder generates the required areal density of localized moments. This result suggests that magnetic flux noise could be reduced by fabricating superconductor-insulator interfaces with less disorder. Support: NSF DMR07-05941, US DOE De-AC02-05CH11231, Samsung Foundation, Teragrid, NERSC.

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

2010-03-01

183

Magnetic flux ropes of Venus - A paradigm for helical magnetic structures in astrophysical systems  

Microsoft Academic Search

The magnetic flux ropes of Venus are small scale (ion gyroradius) cylindrically symmetric structures observed in situ by the Pioneer Venus orbiter in the largely magnetic field-free ionosphere of the planet. The magnetic structure of flux ropes indicates that they are helical kink unstable for lambda > 100 km. The altitude distribution of the fractional volume occupied by ropes can

R. C. Elphic

1985-01-01

184

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

185

Magnetic Barkhausen noise: Stress-dependent mechanisms in steel  

SciTech Connect

Angular dependent magnetic Barkhausen noise energy signal measurements were performed on two similar pipeline steel samples up to tensile stresses of 330 MPa. Variations between the stress dependence of the energy signals at five investigated positions were interpreted in terms of the relative proportion of three stress-dependent mechanisms for modifying the magnetic properties of steel. These were: the change in the number and in the orientation of 180{degree} domain walls and the modification of pinning barriers.

Krause, T.W.; Pulfer, N.; Weyman, P.; Atherton, D.L. [Queen`s Univ., Kingston, Ontario (Canada). Dept. of Physics] [Queen`s Univ., Kingston, Ontario (Canada). Dept. of Physics

1996-09-01

186

Limits on Relativistic Magnetic Monopole Flux from RICE  

NASA Astrophysics Data System (ADS)

The Radio Ice Cherenkov Experiment (RICE) is a radio antenna array at the South Pole. A Monte Carlo simulation of magnetic monopole propagation through polar ice is used to determine RICE's cross-section for monopole detection. We present final results for ultrarelativistic (?>=10^7) magnetic monopole flux upper bounds based on RICE observations from 2001 through 2005. This limit is the strongest direct measurement of ultrarelativistic monopole flux.

Hogan, Daniel

2008-04-01

187

MHD Models of Solar Photospheric Magnetic Flux Concentrations  

Microsoft Academic Search

This contribution has two parts: the first part attempts to review the different approaches and approximations used for the theoretical description of small magnetic flux concentrations. Emphasis is laid on calculations for individual structures (flux tubes) while large-scale numerical simulations are not discussed. In the second part, a few specific topics are considered in somewhat more detail. It is pointed

M. Schüssler

1986-01-01

188

Kink Wave Propagation in Thin Isothermal Magnetic Flux Tubes  

NASA Astrophysics Data System (ADS)

We investigated the propagation of kink waves in thin and isothermal expanding flux tubes in cylindrical geometry. By using the method of radial expansion for fluctuating variables we obtained a new kink wave equation. We show that including the radial component of the tube magnetic field leads to cutoff-free propagation of kink waves along thin flux tubes.

Lopin, I. P.; Nagorny, I. G.; Nippolainen, E.

2014-08-01

189

The Catastrophe of Coronal Magnetic Flux Ropes in CMEs  

Microsoft Academic Search

A brief review is given on the progress made in the study of the catastrophe of coronal magnetic flux ropes with implication in coronal mass ejections (CMEs). Relevant studies have been so far limited to 2.5-D cases, with a flux rope levitating in the corona, either parallel to the photosphere in Cartesian geometry or encircling the Sun like a torus

Y. Q. Hu

2005-01-01

190

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter  

NASA Astrophysics Data System (ADS)

Context: Observations made with the spectropolarimeter onboard the HINODE satellite have detected abundant horizontal magnetic fields in the internetwork quiet Sun. Aims: We compare the results for the horizontal fields obtained at 630 nm with ground-based observations at 1.56 ?m, where the sensitivity to magnetic fields is higher than in the visible. Methods: We obtained 30-s integrated spectropolarimetric data of the quiet Sun on disc centre during a period of extremely stable and good seeing. The data have a rms noise in polarization of around 2 × 10-4 of the continuum intensity. The low noise level allows the spectra to be inverted with the SIR code. We compare the inversion results with proxies to determine the magnetic flux. Results: We confirm the presence of the horizontal fields in the quiet Sun internetwork as reported for the satellite data, including voids without linear polarization signal that extend over an area of a few granules. Voids in the circular polarization signal are only of granular scale. More than 60% of the surface show polarization signals of above four times the rms noise level. We find that the total magnetic flux contained in the more inclined to horizontal fields (? > 45°) is lower by a factor of around 2 than that of the less inclined fields. The proxies for flux determination are strongly affected by the thermodynamic state of the atmosphere, and hence, seem to be unreliable. Conclusions: During spells of good seeing conditions, adaptive optics can render ground-based slit-spectrograph observations at a 70-cm telescope equivalent to the seeing-free space-based data of half-meter class telescopes. We suggest that the difference in the ratio of horizontal to transversal flux between the ground-based infrared data and the satellite-based visible data is due to the different formation heights of the respective spectral lines. We emphasize that the true amount of magnetic flux cannot be derived directly from the spectra. For purely horizontal flux, one would need its vertical extension that has to be estimated by explicit modeling, using the observed spectra as boundary conditions, or be taken from MHD simulations. Time-series of the evolution of the magnetic flux and chromospheric diagnostics are needed to address its possible contribution to chromospheric heating. Appendices A and B are only available in electronic form at http://www.aanda.org

Beck, C.; Rezaei, R.

2009-08-01

191

Barkhausen noise from sintered permanent magnets  

SciTech Connect

The authors introduce the well known Barkhausen technique as a tool for investigation of reversal processes in hard magnetic nucleation-type material. First results of the experiments show the occurrence of correlated grain switching not to be neglectible in the samples of sintered Nd-Fe-B as the volume fraction of multiple Barkhausen jumps reaches 50% near coercivity.

Cuntze, G.; Brendel, H.; Hubert, A. [Univ. Erlangen-Nuernberg, Erlangen (Germany). Inst. fuer Werkstoffwissenschaften] [Univ. Erlangen-Nuernberg, Erlangen (Germany). Inst. fuer Werkstoffwissenschaften

1996-09-01

192

Evaluation of surface decarburization depth by magnetic Barkhausen noise technique  

NASA Astrophysics Data System (ADS)

Industrially unfavorable process of steel surface decarburization was induced by annealing in air. Two methods of after-anneal surface treatment were used: an acid pickling and a sand blasting. The obtained decarburized layers were examined by optical microscope, wave dispersive spectrometer, and surface X-ray diffraction method. Magnetic Barkhausen noise technique was tested for applicability of non-destructive characterization of the decarburized layer depth. A newly introduced parameter, Barkhausen noise coercivity, was proposed for practical use due to its sensitivity to decarburization and stability to measurement conditions. Other magnetic parameters, e.g. number of Barkhausen noise counts, were found to be sensitive to the compressive residual stress caused by the sand blasting.

Stupakov, O.; Perevertov, O.; Tomáš, I.; Skrbek, B.

2011-06-01

193

Magnetic flux density reconstruction using interleaved partial Fourier acquisitions in MREIT.  

PubMed

Magnetic resonance electrical impedance tomography (MREIT) has been introduced as a non-invasive modality to visualize the internal conductivity and/or current density of an electrically conductive object by the injection of current. In order to measure a magnetic flux density signal in MREIT, the phase difference approach in an interleaved encoding scheme cancels the systematic artifacts accumulated in phase signals and also reduces the random noise effect. However, it is important to reduce scan duration maintaining spatial resolution and sufficient contrast, in order to allow for practical in vivo implementation of MREIT. The purpose of this paper is to develop a coupled partial Fourier strategy in the interleaved sampling in order to reduce the total imaging time for an MREIT acquisition, whilst maintaining an SNR of the measured magnetic flux density comparable to what is achieved with complete k-space data. The proposed method uses two key steps: one is to update the magnetic flux density by updating the complex densities using the partially interleaved k-space data and the other is to fill in the missing k-space data iteratively using the updated background field inhomogeneity and magnetic flux density data. Results from numerical simulations and animal experiments demonstrate that the proposed method reduces considerably the scanning time and provides resolution of the recovered B(z) comparable to what is obtained from complete k-space data. PMID:21386139

Park, Hee Myung; Nam, Hyun Soo; Kwon, Oh In

2011-04-01

194

Large scale magnetic helicity fluxes estimated from MDI magnetic synoptic charts  

NASA Astrophysics Data System (ADS)

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 (LCT) to estimate the magnetic helicity evolution over the 23rd solar cycle from 1996 to 2009 by using 795 MDI magnetic synoptic charts. The main results are: 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 negative value at the beginning of the 23rd solar cycle to positive value at the end of the cycle, which also shows the similar trend from the normalized magnetic flux by using the magnetic flux. The net accumulated magnetic helicity is negative in the period between 1996 and 2009.

Yang, Shangbin; Zhang, Hongqi

2013-07-01

195

Stress effects on magnetic Barkhausen noise measurements from magnetized pipeline steel  

Microsoft Academic Search

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

Annie Hsu

2000-01-01

196

Probing magnetization dynamics of strongly interacting magnetic nanoparticles through magnetoresistive current noise measurements  

NASA Astrophysics Data System (ADS)

The dependence of tunnelling current fluctuations on temperature and magnetic field was studied in an organically capped magnetite (Fe3O4) nanocrystal (NC) array deposited between 30 nm spaced gold electrodes. Low-frequency Lorentzian (random telegraph) noise was observed around the magnetization freezing temperature due to magnetic moment switching of the NCs under zero magnetic fields, diminishing with a saturating magnetic field. The temperature dependence of current fluctuations followed the temperature dependence of magnetic susceptibility. This work offers a new tool for locally studying collective magnetization dynamics in strongly interacting magnetic NC arrays.

Taub, Nurit; Markovich, Gil

2010-12-01

197

Siphon flows in isolated magnetic flux tubes. 3: The equilibrium path of the flux tube arch  

NASA Technical Reports Server (NTRS)

The arched equilibrium path of a thin magnetic flux tube in a plane-stratified, nonmagnetic atmosphere is calculated for cases in which the flux tube contains a steady siphon flow. The large scale mechanical equilibrium of the flux tube involves a balance among the magnetic buoyancy force, the net magnetic tension force due to the curvature of the flux tube axis, and the inertial (centrifugal) force due to the siphon flow along curved streamlines. The ends of the flux tube are assumed to be pinned down by some other external force. Both isothermal and adiabatic siphon flows are considered for flux tubes in an isothermal external atmosphere. For the isothermal case, in the absence of a siphon flow the equilibrium path reduces to the static arch calculated by Parker (1975, 1979). The presence of a siphon flow causes the flux tube arch to bend more sharply, so that magnetic tension can overcome the additional straightening effect of the inertial force, and reduces the maximum width of the arch. The curvature of the arch increases as the siphon flow speed increases. For a critical siphon flow, with supercritical flow in the downstream leg, the arch is asymmetric, with greater curvature in the downstream leg of the arch. Adiabatic flow have qualitatively similar effects, except that adiabatic cooling reduces the buoyancy of the flux tube and thus leads to significantly wider arches. In some cases the cooling is strong enough to create negative buoyancy along sections of the flux tube, requiring upward curvature of the flux tube path along these sections and sometimes leading to unusual equilibrium paths of periodic, sinusoidal form.

Thomas, John H.; Montesinis, Benjamin

1989-01-01

198

Analytical Hybrid Model for Flux Switching Permanent Magnet Machines  

Microsoft Academic Search

With the emergence of energy related issues in the automotive sector, there is a tendency to find new efficient solutions to replace existing electrical machinery. One promising candidate is the flux switching permanent magnet machine (FSPMM). Due to its challenging structure and nonlinear characteristic, in the investigation of the machine, generally finite element method (FEM), and rarely the magnetic equivalent

E. Ilhan; B. L. J. Gysen; J. J. H. Paulides; E. A. Lomonova

2010-01-01

199

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

200

Intrinsic Magnetic Flux of the Electron's Orbital and Spin Motion  

Microsoft Academic Search

In analogy with the fact that there are magnetic moments associated respectively with the electron's orbital and spin motion in an atom we present several analyses on a proposal to introduce a concept of intrinsic magnetic flux associated with the electron's orbital and spin motion. It would be interesting to test or to demonstrate Faraday's and Lenz's laws of electromagnetic

K. K. Wan; M. Saglam

2006-01-01

201

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

202

Magnetic flux leakage modeling for mechanical damage in transmission pipelines  

Microsoft Academic Search

This paper presents a two stage FE model for prediction of magnetic flux leakage, resulting from mechanical damage. In the first stage the stress distribution associated with mechanical damage is obtained from a structural model. In the second stage the stress distribution is incorporated into a magnetic FE model, by mapping stress levels to permeability. MFL signals are calculated and

P. A. Ivanov; V. Zhang; C. H. Yeoh; H. Udpa; Y. Sun; S. S. Udpa; W. Lord

1998-01-01

203

The equilibrium structure of thin magnetic flux tubes. I  

NASA Technical Reports Server (NTRS)

A model atmosphere within a thin magnetic flux tube that is embedded in an arbitrarily stratified medium is presently constructed by solving the radiative transfer equation in the two-stream approximation for gray opacity, under the assumption that the magnetic field is sufficiently strong to warrant the neglect of both thermal conduction and convective diffusion; energy inside the flux tube therefore being transported solely by radiation. The structure of the internal atmosphere is determined on the basis of the hydrostatic and radiative equilibrium conditions of the tube embedded in an external atmosphere. The gas temperature along the axis of the tube is computed, and the geometry of the flux tube is determined on the basis of magnetic flux conservation and total pressure equilibrium.

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

1985-01-01

204

Effect of magnetic geometry on ELM heat flux profiles  

NASA Astrophysics Data System (ADS)

In this paper, we explore how precisely the magnetic up/down symmetry must be controlled to insure sharing of edge localized mode (ELM) heat flux between upper and lower divertors in a double-null tokamak. We show for DIII-D, using infrared thermography, that the spatial distribution of Type-I ELM energy is less strongly affected by variations in magnetic geometry than the time-averaged peak heat flux in attached discharges. The degree of control necessary to share ELM heat flux deposition equally between divertors was less stringent than the control needed to balance the time-averaged heat flux. ELM energy is transported more than four times further into the scrape-off layer (SOL) than the time-averaged heat flux.

Lasnier, C. J.; Leonard, A. W.; Petrie, T. W.; Watkins, J. G.

2001-03-01

205

Sigmoidal equilibria and eruptive instabilities in laboratory magnetic flux ropes  

NASA Astrophysics Data System (ADS)

The Magnetic Reconnection Experiment (MRX) has recently been modified to study quasi-statically driven line-tied magnetic flux ropes in the context of storage-and-release eruptions in the corona. Detailed in situ magnetic measurements and supporting MHD simulations permit quantitative analysis of the plasma behavior. We find that the behavior of these flux ropes depends strongly on the properties of the applied potential magnetic field arcade. For example, when the arcade is aligned parallel to the flux rope footpoints, force free currents induced in the expanding rope modify the pressure and tension in the arcade, resulting in a confined, quiescent discharge with a saturated kink instability. When the arcade is obliquely aligned to the footpoints, on the other hand, a highly sigmoidal equilibrium forms that can dynamically erupt (see Fig. 1 and Fig. 2). To our knowledge, these storage-and-release eruptions are the first of their kind to be produced in the laboratory. A new 2D magnetic probe array is used to map out the internal structure of the flux ropes during both the storage and the release phases of the discharge. The kink instability and the torus instability are studied as candidate eruptive mechanisms--the latter by varying the vertical gradient of the potential field arcade. We also investigate magnetic reconnection events that accompany the eruptions. The long-term objective of this work is to use internal magnetic measurements of the flux rope structure to better understand the evolution and eruption of comparable structures in the corona. This research is supported by DoE Contract Number DE-AC02-09CH11466 and by the Center for Magnetic Self-Organization (CMSO). Qualitative sketches of flux ropes formed in (1) a parallel potential field arcade; and (2) an oblique potential field arcade. One-dimensional magnetic measurements from (1) a parallel arcade discharge that is confined; and (2) an oblique arcade discharge that erupts.

Myers, C. E.; Yamada, M.; Belova, E.; Ji, H.; Yoo, J.

2013-12-01

206

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

207

RATES OF PHOTOSPHERIC MAGNETIC FLUX CANCELLATION MEASURED WITH HINODE  

SciTech Connect

Photospheric magnetic flux cancellation on the Sun is generally believed to be caused by magnetic reconnection occurring in the low solar atmosphere. Individual canceling magnetic features are observationally characterized by the rate of flux cancellation. The specific cancellation rate, defined as the rate of flux cancellation divided by the interface length, gives an accurate estimate of the electric field in the reconnecting current sheet. We have determined the specific cancellation rate using the magnetograms taken by the Solar Optical Telescope (SOT) aboard the Hinode satellite. The specific rates determined with SOT turned out to be systematically higher than those based on the data taken by the Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory. The median value of the specific cancellation rate was found to be 8 x 10{sup 6} G cm s{sup -1}-a value four times that obtained from the MDI data. This big difference is mainly due to a higher angular resolution and better sensitivity of the SOT, resulting in magnetic fluxes up to five times larger than those obtained from the MDI. The higher rates of flux cancellation correspond to either faster inflows or stronger magnetic fields of the reconnection inflow region, which may have important consequences for the physics of photospheric magnetic reconnection.

Park, Soyoung; Chae, Jongchul [Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea (Korea, Republic of); Litvinenko, Yuri E. [Department of Mathematics, University of Waikato, P.O. Box 3105, Hamilton (New Zealand)

2009-10-10

208

Limits on upgrade tracker layout from particle fluxes, signal-to-noise, and occupancy  

NASA Astrophysics Data System (ADS)

We have investigated the layout for the inner tracking detector for ATLAS at the high luminosity upgrade of the LHC with respect to radiation damage and occupancy. Using simulated particle fluxes, we evaluate the signal-to-noise ratio based on measured radiation damage data, and estimate the expected occupancy. Tracking performance simulations yield alternative occupancy numbers and we find satisfactory agreement between the both. The dependence of occupancy and signal-to-noise on the location are used to evaluate the layout of the upgraded ATLAS inner detector.

Sadrozinski, Hartmut F.-W.; Bogert, Alex; Nielsen, Jason; Affolder, Anthony; Dawson, Ian; Nicolas, Ludovic

2011-12-01

209

Non-Gaussian dephasing in flux qubits due to 1/f noise  

NASA Astrophysics Data System (ADS)

Recent experiments by Yoshihara [Phys. Rev. Lett. 97, 167001 (2006)] provided information on decoherence of the echo signal in Josephson-junction flux qubits at various bias conditions. These results were interpreted assuming a Gaussian model for the decoherence due to 1/f noise. Here we revisit this problem on the basis of the exactly solvable spin-fluctuator model reproducing detailed properties of the 1/f noise interacting with a qubit. We consider the time dependence of the echo signal and conclude that the results based on the Gaussian assumption need essential reconsideration.

Galperin, Y. M.; Altshuler, B. L.; Bergli, J.; Shantsev, D.; Vinokur, V.

2007-08-01

210

Scaling laws of free magnetic energy stored in a solar emerging flux region  

NASA Astrophysics Data System (ADS)

This Letter reports scaling laws of free magnetic energy stored in a solar emerging flux region which is a key to understanding the energetics of solar active phenomena such as solar flares and coronal mass ejections. By performing three-dimensional magnetohydrodynamic simulations that reproduce several emerging flux regions of different magnetic configurations, we derive power-law relationships among emerged magnetic flux, free magnetic energy and relative magnetic helicity in these emerging flux regions. Since magnetic flux is an observable quantity, the scaling law between magnetic flux and free magnetic energy may give a way to estimate invisible free magnetic energy responsible for solar active phenomena.

Magara, Tetsuya

2014-07-01

211

On the use of magnetic bearings for global control of fan noise  

Microsoft Academic Search

In this paper, a novel approach is presented to reduce fan related noise in an acoustic duct. By installing magnetic bearings on the noise producing machinery, it is possible to collocate the anti-noise source with the disturbance noise source. This approach allows for global noise reduction throughout the duct system

J. M. Watkins; G. E. Piper; D. C. Baumann

1999-01-01

212

The origin of noise and magnetic hysteresis in crystalline permalloy ring-core fluxgate sensors  

NASA Astrophysics Data System (ADS)

6-81.3 Mo permalloy, developed in the 1960s for use in high performance ring-core fluxgate sensors, remains the state-of-the-art for permalloy-cored fluxgate magnetometers. The magnetic properties of 6-81.3, namely magnetocrystalline and magnetoelastic anisotropies and saturation induction are all optimum in the Fe-Ni-Mo system. In such polycrystalline permalloy fluxgate sensors a single phenomenon may cause both fluxgate noise and magnetic hysteresis, explain Barkhausen jumps, remanence and coercivity, and avoid domain denucleation. The phenomenon, domain wall reconnection, is presented as part of a theoretical model. In the unmagnetized state a coarse-grain high-quality permalloy foil ideally forms stripe domains, which present at the free surface as parallel, uniformly spaced domain walls that cross the entire thickness of the foil. Leakage flux "in" and "out" of alternating domains is a requirement of the random orientation, grain-by-grain, of magnetic easy axes' angles with respect to the foil free surface. Its magnetostatic energy together with domain wall energy determines an energy budget to be minimized. Throughout the magnetization cycle the free surface domain pattern remains essentially unchanged, due to the magnetostatic energy cost such a change would elicit. Thus domain walls are "pinned" to free surfaces. Driven to saturation, domain walls first bulge then reconnect via Barkhausen jumps to form a new domain configuration this author has called "channel domains", that are attached to free surfaces. The approach to saturation now continues as reversible channel domain compression. Driving the permalloy deeper into saturation compresses the channel domains to arbitrarily small thickness, but will not cause them to denucleate. Returning from saturation the channel domain structure will survive through zero H, thus explaining remanence. The Barkhausen jumps being irreversible exothermic events are sources of fluxgate noise, powered by the energy available from domain wall reconnection. A simplified domain energy model can then provide a predictive relation between ring core magnetic properties and fluxgate sensor noise power. Four properties are predicted to affect noise power, two of which, are well known: saturation total magnetic flux density and magnetic anisotropy. The two additional properties are easy axes alignment and foil thickness. Flux density and magnetic anisotropy are primary magnetic properties determined by an alloy's chemistry and crystalline lattice properties. Easy axes alignment and foil thickness are secondary, geometrical properties related to an alloy's polycrystalline fabric and manufacture. Improvements to fluxgate noise performance can in principle be achieved by optimizing any of these four properties in such a way as to minimize magnetostatic energy. Fluxgate signal power is proportional to B-H loop curvature (d2B/dH2). The degree to which Barkhausen jumps coincide with loop curvature is a measure of noise that accompanies fluxgate signal. B-H loops with significant curvature beyond the open hysteresis loop may be used to advantage to acquire fluxgate signal with reduced noise.

Narod, B. B.

2014-06-01

213

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

214

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

215

The sinusoidal air-gap magnetic field design of axial flux permanent magnet synchronous motors  

Microsoft Academic Search

The axial flux permanent magnet synchronous motors have features of short axial length, high energy density and great torque. They have found applications in computer peripherals, electric vehicles in-wheel drive and flywheel energy storage devices. The paper research analytically three methods for forming sinusoidal air-gap magnetic field, and put forward that the method with magnets of uniform magnetization, equal thickness

C. S. Zhang; Y. Fan; G. G. Li; H. Y. Chu

2005-01-01

216

Analysis of a Magnetization Effect on the Sensing Signals in Magnetic Flux Leakage type NDT  

Microsoft Academic Search

Because of the strong magnetic field of the PIG in magnetic flux type NDT system, the object itself is magnetized and shows the hysteresis characteristics during the sensing which might cause the distortion of the defect signals. In this paper, the magnetizations of the pipeline in MFL type NDT are analyzed, and the methods are developed to compensate the distorted

G. S. Park; K. Seo; D. S. Kim; P. S. Shim; M. H. Lee; H. R. Yoo; Y. W. Rho

2006-01-01

217

Twenty-three cycles of changing open solar magnetic flux  

NASA Astrophysics Data System (ADS)

This paper presents a comparison of various estimates of the open solar flux, deduced from measurements of the interplanetary magnetic field, from the aa geomagnetic index and from photospheric magnetic field observations. The first two of these estimates are made using the Ulysses discovery that the radial heliospheric field is approximately independent of heliographic latitude, the third makes use of the potential-field source surface method to map the total flux through the photosphere to the open flux at the top of the corona. The uncertainties associated with using the Ulysses result are 5%, but the effects of the assumptions of the potential field source surface method are harder to evaluate. Nevertheless, the three methods give similar results for the last three solar cycles when the data sets overlap. In 11-year running means, all three methods reveal that 1987 marked a significant peak in the long-term variation of the open solar flux. This peak is close to the solar minimum between sunspot cycles 21 and 22, and consequently the mean open flux (averaged from minimum to minimum) is similar for these two cycles. However, this similarity between cycles 21 and 22 in no way implies that the open flux is constant. The long-term variation shows that these cycles are fundamentally different in that the average open flux was rising during cycle 21 (from consistently lower values in cycle 20 and toward the peak in 1987) but was falling during cycle 22 (toward consistently lower values in cycle 23). The estimates from the geomagnetic aa index are unique as they extend from 1842 onwards (using the Helsinki extension). This variation gives strong anticorrelations, with very high statistical significance levels, with cosmic ray fluxes and with the abundances of the cosmogenic isotopes that they produce. Thus observations of photospheric magnetic fields, of cosmic ray fluxes, and of cosmogenic isotope abundances all support the long-term drifts in open solar flux reported by [1999a, 1999b].

Lockwood, M.

2003-03-01

218

Magnetic Sensitivity Beyond the Projection Noise Limit by Spin Squeezing  

NASA Astrophysics Data System (ADS)

We report the generation of spin squeezing and entanglement in a magnetically sensitive atomic ensemble, and entanglement-enhanced field measurements with this system. A maximal mf=ą1 Raman coherence is prepared in an ensemble of 8.5×105 laser-cooled Rb87 atoms in the f=1 hyperfine ground state, and the collective spin is squeezed by synthesized optical quantum nondemolition measurement. This prepares a state with large spin alignment and noise below the projection-noise level in a mixed alignment-orientation variable. 3.2 dB of noise reduction is observed and 2.0 dB of squeezing by the Wineland criterion, implying both entanglement and metrological advantage. Enhanced sensitivity is demonstrated in field measurements using alignment-to-orientation conversion.

Sewell, R. J.; Koschorreck, M.; Napolitano, M.; Dubost, B.; Behbood, N.; Mitchell, M. W.

2012-12-01

219

Magnetic sensitivity beyond the projection noise limit by spin squeezing.  

PubMed

We report the generation of spin squeezing and entanglement in a magnetically sensitive atomic ensemble, and entanglement-enhanced field measurements with this system. A maximal m(f) = ą 1 Raman coherence is prepared in an ensemble of 8.5 × 10(5) laser-cooled (87)Rb atoms in the f = 1 hyperfine ground state, and the collective spin is squeezed by synthesized optical quantum nondemolition measurement. This prepares a state with large spin alignment and noise below the projection-noise level in a mixed alignment-orientation variable. 3.2 dB of noise reduction is observed and 2.0 dB of squeezing by the Wineland criterion, implying both entanglement and metrological advantage. Enhanced sensitivity is demonstrated in field measurements using alignment-to-orientation conversion. PMID:23368463

Sewell, R J; Koschorreck, M; Napolitano, M; Dubost, B; Behbood, N; Mitchell, M W

2012-12-21

220

Analysis of the influence of some magnetizing parameters on magnetic Barkhausen noise using a microscopic model  

NASA Astrophysics Data System (ADS)

A microscopic model of magnetic Barkhausen noise (MBN) in carbon steel is proposed. The model uses the quasi-static magnetic formulation of Maxwell equations for electromagnetic fields combined with a microscopic model of the magnetic Barkhausen noise, and its equations are solved by means of finite difference formulation. The simulated MBN signal obtained presents high similarities to the measured MBN signal. Using this model, the influences of the uniformity and waveform profile of the excitation magnetic field on the envelope of the MBN signal were studied. The results show that the lack of uniformity of the excitation magnetic field increases the amplitude of the MBN envelope at the right of its main peak, and the waveform profile influences the shape of the MBN envelope. The proposed model can be used as a tool for studying the influence of several excitation parameters on the Barkhausen Noise in order to improve this technique.

Pérez-Benítez, J. A.; Espina-Hernández, J. H.; Martínez-Ortiz, P.; Chávez-González, A. F.; de la Rosa, J. M.

2013-12-01

221

Solar magnetic fields and convection. VI - Basic properties of magnetic flux tubes  

Microsoft Academic Search

Some basic properties of magnetic flux tubes are discussed, and Parker's (1976) recent criticisms of earlier papers are answered. It is shown that the helically twisted flux tubes (flux ropes) observed in active regions must be the uppermost sections of submerged ropes whose fields are everywhere greater than 4000 G and that a plausible flow pattern may account for the

J. H. Piddington

1976-01-01

222

Computation of flux switching in magnetic circuits  

Microsoft Academic Search

A number of physical and semiempirical flux-switching models that have appeared in the literature are reviewed briefly and classified. A thin ferrite core is switched by essentially a step mmf (with short rise time Tr). Three components of the totaldot{phi}(t)are distinguished and semiempirical models are proposed for each : 1) an initial elasticdot{phi}spike during the rise time ofF, dot{phi}_{epsilon} =

DAVID NITZAN

1965-01-01

223

Absolute determination of the magnetic flux quantum using superconducting magnetic levitation  

Microsoft Academic Search

The current status of our superconducting magnetic levitation experiment for determining the magnetic flux quantum is described. The flux-up system has been improved significantly by using a Josephson voltage standard. Studies are also in progress to improve the mechanical measurement relevant to the floating body with the goal of reducing the uncertainty to less than 1 ppm

F. Shiota; Y. Miki; A. Namba; Y. Nezu; Y. Sakamoto; T. Morokuma; K. Hara

1995-01-01

224

THE EVOLUTION OF OPEN MAGNETIC FLUX DRIVEN BY PHOTOSPHERIC DYNAMICS  

SciTech Connect

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 coworkers, in the form of a conjecture, argued that truly isolated open flux cannot exist in a configuration with one heliospheric current sheet-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 20 R{sub sun} 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. [Predictive Science, Inc., 9990 Mesa Rim Road, Suite 170, San Diego, CA 92121-2910 (United States); Antiochos, Spiro K., E-mail: linkerj@predsci.com, E-mail: lionel@predsci.com, E-mail: mikicz@predsci.com, E-mail: titovv@predsci.com, E-mail: spiro.antiochos@nasa.gov [NASA Goddard Space Flight Center, NASA/GSFC Code 674, Greenbelt, MD 20771 (United States)

2011-04-20

225

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

226

Magnetic clouds, helicity conservation, and intrinsic scale flux ropes  

NASA Technical Reports Server (NTRS)

An intrinsic-scale flux-rope model for interplanetary magnetic clouds, incorporating conservation of magnetic helicity, flux and mass is found to adequately explain clouds' average thermodynamic and magnetic properties. In spite their continuous expansion as they balloon into interplanetary space, magnetic clouds maintain high temperatures. This is shown to be due to magnetic energy dissipation. The temperature of an expanding cloud is shown to pass through a maximum above its starting temperature if the initial plasma beta in the cloud is less than 2/3. Excess magnetic pressure inside the cloud is not an important driver of the expansion as it is almost balanced by the tension in the helical field lines. It is conservation of magnetic helicity and flux that requires that clouds expand radially as they move away from the Sun. Comparison with published data shows good agreement between measured cloud properties and theory. Parameters determined from theoretical fits to the data, when extended back to the Sun, are consistent with the origin of interplanetary magnetic clouds in solar filament eruptions. A possible extension of the heating mechanism discussed here to heating of the solar corona is discussed.

Kumar, A.; Rust, D. M.

1995-01-01

227

Inference of physical phenomena from FFTF (Fast Flux Test Facility) noise analysis  

SciTech Connect

The source of features observed in noise spectra collected by an automated data collection system operated by the Oak Ridge National Laboratory at the Fast Flux Test Facility (FFTF) can be identified using a methodology based on careful data observation and intuition. When a large collection of data is available, as in this case, automatic pattern recognition and parameter storage and retrieval using a data base can be used to extract useful information. However, results can be limited to empirical signature comparison monitoring unless an effort is made to determine the noise sources. This paper describes the identification of several FFTF noise data phenomena and suggests how this understanding may lead to new or enhanced monitoring. 13 refs., 4 figs.

Thie, J.A.; Damiano, B.; Campbell, L.R.

1989-01-01

228

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

229

Noise-induced magnetization in sigma-delta data converters  

Microsoft Academic Search

We study the behavior of sigma-delta data converters using the Ising model description. In particular, we demonstrate how the magnetization diagram gives an estimate of the input stability boundary, which is an important engineering specification. In sigma-delta data converters, the presence of a small amount of thermal noise can lead to long-range order via a first order phase transition. By

Douglas J. Mar; M. K. Stephen Yeung; Carson C. Chow; Robert W. Adams; James J. Collins

2001-01-01

230

Statistical data analysis of magnetic recording noise mechanisms  

Microsoft Academic Search

Statistical data analysis using empirical eigenfunctions, known as the Karhunen-Loeve (K-L) expansion, is applied to characterize noise mechanisms in magnetic recording. Given any original data set and hence its correlation (covariance) matrix, an empirical orthogonal set of eigenfunctions can be obtained. The original data set can be expressed as an orthonormal expansion of these eigenfunctions. This feature of the K-L

Samuel W. Yuan; H. Neal Bertram

1992-01-01

231

Design Considerations for Tubular Flux-Switching Permanent Magnet Machines  

Microsoft Academic Search

This paper describes a tubular, three-phase, flux-switching permanent magnet (PM) brushless machine that combines salient features from switched reluctance (SR) and conventional PM brushless machines. Feasible slot-pole number combinations, which are also applicable to rotary flux-switching machines, are derived. This paper also examines an alternative stator winding configuration, which is unique to the tubular machine topology. It is shown that

Jiabin Wang; Weiya Wang; Kais Atallah; David Howe

2008-01-01

232

Equilibrium model of thin magnetic flux tubes. [solar atmosphere  

NASA Technical Reports Server (NTRS)

The existence of a physically realizable domain in which approximations that lead to a self consistent solution for flux tube stratification in the solar atmosphere, without ad hoc hypotheses, is proved. The transfer equation is solved assuming that no energy transport other than radiative is present. Convective motions inside the tube are assumed to be suppressed by magnetic forces. Only one parameter, the plasma beta at tau = 0, must be specified, and this can be estimated from observations of spatially resolved flux tubes.

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

1984-01-01

233

A study of magnetic flux-leakage signals  

Microsoft Academic Search

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

K. Mandal; D. L. Atherton

1998-01-01

234

Electron/ion whistler instabilities and magnetic noise bursts  

NASA Technical Reports Server (NTRS)

Two whistler instabilities are investigated by means of the linear Vlasov dispersion equation. They are called the electron/ion parallel and oblique whistler instabilities, and are driven by electron/ion relative drifts along the magnetic field. It is demonstrated that the enhanced fluctuations from these instabilities can explain several properties of magnetic noise bursts in and near the plasma sheet in the presence of ion beams and/or field-aligned currents. At sufficiently high plasma beta, these instabilities may affect the current system in the magnetotail.

Akimoto, K.; Gary, S. Peter; Omidi, N.

1987-01-01

235

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

236

Simulations of Emerging Magnetic Flux. I. The Formation of Stable Coronal Flux Ropes  

NASA Astrophysics Data System (ADS)

We present results from three-dimensional visco-resistive magnetohydrodynamic simulations of the emergence of a convection zone magnetic flux tube into a solar atmosphere containing a pre-existing dipole coronal field, which is orientated to minimize reconnection with the emerging field. We observe that the emergence process is capable of producing a coronal flux rope by the transfer of twist from the convection zone, as found in previous simulations. We find that this flux rope is stable, with no evidence of a fast rise, and that its ultimate height in the corona is determined by the strength of the pre-existing dipole field. We also find that although the electric currents in the initial convection zone flux tube are almost perfectly neutralized, the resultant coronal flux rope carries a significant net current. These results suggest that flux tube emergence is capable of creating non-current-neutralized stable flux ropes in the corona, tethered by overlying potential fields, a magnetic configuration that is believed to be the source of coronal mass ejections.

Leake, James E.; Linton, Mark G.; Török, Tibor

2013-12-01

237

Effects of MHD slow shocks propagating along magnetic flux tubes in a dipole magnetic field  

NASA Astrophysics Data System (ADS)

Variations of the plasma pressure in a magnetic flux tube can produce MHD waves evolving into shocks. In the case of a low plasma beta, plasma pressure pulses in the magnetic flux tube generate MHD slow shocks propagating along the tube. For converging magnetic field lines, such as in a dipole magnetic field, the cross section of the magnetic flux tube decreases enormously with increasing magnetic field strength. In such a case, the propagation of MHD waves along magnetic flux tubes is rather different from that in the case of uniform magnetic fields. In this paper, the propagation of MHD slow shocks is studied numerically using the ideal MHD equations in an approximation suitable for a thin magnetic flux tube with a low plasma beta. The results obtained in the numerical study show that the jumps in the plasma parameters at the MHD slow shock increase greatly while the shock is propagating in the narrowing magnetic flux tube. The results are applied to the case of the interaction between Jupiter and its satellite Io, the latter being considered as a source of plasma pressure pulses.

Erkaev, N. V.; Shaidurov, V. A.; Semenov, V. S.; Biernat, H. K.

238

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

239

Magnetic Flux Emergence In The Quiet Sun Photosphere  

NASA Astrophysics Data System (ADS)

We study the emergence of magnetic flux at very small spacial scales (less than 1 arcsec) in the quiet Sun internetwork. To this aim, several time series of spectropolarimetric maps were taken at disk center using the instrument SP/SOT on board Hinode. The LTE inversion of the full Stokes vector measured in the Fe I 6301 and 6302 lines will allow us to retrieve the magnetic flux and topology in the region of study. We find that the magnetic flux emerges typically within the granular structures. In many cases, the horizontal magnetic field appears prior to any significant amount of vertical field. As time goes on, the traces of the horizontal field dissapear while the the vertical dipoles drift -carried by the plasma motions- towards the surrounding intergranular lanes. Sometimes they stay trapped there for a while but they eventually either disappear by disgregation/cancelation or agregate to other magnetic field concentrations giving rise to larger flux elements. The time scale of these events is of the order of 10-20 minutes.

Centeno, Rebecca; Lites, B.; Socas-Navarro, H.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.; Tsuneta, S.; Katsukawa, Y.; Suematsu, Y.; Kubo, M.; Shimizu, T.

2007-05-01

240

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

241

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

242

Relativistic magnetic monopole flux constraints from RICE  

NASA Astrophysics Data System (ADS)

We report an upper limit on the flux of relativistic monopoles based on the nonobservation of in-ice showers by the Radio Ice Cherenkov Experiment (RICE) at the South Pole. We obtain a 95% C.L. limit of order 10-18(cm2ssr)-1 for intermediate-mass monopoles of 107???1012 at the anticipated energy Etot=1016GeV. This bound is over an order of magnitude stronger than all previously published experimental limits for this range of boost parameters ? and exceeds 2 orders of magnitude improvement over most of the range. We review the physics of radio detection, describe a Monte Carlo simulation including continuous and stochastic energy losses, and compare to previous experimental limits.

Hogan, D. P.; Besson, D. Z.; Ralston, J. P.; Kravchenko, I.; Seckel, D.

2008-10-01

243

Magnetic flux-load current interactions in ferrous conductors  

NASA Astrophysics Data System (ADS)

A modeling technique has been developed to account for interactions between load current and magnetic flux in an iron conductor. Such a conductor would be used in the active region of a normally conducting homopolar machine. This approach has been experimentally verified and its application to a real machine demonstrated. Additionally, measurements of the resistivity of steel under the combined effects of magnetic field and current have been conducted.

Cannell, Michael J.; McConnell, Richard A.

1992-06-01

244

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

245

Magnetic Flux Transfer By The Solar Wind and Heliospheric Current System  

Microsoft Academic Search

We calculated the change of the magnetic flux through the surface encircled by the Earth's orbit. This change is associated with the magnetic flux transfer by the solar wind flow and exhibits clear 22-year periodicity. The magnetic flux transferred by the solar wind is of the same order of magnitude as the flux of the main solar mag- netic field

P. Israelevich; A. Ershkovich; T. Gombosi

2002-01-01

246

Microstructured magnetic materials for RF flux guides in magnetic resonance imaging.  

PubMed

Magnetic resonance imaging and spectroscopy systems use coils, either singly or as arrays, to intercept radio-frequency (RF) magnetic flux from regions of interest, often deep within the body. Here, we show that a new magnetic material offers novel possibilities for guiding RF flux to the receiver coil, permitting a clear image to be obtained where none might otherwise be detectable. The new material contains microstructure designed according to concepts taken from the field of photonic band gap materials. In the RF range, it has a magnetic permeability that can be produced to specification while exhibiting negligible direct-current magnetism. The latter property is vital to avoid perturbing the static and audio-frequency magnetic fields needed to obtain image and spectral data. The concept offers a new paradigm for the manipulation of RF flux in all nuclear magnetic resonance systems. PMID:11157159

Wiltshire, M C; Pendry, J B; Young, I R; Larkman, D J; Gilderdale, D J; Hajnal, J V

2001-02-01

247

Improved measurement of magnetic properties with 3D magnetic fluxes  

NASA Astrophysics Data System (ADS)

This paper, as a pioneering work, presents measurement techniques to characterize soft magnetic materials in three-dimensional (3D) space. A novel 3D magnetic property tester using cubic sample has been constructed and calibrated. Some phenomena and problems of measurement caused by the imperfect winding and misalignment of coils are analyzed. The correction methods are proposed and employed. The new soft magnetic composite SOMALOY™ 500 is investigated.

Zhong, J. J.; Zhu, J. G.; Lin, Z. W.; Guo, Y. G.; Sievert, J. D.

2005-04-01

248

Note: Magnetic noise from the inner wall of a magnetically shielded room.  

PubMed

We measured the thermal magnetic noise generated by the inner high-permeability wall of a magnetically shielded room. This room houses a magnetoencephalogram (MEG), which contains 102 "small" identical magnetometers. For the measurement, we created two large magnetometers by summing the outputs of 46 magnetometers equally on the helmet's left and right side, to look at the summed noise of the right and left vertical walls. From these summed outputs, we calculated the rms noise amplitude due to all six walls at the MEG location to be ~0.5 f T/?Hz at 100 Hz, only slowly rising with lower frequency. This is well below the system noise of each small MEG magnetometer, hence is negligible for the MEG. PMID:23742599

Khan, Sheraz; Cohen, David

2013-05-01

249

Advanced Flux-Switching Permanent Magnet Brushless Machines  

Microsoft Academic Search

This paper overviews the recent development and new topologies of flux-switching (FS) machines, with particularly emphasis on the permanent magnet (PM) type. Specific design issues, including winding configurations, combinations of stator and rotor pole numbers, rotor pole width, split ratio, etc., are investigated, while the torque capability of selected FSPM machines is also compared.

Z. Q. Zhu; J. T. Chen

2010-01-01

250

Large-Scale Solar Cycle Features of Photospheric Magnetic Flux  

Microsoft Academic Search

It is well accepted that the solar cycle originates from a magnetohydrodynamics dynamo deep inside the Sun. Many dynamo models have long been proposed based on a lot of observational constraints. In this Letter, using 342 NSO\\/Kitt Peak solar synoptic charts, we study the large-scale solar cycle features of photospheric magnetic flux to set further constraints. (1) We find the

Wenbin Song; Jingxiu Wang

2006-01-01

251

The Near 160 Day Periodicity in the Photospheric Magnetic Flux  

Microsoft Academic Search

A periodicity near 154 days was discovered in the number of high-energy solar flares detected by Solar Maximum Mission (SMM) and Geosynchronous Operational Environmental Satellites (GOES) during the time interval 1980-1984 (Rieger et al.; Dennis). In this paper, we analyze the historical records of photospheric magnetic flux to show that during solar cycle 21 the periodicity appeared in the photospheric

J. L. Ballester; R. Oliver; M. Carbonell

2002-01-01

252

The flux-rope-fibre theory of solar magnetic fields  

Microsoft Academic Search

The flux-rope theory of solar magnetic fields is reviewed briefly and, together with the dynamo theory, compared with various observational results. Dynamo and related theories are based on fields controlled by the plasma, and it is shown that such fields cannot account for the strong surface fields or even emerge without becoming tangled. Observations which appear uniquely explicable in terms

J. H. Piddington

1978-01-01

253

Simulation on high speed rail magnetic flux leakage inspection  

Microsoft Academic Search

Based on the background of high speed rail, this paper researches the speed effect on rail magnetic flux leakage (MFL) inspection and the recognition of rail oblique cracks by the finite element method (FEM). Firstly, the effect of the testing speed on the MFL signals is discussed theoretically by formulation; secondly, the research model is established in Ansoft software; thirdly,

Zhijun Chen; Jianqing Xuan; Ping Wang; Haitao Wang; Guiyun Tian

2011-01-01

254

3D FEM analysis in magnetic flux leakage method  

Microsoft Academic Search

The magnetic flux leakage (MFL) method is currently the most commonly used pipeline inspection technique. In this paper, 3D FEM is used to analyze the MFL signals, a generalized potential formulation to the magnetostatic field MFL problem is discussed, typical 3D defects are accurately modeled and detail MFL signal in test surface are calculated by the method. The relation between

Huang Zuoying; Que Peiwen; Chen Liang

2006-01-01

255

Magnetic flux leakage inspection of pipelines -- An operator's viewpoint  

Microsoft Academic Search

The use of magnetic flux leakage (MFL) technology for in-line, nondestructive evaluation of natural gas and other pipelines is a competent, cost-effective component in an efficient operation system. MFL provides a comprehensive analysis of metal loss defects, as well as other anomalies that could have a detrimental effect on the pipeline's operation if not discovered and remediated in a timely

1994-01-01

256

Steel pipeline testing using magnetic flux leakage method  

Microsoft Academic Search

With the widespread application and fast development of gas and oil pipeline network in China, the pipeline inspection technology has been used more extensively. The magnetic flux leakage (MFL) method has established itself as the most widely used in-line inspection technique for the evaluation of gas and oil pipelines. This paper presents the typical three dimensional (3D) defects were accurately

Xiang Li; Xunbo Li; Liang Chen; Guangxu Qin; Peifu Feng; Zuoying Huang

2008-01-01

257

Alternative magnetic flux leakage modalities for pipeline inspection  

Microsoft Academic Search

Increasing quality consciousness is placing higher demands on the accuracy and reliability of inspection systems used in defect detection and characterization. Non-destructive testing techniques often rely on using multi-transducer approaches to obtain greater defect sensitivity. This paper investigates the possibility of taking advantage of alternative modalities associated with the standard magnetic flux leakage tool to obtain additional defect information, while

G. Katragadda; W. Lord; Y. S. Sun; S. Udpa; L. Udpa

1996-01-01

258

Design of a Second Harmonic Flux Gate Magnetic Field Gradiometer  

Microsoft Academic Search

An instrument for the measurement of gradients in the earth's magnetic field has been constructed. The gradiometer, of 15-ft base length, is capable of operation for long periods with an error less than 20 ? (5 ??m). Detectors are of the tuned, second harmonic flux gate type. An automatic ambient-field nulling feature has been introduced to ease the requirement for

R. M. Morris; B. O. Pedersen

1961-01-01

259

AXIAL FLUX PERMANENT MAGNET GENERATORS FOR PICO-HYDROPOWER  

Microsoft Academic Search

This paper reviews research being undertaken to apply a novel electrical generator design, the axial flux permanent magnet (AFPM) machine, to pico-hydropower applications. Pico-hydropower is a well established and cost effective means of producing electricity in developing countries, but there is scope for improvement of cost and performance. AFPM machines have been explored in a variety of applications from electric

D. A. Howey

2009-01-01

260

Magnetic flux-load current interactions in ferrous conductors  

Microsoft Academic Search

A modeling technique has been developed to account for interactions between load current and magnetic flux in an iron conductor. Such a conductor would be used in the active region of a normally conducting homopolar machine. This approach has been experimentally verified and its application to a real machine demonstrated. Additionally, measurements of the resistivity of steel under the combined

Michael J. Cannell; Richard A. McConnell

1992-01-01

261

Energy buildup in coronal magnetic flux tubes  

NASA Technical Reports Server (NTRS)

A time-dependent two-dimensional MHD simulation is used to study the response of the magnetic field in coronal loops to photospheric motion. From an initially uniform field, circular sections of the ends of the loop are slowly rotated to represent the photospheric motion. The evolution of the field and flow is characterized by three phases: (1) a phase of negligible kinetic energy where the current and field are predominantly parallel; (2) a phase where the field twist increases, the axial field at and near the axis increases, and the axial field decreases in two cylindrical regions away from the axis; and (3) a phase in which a significant portion of the field makes several rotations at large radii, with a corresponding reducton in the axial field to a few percent of the initial value.

Steinolfson, R. S.; Tajima, T.

1987-01-01

262

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

263

On Signatures of Twisted Magnetic Flux Tube Emergence  

NASA Astrophysics Data System (ADS)

Recent studies of NOAA active region 10953, by Okamoto et al. ( Astrophys. J. Lett. 673, 215, 2008; Astrophys. J. 697, 913, 2009), have interpreted photospheric observations of changing widths of the polarities and reversal of the horizontal magnetic field component as signatures of the emergence of a twisted flux tube within the active region and along its internal polarity inversion line (PIL). A filament is observed along the PIL and the active region is assumed to have an arcade structure. To investigate this scenario, MacTaggart and Hood ( Astrophys. J. Lett. 716, 219, 2010) constructed a dynamic flux emergence model of a twisted cylinder emerging into an overlying arcade. The photospheric signatures observed by Okamoto et al. (2008, 2009) are present in the model although their underlying physical mechanisms differ. The model also produces two additional signatures that can be verified by the observations. The first is an increase in the unsigned magnetic flux in the photosphere at either side of the PIL. The second is the behaviour of characteristic photospheric flow profiles associated with twisted flux tube emergence. We look for these two signatures in AR 10953 and find negative results for the emergence of a twisted flux tube along the PIL. Instead, we interpret the photospheric behaviour along the PIL to be indicative of photospheric magnetic cancellation driven by flows from the dominant sunspot. Although we argue against flux emergence within this particular region, the work demonstrates the important relationship between theory and observations for the successful discovery and interpretation of signatures of flux emergence.

Vargas Domínguez, S.; MacTaggart, D.; Green, L.; van Driel-Gesztelyi, L.; Hood, A. W.

2012-05-01

264

Magneto optical imaging of magnetic flux in superconductors  

NASA Astrophysics Data System (ADS)

Magnetic flux motion in type II superconductors has been studied using magneto-optical imaging (MOI) technique. As a real-space magnetic flux imaging tool, MOI is very suitable for local magnetic characterization of superconductors owing to its large imaging range, capability of dynamic study, and ease of use. In this dissertation project, MOI has been widely used to study the vortex motion in critical state of YBa2Cu 3O7-delta (YBCO) films as well as the dendritic magnetic instability in MgB2 films. As the external magnetic field exceeds the lower critical field, magnetic flux penetrates a superconductor with the form of Abrikosov vortices. Vortices can be pinned by certain structure defects in superconductors, resulting in a non-equilibrium critical state in which a vortex density gradient is present. The behavior of superconductors in critical state is one of the most important subjects in applied superconductivity. Using MOI technique, we were able to directly measure the vortex distribution in a YBCO circular disk. The average flux density profiles were used to test the critical state models. It is well known that the properties of critical state such as flux pinning are strongly coupled to the microscopic structure of superconductors. One of my projects is to investigate the effect of substrate surface roughness on vortex pinning in YBCO films. We imaged the vortex motion in YBCO films that were grown on LaAlO3 (LAO) substrates with naturally formed substrate surface roughness. Enhanced vortex pinning due to substrate surface roughness was observed in YBCO thin films grown on LAO, while there was a tendency to form weak coupling in YBCO thick films. In newly found superconducting MgB2 thin films, we observed dendritic flux-jump due to thermal-magnetic instability of the critical state. In order to associate this instability with material-related superconducting parameters, we studied a series of MgB2 films with different electron scattering. We found that the occurrence of dendritic flux-jump is basically controlled by the flux flow resistivity. Finally, we used fractal concepts to study the scaling behaviors of flux pattern in superconducting films, which is expected to be a complex system with presence of quenched disorders and thermal disorders. Flux penetration into YBCO superconducting films with defect size l ˜ xi (coherence length) and l >> xi was examined by MOI. The scaling analysis of the flux pattern in the pinning disorder-dominated films (l ˜ xi) showed robust self-affine spatial correlation. However, a self-similar flux pattern was found in the porous disorder dominated films (l >> xi). It is demonstrated that scaling analysis is a powerful tool to investigate the microscopic interaction of vortex matter in superconductors.

Ye, Zuxin

265

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

266

GRANULAR-SCALE MAGNETIC FLUX CANCELLATIONS IN THE PHOTOSPHERE  

SciTech Connect

We investigate the evolution of five granular-scale magnetic flux cancellations just outside the moat region of a sunspot by using accurate spectropolarimetric measurements and G-band images with the Solar Optical Telescope (SOT) aboard Hinode. The opposite-polarity magnetic elements approach a junction of the intergranular lanes and then collide with each other there. The intergranular junction has strong redshifts, darker intensities than the regular intergranular lanes, and surface converging flows. This clearly confirms that the converging and downward convective motions are essential for the approaching process of the opposite-polarity magnetic elements. However, the motion of the approaching magnetic elements does not always match with their surrounding surface flow patterns in our observations. This suggests that, in addition to the surface flows, subsurface downward convective motions and subsurface magnetic connectivities are important for understanding the approach and collision of the opposite-polarity elements observed in the photosphere. We find that the horizontal magnetic field appears between the canceling opposite-polarity elements in only one event. The horizontal fields are observed along the intergranular lanes with Doppler redshifts. This cancellation is most probably a result of the submergence (retraction) of low-lying photospheric magnetic flux. In the other four events, the horizontal field is not observed between the opposite-polarity elements at any time when they approach and cancel each other. These approaching magnetic elements are more concentrated rather than gradually diffused, and they have nearly vertical fields even while they are in contact each other. We thus infer that the actual flux cancellations are highly time-dependent events at scales less than a pixel of Hinode SOT (about 200 km) near the solar surface.

Kubo, M.; Low, B. C.; Lites, B. W. [High Altitude Observatory, National Center for Atmospheric Research P.O. Box 3000, Boulder, CO 80307 (United States)

2010-04-01

267

The effect of a transverse magnetic field on 1/f noise in graphene  

NASA Astrophysics Data System (ADS)

The low frequency 1/f noise in graphene devices was studied in a transverse magnetic field of up to B = 14 T at temperatures T = 80 K and T = 285 K. The examined devices revealed a large physical magnetoresistance typical for graphene. At low magnetic fields (B < 2 T), the level of 1/f noise decreases with the magnetic field at both temperatures. The details of the 1/f noise response to the magnetic field depend on the gate voltage. However, in the high magnetic fields (B > 2 T), a strong increase of the noise level was observed for all gate biases.

Rumyantsev, S. L.; Coquillat, D.; Ribeiro, R.; Goiran, M.; Knap, W.; Shur, M. S.; Balandin, A. A.; Levinshtein, M. E.

2013-10-01

268

SIMULATIONS OF TURBULENT DYNAMOS DRIVEN BY THE MAGNETIC HELICITY FLUX  

SciTech Connect

We present results of numerical simulations of driven magnetohydrodynamic turbulence in a periodic box with an imposed large-scale sinusoidal shear. We produce strong large-scale magnetic fields through dynamo action. This dynamo action is highly dependent on the spectral properties of the forcing function. In particular, we can produce either dynamo or antidynamo action by choosing particular forcing functions for the small-scale turbulence. Our results show that the large-scale electromotive force is well correlated with the local accumulation of eddy-scale magnetic helicity. Conversely, the electromotive force is very weakly correlated with the large-scale magnetic field strength or its product with kinetic helicity. We argue that in these simulations large-scale dynamo action is driven by an inverse cascade of magnetic helicity, ultimately caused by the tendency of anisotropic turbulence to create a magnetic helicity flux.

Shapovalov, Dmitry S. [Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 (United States); Vishniac, Ethan T., E-mail: dsh@pha.jhu.edu, E-mail: ethan@mcmaster.ca [Department of Physics and Astronomy, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4M1 (Canada)

2011-09-01

269

Contagious Coronal Heating from Recurring Emergence of Magnetic Flux  

NASA Technical Reports Server (NTRS)

For each of six old bipolar active regions, we present and interpret Yohkoh/SXT and SOHO/MDI observations of the development, over several days, of enhanced coronal heating in and around the old bipole in response to new magnetic flux emerge= within the old bipole. The observations show: 1. In each active region, new flux emerges in the equatorward side of the old bipole, around a lone remaining leading sunspot and/or on the equatorward end of the neutral line of the old bipole. 2. The emerging field is marked by intense internal coronal heating, and enhanced coronal heating occurs in extended loops stemming from the emergence site. 3. In five of the six cases, a "rooster tail" of coronal loops in the poleward extent of the old bipole also brightens in response to the flux emergence. 4. There are episodes of enhanced coronal heating in surrounding magnetic fields that are contiguous with the old bipole but are not directly connected to the emerging field. From these observations, we suggest that the accommodation of localized newly emerged flux within an old active region entails far reaching adjustments in the 3D magnetic field throughout the active region and in surrounding fields in which the active region is embedded, and that these adjustments produce the extensive enhanced coronal heating. We also note that the reason for the recurrence of flux emergence in old active regions may be that active region flux tends to emerge in giant-cell convection downflows. If so, the poleward "rooster tail" is a coronal flag of a long-lasting downflow in the convection zone. This work was funded by NASA's Office of Space Science through the Solar Physics Supporting Research and Technology Program and the Sun-Earth Connection Guest Investigator Program.

Moore, Ronald L.; Falconer, David; Sterling, Alphonse; Whitaker, Ann F. (Technical Monitor)

2001-01-01

270

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 due to photospheric motions. It is suggested that the twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through the observing satellite. We show that the total pressure of several observed twisted tubes resembles the theoretically expected profile. The twist of the isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. We have also found some evidence that the flux tube walls can be associated with local heating of the plasma and elevated proton and electron temperatures. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. Stability analysis of twisted tubes shows that the critical twist angle of the tube with a homogeneous twist is 70°, but the angle can further decrease due to the motion of the tube with respect to the solar wind stream. The tubes with a stronger twist are unstable to the kink instability, therefore they probably cannot reach 1 AU.

Zaqarashvili, Teimuraz V.; Vörös, Zoltán; Narita, Yasuhito; Bruno, Roberto

2014-03-01

271

Magnetic Flux Ropes in the Solar Photosphere: The Vector Magnetic Field under Active Region Filaments  

Microsoft Academic Search

This study is an observational search for evidence of prominence-associated magnetic flux ropes at the photospheric level in plage regions away from sunspots. Although there are a few cases of supposed flux rope geometry in the photosphere of complex sunspot regions (notably ``delta sunspots''), such cases appear to be fairly rare and therefore do not contribute significantly to the suspected

B. W. Lites

2005-01-01

272

Magnetic field dependence of low frequency noise in tunnel magnetoresistance heads  

NASA Astrophysics Data System (ADS)

Field dependence of low frequency noise in tunnel magnetoresistance (TMR) heads with different barrier materials of TiO2, Al2O3, and MgO has been studied. It is found that for MgO-TMR heads, the noise amplitude increases as the sensor is driven from parallel to antiparallel magnetization configurations. However, this field dependence of the noise is not magnetic in origin and can be accounted for by the coherent tunneling mechanism in MgO-TMR heads. For TiO2- and Al2O3-TMR heads, low frequency noise is generally independent of magnetic field when the heads are biased properly by hard bias. A noise spike related to a resistance jump in the transfer curve is measured in a head without proper hard bias. Field-dependent low frequency noise is observed for TMR heads without hard bias. The noise amplitude follows the sensitivity function of the sensor and can be attributed to the thermally activated magnetization fluctuations. Spin transfer (ST) effect on low frequency noise was examined by noise measurements of field dependence at different current directions. Extra noise appears when the TMR head is in antiparallel magnetization configurations. The results suggest that the ST-induced extra noise can be either pure 1/f noise or the combination of both 1/f and random telegraph noises.

Han, G. C.; Zong, B. Y.; Luo, P.; Wang, C. C.

2010-05-01

273

Spin-Torque Effect on Thermally Excited Magnetization Fluctuation Noise in Tunneling Magnetoresistive Read Heads  

Microsoft Academic Search

Thespin-torque(ST)effectonthethermallyexcitedmagnetizationfluctuationnoise(thermalmag-noise)intunnelingmagnetoresistive (TMR) read heads is investigated in the GHz range using noise spectra. The strong noise spectral peak around 3.1 GHz is attributed to thermal mag-noise originating from the free layer. The peak intensity and linewidth strongly depend on both the external magnetic field and current density. These features demonstrate that the ST effect significantly influences the thermal mag-noise as

Yasushi Endo; Tetsuya Abe; Masahiro Yamaguchi

2011-01-01

274

Signal and noise estimation from magnetic resonance images  

NASA Astrophysics Data System (ADS)

This thesis deals with the estimation of noise and signal from Magnetic Resonance (MR) images with a special reference to magnitude MR images. Furthermore, the estimation and improvement of the image signal-to-noise ratio (SNR) and an application to 3D segmentation is discussed. In order to process experimental data in a scientifically justified manner, knowledge of the underlying probability density function (PDF) is indispensable. Therefore, the PDF's of raw as well as processed (i.e., magnitude and phase) MR data are derived to serve as a basis for solving of various estimation problems. The estimation problems include estimation of noise and signal from MR data. Conventional estimation techniques are compared to the Maximum Likelihood (ML) estimation method where use is made of the specific PDF characterizing the MR data. It is shown that conventional estimation methods often yield biased results. In addition, it is shown how the knowledge of the proper PDF can be used to systematic errors during estimation. Applications discussed include the estimation of T1 and T2 maps. Also, an efficient and accurate method for the estimation and improvement of the image SNR is proposed, based on the cross-correlation of two realizations of the same image. Finally, a new segmentation technique is described, which was developed for precise and accurate volume quantization from 3D MR data. Several applications are discussed.

Sijbers, Jan

275

Flux lattice relaxation, noise and symmetry-breaking in frustrated Josephson junction arrays  

SciTech Connect

The purpose of the present report is to study: (1) Relaxation from an initially random flux state. We wish to exhibit the multiple length and time scales involved in this relaxation, and the influence of thermal fluctuations. This noisy relaxation is controlled by the dynamics of various mesoscopic ``defect`` structures defined with respect to the underlying ground-state flux structure. The defects, their mesoscopic collective patterns, and their dynamics control a complex macroscopic response, but are themselves microscopically controlled by the competitions producing the ground-state flux completely. Such multiscaleresponses have been observed in other competing interaction systems e.g., spin glasses, random field magnets, and weakly pinned charge-density waves, as well as noisy responses in high-temperature superconductors. Various ``creep`` and ``stretched-exponential`` regimes have been proposed, as well as phenomenological scaling theories attempting to relate spatial domain sizes with temporal scales. (2) Noisy (i.e.,multitime-scale) voltages responses, and their relation to multilength scales, in current driven JJA`s in the presence of a uniform extended magnetic field which frustrates the flux order.

Gronbech-Jensen, N.; Bishop, A.R.; Lomdahl, P.S.

1993-03-01

276

Flux lattice relaxation, noise and symmetry-breaking in frustrated Josephson junction arrays  

SciTech Connect

The purpose of the present report is to study: (1) Relaxation from an initially random flux state. We wish to exhibit the multiple length and time scales involved in this relaxation, and the influence of thermal fluctuations. This noisy relaxation is controlled by the dynamics of various mesoscopic defect'' structures defined with respect to the underlying ground-state flux structure. The defects, their mesoscopic collective patterns, and their dynamics control a complex macroscopic response, but are themselves microscopically controlled by the competitions producing the ground-state flux completely. Such multiscaleresponses have been observed in other competing interaction systems e.g., spin glasses, random field magnets, and weakly pinned charge-density waves, as well as noisy responses in high-temperature superconductors. Various creep'' and stretched-exponential'' regimes have been proposed, as well as phenomenological scaling theories attempting to relate spatial domain sizes with temporal scales. (2) Noisy (i.e.,multitime-scale) voltages responses, and their relation to multilength scales, in current driven JJA's in the presence of a uniform extended magnetic field which frustrates the flux order.

Gronbech-Jensen, N.; Bishop, A.R.; Lomdahl, P.S.

1993-01-01

277

Effect of Interfacial Disorder on 1\\/f Noise in Magnetic Tunnel Junctions  

Microsoft Academic Search

Magnetic tunnel junctions (MTJs) have the potential for low field ( 1pT\\/Hz^0.5 @ 1 Hz) magnetic sensors. However, 1\\/f noise limits their performance. Here we correlate measured 1\\/f noise with dynamic Lorentz imaging and high-frequency ferromagnetic resonance (FMR) measurements. The measurements show that a large fraction of the 1\\/f noise is due to thermal fluctuations of nano-scale magnetic ripple structure

Stephen Russek; Justin Shaw; Juan Francisco Sierra

2007-01-01

278

Reconnection experiments with flux ropes near 3D magnetic nulls  

NASA Astrophysics Data System (ADS)

Depending on the topology and geometry of the magnetic field, a rich collection of magnetic reconnection scenarios is possible in 3D including reconnection at magnetic nulls. Nulls have been reported in the solar corona [1] and in Earth's magnetosphere [2], yet there are a limited number of laboratory observations. At the Versatile Toroidal Facility (VTF) we have implemented a new magnetic geometry with a pair of 3D null points in the background toroidal field. In the nominal symmetric configuration a field line connects the nulls. We form a flux rope along this field line and observe the rope rapidly restructuring and rewiring as the nulls develop. A suit of diagnostics will be deployed and results presented for the dynamics of the geometry. [1ex] [1] Fletcher et al., Astrophys. J. 554, 451(2001).[0ex] [2] Xiao et al., Nat. Phys. 2, 478 (2006).

Vrublevskis, A.; Egedal, J.; Le, A.

2012-10-01

279

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

280

Magnetic Flux Transport on Active Cool Stars and Starspot Lifetimes  

NASA Astrophysics Data System (ADS)

Rapidly rotating cool stars are known to have large magnetic regions at mid- to high-latitudes. Mid-latitude starspots and magnetic regions have characteristic lifetimes on the order of one month as observed using (Zeeman-) Doppler imaging techniques. The structure and detailed morphology of starspots are not observable at present. In this study, we present numerical simulations of the surface transport of bipolar magnetic regions (BMRs) and magnetic spots on stars which have radii and surface rotational shears of AB Doradus, the Sun, and the HR 1099 primary. The surface flux transport model is based on the magnetic induction equation for radial fields under the effects of surface differential rotation, meridional flow, and turbulent diffusion due to supergranulation. We calculate flux evolution and lifetimes of BMRs with different emergence latitudes, surface shear rates, and tilt angles. For BMRs comparable to the largest ones on the Sun, we find that varying the surface flows and tilt angle modifies the lifetimes over a range of a month. For very large BMRs (area fraction ~ 0.1) the assumption of Joy's law for the tilt angle - as compared to the case with zero tilt - leads to a significant increase of lifetime. Such regions can evolve to form circumpolar spots that live more than a year. Taking the observed weak latitudinal shear and the radius of the active subgiant component of HR 1099, we find longer BMR lifetimes as compared to the more strongly sheared AB Dor case. We have also considered the effect on decay and lifetimes of starspots if they are monolithic or a conglomerate of smaller spots of similar total size. We find these different configurations differ neither in their decay patterns, nor in their lifetimes. We also give an analytical explanation for the linear decay of magnetic flux in the monolithic-spot simulation.

Isik, E.; Schüssler, M.; Solanki, S. K.

2006-08-01

281

Magnetic Bipoles in Emerging Flux Regions on the Sun  

NASA Astrophysics Data System (ADS)

ABSTRACT. We analyse magnetograms and H-alpha filtergrams of an Emerging Flux Region. Small bipoles have been observed on the magnetograms emerging between opposite polarities. Separation velocities of the opposite poles for 45 bipoles observed on June 9, 1985 have been measured and are in the range 0.5 < Vs < 3.5 km/s. A significant magnetic flux increase in the region was observed due to contributions from the emerging bipoles. RESUMEN. Se analizan magnetogramas y filtrogramas en H-alfa de una region de flujo emergente. Se observan pequenos dipolos en los magnetogramas emergiendo entre polaridades opuestas. Se midieron velocidades de separacion de polos opuestos para 45 bipolos observados en junio 9 de 1985 y estan en el intervalo 0.5 < Vs < 3.5 km/s. Se observo un aumento significativo del flujo magnetico en la region debido a contribuciones de los bipolos emergentes. Key words: SUN-CHROMOSPHERE - SUN-MAGNETIC FIELDS

Barth, C. S.; Livi, S. H. B.

1990-11-01

282

Particle signature of magnetic flux transfer events at the magnetopause  

NASA Technical Reports Server (NTRS)

Energetic electron (E greater than 20 keV) and ion (E greater than 25 keV) enhancements have been observed using the Isee 1 and 2 spacecraft during magnetic flux transfer events in the dayside magnetosheath just outside the magnetopause. The ions are seen to be streaming along the magnetic field, filling the 90-180 deg pitch angle region. The electrons are more isotropic and yet exhibit slight anisotropy in the direction opposite to that of the ions. From their intensities and spectra the ions appear to be of magnetospheric origin. With the interpretation of the flux transfer events as 'patchy' interconnection of magnetosheath and magnetospheric field lines, the ions are then seen to be previously trapped magnetospheric particles escaping along freshly opened field lines.

Daly, P. W.; Keppler, E.; Williams, D. J.; Russell, C. T.

1981-01-01

283

Semiclassical gravitational effects near a singular magnetic flux  

NASA Astrophysics Data System (ADS)

We consider the backreaction of the vacuum polarization effect for a massive charged scalar field in the presence of a singular magnetic massless string on the background metric. Using semiclassical approach, we find the first-order (in ? units) metric modifications and the corresponding gravitational potential and deficit angle. It is shown that, in certain region of values of coupling constant and magnetic flux, the gravitational potential and deficit angle can be positive as well as negative over all distances from the string and can even change its sign. Unlike the case of massless scalar field, the gravitational corrections were found to have short-range behavior.

Gorkavenko, Volodymyr M.; Viznyuk, Alexander V.

2004-12-01

284

Coronal Heating and the Magnetic Flux Content of the Network  

NASA Technical Reports Server (NTRS)

Previously, from analysis of SOHO/EIT coronal images in combination with Kitt Peak magnetograms (Falconer et al 1998, ApJ, 501, 386-396), we found that the quiet corona is the sum of two components: the e-scale corona and the coronal network. The large-scale corona consists of all coronal-temperature (T approx. 10(exp 6) K) structures larger than supergranules (>approx.30,000 km). The coronal network (1) consists of all coronal-temperature structures smaller than supergranules, (2) is rooted in and loosely traces the photospheric magnetic network, (3) has its brightest features seated on polarity dividing fines (neutral lines) in the network magnetic flux, and (4) produces only about 5% of the total coronal emission in quiet regions. The heating of the coronal network is apparently magnetic in origin. Here, from analysis of EIT coronal images of quiet regions in combination with magnetograms of the same quiet regions from SOHO/MDI and from Kitt Peak, we examine the other 95% of the quiet corona and its relation to the underlying magnetic network. We find: (1) Dividing the large-scale corona into its bright and dim halves divides the area into bright "continents" and dark "oceans" having spans of 2-4 supergranules. (2) These patterns are also present in the photospheric magnetograms: the network is stronger under the bright half and weaker under the dim half. (3) The radiation from the large-scale corona increases roughly as the cube root of the magnetic flux content of the underlying magnetic network. In contrast, Fisher et A (1998, ApJ, 508, 985-998) found that the coronal radiation from an active region increases roughly linearly with the magnetic flux content of the active region. We assume, as is widely held, that nearly all of the large-scale corona is magnetically rooted in the network. Our results, together with the result of Fisher et al (1999), suggest that either the coronal heating in quiet regions has a large non-magnetic component, or, if the heating is predominantly produced via the magnetic field, the mechanism is significantly different than in active regions. This work is funded by NASA's Office of Space Science through the Solar Physics Supporting Research and Technology Program and the Sun-Earth Connection Guest Investigator Program.

Falconer, D. A.; Moore, R. L.; Porter, J. G.; Hathaway, D. H.; Whitaker, Ann F. (Technical Monitor)

2001-01-01

285

Laser-Driven Magnetic-Flux Compression: Theory and Experiments  

NASA Astrophysics Data System (ADS)

Laser-Driven Flux Compression (LDFC) is a technique used to compress the magnetic field in Inertial Confinement Fusion (ICF) targets driven by a laser. The compressed field in the ICF target is beneficial to the target performance. Embedding a magnetic field in a conventional ICF target reduces the heat loss if the central hot spot becomes magnetized. Higher hot spot temperatures lower the requirements on the implosion velocities, leading to larger shell masses and therefore higher energy gains. For a typical hot spot density of ˜ 10 g/cc, and temperature of ˜ 5 keV, a magnetic field B > 10 MG is required to magnetize the hot spot. Such a strong magnetic field is difficult to be externally generated. Instead of providing the strong magnetic field directly, a seed magnetic field much lower than the required field was provided and compressed by the imploding shell. The field needs to be compressed faster than its diffusion due to the finite resistivity of the fill gas and the shell. This requires the gas in the target being ionized by the shock so that the flux is frozen in the gas region and compressed by the imploding shell. In this thesis, theoretical models, numerical calculations, and basic experiments of flux compression in ICF targets are investigated. A measurable Lawson criterion, developed as a metric to assess the performance of an ICF target, is used to evaluate the benefits of suppressing the heat conductivity. A simple model is used to describe the process of field compression by shock waves during the shell implosion. The magnetohydrodynamics codes, LILAC-MHD and LILAC-MHD-SP, are used to simulate the field compression and the target performance. The Magneto-Inertial-Fusion-Electrical-Discharge-System (MIFEDS), the device providing the seed magnetic field, is described in detail. LDFC experiments using the OMEGA laser at the Laboratory for Laser Energetics are presented. The results include the first demonstration of ˜ 550-fold amplification of a 50 ˜ 60 kG seed field to a ˜ 30 MG compressed field using LDFC, and the first demonstration of 15% and 30 % ion temperature and neutron yield enhancement by compressed magnetic fields.

Chang, Po-Yu

286

Cogging Torque in Flux-Switching Permanent Magnet Machines  

Microsoft Academic Search

The influence of manufacturing tolerances on the cogging torque waveforms, as well as the back-EMF waveforms, in 3- and 5-phase flux-switching permanent magnet (FSPM) machines having different numbers of stator\\/rotor poles is investigated by finite element analyses and experiments. The cause of excessive cogging torque in the prototype FSPM machines has been identified. It is due to modular stator structure

Z. Q. Zhu; A. S. Thomas; J. T. Chen; G. W. Jewell

2009-01-01

287

Novel linear flux-switching permanent magnet machines  

Microsoft Academic Search

Several novel linear, either planar or tubular, flux-switching permanent magnet (FSPM) brushless machines with different winding configurations and mover slot\\/stator pole combinations are proposed and compared, with particular reference to the tubular format. Coil connections are determined from the coil back-EMF vectors. The influence of major design parameters, such as the split ratio, the stator pole width, the mover back-iron

Z. Q. Zhu; X. Chen; J. T. Chen; D. Howe; J. S. Dai

2008-01-01

288

Optimization of linear flux switching permanent magnet motor  

Microsoft Academic Search

Linear planar flux-switching permanent magnet (FSPM) brushless machines have significant potential for transportation application. A 6-slot\\/5-pole linear FSPM motor is optimized in this paper for maximum thrust force at a fixed copper loss and also for minimum cogging force by finite element (FE) analysis. The influence of major design parameters, such as the split ratio between the mover and stator,

W. Min; J. T. Chen; Z. Q. Zhu; Y. Zhu; G. H. Duan

2010-01-01

289

Testing of the Mark 101 magnetic flux compression generator  

Microsoft Academic Search

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

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

1986-01-01

290

Nature of the magnetic flux ropes in the Venus ionosphere  

SciTech Connect

The onset of gutter instability at the moving, accelerating ionopause on Venus may allow magnetic flux tubes of solar wind to penetrate the interior of the ionosphere. As the ionospheric ions cross the lateral tube walls, an axial current of solar-wind electrons will enter, twisting the field lines into a rope. Such a mechanism would explain data recorded on the Pioneer Venus mission.

Dubinin, E.M.; Izrailevich, P.L.; Podgornyi, I.M.; Shkol'nikova, S.I.

1980-03-01

291

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

292

Effect of Lift-off on Pipeline Magnetic Flux Leakage Ins pection 1  

Microsoft Academic Search

On pipeline magnetic flux leakage (MFL) inspection, welds and debris are present in all pipelines and cause lift-off of the sensors and\\/or the m agnetizer of the magnetic flux leakage inspection tool, and have the potential to affect the acqui red magnetic flux leakage data. By adopting ANSYS software to simulate the effect of lift- off on the MFL signal.

Lijian YANG; Guoguang ZHANG; Gang LIU; Songwei GAO

293

The Evolution and Fragmentation of Rising Magnetic Flux Tubes  

NASA Astrophysics Data System (ADS)

From its source at the base of the convection zone magnetic flux is believed to rise buoyantly to the solar surface in the form of isolated tubes. As it rises, the cross section of such a tube will be distorted through its interaction with the surrounding unmagnetized medium. This distortion greatly affects its rate of rise and can ultimately lead to its fragmentation into two parallel tubes. We derive a set of Boussinesq fluid equations for studying the perpendicular dynamics of a rising flux tube. Integrating these numerically shows the tube distorting and then separating into two fragments with opposing senses of fluid circulation. The same behavior was observed in numerical simulations by Schüssler (1979). These counter-rotating elements move apart horizontally from each other and eventually stop rising. A simple picture of isolated buoyant material explains this result and confirms that the rise of the flux is prevented by its fragmentation. This has important consequences for theories of magnetic flux tube emergence.

Longcope, D. W.; Fisher, G. H.; Arendt, S.

1996-06-01

294

Are heliospheric flows magnetic line- or flux-conserving?  

NASA Astrophysics Data System (ADS)

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 conservation and its violation (= magnetic reconnection) in 3-D, already done by other authors with different methods with respect to derivations and interpretations. Then we analyse different heliospheric plasma environments. As a model of the stagnation region/stagnation point in front of the heliospheric nose, we present and discuss the general solution of the ideal MHD Ohm's law in the vicinity of a 2-D stagnation point, which was found by us. We show that ideal MHD either leads necessarily to a diverging magnetic field strength in the vicinity of such a stagnation point, or to a vanishing mass density on the heliopause boundaries. In the case that components of the electric field parallel to the magnetic field do not exist due to the chosen form of the non-ideal Ohm's law, it is always possible to formulate the transport equation of the magnetic field as a modified ideal Ohm's law. We find that the form of the Ohm's law which is often used in heliospheric physics (see e.g. Baranov and Fahr, 2003), is not able to change magnetic topology and thus cannot lead to magnetic reconnection, which necessarily has to occur at the stagnation point. The diverging magnetic field, for instance, implies the breakdown of the flux freezing paradigm for the heliosphere. Its application, especially at the heliospheric nose, is therefore rather doubtful. We conclude that it is necessary to search for an Ohm's law which is able to violate magnetic topology conservation.

Nickeler, D. H.; Karlický, M.

2006-11-01

295

Proposal and Fundamental Analysis of Cylindrical Type Magnetic Flux Pump for High Field Magnet  

Microsoft Academic Search

We have proposed a new type superconducting power supply, cylindrical type magnetic flux pump (CTMFP) for high field magnet. This paper describes the structure and operating principle based on pumping equations, and also presents its fundamental characteristics by the finite element method (FEM). Since the proposed CTMFP that consists of three phase armature coils, an LTS Nb foil and a

Yoon Do Chung; Hyoung Ku Kang; Duck Kweon Bae; Yong Soo Yoon; Eung Ro Lee; Tae Kuk Ko

2010-01-01

296

Solar magnetic flux tube simulations with time-dependent ionization  

NASA Astrophysics Data System (ADS)

In the present work we expand the study of time-dependent ionization previously identified to be of pivotal importance for acoustic waves in solar magnetic flux tube simulations. We focus on longitudinal tube waves (LTW) known to be an important heating agent of solar magnetic regions. Our models also consider new results of wave energy generation as well as an updated determination of the mixing length of convection now identified as 1.8 scale heights in the upper solar convective layers. We present 1D wave simulations for the solar chromosphere by studying tubes of different spreading as a function of height aimed at representing tubes in environments of different magnetic filling factors. Multilevel radiative transfer has been applied to correctly represent the total chromospheric emission function. The effects of time-dependent ionization are significant in all models studied. They are most pronounced behind strong shocks and in low-density regions, i.e. the middle and high chromosphere. Concerning our models of different tube spreading, we attained pronounced differences between the various types of models, which were largely initiated by different degrees of dilution of the wave energy flux as well as the density structure partially shaped by strong shocks, if existing. Models showing a quasi-steady rise of temperature with height are obtained via monochromatic waves akin to previous acoustic simulations. However, longitudinal flux tube waves are identified as insufficient to heat the solar transition region and corona in agreement with previous studies.

Fawzy, D. E.; Cuntz, M.; Rammacher, W.

2012-11-01

297

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

298

Magnetic flux transport on active cool stars and starspot lifetimes  

NASA Astrophysics Data System (ADS)

Context: Many rapidly rotating cool stars show signatures of large magnetic regions at all latitudes. Mid-latitude starspots and magnetic regions have characteristic lifetimes of 1 month or less, as indicated by observations using (Zeeman-) Doppler imaging techniques. Aims: We aim to estimate the lifetimes of bipolar magnetic regions and starspots on the surfaces of cool stars. We consider different possible configurations for starspots and compare their flux variations and lifetimes based on a magnetic flux transport model. Methods: We carry out numerical simulations of the surface evolution of bipolar magnetic regions (BMRs) and magnetic spots on stars, which have radii and surface rotational shears of AB Doradus, the Sun, and the HR 1099 primary. The surface flux transport model is based on the magnetic induction equation for radial fields under the effects of surface differential rotation, meridional flow, and turbulent diffusion due to convective flow patterns. We calculate the flux evolution and the lifetimes of BMRs and unipolar starspots, varying the emergence latitude, surface shear rate, and tilt angle. Results: For BMRs comparable to the largest observed on the Sun, we find that varying the surface flows and the tilt angle modifies the lifetimes over a range of one month. For very large BMRs (area ~10% of the stellar surface) the assumption of a tilt angle increasing with latitude leads to a significant increase of lifetime, as compared to the case without tilt. Such regions can evolve to polar spots that live more than a year. Adopting the observed weak latitudinal shear and the radius of the active subgiant component of HR 1099, we find longer BMR lifetimes as compared to the more strongly sheared AB Dor case. Random emergence of six additional tilted bipoles in an activity belt at 60° latitude enhanced the lifetimes of polar caps up to 7 years. We have also compared the evolution and lifetime of monolithic starspots with those of conglomerates of smaller spots of similar total area. We find similar decay patterns and lifetimes for both configurations.

I?ik, E.; Schüssler, M.; Solanki, S. K.

2007-03-01

299

Influence of High Harmonics of Magnetic Fields on Trapped Magnetic Fluxes in HTS Bulk  

NASA Astrophysics Data System (ADS)

Various kinds of HTS bulk motors are proposed and have been developed. Generally, those motors are driven by semiconductor inverters and currents fed to the armature windings contain high harmonics. Therefore, the bulks are exposed to high harmonics magnetic fields and AC losses are produced in the bulks. The AC losses deteriorate the efficiency of the motors and cause temperature rise of the bulks which decrease the trapped magnetic fluxes of the bulks. Usually, electro-magnetic shielding devices are inserted between the bulks and armature windings. However, the shielding devices degrade compactness of the motors. Therefore, it is important to have knowledge of the influence of the high harmonics magnetic fields on the AC losses and trapped magnetic fluxes of the bulk for optimum design of the shielding devices. In this work, the authors experimentally study the influence of high harmonics magnetic fields.

Yamagishi, K.; Miyagi, D.; Tsukamoto, O.

300

Levitation in the field of a nonsuperconducting coil with magnetic flux stabilization  

NASA Astrophysics Data System (ADS)

A method providing the "frozen flux" conditions in a nonsuperconducting coil is suggested and demonstrated with a model. The feasibility of permanent magnet stable levitation in the field of the coil with magnetic flux stabilization and mean current control is shown. The method allows researchers to exploit permanent magnet-superconducting body interaction in physical devices, for example, to reproduce, using nonsuperconducting coils, the frozen magnetic flux conditions required for the stable levitation of the magnet over a superconducting body.

Koshurnikov, E. K.

2013-09-01

301

Eddy Current Loss in the Frame of a Flux-Switching Permanent Magnet Machine  

Microsoft Academic Search

In flux-switching permanent magnet machines, a significant leakage flux exists at the outer surface of the stator core. Since the leakage flux varies as the rotor rotates, a significant eddy current loss may be induced in the nonmagnetic frame. The leakage flux and the associated eddy current loss in a representative flux-switching machine are investigated by finite element analysis, on

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

2006-01-01

302

Optimization of multiply acquired magnetic flux density B(z) using ICNE-Multiecho train in MREIT.  

PubMed

The aim of magnetic resonance electrical impedance tomography (MREIT) is to visualize the electrical properties, conductivity or current density of an object by injection of current. Recently, the prolonged data acquisition time when using the injected current nonlinear encoding (ICNE) method has been advantageous for measurement of magnetic flux density data, Bz, for MREIT in the signal-to-noise ratio (SNR). However, the ICNE method results in undesirable side artifacts, such as blurring, chemical shift and phase artifacts, due to the long data acquisition under an inhomogeneous static field. In this paper, we apply the ICNE method to a gradient and spin echo (GRASE) multi-echo train pulse sequence in order to provide the multiple k-space lines during a single RF pulse period. We analyze the SNR of the measured multiple B(z) data using the proposed ICNE-Multiecho MR pulse sequence. By determining a weighting factor for B(z) data in each of the echoes, an optimized inversion formula for the magnetic flux density data is proposed for the ICNE-Multiecho MR sequence. Using the ICNE-Multiecho method, the quality of the measured magnetic flux density is considerably increased by the injection of a long current through the echo train length and by optimization of the voxel-by-voxel noise level of the B(z) value. Agarose-gel phantom experiments have demonstrated fewer artifacts and a better SNR using the ICNE-Multiecho method. Experimenting with the brain of an anesthetized dog, we collected valuable echoes by taking into account the noise level of each of the echoes and determined B(z) data by determining optimized weighting factors for the multiply acquired magnetic flux density data. PMID:20400810

Nam, Hyun Soo; Kwon, Oh In

2010-05-01

303

The effect of flux creep on the magnetization field in the SSC diopole magnets  

SciTech Connect

The sextuple fields of model SSC dipole magnets have been observed to change with time when the magnets are held at constant current under conditions similar to injection into the SSC accelerator. The changes in the sextupole component have close to a linear log time dependence, and is felt to be caused by flux creep decay of the magnetization currents in the superconductor filaments. Measurements of this decay have been made under various conditions. The conditions include various central field inductions and changes of field prior to when the decay was measured. The measured field decay in the dipole's sextupole is proportional to the magnitude and sign of the sextupole due to magnetization which was measured at the start of the decay. This suggests that the decay is a bulk superconductivity flux creep. Proximity coupling appears to play only a minor role in the flux creep according to recent LBL measurements with a stable power supply. 4 refs., 6 figs., 3 tabs.

Gilber, W.S.; Althaus, R.F.; Barale, P.J.; Benjegerdes, R.W.; Green, M.A.; Green, M.I.; Scanlan, R.M. (Lawrence Berkeley Lab., CA (USA))

1989-06-01

304

Dilation of force-free magnetic flux tubes. [solar magnetic field profiles  

NASA Technical Reports Server (NTRS)

A general study is presented of the mapping functions which relate the magnetic-field profiles across a force-free rope in segments subjected to various external pressures. The results reveal that if the external pressure falls below a certain critical level (dependent on the flux-current relation which defines the tube), the magnetic profile consists of an invariant core sheathed in a layer permeated by an azimuthal magnetic field.

Frankenthal, S.

1977-01-01

305

Potential micrometeoroid and orbital debris protection system using a gradient magnetic field and magnetic flux compression  

NASA Astrophysics Data System (ADS)

A system for using a magnetic field in conjunction with conventional shielding configurations to protect against micrometeoroid and orbital debris is presented. Analytical, numerical, and experimental studies of a conductor moving through a gradient magnetic field have been performed. The results show that in the high magnetic Reynolds number regime a conducting object will experience large forces that tend to deform it while moving through the gradient field. Additionally a configuration using magnetic flux compression is introduced to act as a magnetic shock absorber. Separately or together, this technology may augment or replace current protection designs for space systems.

Giffin, A.; Shneider, M. N.; Miles, R. B.

2010-08-01

306

Magnetic Flux Leakage Inspection of Gas Pipelines: The Effects of Remanent Magnetization. Topical Report, 1992-1994.  

National Technical Information Service (NTIS)

The Magnetic Flux Leakage (MFL) Technique is the most commonly used technique to inspect large diameter transmission pipelines. A typical MFL inspection system uses permanent magnets to apply an axially oriented magnetic field to the ferromagnetic pipe ma...

J. B. Nestleroth R. J. Davis

1995-01-01

307

Magnetic-field-sensing mechanism based on dual-vortex motion and magnetic noise  

NASA Astrophysics Data System (ADS)

In this study, we report two novel field sensing mechanisms using elliptical permalloy single layer. Using micromagnetic modeling, dual-vortex structure is observed and stabilized in elliptical permalloy single layer by applying hard bias field (along the y-axis) and vertical axis field (perpendicular to plane). During the increasing or decreasing of the hard bias field within certain range, the dual vortices would move away from or approach to each other at a constant velocity, leading to a positive correlation between the hard bias field and the vortex gap. By exploring the magnetic noise properties of the elliptical permalloy single layer under various vortex gap, the vortex gap is found to be positively correlated with both the FMR (Ferromagnetic Resonance) peak positions and the integrated thermally excited mag-noise. Therefore, the combination of the dual-vortex motion and the magnetic noise properties make it possible to measure external field (along hard bias direction) through measuring the FMR peak positions or integrated thermally mag-noise. This FMR-peak-based field sensing mechanism and integrated-noise-based field sensing introduce a simple field sensor structure with expected highest sensitivity to 1.1%/Oe and field detectable range over 1000 Oe, which is promising for potential sensor applications.

Zeng, Tui; Zhou, Yan; Lin, Ko-Wei; Lai, Pui-To; Pong, Philip W. T.

2014-05-01

308

A new stator-flux orientation strategy for flux-switching permanent magnet motor based on current-hysteresis control  

Microsoft Academic Search

A stator-flux orientation strategy based on current hysteresis for the flux-switching permanent magnet (FSPM) motor is proposed, in which the stator-PM FSPM motor is considered as a conventional rotor-PM surface-mounted motor and an equivalent rotor-orientated dq-axes synchronous reference frame is built although there are actually no rotary magnetic motive force produced by the stator magnets in the FSPM motor. Based

Wei Hua; Ming Cheng; Wei Lu; Hongyun Jia

2009-01-01

309

Nonlinear Parker instability of isolated magnetic flux in a plasma  

NASA Technical Reports Server (NTRS)

The nonlinear evolution of the Parker instability in an isolated horizontal magnetic-flux sheet embedded in a two-temperature layer atmosphere is studied by using a two-dimensional MHD code. In the solar case, this two-layer model is regarded as a simplified abstraction of the sun's photosphere/chromosphere and its overlying much hotter (coronal) envelope. The horizontal flux sheet is initially located in the lower temperature atmosphere so as to satisfy magnetostatic equilibrium under a constant gravitational acceleration. Ideal MHD is assumed, and only perturbations with k parallel to the magnetic-field lines are investigated. As the instability develops, the gas slides down the expanding loop, and the evacuated loop rises as a result of enhanced magnetic buoyancy. In the nonlinear regime of the instability, both the rise velocity of a magnetic loop and the local Alfven velocity at the top of the loop increase linearly with height and show self-similar behavior with height as long as the wavelength of the initial perturbation is much smaller than the horizontal size of the computing domain.

Shibata, K.; Tajima, T.; Matsumoto, R.; Hanawa, T.; Horiuchi, T.

1989-01-01

310

The Nature of Kink MHD Waves in Magnetic Flux Tubes  

NASA Astrophysics Data System (ADS)

We examine the nature of MHD kink waves. This is done by determining the frequency, the damping rate and the eigenfunctions of MHD kink waves for three widely different MHD waves cases: a compressible pressure-less plasma, an incompressible plasma and a compressible plasma with non-zero plasma pressure which allows for MHD radiation. The overall conclusion is that kink waves are very robust and do not care about the details of the MHD wave environment. In all three cases the frequency and the damping rate are for most practical purposes the same. In the magnetic flux tube the kink waves are in all three cases, to a high degree of accuracy incompressible waves with negligible pressure perturbations and with mainly horizontal motions. The main restoring force of kink waves in the magnetized flux tube is the magnetic tension force. The gradient pressure force cannot be neglected except when the frequency of the kink wave is equal or slightly differs from the local Alfvén frequency, i.e. in the resonant layer. In a non-magnetic external plasma the wave is of course acoustic. The adjective fast is not the correct adjective to characterize kink waves. If an adjective is to be used it should be Alfvénic. However, it is better to realize that kink waves have mixed properties and cannot be put in one single box.

Goossens, Marcel; Terradas, J.; Andries, J.; Arregui, I.; Ballester, J.

2009-05-01

311

A solar eruption triggered by the interaction between two magnetic flux systems with opposite magnetic helicity  

NASA Astrophysics Data System (ADS)

Context. In recent years the accumulation of magnetic helicity via emergence of new magnetic flux and/or shearing photospheric motions has been considered to play an important role in the destabilization processes that lead to eruptive phenomena occurring in the solar atmosphere. Aims: In this paper we want to highlight a specific aspect of magnetic helicity accumulation, providing new observational evidence of the role played by the interaction of magnetic fields characterized by opposite magnetic helicity signs in triggering solar eruption. Methods: We used 171 Ĺ TRACE data to describe a filament eruption on 2001 Nov. 1 in active region NOAA 9682 and MDI full disk line-of-sight magnetograms to measure the accumulation of magnetic helicity in corona before the event. We used the local correlation tracking (LCT) and the differential affine velocity estimator (DAVE) techniques to determine the horizontal velocities and two methods for estimating the magnetic helicity flux. Results: The chirality signatures of the filament involved in the eruption were ambiguous, and the overlying arcade visible during the main phase of the event was characterized by a mixing of helicity signs. However, the measures of the magnetic helicity flux allowed us to deduce that the magnetic helicity was positive in the whole active region where the event took place, while it was negative near the magnetic inversion line where the filament footpoints were located. Conclusions: These results suggest that the filament eruption may be caused by magnetic reconnection between two magnetic field systems characterized by opposite signs of magnetic helicity. We also find that only the DAVE method allowed us to obtain the crucial information on the horizontal velocity field near the magnetic inversion line.

Romano, P.; Pariat, E.; Sicari, M.; Zuccarello, F.

2011-01-01

312

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

313

Reconstruction of a magnetic flux rope from THEMIS observations  

NASA Astrophysics Data System (ADS)

We investigate a magnetic flux rope (MFR) observed by THEMIS near the duskside magnetopause on 20 May 2007 using the reconstruction technique based on solving the Grad-Shafranov equation. The MFR has characteristics distinct from the adjacent magnetosheath and magnetosphere regions. In spite of these differences, the reconstruction result shows that the MFR is connected simultaneously with both the magnetosheath and the magnetosphere in terms of the magnetic vector potential characteristics. This result provides strong evidence that the MFR represents the union of these two regions. It has a small spatial dimension of ~0.5 R E, a strong core magnetic field of >50 nT, and an intense axial current density of >40 nA/m2 with non-negligible current densities transverse to its axis.

Lui, A. T. Y.; Sibeck, D. G.; Phan, T.; Angelopoulos, V.; McFadden, J.; Carlson, C.; Larson, D.; Bonnell, J.; Glassmeier, K.-H.; Frey, S.

2008-04-01

314

Internal structure of a magnetic flux rope from Cluster observations  

NASA Astrophysics Data System (ADS)

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 center based on the magnetic field signature. This fortuitous situation reveals the irregular magnetic field structure in its inner core, which is a feature reported here for the first time. At the leading edge, the y-component of the electric field was dawnward against the current density direction (dynamo action) and the x-component of the Lorentz force was Earthward. These parameters reversed in direction at its trailing edge (load).

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

2007-04-01

315

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

316

Flux Trapping Properties of Bulk HIGH-TC Superconductors in Static Field-Cooling Magnetization  

NASA Astrophysics Data System (ADS)

The trapping process and saturation effect of trapped magnetic flux of bulk high-temperature superconductors by static field-cooling magnetization (FCM) are reported in the paper. With a cryogenic Bell Hall sensor attached on the center of the bulk surface, the synchronous magnetic signals were recorded during the whole magnetization process. It enables us to know the flux trapping behavior since the removal of the excitation field, as well as the subsequent flux relaxation phenomenon and the flux dissipation in the quench process of the bulk sample. With the help of flux mapping techniques, the relationship between the trapped flux and the applied field was further investigated; the saturation effect of trapped flux was discussed by comparing the peak trapped field and total magnetic flux of the bulk sample. These studies are useful to understand the basic flux trapping properties of bulk superconductors.

Deng, Z.; Tsuzuki, K.; Miki, M.; Felder, B.; Hara, S.; Izumi, M.

2013-06-01

317

Evolution of field-reversed configuration by flux enhancement with rotating magnetic field  

Microsoft Academic Search

We have numerically studied the transient behavior of the evolution of Field Reversed Configuration (FRC) plasma with rotating magnetic field. The plasma formed by a field reversed theta pinch is evolved by increasing the internal magnetic flux with a rotating magnetic field (RMF) and by regulating the axial magnetic flux. The dynamic responses of a plasma pressure, separatrix radius and

M. Ohnishi; J. Kitagaki; Y. Yamamoto; K. Yoshikawa

1997-01-01

318

Rapid single-flux-quantum circuits for low noise mK operation  

NASA Astrophysics Data System (ADS)

Rapid single-flux-quantum (RSFQ) technology has been proposed as control electronics for superconducting quantum bits because of the material and working temperature compatibility. In this work, we consider practical aspects of RSFQ circuit design for low noise low power operation. At the working temperature of 20 mK and operational frequency of 2 GHz, dissipated power per junction is reduced to 25 pW by using 6 ľA critical current junctions available at the Hypres and VTT low Jc fabrication process. To limit phonon temperature to 30 mK, a maximum of 40 junctions can be placed on a 5 mm × 5 mm chip. Electron temperature in resistive shunts of Josephson junctions is minimized by use of cooling fins, giving minimum electron temperatures of about 150 mK for the Hypres process and 70 mK for the VTT process.

Intiso, Samuel; Pekola, Jukka; Savin, Alexander; Devyatov, Ygor; Kidiyarova-Shevchenko, Anna

2006-05-01

319

Magnetic-flux-induced persistent currents in nonlinear mesoscopic rings  

NASA Astrophysics Data System (ADS)

We investigate magnetic-flux-induced persistent currents (PCs) in a one-dimensional nonlinear mesoscopic ring based on the Frenkel-Kontorova (FK) model. By applying a transfer-matrix technique, the energy spectra, the PCs, and the Thouless exponent are theoretically obtained. It is shown that the energy spectrum splits into sub-bands when the on-site energy is gradually increased, and in the flux-dependent energy spectra, the energy levels show different behaviors over the transition by breaking of analyticity. Meanwhile, the PC is determined by the magnetic flux, the on-site energy, and the Fermi level. The increment of the on-site energy leads to a dramatic suppression of the PC. When the Fermi level is in the vicinity of ``band'' gaps, the PC is limited considerably; otherwise, the PC increases by several orders of magnitude. The suppressed PC is related to the electronic localization of the FK ring, which is described by the Thouless exponents. Our investigations provide detailed information about the influence of nonlinear structure on the PC and contribute to its potential application in quantum devices.

Zhang, R. L.; Qi, D. X.; Peng, R. W.; Li, J.; Fan, R. H.; Huang, R. S.; Wang, Mu

2011-04-01

320

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

321

Magnetic flux emergence into the solar photosphere and chromosphere  

NASA Astrophysics Data System (ADS)

Aims: We model the emergence of magnetized plasma across granular convection cells and the low atmosphere, including layers up to the mid-chromosphere. Methods: Three-dimensional numerical experiments are carried out in which the equations of MHD and radiative transfer are solved self-consistently. We use the MURaM code, which assumes local thermodynamic equilibrium between plasma and radiation. Results: In the photosphere, we find good agreement between our simulation predictions and observational results obtained with the Hinode satellite for the velocity and magnetic fields. We also confirm earlier simulation results by other authors. Our experiments reveal a natural mechanism of formation of twisted magnetic flux tubes that results from the retraction of photospheric horizontal fields at new intergranular lanes in decaying granules. In the chromosphere, we present evidence for the non-radiative heating of the emerging magnetized plasma due to the passage of shocks and/or ohmic dissipation. We study the formation of high-temperature points in the magnetic domain. We detect two types of points, classified according to whether they have a photospheric counterpart or otherwise. We also find evidence of those two types in Hinode observations. Using Lagrangian tracing of a large statistical sample of fluid particles, we detect and study episodes of convective collapse of magnetic elements returning to the photosphere. On the other hand, we study the maximum heights reached by all tracers, magnetized or otherwise. Only a small fraction (1.3%) of the magnetic elements reach the mid-chromosphere (z>750 km), while virtually no unmagnetized elements in the sample rise above the level of the reverse granulation (a few 100 km above the photosphere). We find that the rise into the chromosphere occurs in the form of successive jumps with intermediate stops rather than in a smooth continuous fashion and propose a tentative explanation of this behavior. Finally, also using Lagrange tracing, we document the creation of high-temperature points in the chromosphere via rising shock fronts.

Tortosa-Andreu, A.; Moreno-Insertis, F.

2009-11-01

322

Low-frequency noise measurements on commercial magnetoresistive magnetic field sensors  

Microsoft Academic Search

Low-frequency noise was measured in the frequency range from 0.1 Hz to 10 kHz on a variety of commercially available magnetic sensors. The types of sensors investigated include anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) effect devices. The 1\\/f noise components of electronic and magnetic origin are identified by measuring sensor noise and sensitivity at various applied

Nathan A. Stutzke; Stephen E. Russek; David P. Pappas; Mark Tondra

2005-01-01

323

Low-frequency noise measurements on commercial magnetoresistive magnetic field sensors  

Microsoft Academic Search

Low-frequency noise was measured in the frequency range from 0.1 Hz to 10 kHz on a variety of commercially available magnetic sensors. The types of sensors investigated include anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) effect devices. The 1?f noise components of electronic and magnetic origin are identified by measuring sensor noise and sensitivity at various applied

Nathan A. Stutzke; Stephen E. Russek; David P. Pappas; Mark Tondra

2005-01-01

324

Suppression of excess noise in Transition-Edge Sensors using magnetic field and geometry  

Microsoft Academic Search

We report recent progress at NIST on Mo\\/Cu Transition-Edge Sensors (TESs). While the signal-band noise of our sensors agrees with theory, we observe excess high-frequency noise. We describe this noise and demonstrate that it can be strongly suppressed by a magnetic field perpendicular to the plane of the sensor. Both the excess noise and ?=(T\\/R)(dR\\/dT) depend strongly on field so

J. N. Ullom; W. B. Doriese; G. C. Hilton; J. A. Beall; S. Deiker; K. D. Irwin; C. D. Reintsema; L. R. Vale; Y. Xu

2004-01-01

325

SIMULATING THE 'SLIDING DOORS' EFFECT THROUGH MAGNETIC FLUX EMERGENCE  

SciTech Connect

Recent Hinode photospheric vector magnetogram observations have shown that the opposite polarities of a long arcade structure move apart and then come together. In addition to this 'sliding doors' effect, orientations of horizontal magnetic fields along the polarity inversion line on the photosphere evolve from a normal-polarity configuration to an inverse one. To explain this behavior, a simple model by Okamoto et al. suggested that it is the result of the emergence of a twisted flux rope. Here, we model this scenario using a three-dimensional megnatohydrodynamic simulation of a twisted flux rope emerging into a pre-existing overlying arcade. We construct magnetograms from the simulation and compare them with the observations. The model produces the two signatures mentioned above. However, the cause of the 'sliding doors' effect differs from the previous model.

MacTaggart, David; Hood, Alan W., E-mail: dm428@st-andrews.ac.u [School of Mathematics and Statistics, University of St Andrews, St Andrews, Fife KY16 9SS (United Kingdom)

2010-06-20

326

Laser-driven magnetic flux compression simulation with Nautilus  

NASA Astrophysics Data System (ADS)

The Tech-X fluid and MHD framework, Nautilus, is a finite-volume and finite-element shock-capturing code supporting both structured and unstructured meshes. Nautilus is an easy-to-access, parallel, 3D code available to the ICF community. It is incorporated with features relevant to magneto-inertial confinement fusion (MIF) simulations. Laser-driven magnetic-flux compression is an innovative approach to achieve MIF. A cylindrical target with initial seed magnetic field is compressed by energetic laser beams. The magnetic field that is ``frozen-in'' plasma gets compressed with the target. The resulting high magnetic field reduces electron thermal conductivity and improves alpha particle confinement, thus providing an additional thermal insulation of the fuel forming the hot spot. It reduces the energy and driver symmetry requirements for ignition. Features for MIF simulations, such as anisotropic heat conduction , laser ray tracing and energy deposition, have been demonstrated with Nautilus. We discuss our approach to determine the best algorithms for properly modeling laser-driven shock implosions with magnetic fields in conditions relevant to cylindrical MIF.

Zhou, C. D.; Loverich, J.; Hakim, A.

2010-11-01

327

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

328

System having unmodulated flux locked loop for measuring magnetic fields  

SciTech Connect

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

329

Magnetic flux leakage inspection of pipelines -- An operator's viewpoint  

SciTech Connect

The use of magnetic flux leakage (MFL) technology for in-line, nondestructive evaluation of natural gas and other pipelines is a competent, cost-effective component in an efficient operation system. MFL provides a comprehensive analysis of metal loss defects, as well as other anomalies that could have a detrimental effect on the pipeline's operation if not discovered and remediated in a timely manner. This article describes an operator's viewpoint as to the benefits of including this technology in a pipeline quality assurance and integrity program.

Schmidt, J.T. (Texas Eastern Transmission Corp., Houston, TX (United States))

1994-07-01

330

On magnetohydrodynamic thermal instabilities in magnetic flux tubes  

NASA Astrophysics Data System (ADS)

The authors discuss the stability of current-driven filamentary modes in magnetic flux tubes embedded in a plane-parallel atmosphere in LTE and in hydrostatic equilibrium. Within the tube only energy transport by radiation is considered. The dominant contribution to the opacity is due to H- ions and H atoms (in the Paschen continuum). The authors delimit a region in the parameter space of the equilibrium configuration in which the instability is effective, and they discuss the relevance of this process for the formation of structured coronae in late-type stars and accretion disks.

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

1985-12-01

331

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

332

How is open solar magnetic flux lost over the solar cycle?  

Microsoft Academic Search

The Sun's open magnetic field, magnetic flux dragged out into the heliosphere by the solar wind, varies by approximately a factor of 2 over the solar cycle. We consider the evolution of open solar flux in terms of a source and loss term. Open solar flux creation is likely to proceed at a rate dependent on the rate of photospheric

M. J. Owens; N. U. Crooker; M. Lockwood

2011-01-01

333

Decoupling suspension controller based on magnetic flux feedback.  

PubMed

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

334

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.

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

2013-01-01

335

Analysis of amplitudes of equatorial noise emissions and their variation with L, MLT and magnetic activity  

NASA Astrophysics Data System (ADS)

Wave-particle interactions are an important mechanism of energy exchange in the outer Van Allen radiation belt. These interactions can cause an increase or decrease of relativistic electron flux. The equatorial noise (EN) emissions (also called fast magnetosonic waves) are electromagnetic waves which could be effective in producing MeV electrons. EN emissions propagate predominantly within 10° of the geomagnetic equator at L shells from 1 to 10. Their frequency range is between the local proton cyclotron frequency and the lower hybrid resonance. We use a data set measured by the STAFF-SA instruments onboard four Cluster spacecraft from January 2001 to December 2010. We have compiled the list of the time intervals of the observed EN emissions during the investigated time period. For each interval we have computed an intensity profile of the wave magnetic field as a function of frequency. The frequency band is then determined by an automatic procedure and the measured power spectral densities are reliably transformed into wave amplitudes. The results are shown as a function of the McIlwain's parameter, magnetic local time and magnetic activity - Kp and Dst indexes. This work has received EU support through the FP7-Space grant agreement n 284520 for the MAARBLE collaborative research project.

Hrbá?ková, Zuzana; Santolík, Ond?ej; Cornilleau-Wehrlin, Nicole

2013-04-01

336

Low-frequency noise in serial arrays of MgO-based magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

We have studied the low-frequency noise in MgO-based magnetic tunnel junctions (MTJs) in serial configurations. Two types of junctions were compared: MTJ Wheatstone bridges and MTJ discrete resistors closely packed on a wafer die. We have characterized each individual junction to ensure that they have uniform parameters such as linear field sensitivity and noise level. In the array of bridges, the low-frequency noise decreases with an increasing number (N) of bridges, but does not scale with 1/N, as expected from noise theory. The deviation is likely due to the statistical dispersions in MTJ bridge resistance and normalized voltage noise. The total noise of the discrete resistor series does not scale with 1/N either, but rather exhibits a sinusoidal-like variation with N. We attribute it to the possible enhancement of noise from magnetic coupling among the tightly spaced MTJ elements.

Zhang, Wenzhe; Hao, Qiang; Xiao, Gang

2011-09-01

337

Comparison of flux-regulation capability of a hybrid-excited flux-switching machine with different magnet materials  

Microsoft Academic Search

This paper investigates the flux-regulation capability of a novel hybrid-excited flux-switching (HEFS) machine with alternative magnets materials based on finite element analysis. It is found that in the case of Ferrite the air-gap flux can be modulated to a wide range, while in the case of NdFeB, the regulation capability is limited due to high saturation.

Wei Hua; Gan Zhang; Ming Cheng; Xikai Sun

2010-01-01

338

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

339

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

340

Magnetic Barkhausen noise study of domain wall dynamics in grain-oriented 3% Si-Fe  

SciTech Connect

Magnetic Barkhausen noise measurements were performed on various samples of 3% Si steel laminates in order to clarify the relationship between the domain structure, grain orientation, and power losses. The total Barkhausen noise power has been measured versus applied field. The statistical parameters characterizing the Barkhausen noise were related to macroscopic material properties. The results obtained show that a correlation exists between the Barkhausen noise power and the total power losses. This makes it possible to connect the crystallographic structure to the magnetic behavior of grain oriented materials at both microscopic and macroscopic levels.

Birsan, M.; Szpunar, J.A. [McGill Univ., Montreal, Quebec (Canada). Dept. of Metallurgical Engineering] [McGill Univ., Montreal, Quebec (Canada). Dept. of Metallurgical Engineering; Krause, T.W.; Atherton, D.L. [Queen`s Univ., Kingston, Ontario (Canada). Dept. of Physics] [Queen`s Univ., Kingston, Ontario (Canada). Dept. of Physics

1996-03-01

341

Experiment and simulation study of 3D magnetic field sensing for magnetic flux leakage defect characterisation  

Microsoft Academic Search

Magnetic flux leakage (MFL) testing is widely used to detect and characterise defects in pipelines, rail tracks and other structures. The measurement of the two field components perpendicular to the test surface and parallel to the applied field in MFL systems is well established. However, it is rarely effective when the shapes of the specimens and defects with respect to

Yong Li; John Wilson; Gui Yun Tian

2007-01-01

342

A new on-line detection method of magnet flux linkage for permanent magnet synchronous motor  

Microsoft Academic Search

According to the shortcoming that KF and EKF can only estimate the parameter of the PMSM in time domain, a new multi-scale default detection method is developed by combining KF and wavelet transform. By monitoring the voltages and currents of the stator, it is possible to estimate the permanent magnet flux linkage on-line, which has the advantage of KF and

Chuanbo Wen

2009-01-01

343

Implications of Non-cylindrical Flux Ropes for Magnetic Cloud Reconstruction Techniques and the Interpretation of Double Flux Rope Events  

NASA Astrophysics Data System (ADS)

Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) which exhibit signatures consistent with a magnetic flux rope structure. Techniques for reconstructing flux rope orientation from single-point in situ observations typically assume the flux rope is locally cylindrical, e.g., minimum variance analysis (MVA) and force-free flux rope (FFFR) fitting. In this study, we outline a non-cylindrical magnetic flux rope model, in which the flux rope radius and axial curvature can both vary along the length of the axis. This model is not necessarily intended to represent the global structure of MCs, but it can be used to quantify the error in MC reconstruction resulting from the cylindrical approximation. When the local flux rope axis is approximately perpendicular to the heliocentric radial direction, which is also the effective spacecraft trajectory through a magnetic cloud, the error in using cylindrical reconstruction methods is relatively small (? 10?). However, as the local axis orientation becomes increasingly aligned with the radial direction, the spacecraft trajectory may pass close to the axis at two separate locations. This results in a magnetic field time series which deviates significantly from encounters with a force-free flux rope, and consequently the error in the axis orientation derived from cylindrical reconstructions can be as much as 90?. Such two-axis encounters can result in an apparent `double flux rope' signature in the magnetic field time series, sometimes observed in spacecraft data. Analysing each axis encounter independently produces reasonably accurate axis orientations with MVA, but larger errors with FFFR fitting.

Owens, M. J.; Démoulin, P.; Savani, N. P.; Lavraud, B.; Ruffenach, A.

2012-06-01

344

Estimating Total Heliospheric Magnetic Flux from Single-Point in Situ Measurements  

NASA Technical Reports Server (NTRS)

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 in the radial component of the magnetic field suggests that extrapolation from such single-point measurements to total heliospheric magnetic flux is possible. In this study we test this assumption using estimates of total heliospheric flux from well-separated heliospheric spacecraft and conclude that single-point measurements are indeed adequate proxies for the total heliospheric magnetic flux, though care must be taken when comparing flux estimates from data collected at different heliocentric distances.

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

2008-01-01

345

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

346

Signatures of secondary collisionless magnetic reconnection driven by kink instability of a flux rope  

NASA Astrophysics Data System (ADS)

The kinetic features of secondary magnetic reconnection in a single flux rope undergoing internal kink instability are studied by means of three-dimensional particle-in-cell simulations. Several signatures of secondary magnetic reconnection are identified in the plane perpendicular to the flux rope: a quadrupolar electron and ion density structure and a bipolar Hall magnetic field develop in proximity of the reconnection region. The most intense electric fields form perpendicularly to the local magnetic field, and a reconnection electric field is identified in the plane perpendicular to the flux rope. An electron current develops along the reconnection line, in the opposite direction of the electron current supporting the flux rope magnetic field structure. Along the reconnection line, several bipolar structures of the electric field parallel to the magnetic field occur, making the magnetic reconnection region turbulent. The reported signatures of secondary magnetic reconnection can help to localize magnetic reconnection events in space, astrophysical and fusion plasmas.

Markidis, S.; Lapenta, G.; Delzanno, G. L.; Henri, P.; Goldman, M. V.; Newman, D. L.; Intrator, T.; Laure, E.

2014-06-01

347

Magnetic flux leakage inspection of gas pipelines: The effects of remanent magnetization. Topical report, 1992-1994  

SciTech Connect

The Magnetic Flux Leakage (MFL) Technique is the most commonly used technique to inspect large diameter transmission pipelines. A typical MFL inspection system uses permanent magnets to apply an axially oriented magnetic field to the ferromagnetic pipe material. Remanent magnetization affects the applied-magnetization because pipleline steels have sufficient retentivity to influence the magnetization of subsequent inspections. The remanent magnetization affects detection and characterization of pipeline corrosion in two ways. First, remanent magnetization changes the strength of the applied field level for subsequent inspection runs. Second, the remanent magnetization changes the flux leakage from corrosion defects, which affects defect detection and characterization of the defect geometry. Experimental data obtained from the GRI Pipeline Simulation Facility are used to illustrate the effect of remanent magnetization on flux leakage inspections.

Nestleroth, J.B.; Davis, R.J.

1995-04-01

348

Magnetic flux density produced by finite-length twisted-wire pairs  

Microsoft Academic Search

Twisting a wire pair is often used to reduce the wire's low frequency magnetic flux density impinging upon spatially adjacent circuitry. Typically, predictions of the magnetic flux density due to the twisted-wire pair are based upon mathematical formulations for infinite-length wires. In the present work, expressions for predicting the quasistatic magnetic flux density near a finite-length, current-carrying twisted-wire pair are

G. R. Piper

1996-01-01

349

Diffusion of Magnetic Field and Removal of Magnetic Flux from Clouds Via Turbulent Reconnection  

NASA Astrophysics Data System (ADS)

The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology via reconnection in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence reassures that the magnetic field behavior in computer simulations and turbulent astrophysical environments is similar, as far as magnetic reconnection is concerned. This makes it meaningful to perform MHD simulations of turbulent flows in order to understand the diffusion of magnetic field in astrophysical environments. Our studies of magnetic field diffusion in turbulent medium reveal interesting new phenomena. First of all, our three-dimensional MHD simulations initiated with anti-correlating magnetic field and gaseous density exhibit at later times a de-correlation of the magnetic field and density, which corresponds well to the observations of the interstellar media. While earlier studies stressed the role of either ambipolar diffusion or time-dependent turbulent fluctuations for de-correlating magnetic field and density, we get the effect of permanent de-correlation with one fluid code, i.e., without invoking ambipolar diffusion. In addition, in the presence of gravity and turbulence, our three-dimensional simulations show the decrease of the magnetic flux-to-mass ratio as the gaseous density at the center of the gravitational potential increases. We observe this effect both in the situations when we start with equilibrium distributions of gas and magnetic field and when we follow the evolution of collapsing dynamically unstable configurations. Thus, the process of turbulent magnetic field removal should be applicable both to quasi-static subcritical molecular clouds and cores and violently collapsing supercritical entities. The increase of the gravitational potential as well as the magnetization of the gas increases the segregation of the mass and magnetic flux in the saturated final state of the simulations, supporting the notion that the reconnection-enabled diffusivity relaxes the magnetic field + gas system in the gravitational field to its minimal energy state. This effect is expected to play an important role in star formation, from its initial stages of concentrating interstellar gas to the final stages of the accretion to the forming protostar. In addition, we benchmark our codes by studying the heat transfer in magnetized compressible fluids and confirm the high rates of turbulent advection of heat obtained in an earlier study.

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

2010-05-01

350

A new stator-flux orientation strategy for flux-switching permanent magnet motor based on current-hysteresis control  

NASA Astrophysics Data System (ADS)

A stator-flux orientation strategy based on current hysteresis for the flux-switching permanent magnet (FSPM) motor is proposed, in which the stator-PM FSPM motor is considered as a conventional rotor-PM surface-mounted motor and an equivalent rotor-orientated dq-axes synchronous reference frame is built although there are actually no rotary magnetic motive force produced by the stator magnets in the FSPM motor. Based on the proposed model, a vector-control strategy with current hysteresis for the FSPM motor drive is investigated and implemented on a dSPACE-based platform, and both the simulated and experimental results validate the effectiveness. It should be emphasized that the proposed stator-flux orientation strategy can be applied to other stator-PM machines (including doubly salient and flux-reversal PM machines) and other control methods (including space-vector pulsed-width-modification and direct torque control).

Hua, Wei; Cheng, Ming; Lu, Wei; Jia, Hongyun

2009-04-01

351

Siphon flows in isolated magnetic flux tubes. 3. The equilibrium path of the flux tube arch. Annual report, 1989  

SciTech Connect

The arched equilibrium path of a thin magnetic flux tube in a plane-stratified, nonmagnetic atmosphere is calculated for cases in which the flux tube contains a steady siphon flow. The large scale mechanical equilibrium of the flux tube involves a balance among the magnetic buoyancy force, the net magnetic tension force due to the curvature of the flux tube axis, and the inertial (centrifugal) force due to the siphon flow along curved streamlines. The ends of the flux tube are assumed to be pinned down by some other external force. Both isothermal and adiabatic siphon flows are considered for flux tubes in an isothermal external atmosphere. For the isothermal case, in the absence of a siphon flow the equilibrium path reduces to the static arch calculated by Parker (1975, 1979). The presence of a siphon flow causes the flux tube arch to bend more sharply, so that magnetic tension can overcome the additional straightening effect of the inertial force, and reduces the maximum width of the arch. The curvature of the arch increases as the siphon flow speed increases. For a critical siphon flow, with supercritical flow in the downstream leg, the arch is asymmetric, with greater curvature in the downstream leg of the arch. Adiabatic flow have qualitatively similar effects, except that adiabatic cooling reduces the buoyancy of the flux tube and thus leads to significantly wider arches. In some cases the cooling is strong enough to create negative buoyancy along sections of the flux tube, requiring upward curvature of the flux tube path along these sections and sometimes leading to unusual equilibrium paths of periodic, sinusoidal form.

Thomas, J.H.; Montesinis, B.

1989-09-01

352

Spin-dependent shot noise in diluted magnetic semiconductor/semiconductor heterostructures with a nonmagnetic barrier  

NASA Astrophysics Data System (ADS)

We investigate quantum size effect on the spin-dependent shot noise in the diluted magnetic semiconductor (DMS)/semiconductor heterostructure with a nonmagnetic semiconductor (NMS) barrier in the presence of external magnetic and electric fields. The results demonstrate that the NMS barrier plays a quite different role from the DMS layer in the electron transport process. It is found that spin-down shot noise shows relatively regular oscillations as the width of DMS layer increases, while the spin-up shot noise deceases monotonically. However, as the width of NMS layer increases, the spin-down shot noise displays irregular oscillations at first and then decreases while the spin-up shot noise decreases at a quite different rate. The results indicate that the shot noise can be used as a sensitive probe in detecting material type and its size.

Wu, Shuang; Guo, Yong

2014-05-01

353

Optimization of magnetic flux density for fast MREIT conductivity imaging using multi-echo interleaved partial fourier acquisitions  

PubMed Central

Background Magnetic resonance electrical impedance tomography (MREIT) has been introduced as a non-invasive method for visualizing the internal conductivity and/or current density of an electrically conductive object by externally injected currents. The injected current through a pair of surface electrodes induces a magnetic flux density distribution inside the imaging object, which results in additional magnetic flux density. To measure the magnetic flux density signal in MREIT, the phase difference approach in an interleaved encoding scheme cancels out the systematic artifacts accumulated in phase signals and also reduces the random noise effect by doubling the measured magnetic flux density signal. For practical applications of in vivo MREIT, it is essential to reduce the scan duration maintaining spatial-resolution and sufficient contrast. In this paper, we optimize the magnetic flux density by using a fast gradient multi-echo MR pulse sequence. To recover the one component of magnetic flux density Bz, we use a coupled partial Fourier acquisitions in the interleaved sense. Methods To prove the proposed algorithm, we performed numerical simulations using a two-dimensional finite-element model. For a real experiment, we designed a phantom filled with a calibrated saline solution and located a rubber balloon inside the phantom. The rubber balloon was inflated by injecting the same saline solution during the MREIT imaging. We used the multi-echo fast low angle shot (FLASH) MR pulse sequence for MRI scan, which allows the reduction of measuring time without a substantial loss in image quality. Results Under the assumption of a priori phase artifact map from a reference scan, we rigorously investigated the convergence ratio of the proposed method, which was closely related with the number of measured phase encode set and the frequency range of the background field inhomogeneity. In the phantom experiment with a partial Fourier acquisition, the total scan time was less than 6 seconds to measure the magnetic flux density Bz data with 128×128 spacial matrix size, where it required 10.24 seconds to fill the complete k-space region. Conclusion Numerical simulation and experimental results demonstrated that the proposed method reduces the scanning time and provides the recovered Bz data comparable to what we obtained by measuring complete k-space data.

2013-01-01

354

Validation Tests of Data Driven Magnetic Flux Emergence  

NASA Astrophysics Data System (ADS)

The emergence of magnetic flux through the solar photosphere into the corona is a key problem to understanding the energization of the solar corona. With the advent of high spatial and temporal resolution solar vector magnetic field measurements, it may now be possible to simulate the dynamical evolution of the corona by using these measurements as the driving boundary condition for magnetohydrodynamical (MHD) simulations of the corona. First, however, methods for using this data to drive simulations must be validated via quantitative tests. We report here on a series of such tests wherein the driving inputs are taken from self-consistent simulations of the emergence of flux ropes from the upper convection zone through the photosphere and chromosphere into the low corona. Photospheric MHD output from these simulations is then used to drive new simulations, and the driven results are compared against the original results for a variety of driving assumptions and algorithms. We will focus here on the relative advantages and disadvantages of data driving a higher order finite-element-based MHD code (HiFi) versus data driving a lower order Lagrangian remap-based MHD code (LARE3D). For each code, we will report on the necessary input conditions (spatial and temporal resolution, specification of MHD variables, and specification of vertical gradients) needed to reproduce a high level of agreement between the original and the driven simulations. Finally, we will report on the level of agreement achieved when using driving input equivalent to that of current solar observations. Based on these tests, we will address the prospects for using high time and spatial resolution vector magnetogram observations to drive MHD simulations of the solar chromosphere and corona.This work was supported by the ONR 6.1 and the NASA LWS programs.

Linton, Mark; Lukin, Vyacheslav; Leake, James Edward; Schuck, Peter W.

2014-06-01

355

Siphon flows in isolated magnetic flux tubes. IV - Critical flows with standing tube shocks  

NASA Technical Reports Server (NTRS)

Critical siphon flows in arched, isolated magnetic flux tubes are studied within the thin flux tube approximation, with a view toward applications to intense magnetic flux concentrations in the solar photosphere. The results of calculations of the strength and position of the standing tube shock in the supercritical downstream branch of a critical siphon flow are presented, as are calculations of the flow variables all along the flux tube and the equilibrium path of the flux tube in the surrounding atmosphere. It is suggested that arched magnetic flux tubes, with magnetic field strength increased by a siphon flow, may be associated with some of the intense, discrete magnetic elements observed in the solar photosphere.

Thomas, John H.; Montesinos, Benjamin

1991-01-01

356

Estimates of magnetic flux, and energy balance in the plasma sheet during substorm expansion  

NASA Technical Reports Server (NTRS)

The energy and magnetic flux budgets of the magnetotail plasma sheet during substorm expansion are investigated. The possible mechanisms that change the energy content of the closed field line region which contains all the major dissipation mechanisms of relevance during substorms, are considered. The compression of the plasma sheet mechanism and the diffusion mechanism are considered and excluded. It is concluded that the magnetic reconnection mechanism can accomplish the required transport. Data-based empirical magnetic field models are used to investigate the magnetic flux transport required to account for the observed magnetic field dipolarizations in the inner magnetosphere. It is found that the magnetic flux permeating the current sheet is typically insufficient to supply the required magnetic flux. It is concluded that no major substorm-type magnetospheric reconfiguration is possible in the absence of magnetic reconnection.

Hesse, Michael; Birn, Joachim; Pulkkinen, Tuija

1996-01-01

357

Fine resolution soil water fluxes measured with a small Smart Field Lysimeter: The noise removal and further interpretation  

NASA Astrophysics Data System (ADS)

A weighable Smart Field Lysimeter (30 cm diameter, 30 cm depth) with an adaptively regulated suction at its bottom was used to measure soil water fluxes at the surface and at the 30 cm depth of a short grass stand. No overland flow or accumulation of water at the surface were observed and there was no groundwater table within the soil profile. Appropriate distinction between the fluxes of different directions made it possible to separately estimate actual evapotranspiration (upward surface flux), precipitation and condensation (downward surface flux and dew on grass leaves), percolation (downward flux at 30 cm) and capillary rise (upward flux at 30 cm). The primary data were collected at 1 minute intervals but required digital filtering to remove the information noise. Various methods of filtering were tested, with a special regard to intensive rain events. The resulting data have a 10-minute resolution. The lysimeter is capable of self-recovery after a period of drought but the noise of percolation and capillary rise estimates is enhanced for some time during, before and after this period. In these situations, it is important that a porous matrix sensor measures the suction in parallel to the reference tensiometer. Both the precipitation and the actual evapotranspiration derived from the lysimeter data alone are in absolute values higher than the analogous quantities obtained with the help of the directly measured tipping bucket precipitation. These discrepancies are probably due to the rain gauge underestimating true precipitation, but partly also due to numerical noise, however smoothed. If the rain gauge data are used only to distinguish the periods of rain from the rainless periods, than the condensation of water in the soil and on the grass leaves can be estimated. The actual evapotranspiration measured by the lysimeter has a diurnal patterns depending on actual weather. The maximum occurs, on average, shortly after the noon. The percolation curves after rain events make it possible to estimate the soil hydraulic properties on the principle of the outflow method.

Dolezal, Frantisek; Bekere Mekonnen, Getu; Matula, Svatopluk; Mihalikova, Marketa; Fisak, Jaroslav; Teressa Chala, Ayele; Hrkalova, Marketa; Moreira Barradas, Joao Manuel

2014-05-01

358

Embedded Micro-Flux-Gate Magnetic Sensor in Printed Circuit Board  

Microsoft Academic Search

We have developed a micro-flux-gate magnetic sensor using a new printed circuit board (PCB) technology. The flux-gate sensor consists of five PCB stack layers including one layer of magnetic core and four layers of excitation and pickup coils. The center layer as a magnetic core is made of a micropatterned amorphous magnetic ribbon with an extremely high DC permeability of

Won-Youl Choi; Jun-Sik Hwang; Sang-On Choi

2005-01-01

359

A new linear actuator utilizing flux concentration type permanent magnet arrangement  

Microsoft Academic Search

This paper presents a moving-magnet type linear actuator designed by using flux concentration type permanent magnet arrangement, which can generate higher magnetic flux density in air-gap. In this construction, detent force which is induced by both slot-effect and end-effect becomes larger due to strong attractive forces. To reduce cogging force we have employed a modular mover structure of two magnetic

Akira Goto; Takuya Okamoto; Atsushi Ikariga; Takashi Todaka; Masato Enokizono

2010-01-01

360

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

361

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

362

Three-Dimensional Lumped-Parameter Magnetic Circuit Analysis of Single-Phase Flux-Switching Permanent-Magnet Motor  

Microsoft Academic Search

A 3-D lumped-parameter magnetic circuit model is developed for a single-phase flux-switching permanent-magnet motor. Particular attention is given to representing the complex air-gap flux paths by equivalent permeances so as to accurately model the asymmetry in the air-gap field distribution and to determine the back-EMF and inductance waveforms, as well as the average static torque. Leakage fluxes external to the

Yu Chen; Z. Q. Zhu; David Howe

2008-01-01

363

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

364

Magnetic topologies in the solar corona due to four discrete photospheric flux regions  

Microsoft Academic Search

Many dynamic phenomena in the solar corona are driven by the complex and ever-changing magnetic field. It is helpful, in trying to model these phenomena, to understand the structure of the magnetic field, i.e. the magnetic topology. We study here the topological structure of the coronal magnetic field arising from four discrete photospheric flux patches, for which we find that

C. Beveridge; E. R. Priest; D. S. Brown

2004-01-01

365

Identification of magnetizing inrush currents of power transformers based on features of flux locus  

Microsoft Academic Search

In this paper, a new method of identifying magnetizing inrush currents based on features of flux locus is presented. The measured currents and voltages of transformer windings are used to reconstruct the locus of main flux. Its variation range then is examined if within or not the saturation zone by extracting the flux locus features. Based on this, a new

Xiaotan Zhao; Jianyun Chai; Pengsheng Su

2003-01-01

366

BiDirectional Pattern-Dependent Noise Prediction for Heat-Assisted Magnetic Recording With High Jitter Noise  

Microsoft Academic Search

In this paper, we first report the results of an investigation of perpendicular heat-assisted magnetic recording (HAMR) channels containing high jitter noise that is likely to be the case at very high areal densities (e.g., 4 $\\\\hbox{Tb\\/in}^{2}$). To model the HAMR channel, we use the thermal Williams-Comstock model and the microtrack model to derive the transition response, without large thermal

Yibin Ng; B. V. K. Vijaya Kumar; Kui Cai; Rathnakumar Radhakrishnan; Tow Chong Chong

2012-01-01

367

Cogging torque reduction of flux-switching permanent magnet machines without skewing  

Microsoft Academic Search

The flux-switching permanent magnet (FSPM) machine exhibits competitive torque capability and power density due to the high flux density. As a penalty, the cogging torque is relative large, resulting in unfavorable torque pulsation. Due to the simple and robust rotor construction, additional dummy slots are adopted to reduce the cogging torque without skewing magnets or teeth. The optimal dimensions of

Wei Hua; Ming Cheng

2008-01-01

368

Compensation of cogging torque for flux-switching permanent magnet motor based on current harmonics injection  

Microsoft Academic Search

The flux-switching permanent magnet (FSPM) machine exhibits the advantages of high power density and large torque capability. However, due to the high airgap flux density produced by magnets and the doubly-salient structure, the resultant cogging torque is relatively large, which is unfavorable for high performance drive system. In this paper, instead of conventional optimization on the motor itself, a new

Hongyun Jia; Ming Cheng; Wei Hua; Zhengzhuan Yang; Yunqian Zhang

2009-01-01

369

A study of the north-south asymmetry of solar photospheric magnetic flux  

Microsoft Academic Search

Statistical characteristics of the variation of the North-South asymmetry of solar photospheric magnetic flux in 1978–2002 are systematically studied, and they are found to be related to the cycle of solar activity. The degree of the asymmetry during the minimum of solar activity is obviously higher than that during the maximum of solar activity. Moreover, the change of magnetic flux

Wen-Bin Song; Jing-Xiu Wang; Xia Ma

2005-01-01

370

Magnetic Flux Leakage Testing Method for Well Casing Based on Gaussian Kernel RBF Neural Network  

Microsoft Academic Search

Well casing integrity is important for the safe operations of oil wells, and is of great significance to detect well casing defects. Magnetic Flux Leakage (MFL) Detection Technology is widely used to detect the defects of various pipelines. Because the environment where well casing is laid in is usually very complicated, the system which based on magnetic flux leakage technology

Jinzhong Chen; Lin Li; Binggui Xu

2008-01-01

371

GRI pipeline simulation facility magnetic flux leakage test bed vehicle. Final report, September 1990June 1996  

Microsoft Academic Search

To advance the capability of nondestructive internal pipeline inspection devices, commonly called inspection pigs, two test bed vehicles have been built as test platforms for use in the GRI Pipeline Simulation Facility. This report describes the magnetic flux leakage test bed vehicle. Magnetic flux leakage was selected for a specific test bed vehicle implementation because it is the most commonly

J. B. Nestleroth; T. A. Bubenik; A. Teitsma

1996-01-01

372

Magnetic Flux Leakage Detection Technology for Well Casing on Neural Network  

Microsoft Academic Search

Well casing integrity is vital for the safe operations of oil wells, and also significant to detect well casing defects. Magnetic flux leakage (MFL) detection technology is widely-used in detecting the defects of various pipelines. Owing to the very complicated environment where well casing is laid in, the system which based on magnetic flux leakage technology is not mature yet

Jinzhong Chen; Lin Li; Jinan Shi

2008-01-01

373

Characterisation of metal loss defects from magnetic flux leakage signals with discrete wavelet transform  

Microsoft Academic Search

Metal loss defects in a buried pipeline are detected by magnetic flux leakage technique. Characterisation of the defects and sentencing according to the severity is extremely important for organised maintenance of pipelines. In this paper we identify the parameters that characterise a defect and the features of magnetic flux leakage signal (MFL) that are affected by those parameters. We show

S Mukhopadhyay; G. P Srivastava

2000-01-01

374

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

375

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

376

Three-dimensional magnetohydrodynamics of the emerging magnetic flux in the solar atmosphere  

NASA Technical Reports Server (NTRS)

The nonlinear evolution of an emerging magnetic flux tube or sheet in the solar atmosphere is studied through 3D MHD simulations. In the initial state, a horizontal magnetic flux sheet or tube is assumed to be embedded at the bottom of MHD two isothermal gas layers, which approximate the solar photosphere/chromosphere and the corona. The magnetic flux sheet or tube is unstable against the undular mode of the magnetic buoyancy instability. The magnetic loop rises due to the linear and then later nonlinear instabilities caused by the buoyancy enhanced by precipitating the gas along magnetic field lines. We find by 3D simulation that during the ascendance of loops the bundle of flux tubes or even the flux sheet develops into dense gas filaments pinched between magnetic loops. The interchange modes help produce a fine fiber flux structure perpendicular to the magnetic field direction in the linear stage, while the undular modes determine the overall buoyant loop structure. The expansion of such a bundle of magnetic loops follows the self-similar behavior observed in 2D cases studied earlier. Our study finds the threshold flux for arch filament system (AFS) formation to be about 0.3 x 10 exp 20 Mx.

Matsumoto, R.; Tajima, T.; Shibata, K.; Kaisig, M.

1993-01-01

377

A New Method for Reduction of Detent Force in Permanent Magnet Flux-Switching Linear Motors  

Microsoft Academic Search

The permanent magnet (PM) flux-switching linear motor, which is developed from the PM flux-switching rotary machine, possesses similar advantages such as high power density and simple structure. Furthermore, the PM flux-switching linear motor has a potential for mass production at low cost, since the expensive coils and magnets are both set on the short mover. However, the detent force which

Can-Fei Wang; Jian-Xin Shen; Yu Wang; Li-Li Wang; Meng-Jia Jin

2009-01-01

378

Analysis of interior permanent magnet synchronous motor designed for flux weakening operation  

Microsoft Academic Search

The paper presents a finite element method (FEM) based analysis of interior permanent magnet synchronous motor, designed for flux-weakening operation. Flux-weakening performance determined from the FEM analysis on the proposed permanent magnet motor structure is presented in order to verify the suitability of the design concept for flux-weakening operation. The influence of iron losses on the output power capability is

B. Stumberger; A. Hamler; M. Trlep; M. Jesenik

2001-01-01

379

Magnetic flux pinning in superconductors with hyperbolic-tessellation arrays of pinning sites  

NASA Astrophysics Data System (ADS)

We study magnetic flux interacting with arrays of pinning sites (APSs) placed on vertices of hyperbolic tessellations (HTs). We show that, due to the gradient in the density of pinning sites, HT APSs are capable of trapping vortices for a broad range of applied magnetic fluxes. Thus, the penetration of magnetic field in HT APSs is essentially different from the usual scenario predicted by the Bean model. We demonstrate that, due to the enhanced asymmetry of the surface barrier for vortex entry and exit, this HT APS could be used as a “capacitor” to store magnetic flux.

Misko, V. R.; Nori, Franco

2012-05-01

380

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

381

Comparison of flux-switching and doubly-salient permanent magnet brushless machines  

Microsoft Academic Search

This paper compares the topology, operation principle and electromagnetic performance of 12\\/10-pole flux-switching and 12\\/8-pole doubly-salient permanent magnet (PM) 3-phase brushless machines, including airgap field distribution, flux linkage, back-emf, cogging torque, winding inductance and electromagnetic torque etc. It is found that the most significant difference is in the flux linkage waveform. In the doubly-salient PM machines, the phase flux linkage

W. Hua; Z. Q. Zhu; M. Cheng; Y. Pang; D. Howe

2005-01-01

382

Flux-invading behaviors and temperature changes in Dy123 bulk magnet in iterative pulse field magnetization process  

NASA Astrophysics Data System (ADS)

The flux invasion behaviors into the Dy123 based on HTS bulk magnet were precisely measured during the iterative pulsed field magnetizations operated at 31 K. The values of magnetic flux densities were measured at various positions on the bulk magnet sample during and after six successive applications of a peak applied field of 5T. The temperature changes were measured on the sample surface to compare them to the magnetic flux motions in the same manner. The invading flux were apparently repelled by the remaining trapped flux due to the former field application, and the domain where the magnetic flux reached was pushed back outward the sample with increasing the pulse number. The heat generation decreased with the iteration of field applications as well as the magnetic flux invasion expressing the same behavior, and saturated by the sixth field application. The time evolution of temperature measured at various positions on the sample surface showed that the heat generation occurs at the periphery of the sample and propagates toward the center of the sample.

Oka, T.; Ishiduka, D.; Ogawa, J.; Fukui, S.; Sato, T.; Yokoyama, K.; Murakami, A.; Stopfel, H.

2013-11-01

383

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

384

BUILDUP OF MAGNETIC SHEAR AND FREE ENERGY DURING FLUX EMERGENCE AND CANCELLATION  

SciTech Connect

We examine a simulation of flux emergence and cancellation, which shows a complex sequence of processes that accumulate free magnetic energy in the solar corona essential for the eruptive events such as coronal mass ejections, filament eruptions, and flares. The flow velocity at the surface and in the corona shows a consistent shearing pattern along the polarity inversion line (PIL), which together with the rotation of the magnetic polarities, builds up the magnetic shear. Tether-cutting reconnection above the PIL then produces longer sheared magnetic field lines that extend higher into the corona, where a sigmoidal structure forms. Most significantly, reconnection and upward-energy-flux transfer are found to occur even as magnetic flux is submerging and appears to cancel at the photosphere. A comparison of the simulated coronal field with the corresponding coronal potential field graphically shows the development of non-potential fields during the emergence of the magnetic flux and formation of sunspots.

Fang Fang; Manchester, Ward IV; 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)

2012-07-20

385

Estimating the open magnetic flux from the interplanetary and ionospheric conditions  

NASA Astrophysics Data System (ADS)

The open magnetic flux (FPC) is a key parameter to study magnetospheric dynamical process, which is closely related to magnetic reconnections in the dayside magnetopause and magnetotail. Using global MHD simulations, we find that the open magnetic flux FPC can be estimated through a combined parameter f by FPC=0.89f/(f+0.20)+0.52, where the parameter f=vSWBSnSW1/5?P1/3 is a function of the solar wind velocity (vSW), the solar wind number density (nSW), the southern interplanetary magnetic field (IMF) strength (BS), and the ionospheric Pederson conductance (?P). The comparison with the limited observational FPCdata available in the literature shows its promise in estimating the open magnetic flux from the interplanetary and ionospheric conditions. The open magnetic flux (FPC) may be served as a key space weather forecast element in the future.

Wang, C.; Xia, Z. Y.; Peng, Z.; Lu, Q. M.

2013-05-01

386

The Aharonov-Anandan current induced by a time-dependent magnetic flux in graphene rings  

NASA Astrophysics Data System (ADS)

We investigate the current response of a graphene ring in the presence of a time-dependent magnetic flux. Using the linear response theory, we find a nonzero Aharonov-Anandan (AA) phase of carriers in the ring. When the flux increases, the current responses to the flux nonlinearly, which can be applied to translate the external flux signal to the electronic signal. Especially for the huge flux with low frequency, this system can be used as an electronic current square-wave generator. Moreover, a resonant transition can occur in the case of the flux with the proper frequency. And then a current beat frequency phenomenon is observed.

Zhang, Shengli; Chen, Huawei; Zhang, Erhu; Liu, Daqing

2013-09-01

387

Design of Flux-Switching Permanent Magnet Machine Considering the Limitation of Inverter and Flux-Weakening Capability  

Microsoft Academic Search

In this paper, a general design procedure for the flux-switching permanent magnet (FSPM) machine with different topologies is proposed. Firstly, a 3-phase 12-stator-tooth\\/10-rotor-pole topology is introduced and its operation principle is described. Then, the basic design method, including determination of the stator, rotor and magnet dimensions, is proposed under the constraints of some dimensions and electrical parameters. The winding turns

Wei Hua; Ming Cheng; Z. Q. Zhu; D. Howe

2006-01-01

388

Three-dimensional Simulation of Magnetic Flux Dynamics and Temperature Rise in HTSC Bulk during Pulsed Field Magnetization  

NASA Astrophysics Data System (ADS)

We have performed a three-dimensional (3D) numerical simulation of the dynamical motion of the magnetic flux and the heat propagation in the superconducting bulk after applying a pulsed magnetic field. An inhomogeneous Jc distribution was supposed in the bulk; the Jc in the growth sector boundary (GSB) is four times higher than that in the growth sector region (GSR). For lower applied pulsed field, magnetic flux was penetrated and trapped in the GSR, and for higher applied pulsed field, the magnetic flux was trapped more preferentially in the GSB. These results of the simulation reproduce the experimental ones and are valuable for the understanding the flux dynamics in the bulk during pulsed field magnetization.

Fujishiro, H.; Naito, T.; Oyama, M.

389

Starting Torque of Single-Phase Flux-Switching Permanent Magnet Motors  

Microsoft Academic Search

Flux-switching permanent magnet motors are a form of brushless dc motor, with a simple salient-pole rotor, similar to that of a switched reluctance motor, and the excitation coils and permanent magnets on the stator. The influence of the rotor pole-arc and asymmetry on the torque-rotor position characteristic of a single-phase flux-switching permanent magnet motor is investigated by finite-element analysis, and

Y. Chen; S. Chen; Z. Q. Zhu; D. Howe; Y. Y. Ye

2006-01-01

390

Geomagnetic field and cosmic ray variation in association with solar magnetic flux  

Microsoft Academic Search

Summary form only given. Analysis of my work indicates a large transient decrease in geomagnetic field near the arrival of magnetic clouds. We also introduce the fluctuations on the solar magnetic flux over the period 1975-1995. In my investigation we found fluctuation between 1.3 to 1.7 in the solar magnetic flux, which are related with variations solar wind and cosmic

S. K. Mishra; D. P. Tiwari

2004-01-01

391

Emergence of solar magnetic flux from the convection zone into the photosphere and chromosphere  

Microsoft Academic Search

A two-dimensional MHD code is used to study the nonlinear dynamics of solar magnetic flux emerging from the convection zone into the photosphere and chromosphere. An isolated horizontal magnetic flux with beta of about 4 is initially located in a convectively unstable layer (solar convection zone) beneath a two-temperature layered atmosphere (solar corona-chromosphere\\/photosphere). The combined effects of convection and magnetic

K. Shibata; S. Nozawa; R. Matsumoto; A. C. Sterling; T. Tajima

1990-01-01

392

A Giant-Magnetoresistance Sensor for Magnetic-Flux-Leakage Nondestructive Testing of a Pipeline  

Microsoft Academic Search

This paper presents the method of magnetic-flux-leakage (MFL) detection and the characteristics of a giant-magnetoresistance (GMR) sensor. An experimental apparatus that uses a GMR sensor for oil-pipeline inspection is described. A permanent-magnet assembly was used to excite MFL signals. The signal from the GMR sensor is preamplified with an appropriate gain. The result shows the magnetic-flux-leakage method based on a

L. Chen; P.-W. Que; T. Jin

2005-01-01

393

Hybrid excited flux-switching permanent magnet machines  

Microsoft Academic Search

Hybrid excited machines are of interest due to the potential of combining the high power density of PM machines with the flux-adjusting capability of wound field excitation. This paper reviews currently documented hybrid excited machines and compares the flux control performance and torque density of alternative hybrid flux-switching PM machines by means of finite element analysis. The iron loss of

R. L. Owen; Z. Q. Zhu; G. W. Jewell

2009-01-01

394

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

395

Search for a flux of cosmic-ray magnetic monopoles with an eight-channel superconducting detector  

NASA Astrophysics Data System (ADS)

A superconducting detector for cosmic-ray magnetic monopoles originally designed with a cross section of 1.5 m2 (averaged over 4? solid angle) for double-coincident events was active from 5 May 1987 to 5 August 1988. The detector consists of eight independent inductive gradiometers located on the surface of an octagonal prism. The conductor is niobium-titanium foil and the current sensors are rf SQUID's. The signal-to-noise ratio for a single Dirac charge is greater than 50 in a 0.05-Hz bandwidth, and low-noise data was obtained over 547 days of continuous cryogenic operation. Of this time, we obtained ~50% live time. Open circuits in portions of two gradiometers and occasional coupling of disturbances in adjacent gradiometers reduce the active sensing area to 1.1 m2. A closed-cycle helium liquefier eliminates helium transfers and increases the stability of the data. Anticoincidence instrumentation includes strain gauges, a flux-gate magnetometer, an ultrasonic motion detector, and a wideband rms rf voltmeter. The exposure to date represents a limit on the flux of cosmic-ray magnetic monopoles of 7.2×10-13 cm-2 s-1 sr-1 at a 90% confidence level, eliminating most of the phase space for monopole plasma oscillation theories.

Huber, M. E.; Cabrera, B.; Taber, M. A.; Gardner, R. D.

1991-08-01

396

Single image signal-to-noise ratio estimation for magnetic resonance images.  

PubMed

A novel technique to quantify the signal-to-noise ratio (SNR) of magnetic resonance images is developed. The image SNR is quantified by estimating the amplitude of the signal spectrum using the autocorrelation function of just one single magnetic resonance image. To test the performance of the quantification, SNR measurement data are fitted to theoretically expected curves. It is shown that the technique can be implemented in a highly efficient way for the magnetic resonance imaging system. PMID:20703587

Sim, K S; Lai, M A; Tso, C P; Teo, C C

2011-02-01

397

Control of magnetic flux and eddy currents in magnetic films for on-chip radio frequency inductors: Role of the magnetic vias  

NASA Astrophysics Data System (ADS)

In this paper, the role of the magnetic vias for magnetic flux and eddy current control is investigated using both simulations and experiments that used different patterning techniques and by altering the magnetic via width. Improved finger-shaped magnetic vias have been designed and integrated into on-chip radio frequency inductors improving the peak quality factor from 400 MHz to 800 MHz without sacrificing the inductance enhancement. The eddy currents and magnetic flux density in different areas of the magnetic vias are analyzed by 3D electromagnetic simulation. With optimized magnetic vias, the high frequency response of up to 2 GHz has been achieved.

Wu, Hao; Gardner, Donald S.; Zhao, Shirong; Huang, Hai; Yu, Hongbin

2014-05-01

398

Magnetic control of laser ablation plasma for high-flux ion injectors  

NASA Astrophysics Data System (ADS)

We investigated the interaction of a laser ablation plasma with a longitudinal magnetic field aiming to create a directionally moving plasma for high-flux and low-emittance ion injectors. To study the plasma dynamics, time-of-flight measurements and energy analysis of the plasma ion flux were made as functions of the laser intensity and the magnetic field. Moderate magnetic field (˜0.2T) directed the fast and highly charged ions in the target normal. In addition, a slow peak appeared and increased with increasing the magnetic field. These results indicated that directional electric field is formed and recombination increases by the application of longitudinal magnetic field.

Ikeda, Shunsuke; Nakajima, Mitsuo; Hasegawa, Jun; Kawamura, Tohru; Horioka, Kazuhiko

2014-01-01

399

A high-speed induction motor making use of the third harmonic of the magnetic flux  

NASA Astrophysics Data System (ADS)

This paper indicates some possibilities of construction of high-speed induction motors taking advantage of the magnetic flux third harmonic due to a process of magnetization of nonlinear magnetic circuits. Configurations of magnetic frequency triplers, which can be used as basic stator structures and generate a distorted flux, are presented. The paper also describes an experimental high-speed induction motor supplied from a single-phase source via a magnetic frequency tripler that make possible to obtain a rotating field having a synchronous speed equal to 9000 rpm at a supply voltage angular frequency of 314 rd.

Goleman, Ryszard

1994-05-01

400

Structural and magnetic transitions in ensembles of mesoscopic Peierls rings in a magnetic flux  

NASA Astrophysics Data System (ADS)

We investigate effects of a magnetic flux ? threading mesoscopic Peierls rings. Both isolated rings and ensembles of rings are considered and quantum phonon fluctuations are accounted for. Significant qualitative and quantitative modifications to the mean field results are obtained. Quantum fluctuations yield a continuous structural Peierls change occurring without symmetry breaking and a ?-dependent tunneling splitting. The latter yields a ?-modulated ultrasound or hypersound absorption that could be detected experimentally. Important differences exist between an isolated ring and an ensemble of rings. To perform ensemble averaging, we propose a new and simple method. Unlike isolated rings, ensembles possess structural and magnetic properties that do not distinguish between average sizes N=4n and N=4n+2 but rather between smaller and larger N. In ensembles, the flux period is hc/2e, half the period for an isolated ring. An appealing magnetic behavior with an interplay between dia- and paramagnetism is found in ensembles that could be experimentally investigated by means of a SQUID technique.

Bâldea, Ioan; Köppel, Horst; Cederbaum, Lorenz S.

1999-09-01

401

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

NASA Astrophysics Data System (ADS)

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 the 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 ropes, 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.

Petrie, G. J. D.

2007-05-01

402

Modeling and analysis of fractional slot axial flux permanent-magnet machine considering overhang effect  

Microsoft Academic Search

This paper presents the study of a novel axial flux permanent magnet (AFPM) machine, which consist of the segmented stator and fractioned slot windings, with one rotor and double stator. In addition, the overhang effect of permanent magnet of the motor has been analyzed quantitatively. The overhang is used to enhance the force density in permanent magnet machine. According to

Jian Li; Yunhyun Cho

2010-01-01

403

Dual-spacecraft observation of large-scale magnetic flux ropes in the Martian ionosphere  

Microsoft Academic Search

We here report the first dual-spacecraft detection of planetary flux ropes in the ionosphere of Mars. The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on board Mars Express, can measure the magnetic field magnitude near the spacecraft. Typically, these measurements track the known crustal magnetic field strength very well; however, occasionally, MARSIS detects transient, intense magnetic fields that

D. D. Morgan; D. A. Gurnett; F. Akalin; D. A. Brain; J. S. Leisner; F. Duru; R. A. Frahm; J. D. Winningham

2011-01-01

404

A coupled model of magnetic flux generation and transport in stars  

NASA Astrophysics Data System (ADS)

We present a combined model for magnetic field generation and transport in cool stars with outer convection zones. The mean toroidal magnetic field, which is generated by a cyclic thin-layer \\alpha\\Omega dynamo at the bottom of the convection zone is taken to determine the emergence probability of magnetic flux tubes in the photosphere. Following the nonlinear rise of the unstable thin flux tubes, emergence latitudes and tilt angles of bipolar magnetic regions are determined. These quantities are put into a surface flux transport model, which simulates the surface evolution of magnetic flux under the effects of large-scale flows and turbulent diffusion. First results are discussed for the case of the Sun and for more rapidly rotating solar-type stars. Movies are available via http://www.aip.de/AN/movies

I?ik, E.; Schmitt, D.; Schüssler, M.

2007-12-01

405

Effects of creep damage, shot peening, and case hardening on magnetic Barkhausen noise analysis  

SciTech Connect

The micromagnetic emissions, commonly known as Barkhausen noise, are very sensitive to variations in the microstructure and sub-surface stress states of magnetic materials. Steel pipelines at power plants often have creep damage due to microstructural changes in their service life. Early detection of this damage will prevent costly failures. There is also an increasing demand to characterize the sub-surface stress states in structural materials such as high strength materials used in landing gear components in the aerospace industry. Shot peening is used to improve the fatigue strength of these components by the introduction of residual compressive stresses to the surface. Because the magnitude of Barkhausen noise varies with the magnitude of compressive stress, these noise measurements can be used for in-situ evaluation of the effectiveness of the shot peening process. Furthermore, surface modification such as case hardened magnetic samples can be easily observed using micromagnetic Barkhausen noise (MBE) to determine further modification needs.

Sipahi, L.B. (Ames Lab., IA (United States) Iowa State Univ., Ames, IA (United States). Inst. for Physical Research and Technology)

1994-11-01

406

Equivalent magnetic noise in multi- push-pull configuration magnetoelectric composites: model and experiment.  

PubMed

A theoretical model for the multi-push-pull configuration of magnetoelectric (ME) laminated composites comprising magnetostrictive and piezoelectric layers with interdigitated electrodes encapsulated in polyimide film is presented. Analytical solutions for the ME voltage coefficient ?E, ME charge coefficient ?Q, noise charge density and equivalent magnetic noise were derived. Parametric studies are presented to evaluate the influence of material properties and polyimide film geometries. The results show that the value of ?E was determined by the parameters of the magnetostrictive and piezoelectric phases, and that the values of ?Q and noise charge density were determined not only by the component parameters, but also by the volume fraction of the piezoelectric phase and polyimide film geometry. The equivalent magnetic noise had no dependence on the polyimide film geometry, but rather was determined by the component parameters and the volume fraction of the piezoelectric phase. Theoretical and experimental results are compared and shown to have good agreement with each other. PMID:25004486

Yaojin Wang; Hasanyan, D; Menghui Li; Junqi Gao; Jiefang Li; Viehland, D

2013-06-01

407

Uncertainty of reconstructions of spatially distributed magnetic nanoparticles under realistic noise conditions  

NASA Astrophysics Data System (ADS)

Magnetorelaxometry (MRX) is a measurement technique able to sense the magnetic field originating from magnetic nanoparticles (MNPs). The concentration distribution of MNPs can be recovered by interpreting the MRX measurement data with a numerical model, i.e., by solving an inverse problem. We investigate the actual impact of noise on the MNP reconstruction quality when using distributed excitation coil configurations and how the excitation setup needs to be adapted when prior information on the MRX noise is known. Results show that an approximately 4 times larger sensitivity can be attained when adapting the excitation setup to the known realistic noise. The proposed methodology is able to assess the sensitivity limits of the MRX measurement setup more accurately compared to convenient noise models.

Coene, A.; Crevecoeur, G.; Liebl, M.; Wiekhorst, F.; Dupré, L.; Steinhoff, U.

2014-05-01

408

Asymmetry in the current sheet and secondary magnetic flux ropes during guide field magnetic reconnection  

NASA Astrophysics Data System (ADS)

A magnetic reconnection event with a moderate guide field encountered by Cluster in the near-Earth tail on 28 August 2002 is reported. The guide field points dawnward during this event. The quadrupolar structure of the Hall magnetic field within the ion diffusion region is distorted toward the northern hemisphere in the earthward part while toward the southern hemisphere tailward part of X-line. Observations of current density and electron pitch angle distribution indicate that the distorted quadrupolar structure is formed due to a deformed Hall electron current system. Cluster crossed the ion diffusion region from south to north earthward of the X-line. An electron density cavity is confirmed in the northern separatrix layer while a thin current layer (TCL) is measured in the southern separatrix layer. The TCL is formed due to electrons injected into the X-line along the magnetic field. These observations are different from simulation results where the cavity is produced associated with inflow electrons along the southern separatrix while the strong current sheet appears with the outflow electron beam along the northern separatrix. The energy of the inflowing electron in the separatrix layer could extend up to 10 keV. Energetic electron fluxes up to 50 keV have a clear peak in the TCL. The length of the separatrix layer is estimated to be at least 65 c/?pi. These observations suggest that electrons could be pre-accelerated before they are ejected into the X-line region along the separatrix. Multiple secondary flux ropes moving earthward are observed within the diffusion region. These secondary flux ropes are all identified earthward of the observed TCL. These observations further suggest there are numerous small scale structures within the ion diffusion region.

Wang, Rongsheng; Nakamura, Rumi; Lu, Quanming; Du, Aimin; Zhang, Tielong; Baumjohann, Wolfgang; Khotyaintsev, Yuri V.; Volwerk, Martin; André, Mats; Fujimoto, M.; Nakamura, T. K. M.; Fazakerley, Andrew N.; Du, Jian; Teh, Waileong; Panov, Evgeny V.; Zieger, Bertalan; Pan, Yongxin; Lu, San

2012-07-01

409

Suppression of magnetic noise in the fundamental-mode orthogonal fluxgate  

Microsoft Academic Search

A new important advantage has been discovered for the fundamental-mode operation of the orthogonal fluxgate employing an amorphous wire. It has been found that a great enough dc bias practically completely suppresses the magnetic noise generated in the fluxgate core by the ac-bias field; the fluxgate resolution becomes limited only by the excess electric noise in the ac-bias current. As

Eugene Paperno

2004-01-01

410

Low-frequency noise in serial arrays of MgO-based magnetic tunnel junctions  

Microsoft Academic Search

We have studied the low-frequency noise in MgO-based magnetic tunnel junctions (MTJs) in serial configurations. Two types of junctions were compared: MTJ Wheatstone bridges and MTJ discrete resistors closely packed on a wafer die. We have characterized each individual junction to ensure that they have uniform parameters such as linear field sensitivity and noise level. In the array of bridges,

Wenzhe Zhang; Qiang Hao; Gang Xiao

2011-01-01

411

Noise reduction for magnetic resonance images via adaptive multiscale products thresholding.  

PubMed

Edge-preserving denoising is of great interest in medical image processing. This paper presents a wavelet-based multiscale products thresholding scheme for noise suppression of magnetic resonance images. A Canny edge detector-like dyadic wavelet transform is employed. This results in the significant features in images evolving with high magnitude across wavelet scales, while noise decays rapidly. To exploit the wavelet interscale dependencies we multiply the adjacent wavelet subbands to enhance edge structures while weakening noise. In the multiscale products, edges can be effectively distinguished from noise. Thereafter, an adaptive threshold is calculated and imposed on the products, instead of on the wavelet coefficients, to identify important features. Experiments show that the proposed scheme better suppresses noise and preserves edges than other wavelet-thresholding denoising methods. PMID:12956264

Bao, Paul; Zhang, Lei

2003-09-01

412

Magnetic flux compression generator powered electron beam experiments  

SciTech Connect

We have combined several technologies in a rather conservative fashion to perform feasibility experiments to power a vacuum, electron beam diode with an explosive power supply. The magnetic flux compression generator used in these experiments was the 13.2-cm-wide by 52.8-cm-long plate generator. The output of this device was switched into an air-core, foil-wound, step-up transformer with a turn ratio of 30:1. The transformer and diode were connected through a 9-m-long piece of Sieverts high-voltage cable. The diode structure consisted of a high-voltage terminal on a stacked-ring grading structure with a 5-cm-diameter stalk to support a 12.7-cm-diameter field emission cathode. The cathode-anode gap was 2.5 cm, and the anode was 6.35-micron--thick aluminized mylar. Currents of greater than or equal to14 kA and voltages of greater than or equal to540 kV were achieved with a pulse length of greater than or equal to100 ns. Theoretical predictions agree with experimental performance until the diode shorted. However, this event limited the ultimate performance of the experiments. We consider diode shorting to be a major problem to be dealt with in future experiments involving >1 ..mu..s power pulses. This approach to powering electron beams should prove useful for experiments requiring only a few events or for very high-energy diode design and scaling tests. 11 figs.

Freeman, B.L.; Erickson, D.J.; Fowler, C.M.; Hoeberling, R.F.; King, J.C.; Kruse, P.J.; Peratt, A.L.; Rickel, D.G.; Thode, L.E.; Toevs, J.W.

1986-01-01

413

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

414

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

415

Comparison of an axial flux and a radial flux permanent magnet motor for solar race cars  

Microsoft Academic Search

This paper provides analyses, models and a new frame design for the Csiro electric drive for solar cars. The main objective is to determine whether the axial flux Csiro motor (CM) is more efficient than the radial flux Biel motor (BM). To do that, performance tests were conducted and both drives were compared. Results showed that the CM is more

R. Al Zaher; S. de Groot; H. Polinder; P. Wieringa

2010-01-01

416

Properties of a Flux Dam Inserted in the Pickup Coil of a High-Tc Superconducting Quantum Interference Device Magnetometer  

Microsoft Academic Search

The properties of a flux dam inserted in a pickup coil of a high-Tc superconducting quantum interference device (SQUID) are studied. First, the effect of thermal noise at T=77 K on the magnetic flux entry into in the pickup coil through the flux dam is discussed. It is shown that the thermal noise causes the flux entry even when the

Keiji Enpuku; Atsushi Nakahodo; Masahiro Hotta; Shintaro Hijiya; Daishi Tokimizu; Daisuke Kuroda

2001-01-01

417

The Formation of Flux Tubes from a Large Scale Magnetic Field  

NASA Astrophysics Data System (ADS)

One of the key ingredients in the solar cycle is the formation of magnetic flux tubes, which eventually appear as sunspots, from a large scale dynamo-generated field. In order to try and understand this process we have investigated the three-dimensional nonlinear evolution of a layer of magnetic field that is susceptible to a magnetic Rayleigh-Taylor instability. The nonlinear evolution is eventually dominated by vortex interactions, leading to the formation of arched magnetic structures.

Hughes, D. W.; Wissink, J. G.; Matthews, P. C.; Proctor, M. R. E.

418

GRI pipeline simulation facility magnetic flux leakage test bed vehicle. Final report, September 1990-June 1996  

SciTech Connect

To advance the capability of nondestructive internal pipeline inspection devices, commonly called inspection pigs, two test bed vehicles have been built as test platforms for use in the GRI Pipeline Simulation Facility. This report describes the magnetic flux leakage test bed vehicle. Magnetic flux leakage was selected for a specific test bed vehicle implementation because it is the most commonly used in-line inspection technology for the detection of metal loss defects in gas-transmission pipelines. The other test bed vehicle is called the advanced sensor test bed vehicle, which can be used for experiments involving ultrasonics, eddy currents, electromagnetic acoustic transducers, or hybrid techniques (multiple inspection technologies). The magnetic flux leakage test bed vehicle was designed to simulate magnetic flux leakage technology, as well as advance the state of the art of the technology.

Nestleroth, J.B.; Bubenik, T.A.; Teitsma, A.

1996-06-01

419

Magnetic flux shock wave and hydrodynamic dendritic instability in type-II superconducting film  

NASA Astrophysics Data System (ADS)

Non-equilibrium flux penetration into the Meissner state in type-II superconducting film is studied. Effects of heat dissipation and transport on the motion and stability of the interface between the magnetic flux and flux-free domains are considered. It is shown that the magnetic induction and temperature, form shock wave with a narrow normal front moving with a constant velocity. It is shown that tangential instability is responsible for a crush of the shock wave front. For a sufficiently small heat diffusion constant, a ultrafast dendrite-shaped magnetic flux structure is predicted, while for large thermal diffusion the shock wave front is found to be stable. We conclude that the heat ballistic transport rather than magnetic non-locality in the film plays the essential role in the shock wave front properties. The dendritic velocity increases along with a reciprocal thickness of the film.

Deutsch, E.; Shapiro, B. Ya.; Shapiro, I.

2008-01-01

420

Controlled switching of Néel caps in flux-closure magnetic dots.  

PubMed

While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called Néel caps. We demonstrated the controlled switching of these caps under a magnetic field, occurring via the propagation of a surface vortex. For this we considered flux-closure states in elongated micron-sized dots, so that only the central domain wall can be addressed, while domains remain unaffected. PMID:19392153

Cheynis, F; Masseboeuf, A; Fruchart, O; Rougemaille, N; Toussaint, J C; Belkhou, R; Bayle-Guillemaud, P; Marty, A

2009-03-13

421

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