Perpendicular Component Of Magnetic Field Research Articles

Design and simulation of a superconducting machine excitation system taking into account the three-dimensional magnetic leakage

The optimized design of a new high-temperature superconducting rotating pole machine is presented. Its main structural feature is the use of a double stator core which separates the synchronous machine pole shoe from the pole body to rotate separately as the machine rotor, allowing the superconducting coil to operate in a stationary state. The inner stator core, the stationary dewar and the rotor core together form the excitation system of the machine. The excitation coil windings adopt a rectangular cross-section, with flux divertor strategically placed between the high-temperature superconducting coils. This configuration aims to modulate the background magnetic field, specifically reducing the perpendicular magnetic field component. This mitigation minimizes the impact of ambient magnetic fields on the superconducting coil’s current carrying capacity, ensuring an optimized magnetic field environment for its operation. Through the integration of these modifications, the technical and economic parameters of the enhanced high-temperature superconducting machine have been significantly improved. The optimization of design, coupled with detailed calculations of the 3D electromagnetic field, was achieved utilizing the commercial software Ansys EM module.

Dynamic resistance and total loss in small REBCO pancake and racetrack coils carrying DC currents under an AC magnetic field

In many high-temperature superconducting applications, REBCO (Rare-earth barium copper oxide) coils carry DC currents under AC magnetic fields, such as the field winding of rotating machines, linear synchronous motors and the electro-dynamic suspension system of maglev. In such operating conditions, REBCO coils generate AC loss—total loss which includes the magnetization loss due to the shielding currents, and the dynamic loss arising from dynamic resistance caused by the interaction of DC currents and AC magnetic fields. In this work, dynamic resistance and total loss in a small double pancake coil (DPC) and a small double racetrack coil (DRC) are investigated via experiments in the temperature range between 77 K and 65 K. The DC currents are varied from zero to 70% of the self-field critical currents of the REBCO coils, with AC magnetic fields up to 100 mT. The experimental results in the DPC are well supported by the finite element simulation results using 3D T-A formulation. Our results show that the critical current of the DRC is approximately 2%–5% higher than that of the DPC in the temperature range. For given experimental conditions, the magnetization loss in both coils is much greater than the dynamic loss. The dynamic loss and magnetization loss in the DRC are greater than those in the DPC, which we attribute to the large perpendicular magnetic field component in the straight sections of the DRC.

Relationship between drift kinetics, gyrokinetics and magnetohydrodynamics in the long-wavelength limit

Gyrokinetic theories, even ‘global’ models, typically rely on a separation in scale (in perpendicular wavelength) between the fluctuations and the system scale. In such models direct simulation of system-scale dynamics like magnetohydrodynamic (MHD) motion is formally not consistent. Drift-kinetic theory, on the other hand, may be directly applied to model system-scale MHD-ordered behaviour. I review the long-wavelength limit of standard gyrokinetics and drift kinetics to present the relationships between these theories in an elementary fashion. This provides a pathway to global gyrokinetic modelling, resulting in an approach that is structurally similar to kinetic MHD, and I present dynamical equations for solving global field evolution in this framework. Departures from certain earlier global gyrokinetic theories include the appearance of magnetosonic (fast) modes, and the cross-coupling of the parallel and perpendicular currents with perpendicular and parallel magnetic field components. A periodic two-dimensional testcase is outlined as a benchmarking and implementation target, to help clarify practical aspects of these theories, with minimal complexity in terms of boundary conditions, and a proof-of-principle implementation of a field-solver is exhibited. To motivate this work, I first illustrate certain limitations of existing global gyrokinetic frameworks and directly identify how scale separation approximations lead to certain ‘missing’ system-scale field terms in global gyrokinetics, largely as a result of simplifications associated with the field representation in terms of $A_{\|}$ and $B_{\|}$ . As a result, the currents in the gyrokinetic Ampère's law resulting from a gyrokinetic equilibrium distribution do not match the currents implied by Ampère's law in a force-balance MHD equilibrium. I present a simple choice of equilibrium distribution function whose drift-kinetic currents are consistent with MHD currents; a specific Grad–Shafranov equilibrium is used to illustrate the size of the components of the parallel currents.

Effects of Field‐Aligned Cold Plasma Density Filaments on the Fine Structure of Chorus

AbstractThe whistler‐mode chorus emission, a major driver of radiation belt electron energization and precipitation, exhibits significant amplitude modulations on millisecond timescales. These subpacket modulations are accompanied by fast changes in the wave normal angle. Understanding the evolution of wave propagation properties inside chorus elements is essential for modeling nonlinear chorus‐electron interactions, but the origin of these rapid changes is unclear. We propose that the variations come from propagation inside thin, field‐aligned cold plasma enhancements (density ducts), which produce differing modulations in parallel and perpendicular wave magnetic field components. We show that a full‐wave simulation on a filamented density background predicts wave vector and amplitude evolution similar to Van Allen Probes spacecraft observations. We further demonstrate that the commonly assumed wide density ducts, in which wave propagation can be studied with ray tracing methods, cannot explain the observed behavior. This indirectly proves the existence of wavelength‐scale field‐aligned density fluctuations.

Scanning magnetometer based on magnetoimpedance sensor for measuring a remnant magnetization of printed toners

In this work, a scanning magnetometer based on a giant magnetoimpedance sensor was used to study the toner material of a black-and-white laser printer that contains iron oxide nanoparticles. We measured the local perpendicular magnetic field component of various sizes black toner strips, magnetized in a perpendicular magnetic field. Our calibration samples were two-wire current lines, which are the current analogue of magnetized toner strips. Magnetic images of the samples were obtained. It was found that investigated toner samples have an average concentration of magnetic nanoparticles per unit area of order of 6 μg/mm2. At this concentration of nanoparticles, the remnant magnetic moment of the sample 1.6 × 5 mm2 in size was about of 0.046 µA⋅m2 and could be reliably registered by the scanning magnetometer. The magnetic image of the toner sample was compared with a hysteresis loop obtained using a vibrating sample magnetometer.

An investigation on AC loss reduction for permanent-magnet superconducting electrical machine

High temperature superconducting (HTS) tapes could be introduced into large scale electrical machines in order to improve the power density. However, the alternating current (AC) loss of HTS tapes will cause the reduction of efficiency. On the basis of analytical and numerical model calculations, this paper presents an optimal design of the HTS armature winding aiming at lower AC loss. The main contribution of this work is that the relationship between the installation parameters and the AC loss of such HTS armature windings has been figured out based on the analysis of the shape feature of the HTS tape and the external magnetic field. When the tape is placed along a particular direction where the perpendicular component of external magnetic field has the lowest amplitude, the AC loss is the smallest. The modified installation location and angle are found based on the proposed generator. These results are verified using finite element method (FEM).

Numerical Analysis of the Nanoparticle Dynamics in a Viscous Liquid: Deterministic Approach

We study the deterministic dynamics of single-domain ferromagnetic nanoparticles in a viscous liquid induced by the joint action of the gradient and uniform magnetic fields. It is assumed that the gradient field depends on time harmonically and the uniform field has two components, perpendicular and parallel to the gradient one. We also assume that the anisotropy magnetic field is so strong that the nanoparticle magnetization lies along the anisotropy axis, i.e., the magnetization vector is "frozen" into the particle body. With these assumptions and neglecting inertial effects we derive the torque and force balance equations that describe the rotational and translational motions of particles. We reduce these equations to a set of two coupled equations for the magnetization angle and particle coordinate, solve them numerically in a wide range of the system parameters and analyze the role of the parallel component of the uniform magnetic field. It is shown, in particular, that nanoparticles perform only periodic rotational and translational motions if the perpendicular component of the uniform magnetic field is absent. In contrast, the nanoparticle dynamics in the presence of this component becomes non-periodic, resulting in the drift motion (directed transport) of nanoparticles. By analyzing the short and long-time dependencies of the magnetization angle and particle coordinate we show that the increase in the parallel component of the uniform magnetic field decreases both the particle displacement for a fixed time and its average drift velocity on each period of the gradient magnetic field.

Drift of the sensitive direction of Hall-effect devices in (1 0 0)-silicon caused by mechanical shear stress

Conventional Hall effect devices are designed to respond to a single magnetic field component in 3D space. However, if plastic encapsulated Hall effect devices in cubic crystals are exposed to poorly defined and unstable mechanical shear stress, small portions of unwanted, perpendicular magnetic field components will show up in the Hall output signals. Mathematically, any such crosstalk – may it originate from mechanical stress or not – can be expressed as a Taylor series in powers of the magnetic field. The even powers are the well known planar Hall effect at zero mechanical stress. In practice powers of even order are irrelevant, because they are eliminated by the widely used spinning current schemes. In this paper we address the odd powers of the magnetic crosstalk and their drift versus mechanical stress. The effect is small for Hall plates but large for Vertical Hall devices in (100)-silicon. It is fully described by piezo-resistance and piezo-Hall tensors. We present results of numerical simulations and measurements. Thin devices are less affected than thick devices. If magnetic angle sensors are made of Vertical Hall devices, in-plane shear stress leads to small orthogonality errors. This article lays the foundations for compensation circuits to eliminate such shear-stress induced orthogonality errors.

AC loss measurement and simulation in a REBCO coil assembly utilising low-loss magnetic flux diverters

AC loss in high temperature superconducting (HTS) coils affects the performance of HTS devices. Using magnetic flux diverters (MFDs) is an effective way to reduce AC loss in HTS coils. In this paper, measurement and finite element method simulation of AC loss results in a REBCO coil assembly comprising four double pancake coils with two molypermalloy-powder MFDs are presented. Both experimental and numerical results show that MFDs can significantly reduce the AC loss in the REBCO coil assembly while generating negligible loss in themselves. Further, the influence of the distance between the coil assembly and the diverters on AC loss reduction is explored. Compared with the AC loss data in the coil assembly without MFDs, over 80% AC loss reduction is achieved when the distance between the coil assembly and the diverters is at its minimum value, 2 mm. The simulation results reveal that the AC loss reduction in the coil assembly is mainly due to the reduction of the radial (perpendicular) magnetic field component to the surface of REBCO wires in the end windings of the coil assembly.

Shielding Effect of (RE)Ba2Cu3O7-d-Coated Conductors on Eddy Current Loss of Adjacent Metal Layers Under AC Magnetic Fields With Various Orientations

(RE)Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> -d (REBCO) coated conductors are becoming a preferred wire choice for many potential high temperature superconducting (HTS) applications, where additional metal layers are usually attached to the REBCO conductors to improve reliability and stability. In such cases, the eddy current loss in the metal layers is affected by the magnetic field shielding by adjacent REBCO coated conductors. Here we present the ac loss measurement and simulation results in three copper-superconductor stacks comprising a 10-mm-wide Fujikura-coated conductor with one, two, and four 0.1-mm-thick, 10-mm-wide copper strips, under various applied magnetic field angles (defined by the angle between the applied magnetic field and the wide face of the superconductor) from 15° to 90° at 77 K. The amplitude of the magnetic field is up to 100 mT. The shielding effect, which reduces the eddy current loss in the metal layers stacked with a superconductor layer, increases with decreasing field angles, due to the change of the effective penetration field at the different field angles. The eddy current loss in the copper strips in the copper-superconductor stacks under the magnetic fields with various orientations is dominated by the perpendicular magnetic field component.

Shubnikov—de Haas Oscillations in a Three-Dimensional Topological Insulator Based on a Strained HgTe Film in an Inclined Magnetic Field

Specific features in the formation of Landau levels in a three-dimensional topological insulator based on an 80-nm-thick strained mercury telluride film in an inclined magnetic field are studied. The magnetoresistance in the Hall bar geometry with a gate is measured at a temperature 1.9 K and in an applied magnetic field up to 10 T. The gate allows varying the Fermi level position from the valence to conduction band, passing through the bulk band gap. The samples are mounted on a rotating platform that makes it possible to change arbitrarily the angle between the magnetic field direction and the normal to the sample plane within the range of 0°–90°. It is found that the Shubnikov–de Haas oscillations are formed if the perpendicular magnetic field component exceeds 0.4 T, independent of the applied gate voltage. However, the sensitivity of the system to the parallel magnetic field component exhibits a pronounced dependence on the applied gate voltage. Namely, if the Fermi level is in the bulk band gap and the conductivity is determined by the surface states, the amplitude and position of Shubnikov–de Haas oscillations remain independent of the perpendicular magnetic field component even in the situation where the parallel component is twice as large as the perpendicular one. At high magnetic fields, the amplitude of oscillations is suppressed by the parallel field component. On the contrary, if the Fermi level is in the valence or conduction band, the parallel field component not only affects the amplitude of oscillations but also results even in their qualitative changes, e.g., giving rise to new minima related to Zeeman splitting. The observed behavior of the system is in agreement with the modern concepts concerning the spin polarization of the surface state in three-dimensional topological insulators and the spin degeneracy of charge carriers in the bulk.

Non-destructive evaluation of critical current on a Bi-2212 first stage cable based on numerical analysis of the remnant field distributions

The high temperature superconducting material Bi-2212 is considered for use in the next generation of fusion reactors due to an excellent current carrying capacity in a high field. The critical current as well as its inhomogeneity are important factors in evaluating the performance of Bi-2212 wires, cables and conductors. Non-destructive evaluation by using a Hall sensor array is a more effective and faster method for testing the local critical current of a high temperature superconductor compared with a conventional contact-electrical method. In this paper, the coupling and decoupling numerical models of the perpendicular component of the residual magnetic field in Bi-2212 round wire were solved. Then, the perpendicular component of the remnant magnetic field was measured by a Hall sensor array, which was consistent with the results of the coupling model. Our results established the numerical model for a Bi-2212 first stage cable and found that the non-destructive method using four mutual vertical Hall sensor arrays is feasible for evaluation of cable critical current properties.

Dark-bright exciton coupling in asymmetric quantum dots

Symmetry breaking of the confinement in In(Ga)As quantum dots leads to mixing between the bright and dark exciton states and an increase in the oscillator strength of the dark exciton. We demonstrate here that the change in degree of linear polarization of the bright and dark excitons as a function of applied magnetic field, where both parallel and perpendicular magnetic field components are applied, can be used to quantify the dark-bright coupling. Experimental results for a dot where the coupling is weak and the dark exciton is undetectable in zero applied magnetic field are in good agreement with theoretical simulations based on atomistic calculations reported by M. Zieli\ifmmode \acute{n}\else \'{n}\fi{}ski et al. [Phys. Rev. B 91, 085403 (2015)].

Imaging spin-resolved cyclotron trajectories in the InSb two-dimensional electron gas

We consider spin-resolved cyclotron trajectories in a magnetic focusing device with quantum point source and drain contacts defined within the InSb two-dimensional electron gas and their mapping by the scanning gate microscopy. Besides the perpendicular component of the external magnetic field which bends the electron trajectories we consider an in-plane component which introduces the spin-dependence of the cyclotron radius. We demonstrate that the focusing conductance peaks become spin-split by the in-plane magnetic field component of the order of a few tesla and that the spin-resolved trajectories can be traced separately with the conductance mapping.

Experimental and numerical transport AC losses in a four-strand Roebel cable bifilar stack

REBCO Roebel cables are promising candidates for bifilar coils (or non-inductive coils) in large scale high temperature superconducting resistive type fault current limiters (SFCLs), as they enable large current-carrying capability in conjunction with low AC loss. Here we report transport AC loss measurement results in two Roebel cable bifilar stacks (RBS) with different vertical spacing between the turns assembled from a straight 4/2 (four 2 mm width strands) Roebel cable. We also report two-dimensional finite element method AC loss simulation results using the H-formulation in the RBSs, in two parallel bifilar stacks (TPBS) which have the same geometrical dimensions as the RBS with 0.284 mm spacing (g) but have unequal current distribution between the conductors composing the stacks, and in an equivalent bifilar stack (EBS) comprising four 4 mm wide conductors while keeping identical vertical geometrical dimensions with the RBS with g = 0.284 mm but has unequal current distribution between the conductors. We show that the measured transport AC loss values in the RBS with g = 0.284 mm are approximately one fifth of those in the parent straight 4/2 Roebel cable due to cancellation of the perpendicular magnetic field components in the bifilar stack, and also show the AC loss in RBSs increases with increasing g. We then show the calculated transport AC loss values in the RBS with g = 0.284 mm are approximately one third of the values in the TPBS and EBS when It,turn/Ic0,cable > 0.6, where It,turn is the current amplitude for upper or lower turn of the bifilar stacks and Ic0,cable is nominal Ic value for the 4/2 Roebel cable. We numerically demonstrate that the result is caused by flux flow loss in the inner conductors in the TPBS and EBS due to unequal current distributions in conductors. Roebel cables show clear advantage over simple vertical stacks for SFCL applications in terms of AC loss.

Two-dimensional Hybrid Simulations of Filamentary Structures and Kinetic Slow Waves Downstream of a Quasi-parallel Shock

Abstract In this paper, with two-dimensional hybrid simulations, we study the generation mechanism of filamentary structures downstream of a quasi-parallel shock. The results show that in the downstream both the amplitude of magnetic field and number density exhibit obvious filamentary structures, and the magnetic field and number density are anticorrelated. Detailed analyses find that these downstream compressive waves propagate almost perpendicular to the magnetic field, and the dominant wave number is around the inverse of the ion kinetic scale. Their parallel and perpendicular components roughly satisfy (where and represent the parallel and in-plane perpendicular components of magnetic field, is the wave number in the perpendicular direction, and ρ i in the ion gyroradius), and their Alfvén ratio also roughly agrees with the analytical relation (where R Ai and β indicate the Alfvén ratio and plasma beta, respectively), while the corresponding cross helicity and compressibility show good agreement with previous theoretical calculations. All of these properties are consistent with those of kinetic slow waves (KSWs). Therefore, we conclude that the filamentary structures downstream of a quasi-parallel shock are produced by the excitation of KSWs.

The dynamic resistance of YBCO coated conductor wire: effect of DC current magnitude and applied field orientation

Dynamic resistance, which occurs when a HTS coated conductor carries a DC current under an AC magnetic field, can have critical implications for the design of HTS machines. Here, we report measurements of dynamic resistance in a commercially available SuperPower 4 mm-wide YBCO coated conductor, carrying a DC current under an applied AC magnetic field of arbitrary orientation. The reduced DC current, It/Ic0, ranged from 0.01 to 0.9, where It is the DC current level and Ic0 is the self-field critical current of the conductor. The field angle (the angle between the magnetic field and the normal vector of the conductor wide-face) was varied between 0° and 90° at intervals of 10°. We show that the effective width of the conductor under study is ∼12% less than the physical wire width, and we attribute this difference to edge damage of the wire during or after manufacture. We then examine the measured dynamic resistance of this wire under perpendicular applied fields at very low DC current levels. In this regime we find that the threshold field, Bth, of the conductor is well described by the nonlinear equation of Mikitik and Brandt. However, this model consistently underestimates the threshold field at higher current levels. As such, the dynamic resistance in a coated conductor under perpendicular magnetic fields is best described using two different equations for each of the low and high DC current regimes, respectively. At low DC currents where It/Ic0 ≤ 0.1, the nonlinear relationship of Mikitik and Brandt provides the closest agreement with experimental data. However, in the higher current regime where It/Ic0 ≥ 0.2, closer agreement is obtained using a simple linear expression which assumes a current-independent penetration field. We further show that for the conductor studied here, the measured dynamic resistance at different field angles is dominated by the perpendicular magnetic field component, with negligible contribution from the parallel component. Our findings now enable the dynamic resistance of a single conductor to be analytically determined for a very wide range of DC currents and at all applied field angles.

Control of vacuum arc source cathode spots contraction motion by changing electromagnetic field

This paper investigates the magnetic field component impact on cathode spots motion trajectory and the mechanism of periodic contraction. Electromagnetic coils and permanent magnets were installed at the different sides of cathode surface, the photographs of cathode spots motion trajectory were captured by a camera. Increasing the number of magnets and decreasing the distance between magnets and cathode both lead to enhancing cathode spots motion velocity. Radii of cathode spots trajectory decrease gradually with the increasing of electromagnetic coil’s current, from 40 mm at 0 A to 10 mm at 2.7 A. Parallel magnetic field component intensity influence the speed of cathode spots rotate motion, and perpendicular magnetic field component drives spots drift in the radial direction. Cathode spot’s radial drift is controlled by changing the location of the ‘zero line’ where perpendicular magnetic component shifts direction and the radius of cathode spots trajectory almost equal to ‘zero line’.

Hybrid modeling of diffusive shock acceleration in collisionless shocks in multispecies plasma

Diffusive shock acceleration (DSA) is a very efficient mechanism of high energy particle acceleration in heliosphere, supernova remnants, stellar winds and gamma-ray bursts. We present microscopic simulation of particle injection and diffusive shock acceleration which is performed with 3D divergence-conserving second-order accurate hybrid code "Maximus". Hydrogen plasma with admixture of various heavy ions is considered. The injection process is found to start through shock reflection for both hydrogen and heavier ions. However, the reflection process depends on charge-to-mass ratio. While hydrogen ions reflection appears at shock ramp and is governed by the cross-shock potential, the reflection of ions with greater A=Z proceeds deeper down-stream via gyration in perpendicular magnetic field component. The heavy ions appear to inject into the DSA preferentially, but this chemical enhancement saturates with growing A=Z. The protons injection efficiency is estimated within various approaches, and it is shown that about 20% of initial flow energy goes into accelerated particles.

Ferromagnetic Coating for Decreasingof Self-Magnetic Field Acting on HTS Tapes

The decreasing of critical current in the high-temperature tape superconductor (HTS) can be prevented by decreasing of the perpendicular component of self-magnetic field. The coating of this tape by the ferromagnetic material can deform the magnetic field around the HTS tape with flowing transport current. The present report contains the finite-element analysis of the effect of ferromagnetic coating near the HTS tape. The optimal configuration of the ferromagnetic coating for minimizing the perpendicular component of magnetic field was found. In present case, it reduces from 0.04 T to less than 0.01 T. The optimal shape of the coating contains the gap along the lateral side of the HTS tape. The influence of the twisting of the tape coated by ferromagnetic was also investigated. It was found that the difference between twisted and nontwisted cables was not large. The study was also performed for the stack of HTS tapes where neighbor tapes have the opposite direction of the transport current. It was found that the ferromagnetic coating and ribs make the magnetic field near edges smooth.