Magnetic Current Distribution Research Articles

Are there any stable magnetic fields in barotropic stars?

We construct barotropic stellar equilibria, containing magnetic fields with both poloidal and toroidal field components. We extend earlier results by exploring the effect of different magnetic field and current distributions. Our results suggest that the boundary treatment plays a major role in whether the poloidal or toroidal field component is globally dominant. Using time evolutions we provide the first stability test for mixed poloidal-toroidal fields in barotropic stars, finding that all these fields suffer instabilities due to one of the field components: these are localised around the pole for toroidal-dominated equilibria and in the closed-field line region for poloidal-dominated equilibria. Rotation provides only partial stabilisation. There appears to be very limited scope for the existence of stable magnetic fields in barotropic stars. We discuss what additional physics from real stars may allow for stable fields.

Application of a data-driven simulation method to the reconstruction of the coronal magnetic field

Ever since the magnetohydrodynamic (MHD) method for extrapolation of the solar coronal magnetic field was first developed to study the dynamic evolution of twisted magnetic flux tubes, it has proven to be efficient in the reconstruction of the solar coronal magnetic field. A recent example is the so-called data-driven simulation method (DDSM), which has been demonstrated to be valid by an application to model analytic solutions such as a force-free equilibrium given by Low and Lou. We use DDSM for the observed magnetograms to reconstruct the magnetic field above an active region. To avoid an unnecessary sensitivity to boundary conditions, we use a classical total variation diminishing Lax-Friedrichs formulation to iteratively compute the full MHD equations. In order to incorporate a magnetogram consistently and stably, the bottom boundary conditions are derived from the characteristic method. In our simulation, we change the tangential fields continually from an initial potential field to the vector magnetogram. In the relaxation, the initial potential field is changed to a nonlinear magnetic field until the MHD equilibrium state is reached. Such a stable equilibrium is expected to be able to represent the solar atmosphere at a specified time. By inputting the magnetograms before and after the X3.4 flare that occurred on 2006 December 13, we find a topological change after comparing the magnetic field before and after the flare. Some discussions are given regarding the change of magnetic configuration and current distribution. Furthermore, we compare the reconstructed field line configuration with the coronal loop observations by XRT onboard Hinode. The comparison shows a relatively good correlation.

Stored Energy and Quality Factor of Spherical Wave Functions–in Relation to Spherical Antennas With Material Cores

We present closed-form expressions for central properties of spherical wave functions of arbitrary order in relation to arbitrarily sized spherical antennas with lossless solid material cores. These properties are the electric or magnetic spherical surface current distribution radiating a spherical wave, the excitation coefficients for the internal and external spherical waves, the radiated power, the internal and external stored electric and magnetic energies, the difference of total electric and total magnetic energy, the cavity and radiating resonance conditions, and the quality factor. We investigate the variation of the internal/external and electric/magnetic stored energies with the electrical size of the antenna to study their relative significance for the quality factor.

Influence of number of split rings on the leaky radiation of a metamaterial transmission line based on complementary split ring resonators

This article discusses the influence of the number of concentric split rings on the radiation performances of a leaky wave antenna (LWA) created by a microstrip transmission line periodically loaded by series gaps and complementary split ring resonators (CSRRs) in its ground plane.The addition of a second concentric split ring in the CSRR structure strongly modifies the distribution of electric and magnetic currents in each cell of the periodic LWA structure. As a consequence, the far-field radiation pattern of the cell shows a zero in the broadside direction, which is responsible for an apparent split of the LWA radiation pattern at the upper frequency of the backward scanning range, predicted by full-wave simulations and validated by measurements in anechoic chamber.

Transient Magnetic Fields and Current Distributions in an Electric Vehicle Caused by a Lightning Stroke

Summary An electric vehicle has more electronic parts than a gasoline-powered vehicle. Not only the control circuits but also the driving circuits of electric vehicles are electrical, in contrast to those of gasoline-powered vehicles. This means that there is a higher possibility of malfunctions in an electric vehicle due to electromagnetic disturbances caused by a lightning stroke. Therefore, it is important to establish lightning protection methodologies for electric vehicles. To clear up with the mechanisms whereby the lightning current following through the vehicle body and some other parts causes the malfunctions, it is important to clarify the transient magnetic fields and current distributions in electric vehicles. In this paper, the transient magnetic fields and the current distributions in an electric vehicle are simulated by the FDTD method, and the probability of lightning damage is discussed.

高効率加熱を目的としたIH 負荷の磁気特性解析とモデリング

An IH cooking heater should have high efficiency heating and accept all metal pans. In order to develop the high efficiency induction heater for all metals, it is necessary to analyze magnetic characteristics of the working coil of IH cooking heater. However such an analysis has not been discussed until now. This paper presents a magnetic analysis for high efficiency induction heating, which can heat not only iron pans but also aluminum pans. The heating performance of IH cooking heater which heats both iron and aluminum pans is discussed with the analysis result of magnetic flux and eddy current distribution.

Magnetic Nanoparticle Characterization Using Nano-SQUID based on Niobium Dayem Bridges

Abstract Magnetic nano-sensors based on niobium dc SQUID (Superconducting Quantum Interference Device) for nanoparticle characterization are presented. The SQUIDs consists of two Dayem bridges of 90 nm x 250 nm and loop area of 4, 1 and 0.55 μm2. The SQUIDs were designed to have a hysteretic current-voltage characteristic in order to work as a magnetic flux-current transducer. Current-voltage characteristics, critical current as a function of the external magnetic field and switching current distributions were performed at liquid helium temperature. A critical current modulation of about 20% and a current-magnetic flux transfer coefficient (responsivity) of 30 μA/Φ0 have been obtained, resulting in a magnetic flux resolution better than 1 mΦ0. In order to show the effectiveness of sensor for nanomagnetism applications, we performed measurements with and without magnetic nanoparticles on the SQUID loop applying a magnetic field parallel to the SQUID plane. In this configuration the magnetic flux coupled to the SQUID is mainly due to the presence of magnetic nanoparticles. The magnetic nanoparticles can be easily detected and their response to magnetic field studied. Measurements has been performed on Fe3O4 nanoparticles prepared by thermal decomposition method with a nominal particle size of 8 nm. Some examples of magnetization measurements were recorded at low temperature after Zero Field Cooling.

Electromagnetic scattering study for metal/dielectic coated inlet diffuser

As one of strong scattering sources of planes, assessing accurately the characteristic of radar cross-section (RCS) about inlets is still hot in investigation into computational electromagnetics. An iterative physical optics method, which an impedance boundary condition and forward-backward method are added to, is studied and a box-segmentation method of blanking off is proposed. It can compute accurately the electric magnetic current distribution on the wall and the RCS. What is more, by developing a kind of software which can be verified by the relevant examples, the RCS characteristic of any metal or dielectric cavity can be evaluated. On this basis, using the software, the numerical simulation about the metal/coated medium S-diffuser with rectangular shape is carried out, and the RCS variational regulation of diffuser under an incident wave length in 0.03 m is obtained. The improved iterative physical optics method is applicable to the design of any cavity and the study of coating absorbing materials, therefore the design cycle is shortened.

Calculating transport AC losses in stacks of high temperature superconductor coated conductors with magnetic substrates using FEM

In this paper, the authors investigate the electromagnetic properties of stacks of high temperature superconductor (HTS) coated conductors with a particular focus on calculating the total transport AC loss. The cross-section of superconducting cables and coils is often modeled as a two-dimensional stack of coated conductors, and these stacks can be used to estimate the AC loss of a practical device. This paper uses a symmetric two dimensional (2D) finite element model based on the H formulation, and a detailed investigation into the effects of a magnetic substrate on the transport AC loss of a stack is presented. The number of coated conductors in each stack is varied from 1 to 150, and three types of substrate are compared: non-magnetic weakly magnetic and strongly magnetic. The non-magnetic substrate model is comparable with results from existing models for the limiting cases of a single tape (Norris) and an infinite stack (Clem). The presence of a magnetic substrate increases the total AC loss of the stack, due to an increased localized magnetic flux density, and the stronger the magnetic material, the further the flux penetrates into the stack overall. The AC loss is calculated for certain tapes within the stack, and the differences and similarities between the losses throughout the stack are explained using the magnetic flux penetration and current density distributions in those tapes. The ferromagnetic loss of the substrate itself is found to be negligible in most cases, except for small magnitudes of current. Applying these findings to practical applications, where AC transport current is involved, superconducting coils should be wound where possible using coated conductors with a non-magnetic substrate to reduce the total AC loss in the coil.

Magnetic Energy Storage and Current Density Distributions for Different Force-Free Models

In the last decades, force-free-field modelling has been used extensively to describe the coronal magnetic field and to better understand the physics of solar eruptions at different scales. Especially the evolution of active regions has been studied by successive equilibria in which each computed magnetic configuration is subject to an evolving photospheric distribution of magnetic field and/or electric-current density. This technique of successive equilibria has been successful in describing the rate of change of the energetics for observed active regions. Nevertheless the change in magnetic configuration due to the increase/decrease of electric current for different force-free models (potential, linear and nonlinear force-free fields) has never been studied in detail before. Here we focus especially on the evolution of the free magnetic energy, the location of the excess of energy, and the distribution of electric currents in the corona. For this purpose, we use an idealised active region characterised by four main polarities and a satellite polarity, allowing us to specify a complex topology and sheared arcades to the coronal magnetic field but no twisted flux bundles. We investigate the changes in the geometry and connectivity of field lines, the magnetic energy and current-density content as well as the evolution of null points. Increasing the photospheric current density in the magnetic configuration does not dramatically change the energy-storage processes within the active region even if the magnetic topology is slightly modified. We conclude that for reasonable values of the photospheric current density (the force-free parameter α<0.25 Mm−1), the magnetic configurations studied do change but not dramatically: i) the original null point stays nearly at the same location, ii) the field-line geometry and connectivity are slightly modified, iii) even if the free magnetic energy is significantly increased, the energy storage happens at the same location. This extensive study of different force-free models for a simple magnetic configuration shows that some topological elements of an observed active region, such as null points, can be reproduced with confidence only by considering the potential-field approximation. This study is a preliminary work aiming at understanding the effects of electric currents generated by characteristic photospheric motions on the structure and evolution of the coronal magnetic field.

Peristaltic flow of a second‐order fluid in the presence of an induced magnetic field

AbstractIn this paper, we study the effect of the induced magnetic field on peristaltic transport of an incompressible conducting second‐order fluid in a symmetric planar channel. The flow analysis has been developed without low Reynolds number and long wavelength approximations. The analysis consists of a perturbation expansion in terms of the small wave number. Computations have been established for the pressure gradient, stream function, magnetic force function, axial induced magnetic field and current distribution across the channel. The effects of pertinent parameters on the pressure rise per wavelength are investigated by means of numerical integrations. We also study the effect of these parameters on magnetic force function, axial induced magnetic field and current distribution across the channel. The phenomena of trapping and pumping are further discussed. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Computation of Casimir interactions between arbitrary three-dimensional objects with arbitrary material properties

We extend a recently introduced method for computing Casimir forces between arbitrarily shaped metallic objects [M. T. H. Reid et al., Phys. Rev. Lett. 103 040401 (2009)] to allow treatment of objects with arbitrary material properties, including imperfect conductors, dielectrics, and magnetic materials. Our original method considered electric currents on the surfaces of the interacting objects; the extended method considers both electric and magnetic surface current distributions, and obtains the Casimir energy of a configuration of objects in terms of the interactions of these effective surface currents. Using this new technique, we present the first predictions of Casimir interactions in several experimentally relevant geometries that would be difficult to treat with any existing method. In particular, we investigate Casimir interactions between dielectric nanodisks embedded in a dielectric fluid; we identify the threshold surface-surface separation at which finite-size effects become relevant, and we map the rotational energy landscape of bound nanoparticle diclusters.

Influence of induced magnetic field on peristaltic flow in an annulus

This paper looks at the influence of the induced magnetic field on peristaltic transport through a uniform infinite annulus filled with an incompressible viscous and Newtonian fluid. The present theoretical model may be considered as mathematical representation to the movement of conductive physiological fluids in the presence of the endoscope tube (or catheter tube). The inner tube is uniform, rigid, while the outer tube has a sinusoidal wave traveling down its wall. The flow analysis has been developed for low Reynolds number and long wave length approximation. Exact solutions have been established for the axial velocity, stream function, axial induced magnetic field, current distribution and the magnetic force function. The effects of pertinent parameters on the pressure rise and frictional forces on the inner and outer tubes are investigated by means of numerical integrations, also we study the effect of these parameters on the pressure gradient, axial induced magnetic field and current distribution. The phenomena of trapping is further discussed.

Method of Moments Analysis of Slotted Substrate Integrated Waveguide Arrays

A method of moments analysis is described for substrate integrated waveguide slot arrays. The array structure is modeled as a parallel plate waveguide on which via holes and slots are created. The field into the waveguide is expressed by the sum of the parallel plate's contribution and the field scattered by the metallic through holes, with both contributions expressed by means of an expansion in terms of vectorial cylindrical eigenfunctions. The slots are modeled as unknown equivalent magnetic current distributions which are found by solving an integral equation derived from the continuity of the field on the slot surface. Antenna arrays already presented in the literature will be analyzed, and the results compared with HFSS simulations. It will be shown that the proposed method is efficient and gives results in excellent agreement with the most common simulation tools.

A Novel Reconfigurable Cpw-Fed Slot Antenna

A novel low-profile reconfigurable CPW-fed slot antenna that is designed using a half-wavelength slot-line resonator is proposed in this paper. Frequency hopping can be easily achieved by incorporating a pair of PIN diodes, being controlled in either their on or off state. Benefited from the magnetic current distribution of a half-wavelength slot-line resonator, the radiation patterns at the two operating frequencies (1.8GHz and 2.45GHz) are very stable, being almost identical. The proposed antenna is designed and fabricated, and the theoretical and measured results are presented and show good agreement.

NanoSQUID as magnetic sensor for magnetic nanoparticles characterization

Integrated magnetic sensors based on niobium dc SQUID (Superconducting Quantum Interference Device) for nanoparticle characterizations are presented. The SQUIDs consists of two Dayem bridges of 90 nm × 250 nm and loop area of 4, 1, and 0.55 μm2. The devices are realized by using an e-beam lithography nano-fabrication process which can directly pattern the devices in an electron-positive resist and then transferred to a 20 nm single niobium layer by a lift-off post-process. The SQUIDs were designed to have a hysteretic current–voltage characteristic in order to work as a magnetic flux-current transducer. The presence of an integrated niobium coil, tightly coupled to the SQUID, allows us to easily excite the SQUID and to flux bias the SQUID at its optimal working point. Current–voltage characteristics, critical current as a function of the external magnetic field and switching current distributions were performed at liquid helium temperature. A critical current modulation of about 20% and a current-magnetic flux transfer coefficient (responsivity) of 30 μA/Φ0 have been obtained, resulting in a magnetic flux resolution better than 1 mΦ0. The authors performed preliminary measurements with and without iron oxide nanoparticles on the SQUID loop in order to show the device sensitivity in view of nano-magnetism applications. It was showed that the presence of magnetic nanoparticles can be easily detected and the magnetic relaxation curve measured.

Numerical modeling of magnetic induction tomography using the impedance method

This article discusses the impedance method in the forward calculation in magnetic induction tomography (MIT). Magnetic field and eddy current distributions were obtained numerically for a sphere in the field of a coil and were compared with an analytical model. Additionally, numerical and experimental results for phase sensitivity in MIT were obtained and compared for a cylindrical object in a planar array of sensors. The results showed that the impedance method provides results that agree very well with reality in the frequency range from 100 kHz to 20 MHz and for low conductivity objects (10 S/m or less). This opens the possibility of using this numerical approach in image reconstruction in MIT.

Reconfigurable subwavelength waveguide based on magnetic metamaterial

The design and characterization is described of a reconfigurable subwavelength waveguide based on magnetic metamaterial. The waveguide is capable of reconfiguring its propagating mode from right-handed to left-handed, which can be applied in the design of novel microwave and RF devices such as leaky wave antennas with broadened scanning range. The waveguide consists of a rectangular metallic waveguide loaded by two pieces of different magnetic metamaterials, which are structured by the same cells—modified split ring resonators (MSRRs)—with different arrangements. Positions of the two pieces of metamaterials in the waveguide can be reconfigured separately by the control mechanism. The simulated transmission data show that the waveguide has a passband below the cutoff frequency of the hollow waveguide either in the left-handed case or in the right-handed case. The extracted constitutive parameters have demonstrated that the effective permittivity and permeability of the waveguide are simultaneously negative in the left-handed case and positive in the right-handed case. The magnetic field and surface current distributions in the waveguide confirm that the waveguide operates as can be expected. The influence of the control mechanism on the performance of the waveguide is studied. It is shown that the influence is so minor that it can be neglected.

Influence of curvature on transmission properties of finite curved slot arrays

Transmission properties of finite curved slot arrays were investigated based on an extension of mutual admittance approach. One-dimensional curvature effects were analyzed, with the influence of column number and flare angle on magnetic current distribution, scattering patterns and frequency response curves. Results show that the curvature effect depends on the flare angle. When the flare angle is large (above 120°), strong curvature effect leads to violent fluctuation of magnetic current distribution, wide beamwidth and high side lobes, and considerable changes also occur in frequency-response characteristics, including resonant frequency, transmission bandwidth and transmission coefficient. When the flare angle is small enough, generally below 60°, only the magnetic current distributions of marginal elements would get influenced, consequently the scattering patterns and transmission characteristics are similar to those of finite planar arrays. The curvature effect may became very weak and be ignored on this condition.

Curvature, Sphere Theorems, and the Ricci flow

An innovative tool has been realized for the 3D/1D simulation of Ion Cyclotron Radio Frequency (ICRF), i.e. accounting for antennas in a realistic 3D geometry and with an accurate 1D plasma model. The approach to the problem is based on an integral-equation formulation for the self-consistent evaluation of the current distribution on the conductors. The environment has been subdivided in two coupled region: the plasma region and the vacuum region. The two problems are linked by means of a magnetic current (electric field) distribution on the aperture between the two regions. In the vacuum region all the calculations are executed in the spatial domain while in the plasma region an extraction in the spectral domain of some integrals is employed that permits to significantly reduce the integration support and to obtain a high numerical efficiency leading to the practical possibility of using a large number of sub-domain (rectangular or triangular) basis functions on each solid conductor of the system. The plasma enters the formalism of the plasma region via a surface impedance matrix; for this reason any plasma model can be used; at present the FELICE code has been adopted, that affords density and temperature profiles, and FLR effects. The source term directly models the TEM mode of the coax feeding the antenna and the current in the coax is determined self-consistently, giving the input impedance/admittance of the antenna itself. Calculation of field distributions (both magnetic and electric), useful for sheath considerations, is included. This tool has been implemented in a suite, called TOPICA, that is modular and applicable to ICRF antenna structures of arbitrary shape. This new simulation tool can assist during the detailed design phase and for this reason can be considered a Virtual Prototyping Laboratory (VPL). The TOPICA suite has been tested against assessed codes and against measurements and data of mock-ups and existing antennas. The VPL is being used in the design of various ICRF antennas and also for the performance prediction of the ALCATOR C-MOD D antenna.