Large-scale Electromagnetic Field Research Articles

Efficient Fourth-Order PSTD Algorithm with Moving Window for Long-Distance EMP Propagation.

Satellite-borne electromagnetic pulse (EMP) detection technology plays an important role in military reconnaissance, space environment monitoring, and early warning of natural disasters. However, the complex ionosphere greatly distorts the waveform during propagation and poses a challenge to EMP detection. Therefore, it is necessary to conduct theoretical research on EMP propagation in the ionosphere. Conventional second-order pseudo-spectral time-domain (PSTD-2) algorithm has difficulties in keeping the stability and accuracy of waveforms in calculations over hundreds of kilometers of propagation. To overcome the difficulties, a fourth-order PSTD algorithm incorporating the moving window technique (MWPSTD-4) is proposed. In the numerical examples, the performance of MWPSTD-4 is compared with PSTD-4 and PSTD-2 in the long-distance propagation of EMP. The results show that the MWPSTD-4 improves efficiency while guaranteeing accuracy and is suitable for large-scale electromagnetic field simulation. The proposed method provides a basic algorithm to eliminate the numerical dispersion interference for calculating the long-distance propagation of EMP in complex spaces and is helpful for the design and calibration of satellite-borne EMP detectors.

Influence of Fiber Orientation on the Shear Properties of Steel Fiber-Reinforced Reactive Powder Concrete Beams

Steel fiber-reinforced reactive powder concrete (SFRPC) has good mechanical properties and is thus useful for engineering applications. Generally, the fibers in SFRPC are randomly distributed. To study the effects of the amount, length, and orientation of steel fibers on the shear strength of SFRPC beams, nonreinforced aligned steel fiber-reinforced reactive powder concrete (ASFRPC) beams were prepared using a large-scale electromagnetic field orientation device. Shear tests of specimens with a shear span ratio of 1.5 were performed under two fiber contents (1.0% and 2.0%) and three fiber lengths (20, 30, and 40 mm). The full-field strain during the loading process was obtained via the digital image correlation method, and the effects of fiber quantity and orientation on the shear strength of the beams were analyzed. The ASFRPC beams exhibited higher ductility but lower shear capacity than the SFRPC beams under the same conditions. The shear capacity increased with the quantity of steel fibers. According to the test results, the calculation formulas of the shear-bearing capacities of ASFRPC and SFRPC beams were established.

Influence of Non-Uniform Current Distribution Among Layers on AC Loss Characteristics of Multilayer Spiral Copper-Plated Striated Coated-Conductor Cables

In spiral copper-plated striated coated-conductor cables (SCSC cables) carrying ac currents, differences in self-inductance in each layer and mutual inductances between layers cause non-uniform current distribution among layers. This may increase the ac losses of the SCSC cables. We have been developing the electromagnetic field analysis model to evaluate ac losses in SCSC cables carrying ac currents under ac magnetic fields. In the analysis model, a circuit model and a large-scale electromagnetic field analysis model were combined. In the circuit model, each layer of SCSC cables is modeled as a circuit consisting of resistances and inductances. We investigated the effect of non-uniform current distribution among layers on ac losses for a SCSC cable carrying ac currents under ac magnetic fields.

What are the fundamental modes of energy transfer and partitioning in the coupled Magnetosphere-Ionosphere system?

The fundamental processes responsible for energy exchange between large-scale electromagnetic fields and plasma are well understood theoretically, but in practice these theories have not been tested. These processes are ubiquitous in all plasmas, especially at the interface between high and low beta plasmas in planetary magnetospheres and other magnetic environments. Although such boundaries pervade the plasma Universe, the processes responsible for the release of the stored magnetic and thermal plasma energy have not been fully identified and the importance of the relative impact of each process is unknown. Despite advances in understanding energy release through the conversion of magnetic to kinetic energy in magnetic reconnection, how the extreme pressures in the regions between stretched and more relaxed field lines in the transition region are balanced and released through adiabatic convection of plasma and fields is still a mystery. Recent theoretical advances and the predictions of large-scale instabilities must be tested. In essence, the processes responsible remain poorly understood and the problem unresolved. The aim of the White Paper submitted to ESA’s Voyage 2050 call, and the contents of this paper, is to highlight three outstanding open science questions that are of clear international interest: (i) the interplay of local and global plasma physics processes: (ii) the partitioning during energy conversion between electromagnetic and plasma energy: and (iii) what processes drive the coupling between low and high beta plasmas. We present a discussion of the new measurements and technological advances required from current state-of-the-art, and several candidate mission profiles with which these international high-priority science goals could be significantly advanced.

On application of stochastic differential equations for simulation of nonlinear wave–particle resonant interactions

Long-term simulations of energetic electron fluxes in many space plasma systems require accounting for two groups of processes with well separated time-scales: a microphysics of electron resonant scattering by electromagnetic waves and a macrophysics of electron adiabatic heating/transport by mesoscale plasma flows. Examples of such systems are Earth's radiation belts and Earth's bow shock, where ion-scale plasma injections and cross-shock electric fields determine a general electron energization, whereas electron scattering by waves relaxes anisotropy of electron distributions and produces small populations of high-energy electrons. The application of stochastic differential equations is a promising approach for including effects of resonant wave–particle interaction into codes tracing electrons in models of large-scale electromagnetic fields. This study proposes and verifies such equations for the system with non-diffusive wave–particle interactions, i.e., the system with nonlinear effects of phase trapping and bunching. We consider electron resonances with intense electrostatic whistler-mode waves often observed in the Earth's radiation belts. We demonstrate that nonlinear resonant effects can be described by stochastic differential equations with the non-Gaussian probability distribution of random variations of electron energies.

Charged fluids encircling compact objects: force representations and conformal geometries

Charged fluids rotating around compact objects can form unique equilibrium structures when ambient large-scale electromagnetic fields combine with strong gravity. Equatorial as well as off-equatorial toroidal structures are among such figures of equilibrium with a direct relevance for astrophysics. To investigate their geometrical shapes and physical properties in the near-horizon regime, where effects of general relativity play a significant role, we commonly employ a scheme based on the energy–momentum conservation written in a standard representation. Here, we develop its interesting alternatives in terms of two covariant force representations, both based on a hypersurface projection of the energy–momentum conservation. In a proper hypersurface, space-like forces can be defined, following from a decomposition of the fluid four-acceleration. Each of the representations provides us with an insight into properties of the fluid flow, being well reflected in related conformal hypersurface geometries; we find behaviour of centrifugal forces directly related to geodesics of these conformal hypersurfaces and their embedding diagrams. We also reveal correspondence between the charged fluid flow world-lines from an ordinary spacetime, and world-lines determined by a charged test particles equation of motion in a conformal spacetime.

Large-scale electromagnetic field analyses of coils wound with coated conductors using a current-vector-potential formulation with a thin-strip approximation

We developed a novel software for large-scale electromagnetic field analyses of coils wound with coated conductors based on current-vector-potential formulation with thin-strip approximation. Although this formulation was effective for obtaining the precise solutions of the electromagnetic field, the strong nonlinear property of superconducting materials frequently led to highly ill-conditioned linear systems of equations, which were difficult to solve efficiently. Moreover, the memory consumption and computation time required for the analyses rapidly increased with the size of the analysis due to dense matrix operations. In our software, the first difficulty was addressed by a novel preconditioning technique based on the algebraic multigrid method. Algebraic multigrid preconditioning enabled us to efficiently and stably solve the ill-conditioned linear systems of equations encountered in our analyses. It also improved the robustness of the analyses containing multifilament-coated conductors. As regards the second difficulty, the hierarchical matrices representation drastically reduced the memory consumption related to the dense matrices, as well as computation time. Meanwhile, our implementation of the hierarchical matrices representation was quite compatible with parallel computations on distributed memory computers. Finally, we presented some practical examples of large-scale analyses, which became possible by using the new software. For instance, the analysis of a cosine-theta dipole magnet whose number of degrees of freedom was more than 1.5 million was successfully completed in 78 h by 56 parallel processes and with a total memory consumption of 177 GB.

Application of Hierarchical Matrices to Large-Scale Electromagnetic Field Analyses of Coils Wound With Coated Conductors

Because the coefficient matrix is dense in the electromagnetic-field-analysis models of coated conductors using current vector potentials, the analyses of large-scale coils require a large amount of memory and long computation time. In this paper, we introduce hierarchical matrices by using the HACApK library in order to reduce both memory consumption and computation time. The effect of the method with respect to memory consumption and computation time is examined in numerical tests using a pancake coil wound with a coated conductor. The effect of parallel computations is also examined. Finally, we analyzed a large-scale magnet, with 1.5 million unknown values.

The development of magnetic field line wander by plasma turbulence

Plasma turbulence occurs ubiquitously in space and astrophysical plasmas, mediating the nonlinear transfer of energy from large-scale electromagnetic fields and plasma flows to small scales at which the energy may be ultimately converted to plasma heat. But plasma turbulence also generically leads to a tangling of the magnetic field that threads through the plasma. The resulting wander of the magnetic field lines may significantly impact a number of important physical processes, including the propagation of cosmic rays and energetic particles, confinement in magnetic fusion devices and the fundamental processes of turbulence, magnetic reconnection and particle acceleration. The various potential impacts of magnetic field line wander are reviewed in detail, and a number of important theoretical considerations are identified that may influence the development and saturation of magnetic field line wander in astrophysical plasma turbulence. The results of nonlinear gyrokinetic simulations of kinetic Alfvén wave turbulence of sub-ion length scales are evaluated to understand the development and saturation of the turbulent magnetic energy spectrum and of the magnetic field line wander. It is found that turbulent space and astrophysical plasmas are generally expected to contain a stochastic magnetic field due to the tangling of the field by strong plasma turbulence. Future work will explore how the saturated magnetic field line wander varies as a function of the amplitude of the plasma turbulence and the ratio of the thermal to magnetic pressure, known as the plasma beta.

On Electron Adiabaticity in Collisionless Shocks

Collisionless shocks are considered as one of the most efficient phenomena for the electron acceleration in space and astrophysical plasmas, yet the detailed acceleration process and heating mechanism remain unsolved. Various acceleration and heating mechanisms have so far been proposed and discussed to explain the spacecraft measurements in situ at Earth’s bow shock, involving the large-scale electromagnetic fields, plasma turbulence, and wave-particle interactions. A key element of the acceleration and heatingmechanisms is the spatial scale of the shock transition with respect to the gyro-radius of the electrons [1]. In the simplest picture, since the shock transition occurs on a spatial scale of ion gyro-radius, the electrons behave as magnetized at the shock transition. The cross-shock potential (electrostatic potential drop across the shock transition) plays two distinct roles in electron dynamics [2]. First, the cross-shock potential reduces a certain amount of the upstream flow kinetic energy (mostly carried by the ions due to their mass); Second, the kinetic energy is used to heat the electrons (DC heating). On the other hand, the electrons can also be scattered (or demagnetized) by high-amplitude, randomly-oscillating electromagnetic fields such as turbulence or strong inhomogeneities (AC heating). Space turbulence can potentially scatter the electrons randomly leaving upstream or downstream of the shock ramp region. For a quasiperpendicular shock, the electrons become scattered in a non-adiabatic sense perpendicular to the mean magnetic field (Figure 1). In this paper, the adiabaticity in collisionless shocks is addressed to the studies based on particlein-cell-simulation (PIC). Two important parameters enter a PIC simulation carried out for the study of a collisionless shock: the mass ratio mp/me between ions and electrons and the ratio of the electron plasma frequency to gyrofrequency ωpe/ce, see e.g., [3]. Due to the computational limitations, the usage of the realistic values for the both parameters is not yet possible. The common way to overcome this problem is to find a compromise for the two parameters that can mostly optimize the purpose of the study. The impact of using low frequency ratio of the electron plasma frequency to gyrofrequency in numerical simulations has been discussed in Krasnoselskikh et al. [4]: the electric field is considerably overestimated than it should be for a comparison with the Earth bow shock. Here, I address the question, “What is the consequence of using a smaller frequency ratio upon the adiabatic motion of the electrons”?

MHD flows at astropauses and in astrotails

Abstract. The geometrical shapes and the physical properties of stellar wind – interstellar medium interaction regions form an important stage for studying stellar winds and their embedded magnetic fields as well as cosmic ray modulation. Our goal is to provide a proper representation and classification of counter-flow configurations and counter-flow interfaces in the frame of fluid theory. In addition we calculate flows and large-scale electromagnetic fields based on which the large-scale dynamics and its role as possible background for particle acceleration, e.g., in the form of anomalous cosmic rays, can be studied. We find that for the definition of the boundaries, which are determining the astropause shape, the number and location of magnetic null points and stagnation points is essential. Multiple separatrices can exist, forming a highly complex environment for the interstellar and stellar plasma. Furthermore, the formation of extended tail structures occur naturally, and their stretched field and streamlines provide surroundings and mechanisms for the acceleration of particles by field-aligned electric fields.

Comets as solar probes

By rapidly evaporating as they fly through the Sun’s hot outer atmosphere, sungrazing comets reveal an otherwise invisible magnetic field that shapes the very beginnings of the solar wind.

Large-scale time-harmonic electromagnetic field analysis using a multigrid solver on a distributed memory parallel computer

This paper reports on an investigation into large-scale parallel time-harmonic electromagnetic field analysis based on the finite element method. The parallel geometric multigrid preconditioned iterative solver for the resulting linear system was developed on a cluster of shared memory parallel computers. We propose a hybrid parallel ordering method for the parallelization of a multiplicative Schwarz smoother, which is a key component of the multigrid solver for electromagnetic field analysis. The method, using domain decomposition ordering for multi-process parallelism and introducing block multi-color ordering for multi-thread parallel processing, attains a high convergence rate with a small number of message passing interface communications and thread synchronizations. The numerical test confirms that the proposed method attains a solver performance more than twice as good as the conventional method based on multi-color ordering. Furthermore, an approximately 800 million degrees of freedom problem is successfully solved on 256 quad-core processors.

Overlapping computation and communication of three-dimensional FDTD on a GPU cluster

Large-scale electromagnetic field simulations using the FDTD (finite-difference time-domain) method require the use of GPU (graphics processing unit) clusters. However, the communication overhead caused by slow interconnections becomes a major performance bottleneck. In this paper, as a way to remove the bottleneck, we propose the ‘kernel-split method’ and the ‘host-buffer method’ which overlap computation and communication for the FDTD simulation on the GPU cluster. The host-buffer method in particular enables overlapping without any modifications to the update-kernels that are already in use. We also present theoretical formulas to predict the overlap threshold and the total throughput for each method. By using our overlap methods with 6 GPU nodes, we demonstrate that the total performance of 3D FDTD reaches 92% of a six-fold increase, which is the upper limit that would be reached if there were no communication overhead.

EXTREME PARTICLE ACCELERATION IN MAGNETIC RECONNECTION LAYERS: APPLICATION TO THE GAMMA-RAY FLARES IN THE CRAB NEBULA

The gamma-ray space telescopes AGILE and Fermi detected short and bright synchrotron gamma-ray flares at photon energies above 100 MeV in the Crab Nebula. This discovery suggests that electron-positron pairs in the nebula are accelerated to PeV energies in a milliGauss magnetic field, which is difficult to explain with classical models of particle acceleration and pulsar wind nebulae. We investigate whether particle acceleration in a magnetic reconnection layer can account for the puzzling properties of the flares. We numerically integrate relativistic test-particle orbits in the vicinity of the layer, including the radiation reaction force, and using analytical expressions for the large-scale electromagnetic fields. As they get accelerated by the reconnection electric field, the particles are focused deep inside the current layer where the magnetic field is small. The electrons suffer less from synchrotron losses and are accelerated to extremely high energies. Population studies show that, at the end of the layer, the particle distribution piles up at the maximum energy given by the electric potential drop and is focused into a thin fan beam. Applying this model to the Crab Nebula, we find that the emerging synchrotron emission spectrum peaks above 100 MeV and is close to the spectral shape of a single electron. The flare inverse Compton emission is negligible and no detectable emission is expected at other wavelengths. This mechanism provides a plausible explanation for the gamma-ray flares in the Crab Nebula and could be at work in other astrophysical objects such as relativistic jets in active galactic nuclei.

705 数値人体モデルによる大規模full-wave電磁界解析手法の性能評価(OS7-1.計算電磁気学と関連話題(1),OS・一般セッション講演)

705 数値人体モデルによる大規模full-wave電磁界解析手法の性能評価(OS7-1.計算電磁気学と関連話題(1),OS・一般セッション講演)

Dynamics of rising magnetized cavities and ultrahigh energy cosmic ray acceleration in clusters of galaxies

We study the expansion of low density cavities produced by Active Galactic Nuclei jets in clusters of galaxies. The long term stability of these cavities requires the presence of linked magnetic fields. We find solutions describing the self-similar expansion of structures containing large-scale electromagnetic fields. Unlike the force-free spheromak-like configurations, these solutions have no surface currents and, thus, are less susceptible to resistive decay. The cavities are internally confined by external pressure, with zero gradient at the surface. If the adiabatic index of the plasma within the cavity is $\Gamma>4/3$, the expansion ultimately leads to the formation of large-scale current sheets. The resulting dissipation of the magnetic field can only partially offset the adiabatic and radiative losses of radio emitting electrons. We demonstrate that if the formation of large-scale current sheets is accompanied by explosive reconnection of the magnetic field, the resulting reconnection layer can accelerate cosmic rays to ultra high energies. We speculate that the enhanced flux of UHECRs towards Centaurus A originates at the cavities due to magnetic reconnection.

Three-Dimensional Magnetic Field Numerical Calculation by Equivalent $B\hbox{–}H$ Method for Magnetic Field Mitigation

The 3-D magnetic field numerical analysis of a full-scale magnetic shielding room is advantageously performed by using the equivalent B-H method. In the proposed numerical method, the magnetic anisotropy and the nonlinearity due to material magnetic saturation are taken into account, by directly introducing the magnetization characteristics of the considered materials. An equivalent element, which substitutes the plural finite elements having different material properties for a homogeneous one, is introduced to reduce the number of unknowns. When the equivalent B-H method is applied to a full-scale magnetic shielding room (length 5.8 m, width 3.3 m, height 3.3 m), the measured leakage magnetic flux density distribution is found to be in good agreement with the calculated one, proving the reliability of the proposed approach, which can be adopted in many large-scale electromagnetic field problems.

Исследование процессов генерации МГД-волн, их трансформации и переноса волновой энергии при наличии крупномасштабных электромагнитных полей в атмосфере Солнца и гелиосфере

Исследование процессов генерации МГД-волн, их трансформации и переноса волновой энергии при наличии крупномасштабных электромагнитных полей в атмосфере Солнца и гелиосфере

858 大規模電磁場解析における可視化に関する基礎的検討(OS7.大規模並列・連成解析と関連話題(5),オーガナイズドセッション)

858 大規模電磁場解析における可視化に関する基礎的検討(OS7.大規模並列・連成解析と関連話題(5),オーガナイズドセッション)