Macroscopic Magnetic Field Research Articles

Division of the energy and of the momentum of electromagnetic waves in linear media into electromagnetic and material parts

We defend a natural division of the energy density, energy flux and momentum density of electromagnetic waves in linear media in electromagnetic and material parts. In this division, the electromagnetic part of these quantities have the same form as in vacuum when written in terms of the macroscopic electric and magnetic fields, the material momentum is calculated directly from the Lorentz force that acts on the charges of the medium, the material energy is the sum of the kinetic and potential energies of the charges of the medium and the material energy flux results from the interaction of the electric field with the magnetized medium. We present reasonable models for linear dispersive non-absorptive dielectric and magnetic media that agree with this division. We also argue that the electromagnetic momentum of our division can be associated with the electromagnetic relativistic momentum, inspired on the recent work of Barnett [Phys. Rev. Lett. 104 (2010) 070401] that showed that the Abraham momentum is associated with the kinetic momentum and the Minkowski momentum is associated with the canonical momentum.

Effective Maxwell equations from time-dependent density functional theory

The behavior of interacting electrons in a perfect crystal under macroscopic external electric and magnetic fields is studied. Effective Maxwell equations for the macroscopic electric and magnetic fields are derived starting from time-dependent density functional theory. Effective permittivity and permeability coefficients are obtained.

Functional magnetic resonance imaging using RASER

Although functional imaging of neuronal activity by magnetic resonance imaging (fMRI) has become the primary methodology employed in studying the brain, significant portions of the brain are inaccessible by this methodology due to its sensitivity to macroscopic magnetic field inhomogeneities induced near air-filled cavities in the head. In this paper, we demonstrate that this sensitivity is eliminated by a novel pulse sequence, RASER (rapid acquisition by sequential excitation and refocusing) (Chamberlain et al., 2007), that can generate functional maps. This is accomplished because RASER acquired signals are purely and perfectly T2 weighted, without any T2⁎-effects that are inherent in the other image acquisition schemes employed to date. T2-weighted fMRI sequences are also more specific to the site of neuronal activity at ultrahigh magnetic fields than T2⁎-variations since they are dominated by signal components originating from the tissue in the capillary bed. The RASER based fMRI response is quantified; it is shown to have an inherently less noisy time series and to provide fMRI in brain regions, such as the orbitofrontal cortex, which are challenging to image with conventional techniques.

Lattice kinetic simulation of nonisothermal magnetohydrodynamics

In this paper, a lattice kinetic algorithm is presented to simulate nonisothermal magnetohydrodynamics in the low-Mach number incompressible limit. The flow and thermal fields are described by two separate distribution functions through respective scalar kinetic equations and the magnetic field is governed by a vector distribution function through a vector kinetic equation. The distribution functions are only coupled via the macroscopic density, momentum, magnetic field, and temperature computed at the lattice points. The novelty of the work is the computation of the thermal field in conjunction with the hydromagnetic fields in the lattice Boltzmann framework. A 9-bit two-dimensional (2D) lattice scheme is used for the numerical computation of the hydrodynamic and thermal fields, whereas the magnetic field is simulated in a 5-bit 2D lattice. Simulation of Hartmann flow in a channel provides excellent agreement with corresponding analytical results.

THE IMPACT OF MICROSCOPIC MAGNETIC RECONNECTION ON PRE-FLARE ENERGY STORAGE

It is widely accepted that magnetic reconnection releases a large amount of energy during solar flares. Studies of reconnection usually assume that the length scale over which the global (macroscopic) magnetic field reverses is identical to the thickness of the reconnection site. However, in spatially extended high-Lundquist number plasmas such as the solar corona, this scenario is untenable; the reconnection site is microscopic and embedded inside the macroscopic current set up by global fields. We use numerical simulations and scaling arguments to show that embedded effects on reconnection could have a profound influence on energy storage before a flare. From large-scale high-Lundquist number resistive magnetohydrodynamics simulations of reconnection with a diffusion region on a much smaller scale than the macroscopic current sheet, we find that the generation of secondary islands is governed by the local magnetic field immediately upstream of the diffusion region rather than the (potentially much larger) global field. This diminishes the production of secondary islands and leads to a thicker diffusion region than those predicted using the global field strength. Such considerations are crucial for understanding the onset of solar eruptions and how energy accumulates before such eruptions. We argue that if reconnection with secondary islands is fast, the energy storage times before an eruption are too small to explain observations. If reconnection with secondary islands remains slow, embedded effects cause the diffusion region to begin far wider than kinetic scales, so energy storage before a flare can occur while collisional (Sweet-Parker) reconnection with secondary islands proceeds.

Analysis of Collisionless Shock Turbulence by Using Virtual Satellites in 2-D Full Particle-in-Cell Simulations

Two-dimensional full particle electromagnetic particle-in-cell simulations are used in order to reproduce and analyze in details the intrinsic turbulence of the terrestrial collisionless bow shock. In order to compare numerical results and local experimental measurements performed by the four-satellite space mission CLUSTER, one has developed ldquovirtual proberdquo techniques crossing the ldquonumericalrdquo shock in order to mimic the real CLUSTER mission. Numerical results shown in this paper illustrate the measurements of a turbulent macroscopic magnetic field obtained through a time-varying and inhomogeneous shock front.

Unified homogenization theory for magnetoinductive and electromagnetic waves in split-ring metamaterials

A unified homogenization procedure for split ring metamaterials taking into account time and spatial dispersion is introduced. The procedure is based on two coupled systems of equations. The first one comes from an approximation of the metamaterial as a cubic arrangement of coupled LC circuits, giving the relation between currents and local magnetic field. The second equation comes from macroscopic Maxwell equations, and gives the relation between the macroscopic magnetic field and the average magnetization of the metamaterial. It is shown that electromagnetic and magnetoinductive waves propagating in the metamaterial are obtained from this analysis. Therefore, the proposed time and spatially dispersive permeability accounts for the characterization of the complete spectrum of waves of the metamaterial. Finally, it is shown that the proposed theory is in good quantitative and qualitative agreement with full wave simulations.

Control of susceptibility-related image contrast by spin-lock techniques

Macroscopic magnetic field inhomogeneities might lead to image distortions, while microscopic field inhomogeneities, due to susceptibility changes in tissues, cause spin dephasing and decreasing T(2)() relaxation time. The latter effects are especially observed in the trabecular bone and in regions adjacent to air-containing cavities when gradient-echo sequences are applied. In conventional MRI, these susceptibility-related signal voids can be avoided by applying spin-echo (SE) techniques. In this study, an alternative method for the examination and control of susceptibility-related effects by spin-lock (SL) radiofrequency pulses is presented: SL pulses were applied in two different susceptibility-sensitive sequence types: (a) between the jump and return 90 degrees pulses in a 90 degrees (x)-tau-90 degrees (-x) magnetization-prepared Fast Low Angle Shot (FLASH) sequence and (b) between the 90 degrees pulse and the 180 degrees pulse in an asymmetric SE sequence. The range of Larmor frequencies used for spin locking can be determined for different B(1) amplitudes of the SL pulses, allowing control of image contrast by the amplitude of the SL pulses.

Magneto-inertial Approach to Direct-drive Laser Fusion

A magneto-inertial fusion (MIF) approach to inertial confinement fusion (ICF), based on laser-driven magnetic-flux compression (LDFC) is described. This approach benefits from both the high-energy-density characteristic to ICF and the thermal insulation of the fuel by magnetic fields, typical of MFE. The reduction in thermal-conduction losses in the hot spot of an imploding target that has trapped and amplified a pre-seeded magnetic flux leads to increased hot-spot temperatures at lower implosion velocities than required in conventional ICF. This can lead to ignition designs with larger energy gains. This work describes the main concept and the use of a compact magnetic-pulse system to seed a macroscopic magnetic field into cylindrical DD-filled targets, which are radially driven with the OMEGA laser. The compression of the internal magnetic flux is measured with proton deflectometry. Magnetohydrodynamic simulations predict compression of a 0.1-MG seed field to multi-megagauss values, at which levels the radial electron thermal conduction in the hot spot is significantly inhibited. Initial benchmark experiments are described.

T2* measurements in human brain at 1.5, 3 and 7 T

Measurements have been carried out in six subjects at magnetic fields of 1.5, 3 and 7 T, with the aim of characterizing the variation of T2* with field strength in human brain. Accurate measurement of T2* in the presence of macroscopic magnetic field inhomogeneity is problematic due to signal decay resulting from through-slice dephasing. The approach employed here allowed the signal decay due to through-slice dephasing to be characterized and removed from data, thus facilitating an accurate measurement of T2* even at ultrahigh field. Using double inversion recovery turbo spin-echo images for tissue classification, an analysis of T2* relaxation times in cortical grey matter and white matter was carried out, along with an evaluation of the variation of T2* with field strength in the caudate nucleus and putamen. The results show an approximately linear increase in relaxation rate R2* with field strength for all tissues, leading to a greater range of relaxation times across tissue types at 7 T that can be exploited in high-resolution T2*-weighted imaging.

Numerical analysis of falling film absorption with ammonia–water in magnetic field

Absorption strengthen is an effective way to improve the coefficient of performance (COP) of ammonia–water absorption refrigeration system. A mathematical absorption model for ammonia–water falling film absorption in magnetic field was built in this paper to study the influence of magnetic field on the absorption process. Macroscopic magnetic field force was introduced to the mathematical model. Changes in physical properties of ammonia–water solution while absorbing, the variation of falling film thickness along falling and the convection in the direction of thickness of liquid film have been considered in modeling. Numerical results showed that magnetic field have positive effect on the ammonia–water falling film absorption to some degree. When the magnetic induction intensity at the solution’s inlet is 3 Tesla (T), the increment in concentration of ammonia–water solution at outlet is 1.3% and the absorbability increased by 5.9%. The COP of a simple ammonia–water absorption refrigeration system increased by 4.73% and the decrement in circulation ratio is 8.3%.

Advances in detection of magnetic fields induced by electrochemical reactions—a review

The paper summarizes achievements in applications of superconducting quantum interference device (SQUID) magnetometry in electrochemical activity sensing, especially in that related to remote corrosion detection. The studies deal with application of the SQUIDs operating in liquid helium or nitrogen with a spatial resolution of magnetic field detection of the order of a millimeter or greater. This made it possible to observe macroscopic magnetic fields, which originated from the large-scale surface currents (ionic and electronic), which resulted from electrochemical potential gradients on electrochemical interfaces. The gradients owed to variations in temperature, alloy composition, sample geometry, electrolyte flow characteristics (velocity, direction, and turbulence), etc. The measurements demonstrated the capability of SQUIDs to remotely sense corrosion across the integrated media consisting of gaseous and solid dielectrics, metal, and electrolyte. The results have shown the potential of magnetometry for practical corrosion detection in the restricted locations such as in ground or concrete. Despite significant efforts, the field is considered to be at an early stage from both fundamental and practical points of view.

Blipped multi gradient‐echo slice excitation profile imaging (bmGESEPI) for fast T measurements with macroscopic B0 inhomogeneity compensation

With the rapid development of human MRI at field strengths > or = 7 T, knowledge of T(2) (*) relaxation times at such field strengths is needed to optimize acquisition parameters and understand relaxation mechanisms in many applications. However, standard T(2) (*) measurements (e.g., using conventional multiecho gradient-echo (GE) sequences) are affected by macroscopic static magnetic field (B(0)) inhomogeneities, which are particularly severe at high field strength. The multi-GE slice excitation profile imaging (mGESEPI) method was developed for T(2) (*) measurements in the presence of macroscopic B(0) inhomogeneity, but it requires excessive acquisition times at field strengths > or = 7 T. In this paper a more efficient technique, named blipped mGESEPI (bmGESEPI), is proposed. To demonstrate its advantages, T(2) (*) maps were acquired using a conventional multiecho GE method, the mGESEPI method, and the bmGESEPI method in postmortem and in vivo human brains at 8 T.

Magnetometric corrosion sensing under hydrodynamic conditions

SQUID (Superconducting QUantum Interference Device) magnetometer operating in liquid helium with a special resolution of ca. 1 mm was used to study the magnetic fields produced by corrosion reactions under electrolyte flow conditions. Macroscopic magnetic field images were obtained on AA 2024 samples corroding in naturally aerated and oxygen-saturated NaCl solutions and 0.1 M NaOH. The fields originated from the macroscopic surface currents (ionic and electronic), which appear due to corrosion potential differences that resulted from differences in electrolyte flow characteristics (velocity, direction, and turbulence). The measurements demonstrated SQUID capability of remote corrosion sensing across the integrated media consisting of gaseous and solid dielectrics, metal, and electrolyte. The results have shown the potential of magnetometry for practical corrosion detection in the restricted locations (“hidden” corrosion) where solution flow is applied.

Acceleration of Plasma Flows Due to Reverse Dynamo Mechanism

The "reverse dynamo" mechanism—the amplification/generation of fast plasma flows by microscale (turbulent) magnetic fields via magneto-fluid coupling—is recognized and explored. It is shown that macroscopic magnetic fields and flows are generated simultaneously and proportionately from microscopic fields and flows. The stronger the microscale driver is, the stronger the macroscale products will be. Stellar and astrophysical applications are suggested.

Orbital Magnetization in Extended Systems

While the orbital magnetic dipole moment of any finite sample is well-defined, it becomes ill-defined in the thermodynamic limit as a result of the unboundedness of the position operator. Effects due to surface currents and to bulk magnetization are not easily disentangled. The corresponding electrical problem, where surface charges and bulk polarization appear as entangled, was solved about a decade ago by the modern theory of polarization, based on a Berry phase. We follow a similar path here, making progress toward a bulk expression for the orbital magnetization in an insulator represented by a lattice-periodic Hamiltonian with broken time-reversal symmetry. We therefore limit ourselves to the case where the macroscopic (i.e. cell-averaged) magnetic field vanishes. We derive an expression for the contribution to the magnetization arising from the circulating currents internal to the bulk Wannier functions, and then transform to obtain a Brillouin zone integral involving the occupied Bloch orbitals. A version suitable for practical implementation in discretized reciprocal space is also derived, and the gauge invariance of both versions is explicitly shown. However, tests on a tight-binding model indicate the presence of additional edge currents, and it remains to be determined whether these can be related to the bulk band structure.

Electric and magnetic microfields inside and outside space-limited configurations of ions and ionic currents

The problem of electric and magnetic microfields inside finite spherical systems of stochastically moving ions and outside them is studied. The first possible field of applications is high temperature ion clusters created by laser fields [1]. Other possible applications are nearly spherical liquid systems at room-temperature containing electrolytes. Looking for biological applications we may also think about a cell which is a complicated electrolytic system or even a brain which is a still more complicated system of electrolytic currents. The essential model assumption is the random character of charges motion. We assume in our basic model that we have a finite nearly spherical system of randomly moving charges. Even taking into account that this is at best a caricature of any real system, it might be of interest as a limiting case, which admits a full theoretical treatment.For symmetry reasons, a random configuration of moving charges cannot generate a macroscopic magnetic field, but there will be microscopic fluctuating magnetic fields. Distributions for electric and magnetic microfields inside and outside such space- limited systems are calculated. Spherical systems of randomly distributed moving charges are investigated. Starting from earlier results for infinitely large systems, which lead to Holtsmark- type distributions, we show that the fluctuations in finite charge distributions are larger (in comparison to infinite systems of the same charge density).

Beltrami fields in a three-species magnetoplasma

Nonlinear couplings between Beltrami fields (magnetic fields and ion flows) in a three-species plasmas are considered. By using a multi-fluid plasma model, a set of evolution equations for nonlinearly coupled magnetic fields and the ion vorticity is derived. Solutions of the coupled equations in the steady state are obtained and analyzed. Implications of the present investigation to macroscopic magnetic fields in the Earth's magnetotail/magnetosheath and in the heliosphere are discussed.

Determination of transverse relaxation rate for estimating iron deposits in central nervous system

To determine the amount of iron deposits in the basal ganglia, we examined 13 healthy volunteers with a 1.5 T MRI system using three transverse relaxation rates measured with two sequences. The transverse relaxation rates comprise the reversible contribution (R2') and irreversible contribution (R2) to a phase-reversal 180 degrees -pulse sequence. The transverse relaxation rates with the estimated iron indicated that both R2 and R2' had a robust relationship with brain iron level as determined from published post mortem data. This was the case only when the analyses are limited to the subcortical gray matter regions, however. R2' was affected by macroscopic magnetic field inhomogeneity arising from the skull bases, so that it was less robust for estimating the amount of iron deposits in the basal ganglia.

Formation of macroscopic structures in partially ionized magnetoplasmas

It is shown that the governing nonlinear equations for a partially ionized magnetoplasma admit large-scale (in comparison with the ion skin depth) structures on a scale size that is controlled by the neutral fluid mass density. Implications of our investigation to macroscopic magnetic field structures in the interstellar plasma are discussed.