Quasistatic Magnetic Field Research Articles

Magnetic Shielding With YBCO Coated Conductors: Influence of the Geometry on Its Performances

A superconducting magnetic shield can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong attenuation of external dc magnetic fields. The purpose of the present work is to study experimentally several geometries of such magnetic shields made out of YBa2Cu3O7 (YBCO) coated conductors from SuperPower. Our aim is to investigate in detail the influence of the aspect ratio and the number of layers of the assembly on the magnetic shielding properties. In order to do so, the magnetic shield is subjected to an axial quasi-static (“dc”) magnetic field ramped slowly at a fixed sweep rate. A Hall probe is used to measure the local magnetic induction inside the assembly as a function of the applied magnetic induction. Results show that the shielding factor, SF, (defined as the ratio between the applied magnetic induction and the magnetic induction measured inside the shield) is improved for increasing aspect ratios of the global coated conductor assembly and that the threshold magnetic induction (defined for SF = 10) increases with the number of layers. Using a double layer of 18 sections at T = 77 K, dc magnetic fields up to 56 mT can be shielded by a factor larger than 10. Finally, the effect of an air gap of constant width between coated conductor sections is also characterized.

Broken ergodicity, magnetic helicity, and the MHD dynamo

We consider an unforced, incompressible, turbulent magnetofluid constrained by concentric inner and outer spherical surfaces. We define a model system in which normal components of the velocity, magnetic field, vorticity, and electric current are zero on the boundaries. This choice allows us to find a set of Galerkin expansion functions that are common to both velocity and magnetic field, as well as vorticity and current. The model dynamical system represents magnetohydrodynamic (MHD) turbulence in a spherical domain and is analyzed by the methods similar to those applied to homogeneous MHD turbulence. We find a statistical theory of ideal (i.e. no dissipation) MHD turbulence analogous to that found in the homogeneous case, including the prediction of coherent structure in the form of a large-scale quasistationary magnetic field. This MHD dynamo depends on broken ergodicity, an effect that is enhanced when total magnetic helicity is increased relative to total energy. When dissipation is added and large scales are only weakly damped, quasiequilibrium may occur for long periods of time, so that the ideal theory is still pertinent on a global scale. Over longer periods of time, the selective decay of energy over magnetic helicity further enhances the effects of broken ergodicity. Thus, broken ergodicity is an essential mechanism and relative magnetic helicity is a critical parameter in this model MHD dynamo theory.

Kinetic theory of quasi-stationary collisionless axisymmetric plasmas in the presence of strong rotation phenomena

The problem of formulating a kinetic treatment for quasi-stationary collisionless plasmas in axisymmetric systems subject to the possibly independent presence of local strong velocity-shear and supersonic rotation velocities is posed. The theory is developed in the framework of the Vlasov-Maxwell description for multi-species non-relativistic plasmas. Applications to astrophysical accretion discs arising around compact objects and to plasmas in laboratory devices are considered. Explicit solutions for the equilibrium kinetic distribution function (KDF) are constructed based on the identification of the relevant particle adiabatic invariants. These are shown to be expressed in terms of generalized non-isotropic Gaussian distributions. A suitable perturbative theory is then developed which allows for the treatment of non-uniform strong velocity-shear/supersonic plasmas. This yields a series representation for the equilibrium KDF in which the leading-order term depends on both a finite set of fluid fields as well as on the gradients of an appropriate rotational frequency. Constitutive equations for the fluid number density, flow velocity, and pressure tensor are explicitly calculated. As a notable outcome, the discovery of a new mechanism for generating temperature and pressure anisotropies is pointed out, which represents a characteristic feature of plasmas considered here. This is shown to arise as a consequence of the canonical momentum conservation and to contribute to the occurrence of temperature anisotropy in combination with the adiabatic conservation of the particle magnetic moment. The physical relevance of the result and the implications of the kinetic solution for the self-generation of quasi-stationary electrostatic and magnetic fields through a kinetic dynamo are discussed.

Resonances for activity waves in spherical mean field dynamos

We study activity waves of the kind that determine cyclic magnetic activity of various stars, including the Sun, as a more general physical rather than a purely astronomical problem. We try to identify resonances which are expected to occur when a mean-field dynamo excites waves of quasi-stationary magnetic field in two distinct spherical layers. We isolate some features that can be associated with resonances in the profiles of energy or frequency plotted versus a dynamo governing parameter. Rather unexpectedly however the resonances in spherical dynamos take a much less spectacular form than resonances in many more familiar branches of physics. In particular, we find that the magnitudes of resonant phenomena are much smaller than seem detectable by astronomical observations, and plausibly any related effects in laboratory dynamo experiments (which of course are not in gravitating spheres!) are also small. We discuss specific features relevant to resonant phenomena in spherical dynamos, and find parametric resonance to be the most pronounced type of resonance phenomena. Resonance conditions for these dynamo wave resonances are rather different from those for more conventional branches of physics. We suggest that the relative insignificance of the phenomenon in this case is because the phenomena of excitation and propagation of the activity waves are not well-separated from each other and this, together with the nonlinear nature of more-or-less realistic dynamos, suppress the resonances and makes them much less pronounced than resonant effects, for example in optics.

Self Generated Magnetic Field in a Laser Produced Plasma Embedded with Atomic Clusters

Self generated magnetic field by an amplitude modulated intense laser beam in a tunnel ionized inhomogeneous plasma embedded with atomic clusters is studied. The transverse profile of the laser (along \( {\hat{{y}}}\)) is cosine and the clusters are taken to undergo Coulomb explosion thereby creating a plasma with decreasing density with time. The laser exerts a ponderomotive force \( {\vec{{F}}}_{{p}} \) on the electrons, imparting them a transverse velocity \( {\vec{{v}}} \). The current density \( {\vec{{J}}} \), thus produced is irrotational i.e. \( \nabla \times {\vec{{J}}}\,\, \ne \,\, 0 \), and gives rise to a quasistatic magnetic field along \( {\vec{{F}}}_{{p}} \, \times \,\nabla \,{{n}}_{ 0} \). The generated magnetic field amplitude is sensitive to electron plasma frequency and laser modulation frequency. The peak value of generated magnetic field at resonance is ~40 MGauss at laser intensity 1015 W/cm2.

Localization in 2D Using Beacons of Low Frequency Magnetic Field

In this work we propose methods for object localization in 2D using beacons of low frequency quasi-static magnetic field. From a practical point of view, localization in 2D is sufficient for many applications, requiring much less calculations than in 3D, making it more robust and easier to implement in real-time low power applications. The low frequency magnetic field may penetrate foliage, soil, buildings, and many other types of media. This is an important advantage over traditional localization methods such as sonar or radar, where effective operation requires line-of-sight. Another advantage of the low frequency magnetic fields is that there is no direct influence by bad weather conditions and diurnal variations. Opposite to traditional electromagnetic methods, where operational range is usually more than a wavelength, low frequency induction approach results in a relatively limited localization range. Each beacon comprises a coil generating a magnetic field of a unique frequency in the ULF band. The generated magnetic fields are sensed by a search-coil magnetometer. The magnetometer readings are processed to estimate the magnitude and phase of the received beacons signals, which are used to localize the magnetometer. For a moving object, we propose to combine localization together with tracking algorithm using a data fusion approach. The proposed methods have been tested using numerous computer simulations, showing accurate localization results. A prototype was developed and used in field experiments, validating simulation results. The good accuracy together with a simple implementation makes the proposed methods attractive to many real-time low power field applications.

Coupling of laser energy into hot-electrons in high-contrast relativistic laser-plasma interactions

We use particle-in-cell simulations to explain the mechanisms responsible for the coupling of laser energy into relativistic electrons for the case of sharp interface, solid density metal targets free of pre-plasma. For perfectly flat interfaces, the accelerated electron trajectories are dominated by the standing-wave (SW) field structure formed by interference between incident and reflected pulses. We find that quasi-static magnetic fields that develop near the interface play only a minor role in perturbing the relativistic electron trajectories but can contribute to enhanced absorption. Target surfaces that are structured exhibit enhanced absorption, and the acceleration mechanism deviates from the clean standing-wave acceleration mechanism leading to more stochastic electron heating and larger divergence angles.

An Electromagnetic Localization and Orientation Method Based on Rotating Magnetic Dipole

The electromagnetic localization and orientation method is based on 2-axis generation and 3-axis sensing of a quasi-static magnetic field. Two mutually orthogonal coils fed with phase-quadrature signals comprise the excitation source, which is equal to a rotating magnetic dipole. Using the amplitude and phase information of the sensing signals from the 3-axis sensing coils, the position of the sensor can be calculated directly by simple and explicit analytical expressions, then the orientations of the sensor can be determined by using rotation matrix. In this paper, a novel method is proposed to realize the direct calculation, which is easy and fast to determine the position and orientation of the 3-axis sensor in a single period of excitation. Experimental results demonstrate a promising performance of this method.

Specific features of the magnetic and electric properties and the magnetically inhomogeneous state of Nd0.5Sr0.5MnO3 single crystal

The hysteretic behavioral features of magnetization and resistance upon remagnetization in quasistatic (up to 9 T) and pulse (up to 14 T) magnetic fields have been investigated. The relaxation processes of magnetization and resistance after the effect of a 9 T magnetic field have also been studied. A mechanism of the remagnetization of antiferromagnetic insulating-ferromagnetic metallic (AFM/I-FM/M) phases and the existence of high-conductivity state of the sample after removal of the magnetizing field is proposed for low temperatures. This mechanism is due to slow relaxation of the nonequilibrium lattice corresponding to the FM phase (of larger volume) toward the equilibrium AFM lattice (of lesser volume).

Magneto-Quasistatic Tracking of an American Football: A Goal-Line Measurement [Measurements Corner

An American football was tracked using a long-range magneto-quasistatic position and orientation measurement system. A low-weight emitter that emitted a low-frequency quasistatic magnetic field was embedded within an American football. The emitter weighed a total of 26.5 g, which was within the manufacturing tolerance of an American football, and did not alter the dynamics of the ball. Measurements of a person carrying the football along the goal line of an American football field are described, along with a description of the construction of the magneto-quasistatic tracking system. The technique demonstrated measurements with a distance accuracy of 15 cm and an azimuthal orientation accuracy of 2.45° for measurements conducted along the goal line of an American football field.

Generating overcritical dense relativistic electron beams via self-matching resonance acceleration.

We show a novel self-matching resonance acceleration regime for generating dense relativistic electron beams by using ultraintense circularly polarized laser pulses in near-critical density plasmas. When the self-generated quasistatic axial magnetic field is strong enough to pinch and trap thermal relativistic electrons, an overdense electron bunch is formed in the center of the laser channel. In the trapping process, the electron betatron frequencies and phases can be adjusted automatically to match the resonance condition. The matched electrons are accelerated continuously and a collimated electron beam with overcritical density, helical structure, and plateau profile energy spectrum is hence generated.

Laser-driven collimated tens-GeV monoenergetic protons from mass-limited target plus preformed channel

Proton acceleration by ultra-intense laser pulse irradiating a target with cross-section smaller than the laser spot size and connected to a parabolic density channel is investigated. The target splits the laser into two parallel propagating parts, which snowplow the back-side plasma electrons along their paths, creating two adjacent parallel wakes and an intense return current in the gap between them. The radiation-pressure pre-accelerated target protons trapped in the wake fields now undergo acceleration as well as collimation by the quasistatic wake electrostatic and magnetic fields. Particle-in-cell simulations show that stable long-distance acceleration can be realized, and a 30 fs monoenergetic ion beam of >10 GeV peak energy and <2° divergence can be produced by a circularly polarized laser pulse at an intensity of about 1022 W/cm2.

Suppression effects of Weibel instability for fast electron divergence

Quasi-static magnetic fields, which are induced by the Weibel instability and grow to more than hundred Megagauss, lead to large divergence angle of fast electrons, hence lower energy coupling. To suppress the divergence, two different structures, namely density trough and punched out holes, are introduced to targets. In the density trough target, the Weibel instability is enhanced and the divergence is getting worse. On the other hand, the divergence angle is improved but the number of electrons is degraded for fast electrons (<3 MeV) in the punched out target.

用于磁压剪技术的10 T准静态磁场发生器

用于磁压剪技术的10 T准静态磁场发生器

Dynamics of vortex nucleation in nanomagnets with broken symmetry

We investigate fundamental processes that govern dynamics of vortex nucleation in sub-100 nm mesoscopic magnets. We focus on a structure with broken symmetry - Pacman-like nanomagnet shape - in which we study micromagnetic behavior both by means of a simple model and numerically. We show that it is possible to establish desired vortex chirality and polarity by applying only quasi-static in-plane magnetic field along specific directions. We identify the modes of vortex nucleation that are very robust against external magnetic field noise. These vortex nucleation modes are common among wide range of sub-100 nm magnets with broken rotational symmetry.

Magnetostrictive–piezoelectric composite structures for energy harvesting

In this paper, harvesters coupling magnetostrictive and piezoelectric materials are investigated. The energy conversion of quasi-static magnetic field variations into electricity is detailed. Experimental results are exposed for two macroscopic demonstrators based on the rotation of a permanent magnet. These composite/hybrid devices use both piezoelectric and magnetostrictive (amorphous FeSiB ribbon or bulk Terfenol-D) materials. A quasi-static (or ultra-low frequency) harvester is constructed with exploitable output voltage, even in quasi-static mode. Integrated micro-harvesters using sub-micron multilayers of active materials on Si have been built and are currently being characterized.

Generation of a magnetic field in a weakly inhomogeneous plasma interacting with a short laser pulse

The generation of a quasistationary magnetic field in a plasma interacting with a weakly focused low-intensity short laser pulse has been studied. It has been shown that the magnetic field changes direction at times comparable with the free path time of effective electrons. Generation also occurs after the switching off of the short pulse and the maximum field is proportional to the duration of the pulse and is reached at times larger than the free path time of the suprathermal electrons.

A Novel Positioning and Orientation System Based on Three-Axis Magnetic Coils

The positioning and orientation system consists of three-axis generating coils and three-axis sensor coils in quasi-static magnetic field. The three-axis generating coils are fixed orthogonally and excited by the alternating current (AC) signals with different frequencies. They create the magnetic field that is equivalent to that generated by three orthogonal magnetic dipoles. Using the amplitude and phase information of the sensing signals in the sensor coils, the position and orientation parameters of the sensor coils can be computed by using an appropriate algorithm. In this paper, a novel algorithm is proposed to determine the position of the sensor coil object by some equations directly, so that its position and orientation parameters can be calculated much easier and faster. Based on this method, a system with support circuitry is designed with some special signal acquisition and sampling methods. Especially, a signal extraction (function fitting) method is proposed to pick up the coupling AC signal magnitude of the sensor coils, which simplifies the hardware circuitry and improves the signal acquisition accuracy. The simulation and real experimental results show that the system works satisfactorily with good accuracy.

A utilization of internal/external quasi-static magnetic field gradients: transport phenomenon and magnetic resonance imaging of solid polymers

Two topics concerning the utilization of internal and external magnetic field gradients are discussed. The first topic concerns the transport of conformons in the quasi-ordered phase of a regioregular, π-conjugated polymer, poly(4-methylthiazole-2,5-diyl), using a combination of longitudinal and transverse relaxation dispersion measurements. The effects of conformon diffusion on the relaxation exponent were successfully estimated via a numerical approach that used the Fourier spectrum solution of the one-dimensional Bloch-Torrey equation. The second topic demonstrates a low-cost magnetic resonance imaging method using a magnetic field gradient from a neodymium ferromagnet. Spin echo experiments are shown for three-layered thin films of PDMS/PTFE/PDMS under a large magnetic field.

Characterization of relativistic electrons generated by a cone guiding laser pulse

We demonstrated the interaction of a gold cone target with a femto second (fs) laser pulse above the relativistic intensity of 1.37 × 1018 μm 2W/cm2. Relativistic electrons with energy above 2 MeV were observed. A 25%-40% increase of the electron temperature is achieved compared to the case when a plane gold target is used. The electron temperature increase results from the guiding of the laser beam at the tip and the intense quasistatic magnetic field in the cone geometry. The behavior of the relativistic electrons is verified in our 2D-PIC simulations.