Oblique Magnetic Field Research Articles

Sheath structure of a magnetized plasma

The structure of plasma sheath in an oblique magnetic field is numerically investigated by using fluid dynamics method. The simulation results reveal that the magnetic field has significant effects on the plasma sheath. Especially, the ion density distribution near the sheath edge changes obviously. Two cases of ion incidence are considered here. The action states of ions are more complicated when the ions enter the sheath with an oblique incidence angle. Under suitable conditions, Lorentz force induces fluctuations in the ion density. Ions could gather in some regions.

경사 외부자장에 대한 600 kJ급 SMES용 HTS도체의 DC V-I 특성

We are developing a small-sized high temperature superconducting magnetic energy storage (HTS-SMES) magnet with the nominal storage capacity of 600 kJ, which provides electric power with high quality to sensitive electric loads. Critical current and N-value of a high temperature superconductor with large current, which was selected for the development of the 600 kJ HTS-SMES magnet, were investigated in various oblique external magnetic fields. Based on the critical current and N-value measured for the short sample conductor, we discussed the DC V - I characteristic of a model coil fabricated with the same conductor of 500 m. The results show that the measured critical current and N-value of the conductor for parallel field are constant in external magnetic fields less than about 0.2 T. However, for oblique fields, its critical current and N -value abruptly decrease in all external magnetic fields. Moreover, the measured critical current of the model coil well agrees with the numerically calculated one based on the DC V - I characteristic measured for the short sample conductor. This suggest that losses and critical currents for an HTS-SMES magnet made up of a high temperature superconductor with anisotropic characteristic are predictable from the data of a short sample conductor.

Nonresonant microwave absorption in epitaxialLa0.7Sr0.3MnO3films and its relation to colossal magnetoresistance

We study magnetic-field-dependent nonresonant microwave absorption and dispersion in thin ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ films and show that it originates from colossal magnetoresistance. We develop a model for the magnetoresistance of a thin ferromagnetic film in an oblique magnetic field. The model accounts fairly well for our experimental findings, as well as for the results of other researchers. We demonstrate that nonresonant microwave absorption is a powerful technique that allows contactless measurement of magnetic properties of thin films, including magnetoresistance, anisotropy field, and coercive field.

Ground state of theH2+molecule in oblique magnetic fields

Calculations in the Born-Oppenheimer approximation of curves for the ground-state energy of the $\mathrm{H}_{2}{}^{+}$ ion vs the separation, $R$, of its protons in magnetic fields making oblique angles $\ensuremath{\theta}$ with the line connecting the protons have recently been published. It is shown here that these curves are qualitatively incorrect. The variational trial function employed fails to approximate well the wave function of the $\mathrm{H}_{2}{}^{+}$ ground state for oblique fields, even though this function gives very accurate results for $\ensuremath{\theta}=0$. A trial function is proposed which gives much more accurate potential curves in oblique fields. It is used to plot and tabulate ground-state binding energies for various values of $\ensuremath{\theta}$, $R$, and magnetic field strength.

Anisotropy of the electron Landé g factor in InAs/GaAs self-assembled quantum dots

We report on the precise determination of the electron Landé g factor and its anisotropy in self-assembled InAs/GaAs quantum dots (QDs), by means of measurements of Larmor precession frequencies in photoinduced circular dichroism (PCD) experiments performed with an oblique magnetic field. This optical technique, applied to an ensemble of QDs, is able to measure the electronic g factor corresponding to ground-state electrons resident in QDs emitting at a given energy of fundamental optical transition. In good agreement with recent theoretical results, we measure | g ⊥|=0.397±0.003 and | g ∥|=0.18±0.02 for QDs emitting at 1.32 eV.

Spin quantum beats of bright and dark excitonic states in neutral InAs quantumdots

The energy eigenvalues and eigenfunctions of heavy-hole (HH) excitons and theexpression of spin quantum beats (QBs) in neutral InAs quantum dots (QDs) inan oblique magnetic field have been calculated and derived in terms of a spinHamiltonian model. It is shown from the oscillating curves of spin QBs that theamplitude and period of QBs are determined by the magnitude and direction of themagnetic field, and that the impact of the coupling component of electrons and holes,δ0, on QBs should not be neglected even if in an intense magnetic field likeB = 4 T, so longas the angle θ≤30°; θ is the angle between the direction of the magnetic field and thez-direction (quantized direction). The influence of anisotropicg factors of electron and hole on QBs has also been studied. The calculated results show thatg-factormarkedly affected the amplitude and periodicity of QBs and the periodic oscillating of QBs vanishesat Δgx = ge,x−gh,x = 0.6, which shows that whether or not the periodic signal of QBs canbe observed is related to the ‘matching’ of the electron and holeg-factors.

Anisotropic spin splitting of the electron ground state in InAs quantum dots

Photoinduced circular dichroism experiments in an oblique magnetic field allow measurements of Larmor precession frequencies, and so give a precise determination of the electron Landé g factor and its anisotropy in self-assembled InAs∕GaAs quantum dots emitting at 1.32eV. In good agreement with recent theoretical results, the authors measure ∣g⊥∣=0.397±0.003 and ∣g‖∣=0.18±0.02.

Modified Korteweg-deVries soliton evolution at critical density of negative ions in an inhomogeneous magnetized cold plasma

Soliton propagation at critical density of negative ions is studied for weakly inhomogeneous magnetized cold plasma having positive ions, negative ions, and electrons. A general phase velocity relation is obtained and possible modes are studied for different cases involving different constituents of the plasma. Two types of modes (fast and slow) are found to propagate for the equal mass of the positive and negative ions. However, a limit on the obliqueness of magnetic field is obtained for the propagation of slow mode. For both types of modes, a variable coefficient modified Korteweg-deVries equation with an additional term arisen due to the density gradient is realized, which admits solutions for compressive solitons and rarefactive solitons of the same amplitudes at critical negative ion density. The propagation characteristics of these solitons are studied under the effect of densities of ions, magnetic field, and its obliqueness. The amplitudes of fast and slow wave solitons show their opposite behavior with the negative ion concentration, which is consistent with the variation of phase velocities with the negative ion density.

ULF Doppler oscillations in the low latitude ionosphere

An array of magnetometers and Doppler sounders spanning L = 1.56–2.77 was used to examine perturbations in the daytime ionosphere driven by downgoing ULF waves. For frequencies away from the local field line resonance (FLR), the waves caused mostly vertical motions of the F‐region plasma with amplitude ∼0.03–0.06 Hz/nT, with a phase delay of 30° to 40° at the ground. At the local resonant frequency and harmonics the amplitude and phase delay increased markedly. We modeled this by considering an admixture of ULF wave modes and oblique magnetic field geometry, and using actual ionospheric parameters. The model results agree well with observations when the downgoing wave mode varies smoothly from pure fast mode away from the resonance to mostly transverse mode at resonance. These results provide new information on the interaction between downgoing ULF waves and the ionospheric plasma.

Composite to Tilted Vortex Lattice Transition inBi2Sr2CaCu2O8+δin Oblique Fields

Precision measurements of the vortex phase diagram in single crystals of the layered superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ in oblique magnetic fields confirm the existence of a second phase transition, in addition to the usual first order vortex lattice melting line $H_{m}(T)$. The transition has a strong first order character, is accompanied by strong hysteresis, and intersects the melting line in a tricritical point ($H_{m}^{\perp}$, $H^{\parallel}_{cr}$). Its field dependence and the changing character of the melting line at the tricritical point strongly suggest that the ground state for magnetic fields closely aligned with the superconducting layers is a lattice of uniformly tilted vortex lines.

Steady flows of a laterally heated ferrofluid layer: Influence of inclined strong magnetic field and gravity level

A horizontal ferrofluid layer is submitted to a lateral heating and to a strong oblique magnetic field. The problem, combining the momentum and heat balance equations with the Maxwell equations, introduces two Rayleigh numbers, Ra the gravitational one and Ram the magnetic one, to represent the buoyancy and the Kelvin forces, which induce motion, versus the momentum viscous diffusion and heat diffusion. Whatever the inclination of the magnetic field, the steady solution of the problem is presented as a power series of a small parameter ϵH measuring the ratio of variation of the magnetization across the layer divided by the magnitude of the external imposed field. For cases of physical relevance, comparisons between analytical and numerical studies have lead to a major statement: in the strong field region (ϵH≪1) the zero order solution is the product of the Birikh solution that corresponds to the usual Newtonian fluid submitted to a lateral gradient, multiplied by a modulating factor accounting for inclination and both Rayleigh numbers. Physically, this simplified solution is valid for microgravity conditions where the magnetic field competes enough with microgravity effects to invert the laminar flow and thus suppress the motion for two specific values of the inclination angle.

Critical currents at the grain boundary of (Sm0.8Dy0.2)Ba2Cu3O7 film under oblique magnetic fields

We measured critical current densities (Jcb) at the 30° grain boundary of a bicrystalline (Sm0.8Dy0.2)Ba2Cu3O7 film under various magnetic fields (H→a), which were applied obliquely. We varied the field from −0.7kOe to +0.7kOe while the angles (θ) of the fields were 2°, 22.5°, 45°, 67.5° and 90° with respect to the film surface. The curves of Jcb vs H→a showed the well known butterfly-like hysteretic curves. We separated the two components of field, H⊥ and H∥, which are normal and parallel to the film surface, respectively. Our data indicate that the roles of these two components for the field dependence of Jcb are different. We combined the effect of H⊥ deduced from the data for the normal field (θ=90°) and the effect of H∥ deduced from the data for the almost parallel field (θ=2°). Multiplying the independent reduction factors deduced from these two cases, we found a new formula, which expresses Jcb vs H→a for general θs. All the experimental data for various θs fit well to this new formula.

Evolution of Rotating Molecular Cloud Core with Oblique Magnetic Field

We studied the collapse of rotating molecular cloud cores with inclined magnetic fields, based on three-dimensional numerical simulations. The numerical simulations start from a rotating Bonnor-Ebert isothermal cloud in a uniform magnetic field. The magnetic field is initially taken to be inclined from the rotation axis. As the cloud collapses, the magnetic field and rotation axis change their directions. When the rotation is slow and the magnetic field is relatively strong, the direction of the rotation axis changes to align with the magnetic field, as shown earlier by Matsumoto & Tomisaka. When the magnetic field is weak and the rotation is relatively fast, the magnetic field inclines to become perpendicular to the rotation axis. In other words, the evolution of the magnetic field and rotation axis depends on the relative strength of the rotation and magnetic field. Magnetic braking acts to align the rotation axis and magnetic field, while the rotation causes the magnetic field to incline through dynamo action. The latter effect dominates the former when the ratio of the angular velocity to the magnetic field is larger than a critical value Ω0/B0 > 0.39G1/2c, where B0, Ω0, G, and cs denote the initial magnetic field, initial angular velocity, gravitational constant, and sound speed, respectively. When the rotation is relatively strong, the collapsing cloud forms a disk perpendicular to the rotation axis and the magnetic field becomes nearly parallel to the disk surface in the high-density region. A spiral structure appears due to the rotation and the wound up magnetic field in the disk.

Flux penetration into superconductingNb3Snin oblique magnetic fields

Penetration of magnetic flux into a rectangular platelet of superconducting ${\mathrm{Nb}}_{3}\mathrm{Sn}$ subject to an oblique magnetic field is studied by magneto-optical imaging. Flux patterns are observed for three distinct scenarios in which the final magnetic field vector is applied differently: (A) the vector has a fixed direction but its magnitude changes from zero to the final value, (B) first the perpendicular component is applied and then the in-plane component, and (C) vice versa. These scenarios lead to distinctly different final states that are consistent with theoretical predictions.

Jeans’ Gravitational Instability of a Thermally Conducting, Unbounded, Partially Ionized Plasma

The gravitational instability of an infinitely extending homogenous, partially ionized plasma, permeated by an oblique magnetic field, has been studied to investigate the effects of Hall currents, finite conductivity, viscosity, collision with neutrals and thermal conductivity on the growth rate of the disturbance. The dispersion relation obtained has been solved numerically, and it has been found that Hall currents and collision with neutrals have a destabilizing influence on the growth rate while the other mechanisms reinforce the gravitational instability. Jeans’ criterion, derived within a purely hydrodynamic framework, has been rediscussed along a nonextensive kinetic theory. A new Jeans’ criterion was deduced, which depends on the nonextensive parameter q and the standard Jeans’ wave number is recovered in the limiting case q = 1

Numerical Evaluation of Total AC Loss in Parallel Tape Conductor With Transport Current in Oblique Magnetic Field

We numerically calculate AC losses in a three-strand parallel tape conductor with optimum transposition by means of the finite-element method that directly analyzes the magnetic field distribution. The parallel conductor carries an alternating transport current, whose amplitude is 70% of the critical current, in an in-phase external AC magnetic field that has an arbitrary angle to the flat face of Bi-2223 tapes. It is confirmed that the AC losses in the range of very small and large field amplitude are almost equal to those in cases of applying only a transport current to the parallel conductor and only an external magnetic field to a single Bi-2223 tape, respectively. However it is found that the AC losses are affected by the interaction among the transport current, external magnetic field, and strand number in the range around the full penetration field. Our proposed finite-element analysis helps us to evaluate the AC loss generated in such the middle range of field amplitude quantitatively.

Fundamental solution for coupled magnetohydrodynamic flow equations

In this paper, a fundamental solution for the coupled convection–diffusion type equations is derived. The boundary element method (BEM) application then, is established with this fundamental solution, for solving the coupled equations of steady magnetohydrodynamic (MHD) duct flow in the presence of an external oblique magnetic field. Thus, it is possible to solve MHD duct flow problems with the most general form of wall conductivities and for large values of Hartmann number. The results for velocity and induced magnetic field is visualized in terms of graphics for values of Hartmann number M ⩽ 300 .

Magnetic field profiles of a superconducting strip in an oblique magnetic field

We consider the critical state of a thin superconducting strip in an oblique applied magnetic field Ha without any restrictions on the dependence of the critical current density jc on the local magnetic induction B. In such a strip, jc in general is not constant across the thickness of the sample and differs from Jc/d, where Jc is the critical sheet current. It is shown that in the case of B-dependent jc, the profiles Hz(x) of the magnetic-field component perpendicular to the strip plane generally depend on the in-plane component Hax of the applied magnetic field Ha. On the basis of this analysis, we explain how one can extract jc(B) from the magnetic-field profiles Hz(x) measured by magneto-optical imaging at the upper surface of the strip.

Electron surfing acceleration in oblique magnetic fields

Initially, inhomogeneous plasma jets, ejected by active galactic nuclei and associated with gamma-ray bursts, are thermalized by the formation of internal shocks. Jet subpopulations can hereby collide at Lorentz factors of a few. As the resulting relativistic shock expands into the upstream plasma, a significant fraction of the upstream ions is reflected. These ions, together with downstream ions that leak through the shock, form relativistic beams of ions that outrun the shock. The thermalization of these beams via the two-stream instability is thought to contribute significantly to plasma heating and particle acceleration by the shock. Here, the capability of a two-stream instability to generate relativistic field-aligned and cross-field electron flow, is examined for a magnetized plasma by means of a particle-in-cell (PIC) simulation. The electrons interact with the developing quasi-electrostatic waves and oblique magnetic fields. The simulation results bring forward evidence that such waves, by their non-linear interactions with the plasma, produce a highly relativistic field-aligned electron flow and electron energies, which could contribute to the radio synchrotron emissions from astrophysical jets, to ultrarelativistic leptonic subpopulations propagating with the jet and to the halo particles surrounding the accretion disc of the black hole.

Lateral motion of a dust particle in magnetized plasma sheath

In this paper, lateral mobilization of a dust particle through application of an oblique magnetic field in a plasma sheath with infinitely long lateral dimensions is studied. A weakly collisional two-fluid model is implemented to simulate the plasma. A single submicrometer-sized particle is released at the sheath edge with zero initial velocity and is tracked in Lagrangian frame, taking into account various forces. Various characteristics of the particle lateral motion such as time history of its horizontal position, velocity, and acceleration components are presented and discussed. Through investigation of power spectra of plasma and particle variables, the particle drift in the horizontal plane is explained. While varying parameters such as magnetic field intensity, particle density, and its radius, the particle horizontal velocity components are calculated. The variation of these velocity components is then explained by considering the ion velocity components at the particle location.