Shape Of Magnetic Field Research Articles

Empirical Radial Dependencies of the Energy Components in Magnetized Young Stellar Objects

Interstellar gas comes with empirical physical relations governing mean parameters such as the gas density n, diameter D, magnetic field B, and gas line width σ, with the forms n ~ Dc, B ~ Dp, B ~ nk, σ ~ Dq, and p = ck. Using these results, I derive simple formulae for the energy components in turbulent young stellar objects of Classes 0 and I as a function of clump size D (D 0.1 pc) turbulent and gravitational energies become dominant (x = 2). In addition, the expected relationship between turbulent width and magnetic field, σ ~ Bq/p, is not steep, and the observational data to date bear this out. Damping scenarios, magnetic field shapes, and ambipolar diffusion relevant to observations are discussed.

低電力ホールスラスタの基本作動特性と推進性能

Laboratory-model low-power Hall thrusters, named the THT-III-series, were operated in order to examine influences of ceramics species of the acceleration channel walls, and shape and strength of radial magnetic field in the acceleration channels on the operational characteristics. For the THT-IIIA-BN thruster both with acceleration channel walls made of boron nitride and with more intensive concentration of magnetic field lines near the acceleration channel exit, a higher thrust efficiency was achieved with a lower discharge current and a higher thrust for an optimum magnetic field strength regardless of discharge voltage at a constant xenon mass flow rate. Accordingly, both the thrust and the specific impulse ranged from 10 to 70mN and from 1,200 to 2,300s, respectively, at discharge voltages of 200–500V with mass flow rates of 1–3mg/s in a wide input power range of 250–1,800W. The thrust efficiency ranged from 30 to 45%.

Thrust performance and plasma features of low-power Hall-effect thrusters

The closed-electron-drift Hall-effect accelerator is a promising thruster in space. The effects of magnetic field shape and strength on Hall thruster performance were studied using 1-kW-class THT-series Hall thrusters. The discharge current characteristics were sensitive to magnetic field shape and strength. The thrust for the THT-IIIA thruster with more intensive concentration of magnetic field near the acceleration channel exit, i.e., with a smaller magnetic field at the anode was slightly higher than that for the THT-III thruster. The THT-IIIA thruster could be stably operated at a higher magnetic field strength than that for the THT-III thruster. As a result, for the THT-IIIA thruster, a higher thrust efficiency was achieved with a lower discharge current and a higher thrust. Furthermore, Hall thruster flowfield analysis was made to understand inner physical phenomena. Both ionization and acceleration were found to efficiently occur in a thin region with a few mm thick near the acceleration channel exit. The calculated thruster performance roughly agreed with the experimental ones.

Transition from the C3-dominated discharge to the sooting plasma.

Mass spectrometry and photoemission spectroscopy of a graphite hollow cathode source identify the parameters of the transition from the C3-dominated discharge to the sooting plasma. The transition is a function of the shape and profile of a special cusp magnetic field B(z)(r, theta), the geometry of the source, the discharge current, and pressure. Characteristic atomic and molecular emission lines and bands in the C3 discharge transform into broad bands emitted by the excited soot. We identify four prominent emission bands between 300-400 nm to be the hallmark of the sooting plasma.

Critical state in superconductor thin plates with elliptic shape

An analytic solution of the Bean critical state model is given for superconductor thin films or plates with elliptic shape and constant thickness in a perpendicular magnetic field. This two-dimensional solution exhibits qualitatively novel features and reproduces the known one-dimensional solutions for long strips and circular disks.

Analytic solution for the critical state in superconducting elliptic films

A thin superconductor platelet with elliptic shape in a perpendicular magnetic field is considered. Using a method originally applied to circular disks, we obtain an approximate analytic solution for the two-dimensional critical state of this ellipse. In the limits of the circular disk and the long strip this solution is exact, i.e., the current density is constant in the region penetrated by flux. For ellipses with arbitrary axis ratio the obtained current density is constant to typically ${10}^{\ensuremath{-}3}$, and the magnetic moment deviates by less than ${10}^{\ensuremath{-}3}$ from the exact value. This analytic solution is thus very accurate. In increasing applied magnetic field, the penetrating flux fronts are approximately concentric ellipses whose axis ratio $b/a<~1$ decreases and shrinks to zero when the flux front reaches the center, the long axis staying finite in the fully penetrated state. Analytic expressions for these axes, the sheet current, the magnetic moment, and the perpendicular magnetic field are presented and discussed. This solution applies also to superconductors with anisotropic critical current if the anisotropy has a particular, rather realistic form.

Magnetic field behavior of SNS edge Josephson junctions

Measurements of the response of critical current of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///PrBa/sub 2/Cu/sub (3-x)/(Ga)/sub x/O/sub 7-/spl delta///YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// SNS edge Josephson junctions to external magnetic field reveal periodic dependence of critical current on magnetic field. Theoretically the shape of critical current versus magnetic field for uniform current flow across the junction should resemble a Fraunhofer diffraction pattern. Using analysis of depth of modulation as an indication of how well the experimental modulation pattern fits the diffraction model, our experiments show a correlation between the depth of modulation and the type of junction. RSJ-like I-V curves, with their sharp concave upward curvature and I/sub ex//I/sub c/ 1. Also the response of single junctions as a function of temperature is analyzed.

Thermopower of a Chaotic Quantum Dot

The thermovoltage of a chaotic quantum dot is measured using a current heating technique. The fluctuations in the thermopower as a function of magnetic field and dot shape display a non-Gaussian distribution, in agreement with simulations using Random Matrix Theory. We observe no contributions from weak localization or short trajectories in the thermopower.

Undulator scheme with variable composition of the magnetic field harmonics

A method of variation of the undulator radiation wavelength is considered. The method is based on the influence of the undulator magnetic field shape on the radiation frequency. It has been shown that the change of the partial contribution of higher harmonics (mainly of the third harmonic) into the magnetic field shape leads to smooth tuning of the radiation frequency within a range adequate to three-fold changes of the undulator period. An undulator scheme providing such properties without reconstruction of the undulator design has been proposed.

Electromagnetic Charactristics and Application of a Single-Phase Magnetic Field Generator

The present paper examines a permanent-split-capacitor type single-phase induction motor that does not contain a rotor. The stator of this motor is capable of generating an elliptical magnetic field inside the housing by exciting 2-stator windings, a main winding and an auxiliary winding. Magnetic field shape can be varied by changing the value of the capacitor in series with the auxiliary winding. The stator of this motor is referred to as a single-phase magnetic field generator, and the electrical and magnetic characteristics of the generator were investigated. An expression was derived for determining of the magnetic field based on the magnetic circuit of the generator. The derived equation was applied to calculate the Lissajous elliptical magnetic field produced in a prototype magnetic field generator. The calculated results were in good agreement with those obtained experimentally using an oscilloscope to generate the Lissajous patterns. In addition, the calculated value of the magnetic flux density was in good agreement with that measured experimentally using a Gauss meter. Information on a new magnetic barrel finishing machine was obtained from the results of the experiments, and used to manufacture the new machine. This machine has succeeded in finishing various metals with complex shapes.

Choice of drives for electric vehicles: a comparison between two permanent magnet AC machines

Most motors for electric vehicles are based on the DC machine, nevertheless, the brushless surface mounted permanent magnet AC machine seems to be a good alternative. The authors propose a method of design and a comparison of two types of brushless machine that takes into account the requirements for the propulsion of electric vehicles. Two types of AC machine are designed taking into account the limitations due to the limited voltage power supply in terms of the airgap magnetic field shape and the corresponding stator current waveforms. Finally, a very simple analytical optimal design is proposed and verified both by numerical simulations and prototypes realisation.

Experimental study of the parallel velocity shear instability

The parallel velocity shear instability (PVSI) is experimentally investigated in a double-ended Q machine in which the shear in the ion flow is produced by suitable shaping of the axial magnetic field. Instability fluctuation levels approaching nearly 100% are often observed.

Optimal Larmor precession magnetic field shapes: application to neutron spin echo three-axis spectrometry

Resolution characteristics of neutron spectrometers using Larmor precession of the neutron spin are limited by magnetic field homogeneities of a special type. The line integral of the modulus of the magnetic induction along a neutron trajectory of length L is a measure of the amount of precession performed by the neutron. Hence it should be precisely the same for all neutron trajectories in a diverging beam. We present an analytical solution to the variational problem , for the case of cylindrical magnets (better than any lower symmetry geometry) coaxial to the beam axis. This solution describes the best irrotational (rot B = 0) field shape along the beam axis z. It can be obtained in practice by superposing a number of solenoids of different lengths. The optimal homogeneity is significantly better than for a simple solenoid of comparable dimensions, the only magnets used until now. For realistic lengths L and beam radii r, it is however not good enough for very high-resolution measurements. We therefore introduce a technique to correct both for the residual inhomogeneities of optimized cylinder magnets and the line integral variations due to path length differences resulting from finite angular beam divergence. Such corrections can only be done by introducing current distributions in the beam. Their optimal distributions can also be calculated analytically. Until now only the residual inhomogeneities have been corrected by in beam currents. With the two concepts of optimal field shape (OFS) and path length corrections described here, the resolution properties of Larmor precession techniques can be pushed to their intrinsic limits. As a further result of the correction technique introduced here, wider angular divergences can be used, for example using multi-detectors, resulting in substantially improved neutron economy. Several neutron spin echo (NSE) spectrometers based on above ideas have in the meantime been constructed by different working parties. The experimental results confirm the calculations reported here; no significant polarization drop is observed at maximum field. For the investigation of dispersive elementary excitations in solids where the neutron energy change depends on the momentum transfer, a special type of gradient coils is needed. We describe their design for OFS precession magnets.

Computational Studies on Plasma Generation and Erosion in a Rectangular Magnetron Sputtering System

A 3-dimensional simulation of electron orbits is applied to a planer magnetron sputtering system with rectangular magnets. Collisions between electrons and neutral gas atoms of Ar are considered and the ionization points are assumed to approximate the distribution of sputtering plasmas. The effect of external magnetic fields on the control of the shape of magnetic fields in this calculation model is examined. By applying the external magnetic fields, the distribution of plasma density as well as the position of erosion on the target can be controlled.

Control of Plasma Damage to Gate Oxide during High Density Plasma Chemical Vapor Deposition

Gate oxide damage resulting from high density plasma chemical vapor deposition of silicon oxide was investigated using damage sensitive antenna structures with area ratios up to 200,000:1. Significant damage was detected from an unoptimized oxide deposition process. A 24−1 fractional factorial experimental design was used to screen the effect of four parameters: radio frequency power, microwave power, electrostatic chuck potential, and magnetic field. RF power and electrostatic chuck potential made no contribution to oxide degradation. The main factor was microwave power, and further experiments with microwave power ranging from 1500 to 2500 W showed that gate charging damage increased with microwave power, with the extent and distribution of damage depending on the magnetic field shape.

An unbalanced magnetron sputtering device for low and medium pressures

The discharge characteristics of an unbalanced magnetron sputtering device, equipped with a 100-mm-diam planar circular target and with two electromagnetic coils, have been analyzed in a range of operation pressures from 0.02 to 1 Pa. The variation of discharge extinction and ignition pressures and target voltage is presented as a function of the ratio of currents in the magnetron coils and discharge current. A maximum discharge current beyond which it is not possible to sustain the discharge at low pressures has been observed. This extinction current increases with operating pressures. An explanation for the effect is based on the loss of gas from the plasma due to gas heating with energetic sputtered particles and subsequent gas density rarefaction in the near target region. Higher pressure is thus necessary to keep constant gas density in the magnetron plasma when the discharge current is increased. The magnetic field configuration is presented for several values of the ratio of currents in the magnetron coils and correlated to the variation of discharge extinction and ignition pressures, extinction current, and target voltage. The low pressure operation of the magnetron strongly depends on optimization of the magnetic field shape on the sputtered target. The deposition rate of titanium films is a linear function of magnetron power up to 2.5 kW and does not depend on Ar pressure in the pressure range studied (0.08–0.6 Pa).

Optimized magnetic field shape for low pressure magnetron sputtering

Discharge characteristics at low pressures (<0.1 Pa) of an unbalanced magnetron with two electromagnets have been studied. The discharge extinction and ignition pressures are a function of the currents I1 and I2 in both electromagnet coils. Minimum pressure is obtained for a particular ratio I2/I1. Extinction pressures range down to 1.5×10−2 Pa. The magnetic field of the magnetron was measured. Its shape was fitted using Bessel functions and the field line pattern solved using Runge–Kutta method. The magnetic field for the minimum extinction and ignition pressures features maximum area of the target covered with the tunnel of magnetic field lines.

Surface orbital magnetism

We compute the surface correction to the density of states of a particle in a convex box subjected to a magnetic field. Applying these results to orbital magnetism, we find that at high temperatures or weak magnetic fields the surface magnetization is always paramagnetic, but oscillations appear at low temperatures. In two dimensions they can give very large paramagnetic contributions near integer values of the filling factor. Explicit formulas are given for the zero-field susceptibility and for samples with a cylindrical shape in arbitrary magnetic field.

Magnet for a MHD linear generator

In this paper the authors analyse the performance of a magnetohydrodynamic (MHD) linear generator excited by means of various saddle shaped superconducting (sc) coils. In particular the device has been excited using: a prototype magnet, a demonstrative magnet and a commercial size magnet. The various magnet shape and size determine a quite different distribution of the magnetic flux density along the channel. To show the effects of the magnetic field shape on the device behaviour, we have simulated the fluid by means of a three-dimensional (3D) lumped parameter network representative of all major physical phenomena. The equivalent network has been determined using an innovative approach with respect to the parameter evaluation. The reliability of the proposed model has been verified using experimental data. The aim of this paper is to show the importance of the device simulation in the magnet optimization design to quantify the effects of a nonhomogeneous magnetic field.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

X-ray and vacuum ultraviolet imaging for electron cyclotron resonance processing plasmas

Electron cyclotron resonance (ECR) plasma systems are currently being investigated for plasma etching and plasma enhanced chemical vapor deposition. ECR generates high charged-particle densities at lower operating pressures which permits a higher degree of directionality for plasma etching, but produces low-energy x rays and vacuum ultraviolet radiation (VUV). Both types of radiation may cause damage to the wafer being processed. To reduce or shield the radiation damage, one would need to determine the spectrum and locations where the radiation is produced. The regions of radiation production can be determined by the image obtained from a pinhole camera placed along and across the center line of the dc magnetic field. The location of x-ray production could be changed by altering the shape of the dc magnetic field. The results verify that x rays are produced whenever hot electrons that travel along magnetic field lines strike the chamber walls. Similar methods were used to image the VUV radiation. The images show that the VUV is produced over the entire bulk plasma in addition to locations of wall bremsstrahlung where the x rays were produced.