Effect Of Axial Magnetic Field Research Articles

Axial Magnetic Field Effects on Xenon Short-Arc Lamps

The effect of an axial magnetic field (AMF) on an old xenon short-arc lamp is experimentally investigated in this work. As the AMF increases up to 18 mT, the visible radiation power and electric power ascend more than 80% and 70% respectively, and the radiation efficiency is improved by 23% for the best increment at 12 mT AMF. The measurement of radiation intensity shows that the increment of radiation intensity comes mostly from the plasma area close to the cathode tip, and partially from the other area of the arc column. Successive images of the arc indicate that the arc column not only rotates about its axis, but revolves around the axis of electrodes with the AMF. The arc column structure is constricted, distorted and elongated as the AMF increases. It is suggested that the improvements of the radiation intensity and radiation efficiency are attributed to the constriction of the arc column, which is mainly induced by the enhanced cathode jet.

Numerical study of MHD Taylor vortex flow with low magnetic Reynolds number in finite-length annulus under uniform magnetic field

The effects of axial uniform magnetic field on Taylor vortex flow with low magnetic Reynolds number in a concentric finite-length annulus are investigated numerically. The inner cylinder is rotating and the outer one is held at rest. As the endwall effects play a significant role in determining the vortical structure, three endwall conditions are taken into account in present study: all are fixed, all are rotating with the inner cylinder, one is fixed but the other is rotating with the inner cylinder. All the boundaries are assumed to be insulating, and the chosen aspect ratio, the height of the cylinder to the gap width, is moderate Γ=10, hence the endwall effects and the Taylor vortex can be both considered. The governing equations are solved using collocation spectral method and projection method which allows pressure–velocity decoupling. The flow patterns and developments of vortical structure with different endwall conditions are presented at various strength of axial magnetic field. The effects of axial magnetic field on magnetohydrodynamics flow are discussed.

Thermocapillary Convection in Floating Zone with Axial Magnetic Fields

Numerical simulations on the effects of axial magnetic fields on the thermocapillary convection in a liquid bridge of silicone-oil-based ferrofluid under zero gravity have been conducted. The Navier-Stokes equations coupled with the energy conservation equation are solved on a staggered grid, and the mass conserving level set approach is used to capture the free surface deformation of the liquid bridge. The obvious effects of the magnetic fields on the flow pattern as well as the velocity and temperature distributions in the liquid bridge can be detected. The axial magnetic fields suppress the thermocapillary convection and a stagnant flow zone is formed between the circulating flow and the symmetric axis as the magnetic fields increase. The axial magnetic fields affect not only the velocity level inside the liquid bridge but also the velocity level on the free surface. The temperature contours near the free surface illustrates conduction-type temperature profiles at moderate strength fields.

Optical absorption of zigzag single walled boron nitride nanotubes in axial magnetic field

We have investigated the effect of axial magnetic field on the band structure, dipole matrix elements and absorption spectrum in different energy ranges, using tight binding approximation. It is found that magnetic field breaks the degeneracy in the band structure and creates new allowed transitions in the dipole matrix which leads to creation of new peaks in the absorption spectrum. It is found that, unlike to CNTs which show metallic–semiconductor transition, the BNNTs remain semiconductor in any magnetic field strength. By calculation the diameter dependence of peak positions, we found that the positions of three first peaks in the lower energy region (E <5.3 eV) are proportional to n−2. In the middle energy region (7 < E < 7.5 eV) all (n, 0) zigzag BNNTs, with even and odd nanotube index, have two distinct peaks in the absence of magnetic field which these peaks may be used to identify zigzag BNNTs from other tube chiralities. For odd (even) tubes, in the middle energy region, applying the magnetic field leads to splitting of these two peaks into three (five) distinct peaks.

Influence of AMF on the Expansion Speed of Cathode Spots in High-Current Triggered Vacuum Arc

In this paper, the influence of an axial magnetic field (AMF) on the expansion speed of cathode spots (CSs) in the initial expansion stage of high-current triggered vacuum arc is investigated experimentally in a demountable vacuum chamber. Butt contacts with contact plate material of oxygen-free high-thermal-conductivity Cu and CuCr25 were used in experiments. Images of CSs are photographed with a high-speed digital camera with an exposure time of 2 $\mu{\rm s}$ . The uniform constant AMF in the intercontact region is supplied by external magnetic field coil. Experimental results indicate that, in an arc stabilized by AMF, CS expansion speed also increases with the increase of $di/dt$ , which is the same as that in arc burning freely. In addition, quasi-quantitative analysis of the experimental results show that the bending and lengthening of CS motion trajectory in AMF are not sufficient to explain the decrease of CS expansion speed with AMF. Another reason might be related to the inhibitory effect of AMF on the loss of arc plasma, which could also contribute to the decrease of CS expansion speed.

Effect of Axial Magnetic Field Strengths on Radial Velocity Field in Liquid Bridge under Microgravity

This article studies on the effect of magnetic field strengths on the flow field in a liquid bridge under zero gravity. The mass conservation level set method is used to track the two-phase interface. The results show that inhibitory effect of additional axial magnetic field on thermocapillary convection within liquid bridge is obvious, and this kind of inhibitory effect increasing as the magnetic field strength is strengthened.

Numerical investigation of magnetic field effect on pressure in cylindrical and hemispherical silicon CZ crystal growth

AbstractThe effect of axial magnetic field of different intensities on pressure in silicon Czochralski crystal growth is investigated in cylindrical and hemispherical geometries with rotating crystal and crucible and thermocapillary convection. As one important thermodynamic variable, the pressure is found to be more sensitive than temperature to magnetic field with strong dependence upon the vorticity field. The pressure at the triple point is proposed as a convenient parameter to control the homogeneity of the grown crystal. With a gradual increase of the magnetic field intensity the convection effect can be reduced without thermal fluctuations in the silicon melt. An evaluation of the magnetic interaction parameter critical value corresponding to flow, pressure and temperature homogenization leads to the important result that a relatively low axial magnetic field is required for the spherical system comparatively to the cylindrical one.

Effect of Axial Magnetic Field on Demixing of a Binary Fluid Mixture due to the Rotation of a Heated Sphere

The paper presents numerical investigation of effects of pressure gradient and temperature gradient on demixing of a binary mixture of thermally and electrically conducting incompressible viscous fluids set in motion due to the rotation of a heated sphere in presence of a uniform magnetic field applied in a direction parallel to the axis of rotation. The boundary layer equations governing the motion, temperature and concentration distribution are solved numerically with MATLAB’s built – in solver bvp4c. The numerical data is represented graphically and the effects of various parameters on demixing of the binary fluid mixture are discussed.

Simulation of Vacuum Arc Characteristics Under Four Kinds of Axial Magnetic Fields and Comparison with Experimental Results

Based on a two-dimensional axisymmetric magnetohydrodynamic (MHD) model, the vacuum arc characteristics under four kinds of axial magnetic fields (AMFs) are analyzed, which include a bell-shaped AMF generated by a pair of commercial cup-shaped electrodes, and three kinds of saddle-shaped ones generated by three pairs of newly designed electrodes. The simulation result indicates that the effect of AMF on the vacuum arc characteristics is significant. A comparison between the simulation result and experimental one shows that the distribution of the simulated ion density integrated along the viewing path is in agreement with the image of the arc column. Both the simulation result and the experimental one show that, among the four kinds of AMFs, the saddle-shaped one with the highest strength is the best, which could resist the constriction of the vacuum arc more efficiently, while the saddle-shaped one with the lowest strength and the bell-shaped one are the least desirable.

Effects of axial magnetic field on the electronic and optical properties of boron nitride nanotube

The splitting of band structure and absorption spectrum, for boron nitride nanotubes (BNNTs) under axial magnetic field, is studied using the tight binding approximation. It is found that the band splitting ( Δ E ) at the Γ point is linearly proportional to the magnetic field ( Φ / Φ 0 ) . Our results indicate that the splitting rate ν ii , of the two first bands nearest to the Fermi level, is a linear function of n −2 for all (n,0) zigzag BNNTs. By investigation of the dependence of band structure and absorption spectrum to the magnetic field, we found that absorption splitting is equal to band splitting and the splitting rate of band structure can be used to determine the splitting rate of the absorption spectrum.

Experimental observation and numerical analysis of arc plasmas diffused by magnetism

At atmospheric pressure, an intensified charge-coupled device (ICCD) camera with a narrow-band filter is used to capture the unsaturated images of a magnetically rotating arc. Comparison of its configurations with different arc current and external axial magnetic field (AMF) strength shows that the strong electromagnetic force may impel the arc to diffuse. Under the fully diffuse mode, the plasma is distributed throughout the electrode gap and no anode attachment can be seen in the cross-section of the torch. The fully diffuse plasma runs more steadily and its intensity distribution is more uniform, while its voltage fluctuation is reduced significantly. Using a commercial CFD (computational fluid dynamics) code FLUENT, the fluid flow and heat transfer of the fully diffuse plasma in an assumed magnetron torch have been simulated for qualitatively discussing the AMF effects. Numerical results show that the AMF significantly impels the plasma to retract axially and expand radially. As a result, the plasma intensity distribution on the cross section of the torch gets to be more uniform.

The effect of axisymmetric two-dimensional magnetic field on the configuration of vacuum-arc plasma

An experimental investigation is made of the effect of axisymmetric two-dimensional magnetic field on the forming of plasma and on the configuration of cathode spots in a vacuum-arc discharge. It is demonstrated that a magnetic field with a transverse (relative to the discharge axis) component has a significant effect on the shape of plasma column and on the rate of expansion of the cathode spot region. In a magnetic field, arc plasma has the form of truncated cone expanding toward the anode. The cathode spots take up a part of the cathode area which decreases with increasing magnetic field. Arguments are given in support of the assumption that the arrangement of cathode spots and the form of arc plasma are defined by the minimum principle similar to the Steinbeck principle. In so doing, the displacement of spots is caused by their emergence in a new region corresponding to a lower arc voltage. Also discussed is the mechanism associated with retrograde motion of cathode spot in view of the effect of azimuthal magnetic field on the axial component of current and of the effect of axial magnetic field on the azimuthal component of current.

The effects of axial magnetic field on electronic properties of carbon nanotubes

The effects of the external magnetic field on the energy gap of carbon nanotubes were investigated by using the tight-bonding method. It was found that, under the applied magnetic field, the peak of energy gap induced by magnetic field and the slope of energy gap versus magnetic field depend on their diameters tightly; when tubes with the same diameters are used, the metal type tubes have the highest energy gap peak. Furthermore, under the magnetic field, the density of states of carbon nanotubes near the Fermi level are also calculated, showing that the Van Hove singularities will exhibit a periodic phenomenon of splitting, moving and merging. Our calculations meet the results from Green's function method, so they are reasonable.

Photoresist stripping after low-k dielectric layer patterning using axial magnetic field assisted reactive ion etching

Effects of axial magnetic field added to the conventional reactive ion etcher (RIE) are studied in terms of plasma characteristics and applied to anisotropic photoresist strip process. Photoresist strip process of dual damascene Cu interconnection technology requires little damage to the low-k dielectric layers after plasma exposure. The magnitude and direction of axial magnetic field can be controlled by the current to a pair of Helmholtz coils. Compared to the conventional RIE, it is found that etch rate is increased by 30% and its uniformity is improved for 300 mm wafers. Selectivity of photoresist over SiOC-H low-k dielectric film and the surface modification and damage of low-k dielectric layers by O2∕N2 plasma are also investigated. The addition of the axial magnetic field to RIE can increase photoresist etch rate by increasing the plasma density and reduce damage to low-k dielectric layer by reducing the dc self bias.

Mechanism of diffusion of positively charged dust particles in a photoemission cell under microgravity conditions

The decay of a dusty plasma in a photoemission cell under microgravity conditions is investigated on the basis of the method of nonlocal moments. It is founds that plasma decay in space experiments occurs in accordance with the mechanism of free electron diffusion followed by dust particle drift. An analytic solution is found for the evolution of radial distributions of the dust particle concentration and the electric field under the experimental conditions. The effect of abnormally high temperatures of dust particles is considered. The effect of axial magnetic fields on the decay of dusty plasma is investigated. It is shown that the plasma decay in a magnetic field is governed by the ambipolar diffusion mechanism, the decay being prolonged up to 103 s in a magnetic field on the order of 103–104 G in strength.

Numerical study of 3D unsteady melt convection during industrial-scale CZ Si-crystal growth

We present a computational model of 3D turbulent melt convection in Czochralski Si-crystal growth systems, based on the hybridization of Reynolds-averaged approach and large eddy simulation. The effect of superimposed magnetic field action on the melt flow is introduced in the model to account for the suppression of turbulent melt fluctuations. The model has been verified using experimental data for temperature in the melt and along the melt–crucible surface. Effects of axial magnetic field on the change in melt convection are studied in an industrial configuration.

Effect of axial magnetic field on three-dimensional instability of natural convection in a vertical Bridgman growth configuration

A study of the effect of an externally imposed magnetic field on the axisymmetry-breaking instability of an axisymmetric convective flow, associated with crystal growth from bulk of melt, is presented. Convection in a vertical cylinder with a parabolic temperature profile on the sidewall is considered as a representative model. A parametric study of the dependence of the critical Grashof number Gr cr on the Hartmann number Ha for fixed values of the Prandtl number (Pr=0.015) and the aspect ratio of the cylinder ( A=height/radius=1, 2 and 3) is carried out. The stability diagram Gr cr(Ha) corresponding to the axisymmetric—three-dimensional transition for increasing values of the axial magnetic field is obtained. The calculations are done using the spectral Galerkin method allowing an effective and accurate three-dimensional stability analysis. It is shown that at relatively small values of Ha the axisymmetric flow tends to be oscillatory unstable. After the magnitude of the magnetic field (Ha) exceeds a certain value the instability switches to a steady bifurcation caused by the Rayleigh–Bénard mechanism.

Polarization gradient induced electron shelving in cold helium atoms in an optical lattice: effect of axial magnetic field

We report the effect of atomic motion and an axial magnetic field on the spectrum of resonance fluorescence emitted by cold helium atoms in an optical lattice. The polarization gradient induces electron shelving at selected points on the optical lattice, giving rise to light and dark periods. We find in particular that localization of the atoms in the optical lattice inhibits electron shelving but can be recovered in the presence of an axial magnetic field.

Effects of axial magnetic field on the oscillatory convection coupled with crucible rotation in Czochralski crystal growth

Abstract Experimental results are presented of the oscillatory convection of mercury melts coupled with crucible rotation under an imposed axial magnetic field in Czochralski crystal growing configuration. We have observed that baroclinic traveling thermal wave induced by crucible rotation turns into a standing wave by the action of axial magnetic field, when the crucible rotation rate is relatively low. It is found that a significant damping of the temperature oscillation underneath a crystal disk can be obtained after the transition from the traveling thermal wave to the standing wave.

Rotation and magnetic effect on the non-axisymmetric temporal instability of a capillary jet

The non-axisymmetric capillary instability of a cylindrical jet and its break-up in another fluid are investigated in the case when the jet is subject to the combined effect of rotation, axial motion, and axial magnetic field. The criterion for maximum instability enables the determination of the intact jet length and size of the droplets produced in the course of the jet break-up process. Computational results for modes m = 1, 2, 3 and 4 show that non-axisymmetry postpones the onset of instability and for m = 3 and m = 4 the occurrence of instability is made possible only by substantially increasing both the angular velocity Ω 1 and by a limited permissible increase of | W 1 – W 2|, the difference between axial vel These results also show that the droplet radius and jet length are increasing due to the stabilizing effect of the axial magnetic field. Rotation will cause these two quantities to diminish or enlarge according to the relative angular velocities of the jet and the surrounding medium.