Presence Of Axial Magnetic Field Research Articles

CFD analysis of Joule heating effect in a confined axi-symmetric swilling flow under the influence of axial magnetic field

The effect of axial magnetic field along with axial temperature gradient, in a confined cylindrical cavity packed with incompressible electrical conducting fluid having a top rotating lid, has been investigated. Due to the presence of axial magnetic field, Joule heating effect is considered in this study. The governing parameters ranges are as follows: [Formula: see text], [Formula: see text] at Re[Formula: see text]1000, Pr[Formula: see text]0.015. It is found that the internal heat generation due to Joule heating in mixed convection is governed by interaction parameter and temperature gradient. However, the Joule heating effect has very less effect on primary flow in comparison to the secondary flow along the meridional plane.

A Numerical Study on the Influence of an Axial Magnetic Field (AMF) on Vacuum Arc Remelting (VAR) Process

A comprehensive numerical model is proposed to study the influence of an axial magnetic field (AMF) on the solidification behavior of a Titanium-based (Ti–6Al–4V) vacuum arc remelting (VAR) ingot. Both static and time-varying AMF are examined. The proposed 2D axisymmetric swirl model includes calculating electromagnetic and thermal fields in the entire system composed of the electrode, vacuum plasma, ingot, and mold. A combination of vector potential formulation and induction equation is proposed to model the electromagnetic field accurately. Calculations of the flow in the melt pool and solidification of the ingot are also carried out. All governing equations are presented in cylindrical coordinate. The presence of a weak AMF, such as the earth magnetic field, can dramatically influence the flow pattern in the melt pool. The “Electro-vortex flow” is predicted ignoring AMF or in the presence of a time-varying AMF. However, the flow pattern is “Ekman pumping” in the presence of a static AMF. The amount of side-arcing has no influence on the pool depth in the presence of an AMF. Modeling results are validated against experiments.

Experimental Investigation on the Current Carried by a Cathode Spot of Vacuum Arc in Axial Magnetic Fields

The splitting and threshold currents of vacuum arc cathode spots in axial magnetic fields (AMFs) up to 220 mT are investigated experimentally. Measurements are made on electrodes of Cu and CuCr25 with electrode separations of 2 and 6 mm. The electrode diameter is 30 mm. All the experiments are conducted in a demountable vacuum chamber evacuated continuously by a turbo-molecular pump to a pressure of the order of 10−3 Pa. The spots are photographed using a high-speed digital camera with an exposure time of $2~\mu \text{s}$ . The AMF is applied by means of two permanent magnets placed symmetrically inside the chamber. The experimental results indicate that the splitting and threshold current decrease in the presence of AMF and tend to certain constant values when the field gets stronger. The splitting and threshold currents are considerably larger than Cu electrodes than those with CuCr25 electrodes. With increasing electrode separation, the splitting current decreases while the threshold current increases. Possible explanations are proposed to account for the experimental observations.

Magnetogasdynamic exponential shock wave in a self-gravitating, rotational axisymmetric non-ideal gas under the influence of heat-conduction and radiation heat-flux

Similarity solutions are obtained for one-dimensional cylindrical shock wave in a self-gravitating, rotational axisymmetric non-ideal gas with azimuthal or axial magnetic field in the presence of conductive and radiative heat fluxes. The total energy of the wave is non-constant. It is obtained that the increase in the Cowling number, in the parameters of radiative as well as conductive heat transfer and the parameter of the non-idealness of the gas have a decaying effect on the shock wave however increase in the value of gravitational parameter has reverse effect on the shock strength. It is manifested that the presence of azimuthal magnetic field removes the singularities which arise in some cases of the presence of axial magnetic field. Also, it is observed that the effect of the parameter of non-idealness of the gas is diminished by increasing the value of the gravitational parameter.

The Effect of Axial Magnetic Field on The Breakdown Voltage of Air at Low Pressure

To add more details about the effect of the axial magnetic field on the plasma profile, the breakdown voltage of air was investigated at low pressure (9-15 Pa) in the presence of axial magnetic field (0.01-0.04T). The air was ignited by a DC voltage between two plain electrodes of aluminum separated by a distance (8.5cm). The measurements showed that the discharge voltage decreases to a minimum value, then returns to increase over the minimum with increasing the magnetic field strength, at all pressures in the range. It was also observed that a maximum decrease in the discharge voltage is obtained near the minimum of Paschen curve from the right side. The decrease in the discharge voltage was caused mainly by the effect of magnetic flux density on the diffusion of charged particles to the walls, especially on that of free electrons which were borne during the breakdown process.

Limitations of paraxial approximation to model elec-tron acceleration by a laser pulse in vacuum in the presence of an axial magnetic field

We model electron acceleration using paraxial approximation (PA) and seventh order correction description (O7) of a laser field in vacuum in the presence of an axial magnetic field. The effect of initial momentum, laser intensity, spot size, and initial position of electron on optimum value of magnetic field and electron energy for linearly and circularly polarized laser pulse has been investigated. We show that PA fails to obtain correct values of optimum magnetic field and electron energy. The amplitude of oscillations of the electron increases with time in the presence of axial magnetic field and PA fails to correctly take into account focusing and defocusing of laser and obtain correct results.

Performance of the cost-effective Planacon ® MCP-PMTs in strong magnetic fields

We present the behavior of the cost-effective Planacon MCP-PMTs with 25μm pore diameter in the presence of axial magnetic fields up to 0.5 T. Having a batch of 62 devices of the same type, two MCP-PMTs were selected and their gain variation measured in different magnetic fields. These two otherwise identical devices satisfied the selection criteria by requiring the lowest (1.15 kV) and one of the highest (1.4 kV) bias voltage values to achieve a given gain. Both MCP-PMTs have a nearly identical tolerance of the strong magnetic field despite the significant difference in the bias voltage. This clarifies the mechanism of the B-field influence on the MCP-PMT gain, emphasizing the importance of the intrinsic parameters of the MCP emissive coating rather than external parameters, such as the total bias voltage. By evaluating the dependence of both gain and timing parameters on the magnetic field strength, we confirm the operability of such MCP-PMTs in strong magnetic fields in spite of the relatively large pore diameter and low bias voltage required for a given gain.

Enhanced electron acceleration by a chirped tightly focused laser in vacuum in the presence of axial magnetic field

The paper presents a scheme of vacuum electron acceleration due to tightly focused linearly polarized (LP) frequency chirped laser in the presence external magnetic field. The focused terawatt chirped laser pulse and the external axial magnetic field enable the electron of few MeV initial energy to attain high energy in GeV range, hence, explore the possibility of efficient electron acceleration. This can be achieved by optimizing the focused terawatt chirped laser and axial magnetic field parameters for resonance, which is realized between the electron and electric field of the tightly focused laser pulse. The frequency chirp plays a crucial role in enforcing resonance condition for longer duration and the applied magnetic field strongly enforces the electron to remain in the accelerating phase, thereby, both factors are equally imperative for accomplishing high electron energy gain in GeV. The presence of optimum axial magnetic field (~8.5 MG) along with optimum value of linear frequency chirp for laser intensity 1.38 × 1020 W/cm2 results in electron beam with energy ~5.78 GeV. During acceleration process, electron energy gain is highly sensitive to the initial phase of laser along with the frequency chirp and magnetic field. Electron trajectories in the influence of frequency chirp and magnetic field are also analyzed.

One-dimensional cylindrical shock waves in non-ideal gas under magnetic field

In the present paper, we analyze the evolutionary behavior of imploding strong shock waves propagating through a non-ideal gas in the presence of axial magnetic field. An evolution equation has been constructed by using the method based on the kinematics of one-dimensional motion of shock waves. The values of similarity exponents have been calculated by using the first order truncation approximation which describes the decay behavior of strong shocks. The approximate values of the similarity exponents are compared with the similarity exponents calculated by the CCW approximation, the exact similarity solution and perturbation technique.

Electron acceleration by cosh-Gaussian laser beam in the presence of axial magnetic field

The present study was conducted to determine electron acceleration by cosh-Gaussian laser beam (CGLB) and axial magnetic field. CGLB possesses higher power and has ability to focus earlier than the Gaussian beam. Nonlinear differential equations are derived for CGLB for energy and velocity. The dependence of electron energy gained for different values of laser intensity parameter ‘ao’ has been graphically plotted. It is examined that both laser and axial magnetic field significantly affects the electron energy.

Effect of Mean Taylor Vortex Motion on Turbulent Fluctuations in the Presence of Axial Magnetic Field

Effect of Mean Taylor Vortex Motion on Turbulent Fluctuations in the Presence of Axial Magnetic Field

Optoelectronic properties of silicon hexagonal nanotubes under an axial magnetic field

In this work, we derive a clear and easy-to-use analytic expression for optical transition matrix elements for the zigzag silicon hexagonal nanotubes (Si h-NTs) in the presence of axial magnetic field. The optical dipole matrix elements and optical absorption are analytically derived in terms of one-dimensional wave vector, kz and subband index l for a light polarization parallel to the tube axis. The model is based on a single-orbital π-electron tight-binding Hamiltonian extended up to the first nearest-neighbors. By comparing the band structure of tubes in different magnetic fields, one can see that the band gap is modified and the degenerated bands are split and create new allowed transition corresponding to the band modifications. Moreover, it is found that the system is metallic in the absence of magnetic field, but with increasing the magnetic flux the system tends towards the semiconducting behavior and the metal–semiconductor phase transition is observed.

Sensitiveness of axial magnetic field on electron acceleration by a radially polarized laser pulse in vacuum

We examine the electron acceleration by a radially polarized (RP) laser pulse in vacuum under influence of an intense axial magnetic field. The electron while interaction with a RP laser pulse gets accelerated with high energy gain. The attained energy gain further enhanced up-to the order of GeV with an intense RP laser pulse. We observe a significant enhancement in energy gain in the presence of an intense axial magnetic field in the direction of propagation of laser pulse. The presence of axial magnetic field improves the strength of v→×B→ force which supports the retaining of betatron resonance for longer durations. This improves the electron acceleration with an enhanced energy gain up to 5.2GeV. It is noticed that the axial magnetic field is sensitive to electron acceleration, small change in magnetic field leads to enhance electron energy gain significantly. Our results also show relatively smaller scattering of the electrons in the presence of axial magnetic field.

Viscous potential flow analysis of magnetohydrodynamic capillary instability with heat and mass transfer

Abstract A linear analysis of capillary instability of a cylindrical interface in the presence of axial magnetic field has been carried out when there is heat and mass transfer across the interface. Both fluids are taken as incompressible, viscous and magnetic with different kinematic viscosities and different magnetic permeabilities. Viscous potential flow theory is used for the investigation and a dispersion relation that accounts for the growth of axisymmetric waves is derived. Stability criterion is given by critical value of applied magnetic field as well as critical wave number and stability is discussed theoretically as well as numerically. Various graphs are drawn showing the effect of various physical parameters such as magnetic field strength, heat transfer capillary number, and permeability ratio, on the stability of the system. It has been observed that the axial magnetic field and heat and mass transfer both have stabilizing effect on the stability of the system.

Competing charge-density wave, magnetic, and topological ground states at and near Dirac points in graphene in axial magnetic fields

In the presence of axial magnetic fields that can be realized in deliberately buckled monolayer graphene, quasi-relativistic Dirac fermions may find themselves in a variety of broken symmetry phases even for weak interactions. Through a detailed Hartree self-consistent numerical calculation in finite strained graphene with cylindrical and open boundaries we establish the possibility of realizing a charge-density-wave order for the spinless fermions in the presence of weak nearest-neighbor repulsion. Such an instability gives rise to a staggered pattern of average fermionic density between bulk and boundary of the system as well as among two sublattices of graphene, due to the spatial separation of the zero energy states localized on opposite sublattices. Although with fermions spin restored, an unconventional magnetic order driven by the onsite repulsion, possibly leads to the dominant instability at the Dirac point, the proposed charge-density-wave order can nevertheless be realized at finite doping, which is always accompanied by a finite ferromagnetic moment. Additionally, the charge-density-wave phase supports a quantized charge or spin Hall conductivity when its formation away from the Dirac point is further preceded by the appearance of topological anomalous or spin Hall insulator respectively. The topological orders in strained graphene can be supported by weak second neighbor repulsion, for example. Therefore, depending on the relative strength of various short-range components of the Coulomb interaction a number of broken symmetry phases can be realized within the zero energy manifold in strained graphene.

Selection rules and linear absorption spectra of carbon nanotubes in axial magnetic fields

We derive a transparent and easy-to-use analytic expression for the selection rules and the optical dipole matrix elements for carbon nanotubes of arbitrary chirality in the presence of axial magnetic fields using a single-orbital \ensuremath{\pi}-electron tight-binding model. From this, we calculate the linear absorption spectrum for arbitrary polarization directions of the incident light, providing insight into all optically allowed transition. We show that the transverse absorption peaks can be selectively excited with circularly polarized light and spectrally resolved in an axial magnetic field.

Two interacting electrons confined in a 3D parabolic cylindrically symmetric potential, in presence of axial magnetic field: A finite element approach

In this work, we explore the performance and accuracy of finite element numerical simulations for the relevant and well-known problem of two interacting electrons confined in a parabolic cylindrically symmetric potential, and under the influence of axial magnetic field. By using a full configuration interaction method as reference, we compare two different ways to obtain the eigenvalues of the system by finite element simulations, one of which approximately separates the Coulomb interaction and averages the longitudinal part. We find that in the regime of low aspect ratios, the results from the approximate scheme with approximation are quite optimal (<0.05% of difference respect to the reference), but once the dots turn more and more thicker, the results become just acceptable (>0.5% of difference), due to underestimation of the Coulomb interaction. On the other hand, for the non-approximated finite element approach the results are consistently reliable along the different field and aspect ratio regimes (<0.02% of difference respect to the reference). This allows us to present a novel, efficient and highly accurate method for obtaining electronic structure of interacting particles in 0D nanostructures.

Magneto-rotational instability in differentially rotating liquid metals

We study the stability of Couette flow between two cylinders in the presence of axial magnetic field in local WKB approximation. We find the analytical expression of the critical angular velocity minimized over the wave number and the imposed magnetic field as a function of the measure of deviation of the rotation law from the Rayleigh line. The result found is in a good agreement with the previously known numerical results based on the global analysis. We perform a minimization of the critical Reynolds number over the wave number at fixed magnetic field both analytically and numerically. We show that a compromise between resistive suppression of magneto-rotational instability at weak magnetic field and the increase of the critical Reynolds number with the increase of magnetic field is possible. It takes place at moderate values of magnetic field of order 3 × 10 2 gauss giving the critical Reynolds number of order 4 × 10 4 .

Couette flow in ferrofluids with magnetic field

Instability of a viscous, incompressible ferrofluid flow in an annular space between two coaxially rotating cylinders in the presence of axial magnetic field has been investigated numerically. The magnetic field perturbations in fluid in the gap between the cylinders have been taken into consideration and these have been observed to stabilize the Couette flow.

Effect of time-varying axial magnetic field on photoresist ashing in an inductively coupled plasma

Time-varying axial magnetic field is added to an inductively coupled plasma. Weak axial magnetic field of about 10–15 G can be obtained by a pair of Helmholtz coils attached to the chamber. This scheme has been applied to etch SiO2 and silylated photoresist, where processing pressures are generally below 7 Pa, and ions are major reaction species. In order to extend this concept to other etching conditions, this method is applied to photoresist ashing, where processing pressure is usually higher than 133 Pa, and downstream oxygen radicals are important species. It is found that 60 Hz of the ac current gives the maximum ash rate and that ash rate is increased from 6.1 to 7.0 μm/min, and uniformity is improved from 8% to 4.5% over an 8 in. wafer. When aluminum baffle is placed between plasma source and reaction chambers, the ash uniformity improves further to 2.0% over a 300 mm wafer. The presence of axial magnetic field increases the plasma density and thus ash rate, and better uniformity comes from redistribution of ions and radicals during the off period in a periodic magnetic field.