Strong Axial Magnetic Field Research Articles

Thermoelectromagnetic convection in floating zone silicon growth with a nonaxisymmetric temperature and a strong axial magnetic field

Thermoelectromagnetic convection (TEMC) is the melt motion driven by the interaction of an externally applied magnetic field and the thermoelectric current due to variations of the absolute thermoelectric power and of the temperature. This paper presents numerical solutions for the three-dimensional TEMC in a simplified model of the floating zone process with a nonaxisymmetric temperature variation over the crystal–melt interface. The axial magnetic field is sufficiently strong that inertial effects are negligible, so that each azimuthal Fourier component of the TEMC can be treated independently. Results are presented for the cos θ and cos(2 θ) components of the TEMC. The results show that a very small deviation from axisymmetry in the temperature produces a very large deviation from axisymmetry in the radial and axial components of the TEMC velocity.

Inverse Faraday effect in a relativistic laser channel

Interaction between energetic electrons and a circularly polarized laser pulse in a relativistic plasma channel is studied. Laser radiation can be resonantly absorbed by electrons executing betatron oscillations in the channel and absorbing angular momentum from the laser. The absorbed angular momentum manifests itself as a strong axial magnetic field (inverse Faraday effect). The magnitude of this magnetic field is calculated and related to the amount of the absorbed energy.

Non-uniform 3He polarization formed by multiple collisions of a fast 3He + ion with polarized Rb vapor in a strong magnetic field

We investigated the spatial distribution of a polarization in 3He beam expected from a novel polarized 3He ion source based on electron pumping, i.e., multiple electron capture and stripping collisions of an incident fast 3He + ion with a polarized Rb vapor in a strong axial magnetic field. For this purpose, a Monte Carlo simulation was carried out for 19 keV 3He + ions with varying Rb vapor thickness, magnetic field, and beam emittance. The calculated results showed a distribution of the 3He polarization that we call a “polarization hole”, which has a low polarization area around the beam axis. The parameters characterizing the polarization hole, i.e., the polarization and radius of the hole, were found to depend on the Rb vapor thickness, the magnetic field, the beam size, and the angular divergence of the initial beam. These parameters were successfully reproduced with analytical functions deduced from a probability density function prescription. This provides a powerful tool to treat complex phenomena of multiple collisions in strong magnetic fields without performing time-consuming Monte Carlo calculations.

Splitting of the spectra of MHD waves and the structure of a satellite alfvén resonance in a cold plasma in a strong axial magnetic field and a weak helical field

The possibility is demonstrated of splitting the eigenfrequencies of MHD plasma waves in a stellarator with a weakly rippled helical confining magnetic field. The distribution of the fields of an Alfven wave in the satellite Alfven resonance region is investigated when the influence of the helical ripple in a confining magnetic field on the resonance structure is comparable with the effects of the finite ion Larmor radius, electron inertia, and collisions between plasma particles.

Inertial effects in the rotationally driven melt motion during the Czochralski growth of silicon crystals with a strong axial magnetic field

This article treats the melt motion driven by the rotations of the crystal and crucible about their common vertical axis during the Czochralski growth of silicon crystals with a strong, uniform, vertical magnetic field produced by a solenoid around the crystal growth furnace. Since molten silicon is an excellent electrical conductor, the interaction parameter N and the Hartmann number Ha are both large for typical magnetic field strengths, so that composite singular perturbation techniques for \(N\gg1\) and \(Ha\gg1\) are appropriate. An inertialess solution, which also assumed that \(N\gg Ha^{3/2}\), was presented in a previous paper. In the inertialess solution, the largest gradient of the azimuthal velocity \(v_\theta\) and the largest secondary flow with radial and axial velocities vr and vz both occur inside an interior layer with an \(O(Ha^{-1/2})\) radial thickness at the vertical cylinder directly beneath the periphery of the crystal, where the growth interface meets the free surface. For all current experimental studies, the assumption that \(N\gg Ha^{3/2}\) is not satisfied. The appropriate assumption is that \(N=O(Ha^{3/2})\), and inertial effects are not negligible inside the interior layer. An intersection region, which is formed by the intersection of the interior layer and a Hartmann layer with an \(O(Ha^{-1})\) axial thickness adjacent to the crystal-melt interface and free surface, is intrinsically coupled to the interior layer. This article treats inertial effects in the interior layer and intersection region for \(N=O(Ha^{3/2})\) and \(Ha\gg1\). Non-linear governing equations were derived and solved numerically. A fourth-order Adams-Bashforth-Moulton predictor-corrector method was used to solve the transport equations for the primary azimuthal velocity and for the secondary-flow vorticity. Poisson equations, which govern the stream functions for both the secondary flow and the electric current density, were solved using a matrix diagonalization technique. The effects of inertia on the melt motion are discussed. This type of study provides for a fuller understanding of the melt motion, without which defect-free crystals will be difficult to grow on a consistent basis.

Stability of magnetohydrodynamic Dean Flow as applied to centrifugally confined plasmas

Dean Flow is the azimuthal flow of fluid between static concentric cylinders. In a magnetized plasma, there may also be radial stratification of the pressure. The ideal magnetohydrodynamic stability of such a flow in the presence of a strong axial magnetic field and an added radial gravitational force is examined. It is shown that both the Kelvin–Helmholtz instability and pressure-gradient-driven interchanges can be stabilized if the flow is driven by a unidirectional external force and if the plasma annulus is sufficiently thin (large aspect ratio). These results find application in schemes using centrifugal confinement of plasma for fusion.

Performance of fine-mesh photomultiplier tubes designed for an undoped-CsI endcap photon detector

We have developed two types of fine-mesh photomultiplier tubes for an undoped-CsI endcap photon detector installed in a strong axial magnetic field. These photomultiplier tubes have ultraviolet-transmitting glass windows, backed by large-area bialkali photocathodes, in order to efficiently detect scintillation light from undoped-CsI crystals. A gain of more than 2 × 10 5, in a magnetic field of 1.0 T, was obtained from these tubes with 19-stage fine-mesh dynodes. Gain drops caused by the magnetic field and by high anode currents were investigated. Several performance criteria of the tubes were studied: gain, quantum efficiency, linearity, noise, and transit time spread.

Microwave plasma source for the negative hydrogen ion production

Microwave plasma source for the negative hydrogen ion production was constructed and tested. The plasma source consists of two chambers: a plasma production chamber and a plasma confinement chamber. The production chamber is placed into the strong axial magnetic field and the microwaves (2.45 GHz, up to 5 kW) are introduced through the quartz window along the magnetic field lines. It is found that the suppression of the fast electron loss to the window holder is important to improve the efficiency of hydrogen plasma production. The plasma density increases with the magnetic field strength in the plasma production chamber. For the microwave power of 4 kW the uniform plasma of 3×1012 cm−3 for argon and 3×1011 cm−3 for hydrogen is obtained in the area of 20×20 cm. Electron temperature in the plasma grid region is around 2 eV. The optimum gas pressure is around 6 mTorr.

Submicrochannel plate multipliers

A theoretical investigation of alumina micron and submicron multichannel electron amplifiers was carried out. The theoretical study was based on multiplier performance Monte-Carlo simulation and general concepts of secondary electron emission theory. The simulation was performed for multipliers for which the length-to-diameter ratio was varied from 40 to 60 and the bias voltage from 600 to 1400 V. Calculations were made for the linear mode of multiplier operation under single electron stimulus. The simulation showed that alumina plates of the considered configurations are expected to possess a high gain (> 1000 at applied voltages exceeding 1.2 kV). The simulation also demonstrated that submicrochannel multipliers have faster response (< 30 ps), better temporal resolution (< 10 ps) and higher tolerance to strong axial (parallel with respect to channel axis) magnetic fields (up to ∼ 4 T) than microchannel plates with the same length-to-diameter ratios. Anodic alumina submicrochannel plates with channel diameter 0.07–0.4 μm, length-to-diameter ratio up to 300 and an effective open area up to ∼ 50% have been produced.

Influence of the illumination characteristics on the laser plasma dynamics in a strong axial magnetic field

The paper presents results of investigations and the analysis of the behavior of a laser produced plasma in a strong axial magnetic field of the induction 10T. The studied plasma was generated by means of a Nd laser of the energy 5 J and a pulse duration of about 1 ns. Three frame laser interferometry was used for determining the dynamic and electron density distributions of the expanding plasma. An analysis of the interferometric measurement results allowed us to distinguish two characteristic regimes of the plasma expansion: angular and axial. Using 2D MHD computer simulation it was found that these regimes are connected with the character of the target illumination, namely the laser beam spatial intensity distribution in the focal area.

A fluid model of the bulk plasma in a cylindrical magnetized low-pressure RF discharge and comparison with PIC - MC simulation results

The modelling of a cylindrical low-pressure RF discharge in a strong axial magnetic field is analysed by a comparison of previously obtained kinetic particle-in-cell - Monte Carlo (PIC - MC) simulation results to a newly developed hydrodynamic model. We show that such a model is possible in the bulk of the discharge, where the electron distribution function is Maxwellian to good accuracy. The small electron diffusion coefficient perpendicular to the magnetic field is essential for the validity of the hydrodynamic analysis. As a result, we obtain good agreement between the hydrodynamic model and our kinetic simulation data. We furthermore exemplify the usefulness of the hydrodynamic approach by discussing the properties of the discharge bulk in terms of macroscopic quantities; we thus can show that the contribution of thermal diffusion to the electron current is of the same importance as that of ordinary diffusion, an issue usually overlooked in the analysis of magnetized discharges.

Van der Pol model of a Cherenkov maser

Abstract A non-linear analysis of a Cherenkov maser is presented. The system consists of a ring configuration of a cylindrical waveguide filled with a dielectric material. A single transverse-magnetic mode is assumed to propagate in the system. A low-density pencil electron beam travels in part of the ring, confined by a strong axial magnetic field. Using the single-particle description for the beam and the wave equation for the field, we obtain a set of two coupled non-linear differential equations describing the slowly varying amplitude and phase of the electromagnetic mode. The gain per path is assumed to be small and the spatial growth of the field is neglected. The resulting time dependent amplitude includes the exponential gain of the linear stage and the saturation to its maximum value. The time dependent frequency is also calculated. The two equations are combined to a single Van der Pol equation with a non-linear restoring force. This description demonstrates the similarities and differences between the Cherenkov maser and other lasing systems.

The plasma for material processing

A new plasma source is developed by introducing `Buckminsterfullerene', , particles into a low-temperature (about 0.2 eV) potassium plasma column confined by a strong axial magnetic field, which consists of electrons, positive ions and large negative ions. Drastic changes in characteristics of thin films formed by using the plasma are observed in accordance with bias control of fluxes and energies of and ions coming into contact with substrates. It is hoped that this ultra-fine-particle plasma will be useful for producing -based materials with unique functions such as alkali fullerides, endohedral metallofullerenes and polymers.

Using x-ray spectroheliograph technique for investigations of laser-produced plasma under interaction with strong magnetic field

A dense jet of a plasma consisting of multiply charged ions was generated in the interaction of a laser plasma with a strong external axial magnetic field. It is shown that using the high-luminosity x-ray spectroheliograph technique allows to measure plasma emission spectra with 2-dimensional spatial resolution even in the cases when these spectra have small intensities. The x-ray spectroscopy and interferometry methods are used to measure plasma parameter distributions. The dependencies of Ne(z) and Te(z) measured in this paper can be used to calculate the evolution of plasma ionization state during plasma expansion. The quasihomogeneous laser jet, which appears when a laser plasma interacts with an external magnetic field can be used not only to form an active medium of a short wavelength laser, but probably also to tackle the urgent problem of transport in a laser ion injector.

Production and control of K–C60 plasma for material processing

An ultrafine-particle plasma including large negative fullerene ions is produced by introducing fullerene particles into a low-temperature (≊0.2 eV) potassium plasma column confined by a strong axial magnetic field. The density ratio of the negative fullerene to positive potassium ions is controlled by changing the oven temperature for fullerene sublimation and the plasma density being varied in the range from 0 to 0.97 in the central region of the plasma cross section. Since the electrons are well confined in the central region, the ratio increases radially and is more than 0.9999 in the radial edge region. A thin film consisting of the potassium and fullerene particles is formed on a metal end plate which terminates the plasma column. Depending on the end plate potential and radial position, the film is verified to contain metal-doped fullerenes with the potassium atom in the endohedral cavity, which are formed as a result of the potassium-atom insertion into the stable cage of fullerene particles.

Formation of a plasma jet of multiply charged ions in the interaction of a laser plasma with an external pulsed magnetic field

A dense jet of a plasma consisting of multiply charged ions was generated in the interaction of a laser plasma with a strong external axial magnetic field. Images were formed by spectral lines and the soft x-ray spectrum range of the plasma jet was obtained with a large-aperture spectrograph containing a mica crystal bent to form a spherical surface with a radius of R = 10 cm. A tenfold increase in the density of the He-like Mg XI plasma, compared with a freely expanding plasma, was observed at a distance of 5 mm from the target.

Production of C60 plasma

An ultrafine-particle plasma consisting of electrons, positive K+ ions, and large negative C−60 ions is produced by introducing ‘‘Buckminsterfullerene, C60’’ particles into a low-temperature (≂0.2 eV) potassium plasma column confined by a strong axial magnetic field. With an increase in the C−60 fraction, the electron shielding decreases, yielding clear effects on plasma collective phenomena, which are demonstrated for low-frequency electrostatic plasma-wave propagations and instabilities. This plasma might be useful for producing new C60-based materials.

Propagation of short electron pulses in underdense plasmas.

Dense relativistic electron beams traversing a plasma, in what is known as the underdense, or ion focusing, regime experience a strong, linear transverse restoring force. This force arises from the nearly immobile ions which form a channel of uncompensated positive charge when the plasma electrons are ejected in response to the introduction of the beam charge. This phenomenon can be used for focusing the electron beam to very high densities over long propagation distances. Several schemes have been proposed, including the nonlinear plasma wake-field accelerator, the adiabatic plasma lens, and the ion-channel laser, whose viability is based on this focusing effect for very short pulse, high current electron beams propagating in plasma. In this paper we examine, analytically and numerically, the self-consistent requirements on plasma density, beam current, length, and transverse emittance which must be satisfied in order for ion-channel formation and near equilibrium beam propagation to exist over the majority of the length of the electron beam. The dynamics of the beam-plasma system are modeled by a simultaneous solution of the plasma electron cold-fluid equations, and the Maxwell-Vlasov equation governing the beam's thermal equilibrium. The effects of introducing a strong axial magnetic field on the plasma response and beam equilibria are examined. In addition to developing criteria for self-consistent equilibrium focusing, a time-dependent analysis where the beam particles are treated as mobile particles in cells is developed in order to study the dynamical approach of this equilibrium. Inherently time-dependent phenomena, such as matching of the beam into the plasma and adiabatic lenses, are then examined with this method.

Microwave generation from an axially extracted virtual cathode oscillator with a guide magnetic field

The operation of a virtual cathode oscillator (vircator) with strong axial magnetic field has been experimentally studied. Depending on the cathode–anode gap and cathode diameter, the operating voltage varies from 200 kV up to 480 kV with 2–7 kA diode current. Microwave emission is produced by the oscillating virtual cathode. The central microwave frequency follows the beam plasma frequency. It varies by 11.5 GHz up to 22 GHz, depending on the current density. The oscillation frequency does not depend on the guide magnetic field magnitude. A maximal output power of 15±5 MW in asymmetric transverse magnetic (TM) modes is achieved by the axially extracted vircator. Variation of the magnetic field intensity in a range of 0–40 kG has an insignificant effect upon the emitted microwave power. An electron beam power to microwave power conversion efficiency of approximately 1% is obtained.

A liquid-metal free surface in a sliding electrical contact with a strong magnetic field

A low-resistance electrical contact is provided by a liquid metal in a small gap between the perimeter of a rotating disk (rotor) and a static surrounding surface (stator). The liquid metal extends radially inward on both sides of the rotor to free surfaces with an inert cover gas, and there is a strong axial magnetic field. This paper presents results for the shape of the free surface and for the liquid-metal velocity and pressure adjacent to the free surface. The results depend on the magnetic-field strength, the surface tension, the wetting angle at the free-surface–solid intersections, and the voltage difference between the stator and rotor. The copper stator and rotor are perfect conductors compared to the liquid metal. Two cases are considered, with and without electrically insulating coatings on the sides of the rotor.