Model of a disk magnetohydrodynamic channel using pure inert gases and Faraday current commutation

The purpose of this experiment was to find a way to increase the effective conductivity in a nonequilibrium MHD channel. The working material used was inert gas without admixtures of alkali metals. The experiment was carried out in a disk MHD channel coupled to a shock tube, with radial expansion of the gas and an axial magnetic field. An inset was located in a sector of the disk channel with electrodes, to which a load was connected to extract power into an external circuit. Voltage-current characteristics were recorded in this experiment along with the structure of luminous nonuniformities. The results of the experiment showed that as ionization instabilities develop, the effective conductivity of the plasma increases with increasing magnetic induction, and the extracted power density for fields above critical turns out to be roughly 30% higher than that calculated by assuming that the conductivity is a constant. Zh. Tekh. Fiz. 67, 12–15 (December 1997)     

Effect of unstable MHD modes on the confinement of a stellarator plasma

The results of an experimental study of the influence of unstable MHD modes on plasma confinement in an L-2M stellarator are presented. The spectral and statistical characteristics of turbulent plasma simultaneously at both the edge and center of the plasma are investigated. It is shown that at constant power of electron-cyclotron heating of the plasma the energy content of the plasma depends strongly on the strength of the externally applied vertical magnetic field used to adjust the position of the plasma column. Appreciable degradation of plasma confinement is observed for values of the vertical field such that ideal MHD modes become unstable in the greater part of the plasma column. This in turn is due to the formation of a magnetic configuration with a magnetic “hump.” At the same time, in the edge plasma the instability of resistive-balloon modes grows, and turbulent particle transport increases. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 6, 407–412 (25 March 1999)     

Coherent transients in gases

A great variety of coherent transient processes in atomic and molecular gases is investigated. The degeneracy with respect to magnetic sublevels, which is typical of working levels of atomic and molecular transitions in gases, leads to new phenomena. In particular, the non-Faraday rotation of the coherent-response polarization vector at the 0 ↔ 1 transition is investigated in the presence of the longitudinal magnetic field in the ¹⁷⁴Yb vapor for both photon echo and stimulated photon echo. A specific relaxation channel that involves the depolarizing collisions emerges in a gas due to the degeneracy of working levels. Such collisions are especially important for atoms and are also observed in molecules. The anisotropy of the depolarizing collisions leads to a new phenomenon of the collision photon echo, which is experimentally demonstrated for the 0 ↔ 1 transition in the ¹⁷⁴Yb mixtures with atomic buffers. The velocity-dependent relaxation rates are observed for various coherent phenomena in molecular gases. The coherent control of a few coherent transient processes is implemented.     

On the electron mobility of δ layers in the presence of diamagnetic “ejection” of size-quantization levels

The Hall mobility of electrons is investigated as a function of the population of size-quantization subbands in the two-dimensional electron gas of a δ-doped layer in GaAs with constant total electron density N _s =3.2×10¹² cm⁻² (three initially filled subbands) at T=4.2 K. The population of the subbands is varied by diamagnetic “ejection” of size-quantization levels (i.e., pushing them over the Fermi level) by a magnetic field oriented parallel to the plane of the δ-doped layer. The measurements are made in magnetic fields making small angles (5°) with the plane of the doping. The magnetic field component normal to the plane was used to measure the Hall mobility and density. It is found that the measured Hall mobility as a function of the ejecting magnetic field has a distinct maximum. This maximum is due to an increase in the electron mobility in the first subband (the ground subband is assigned the index 0) and electron redistribution between subbands with in increasing ejecting magnetic field parallel to the plane of the δ layer. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 704–708 (10 December 1997)     

Design of Closed-Cycle MHD Generator with Nonequilibrium Ionization and System

A closed-cycle MHD generator topping a steam bottoming plant is analyzed. The combined power plant involves three working fluids in three loops. The MHD loop is investigated more thoroughly since it is the least conventional of the three. Equations are developed to determine the geometric and thermodynamic variables throughout the MHD channel for inlet conditions of mass flow, temperature, pressure, and velocity. Limiting design parameters are output power, channel length, channel aspect ratio, Hall parameter, and interaction parameter. The basic closed-cycle MHD loop working fluid can consist of either argon or helium seeded with cesium. Both non-equilibrium ionization produced by the elevation of the electron temperature from joule heating of the plasma and thermal ionization are considered. Equations used to calculate the electrical conductivity and the elevation of electron temperatures are derived. These equations are coupled with the one-dimentional differential equations applicable to an MHD generator. The chief interest is in determining those MHD channel conditions which result in the most thermodynamically efficient MHD-steam plantcombination. Thus an overall heat balance forthe system is required. Equations are developed to calculate the gas properties at the various stations of the closed loop and to determine the overall efficiency of the cycle. A rather flexible computer program written in Fortran is used to solve the MHD generator equations and to make the overall heat balance. Some typical results presented demonstrate the feasibility and adaptability of the analysis for optimizing the thermal efficiency and the sensitivity of thermal efficiency to various parameters.     

Cold-hollow-cathode arc discharge in crossed electric and magnetic fields

A crossed-field cold-hollow-cathode arc is stable at low working gas pressures of 10⁻²–10⁻¹ Pa, magnetic-field-and gas-dependent arcing voltages of 20–50 V, and discharge currents of 20–200 A. This is because electrons come from a cathode spot produced on the inner cathode surface by a discharge over the dielectric surface. The magnetic field influences the arcing voltage and discharge current most significantly. When the plasma conductivity in the cathode region decreases in the electric field direction, the magnetic field increases, causing the discharge current to decline and the discharge voltage to rise. The discharge is quenched when a critical magnetic field depending on the type of gas is reached. Because of the absence of heated elements, the hollow cathode remains efficient for long when an arc is initiated in both inert and chemically active gases.     

Three-dimensional structures of MHD flow in a disk generator

Three-dimensional nonuniform plasmas and boundary layers have been studied numerically under an MHD interaction. The nonuniform plasma of “streamer” owing to weak ionization of seed material has a spiral structure in the r-&thetas; plane, and the plasma becomes almost uniform between the walls in the r-z plane. This structure is almost the same as that in our previous paper (1997), where the gas (heavy particle) properties are assumed to be invariant and steady. In addition to the streamer, the nonuniform plasma of “domain” owing to weak ionization of noble gas is revealed. The domain has the structure perpendicular to the streamer. In a strong MHD interaction case, the static pressure considerably increases in the upstream region of a generation channel, and the pseudo-shock waves appear in the generator, but the plasma is almost uniform along the &thetas; direction. The boundary layer in the strong MHD interaction is considerably thick, and in the broad region near the wall the Hall current flows reversely. In the weak MHD interaction case, the plasma forms a nonuniform structure along the &thetas; direction, and the Hall current does not always flow in the opposite direction even on the insulator wall since the azimuthal electric field is not zero     

Galvanomagnetic phenomena in layered conductors

The dependence of the resistance and the Hall field in a layered conductor with a quasi-two-dimensional electron energy spectrum of arbitrary shape on the magnitude and orientation of the magnetic field in relation to the layers is analyzed. It is found that when current flows perpendicular to the layers, the resistance of the specimen strongly depends on the angle ϑ between the normal and the vector of a strong magnetic field. The Kapitza law is shown to hold within a fairly broad range of magnetic fields in the plane of the layers, i.e., the resistance increases linearly with the magnetic field strength. The Hall field proves to be insensitive to the emergence of open sections of the Fermi surface, and the Hall constant in strong magnetic fields is the same for any orientation of the magnetic field and the current. Zh. éksp. Teor. Fiz. 112, 618–627 (August 1997)     

Current drive for rotamak plasmas

Experiments which have been undertaken over a number of years have shown that a rotating magnetic field can drive a significant non-linear Hall current in a plasma. Successful experiments of this concept have been made with a device called rotamak. In its original configuration this device was a field reversed configuration without a toroidal magnetic field but with a vertical field to establish the MHD equilibrium. However, modifications have shown that current can also be driven if a central current-carrying rod is used to provide an applied toroidal field. The new rotamak has then a spherical tokamak magnetic field structure. Article presented at the International Conference on the Frontiers of Plasma Physics and Technology, 9–14 December 2002, Bangalore, India.     

Anisotropy of the upper critical field and specific heat in V3Si — a superconductor with cubic symmetry

The specific heat of a V₃Si single crystal (T _c=17 K, H _c2=20 T) in magnetic fields up to 8 T isinvestigated experimentally for three orientations of the field relative to the crystallographic directions — H∥〈001〉, H∥〈110〉, and H∥〈111〉. Both the upper critical magnetic field and the specific heat of the mixed state are observed to depend on the orientation of the magnetic field relative to the crystallographic directions (anisotropy): The critical field reaches its maximum value and the specific heat its minimum value in a field along the 〈001〉 direction. The anisotropy scale in both phenomena increases as the magnetic field and reaches 3% in a 6 T field. The interrelationship of the upper critical field anisotropy and the specific-heat anisotropy in type-II superconductors is studied. It is shown that the anisotropy of the specific heat in the mixed state in weak fields can serve as a criterion for nontrivial pairing. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 26–29 (10 January 1999)     

Hall effect and negative magnetoresistance in thin crystals of NbSe<Subscript>3</Subscript>

We have measured the Hall effect and the transverse magnetoresistance in NbSe₃ single crystals. In the liquid helium temperature range we observed an absolute negative magnetoresistance (NMR) — the value of the resistance under magnetic field being much lower than that at zero field — in NbSe₃ single crystals with a thickness less than 5 μm with the magnetic field oriented in the (b, c) plane. We show that this NMR effect is observed in the magnetic field range in which the Hall constant changes its sign. The results are qualitatively explained by the change of the surface scattering contribution to the magnetoconductance in the magnetic field range near the Hall voltage zero crossing.     

The role of helicity in turbulent MHD flows

We have studied the behavior of a helical homogeneous small-scale MHD turbulent flow under the influence of a weak inhomogeneous large-scale disturbance. We have shown that turbulent energy redistribution in the presence of nonzero helicity occurs mainly over large scales. Helicity increases correlation time, leading to the weakening of a direct cascade and to the formation of steep spectra over small scales, with simultaneous turbulent energy growth over large scales. Furthermore, an expression for the effective viscosity of the mean flow is derived. It is shown that the magnetic field, in addition to the helicity, reduces the effective viscosity of the medium. This may be important in the study of MHD flow around obstacles in the presence of an external magnetic field. Zh. éksp. Teor. Fiz. 114, 171–181 (July 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Out-of-plane bipolar and quadrupolar magnetic fields generated by shear flows in two-dimensional resistive reconnection

Shear flows perpendicular to the anti-parallel reconnecting magnetic field are often observed in magnetosphere and interplanetary plasmas, and in laboratory plasmas toroidal differential rotations can also be generated in magnetic confinement devices. Our study finds that such shear flows can generate bipolar or quadrupolar out-of-plane magnetic field perturbations in a two-dimensional resistive MHD reconnection without the Hall effects. The quadrupolar structure has otherwise been thought a typical Hall MHD reconnection feature caused by the in-plane electron convection. The results will challenge the conventional understanding and satellite observations of the signature of reconnection evidences in space plasmas.     

A direct connection between quantum Hall plateaus and exact pair states in a 2D electron gas

It is previously found that the two-dimensional (2D) electron-pair in a homogeneous magnetic field has a set of exact solutions for a denumerably infinite set of magnetic fields. Here we demonstrate that as a function of magnetic field a band-like structure of energy associated with the exact pair states exists. A direct and simple connection between the pair states and the quantum Hall effect is revealed by the band-like structure of the hydrogen “pseudo-atom”. From such a connection one can predict the sites and widths of the integral and fractional quantum Hall plateaus for an electron gas in a GaAs-Al _x Ga_1−x As heterojunction. The results are in good agreement with the existing experimental data.     

Development of ionization in nonequilibrium inert-gas plasmas in magnetogasdynamic channels

Effects accompanying the interaction of a flow of preionized inert gas with a magnetic field are studied: selective electron heating, the development of nonequilibrium ionization, and the onset of the ionization instability. Local and average densities and temperatures of the electrons are measured and the average ionization rate is determined. It is found that the average electron density increases as the magnetic induction is raised, in both stable and ionization unstable plasmas. The difference in the rates at which ionization develops in these two states is revealed. The mechanism for the coupling between the average ionization rate in an ionization unstable plasma and the spatial-temporal characteristics of the plasma inhomogeneities is established. Zh. Tekh. Fiz. 69, 56–61 (November 1999)     

Hall effects on steady hydromagnetic flow in a rotating channel in the presence of an inclined magnetic field

Effects of Hall current on a steady hydromagnetic (MHD) fully developed flow in a rotating environment within a parallel plate channel in the presence of an inclined magnetic field is studied. From an extension of literature [13] subject to a forced oscillation it is observed that the present paper is methodically more correct to work first in the steady state where forced oscillation becomes insignificant and then new results are expected for an unsteady MHD flow under the influence of a pulse-oscillator. Exact solutions of the governing equations are obtained in a closed form. The graphical representation for the velocity and the induced magnetic field are depicted graphically and the heat transfer at both the plates are presented in tables.     

Hall resistance anomalies induced by a radiation field

A study is made of the four-terminal Hall junction under the influence of a radiation field. The frequency of the radiation field is tuned to a transition between the energy of a bound state below a conduction subband and the Fermi energy of the incident electrons. Radiation-field-induced resonant dips of the Hall resistance are exhibited at low magnetic fields. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 403–408 (25 September 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Experimental observation of cyclotron superradiance

Intense (several hundreds of kilowatts) subnanosecond coherent microwave radiation — cyclotron superradiance of an electron swarm moving in a uniform magnetic field — was recorded. The maximum power of the radiation was observed under group synchronization conditions, when the translational velocity of the swarm is equal to the group velocity of the waves in the waveguide channel. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 322–325 (10 March 1996)     

Energy absorption by a magnetic nanoparticle suspension in a rotating field

Heat generation by viscous dissipation in a dilute suspension of single-domain ferromagnetic particles in a rotating magnetic field is analyzed by assuming that the suspended particles have a high magnetic rigidity. The problem is solved by using a kinetic approach based on a rotational diffusion equation. Behavior of specific loss power (SLP) as a function of field strength H and frequency ω is examined at constant temperature. SLP increases as either of these parameters squared when the other is constant, eventually approaching a saturation value. The function SLP(H, ω) can be used to determine optimal and admissible ranges of magnetically induced heating.     

Effects of Hall current on unsteady MHD flows of a second grade fluid

The aim of this present paper is to construct exact solutions corresponding to the motion of magnetohydrodynamic (MHD) fluid in the presence of Hall current, due to cosine and sine oscillations of a rigid plate as well as those induced by an oscillating pressure gradient. A uniform magnetic field is applied transversely to the flow. By using Fourier sine transform steady state and transient solutions are presented. These solutions satisfy the governing equations and all associated initial and boundary conditions. The results for a hydrodynamic second grade fluid can be obtained as a limiting case when B ₀ → 0 and for a Newtonian fluid when α ₁ → 0.