Table of contents

The authors examine the magnetic configuration of a one-turn helical magnetic field in the presence of an axial current. Particular attention is paid to the properties of the rotational transform of the magnetic field lines and to the minimum of the mean magnetic field. A relationship is found between negative V"(ø) and shear, on the one hand, and the structural features of the configuration, on the other.

It is shown at what parameter values the configuration in question possesses closed magnetic surfaces surrounding the helical axis, an angle of rotational transform, shear and a minimum of the mean magnetic field.

The authors investigate the non-linear excitation of ion-acoustic oscillations in a plane plasma layer contained by an external high-frequency electromagnetic field of the TE wave type. The plasma is assumed to be collision-free and non-isothermal (T_e » T_i). The time-averaged density distribution in the plasma layer is found. The conditions are ascertained for the resonance excitation of ion-acoustic waves at the second harmonic of the external field; near the resonance, the amplitudes of the excited waves can be fairly large, not only within the skin layer but also through the entire plasma volume.

The article is a theoretical study of ohmic resistance and the skin effect in a plasma confined by an axially symmetric magnetic field. Simultaneous solution of the drift kinetic equation and the quasi-steady-state Maxwell equations shows that, when the electron free-path lengths exceed the scale of the magnetic-field inhomogeneity, strong corrugation leads to a considerable rise in ohmic resistance and skin-layer thickness. An analytical expression is found for the electrical conductivity of the plasma and the thickness of the skin layer which takes account of both electron-ion and electron-electron collisions. The spatial distribution of current density has been studied over a wide frequency range. The results obtained here are of interest for experiments on containment and heating of plasma in traps with corrugated fields.

Experiments with a magnetic well using a gun-injected plasma are described. Typical parameters are B_z ≃ 1.8 kG, n = 5×l0¹² cm⁻³, T_i = 1.5 keV, T_e = 15 _eV and length of trap = 1.5 m. A wide range of diagnostics have been used including atomic beams of potassium, krypton and xenon for measuring ∫ n dℓ and T_e. An initial rapid loss of plasma is associated with the method of filling the trap which results in an ion velocity space distribution peaking near the loss cone angle; however, detailed analysis shows that the ion scattering process is not caused by Coulomb collisions although the energy transfer rate between ions and electrons is not inconsistent with classical collisions. Electric field fluctuations of ∼ 1 V · cm⁻¹ near ω_ci are observed.

Evaluations are given of the level of turbulent pulsations in a plasma with current flowing perpendicular to the magnetic field, in which longitudinal oscillations are excited with characteristic frequency

Re ω ∼ Im ω ∼ min{ ω_pi} ≫ ω_Hi

It is shown that in such a plasma it is possible to have an effective stochastic heating of ions and electrons resulting in an increase in the transverse ion temperature and longitudinal electron temperature.

We give a fluid description of a uniform, bounded, hot ion-electron plasma which includes collision effects and a full electromagnetic treatment. The absorbed r.f. power is calculated in the magneto-ion domain for a plasma column excited by a cylindrical TE wave. Axisymmetric and multipolar azimuthal excitation are considered. A resonance spectrum is obtained which is strongly dependent on the geometry and on the hot character of the plasma. These results are compared with those of an earlier collisionless quasi-static model, and the influence of the various parameters is systematically investigated. The r.f. power absorbed at some well-chosen resonances can be quite considerable when the intervening parameters are optimized.

The following theorem is proved in the article: An MHD-unstable configuration with a dissipationless plasma surrounded by vacuum and possibly superconducting walls cannot be stabilized by introducing walls of finite electrical conductivity.

The laminar wave-train structure of collisionless magnetic slow shocks is investigated using two-fluid hydromagnetics with ion-cyclotron-radius dispersion. For shock strengths less than the maximally strong switch-off shock, in the shock-leading edge, dispersive steepening forms a magnetic-field gradient, while in the downstream flow dispersive propagation forms a trailing wave train; dispersion scale lengths are the ion inerrial length if β > 1 and the ion cyclotron radius if β > 1. In the switch-off slow-shock leading edge, dispersion only produces rotations of the magnetic-field direction; the gradient of the magnetic-field magnitude, and hence the shock-steepening length, is determined solely by resistive diffusion. The switch-off shock structure consists of a long trailing train of magnetic rotations which are gradually damped by resistivity. The low-6 parallel fast switch-on shock has a similar wave-train structure with the magnitude of the field rotations gradually increasing toward the downstream flow.

The theoretical analysis of the propagation of magnetohydrodynamic waves in a cylindrical plasma immersed in a steady inhomogeneous magnetic field is here undertaken by assuming a model field of two constant field regions joined by the field = A∇(l/r). We thus consider 'beach' and 'cliff' magnetic field configurations. The general solution of the coldplasma equations in the central (inhomogeneous field) region must be found numerically, but in the low-frequency case analytic solutions are obtained. The wave reflection and transmission found by applying boundary conditions at the constant field.

Solutions are so obtained for a variety of cases, including simple reflection of the compressional and torsional modes, reflection of the compressional mode when the cutoff condition is encountered, absorption of the torsional mode at the ioncyclotron resonance, and conversion of one mode into the other after propagation through both of the above conditions. Finally, comparison is made with several recent experiments, which shows that the above theory is sufficient in the case that the wave frequency is not too close to the resonance frequency.

The case of reflection of compressional waves from the cutoff shows good agreement between theory and experiment, except in the high field region, where apparently the waves are not confined by the magnetic field lines. Reflection of torsional waves close to the resonance condition shows disagreement in the transition region, where the experimental wave energy is as great as 4 times as large as that predicted both by the above theory and the WKB method.

The effect of an r.f. electric field of frequency Ω, oriented parallel to a static magnetic field, having a gradient in the radial direction of the plasma column, on the drift dissipative instability is studied using a two-fluid macroscopic approach. This method is identical to that developed by two of the authors in their 1969 paper on the stabilization of drift oscillations by a uniform r.f. field. When Ω is greater than the collision frequency of the electrons, the presence of a gradient of the r. f. field reduces the minimum value of this field required for complete stabilization by more than an order of magnitude.

The paper deals with the propagation of fast magneto-acoustic waves in a low-density plasma (p_B ≫ p) situated in a magnetic field created by two parallel or anti-parallel currents.

The transparence of different regions of the magnetic trap for magneto-acoustic waves is studied. The existence of ring-like traps for magneto-acoustic waves is stated.

A translation of the abstracts of all the articles for this issue into French, Russian and Spanish.