Table of contents

A five fluid MHD stationary model to describe the structure of the current sheath produced in plasma focus devices is presented. Electronic collisional excitation and ionization (with its respective inverse processes) has been included in the treatment and also radiative processes, but with the assumption that the current sheath becomes opaque to line radiation and transparent to continuous (free-free and free-bound) radiation. Numerical solutions of the equations are presented in terms of two parameters: the pressure of the neutral gas and the magnitude of the pushing magnetic field. For a given value of the magnetic field the results of the model are reliable as long as the pressure is smaller than a certain critical value for which recombination radiation to be fundamental level beings to be strongly absorbed by the plasma. For pressures higher than this critical value the results of the model are meaningless and a generalized treatment of the radiation (taking into account radiative transfer equations) should be required.

The transport of charged test-particles in the presence of given magnetic fluctuations and collisions is investigated from two approaches, namely, the Langevin and corresponding Fokker-Planck equations. Analytic discussion of the Fokker-Planck equation shows the radial particle and energy fluxes to be purely diffusive. The Langevin approach confirms the validity of the Fokker-Planck perturbative calculation for sufficiently small fluctuation amplitudes. From the Fokker-Planck theory the authors derive an effective parallel thermal diffusivity valid for all collisionalities, and show that the high collisionality results of Braginskii and the low collisionality form previously conjectured by the authors, are recovered. Application of the theory to TFR lead to qualitative agreement with published observations.

Discusses RF heating of three classes of Tokamak flux surfaces which intersect the electron cyclotron resonance heating region: rational-q, near rational-q and irrational-q. On irrational-q surfaces, the degree of heating is the same for all electrons. One and near rational-q surfaces, heating is localised to closed helical bands of electrons. These hot bands support local enhancements of current density, and act as current-carrying filaments within the plasma. The poloidal magnetic field associated with these currents has X-points. The use of ECRH as a current drive mechanism will enhance these effects, as will an increase in the size of Tokamak major radius.

MHD stability properties of previously calculated equilibria for Extrap type of configurations, generalized to an arbitrary number (N) of external conductors, are investigated. A perturbation technique with a small parameter epsilon measuring the non-circularity of the plasma, is used. An important issue concerns the degeneracy of modes corresponding to various poloidal model numbers m, and some general results, due solely to the angular periodicity of the configuration, are derived. In particular, it is shown that for the mode m=1 the degeneracy prevails to all orders in epsilon if N>2 or, equivalently, if the equilibrium has no quadrupole component. This turns out to be extremely important for the possibility of stabilizing long kink modes, and probably explains why only 'square-shaped' equilibria have been found to be stable in earlier numerical computations. In the four-conductor Extrap, the mode which is most strongly affected by the external field is m=2, for which the degeneracy is removed already in first order. Leading order frequency shifts for the modes m=1 and m=2 are explicitly calculated for surface- and constant-current pinches.

Strong heating and improved confinement of ions have been observed in an axisymmetric magnetic mirror field supplemented with an electrostatic DC field across and along the magnetic field lines. The increased life time of ions is understood as trapping of ions in the electrostatic potential well in the magnetic mirror field. The efficient heating of ions results from thermalization through weak collisions and/or finite correlation time of the strong E*B type instabilities excited by the radial current flowing into the outer electrode. The ion temperature increases in proportion to V_H², the square of the applied DC voltage for heating.

Wave scattering by density fluctuations is enhanced if density fluctuations are localized near the reflection level of the incident wave. The possibility of measuring local density fluctuations from the reflected wave is discussed. Preliminary results on density fluctuations induced by drift waves and MHD modes obtained in the TFR Tokamak with a microwave reflectometer are presented.

The toroidal equilibrium in reversed field pinches (RFP) is usually obtained with image currents in a conducting shell induced by an outward displacement Delta of the plasma. For tight aspect ratio a/R approximately 1/3 (a and R are the minor and major radii) as in HBTX1A the displacement is significant with Delta /a approximately 0.1-0.15 causing a reduction in plasma cross-section area, increasing the average plasma to shell distance and giving plasma wall interaction predominantly on the toroidal outside. The authors describe experiments in which a steady vertical field is used to centre the plasma (i.e. Delta =0). Centering is demonstrated by magnetic measurements with discrete and cosine coils and the radial distribution of soft X-rays. Significant reductions in plasma resistivity, radiation, wall impurities and fluctuations are obtained.

The low frequency wave and instability spectrum of a cylindrical, uniform density, zero beta plasma in a Bessel function magnetic field is analysed. The analysis has application to wave heating of reversed field pinches and energy propagation in, and instabilities of, twisted solar coronal loops. A surface Alfven wave which can be Alfven resonance damped is identified, and its connection to the unstable external kink mode is discussed. A distinctive extra mode is identified as a poloidal fast Alfven wave with wave fields concentrated at the position of zero axial equilibrium magnetic field and with a dispersion relation dependent dominantly on the poloidal wavenumber and only weakly on the axial wavenumber. Finally, the spectrum of discrete Alfven waves, or global Alfven eigenmodes, is also identified.

Values of the total energy confinement time tau _E are presented for ohmic hydrogen discharges on the TEXT Tokamak, spanning a range of chord-averaged densities of 2.0-8.5*10¹³ cm^-3, plasma currents of 150-400 kA, and toroidal fields of 15-28 kG. At the higher densities tau _E exhibits a saturation which corresponds well with a recently proposed scaling law (Goldston, 1984) in which the saturated value is given by an empirical expression for beam-heated limiter discharges with P_tot identical to P_OH. The relationship between this observed saturation and density-profile widths is addressed.

To investigate how torsion of a magnetic axis influences the equilibrium of high beta plasmas the authors have built a figure-eight stellarator consisting of four half tori. The connection angles of the half tori can be altered, so that the angle of the rotational transform of the magnetic field lines can be changed from 0 degrees to 160 degrees . If the angle of the rotational transform is small, the plasma column, produced by a theta pinch, drifts away and finally is reflected at the tube wall. In contrast, for a larger angle of the rotational transform there are indications that an equilibrium state may be obtained after a certain degree of radial drift. This difference may be qualitatively explained by the fact that magnetic surfaces exist only in the latter case.