Table of contents

An equation describing the evolution of 2-D inhomogeneous magnetized plasma has been derived. In the cold ion case, it is merely a Euler equation or convective cell equation. There is a solitary flute vortex solution to it. Moreover, the structure of the solution is similar to that of the oblique vortex solution obtained by Makio et al. (1981) and Meiss and Horton (1982, 1983), but its properties differ from the latter in several aspects. There is no restriction on the speed of the vortex and it can propagate in any direction in a plane with magnetic field B as it is normal, although the effect of a warm ion makes the vortices propagate anisotropically in the plane. The plasma appears to be adhered to vortices and moving along with them. This means that the existence of flute vortices could be connected with convection of particles and energy of plasma across magnetic surfaces.

The linear and nonlinear characteristics of pressure-driven magnetohydrodynamic instabilities are examined for a high-beta ( beta _T=4.5%) limiter discharge and for a typical high confinement time diverted discharge in Doublet III. These discharges are found to be linearly unstable to high-beta external kink-tearing modes even with the stabilizing influence of a conducting shell and a conducting mantle. Nonlinear calculations show that tearing mode islands are too small to overlap, possibly related to the experimentally observed stability against disruption at the discharge time of interest.

The effects of plasma density inhomogeneities on the growth of Rayleigh-Taylor instability of an ablatively accelerated inertial confinement fusion target are studied part analytically. The density profile is simulated by means of suitable exponentials in various spatial regions of interest and analytical equations, determining the growth rate of the instability, are derived. Results for the fastest growth rate are presented and discussed for a wide class of realistic profile parameters. A reduction in the growth rate of the instability is predicted particularly at low wavenumbers. A comparison of the present results with the analytical results for a simple step-wise density profile indicates that this reduction in the growth rate may be attributed to the finite density gradients present.

Electron density and temperature measurements by repetitively pulsed 90 degrees Thomson scattering from a steady state, low-density plasma with superimposed magnetic field are reported. The density is calibrated by Rayleigh scattering from argon; a pressure scan shows that the stray-light signal is the same as the Rayleigh scattering from 2.7 Pa argon. The detection sensitivity of the Thomson experiment reaches 10¹⁰ electrons and an accuracy of 10% is achieved by a nonlinear three-parameter fitting procedure. Electron data are presented resulting from measurements in the middle of the arc. To study plasma-wall interaction the vicinity of the plane anode is investigated, too.

Small amplitude electron acoustic double layers have been discussed for the first time in a plasma containing two electrons and one ion species. Rarefactive and compressive DL solutions have been found to exist when the hot component of the electrons are considered isothermal and non-isothermal respectively.

Ion drift and ion cyclotron drift waves have been observed in the density gradient of a plasma column. Launching RF near the lower hybrid frequency into the plasma results in a suppression of the ion drift waves and a parametric amplification of the ion cyclotron drift waves. Spectral analysis of probe signals monitoring the plasma density fluctuations demonstrates the importance of three wave coupling for the saturation of the observed instabilities.

Drift wave instability in a magnetic quadrupole is analysed. Formulas are derived for the growth rate due to a dissipative trapped-electron mechanism and the damping rate due to ion viscosity. A numerical study is made of the drift waves which occur spontaneously in the shared flux region of the UMIST quadrupole. The calculated growth rate is of the same order as the experimental estimates but the viscous damping is not sufficiently large to account for the upper limit on the axial wave number of the observed spectrum.

Measurements are presented of electromagnetic radiation emitted perpendicular to a strong magnetic field in a gas discharge. The discharge plasma is characterized by electron densities in the range 10¹⁰-3*10¹² cm^-3, an electron temperature of approximately 2 eV, and a dilute energetic electron tail extending out to the discharge voltage (<or approximately=200 V). Emission measurements are made in the frequency range 12-18 GHz. It is shown that the emission is predominantly in the extraordinary mode, originates at the upper hybrid layer in the radially inhomogeneous plasma, and is generated by the energetic electron tail. Combined with earlier detailed measurements of the energetic electron properties it is shown that the emitted power varies linearly with the hot electron density in the vicinity of the upper hybrid radius and varies as the 3/2 power of the tail temperature. Absolute power estimates are provided to link the authors measurements to the fundamental emission parameters. There are indications that tunneling of radiation is not playing a role in these experiments, but this conclusion requires a lot of further study.

A variational principle for a cold plasma, formulated by Taylor (1974), minimizes the magnetic energy of a cylindrical plasma confined by a infinitely conducting metal wall, subject to the constraints of constant longitudinal flux and magnetic helicity. The paper extends this minimum energy principle to the case of a vacuum layer separating the plasma from the wall. The ensuing shift of the F-I-curve is in qualitative agreement with the experiments.

The ideal magnetohydrodynamic equations are used to investigate the equilibrium configurations of a plasma when it is subjected to an external rotating magnetic field. It is shown that the rigid rotor model for the steady toroidal current is an exact solution of the equations provided that there are no oscillating secondary currents within the plasma and that there is no toroidal component of the steady magnetic field. Experimental observations of rotamak plasmas indicate that these conditions are often not satisfied. The rigid rotor model may therefore not be applicable to these plasmas. A perturbation method is used to generalise the theory to include the effects of small oscillating secondary currents and small toroidal magnetic fields.

An experiment for calibrating the high-frequency Stark effect in a turbulent plasma is presented. The electrostatic turbulence is created by injecting an electron beam into a pulsed low pressure arc discharge. Microwave measurements of the unstable wave yield information on the validity of the applied theoretical model and permit a determination of the field strengths. Spectroscopic observation of plasma satellites as well as of an enhanced inelastic excitation of spectral lines confirm these values.

Scaling laws for the ablative corona generated by the incidence of an intense light ion beam on a spheric solid target are obtained assuming that a self-regulating regime of the corona opacity is created. The results are in agreement with a recent numerical model.

Attention has been drawn to a spatial filtering method for plasma density profiling. This method does not need interferometric set-up and gives a derivative of the phase shift directly.

The collisional Weibel instability growth rate is calculated for an overdense plasma slab heated by inverse-bremsstrahlung. Results are obtained for a background electron distribution function from a Fokker-Planck code and from linear transport. The former indicates inhibition in the growth rate close to critical density and enhancement near the foot of the heat front.