Table of contents

An account is given of measurements on fluctuations occurring in the Sceptre III and Sceptre IV discharges, as recorded by time-resolved spectroscopic techniques and by magnetic and Langmuir probes. Systematic properties of repeatability and movement parallel or anti-parallel to the direction of current flow were identified in some cases and measurements of these properties are presented. It is shown that the motion of fluctuations in magnetic field and in spectral line emission are in opposite directions to each other in different regions of the discharge; these results are discussed in the light of recent theoretical work.

Consideration is given to the types of MHD instability expected in the diffuse toroidal pinch discharges observed in the Sceptre experiments. In the central region where there is little or no magnetic shear but an appreciable negative pressure gradient, the instability will be either a pure interchange mode, or a quasi-interchange mode with properties very similar to the pure interchange. In the outer region where there is some magnetic shear, a kink instability is expected. The wave velocity which will be imparted to the interchange mode due to the finite Larmor radius effects is computed with all the first-order effects included. In the few cases where sufficient experimental data is available, there is agreement in sign and magnitude with the observed velocities for fluctuations in the discharge core. There is insufficient experimental data for the outer region, but the wave velocity imparted to the kink instability due to the Hall effect only is shown to be sensitive to the orientation of k with respect to B. This explains why the observed wave velocity is often of opposite sign in the outer region.

In the simple case of a cylindrical plasma with superficial current shear, a new instability due to a coupling between compressibility and resistivity has been found. Near acoustic velocity phase, the growth rate of this instability is in η 1/4. In toroïdal systems, this instability can appear under Kruskal limit, even for the mode m = 1, for which the stability limit is 1/2 in i/2π units, and the stability limit for m > 1 modes is the closer to 0 the greater is m.

The damping length for an ion cyclotron wave in the neighbourhood of its resonance is known in two limiting cases: Landau-like damping in a collisionless plasma (as found, for example, by Stix): and collisional damping in a zero-temperature plasma.

We have used a simplified model for collisions (in the manner of Gross and Krook) to effect an investigation which includes both finite temperature and collisions. It is found that the limiting cases above are joined by a transition region in which damping depends on both effects - i.e. a region where the damping length is given by

l ∼ c((κT/m_ec²)(1/ω_p²λΩ))^1/4

λ being the electron-ion collision frequency, and Ω the ion cyclotron frequency.

Numerical results in ranges of experimental interest are presented in graphical form for the potential profile around negatively biased spherical and cylindrical probes in a collisionless plasma and for the saturation ion current-voltage characteristics. The computations were made on the basis of the theories of Allen, Boyd and Reynolds (1957) for zero ion temperature and of Bernstein and Rabinowitz (1959) for monoenergetic ions. These theories are useful primarily for small probes. For large probes the theory of Lam (1964) is applicable. For completeness we have also included whatever curves are necessary for the use of Lam's theory.

For the potential equation of the two-dimensional stationary potential flow of a plasma of infinite conductivity, a method of characteristics has been derived which is set up in analogy to the Prandtl-Busemann method of gas dynamics.

The results are described of an investigation of non-thermal microwave radiation from the plasma of an electrodeless induction discharge. A correlation between the microwave and the X-ray radiation is established. The microwave radiation appears at the instant of formation of the critical density in the plasma, for the frequency of detection. A suggestion is made concerning the appearance in the plasma of a beam instability, leading to the excitation of electron plasma oscillations which become transformed into transverse electromagnetic waves.

The effect of non-linear effects on electron cyclotron resonance is considered. Special attention is given to determining the maximum velocity acquired by the plasma electrons under cyclotron resonance conditions.