Table of contents

After a brief general discussion of double layers, Langmuir's original theory (1929) is described and a review is then given of relevant work carried out in the Department of Engineering Science at Oxford.

Some of the plasma physical problems of intense astrophysical particle streams, which occur in diverse cosmic situations, are reviewed. The author pays particular attention to extragalactic and stellar jets and solar flares.

In a new departure in experimental Space Plasma Physics, lithium and barium plasmas were injected into the solar wind approximately 110000 kms above the Earth's surface. The effects, monitored in-situ by two spacecraft of the AMPTE (Active Magnetospheric Particle Tracer Explorers) mission, included the formation of diamagnetic cavities, a slowing down locally of the solar wind, generation of plasma waves and heating of solar-wind electrons. These comet-like interactions between the solar wind and obstacles presented by the injected plasmas are discussed and compared with the effect of the natural obstacle of the Earth's magnetosphere first encountered by the solar wind at the bow shock. Particular reference is made to the 'artificial comet' created by a barium-ion release on 27 December 1984.

As a first step in the JET ICRF programme, two antenna generator units have been installed at JET and operated up to the design specification of 3 MW coupled in the Torus for 1 second. After a brief description of the system, the experimental results of wave coupling to the plasma and matching the plasma loaded antenna are discussed. The first heating results are presented concentrating on the analysis of a case where the total power to the plasma increases by a ratio of 2.5.

A brief review of the basic principles of the free electron laser is followed by a description of the UK Free Electron Laser presently being developed at the Kelvin Laboratory, Glasgow University, jointly by staff from Heriot-Watt University, Daresbury Laboratory and Glasgow University. The research programme is reviewed and future plans of the project outlined.

Interaction of lower hybrid waves at 1.3 GHz with ions and electrons was studied in the density range 0.2-5*10¹³ cm^-3 in the ASDEX tokamak. At high densities, n_e>or approximately=4*10¹³ cm^-3, fast ions with mainly perpendicular velocities are produced by the RF power at the plasma periphery. They are not well confined and do not lead to any bulk plasma heating. At lower densities, 2*10¹³<or approximately=n_e<or approximately=4*10¹³ cm^-3, electron and ion heating is observed. The heating is better in deuterium than in hydrogen plasmas. At very low densities, n_e<or approximately=2*10¹³ cm^-3, the discharge becomes suprathermal as soon as the RF power is switched on. Launching an asymmetric spectrum of waves in a low density plasma leads to the generation of an RF-driven DC-plasma current.

The modelling used in the boundary layer depends on a number of factors including whether energy loss is dominated by convection through the sheath, by conduction, or by radiation from impurities. A number of criteria are derived for assessing these factors and are then compared with the experimental data from a range of tokamaks. The comparison is limited to tokamaks operating with limiters, although the criteria also apply to tokamaks with divertors. In most tokamaks, under normal operating conditions, the dominant impurity process is sputtering. Impurities from the wall and limiter enter as neutral atoms and undergo ionization at radial positions which depend on the boundary plasma density and temperature profiles. When impurity densities are sufficiently high they can reduce the edge temperature by increasing the radiation level resulting in a feedback effect on impurity production. The impurity source profiles and impurity transport in the boundary are discussed.

The development of laser action in the XUV spectral region is briefly reviewed, with particular attention to the prospective devices operating below the carbon edge. Design constraints on expansion cooled recombination lasers are discussed. Detailed scaling laws for the carbon fibre system are identified, and the constraints on aluminium devices described. Experimental verification of the theoretical modelling is presented.

The author studies the concept of plasma relaxation by magnetic reconnection which has been applied to many laboratory plasmas, but whose origin lies in observations on the Toroidal Pinch. This is one of the simplest of plasma confinement systems. In essence, it involves only a toroidal vessel in which a toroidal magnetic field B₀ is first created by external coils then, after a suitable ionizing process, a toroidal current I is induced. It is this current which is responsible for plasma heating, compression, and confinement.

The prospects for electron cyclotron resonance heating (ECRH) and high power, high frequency gyrotrons are examined. Recent experimental and theoretical progress at MIT on the development of gyromonotrons and gyroklystrons is described. It is shown that CW gyrotrons capable of MW powers and high efficiency are consistent with the technological constraints of the device. The design parameters of 1 and 10 MW, 120 GHz monotrons are given. An ECRH system based on such RF sources is described and compared with alternative plasma heating techniques.