Table of contents

Current drive by electron cyclotron heating of the far end of a lower-hybrid-sustained electron tail is investigated. The current density profile is determined by means of the kinetic theory and the ray-tracing code for the PLT and the Alcator C parameters. It is shown that the value of the ratio W/I for the combined effects of lower hybrid and electron cyclotron wave absorption can be improved compared to that of the present lower hybrid experiments. Suitable shaping of the current density profiles can be achieved by adjusting the wave-launching parameters.

Confinement studies on the Impurity Study Experiment (ISX-B) in beam-heated plasmas contaminated with small quantities of low-Z impurities are reported. Experimental results on the correlation of particle and energy confinement are presented. A linear relationship of energy confinement and plasma density is observed. As density is increased further, this effect saturates and energy confinement becomes independent of electron density. The experiments have been extended to higher beam power, resulting in an expansion of the ISX-B operating space. Impurities other than neon (carbon and silicon) have been tried and do not produce an enhancement in confinement. Edge cooling by the introduction of impurities has been demonstrated. The change in confinement has been shown to be correlated with changes in the normalized poloidal field fluctuation level (_θ/B_θ) but not with the density fluctuation level (ñ_e/n_e). The experimental results are compared with models of drift-wave and resistive ballooning turbulence and an explanation is offered for the difference between the results with recycling and non-recycling impurities.

A comprehensive analysis of the stability properties of the appropriate kinetically generalized form of MHD ballooning modes, together with the usual trapped-particle drift modes, is presented. The calculations are fully electromagnetic and include the complete dynamics associated with compressional ion acoustic waves. Trapped-particle effects, together with all forms of collisionless dissipation, are taken into account without approximations. The influence of collisions is estimated with a model Krook operator. Results from the application of this analysis to realistic tokamak operating conditions indicate that unstable short-wavelength modes with significant growth rates can extend from β = 0 to values above the upper ideal-MHD critical beta associated with the 'second stability regime'. Since the maximum growth rates of the relevant modes appear to vary gradually with beta, these results support a 'soft' beta-limit picture involving a continuous (rather than abrupt or 'hard') modification of anomalous transport already present in low-beta tokamaks. However, at higher beta the increasing dominance of the electromagnetic component of the perturbations indicated by these calculations could also imply significantly different transport scaling properties.

An investigation is made of the effect of selective ICRF heating of neutral-beam-injected ions. The minority heating scheme as well as the second harmonic heating scheme are considered. Significant ion tail enhancement and subsequent increase in the fusion power multiplication are found. The cut-off in the RF-induced velocity space diffusion at small wavelengths is also investigated.

Measurements of the spatial structure of tokamak edge density fluctuations ñ have been made using compact probe arrays in the Caltech tokamak. The results indicate that the structure of ñ in the tokamak edge is characterized by very short auto-correlation times and short cross-correlation lengths, i.e. that the edge density fluctuations consist of small-scale 'turbulence'. The space-time structure of this ñ turbulence is consistent with several recent non-linear edge instability models. The structure of the local electrostatic potential is measured to be qualitatively similar to that of ñ, and local cross-correlation between ñ and indicates large outward radial particle transport similar to predictions of the edge instability models.

In tokamaks where auxiliary ion heating leads to T_i> T_e, the neglect of electron collisions is no longer a good approximation in determining ion transport coefficients. The enhancement of the ion heat conduction due to electron collisions is determined for (a) the Pfirsch-Schliiter regime and (b) the banana regime for the case where is large. The enhancement of ion transverse viscosity (P|) is particularly important; the contribution due to ion-electron collisions is approximately equal to the ion-ion collision term even for T_i = T_e.

The visible continuum near λ = 5360 Å is monitored during sawtoothing discharges on the Alcator C tokamak. Because of the typically flat continuum emission profiles, effects of internal disruptions on the central-chord continuum brightness are small. However, by averaging over many internal sawtooth periods from similar discharges, sawtooth oscillations on the continuum brightness are observed. The continuum sawteeth obtained closely resemble soft-X-ray sawteeth in both phase and structure. The relative change in the central-chord brightness (ΔB/B) due to the internal disruption is |ΔB/B| ≈ 0.5% ± 0.085%, which is found to be consistent with the predictions of a simple sawtooth model.

Several assumptions made in our previous study of the correlation between the H_α-line emission rate and the ablation rate of a hydrogen pellet injected into a tokamak discharge led us to take the emission layer of the ablatant as optically thin with respect to all levels of the principal quantum number, n. While this may be justified for higher values of n, it requires some consideration about the resonance line, L_α, the Lyman alpha line. To study its possible absorption effect, we shall consider the extreme case that the L_α-line is completely absorbed.

This Conference was made particularly memorable in that experimental results from a new generation of fusion devices, the large tokamaks TFTR and JET, were reported for the first time in this series of conferences. It was also stimulated by its timeliness since many of the existing tokamak devices have reached an elevated state of machine capability and operational efficiency and have succeeded in raising their productivity. Thus, the achieved level of plasma performance parameters such as T, nτ, and nτT has readily surpassed the level of the preceding conferences, and the extent of experimental research carried out has allowed an improvement of plasma understanding and an exploration of the possibilities of new concepts.