Table of contents

The non-linear behaviour of the collisionless drift instabilities and the resultant anomalous plasma diffusion have been studied by means of computer simulations and analytical theory. The simulation model used is a full three-dimensional electrostatic model of cylindrical geometry in an external magnetic field. Full dynamics is employed for ion motion while guiding-centre approximations are used for electron motion, which allows us to use rather realistic plasma parameters in the simulations. The simulation results indicate that a strong turbulence develops through non-linear interaction of the drift instabilities, which results in the formation of convective cells and anomalous particle diffusion. The broad frequency spectrum resembles that observed recently in toroidal confinement devices. Analytical theory is developed, based on the modecoupling process, to explain the generation of convective cells and strong plasma turbulence, along with an estimate of the resultant particle diffusion.

The authors study the hypothesis that sawtooth oscillations or internal disruptions are the result of a cyclic process in which the plasma core is resistively heated until the safety factor drops below unity, causing the m = 1 tearing mode to become unstable, to grow with an accelerating growth rate, and ultimately to flatten the electron temperature and safety factor profiles. A model based on this hypothesis compares favourably with experimental data from the Oak Ridge Tokamak (ORMAK) in explaining (1) the rate at which a sawtooth rises, (2) the radial dependence of the precursor and main sawtooth oscillation amplitudes, (3) the accelerating growth of the m = 1 precursor oscillations, and (4) the repetition time of the sawteeth. The heat lost from the central region during the internal disruption is found to transport diffusively through the exterior plasma with a conduction coefficient that agrees within a factor of two with the value inferred from the observed electron power balance.

A kinetic equation is derived for fast electrons in toroidal, tokamak- or stellarator-type magnetic devices. It is of a different type for different groups of electrons, i.e. passing or trapped electrons, in a magnetic field. The solution of the kinetic equation is found, and analytic expressions for the electron distribution function are obtained for the case of an ideal (non-rippled) torus and an electric field weaker than the critical Dreicer field. The runaway flux of electrons is found. The case of a rippled torus is investigated by taking account of the vertical drift resulting in electrons blocked in the ripples being lost to the chamber walls. Considerable plasma loss is shown to be possible only for electrons of an energy exceeding some quantity _d, which depends on the magnetic field, the depth of the ripple, the plasma density, and the parameters of the device.

Tokamak plasma diagnostics is reviewed. Subjects are classified according to the physical quantities to be measured. Instrumental techniques and theoretical considerations are developed at length only when the development in question is relatively new. Present trends are emphasized, as well as diagnostic limitations and needs for future large tokamaks.

A set of ring conductors placed inside a solenoid is being proposed as an efficient end-stopper. The rings can stabilize strong end-mirrors by creating a minimum-B situation, and via non-adiabatic effects they will tend to trap the end-loss plasma. Some of the properties of such configurations are examined.

Sawtooth oscillations appear to be the result of a cyclic process in which the plasma core is resistively heated until the safety factor drops below unity, causing the m = 1 tearing mode to become unstable and ultimately to flatten the electron temperature and safety factor profiles. A systematic model based on this hypothesis is shown to provide a consistent interpretation of ORMAK experimental data, including for the first time an explanation of the accelerating growth rate of the m = 1 precursor oscillations and the repetition time of the sawteeth.

An asymptotic expansion for the electrostatic potential in cold plasma due to excitation near the lower-hybrid frequency is developed which is valid in the presence of two-dimensional field and density inhomogeneities. The lowest-order terms of the expansion yield the resonance cone surfaces and WKB amplification factors associated with finite-sized sources. The technique is applied to investigate phased waveguide array excitation.