No abstract

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 number of new methods are discussed for determining the electron heat conduction coefficient χe in a tokamak from the experimental observation of the space-time evolution of the temperature perturbations induced by internal disruptions. In the Oak Ridge Tokamak (ORMAK) the various average values of χe and the radial dependence of χe are found to be consistent with and more precise than the χe(r) determined by conventionally analysing the electron power balance equation. The net result of these measurements is to conclusively prove that the dominant radial electron heat transport mechanism in tokamaks is a microscopic, diffusive process.