Studies on macroinstabilities in a theta pinch with antiparallel magnetic field

Abstract

The plasma column of a theta pinch with an antiparallel trapped magnetic field, has a tendency to break up into several discrete plasma rings. With a smaller theta-pinch apparatus experiments were done to search for mechanisms responsible for triggering such macroinstabilities. Probe ports inserted into the plasma and also external side pockets on the discharge tube cause an early and drastic breakup of the plasma column. But even in smooth tubes discrete plasma rings occur. They could be suppressed by having some discharges with a parallel magnetic field preceding those with a reversed one. This “memory” effect could be explained by the contamination of the inner tube wall by water after each discharge. In this way the plasma column with an antiparallel trapped magnetic field was stable until peak magnetic field. This stability proved to be insensitive to adding amounts of oxygen–as much as 4.6%—to the hydrogen gas and also to lowering the degree of pre-ionization. Finally comparison has been made for the observed formation time of the plasma rings and their number with theoretical predictions. With plasma temperatures derived from the appearance of O IV lines the formation time can be explained by the resistive diffusion time whereas the theoretical predictions of Furth, Killeen and Rosenbluth for tearing-mode resistive instabilities seem rather to give too small values.