Particle drifts and the transition to containment in the LT-3 Tokamak

The Canberra Tokamak LT-3 exhibits a delay between the application of the toroidal electric field and the main rise in gas current indicating bulk ionization of the gas. During this "prebreakdown" phase the electric field is high and the current is dominated by runaway electrons. The rotational transform is very low, however, and the condition for compensation of transverse particle drifts is not satisfied. A theoretical model is considered which includes only the curvature drift of the runaways. This provides estimates of the maximum energy an electron achieves before it is lost to the walls and a critical condition on the current when the transition to particle containment and total gas breakdown should occur. Measurements of runaway electron energies and gas currents in LT-3 are in agreement, within experimental and theoretical accuracy, with the predictions of the model. The effect of a compensating perpendicular magnetic field also confirms the model. The mechanisms governing the duration of the delay to breakdown remain obscure. In view of the short particle lifetime a means of re-introducing electrons, such as secondary emission, seems indicated. The prebreakdown phase may be avoided by pre-ionization to densities above a critical value. The particle drifts are then compensated and no delay occurs before the onset of exponential ionization increase.