Breakdown and preionization experiments in the SINP tokamak

Breakdown phenomena in the SINP tokamak are studied experimentally for different values of filling pressure (p), toroidal electric field (E), toroidal magnetic field and vertical magnetic field by using a hot filament preionization system. Breakdown had been achieved for a wide range of E/p, starting from 100 up to 4000 V.cm-1.torr-1. In the present experiment it has been observed for the first time that there exists an optimum value of the electric field on either side of which the breakdown time increases. The experimental results have been explained in terms of a model by Hutchinson and Strachan (1974), invoking particle drift compensation by runaway electrons during the breakdown phase in the tokamak. Previously, in this kind of runaway electron dominated breakdown a monotonic increase of the breakdown time: with E was observed. The effect of the preionization strength has also been studied with the help of a radio frequency (RF) system. The breakdown time could be shortened to within a few ionization times ( approximately=15 mu s) by RF preionization