Study of sawtooth oscillations in the HT-7 superconducting tokamak

Abstract

Experiments have been carried out with a moveable graphite limiter on the HT-7 tokamak in order to investigate the influence of the horizontal plasma position on the density limit. The observed sawtooth oscillation at values of the edge safety factor 3.5 < q ( a ) < 4 , has been studied in this Letter, which shows that oscillation with m = 1 and n = 1 , where m and n are the poloidal and toroidal mode numbers, respectively, trigger m = 2 and n = 1 , in sawtooth collapse which finally cause disruption in the HT-7 tokamak. We propose that the mode may be destabilized by the current density gradient. The q = 1 surface is appears at about r = 0.5 a , than the q = 2 surface must be close to the q = 1 surface. Therefore, the sawtooth collapse can greatly change the current profile near the q = 2 surface and then trigger the m = 2 mode that finally causes the disruption.