Density limits and disruptions in the TCA tokamak

Density limit disruptions have been studied during the current flat-top of TCA. Disruption occurs at two different densities, depending on discharge programming. The highest densities are achieved by a strong influx of neutral gas called 'hard gas puffing'. This allows the plasma density to cross a range which is otherwise unstable and to reach a second region of stability with high internal inductance. When disruption occurs after hard gas puffing, the internal inductance decreases in time, in contrast to a slower density rise where the inductance increases continuously. The character of the disruptions depends also on the safety factor at the plasma edge, qa; the disruptions are much more severe for qa < 3, while the density limit is often non-disruptive for large qa. At large values of qa ≥ 4.5 the density is limited by increased mode activities. (author). 30 refs, 28 figs, 3 tabs