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, q(a); the disruptions are much more severe for q(a) < 3, while the density limit is often non-disruptive for large q(a). At large values of q(a) greater-than-or-equal-to 4.5 the density is limited by increased mode activities.