Disruptions and disruption avoidance in the CTH torsatron/tokamak experiment

It has long been known that disruptions are rare in current-carrying toroidal plasmas if the discharge is performed within a stellarator equilibrium with a modest vacuum rotational transform of $\iota \quad \le $ 0.14 [1]. Experiments are performed in the current-carrying Compact Toroidal Hybrid (CTH) torsatron/tokamak (R = 0.75 m, a $\sim $ 0.2 m, B $\le $ 0.7 T, I$_{p} \quad \le $ 65 kA, n$_{e} \quad \le $ 10$^{19}$ m$^{-3})$ to better understand the conditions for passive disruption. As in previous experiments, we find that disruptions characterized by a rapid current quench do not occur if the applied vacuum rotational transform $\iota _{VAC}$(a) exceeds a value of $\sim $0.15 even though stable plasmas with q$_{edge}$ of 1.6 are obtained. Below this threshold transform, disruptions can be triggered by ramping the plasma density to levels comparable to the Greenwald density limit. These and other disruptions also arise from a vertical instability in which the plasma channel drifts upward. The vertical instability is reduced or eliminated with (1) sufficient external transform, or (2) reduction of the vertical elongation using a pair of quadrupole shaping coils. \\[4pt] [1] W VII-A Team, Nucl. Fusion \textbf{20} 1093 (1980)