Disruptions in current-driven discharges in the Compact Toroidal Hybrid experiment

Disruption avoidance in stellarators is relevant to helical configurations with tokamak-like levels of bootstrap current, e.g. quasi-axisymmetric devices, and stellarator-tokamak hybrids. Disruptions are investigated in the Compact Toridal Hybrid (CTH) experiment (R$_{0}$ = 0.75 m, a $\sim $ 0.2 m, B$_{0} \quad \le $ 0.7 T, $\bar {n}_e $ = 0.2 -- 1.5 x 10$^{19}$ m$^{-3})$, a flexible heliotron operating with significant ohmic current. At B$_{0 }$= 0.5 T, the edge vacuum rotational transform is variable from $\iota _{VAC}$(a) = 0.05 to 0.5${\rm g}$nd plasma currents up to 40 kA are driven in plasmas generated by electron-cyclotron resonant heating at 14 GHz. At the lowest vacuum transform $\iota _{VAC}$(a) = 0.05, current-driven disruptions leading to a complete loss of plasma can be induced for a total rotational transform $\iota _{TOT}$(a) $>$ 0.3 at plasma densities $\bar {n}_e \quad \ge $ 0.8 x 10$^{19}$ m$^{-3}$. Disruptions are preceded by m = 2 tearing activity. Complete disruptions have not yet been observed in CTH with h $\iota _{VAC}$(a) $\ge $ 0.2, although partial current collapses can take place. Efforts are underway to study the transitional behavior of the disruptions as the vacuum transform is continuously raised.