Stabilization of the (2, 1) tearing mode and of the current disruption in the W VII–A stellarator

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Abstract
A numerical code based on a Δ'-analysis is applied to calculate the saturated amplitude of tearing modes dependent on the current density profile. The only stellarator effect.included is the additional, shearless external rotational transform ι̵0 in the safety factor profile q(r). In this way, the stellarator field shifts the resonant q = 2 surface toward the outside, where the current density gradient is smaller, and stabilizes the (2, 1) mode as observed experimentally. Also the measured dependence of the (2, 1) mode amplitude on electron density and plasma current can be absolutely predicted by the calculations. – In addition to stabilizing the (2, 1) tearing mode, the current disruption is suppressed in Ohmically heated W VII-A discharges for ι̵0 0.15. The experimental findings, together with the calculated island widths, are compared with the predictions of a theoretical model proposed by several authors to explain the current disruption.