Resistive wall modes in a reversed field pinch with interchangeable shells

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Abstract
Resistive wall modes have been investigated in a reversed field pinch (Reversatron IIi [IEEE Trans. Plasma Sci. PS-16, 667 (1988)]) operated with three different boundary conditions: (1) a copper shell having a magnetic penetration time τs comparable to the discharge duration, (2) a brass shell having τs comparable to the setting-up time, and (3) no shell, which gives τs much shorter than the setting-up time. The brass shell is found to impede the growth of the m=1, n=−6 ‘‘on-axis’’ mode observed to grow in the setting-up phase of no-shell discharges. In the sustainment phase, there is a higher level of m=1 modes than with the copper shell and the discharge duration, ion temperature, and plasma current are reduced. The mode spectrum is broad in the sustainment phase and the degradation in discharges cannot be attributed to any single mode. There is evidence for transient mode locking resulting in a slinky mode in copper-shell discharges.