Studies of large, non-circular, reversed field pinch discharges

Reversed field pinch (RFP) discharges have been produced in a large (1.39 metre major radius, 0.56 metre average minor radius), thick walled (5 cm), aluminium vacuum vessel with indented sides. The discharges are self-reversed and ramped up to a current of 300 kA over a time of 10 ms. Reversal is sustained for > or approx. 10 resistive diffusion times, despite the presence of large magnetic fluctuations. The influence of the bad poloidal magnetic curvature on RFP stability is examined by measurement of magnetic fluctuations near the plasma edge in the separate bad and good curvature regions of the non-circular plasma for RFP and non-reversed discharges with an edge safety factor, qa, of 0.4 and 1.4. For qa ∼ 1.4 discharges, the poloidal field curvature is small. The large device size permits RFP startup at a low toroidal loop voltage (< or approx. 200 V), which is applied to a gap exposed to plasma, but successfully protected against arcing (up to 300 V). RFP plasmas have also been obtained with a toroidal limiter. (author). 14 refs, 8 figs, 1 tab