Extended operational regimes and MHD behavior in a low-aspect-ratio reversed field pinch in RELAX

Operational regimes have been investigated over a wide range of discharge parameters in a low-aspect-ratio (low-A) reversed field pinch (RFP) RELAX. Two distinctive regimes have been identified, possibly characteristic to low-A RFP. One is a very shallow-reversal regime, and the other is an extremely deep-reversal regime where a field-reversal parameter lower than-1 could be sustained. Innewly attained extremely deep-reversal plasmas, the amplitudes of the resonant modes were suppressed to a lower level with enhanced soft-x-ray emission intensity. The extremely deep-reversal regime in low-A RFP may have a potential to become a new operational regime with improved plasma performance.