Loop voltage and MHD activity during the start-up of an RFP

During the start-up phase of a reversed field pinch discharge (RFP), the configuration undergoes relaxation whichis the dynamic process by which the configuration evolves to states near the minimum magnetic energy. Experiments in the Extrap T1 device show that the MHD activity varies during start-up apparently dependent on current profile effects at the time when the reversal surface is produced at the plasma edge. Under unfavourable conditions characterized by a pinch parameter, Θ = Bθ(a)/ left angle BΦ right angle , less than about 1.5 at the time of reversal, m = 0 mode activity is large and the loop voltage increases substantially resulting in a suppressed plasma current level. The increased loop voltage is believed to be the result of m = 0 tearing modes resonant at the reversal surface and driven by a large parallel current gradient at the edge which become particularly pathological when Θ is comparatively low. (orig.)