Confinement improvement in the MST reversed field pinch

Abstract

Summary form only given. The energy loss in the reversed field pinch (RFP) predominantly results from parallel streaming in a stochastic magnetic field. This stochasticity results from B/spl tilde//B/spl sim/1% magnetic fluctuations which accompany m=1, n/spl sim/2R/a tearing (or resistive kink) instabilities in the plasma core. Major research goals in the MST are to understand fluctuation induced transport and to improve plasma confinement using this understanding. Magnetic fluctuation induced transport in the plasma core is studied during a period of high magnetic activity preceding sawteeth events. The flow dynamics of bulk plasma rotation is examined by measuring Doppler shifts of impurity ions spectral lines. Both, spontaneous and actively driven confinement improvement regimes have been observed. After machine conditioning with solid-target boronization, a high confinement regime, characterized by the absence of sawteeth, spontaneously appears during low-density discharges. Similar improvements result by actively applying a transient auxiliary inductive electric field to the MST plasma. The current density gradient is reduced, the growth of the m=1 tearing fluctuations slows, and the energy confinement time doubles. To sustain and enhance the improved plasma, electrostatic and Rf current drivers are being developed.