EXB FLOW SHEAR AND ENHANCED CONFINEMENT IN THE MADISON SYMMETRICAL TORUS REVERSED-FIELD PINCH

Strong ExB flow shear occurs in the edge of three types of enhanced co nfinement discharge in the Madison Symmetric Torus [Dexter ct al., Fus ion Technol. 19, 131 (1991)] reversed-field pinch. Measurements in sta ndard (low confinement) discharges indicate that global magnetic fluct uations drive particle and energy transport in the plasma core, while electrostatic fluctuations drive particle transport in the plasma edge . This paper explores possible contributions of ExB flow shear to the reduction of both the magnetic and electrostatic fluctuations and, thu s, the improved confinement. In one case, shear in the ExB flow occurs when the edge plasma is biased. Biased discharges exhibit changes in the edge electrostatic fluctuations and improved particle confinement. In two ether cases, the flow shear emerges (1) when auxiliary current is driven in the edge and (2) spontaneously, following sawtooth crash es. Both edge electrostatic and global magnetic fluctuations are reduc ed in these discharges, and both particle and energy confinement impro ve. (C) 1998 American Institute of Physics.