The physics of high-density, high-beta reversed-field pinch plasmas

The use of pellet injection to achieve high-density, high-beta discharges in the Madison Symmetric Torus has been investigated. The physics goals motivating this work are split into two primary and two secondary thrusts. The primary goals are the use of pellet fueling in conjunction with improved confinement plasmas to attain higher plasma beta and to investigate the consequences for stability at higher beta. The secondary research thrusts are to compare pellet-fueling of standard RFP discharges to edge-fueled plasmas and to begin the search for a density limit in MST.;Following are the results of the primary and secondary goals. Pellet injection has been used to increase the density in improved confinement discharges fourfold while maintaining low magnetic fluctuations, and data suggest that even higher density is possible. A record plasma beta has been achieved for the improved confinement RFP in the process. A portion of the beta increase is attributed to a rising ion temperature (not seen in low density improved confinement) caused in part by the improved thermal coupling between electrons and ions. At this high beta, a new regime for instabilities is accessed. Both local interchange and global tearing instabilities are calculated to be linearly unstable. The tearing instability, normally driven by the current gradient, is driven by the pressure gradient in this case and appears to be the cause of a soft beta-limit. This beta-limit occurs as a reduction in the energy confinement time in moving to high beta during improved confinement plasmas. In standard (non-improved) confinement discharges, pellet fueling can peak the density profile where edge fueling cannot. The core-fueling of pellet injection alters the nature of the MHD activity in a standard discharge, but confinement appears unchanged from an edge-fueled discharge. For a limited range of plasma currents, MST discharges with edge fueling are constrained to a maximum density corresponding to the Greenwald limit. This limit is surpassed in pellet-fueled improved confinement discharges.