Enhanced confinement in the MST reversed-field pinch

Abstract

Summary form only given. The Madison Symmetric Torus (MST) reversed-field pinch is a toroidal, ohmically-heated, magnetic fusion research device capable of plasma currents of I 700 kA and plasma temperatures of several hundred eV. Recent MST discharges have exhibited energy confinement times, /spl tau//sub E/, that are 2-3 times greater than normal. An important benchmark of the MST's performance, /spl tau//sub E/=(total plasma energy)/(input ohmic heating power) and is about 1 ms in normal discharges. /spl tau//sub E/ is limited by transport of energy and particles out of the plasma core due primarily to fluctuations in the structure and magnitude of the internal magnetic field. The dominant magnetic fluctuations are sawtooth events, so-named for the shape of their waveform in many plasma parameters. Sawteeth arise due to overpeaking of the current-density profile on the magnetic axis. During each sawtooth, the total plasma energy decreases, and the ohmic heating power increases, resulting in a sharp drop in /spl tau//sub E/. Recently, discharges with sawtooth-free periods have been achieved, resulting in enhanced confinement. One method of preventing sawteeth is to actively drive current in the edge plasma. This flattens the normally-peaked current-density profile. The best results with this method have been achieved with careful conditioning by solid-target boronization of the plasma-facing inner wall of the vacuum vessel.