An evaluation of limiter configurations in ZT-40M

Abstract

Three limiter configurations were evaluated in the reversed-field pinch, ZT-40M. A bare graphite mushroom-shaped probe located in a pump port measured changes in the energy flux profiles resulting from three different limiter configurations. The first configuration was a graphite segmented poloidal ring limiter located 90° toroidally upstream from the mushroom probe, and it protruded into the plasma chamber ~ 0.95 cm beyond the bellows. The second configuration consisted of three graphite tiles which were positioned in the bottom of the convolution section immediately upstream from the mushroom probe. These tiles were electrically floating with respect to the metal liner and protruded radially inwards ~1.27 cm beyond the bellows. For the third configuration four graphite tiles were placed on each side of the pump port section where the mushroom probe was located. These tiles extended 25 cm in both toroidal directions from the pump port section and protruded ~ 0.95 cm into the plasma region. These tiles were also electrically floating. The introduction of the poloidal ring limiter resulted in the mushroom probe absorbing 28% less energy at the major radial position where the tip of the mushroom was even with the shadow of the liner bellows. The tile limiters produced an additional, but small, reduction in the energy absorbed at this location. The tiles produced an increase in the plasma resistance and radiation late in the discharge. Significant amounts of arcing occurred between the tiles and the liner. The experimental results led to a configuration which utilized four graphite poloidal ring limiters. These four limiters protected the pump port areas; however, the occasional increase in the liner temperature at a location approximately halfway between two limiters was not eliminated. The fraction of input energy radiated during the constant-current phase of the discharge increased from 20% to 30% with the four poloidal limiters, owing to an increase in electron density. During start-up, the energy radiated per electron was less for the four-ring case than for the case with no limiters, indicating that the discharge may have been cleaner with the four rings.