SPHEROMAK FORMATION STUDIES IN SSPX*

We present results from the Sustained Spheromak Physics Experiment (SSPX) at LLNL, which has been built to study energy confinement in spheromak plasmas sustained for up to 2 ms by coaxial DC helicity injection. Peak toroidal currents as high as 600kA have been obtained in the 1m dia. (0.23m minor radius) device using injection currents between 200Ð400kA; these currents generate edge poloidal fields in the range of 0.2Ð0.4T. The internal field and current profiles are inferred from edge field measurements using the CORSICA code. Density and impurity control is obtained using baking, glow discharge cleansing, and titanium gettering, after which long plasma decay times (t 1.5ms) are observed and impurity radiation losses are reduced from ~50 % to <20 % of the input energy. Thomson scattering measurements show peaked electron temperature and pressure profiles with Te (0)~120eV and be~7%. Edge field measurements show the presence of n=1 modes during the formation phase, as has been observed in other spheromaks. This mode dies away during sustainment and decay so that edge fluctuation levels as low as 1 % have been measured. These results are compared with numerical simulations using the NIMROD code. In this paper we present results of formation and sustainment experiments in the SSP