Performance improvement with plasma edge biasing on SPECTOR

Physics of Plasmas, Volume 28, Issue 3, March 2021. A description of an edge-biasing experiment conducted on the spherical compact toroid plasma injector is presented with results. The insertion of a disk-shaped molybdenum electrode (probe), biased at up to +100 V, into the edge of the Compact Torus (CT), resulted in up to 1.2 kA radial current being drawn from the probe to the wall. Core electron temperature, as measured with a Thomson-scattering diagnostic, and CT lifetime, more than doubled in the optimal configuration tested. Temperature and density profiles become more centrally peaked with biasing. In contrast to previous biasing experiments, a significant reduction in electron density was observed; this may be due to the effect of a transport barrier impeding CT fueling and associated cooling, where, as verified by MHD simulation, a principal fueling source is neutral gas that remains concentrated around the gas valves after CT formation.