Effects of Density Control on Internal Plasma Dynamics and Current Drive in HIT-SI

Abstract

The Helicity Injected Torus with Steady Inductive Helicity Injection (HIT-SI) experiment investigates a method of current drive and spheromak formation through constant inductive helicity injection into a confinement volume of major radius 0.3 m. Using Steady Inductive Helicity Injection (SIHI) a DC plasma current is generated and formed in the confinement volume from the AC driven plasma injectors. Recent results have shown that helium plasmas can be used to condition the walls for deuterium plasmas. These deuterium plasmas have produced record toroidal currents (> 50 kA) and current amplifications (approximately 3) on HIT-SI. In addition density measurements indicate that deuterium plasmas are the first plasmas on HIT-SI to achieve a ratio of current density to electron density, j/n>10<super>-14</super> A·m, an important indication of plasma quality. Internal magnetic field measurements show these high performance plasmas have suppressed fields indicative of a region of low λ and that this region is built up and sustained during the injector drive. Currents in this low λ region that do not link the injectors---named separatrix currents---are sustained up to 40 kA. Finally a model that predicts the build up of toroidal current from the density and injector current is developed. The implication of this model is that dynamo current drive can be entirely applied and does not need to result from instabilities in the plasma.