Initial studies of steady inductive helicity injection on the HIT-SI experiment

The helicity-injected torus (HIT-SI) is designed to produce a "bow tie" spheromak, formed and sustained by steady inductive helicity injection (SIHI) current drive (Jarboe, 1999). SIHI is implemented via two "helicity injectors." Each injector acts as a 180° segment of a reversed-field pinch (RFP), with a typical peak current of 11 kA and a peak flux of 0.5 mWb. The toroidal loop voltage, toroidal injector plasma current, and toroidal flux of a single injector all oscillate sinusoidally at a frequency of 5 kHz and nearly in phase, so the single injector helicity injection rate and power input to the plasma are always positive. Operating the two injectors 90° out of phase in time provides nearly constant power input and helicity injection rate. To date, injector plasmas have been sustained in this manner for over 5 ms. An internal magnetic probe array has been installed in one of the injectors. Observations of the injector fields will be presented, illustrating the evolution of the field structure through the injector cycle.