Inductively-Coupled Preionization Sources for the HIT-SIU Spheromak Experiment

The HIT-SI3 and HIT-SIU plasma experiments are designed to study spheromak plasmas created and sustained through steady inductive helicity injection. The oscillating fields inside the inductive helicity injectors tend to expel plasma inside them, meaning that it must constantly be replenished to avoid starvation. HIT-SI3 accomplished this by injecting neutral gas using puff valves and relying on Paschen breakdown for initial ionization, limiting the range of accessible densities. The HIT-SIU experiment attempts to expand the range of accessible operating densities by supplementing its neutral sources with a set of high power inductively-coupled plasma sources to feed pre-ionized gas into the injectors. This system aims to improve control of plasma density, which has a strong impact on current drive performance. The electron density and temperature inside the sources was measured across a range of values for applied magnetic field and input power, demonstrating a high-density, uniform source plasma for the injectors, over a wide range of fueling rates. Results comparing operation of HIT-SIU, with and without pre-ionization sources, will be presented along with comparison to the HIT-SI3 experiment.