Characterization of neutral transport in the HIT-SI3 and HIT-SIU spheromak devices

Neutral gas is an important energy loss mechanism in quasi steady-state plasma confinement devices, making its control an important consideration in fusion devices. The HIT-SI3 and HIT-SIU devices study spheromak plasmas created and sustained through steady inductive helicity injection. Over the short duration of the discharge, neutral density inside the confinement volume is controlled primarily by passive pumping volumes which relieve neutral pressure at the edge. A langmuir probe is used to characterize plasma parameters inside these volumes, which are compared to a multifluid transport model to estimate their pumping rate. The HIT-SIU experiment improves on control of neutral and plasma density by using helicon sources to pre-ionize gas fueling the helicity injectors. Plasma parameters in the helicon plumes will be characterized with a langmuir probe and ion doppler spectroscopy diagnostics [1]. Additionally, neutral density will be measured with a fast ionization gauge and optical imaging of the plume shape. This will support optimization of helicon operating parameters to provide sufficient density for efficient operation of the helicity injectors while controlling plasma and neutral density in the confinement volume.