Overview of the HIT-SI3 spheromak experiment

The HIT-SI and HIT-SI3 spheromak experiments ($a=$23~cm) study efficient, steady-state current drive for magnetic confinement plasmas using a novel method which is ideal for low aspect ratio, toroidal geometries. Sustained spheromaks show coherent, imposed plasma motion and low plasma-generated mode activity, indicating stability. Analysis of surface magnetic fields in HIT-SI indicates large $n=$0 and 1 mode amplitudes and little energy in higher modes. Within measurement uncertainties all the $n=$1 energy is imposed by the injectors, rather than being plasma-generated. The fluctuating field imposed by the injectors is sufficient to sustain the toroidal current through dynamo action whereas the plasma-generated field is not (Hossack \textit{et al.}, Phys. Plasmas, 2017). Ion Doppler spectroscopy shows coherent, imposed plasma motion inside $r\approx $10~cm in HIT-SI and a smaller volume of coherent motion in HIT-SI3. Coherent motion indicates the spheromak is stable and a lack of plasma-generated $n=$1 energy indicates the maximum $q$ is maintained below 1 for stability during sustainment. In HIT-SI3, the imposed mode structure is varied to test the plasma response (Hossack \textit{et al}., Nucl. Fusion, 2017). Imposing $n=$2, $n=$3, or large, rotating $n=$1 perturbations is correlated with transient plasma-generated activity.