Numerical studies and validation of non-axisymmetric perturbations and self-organized spheromak formation in the HIT-SI and HIT-SI3 devices.

Finite-beta extended MHD modeling using the NIMROD code is validated against experimental results and used to enhance understanding of the details of plasma self-organization, where Hall physics and other two-fluid effects are found to be important. The Helicity Injected Torus with Steady Inductive helicity injection experiments use a set of inductively driven helicity injectors to apply oscillating non-axisymmetric current drive on the edge of the plasma, both forming and sustaining a self-organized, stable, steady state axisymmetric spheromak equilibrium in a central flux-conserving volume. The two devices differ on both the spatial and temporal details of the AC-driven helicity injection but produce similar final plasma equilibrium. It is shown both experimentally and in simulation that a spheromak equilibrium can be formed for every spatial and temporal variation of the helicity injection scheme possible on the experiments, which combine n=1, 2, and 3 toroidal perturbations. While each case forms a similar stable equilibrium, differences in the observed equilibria generated are also seen. The detailed study of these differences through a combination of simulation and experimental probe measurements allow improved understanding of the details of plasma self-organization.