The role of non-axisymmetric perturbations on operations of the HIT-SI3 device

The HIT-SI3 device at the University of Washington uses three steady inductive helicity injectors to form and sustain spheromak plasma equilibria. Each injector is a semi-toroid connected to the main confinement volume and is operated by two sets of coils oscillating in phase to inductively drive helical magnetic structures that inject magnetic helicity which forms and sustains a toroidally symmetric spheromak equilibrium. The toroidal spectrum of the imposed perturbation has been found to be primarily dependent on the temporal phase of the helicity injection waveforms. A recently implemented control system [1] has allowed the operation of a variety of spectra involving significant amounts of n=1, n=2, and n=3 perturbations. These results are compared with several linear and nonlinear models, including extended MHD simulations using the NIMROD code and composite Taylor state equilibria computed using the PSI-Tet code [2].

1. Morgan et al, Rev. Sci. Instrum. 92, 053530 (2021)

2. Benedett et al, Nuc. Fus., 61, 036022 (2021)