Overview of the Helicity Injected Torus Program

Summary form only given. The helicity injected torus with steady inductive helicity injection (SIHI) spheromak experiment (HIT-SI) addresses critical issues for spheromaks, including current drive, operation at high beta, and confinement. HIT-SI features an optimal high-beta plasma shape and current profile, steady-state inductive operation, minimal plasma-wall interaction, and injected power always flowing into the plasma. HIT-SI has a "bow-tie" shaped 1.3 cm thick Cu flux conserver with major radius R=0.33 m and axial extent of 0.57 m. A half torus helicity injector at each end of the flux conserver produces conjugate sinusoidal flux (4 MW peak) and loop voltages (20 MW peak) at 5 kHz by IGBT-based switching power amplifiers. The injector loop voltage is provided by a series LC resonant tank "voltage circuit" with a novel amplitude feedback control method. The injector flux is generated by a direct pulse-width-modulated feedback "flux circuit". Injector flux and loop voltages are phase controlled to maintain power flow always inward. Insulating breaks for the oscillating flux and loop voltage are provided by a novel double viton o-ring system. HIT-SI uses the diagnostic suite previously used by the HIT-II experiment. Recent results show increased injector current with increased neutral gas fuelling in the injectors. Efforts are underway to improve wall conditioning by RF glow discharge cleaning and installation of an improved pumping system. Studies are underway to evaluate replacement of the present series resonance on the voltage circuit with a parallel resonance. This should eliminate the voltage steps of the square-wave driver from appearing on the transformer primary coil. The NIMROD code is used to perform 3D MHD simulations of HIT-SI. The HIT-SI injectors are modeled using high toroidal mode number (up to n=10) boundary conditions. Previous results have shown an increase in the spheromak n=0 component with increasing Lundquist number, and also after the in- ectors are shut off. Present modeling work includes extending the duration of injector sustainment, and continued simulation of spheromaks decay after injector shut off