Non-Solenoidal Tokamak Startup via Inboard Local Helicity Injection on the Pegasus ST

Local helicity injection (LHI) is a non-solenoidal startup technique utilizing small injectors at the plasma edge to source current along helical magnetic field lines. Unstable injected current streams relax to a tokamak-like configuration with high toroidal current multiplication. Flexible placement of injectors permits tradeoffs between helicity injection rate, poloidal field induction, and magnetic geometry requirements for initial relaxation. Experiments using a new set of large-area injectors in the lower divertor explore the efficacy of high-field-side (HFS) injection. The increased area (4 cm$^{\mathrm{2}})$ current source is functional up to full Pegasus toroidal field ($B_{T,inj} =0.23$ T). However, relaxation to a tokamak state is increasingly frustrated for $B_{T,inj} >0.15$ T with uniform vacuum vertical field. Paths to relaxation at increased field include: manipulation of vacuum poloidal field geometry; increased injector current; and plasma initiation with outboard injectors, subsequently transitioning to divertor injector drive. During initial tests of HFS injectors, achieved $V_{inj} $ was limited to $\sim 600$ V by plasma-material interactions on the divertor plate, which may be mitigated by increasing injector elevation. In experiments with helicity injection as the dominant current drive $I_{p} \sim 0.13$ MA has been attained, with $\overline T_{e} >100$ eV and $\bar{{n}}_{e} \sim 10^{19}$ m$^{\mathrm{-3}}$. Extrapolation to full $B_{T} $, longer pulse length, and $V_{inj} \sim 1$ kV suggest $I_{p} >0.25$ MA should be attainable in a plasma dominated by helicity drive.