The Physics of Local Helicity Injection Non-Solenoidal Tokamak Startup

Non-solenoidal startup via Local Helicity Injection (LHI) uses compact current injectors to produce toroidal plasma current $I_{p} $ up to 170 kA in the \textsc{Pegasus} Toroidal Experiment, driven by 4--8 kA injector current on timescales of 5--20 milliseconds. Increasing the $I_{p} $ buildup duration enables experimental demonstration of plasma position control on timescales relevant for high-current startup. LHI-driven discharges exhibit bursty MHD activity, apparently line-tied kinking of LHI-driven field lines, with the bursts correlating with rapid equilibrium changes, sharp $I_{p} $ rises, and sharp drops in the injector impedance. Preliminary NIMROD results suggest that helical LHI-driven current channels remain coherent, with $I_{p} $ increases due to reconnection between adjacent helical turns forming axisymmetric plasmoids, and corresponding sharp drops in the bias circuit impedance. The DC injector impedance is consistent with a space charge limit at low bias current and a magnetic limit at high bias current. Internal measurements show the current density profile starts strongly hollow and rapidly fills in during $I_{p} $ buildup. Simulations of LHI discharges using the Tokamak Simulation Code (TSC) will provide insight into the detailed current drive mechanism and guide experiments on \textsc{Pegasus} and NSTX-U.