Non-solenoidal startup in Pegasus discharges

Recent Pegasus experimental studies are directed at developing non-solenoidal startup techniques for ST and tokamak applications. High-field-side magnetic helicity injection with washer-stack current-sources (plasma guns) produces discharges with toroidal current $I_p$ up to 50 kA, using static coil currents and only 3 kA of injected current $I_{inj}$. Discharges driven by low-field-side injection typically require outer-PF ramps for radial force balance, also providing inductive current drive, and have achieved $I_p$=80 kA using $I_{inj} \le$ 2 kA. In either injection geometry, $I_p$ persists for a significant interval after gun shutoff, while the plasmas relax into typical tokamak equilibria with well-defined edges. According to a semi-empirical model, the maximum gun-driven $I_p$ is determined by the helicity injection rate, radial force balance, kink stability, and the Taylor relaxation criterion. Higher helicity injection rates will extend the Pegasus operating space, allowing higher $I_p$ and normalized current $I_N$, and enabling both flux amplification studies and predictive testing of the $I_p$ model.