Overview of Non-Solenoidal Startup Studies in the Pegasus ST

Local helicity injection (LHI) is a non-solenoidal startup method pursued on Pegasus utilizing compact, high power current sources ($A_{inj} \sim 2-4$ cm$^{\mathrm{2}}$, $I_{inj} \sim 10$ kA, $V_{inj} \sim 1$ kV) at the plasma edge. Outboard injectors ($N_{inj} =4$, $A_{inj} =8$ cm$^{\mathrm{2}})$ produce $I_{p} \sim 170$ kA plasmas compatible with Ohmic drive. A 0-D model that treats the plasma as a resistive element with time-varying inductance and enforces $I_{p} $ limits from Taylor relaxation is used to interpret experimental $I_{p} (t)$ in several scenarios. Strong inductive drive arises from the plasma shape evolution, in addition to poloidal field induction. A new injector system has recently been installed in the lower divertor region ($N_{inj} =2$, $A_{inj} =8$ cm$^{\mathrm{2}})$ to explore the implications of geometric placement of the helicity injectors on LHI startup. This geometry supports tests of reconnection dynamics seen in NIMROD simulations, high-$B_{T} $ effects expected in larger devices, and LHI electron confinement with and without inductive assist. Plasmas with $I_{p} >130$ kA, $V_{inj} \sim 0.5$ kV, $\Delta t_{pulse} \sim 8$ ms and $B_{T} /B_{T,max} \le 50\% $ are produced with the inboard system to date, consistent with performance expectations. Higher $I_{p} $ is expected with increased $B_{T} $, $V_{inj} $, and $\Delta t_{pulse} $. Thomson scattering data in both geometries indicate high $T_{e} \ge 100$ eV during LHI, suggesting the confinement is not strongly stochastic. Conceptual design work is exploring the feasibility of coaxial helicity injection and ECH heating on Pegasus in addition to LHI.