Non-solenoidal Startup via Local Helicity Injection on Pegasus: Progress and Plans

Non-solenoidal plasma startup via local helicity injection (LHI) at the Pegasus toroidal experiment now provides routine operation at I$_{\mathrm{p}}\approx $ 0.17MA with I$_{\mathrm{inj}}\approx $ 5kA and V$_{\mathrm{inj}}\approx $ 1kV from four active arc injectors. Experiments in the past year have advanced the understanding of the governing physics of LHI and its supporting technology. Injector impedance scales as V$_{\mathrm{inj}}^{\mathrm{3/2}}$ and is governed by two effects: a quasineutrality constraint on electron beam propagation, related to the tokamak edge density, and double-layer sheath expansion, related to n$_{\mathrm{arc}}$. Injector design improvements permit operation at V$_{\mathrm{inj}} \quad \ge $ 1 kV without deleterious PMI or impurity generation. Discharges with varied shape, I$_{\mathrm{p}}$(t), and helicity input test a predictive 0D power-balance model for LHI startup. Anomalous, reconnection-driven T$_{\mathrm{i}}$ \textgreater 800 eV and strong MHD activity localized near the injectors are observed during LHI. Preliminary core Thomson scattering measurements indicate surprisingly high T$_{\mathrm{e}}$ \textgreater 300 eV, which if verified may indicate the dominance of high-energy electron fueling from the injector current streams. A new divertor injector system has been designed to substantially increase the available helicity input rate and support critical studies of confinement during LHI and reconnection activity at high I$_{\mathrm{p}}$. A proposed upgrade to the Pegasus experiment will extend these studies to NSTX-U relevant parameters.