Physics of Plasma Cathode Current Injection During LHI*

Localized helicity injection (LHI) ST startup employs current sources at the tokamak edge. Max $I_{\mathrm{p}}$ in LHI scales with injection voltage $V_{\mathrm{inj}}$, requiring an understanding of injector impedance. For the arc-plasma cathode electron injectors in Pegasus, impedance is plasma-determined, and typically $V_{\mathrm{inj}}$\textgreater 1kV for $I_{\mathrm{inj}}=$2kA. At low $I_{\mathrm{inj}}$, $I_{\mathrm{inj}}\propto V_{\mathrm{inj}}^{\mathrm{3/2}}$, an indication of a double layer (DL) common to such devices. However, at $I_{\mathrm{inj}}$\textgreater $\sim $1kA, $I_{\mathrm{inj}}\propto V_{\mathrm{inj}}^{\mathrm{1/2}}$ occurs, a scaling expected for limited launched beam density, $n_{b} \equiv {I_{inj} } \mathord{\left/ {\vphantom {{I_{inj} } {(e\sqrt {{2eV_{inj} } \mathord{\left/ {\vphantom {{2eV_{inj} } {m_{e} }}} \right. \kern-\nulldelimiterspace} {m_{e} }} A_{inj} )}}} \right. \kern-\nulldelimiterspace} {(e\sqrt {{2eV_{inj} } \mathord{\left/ {\vphantom {{2eV_{inj} } {m_{e} }}} \right. \kern-\nulldelimiterspace} {m_{e} }} A_{inj} )}\sim I_{\mathrm{inj}}$/$V_{\mathrm{inj}}^{\mathrm{1/2}}$. An ohmic discharge injection target was created to test this hypothesis. Langmuir probe data showed $I_{\mathrm{inj}}$/$V_{\mathrm{inj}}^{\mathrm{1/2}}\propto n_{\mathrm{edge}}$ at low $n_{\mathrm{edge}}$, consistent with a limit ($n_{\mathrm{edge}}\ge n_{\mathrm{e,b}})$ imposed by quasineutrality. If edge fueling maintained $n_{\mathrm{edge}}\ge n_{\mathrm{e,b}}$, spectroscopic measurements of source density $n_{\mathrm{arc}}$ indicated $I_{\mathrm{inj}}$/$V_{\mathrm{inj}}^{\mathrm{1/2}}\propto n_{\mathrm{arc}}$, as expected from DL expansion. Thus $n_{\mathrm{b}}$ established by $n_{\mathrm{arc}}$ or $n_{\mathrm{edge}}$ determines $V_{\mathrm{inj}}$ up to the onset of cathode spot (CS) arcing. Technology development has increased obtainable V$_{\mathrm{inj}}$ and reduced CS damage using new ring shielding and a cathode design drawing CS's away from insulators. This involved a novel optimization of conical frustum geometry. Finally, consistent with NIMROD predictions of coherent streams in the edge during LHI, pairwise triangulation of outboard Mirnov data assuming beam m$=$1 motion has allowed an estimate of beam R(t), Z(t) location that is near the injector R, and consistent across the array.