Point-source helicity injection for ST plasma startup in {\sc Pegasus}*

Plasma current guns are used as point-source DC helicity injectors for forming non-solenoidal tokamak plasmas in the {\sc Pegasus} Toroidal Experiment. Discharges driven by this injection scheme have achieved $I_p \ge$ 100 kA using $I_{\rm inj} \le$ 4 kA. They form at the outboard midplane, transition to a tokamak-like equilibrium, and continue to grow inward as $I_p$ increases due to helicity injection and outer- PF induction. The maximum $I_p$ is determined by helicity balance (injection rate {\em vs} resistive dissipation) and a Taylor relaxation limit, in which $I_p \propto \sqrt{I_{\rm TF} I_{\rm inj}/w}$, where $w$ is the radial thickness of the gun-driven edge. Preliminary experiments tentatively confirm these scalings with $I_{\rm TF}$, $I_{\rm inj}$, and $w$, increasing confidence in this simple relaxation model. Adding solenoidal inductive drive during helicity injection can push $I_p$ up to, but not beyond, the predicted relaxation limit, demonstrating that this is a hard performance limit. Present experiments are focused on increasing the injection voltage ({\em i.e.}, helicity injection rate) and reducing $w$. Near-term goals are to further test scalings predicted by the simple relaxation model and to study in detail the observed bursty $n$=1 activity correlated with rapid increases in $I_p$.