Recent Results from the Pegasus Toroidal Experiment

Pegasus is an ultra-low aspect ratio spherical tokamak (A=1.15), used to study the physics of low-A plasmas and develop non-solenoidal tokamak startup techniques. A combination of point-source magnetic helicity injection and poloidal field induction produces non-solenoidal tokamak plasmas with $I_p \le 0.17$ MA using $~4$ kA injected current $I_{inj}$, consistent with the heilicity injection rate and a Taylor relaxation limit. A strong double layer sheath at the injectors describes the impedance of the injection circuit, implying the helicity injection rate for a given $I_{inj}$ is a strong function of the local plasma density. Impurity ion spectroscopy indicates strong heating ($T_i$ $\sim$ 0.5 keV) during helicity drive. Passive gas-fueled electrodes can be used as helicity injectors, complementing active current sources and affording a means to optimize the Taylor and helicity injection limits. Ohmic plasmas in Pegasus are often unstable to peeling modes, an instability underlying edge localized modes (ELMs) in larger tokamaks. Time-resolved current profile measurements show that ELM-like, current-carrying filaments form at the plasma edge, which then detach and propagate outward.