Non-Inductively Driven Tokamak Plasmas at Near-Unity Toroidal Beta in the Pegasus Toroidal Experiment*

A major goal of the spherical tokamak research program is accessing a state of low internal inductance $l_{i}$, high elongation $\kappa $, high toroidal and normalized beta ($\beta_{t}$ and $\beta_{N})$, and low collisionality without solenoidal current drive. A new local helicity injection (LHI) system in the lower divertor region of the ultra-low aspect ratio Pegasus ST provides non-solenoidally driven plasmas that exhibit most of these characteristics. LHI utilizes compact, edge-localized current sources ($A_{inj}\sim $ 4 cm$^{\mathrm{2}}$, $I_{inj}\sim $ 8 kA, $V_{inj}\sim $ 1.5 kV) for plasma startup and sustainment, and can sustain more than 200 kA of plasma current. Plasma growth via LHI is enhanced by a transition from a regime of high kink-like MHD activity to one of reduced MHD activity at higher frequencies and presumably shorter wavelengths. The strong edge current drive provided by LHI results in a hollow current density profile with low $l_{i}$. The low aspect ratio ($R_{0}/a\sim $ 1.2) of Pegasus allows ready access to high $\kappa $ and MHD stable operation at very high normalized plasma currents ($I_{N}= I_{p}$\textit{/aB}$_{T}$ \textgreater $_{\mathrm{\thinspace }}$15). Thomson scattering measurements indicate $T_{e}\sim $ 100 eV and $n_{e}_{\mathrm{\thinspace }}\sim $ 1$\times $19 m$^{\mathrm{-3}}$. The impurity $T_{i}$ evolution is correlated in time with high frequency magnetic fluctuations, implying substantial reconnection ion heating is driven by the applied helicity injection. Doppler spectroscopy indicates $T_{i}\ge T_{e}$ and that the anomalous ion heating scales consistently with two fluid reconnection theory. Taken together, these features provide access to very high $\beta_{t}$ plasmas. Equilibrium analyses indicate $\beta_{t}$ up to $\sim $100{\%} and $\beta _{N}\sim $ 6.5 is achieved. At increasingly low $B_{T}$, the discharge disrupts at the no-wall ideal stability limit. In these high $\beta_{t}$ discharges, a minimum \textit{\textbar B\textbar } well forms over $\sim $50{\%} of the plasma volume. This unique magnetic configuration may be of interest for testing predictions of stabilizing drift wave turbulence and/or improving energetic particle confinement.