Pellet fueling of axisymmetric and non-axisymmetric MST plasmas

Deuterium pellet injection into toroidally axisymmetric MST plasmas with a broadband reduction in magnetic tearing fluctuations and improved confinement has resulted in a total $\beta $ of 26{\%} with a pressure gradient that exceeds the Mercier criterion. The density limit has been exceeded by 50{\%} in 200kA discharges and by 20{\%} in 500kA discharges, with the latter case having a density exceeding 7.5e19 m$^{-3}$. Simulations in toroidal geometry with NIMROD reveal that the plasma is linearly unstable to pressure-driven tearing and interchange modes. Pellets have also been injected into a new class of plasmas in which toroidal axisymmetry is broken by a 3D helical structure in the core. This structure emerges when the innermost-resonant tearing mode grows to large amplitude and dominates the mode spectrum. Pellet injection during growth of this mode can trigger a rapid change in that mode's growth rate. Pellet fueling after the mode has saturated leads to substantial density gradients. Supported by USDoE.