Anomalous Ion Heating of Ultra-low Safety Factor Tokamak Plasmas in the Madison Symmetric Torus

Toroidal plasmas in the ultra-low-q (ULq) regime can now be produced in MST with edge-q in the range 0.25 < q(a) < 1 using high-bandwidth feedback power supplies that sustain the plasma current at ≤500 kA and toroidal field of 0.13 T. Compared to a standard tokamak with q(a) > 2, ULq plasmas generally have high resistivity, strong turbulence, weak magnetic shear, weak internal gradients in temperature and density, static mode activity without sawteeth, and resistance to disruption. Impurity emissions, measured with a high-throughput multi-chord spectrometer, show that ion temperatures (T_i) are anomalously large with T_i ≥ 3*T_e and strongly dependent on q(a). T_i does not exhibit strong temporal dynamics, implying the underlying heating mechanism is more steady than seen in reversed field pinch plasmas. We present the relations between T_i and q(a), plasma density, toroidal field, density fluctuations, and magnetic fluctuations. A 25 kV neutral beam is used to introduce a population of fast ions which generate a flux of fusion products. Initial measurements of neutron flux for a standard tokamak, reversed field pinch, and ULq plasmas are presented to compare the fast ion confinement properties of the different regimes.