MHD computations of ultra-low safety factor plasma

Using the nonlinear visco‐resistive MHD code NIMROD, global‐scale fluctuations in the edge safety‑factor regime 0<q(a)<2 are investigated with self‑consistent, evolving profiles. This intermediate regime, lying between conventional tokamak and reversed‐field‐pinch operation, is accessible in the Madison Symmetric Torus (MST) due to its feedback‐controlled programmable power supply and close‐fitting conducting shell. Starting from vacuum field in the simulations, plasma current is driven via a proportional–differential loop‑voltage controller that mimics MST experiments. Ohmic heating and anisotropic thermal conduction self-consistently models transport with magnetic relaxation. Simulations with constant thermal diffusivities reproduce key features: a sawtooth-like is observed throughout most of 1<q(a)<2, while for 0<q(a)<1 plasma self-organize about major rational numbers. A kinetic-energy integral analysis allows the comparison of current and pressure drives. Computations with temperature‐dependent (Braginskii) transport coefficients underestimate thermal energy transport. Allowing nonzero normal magnetic component at the boundary and implementing steady-state impurity radiation yields more realistic profile relaxation. Work supported by US DOE award DE-FG02-85ER53212.