Comparing resistive MHD simulations and experiments of Coaxial Helicity Injection (CHI) in NSTX

NSTX generates CHI plasmas with current, density, and temperature appropriate for ST startup.\footnote{R. Raman, et al., PRL \textbf{104}, 095003 (2010).} Whole-device simulations of CHI using the NIMROD MHD code\footnote{C.R. Sovinec, et al., J. Comp. Phys \textbf{195}, 355 (2004).} extend the HIT-II model.\footnote{A. Bayliss, et al., submitted for publication.} A model power supply generates time-dependent voltage and current at the injection gap. Absorber gap voltage maintains a constant vacuum toroidal flux. Simulation physics includes ohmic heating and thermal conductivity along and across the magnetic field and generation of nonaxisymmetric fields and flows. A flux bubble expands in the simulation with current and plasma temperature similar to experiment; an n=1 mode is observed to generate an helical ribbon of current and velocity vortices on the flux bubble surface. Time-dependent poloidal-field boundary conditions for interesting NSTX discharges are used for quantitative comparisons with experiments.