Multiphysics Analysis of a Non-Resonant Internal Mode in NSTX

One class of tearing modes in the National Spherical Torus eXperiment (NSTX) occurs without any evidence of the usual triggering modes. Using several components of the SWIM framework [1], we have performed extensive linear and nonlinear MHD and hybrid MHD-kinetic analyses showing that these may be accounted for by a non-resonant mode with toroidal mode number $n=$1 that develops at moderate normalized \textit{$\beta $}$_{N}$ when the shear is low and the central safety factor $q_{0}$ is close to but greater than one. This mode, which is related to previously identified ``infernal'' modes [2], will saturate and persist, and can develop poloidal mode number $m=$2 magnetic islands in agreement with experiments. Our analysis includes a free-boundary transport simulation of an entire discharge showing that, with reasonable assumptions, we can predict the time of mode onset. Implications for ITER hybrid and AT regimes will be discussed. \\[4pt] [1] See web site http://cswim.org. \\[0pt] [2] J. Manickam, et al., \textit{Nucl. Fus}. \textbf{27}, 1461 (1987).