Energetic particle effects on n=1 MHD instabilities in a DIII-D hybrid discharge*

The $\delta f$ kinetic-MHD model in the 3-D extended MHD code NIMROD is used to perform a simulation study of energetic particle effects on the $n=1$ mode in a DIII-D hybrid discharge. The hybrid has low $q_{min} >\sim 1$ at high confinement, and is a candidate operational scenario for burning plasma experiments. However hybrid discharges are limited to moderate $\beta_N$ by the $m/n=2/1$ instability. Using realistic DIII-D equilibria, the stability of the $n=1$ mode is computed over a ($q_{min}$,$\beta_{N}$) space. Unstable modes are driven by energetic particles far into the MHD stable region in this space. The drive is associated with the fishbone mode or BAE mode, depending on $q_{min}$. The stability boundary is found near the experimental ($q_{min}$,$\beta_N$), where the unstable mode has a $m/n=1/1$ component localized near the axis. Experimentally, a $m/n=1/1$ structure is observed in agreement with the computed mode in key physical respects. At higher $q_{min}$ and $\beta_N$ a mode with a broad $m/n=2/1$ structure is unstable. This suggests that the $m/n=2/1$ mode is triggered by energetic particles in these discharges, as $\beta_N$ is increased. A group of several similar discharges shows strong agreement with this computational explanation of onset.