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

Abstract

The δf kinetic-magnetohydrodynamic (MHD) model in the 3D extended-MHD code NIMROD is used to perform a linear simulation study of energetic particle effects on the n = 1 mode in a DIII-D hybrid discharge. The hybrid discharge has a long steady state with low qmin ≳ 1 at high confinement, is useful for numerous physics studies, and is a candidate operational scenario for burning plasma experiments. But hybrid discharges are limited to moderate βN by the m/n = 2/1 instability, which onsets with small increases in βN. Using realistic equilibria based on experimental reconstructions from DIII-D, the stability of the n = 1 mode during the steady state of a hybrid discharge is computed over a (qmin, βN) space. MHD stability analyses do not indicate instability to the n = 1 for small increases in βN above that of the experimental discharge. Our results show that energetic particles significantly change the stability map in (qmin, βN) parameter space from the MHD-only result. Unstable modes are driven by energetic particles far into the MHD stable region in (qmin, βN) space. Three different unstable regions are identified, each being defined by the fastest growing mode and distinctly different frequencies. We examine sample eigenmodes from these three regions. At low qmin ∼ 1 the drive is associated with the fishbone mode, while the higher qmin ≳ 1.12 the drive is associated with the Beta induced Alfven eigenmode. Overlaying the experimental trajectory in the same (qmin, βN) parameter space shows that all three regions are in proximity to the trajectory. In the higher qmin region a mode with a broad m/n = 2/1 dominant structure is most unstable, while in the region of the trajectory the most unstable mode has a m/n = 1/1 component localized near the axis. Experimentally, a nonlinearly saturated m/n = 1/1 structure is observed localized near the magnetic axis. This agreement suggests that the m/n = 2/1 mode may be triggered by energetic particles in these discharges.