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 q min ≳ 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 (q min, 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 (q min, βN) parameter space from the MHD-only result. Unstable modes are driven by energetic particles far into the MHD stable region in (q min, 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 q min ∼ 1 the drive is associated with the fishbone mode, while the higher q min ≳ 1.12 the drive is associated with the Beta induced Alfven eigenmode. Overlaying the experimental trajectory in the same (q min, βN) parameter space shows that all three regions are in proximity to the trajectory. In the higher q min 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.

A model of energetic ion effects on pressure driven tearing modes in tokamaks

“…Simulations of tokamaks have shown that energetic particles can drive the development of resistive instabilities as shown in Ref. [14], but such particle contributions have also been shown to dampen the mode in simulations as in Ref. [15].…”

“…Using a "slowing-down" distribution is not only physically justified, but also gives this study a point of consistency with Refs. [15] and [14] among many others.…”

“…In Ref. [14] it was found that the addition of energetic ions in weakly sheared or slightly reversed shear equilibrium configurations can drive n = 1 instabilities. For low q min 1, a non-resonant m/n = 1/1 instability was driven by energetic ions with fishbone characteristics, where similar results have been found by other groups [16].…”

“…These results for the 2/1 mode, between Refs. [15] and [14], would seem to conflict, given an intuition that the energetic ion contributions to the 2/1 mode stability would dominantly originate from the vicinity surrounding the q = 2 surface, which does not differ greatly in characteristics between the two cases. However, this paper addresses how the differences in the equilibrium shear configurations in the core alter the contribution of energetic ions to the stability of the 2/1 mode.…”

“…On the other hand, hybrid discharges in DIIID, on which the work of Ref. [14] was inspired, have flat or weakly reversed q profiles, and the energetic ions appear to destabilize the 2/1 mode in these discharges.…”