Linear stability analysis of ion-scale microturbulence in low and high collisionality NSTX discharges

C.F. Clauser, W. Guttenfelder, T. Rafiq, E. Schuster

Division of Plasma Physics (DPP) Annual Meeting of the American Physical Society (APS)

Pittsburgh, PA, USA (Remote), November 8-12, 2021

Future NSTX-U discharges will focus on low collisionality regimes where turbulence-driven thermal transport could play a critical role. The gyrokinetic code CGYRO is used to analyze a high-beta and high-collisionality NSTX discharge in both electrostatic and electromagnetic limits to better understand the transition from high to low collisionality regimes and the influence of anomalous thermal transport on the energy confinement time. Microinstabilities such as ion temperature gradient and trapped electron modes are shown to be stable in the electrostatic limit due to the effects of flow shear. In the electromagnetic limit, on the other hand, kinetic ballooning modes and microtearing modes are shown to be unstable, with the latter being the dominant instability when experimental profile values are used. Scans of a variety of parameters, including ion temperature gradient, pressure gradient, electron beta, and wave number, are performed to examine these instabilities and their stability thresholds. The results of a low collisionality NSTX discharge are also compared to those of a high collisionality discharge. Finally, the NSTX-U low collisionality expecting conditions are investigated. The results are contrasted to gain a better understanding of the impact of collisionality on these regimes.

*Supported by the US DOE under DE-SC0021385.