Linear ion-scale micro-stability analysis of high and low-collisionality NSTX discharges and NSTX-U projections
C. Clauser, W. Guttenfelder, T. Rafiq and E. Schuster
Physics of Plasmas 29, 102303 (2022)
Abstract
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Linear gyrokinetic simulations were conducted to investigate ion-gyroradius-scale
micro-instability predictions for high-beta NSTX discharges and NSTX-U
projections that span over an order of magnitude variation in collisionality.
A complex mix of microtearing modes and hybrid trapped electron modes/kinetic
ballooning modes (TEM/KBM) is predicted for all experimental or projected
conditions. Ion temperature gradient (ITG) instabilities are typically
stable in the NSTX discharges investigated, consistent with the observed
neoclassical ion thermal transport. ITG thresholds inferred from the
simulations are typically much higher than the experimental NSTX gradients,
as well as the projected gradients in the NSTX-U scenario, which assumed
ion temperatures limited by neoclassical transport only. The analysis
suggests ITG instabilities are unlikely to contribute significant anomalous
thermal losses in high-beta, lower collisionality NSTX-U scenarios. On
the other hand, the NSTX experimental profiles and NSTX-U projections
are predicted to be very close to the predicted onset of unstable KBM at
most radii investigated. The proximity of the various discharges to the
KBM instability threshold implies it may play an important role in setting
profile shapes and limiting global energy confinement. It remains to be
understood and predicted how KBM contributes to multichannel transport
(thermal and particle transport, for both ions and electrons) in a way
that is consistent with experimental inferences.