Linear ion-scale micro-stability analysis of high and low-collisionality NSTX discharges and NSTX-U projections

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

11th ITER International School on “ITER Plasma Scenarios and Control”

San Diego, California, USA, July 25-29, 2022

NSTX high β and high collisionality discharges have shown ion transport near neoclassical levels. In addition, data from NSTX discharges indicate that the normalized energy-confinement time scales nearly inversely with collisionality. One of the main motivations for NSTX-U is to investigate increasingly reduced collisionality regimes in order to confirm this trend, which might have a direct impact on the performance and success of future low aspect ratio power fusion reactors. Therefore, it is of primary interest to better understand the role of turbulent ion transport in those regimes and to determine whether it can be relevant and if its presence will degrade the confinement. These results can also enlighten the election of reduced models when conducting predictive simulations for scenario planning. As a first step, a broad series of linear gyrokinetic simulations were used to investigate micro-stability properties for NSTX discharges and NSTX-U projections over a range of collisionalities. The simulations were performed using the code CGYRO and were focused on the confinement region in which unstable modes in the ion-scale wavenumber range, were identified. Three different NSTX discharges with high, medium and low collisionality were chosen, as well as an NSTX-U projection with collisionalities even lower than those of NSTX. Throughout all the cases that were analyzed, a complex mode zoology was found to be present at the experimental or projected conditions, including microtearing modes, which can contribute to electron transport as well as hybrid trapped electron modes/kinetic ballooning modes (TEM/KBM), which can also contribute to ion transport. Thresholds of ion-directed modes such as kinetic ballooning modes (KBMs) and ion temperature gradient modes (ITGs) were determined in most cases. KBMs were found present in many cases, with their thresholds close or at the experimental or projected value, providing further evidence that they may play a role limiting the confinement. On the other hand, ITG were never found to be unstable at the experimental or projected conditions and their thresholds occur at increased ion temperature gradients, even for the chosen NSTX-U condition, which has the steepest projected gradients and lowest collisionalities expected for NSTX-U. In almost all cases KBM thresholds are found to be triggered first than ITGs. Diagrams summarizing the results with dominant modes and thresholds are presented.

*Supported by the US DOE under DE-SC0021385 and DE-SC0013977.