The dominant micro-turbulence instabilities in the lower q95 high βp plasmas on DIII-D and predict-first extrapolation

Abstract

Large-radius internal transport barriers (ITB) are the signature of high scenarios on DIII-D. Previous studies show that a large Shafranov shift, rather the shear, suppresses the turbulence and helps in the formation of the large-radius ITB. New gyrokinetic simulations suggest that the remaining micro-instabilities in lower q95 (<7.0), high ITB plasmas are drift wave instabilities, including the collisionless trapped electron mode in the core and ITB peak gradient region, the electron temperature gradient mode in the ITB peak gradient region and at the ITB foot and the ion temperature gradient mode at the ITB foot. Gyrokinetic simulation results qualitatively agree with the density fluctuation analysis from beam emission spectroscopy, which suggests the existence of a low-k ion mode at the ITB foot. The gyrokinetic simulations also predict that a larger Shafranov shift can overwhelm the driving sources for turbulence from the profile gradients at higher , leading to stronger turbulence suppression and stronger ITBs in lower q95, high plasmas.