Linear gyrokinetic analysis of a DIII-D H-mode pedestal near the ideal ballooning threshold

Abstract

Recent advances in GYRO allow simulations to map out the linear stability of many eigenvalues and eigenvectors of the gyrokinetic equation (as opposed to only the most unstable) at low computational cost. In this work, GYRO's new linear capabilities are applied to a pressure scan about the pedestal region of DIII-D shot 131997. MHD calculations in the infinite-n limit of the ideal ballooning mode, used in the very successful EPED model to predict pedestal height and width, demonstrate clear onset of the instability at 70% of the experimental pressure. Presented GYRO results first demonstrate that the ion temperature gradient driven mode and microtearing mode are dominant at the top of the pedestal, while an unnamed group of drift waves are found to be most unstable in the peak gradient region of the pedestal. The peak gradient modes have very extended ballooning structure, peak near the inboard midplane and have drift frequencies at or near the electron diamagnetic drift direction, even for very low wavenumbers (kθρs ∼ 0.2). Connection is made to the MHD calculations by demonstrating the kinetic ballooning mode (KBM) is present but subdominant in the DIII-D pedestal, and the pressure required for onset of the KBM in the gyrokinetic limit is in near agreement with MHD predictions. Finally, comparisons and analysis of GYRO with two independent gyrokinetic codes, GEM (initial value) and HD7 (1D eigenvalue), are presented.