Stabilizing Effects of Negative Triangularity on Microinstability-Driven Turbulence

Negative triangularity shaping of tokamak plasmas has been shown to improve confinement both experimentally and numerically. This is theorized to be due to stabilization of ion-scale microinstabilities, leading to the suppression of turbulent transport. We investigate the stability properties of collisionless ion-temperature-gradient (ITG), trapped-electron-mode (TEM), and kinetic-ballooning-mode (KBM) instabilities in axisymmetric equilibria. The global electromagnetic gyrokinetic code XGC is used to simulate up-down symmetric positive- and negative-triangularity geometries of DIII-D-like equilibria at varying temperature and density profiles and plasma beta. We compare critical gradients, growth rates, and stability boundaries of these collisionless microinstabilities to study the strength and extent of the negative-triangularity-driven stabilization mechanism in different regimes. At low beta, kinetic electron simulations show that both ITG and TEM are linearly stabilized by negative triangularity. We also compare critical gradients for ideal ballooning modes with those for KBM instabilities and assess the significance of negative triangularity effects within experimental operating regimes.