H-mode Access and Limit Cycle Oscillations in DIII-D Negative Triangularity Plasmas

Negative triangularity L-mode plasmas in DIII-D exhibit H-mode-like thermal confinement and normalized pressure beta_n [1]. Their increased scrape-off layer width potentially opens a path for mitigating divertor power loading in future burning plasmas. Calculations show a reduced ballooning mode threshold prevents access to high pedestal pressure [2]. At moderate negative upper triangularity d_U ~-0.2, ELMing H-mode is still sustained in DIII-D, with a moderately increased power threshold P_LH ~3.5 MW compared to positive d_U plasmas with similar NBI torque. However, plasmas with high negative upper triangularity (d_U= -0.325) access ELMing H-mode only transiently, followed by extended limit cycle oscillations (LCO [3]), The edge electric field and normalized edge pressure gradient periodically increase but remain well below H-mode values up to the highest coupled power (~11 MW). Detailed gyrofluid stability analysis of the outer core plasma is presented.