Properties and Limits of the ELM-free Negative Triangularity Edge on DIII-D

The edge region of tokamak plasmas with strong negative triangularity (NT) shaping is fundamentally different from conventional L-mode or H-mode plasmas. Most prominently, NT plasmas are inherently free of edge localized modes (ELMs), even at injected powers well above the predicted L-H power threshold [1]. Though the edge pressure gradient is therefore reduced compared to ELMy H-mode plasmas, NT plasmas are still able to support small pedestals and are typically characterized by an enhancement of edge pressure gradients beyond those found in traditional L-mode plasmas. On DIII-D, the pressure gradient inside of this small pedestal is unusually steep - sometimes maintaining similar gradients to the pedestal itself well into the core region. This allows NT configurations to access performance measures competitive with other ELM-free regimes previously achieved on DIII-D that typically have larger edge pressures. Gyrokinetic and magnetohydrodynamic modeling of the edge region are used to develop physics-informed scalings for the edge pressure of NT discharges that can be used to inform extrapolations to NT-based fusion energy systems.

[1] A.O. NELSON, et. al, Phys. Rev. Lett. 131, 195101 (2023)