Mechanisms Defining Pedestal Structure in the DIII-D Negative Triangularity Edge

The edge region of tokamak plasmas with strong negative triangularity (NT) shaping maintains pedestal-like structures while being inherently free of edge localized modes (ELMs), even at injected powers well above the predicted L-H power threshold. Though the edge pressure gradient present in NT plasmas is reduced compared to ELMy H-mode plasmas, it is enhanced compared to traditional L-mode plasmas. This results in a small NT pedestal that correlates with improved confinement and performance in NT regimes, mimicking the performance improvement commonly associated with H-mode pedestals. In order to understand the physics effects responsible for setting the form of the NT pedestal, NT discharges from the DIII-D tokamak are studied with various gyrokinetic and magnetohydrodynamic modeling tools. Resistive modeling with BOUT++ suggests that low- and intermediate-n modes may play a larger role in the steep-gradient region of the pedestal whereas high-n modes are likely active near the separatrix. Scans of the edge pressure with gk_ped further indicate that kinetic ballooning modes may play a role in setting the pedestal top limit for a selection of the NT discharges. For most cases on DIII-D, flux-matched simulations with GKNN are able to adequately reproduce NT profiles from rho=0.1 to rho=0.98. Based on these results, progress toward the development of physics-informed scalings for the edge pressure of NT discharges that can be used to inform extrapolations to future ELM-free NT machines is reported.