Robust Avoidance of Edge-Localized Modes alongside Gradient Formation in the Negative Triangularity Tokamak Edge

Abstract

In a series of high performance diverted discharges on DIII-D, we demonstrate that strong negative triangularity (NT) shaping robustly suppresses all edge-localized mode (ELM) activity over a wide range of plasma conditions: $⟨n⟩=0.1--1.5\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{ }{\mathrm{m}}^{\ensuremath{-}3}$, ${P}_{\mathrm{aux}}=0--15\text{ }\text{ }\mathrm{MW}$, and $|{B}_{\mathrm{t}}|=1--2.2\text{ }\text{ }\mathrm{T}$, corresponding to ${P}_{\mathrm{loss}}/{P}_{\mathrm{LH}08}\ensuremath{\sim}8$. The full dataset is consistent with the theoretical prediction that magnetic shear in the NT edge inhibits access to ELMing $H$-mode regimes; all experimental pressure profiles are found to be at or below the infinite-$n$ ballooning stability limit. Our present dataset also features edge pressure gradients in strong NT that are closer to an $H$-mode than a typical $L$-mode plasma, supporting the consideration of NT for reactor design.