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: $⟨n⟩=0.1-1.5\times 10^{20}$m${}^{-3}$, $P_{\mathrm{aux}}=0-15$MW and $|B_{\mathrm{t}}|=1-2.2$T, corresponding to $P_{\mathrm{loss}}/P_{LH08}\sim 8$. 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. Importantly, we also report enhanced edge pressure gradients at strong NT that are significantly steeper than in traditional ELM-free L-mode plasmas and provide significant promise for NT reactor integration.

5 pages, 5 figures