Effective Transport Barriers without H-mode Bifurcation in DIII-D Negative Triangularity Plasmas*

Diverted negative triangularity (NT) plasmas in DIII-D achieve high thermal confinement (H_98y2 >~1) and normalized pressure β_N >2.5 [1]. At strongly negative average triangularity (δ_Ave ≤ -0.35) these plasmas do not bifurcate to H-mode even at high heating power P_loss/P_LH-08 ≥10, but exbibit a narrow edge transport barrier on the order of ~2 ion gyro-radii wide. This barrier is supported by strong counter-current intrinsic rotation shear, consistent with theory [2] based on preferential turbulent loss of co-current passing ions in NT plasmas. Doppler Backscattering turbulence data demonstrate very short radial turbulence correlation lengths and the suppression of turbulence streamers/avalanches within the NT barrier, evidenced by a locally reduced Hurst exponent. Experimental evidence and infinite-n ballooning mode stability calculations suggest that the barrier layer cannot expand inward [3]; hence strong NT plasmas do not approach the peeling-ballooning stability limit and remain free of Edge Localized Modes (ELMs). In contrast, modest NT plasmas (δ_Ave ≥ - 0.2) are characterized by intrinsic co-current edge rotation and edge profiles similar to typical PT plasmas, accessing ELMing H-mode at a power threshold below the 2008 ITPA power threshold scaling P_LH-08.

[1] M. Austin, M. Marinoni, M.L. Walker et al., Phys. Rev. Lett. 122 115001 (2019).

[2] T. Stoltzfus-Dueck, A.N. Karpushov,  O. Sauter et al., Phys. Rev. Lett. 114 245001 (2015).

[3] A.O. Nelson, L. Schmitz, C. Paz-Soldan et al., Phys. Rev. Lett. 131 195101 (2023).