Full orbit calculations show that the negative triangularity and H-mode plasma edge are equivalent

In DIII-D negative triangularity (NT) shaped plasmas, steep density and temperature gradients are observed, similar to those that can be present in H-mode. Despite the similar edge gradients, no phenomena that are characteristic for H-mode transition, such as a drop in the H_α-light and Edge Localized Modes (ELM) are observed when the triangularity is less than -0.18 [1].

In this presentation full ion orbit calculations using the SPIRAL code [2] are used to determine the maximum sustainable edge gradients for various triangularities (ranging from +0.6 to -0.5) and ion temperatures. It is shown that without the inclusion of particle-particle interactions, the gradients are set by finite orbit width effects but when scattering effects are included the maximum obtainable edge gradients decrease with decreasing triangularity.

The observed changed in the maximum sustainable edge gradients can be explained by triangularity-dependent changes in (parallel,perpendicular) velocity space: the loss cone region increases with decreasing triangularity thereby limiting the maximal obtainable edge gradients.

Although the edge gradients are reduced in NT plasma shapes, an edge pedestal similar to H-mode plasmas is still present albeit with reduced pressure gradients. The reduced pressure gradients can be favorable for the suppression of pressure driven edge modes.

[1] A.O. Nelson et al. NF 62 (2022) 096020

[2] G.J. Kramer et al. PPCF 55 (2013) 025013