Full-orbit simulation of charge separation for negative triangularity plasma shapes in DIII-D

In DIII-D negative triangularity (NT) shaped plasmas, H-modes are not observed when the triangularity is stronger than about -0.18 [1]. A characteristic of the H-mode is the presence of a radial electric field at the plasma edge. This field suppresses the edge fluctuations via the ExB shear flow mechanism and creates a a transport barrier. Ion finite orbit width (FOW) effects can cause charge separation at the plasma edge and form the source for the observed electric field. In this presentation, the effects of charge separation due to FOW effects near the plasma edge are presented. Ion orbits are calculated for thermal distributions using the full-orbit following SPIRAL code [2]. Ion densities outside the last closed flux surface (LCFS) were calculated as a function of the the plasma shape. It was found that the ion densities outside the LCFS are comparable for positive triangularity and weakly NTs, where H-modes were observed. For strongly NT shaped without H-mode characteristics, the ion density outside the LCFS is decreased, indicating that the electric field is reduced and therefore, less effective in suppressing edge fluctuations.

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

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