Edge stability of stationary ELM-suppressed regimes on DIII-D

Abstract

We discuss the MHD stability of the H-mode pedestal region in two edge localized mode (ELM)-suppressed H-mode regimes on DIII-D, the quiescent (Q)H-mode and resonant magnetic perturbation (RMP) H-mode. The QH-mode is obtained at low density with most of the neutral beam power injected counter to the plasma current, and is characterized by a continuous, low toroidal mode number (n), edge harmonic oscillation (EHO). QH-mode is observed to transition back to ELMing H-mode as the rotational shear is decreased by increasing the co-current neutral beam power fraction or by increasing the plasma to conducting wall distance, consistent with a model for the EHO as a peeling instability with added drive from rotational shear, which saturates through self transport of momentum or damping of rotation through drag of the mode on the conducting wall. In RMP H-mode, an n = 3 coil provides the non-axisymmetric perturbation. Two regimes of RMP ELM suppression are observed. At low collisionality with large resonant field perturbation, the pedestal pressure gradient and current density are reduced below the peeling-ballooning mode stability limit; while at higher collisionality with large non-resonant perturbation there is little change in the pedestal parameters and type II ELMs are enhanced.