Linear Gyrokinetic Analysis of the Grassy-ELM Regime with Edge Resonant Magnetic Perturbations in DIII-D

The gyrokinetic code CGYRO is used to study the linear stability of the pedestal during application of resonant magnetic perturbations (RMPs). For the studied discharge, type-I ELMs are suppressed and the pedestal exhibits cyclic pulsations between a grassy-ELM state and an ELM suppressed state. While the supressed state has a more conventional DIII-D H-mode pedestal, the grassy-ELM state has a wider pedestal with a locally flattened region in the density and temperature profiles, in the steep gradient region (SGR). For both states, simulations show that at pedestal top microtearing modes (MT) are dominant for k_θρ_S<1, and ion temperature gradient driven instabilities (ITG) are dominant over the broader ion scales, up to k_θρ_S~2. In the SGR the dominant modes are trapped electron modes (TEM) which transition to electron temperature gradient modes at k_θρ_S~0.3, except in the flattened region in the SGR of the grassy-ELM state. Here, for k_θρ_S<0.1 MT are dominant, whereas for k_θρ_S>0.1 TEM dominate. Although here TEM have higher growth rates than MT, a general mixing length estimate for electron energy flux is larger for MT, suggesting these modes may have a role in the turbulent transport of the grassy-ELM state.