The Density Dependence of Edge-Localized-Mode Suppression and Pump-out by Resonant Magnetic Perturbations in the DIII-D Tokamak

TL;DR

This study combines experiments and simulations to understand how density affects ELM suppression and density pump-out in the DIII-D tokamak, highlighting the role of narrow magnetic islands and their impact on pedestal stability.

Contribution

It demonstrates that low pedestal density is essential for RMP penetration and ELM suppression, with simulations predicting magnetic island effects consistent with experimental observations.

Findings

Low pedestal density is required for ELM suppression.

Magnetic islands cause pedestal pressure reduction stabilizing PBMs.

Density pump-out decreases with increasing density, matching experiments.

Abstract

The density dependence of edge-localized-mode (ELM) suppression and density pump-out (density reduction) by n = 2 resonant magnetic perturbations (RMPs) is consistent with the effects of narrow well-separated magnetic islands at the top and bottom of the H-mode pedestal in DIII-D low-collisionality plasmas. Nonlinear two-fluid MHD simulations for DIII-D ITER Similar Shape (ISS) discharges show that, at low collisionality, low pedestal density is required for resonant field penetration at the pedestal top, consistent with the ubiquitous low density requirement for ELM suppression in these DIII-D plasmas. The simulations predict a drop in the pedestal pressure due to parallel transport across these narrow width (0.02) magnetic islands at the top of the pedestal that is stabilizing to Peeling-Ballooning-Modes (PBMs), and comparable to the pedestal pressure reduction observed in experiment at the onset of ELM suppression. The simulations predict density pump-out at experimentally relevant levels (-20%) at low pedestal collisionality due to very narrow (~0.01-0.02) RMP driven magnetic islands at the pedestal foot at ~0.99. The simulations show decreasing pump-out with increasing density, consistent with experiment, resulting from the inverse dependence of parallel particle transport on resistivity at the foot of the pedestal. The robust screening of resonant fields is predicted between the top and bottom of the pedestal during density pump-out and ELM suppression, consistent with the preservation of strong temperature gradients in the edge transport barrier as seen in experiment.