Magnetic Barriers and their q95 dependence at DIII-D

TL;DR

This study investigates how magnetic barriers formed by resonant magnetic perturbations at DIII-D depend on q95 and their potential role in ELM control, using field-line tracing with experimental equilibria.

Contribution

It provides the first corroboration of magnetic barriers at DIII-D and analyzes their dependence on q95, challenging previous assumptions about their role in ELM suppression.

Findings

Magnetic barriers form at DIII-D and depend on q95.

Barriers may narrow the stochastic edge layer.

No clear anti-correlation between barriers and ELM suppression.

Abstract

It is well known that externally generated resonant magnetic perturbations (RMPs) can form islands in the plasma edge. In turn, large overlapping islands generate stochastic fields, which are believed to play a role in the avoidance and suppression of edge localized modes (ELMs) at DIII-D. However, large coalescing islands can also generate, in the middle of these stochastic regions, KAM surfaces effectively acting as "barriers" against field-line dispersion and, indirectly, particle diffusion. It was predicted in [H. Ali and A. Punjabi, Plasma Phys. Control. Fusion 49 (2007), 1565-1582] that such magnetic barriers can form in piecewise analytic DIII-D plasma equilibria. In the present work, the formation of magnetic barriers at DIII-D is corroborated by field-line tracing calculations using experimentally constrained EFIT [L. Lao, et al., Nucl. Fusion 25, 1611 (1985)] DIII-D equilibria perturbed to include the vacuum field from the internal coils utilized in the experiments. According to these calculations, the occurrence and location of magnetic barriers depends on the edge safety factor q95. It was thus suggested that magnetic barriers might contribute to narrowing the edge stochastic layer and play an indirect role in the RMPs failing to control ELMs for certain values of q95. The analysis of DIII-D discharges where q95 was varied, however, does not show anti-correlation between barrier formation and ELM suppression.