Stability and dynamics of the edge pedestal in the low collisionality regime: physics mechanisms for steady-state ELM-free operation

TLDR

Understanding edge pedestal physics and ELMs is critical for ITER, with the peeling–ballooning model explaining ELM onset and pedestal limits, and recent discoveries of low‑collisionality, ELM‑free regimes such as QH and RMP. The paper reviews recent progress in the peeling–ballooning model, experimental validation, nonlinear 3D ELM simulations, and new studies of pedestal power dependence and sheared toroidal flow, and proposes a model for QH mode driven by a saturated kink/peeling edge harmonic oscillation. The authors use the ELITE code to evaluate peeling–ballooning stability, perform nonlinear 3D simulations of ELM dynamics, and develop a model in which the QH edge harmonic oscillation is destabilized by current and rotation. DIII‑D observations show density, shape, and rotation dependence of QH discharges, with the model accurately predicting density dependence and qualitatively matching mode structure, rotation, and outer gap, while low‑density RMP discharges exhibit a similar regime with the EHO replaced by an applied magnetic perturbation.

Abstract

Understanding the physics of the edge pedestal and edge localized modes (ELMs) is of great importance for ITER and the optimization of the tokamak concept. The peeling–ballooning model has quantitatively explained many observations, including ELM onset and pedestal constraints, in the standard H-mode regime. The ELITE code has been developed to efficiently evaluate peeling–ballooning stability for comparison with observation and predictions for future devices. We briefly review recent progress in the peeling–ballooning model, including experimental validation of ELM onset and pedestal height predictions, and nonlinear 3D simulations of ELM dynamics, which together lead to an emerging understanding of the physics of the onset and dynamics of ELMs in the standard intermediate to high collisionality regime. We also discuss new studies of the apparent power dependence of the pedestal, and studies of the impact of sheared toroidal flow. Recently, highly promising low collisionality regimes without ELMs have been discovered, including the quiescent H-mode (QH) and resonant magnetic perturbation (RMP) regimes. We present recent observations from the DIII-D tokamak of the density, shape and rotation dependence of QH discharges, and studies of the peeling–ballooning stability in this regime. We propose a model of the QH-mode in which the observed edge harmonic oscillation (EHO) is a saturated kink/peeling mode which is destabilized by current and rotation, and drives significant transport, allowing a near steady-state edge plasma. The model quantitatively predicts the observed density dependence and qualitatively predicts observed mode structure, rotation dependence and outer gap dependence. Low density RMP discharges are found to operate in a similar regime, but with the EHO replaced by an applied magnetic perturbation.