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

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 to observation and predictions for future devices. We briefly present recent progress in the peelingballooning model, including studies of the apparent power dependence of the pedestal, and studies of the impact of sheared toroidal flow. Nonlinear 3D simulations of ELM dynamics using the BOUT code are also described, leading to an emerging understanding of the physics of the onset and dynamics of ELMs in the standard intermediate to high collisionality regime. 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 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.

Edge pedestal control in quiescent H-mode discharges in DIII-D using co-plus counter-neutral beam injection

“…4, the QH-mode operating point is near the peeling stability boundary; this result has been seen for all cases where the edge stability has been analyzed with ELITE [1,8,12,13]. The theory predicts that the EHO should exist near this boundary because the low n modes are the most unstable along this boundary and these are the modes which are further destabilized by rotational shear [13].…”

“…The EHO is an electromagnetic mode localized in the edge pedestal region which provides the extra particle transport necessary to keep the edge operating point below the peeling-ballooning boundary in QH-mode [1,12]. Based on the observation that the predicted growth rates for modes with low toroidal mode number € n increase with increasing shear in the edge rotation [13], the theory predicts that the EHO is a low € n peeling mode driven unstable by rotational shear at edge conditions slightly below the ELM stability limit in the absence of rotation. As the mode grows to finite amplitude, its magnetic fields interact with the vacuum vessel wall, slowing the plasma rotation, decreasing the rotational shear and, hence, reducing the drive for the mode, thus allowing the mode to saturate at finite amplitude.…”

Effect of island overlap on edge localized mode suppression by resonant magnetic perturbations in DIII-D

“…Linear peeling-ballooning ͑P-B͒ stability analysis with the ELITE code [56][57][58] indicates that in the ISS plasmas, suppression of Type-I ELMs occurs because application of n = 3 RMPs moves the pedestal plasma operating point from the unstable region to the stable region, 10,59 as was observed in lower triangularity plasmas with the ITER pedestal collisionality. The linear P-B calculations indicate that the ELM stabilization by the RMPs is due to reductions of both the pedestal pressure gradient and the calculated edge bootstrap current.…”

21st IAEA Fusion Energy Conference: summary of sessions EX/D, EX/S and EX/W

“…ELM physics studies are intimately linked to the characterisation of the filamentary structures as described above in section 2.1. While nonlinear models to describe the ELM process in detail are just starting to be developed, linear stability that predicts the mode onset is now wellestablished in terms of the peeling-ballooning model that can explain many features of ELM stability [44].…”