Advances in the physics understanding of ELM suppression using resonant magnetic perturbations in DIII-D

Abstract

Recent experiments on DIII-D have increased confidence in the ability to suppress edge-localized modes (ELMs) using edge-resonant magnetic perturbations (RMPs) in ITER, including an improved physics basis for the edge response to RMPs as well as expansion of RMP ELM suppression to more ITER-like conditions. Complete ELM suppression has been achieved utilizing n = 3 RMPs in the ITER baseline scenario. In addition, RMP ELM suppression has been expanded to include plasmas with helium concentrations near 25% and the use of n = 2 RMPs. Analysis of the kinetic profile response suggests that ELM suppression is correlated with the co-alignment of the ω ⊥e = 0 location, an n = 3 rational surface, and the top of the pedestal. Modelling predicts that such a co-alignment could potentially lead to island (or island chain) formation just inside the top of the pedestal, inhibiting the growth of the pedestal and thereby maintaining the ELM-free state. Detailed analysis of data obtained during toroidal phase variations of the applied n = 3 RMPs have provided further evidence of an island-like structure at the top of the pedestal. In addition, nearly matched discharges with co-neutral-beam-injection (co-NBI) and counter-NBI have demonstrated the importance of the presence of the ω ⊥e = 0 location for ELM suppression. In the counter-NBI cases, the toroidal rotation profile is such that there is no ω ⊥e = 0 location and ELMs are not suppressed in conditions in which ELM suppression is generally observed with co-NBI.

Non-linear modeling of the threshold between ELM mitigation and ELM suppression by resonant magnetic perturbations in ASDEX upgrade

“…Ref. [29]) suggest that ELM suppression is obtained when RMPs sufficiently enhance the edge transport, such that the pressure gradient and current density are reduced below the ELM stability limit at the pedestal top, similarly to the initial goal of RMPs described in introduction. On the other hand, this paper describes an alternative mechanism of ELM suppression, in which the reduction of the pressure gradient and current density below the stability limit is not a necessary condition.…”

Two-fluid nonlinear theory of response of tokamak plasma to resonant magnetic perturbation

“…17 It is hypothesized that pedestal expansion is halted by a region of enhanced transport at the top of the pedestal that is not suppressed by E × B velocity shear, and that this region is associated with the presence of an RMP-driven magnetic island chain located just outside, but close to the top of, the pedestal. 18 The aim of this paper is to develop a physical understanding of the dynamics of magnetic reconnection driven at a single rational surface that is resonant with one of the dominant helical harmonics of an applied RMP. The rational surface is assumed to be located just outside, but close to the top of, the pedestal of an H-mode tokamak plasma.…”

Modelling of plasma response to 3D external magnetic field perturbations in EAST

“…The last, and so far the most mature and reliable technique, is the application of the resonant magnetic perturbation (RMP) fields. The 3 ELMs can often be well mitigated [9], or even fully suppressed [10] using the RMP coils.…”