Results from radiating divertor experiments with RMP ELM suppression and mitigation

Abstract

The range in density and collisionality for which resonant magnetic perturbations (RMPs) are effective in suppressing edge-localized modes (ELMs) in the presence of a radiating divertor was found to be modest for representative H-mode plasmas in DIII-D. When deuterium and argon gas injection rates were increased during RMP, both the electron collisionality in the pedestal and the maximum electron pressure gradient (∇P e,MAX) in the pedestal also increased. As ∇P e,MAX approached values consistent with the peeling–ballooning stability limit, as determined by edge stability analysis, ELMing activity re-emerged. For cases with the same injected neutral beam power, argon accumulation in the main plasma was greater in the RMP ELM-suppressed cases than in comparable non-RMP ELMing H-mode cases. Reductions in the core concentration of injected argon were observed for both RMP and non-RMP H-mode cases when their respective deuterium injection rates were increased. Although complete ELM suppression in RMP radiating divertor plasmas in DIII-D was only accessible over a limited range in pedestal density and collisionality, significant ELM mitigation with heat flux reduction was possible over a wider range. Comparing RMP radiating divertor discharges after the re-appearance of ELMing activity during gas puffing with a standard ELMing plasma for cases with the same pedestal density reveals that the RMP discharges have (1) lower average electron temperature at the midplane separatrix, implying lower average electron temperature at the divertor target, (2) lower time-averaged peak heat flux and (3) lower transient peak heat flux from ELMs even at the same pedestal collisionality.

Impurity ion flow and temperature measured in a detached divertor with externally applied non-axisymmetric fields on DIII-D

“…It has been shown in NSTX [2] that RMPs increase the amount of gas needed to achieve divertor detachment. Similar results were seen in DIII-D [3], where applying RMPs caused a reduction in the pedestal density and promoted attachment of the inner divertor leg. Subsequent gas puffing to increase the pedestal density caused "early" detachment of the inner leg at a pedestal density below the value needed to achieve detachment without RMPs.…”

“…Neither ELM suppression nor mitigation was seen when the RMPs were applied for these cases. ELM suppression in DIII-D is generally only seen when the density at the top of the pedestal (n e ped ) is below 5 about 2.5x10 19 m -3 , [3] which is much lower than the pedestal density, shown in Fig. 1(c), used in this experiment to achieve detachment of both strike points.…”

Physics news on the Internet (based on electronic preprints)

“…Externally applied 3D MPs have been shown to either suppress or mitigate large, type I ELMs in DIII-D [16,17,[59][60][61][62][63], JET [64][65][66][67], ASDEX-Upgrade [20,[68][69][70][71], KSTAR [72,73], and MAST [74][75][76][77]. There are common characteristics in the way that the plasmas respond to the 3D fields, but also apparent differences.…”

“…Type I ELMs can be completely suppressed in DIII-D (ν * e < 0.35 and also ν * e > 1) [62,63] and in KSTAR (ν * e < 0.5 − 1) [72]. The key requirements are (1) the edge safety factor is maintained in one of several resonant windows and (2) the resonant magnetic perturbations (RMP) amplitude exceeds a threshold normalized radial field perturbation, δB r /B t .…”

Guiding Center Orbit Following Calculation of Edge Particle and Heat Transport in Stochastic Magnetic Field

“…These puzzling features are further confirmed in recent DIII-D experiments. [3,4] A few possible ingredients are considered in this work to account for the inconsistency: (1) magnetic field lines are not completely stochastic [5] and the remnants of magnetic islands play a role, (2) trapped particles effects due to the equilibrium magnetic field inhomogeneity, (3) radial electric field and convective transport. [6] To investigate these latter effects, we employ a guiding center particle model.Being different from self-generated tearing instabilities, [7] in the RMP experiments, magnetic perturbations are given by the external magnetic I-coils.…”