ELM mitigation by supersonic molecular beam injection: KSTAR and HL-2A experiments and theory

Abstract

We report recent experimental results from HL-2A and KSTAR on ELM mitigation by supersonic molecular beam injection (SMBI). Cold particle deposition within the pedestal by SMBI is verified in both machines.The signatures of ELM mitigation by SMBI are an ELM frequency increase and ELM amplitude decrease.These persist for an SMBI influence time τI. Here, τI is the time for the SMBI influenced pedestal profile to refill. An increase in f SMBI ELM /f 0 ELM and a decrease in the energy loss per ELM WELM were achieved in both machines. Physical insight was gleaned from studies of density and vφ(toroidal rotation velocity) evolution, particle flux and turbulence spectra, divertor heat load. The characteristic gradients of the pedestal density soften and a change in vφwas observed during a τI time. The spectra of the edge particle flux Г~ < ˜vr˜ne> and density fluctuation with and without SMBI were measured in HL-2A and in KSTAR, respectively. A clear phenomenon observed is the decrease in divertor heat load during the τI time in HL-2A. Similar results are the profiles of saturation current density Jsat with and without SMBI in KSTAR. We note that τI/τp (particle confinement time) is close to ~1, although there is a large difference in individual τI between the two machines. This suggests that τI is strongly related to particle-transport events. Experiments and analysis of a simple phenomenological model support the important conclusion that ELM mitigation by SMBI results from an increase in higher frequency fluctuations and transport events in the pedestal.

Propagation Dynamics Associated with Resonant Magnetic Perturbation Fields in High-Confinement Mode Plasmas inside the KSTAR Tokamak

“…3. In contrast, without RMP, applying SMBI only led to a uniform reduction in toroidal rotation (not a localized V ϕ braking around the rational surface) [12].…”

“…Although no phase minima are observed when the modulated SMBI is applied without RMP in KSTAR H-mode plasmas, there is a uniform reduction in the rotation profile due to the edge localized drag from the SMBI [12]. Thus the presence of the RMP field in plasma and the presence of a phase minima in the plasma response to RMP can be uniquely determined by using small edge perturbation caused by SMBI pulses.…”

“…In this Letter, to probe the propagation of the RMP fields in plasma, we actively induce perturbations in the plasma response by applying small edge plasma perturbations with a train of pulsed supersonic molecular beam injections [12] during a steady state RMP. The use of modulated SMBI provides a time varying perturbation to both the plasma density source in the region just inside the last closed flux surface (via modulation of the neutral density) and a modulated flow damping rate (via ion-neutral charge exchange and elastic scattering).…”

“…In perturbative plasma transport experiments, a minimum phase position generally indicates the position of an induced perturbation source [13,15]. The induced minimum phase in v ϕ is unlikely to be due to a modulation of the "neutral particle source" because SMBI neutral particles cannot penetrate through the edge plasma to reach the q ¼ 3 position, let alone q ¼ 2 positions in KSTAR H-mode discharges [12]. Thus, the perturbation source in the plasma, corresponding to the minimum phase location, must be induced by a difference in plasma response due to the static RMP field with and without the edge SMBI perturbation.…”

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“…SMBI has been used to ameliorate ELMs in a number of devices, including HL-2 A [137,168,169] and KSTAR [73,169]. An example of ELM mitigation with SMBI from HL-2 A is shown in figure 32.…”