Pellet triggering of edge localized modes in low collisionality pedestals at DIII-D

Abstract

Edge localized modes (ELMs) are triggered using deuterium pellets injected into plasmas with ITER-relevant low collisionality pedestals, and the resulting peak ELM energy fluence is reduced by approximately 25-50% relative to natural ELMs destabilized at similar pedestal pressures. Cryogenically frozen deuterium pellets are injected from the low-field side of the DIII-D tokamak at frequencies lower than the natural ELM frequency, and heat flux is measured by infrared cameras. Ideal MHD pedestal stability calculations show that without pellet injection, these low collisionality pedestals were limited by their current density (peeling-limited) rather than their pressure gradient (ballooning-limited). ELM triggering success correlates strongly with pellet mass, consistent with the theory that a large pressure perturbation is required to trigger an ELM in low collisionality discharges that are far from the ballooning stability boundary. For sufficiently large pellets, both instantaneous and time-integrated ELM energy deposition measured by infrared cameras is reduced with respect to naturally occurring ELMs at the inner strike point, which is the position where it is largest for natural ELMs. Energy fluence at the outer strike point is less effected. Cameras observing both heat flux and D-alpha emission often find significant toroidally asymmetric striations in the outboard far scrape-off layer resulting from ELMs that are triggered by pellets. Toroidal asymmetries at the inner strike point are similar between natural and pellet-triggered ELMs, suggesting that the reduction in peak heat flux and total fluence at that location is robust for the conditions reported here.

Simulation of pellet ELM triggering in low-collisionality, ITER-like discharges

“…The big pellet then triggers an ELM 20 ms later, identified by the spikes in all signals as highlighted by the vertical red bar. Also the ELM energy fluence changes for pellet triggered ELMs [22] like this, compared to natural ELMs. This observation repeats more or less throughout the discharge.…”

“…In the experiment [22], cryogenically frozen deuterium pellets were injected at the outboard midplane directed towards the magnetic axis. Two sizes of cylindrical pellets were used, each produced using a separate pellet gun but launched from the same injection point inside the A traditionally pacingsized pellet, with diameter 1.3 mm and length 0.9 mm, referred to as 'small' pellet from here on, is followed 20 ms later by a traditionally fueling-sized pellet, with diameter and length both 1.8 mm, henceforth referred to as 'big' pellet.…”

“…To demonstrate and predict the feasibility of pellet ELM triggering in ITER, it is necessary to study it in present-day machines under lowcollisionality conditions. Recently such experiments have been performed at DIII-D [22] and are studied numerically in this publication.…”

The effectiveness of D2 pellet injection in reducing intra-ELM and inter-ELM tungsten divertor erosion rates in DIII-D during the Metal Rings Campaign

“…The peak intra-ELM heat flux associated with the ELM decreased steadily with increasing D 2 mass injection. In past MRC and other DIII-D affiliated experiments, the peak intra-ELM heat flux and average pedestal stored energy drop monotonically decrease with increasing D 2 mass injection, while the ELM frequency increases [5,6]. In both figures 9 and 10, the error is considered the standard deviation of measurements associated with each ELM.…”

“…D 2 pellets injected at thermal velocities (approximately 100-200 m s −1 ) near the low-field side midplane of the tokamak, where instabilities are significant, have been demonstrated to mitigate ELM effects by triggering smaller and more frequent ELMs. D 2 pellet injection destabilizes the pedestal, perturbing the ballooning stability boundary [5,6]. Smaller ELMs result in a lower energy deposited in the divertor per ELM, providing a mechanism to remain below damage threshold of impacted PFMs.…”