PLASMA-SURFACE INTERACTIONS 21 — Proceedings of the 21st International Conference on Plasma-Surface Interactions in Controlled Fusion Devices Kanazawa, Japan May 26-30, 2014
The triggering of rapid small edge localized modes (ELMs) by high frequency pellet injection has been
proposed as a method to prevent large naturally occurring ELMs that can erode the ITER plasma facing
components (PFCs). Deuterium pellet injection has been used to successfully demonstrate the ondemand
triggering of edge localized modes (ELMs) at much higher rates and with much smaller intensity
than natural ELMs. The proposed hypothesis for the triggering mechanism of ELMs by pellets is the local
pressure perturbation resulting from reheating of the pellet cloud that can exceed the local high-n
ballooning mode threshold where the pellet is injected. Nonlinear MHD simulations of the pellet ELM
triggering show destabilization of high-n ballooning modes by such a local pressure perturbation.
A review of the recent pellet ELM triggering results from ASDEX Upgrade (AUG), DIII-D, and JET reveals
that a number of uncertainties about this ELM mitigation technique still remain. These include the heat
flux impact pattern on the divertor and wall from pellet triggered and natural ELMs, the necessary pellet
size and injection location to reliably trigger ELMs, and the level of fueling to be expected from ELM
triggering pellets and synergy with larger fueling pellets. The implications of these issues for pellet
ELM mitigation in ITER and its impact on the PFCs are presented along with the design features of the
pellet injection system for ITER.
2014 Elsevier B.V. All rights reserved.
The H-mode confinement regime of plasma operation is planned
for ITER in order to achieve high fusion performance. It is characterized
by a steep pressure gradient and ‘‘pedestal’’ at the plasma edge
that is expected to lead to the quasi-periodic instability of edgelocalized
modes (ELMs) . ELMs expel periodic bursts of particles
and energy from the plasma, which if large enough can pose a serious
threat to the PFCs by erosion and melting from the high heat