Comparison of Lithium Granule Induced ELM triggering threshold levels for EAST and DIII-D

摘要

Introduction - On both the EAST and DIII-D tokamaks the radial injection of lithium granules has been shown to trigger Edge Localized Modes (ELMs)[1,2]. The granules ablate upon entering the steep gradient region and the rapidly ionized ablatant becomes entrained within an overdense flux tube. This can lead to the generation of a ballooning instability, thus initiating an ELM. The control of ELMs in a manner that mitigates their impact on plasma facing surfaces is required for next step fusion devices such as ITER. It is anticipated[3] that the thermal cycling of unmitigated ELMs will rapidly shorten the effective divertor lifetime. To reduce this effect, one of the baseline strategies for ELM control is a rapid stimulated triggering of the ELMs, termed "pacing". The utilization of pacing as an effective mitigation strategy is dependent upon two connected tenets. First, there must be a demonstrated inverse relationship between the frequency of the triggered ELMs and the peak heat flux delivered to the PFCs and second, the ELMs must be triggered regularly and reliably. Any cessation in the production of stimulated ELMs could lead to a return of the natural ELM cycle, thus resulting in an intolerable energy release. While the former question is by no means settled, it is this second condition which we focus on here.