The impact of ELMs on the ITER divertor

Edge-Localized-Modes (ELMs) are expected to present a significant transient flux of energy and particles to the ITER divertor. The threshold for ablation of the graphite target will be reached if the ELM transient exceeds Q/t(1/2) similar to 45 MJ m(-2) s(-1/2) where Q is the ELM deposition energy density and t the ELM deposition time. The ablation parameter in ITER can be determined by scaling four factors from present experiments: the ELM energy loss from the core plasma, the fraction of ELM energy deposited on the divertor target, the area of the ELM profile onto the target, and finally the time for the ELM deposition. Review of the ELM energy loss of Type I ELM data suggests an ITER ELM energy loss of 2-6% of the stored energy or 25-80 MJ. The fraction of heating power crossing the separatrix due to ELMs is nearly constant (20-40%) resulting in an inverse relationship between ELM amplitude and frequency. Measurements on DIII-D and ASDEX-Upgrade indicate that 50-80% of the ELM energy is deposited on the target. There is currently no evidence for a large fraction of the ELM energy being dissipated through radiation. Profiles of the ELM heat flux are typically 1-2 times the width of steady heat flux between ELMs, with the ELM amplitude usually larger on the inboard target. The ELM deposition time varies from about 0.1 ms in JET to as high as 1.0 ms in ASDEX-Upgrade and DIII-D. The ELM deposition time for ITER will depend upon the level of conductive Versus convective transport determined by the ratio of energy to particles released by the ELM. Preliminary analysis suggests that large Type I ELMs for low recycling II-mode may exceed the ablation parameter by a factor of 5. Promising regimes with smaller ELMS have been found at other edge operational regimes, including high density with gas puffing, use of RF heating and operation with Type III ELMs. (C) 1999 Elsevier Science B.V. All rights reserved.