Infoscience

Conference paper

The impact of large ELMs on JET

To ensure sufficient divertor target lifetime, the loss in plasma stored energy due to ELMs in ITER should be restricted to Delta W-ELM <= 1 MJ. Only in JET, by virtue of its size, can such energies be approached. This contribution examines the impact of large, Type I ELMs in high current H-mode JET discharges with ITER-relevant pedestal characteristics. The ELMs provoke strong radiation losses, mostly confined to the inner divertor volume. Although the data are scattered, the average magnitude of this loss is Delta E-RAD similar to 0.5 Delta W-ELM, for Delta W-ELM < 0.55 MJ. For higher Delta W-ELM, greater fractional radiation losses occur, reaching similar to 0.7 Delta W-ELM at Delta W-ELM similar to 0.9 MJ, indicating enhanced impurity release. Even at the highest ELM energies, peak divertor target surface temperatures are too low for carbon sublimation, suggesting that thermal decomposition and/or ablation of thick co-deposited layers on the inner target may be occurring. On average. across the range of energies studied, ELMs are found to deposit between 3-4.5% of Delta W-ELM on main wall limiters. When applied to the data for a specific discharge in the series, the model of ELM filament parallel energy losses developed at JET requires radial ELM velocities in the interval 0.1-0.65 km s(-1) to explain these deposited energy fractions. (C) 2009 Elsevier B.V. All rights reserved.

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