The impact of large ELMs on JET

Abstract

To ensure sufficient divertor target lifetime, the loss in plasma stored energy due to ELMs in ITER should be restricted to Δ 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 Δ E RAD ∼ 0.5Δ W ELM, for Δ W ELM < 0.55 MJ. For higher Δ W ELM, greater fractional radiation losses occur, reaching ∼0.7Δ W ELM at Δ W ELM ∼ 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 Δ 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.