Assessment of the Use of Injected Impurities for Heat Flux Mitigation in JET ELMy H-modes.

One of the most severe problems for the operation of next step fusion devices in H-mode is the transient power load to the divertor target plates due to Type I ELMs. The purpose of this paper is to assess the use of impurity seeding to reduce Type I ELM heat fluxes through enhanced radiation, either in a narrow region just inside the separatrix, or in the SOL and divertor regions, while maintaining good core confinement. An analysis of the divertor heat load during Type I ELMs in argon seeded H-modes on JET has already been reported. In these discharges, with Ar and Prad /Ptot 0.6, the peak heat flux due to Type I ELMs was reduced by a factor of 2, and the confinement was 5% less than in the deuterium reference pulse. This effect is due to a reduced pedestal energy, and hence a reduced ELM energy release. From the limited set of data analysed in, no clear evidence of any additional radiative dissipation in the SOL and divertor region was observed. In the present paper, the previous analysis of argon seeded H-modes is compared with new experiments where nitrogen is used as radiating species as a means to change the localisation of the radiation from the region just inside the separatrix to the SOL and divertor, the underlying physics idea being that, in that case, some fraction of the ELM energy might be radiated before reaching the target plates. The results are modelled using the time-dependent SOLPS5.0 code. Previous calculations of this possible buffering effect have been reported for JET and ITER, both predicting it to be large (> factor 2) only for small ELMs (DWdia <20kJ in JET).
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