Radiative dissipation of small ELMs in JET-ILW highly radiating impurity seeded H-mode scenarios*

Volumetric dissipation of ELM-induced transient heat fluxes via extrinsic impurity (Ar, Ne, N) radiation in highly radiating JET-ILW H-mode scenarios can contribute to a significant reduction in the deposited energy on the divertor targets during small ELMs, which occur in pronounced detachment conditions and can coexist with the X-point radiator (XPR) regime. Combining ELM-resolved measurements of the relative changes to the pedestal pressure, radiated power, heat and particle fluxes on the divertor targets, as well as novel near-UV spatially resolved divertor spectroscopy capabilities, the extent of ‘re-attachment’ during fast transient events is analyzed. An upper limit on the ELM size that can be radiatively dissipated (buffered) is derived based on the spatial and temporal dynamics of the Ar¹⁺-Ar²⁺, Ne¹⁺-Ne³⁺ and N¹⁺-N³⁺ spectral line emission in the near-UV range and the magnitude of the measured heat fluxes as they approach the noise levels of the infra-red camera systems. The extent to which the low charge state spectral lines can be used as proxies for the impurity radiation front position in the outer divertor is examined, as are the intra-ELM plasma conditions near the outer target. The experiment data and its interpretation will be invaluable for validating time-dependent SOLPS-ITER simulations with the aim of developing capability for quantifying the impact of partially and fully buffered small ELMs on plasma facing component lifetime evaluation in next-step devices.