ELM buffering in the MAST Upgrade Super-X divertor

The MAST Upgrade Super-X divertor is typically seen to detach in steady state discharges. However, divertor re-attachment is observed to occur during fast transient heat loads. In this paper the effect of edge localised modes (ELMs) on the divertor are diagnosed on fast time scales with Thomson scattering and with a new ultrafast divertor spectroscopy (UFDS) diagnostic. The Thomson scattering data show full ionisation of the detached neutral buffer during large ELM events (>2-3 kJ) in a ∼1 ms time window after ELMs. Plasma temperature at the strikepoint varies depending on the ELM size and timing reaching up to 10 eV, which is significantly higher than inter-ELM levels but much lower than the pedestal temperatures of ∼200 eV. The UFDS diagnostic allows determination of ELM induced reattachment (burn-through) by monitoring the spatial distribution of D 2 Fulcher emission across the divertor. In this initial investigation the ELM energy and divertor neutral gas pressure (P gas ) are hypothesised to be the most influential parameters on whether an ELM causes burn-through or not without extrinsic impurities. The relationship of these parameters to burn-through as measured by UFDS is examined by a database of ELMs from MAST-U pulses. For the MAST-U Super-X divertor, the required P gas (in Pa) to buffer an ELM of energy ΔW ELM (in kJ) is estimated to follow a limit of P gas ≥ 0.67ΔW ELM by a simple model, which is shown to agree well with the experimental results.