Quantification of ELM and inter-ELM heat and particle flux to a secondary divertor and its consequences for future fusion machines

We show that the ELM-driven heat flux to the secondary divertor of DIII-D can be up to 1/3 of the total heat flux, which could reach several tens of MW/m² in ITER. Resolving the mechanisms that deliver heat and particle fluxes to the divertor target is crucial because ITER is designed to operate in lower-single-null (LSN) but with a secondary X-point (XPT) inside or near the tiles coated with beryllium, possessing a lower heat flux tolerance (2 MW/m²) than tungsten. DIII-D type-I ELMing discharges with a secondary XPT inside the vessel were used for a quantitative analysis with infra-red thermography and fast reciprocating probe measurements of the ELM and inter-ELM power distribution between the primary and secondary divertors by varying the up-down magnetic balance (dRsep) from -5 (LSN) to +16mm (upper-single-null, USN). We experimentally demonstrate for the first time that, with dRsep < 10mm, ELM plasma is transported to the secondary inner target, which is magnetically isolated from the outer leg, leading to ELM peak heat values that are comparable to those at the secondary outer strike point. Both the integrated and the ELM peak heat flux to the secondary divertor decay below ~50% of the maximum as dRsep is varied from -5 to +6mm, but the integrated heat flux decay asymptotes. Values of dRsep above ~25mm are needed to reduce the ELM heat flux to the secondary divertor below 10% of heat flux deposited to a well-defined SN. These findings imply that the up-down magnetic balance will have to be accurately controlled (+/- 5mm) in order to preserve the upper wall. This poses a concern for any future tokamak that will operate in quasi-DN configuration and where the secondary inner target is not yet designed to withstand significant heat loads.