ELM and inter-ELM heat and particle flux to a secondary divertor in the DIII-D tokamak

Abstract

DIII-D edge-localized-modes (ELM) discharges with a secondary X-point inside the vessel were used to evaluate the power distribution between the primary and secondary divertors. The magnetic balance, defined by the distance between the primary and secondary separatrices at the outer midplane (dRsep), was varied from −5 mm (lower-single-null, LSN) to +16 mm (upper-single-null, USN). We find that the secondary divertor receives up to 1/3 of the total heat flux (primary + secondary divertors), and it is dominated by the ELM-driven flux. Both the radially-integrated and the peak heat flux to the secondary divertor decay below ∼50% of the maximum as dRsep is varied from −5 to +6 mm, but the integrated heat flux decay flattens out and it would take dRsep above ∼25 mm to reduce the ELM heat flux to the secondary divertor below 10% of the one deposited to a well-defined SN. Both the secondary inner strike point (ISP) and outer strike point receive heat flux during ELMs. The peak heat flux is comparable in both strike points for dRsep < 10 mm, but the ISP receives less than 15% of the total flux. This is experimental evidence that the secondary inner divertor region receives significant ELM flux which should be taken into account for the design of future generation tokamaks.