Power exhaust concepts and divertor designs for Japanese and European DEMO fusion reactors

Concepts of the power exhaust and divertor design have been developed, with a high priority in the pre-conceptual design phase of the Japan-Europe broader approach DEMO design activity (BA DDA). Common critical issues are the large power exhaust and its fraction in the main plasma and divertor by the radiative cooling (P (rad) (tot)/P (heat) > 0.8). Different exhaust concepts in the main plasma and divertor have been developed for Japanese (JA) and European (EU) DEMOs. JA proposed a conventional closed divertor geometry to challenge large P (sep)/R (p) handling of 30-35 MW m(-1) in order to maintain the radiation fraction in the main plasma at the ITER-level (f (rad) (main) = P (rad) (main)/P (heat) similar to 0.4) and higher plasma performance. EU challenged both increasing f (rad) (main) to similar to 0.65 and handling the ITER-level P (sep)/R (p) in the open divertor geometry. Power exhaust simulations have been performed by SONIC (JA) and SOLPS5.1 (EU) with corresponding P (sep) = 250-300 MW and 150-200 MW, respectively. Both results showed that large divertor radiation fraction (P (rad) (div)/P (sep) > 0.8) was required to reduce both peak q (target) (<= 10 MW m(-2)) and T (e,i) (div). In addition, the JA divertor performance with EU-reference P (sep) of 150 MW showed benefit of the closed geometry to reduce the peak q (target) and T (e,i) (div) near the separatrix, and to produce the partial detachment. Integrated designs of the water cooled divertor target, cassette and coolant pipe routing have been developed in both EU and JA, based on the tungsten (W) monoblock concept with Cu-alloy pipe. For year-long operation, DEMO-specific risks such as radiation embrittlement of Cu-interlayers and Cu-alloy cooling pipe were recognized, and both foresee higher water temperature (130 degrees C-200 degrees C) compared to that for ITER. At the same time, several improved technologies of high heat flux components have been developed in EU, and different heat sink design, i.e. Cu-alloy cooling pipes for targets and RAFM steel ones for the baffle, dome and cassette, was proposed in JA. The two approaches provide important case-studies of the DEMO divertor, and will significantly contribute to both DEMO designs.