Plasma exhaust and divertor studies in Japan and Europe broader approach, DEMO design activity

Power exhaust scenario and divertor design for a steady-state Japan (JA) DEMO and a pulse Europe (EU) DEMO1 have been investigated as one of the most important common issues in Broader Approach DEMO Design Activity. Radiative cooling is a common approach for the power exhaust scenario. For the JA DEMO, development of the divertor design appropriate for high P_sep/R_p ∼30 MW m⁻¹ is required, while the radiation fraction in the main plasma (f_rad ^main = P_rad ^main/P_heat) is ITER-level (0.40–0.45) and the exhaust power above the L- to H-mode power threshold (f_LH = P_sep/P_th ^LH) is large margin (∼2). For the EU DEMO1, larger f_rad ^main (=0.67) and smaller f_LH (=1.2) plasma is required, using higher-Z impurity seeding, in order to apply ITER-level divertor (P_sep/R_p = 17 MW m⁻¹). ITER technology, i.e. water cooling with W-monoblock and Cu-alloy (CuCrZr) heat sink, is a baseline for JA and EU to handle the peak heat load of 10 MW m⁻²-level, and neutron flux and irradiation dose are comparable. For the JA DEMO, two different water-cooling pipes, i.e. CuCrZr and F82H steel, are proposed. For the EU DEMO1, the heat sink consists of all Cu-alloy pipe, and the divertor size is reduced with replacing the baffles by the breeding blankets. Choices of the heat sink components have been developed appropriate to the high irradiation dose condition. These JA and EU approaches of the power exhaust scenario will provide important case studies for the future decision of the DEMO divertor design.