Simulation studies of divertor detachment and critical power exhaust parameters for Japanese DEMO design

Highlights: • Simulation study for the divertor design of Japanese fusion DEMO reactor. • Systematic scans of critical power exhaust parameters for the divertor operation. • Heat load and detachment plasma profiles were evaluated in the low density condition. • Restrictions of the divertor operation under more severe conditions were determined. Handling of a large thermal power exhausted from the confined plasma is one of the most important issues for ITER and DEMO. A conventional divertor, which has the closed geometry similar to that of ITER and longer leg of 1.6 m, was proposed for the Japanese (JA) DEMO reactor (R_p/a_p = 8.5/2.42 m). A radiative cooling scenario of Ar impurity seeding and the divertor performance have been demonstrated by SONIC simulation, in order to evaluate the power exhaust in JA-DEMO 2014 (primary design with P_sep ~ 283 MW) and JA-DEMO with higher plasma elongation (a revised design with P_sep ~ 235 MW). The divertor operation with the peak q_target ≤ 10 MWm⁻² was determined in the low n_e^sep of 2–3 × 10¹⁹ m⁻³ under the severe conditions of reducing radiation loss fraction, i.e. f*_rad^div = (P_rad^sol + P_rad^div)/P_sep, and diffusion coefficients (χ and D). The divertor geometry and reference key parameters (f*_rad^div ~ 0.8, χ = 1 m²/s and D = 0.3 m²/s) were so far consistent with the power exhaust concepts in the n_e^sep range, and the revised JA-DEMO design has advantages of wider n_e^sep range and enough margin for the divertor operation. For either severe assumption of f*_rad^div ~ 0.7 or χ and D to the half value, higher n_e^sep operation was required for the primary design in order to control the peak q_target ≤ 10 MWm⁻², i.e. the operation window was reduced. Applying the two severe assumptions, the divertor operation was difficult in the low n_e^sep range for the both designs.