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

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 similar to 283 MW) and JA-DEMO with higher plasma elongation (a revised design with P-sep similar to 235 MW). The divertor operation with the peak q(target) <= 10 MWm(-2) was determined in the low n(e)(sep) of 2(-3) x 10(19) m(-3) under the severe conditions of reducing radiation loss fraction, i.e. f*(div)(rda) = (P-rad(sol) + P-rda(div))/P-sep, and diffusion coefficients (chi and D). The divertor geometry and reference key parameters (f*(div)(rda) similar to 0.8, chi = 1 m(2)/s and D = 0.3 m(2)/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*(div)(rda) similar to 0.7 or chi 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.

Automated summary

The management of large thermal power exhausted from the confined plasma is crucial for ITER and DEMO. A conventional divertor with a closed geometry was proposed for the Japanese DEMO reactor to address this issue, and its performance was evaluated through SONIC simulation. The study demonstrated that Ar impurity seeding and radiative cooling could effectively handle the power exhaust in different operating conditions.