First SOLPS‐ITER Simulation Study of Japanese DEMO on Leakage of Seeded Ar Impurity

For JA DEMO, one of the grand challenges is efficiently handling the extreme power loads onto plasma‐facing components, particularly the divertor targets. Impurity seeding is an essential strategy for increasing the radiation losses and thereby reducing the heat load on the targets. A critical remaining issue is the trade‐off between the retention of radiative impurities in the divertor region to maximize the radiation loss, and their leakage to the upstream region, which degrades the core plasma confinement. To address this issue, based on the current baseline divertor geometry of JA DEMO, the SOLPS‐ITER code is applied for the first time to evaluate the divertor power exhaust and Ar impurity distribution across various Ar seeding levels in detail. The simulation results show that increasing the Ar seeding level is highly beneficial for reducing the heat load on the targets. Moreover, in the near SOL there is a strong flow reversal of the main D ions and Ar impurity ions from the lower outer divertor target toward the upstream. This flow reversal promotes the transport of Ar ions out of the lower outer divertor region, leading to significant core contamination. As a result, such leakage severely constrains the viability of using high seeding levels for heat‐load control in the current JA DEMO divertor geometry and plasma operating scenario.