Divertor detachment with neon seeding in grassy-ELM H-mode in EAST

Abstract

Simultaneous control of the transient heat load induced by the large-amplitude edge-localized modes (ELMs) and the time-averaged heat and particle fluxes to the divertor targets is a critical issue for the steady-state operation of a future tokamak fusion reactor. The combination of divertor detachment and grassy-ELM regime provides a candidate solution to this issue. The strong particle exhaust capability of the grassy-ELM regime greatly facilitates the achievement of steady-state operation of a detached plasma with divertor impurity seeding. Stable complete detachment at inner target and partial detachment at outer target in the grassy-ELM regime have been achieved with seeding of 5% neon (Ne) and 95% D2 mixture since 2018 in the EAST superconducting tokamak with ITER-like tungsten monoblock upper divertor. The peak ion fluxes (Γion) on the upper outer and inner divertor targets were reduced by 55% and 92%, respectively. However, it was accompanied by confinement degradation with 18% reduction in the plasma stored energy WMHD. In contrast, partial detachment on the upper outer divertor target with confinement improvement has been achieved with the power across the separatrix (Psep) around the H-mode threshold power. In addition, at relatively lower q95 (∼5.7) with unfavorable Bt direction, Ne seeding leads to a transition from mixed (large/small) ELM regime into grassy-ELM regime with significantly increased ELM frequency, which extends the accessible parameter range of the grassy-ELM regime towards lower q95.