Recovery of vacuum condition and plasma performance after vessel vent in EAST tokamak

Abstract

The plasma recovery after vessel vent is studied in the EAST 2019 spring campaign. The removal of condensate water is important for the fully superconducting tokamak. The low-Z impurities are mostly removed after a baking cycle for 200 h and GDC for 111 h. About 49% of H2 and 40% of H2O molecules are removed in this phase. The clean ultra-high vacuum pressure is re-obtained to 3.0 × 10−6 Pa. In a follow-up process, about ~550 shots with an accumulated plasma duration of 4100 s are carried out to further remove implanted impurities. In the initial discharge stage, the removal rates of H2 and H2O particles are 4.7 × 1021 molecules/(m2·h) and 3.1 × 1021 molecules/(m2·h), respectively, which are ~4 times higher than that during D2/He-GDC and baking. About 31% of H2 and H2O impurities are removed due to the effect of initial plasma shots. And the repeatable plasma shots are achieved with the decrease of impurity level. The results provide reference for the arrangement of recovery plan after vacuum failure of EAST, ITER, CFETR and superconducting fusion devices.