Investigations of impurity seeding and radiation control for long-pulse and high-density H-mode plasmas in JT-60U

Reduction of heat loading appropriate for the plasma facing components (PFCs) such as the divertor and the first wall is crucial for a fusion reactor. Impurity gas seeding is one of techniques to decrease peak heat flux to the divertor both in steady-state and transient phases. Power handling by large radiation power loss has been studied in the ELMy H-mode plasmas on JT-60U with argon (Ar) gas seeding [1, 2] since good confinement (HH≥0.85) was obtained up to high density ( € n e /n~0.8-0.9, n is the Greenwald density). On the other hand, it was not clearly understood to sustain the good confinement plasma with the large radiation power under the wall saturated condition where particle recycling flux changes during the long discharge. In this paper, control of the large radiation in the good energy confinement plasma was, for the first time, investigated. Total radiation fraction of Prad/Pabs = 0.8-0.9 was maintained continuously during Ar gas puffing (up to 13 s so far), and control of radiation power in the main plasma has been investigated during outgas condition from the PFCs.