Wall pumping and saturation in divertor tokamaks

Recent investigations of the wall pumping and saturation during tokamak discharges have been summarized, focusing on wall saturation in long pulse (30 s) ELMy H-mode discharges in JT-60U. The ELMy H-mode discharges in divertor tokamaks such as ASDEX-Upgrade and JT-60U showed large deuterium retention (Nwall = (3–5) × 1022 D) and a large retention ratio (Rwall = 0.3–0.5) for cases with large gas puffing into the vacuum vessel. In short discharges of ASDEX-Upgrade, the deuterium retention and the retention ratio were increased with gas puff flux. On the other hand, in some long-pulse discharges of JT-60U, the net wall pumping flux decreased to zero during the later phase, i.e. wall saturation was observed. The net wall pumping flux in the later phase changed the operation history, which determined the variation of the deuterium retention. Wall pumping at low temperature regions in the divertor or private dome rather than co-deposition with carbon is important in explaining the large change in deuterium retention in the series of discharges.Under saturated wall conditions, significant changes were not seen in SOL and divertor plasmas. However, increases in the recycling flux and carbon generation were locally observed in the private flux region. The increases in particle recycling and carbon influx were larger than those expected on increasing the surface temperature of the dome tiles. Understanding the carbon generation process in long-pulse discharge is important for controlling the divertor plasma, while the increase in carbon contamination in the main plasma was small under the attached divertor condition.