Selection of Plasma-Facing Materials

PFM has been changing with improvement of plasma confinement, targeting higher density and temperature for self-burning. Now major criteria for the choice of plasma-facing materials (PFM) are (1) tolerance to high-power load, (2) long lifetime including low erosion and tolerance to neutron damage, and (3) low tritium retention. At present, W seems to be a very good PFM, if its central accumulation could be avoided by suitable impurity transport control. Prompt deposition of W is very promising because it reduces the net erosion and long-range transport. Consequently, deposition at remote area is suppressed. However, effects of neutron irradiation and Tritium (T) retention require different assessment for PFM selection. Although low T retention is better and has been one of the most important criteria for the selection of PFM of ITER, easy recovery would be more important for T fuel self-sufficiency and T safety in a reactor. In this respect, C-wall seems better, because most of T is retained in deposited layers and near-surface regions of eroded area, while bulk retention is dominant for W-wall and difficult to recover. The selection is not easy. Hopefully, ITER could be used for researches for selection of PFM in a reactor.