An overview of recent erosion/deposition and hydrogen retention studies in JT-60U

Abstract

Recent erosion/deposition and hydrogen retention studies in JT-60U have been summarized. Inner divertor tiles were covered by thick redeposited layers, whereas outer divertor tiles eroded, and significant carbon deposition was observed at the outer exhaust slot, i.e. the bottom side of the outer dome wing. The outboard first wall tiles were mostly eroded. The mass balance of carbon erosion and redeposition in the whole vessel is still missing.H+D retention in the redeposited layers on inner divertor tiles correlates well with their thickness, and hydrogen concentration in the layers, (H+D)/C, was uniformly ∼0.03 except for a few μm top surface layers with ∼0.05. Surface D within a few μm in depth was mostly replaced by H owing to H–H (H plasma +H neutral beam injection (NBI)) discharges made for tritium removal, indicating possible T removal by D–D discharges. Rather high D retention was observed on the outer dome wing probably owing to higher energy deuteron implantation originating from NBI.JT-60U seems a good example to reduce carbon deposition as well as tritium retention. Well-aligned tiles with more or less no steps between neighboring tiles in the toroidal and poloidal directions could reduce net erosion and redeposition not only on the plasma-facing surface but also at the plasma-shadowed area. During plasma discharges, the surface temperature in the divertor area becomes higher than the base temperature (600 K), which could reduce tritium retention significantly.