Hydrogen Isotopes Retention in Gaps at the JT-60U First Wall Tiles

Retentions of Hydrogen (H) and deuterium (D) in the side surfaces (gaps between tiles) of the carbon tiles used as first wall in JT-60U were measured by the thermal desorption spectroscopy. In the gaps, the H and D retention were dominated in carbon deposited layer. The gap retention was less than that of the eroded plasma facing surface, where the retention was saturated, and linearly increased with exposure time. Overall retention rate in the gaps of the first wall tiles was determined to be 4.0 × 10¹⁹ H+D/s, and was comparable or larger than those in the re-deposited layers on the plasma facing surfaces and in the shadowed areas in the divertor region.