Tritium retention and erosion/deposition of first wall materials

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Retention of hydrogen isotopes (H, D, and 1) and erosion/deposition in plasma facing carbon materials as well as dust sampling in JT-60U and the results are studied and compared with those of JET. In D-D discharge machines like JT-60U, tritium produced by D-D reaction does not fully lose its initial energy of 1 MeV and is implanted into plasma facing wall more than 1μm in depth. Hence tritium retention profile is separated from carbon deposition profiles. On the other hand, in JT-60U, H+D retention profiles in the diverter region are well correlated with the carbon deposition profiles and the deposition dominated inner diverter tiles show high retention, while the erosion dominated outer diverter tiles show low retention. Nevertheless the retained amount in the redeposited layers is smaller than those observed in JET, which is attributed to temperature rise of the redeposited layers probably owing to their poor thermal contact to the substrate. Carbon deposition on the plasma shadowed … More area and remote area like pumping ducts is very small and only small amount of dust is collected. Low deposition in the remote area in JT-60U could be attributed to the precise alignment of diverter tiles both toroidally and poloidally. It is suggested that divertor geometry could have rather strong effects on carbon deposition or carbon transport.Pulsed-laser induced desorption of hydrogen from co-deposits on JT-60 open-divertor tile was also investigated. The fundamental (1064 nm) and fourth harmonic (266 nm) emission of a 20 ps-Nd: YAG laser were used, and dependences of hydrogen desorption on laser intensity IL and wavelength λ were studied. Hydrogen-desorption efficiency, defined as the ratio between the number of desorbed hydrogen by laser irradiation and that of hydrogen retained in the ablated volume, was largest in the region, where strong ionization of C+ occurred, and was larger for λ = 266 nm, in which a laser photon can cut C-H bond, compared with that for λ=1064 nm. For the ablative removal of hydrogen, a short-wavelength and high-power laser is desirable. Less