Hydrogen retention in divertor tiles used in JT-60 for hydrogen discharge period

Hydrogen retention characteristics of graphite divertor tiles used in JT-60 for hydrogen discharge period were investigated by the thermal desorption spectroscopy. Most of the tiles were covered with re-deposited layers of the maximum thickness of 70 μm. Total amount of hydrogen desorbed from each tiles was proportional to the thickness of the re-deposited layer on the tiles. Hydrogen desorption from substrate determined after the removal of the re-deposited layer was quite small compared to that from the re-deposited layers. Assuming homogeneous hydrogen distribution in the re-deposited layers, H/C ratio in the re-deposited layer was approximately 0.015. This is much smaller than those observed in JET and other devices. The re-deposited layers are very likely subjected to high temperatures because of their poor thermal contact to the substrate owing to their porous structure.