Structure of materials deposited on the plasma facing surface in TRIAM-1M tokamak and the effect on hydrogen recycling

Microstructure and chemical composition of depositions in TRIAM-1M under long pulse discharges have been examined. It is found that the deposition has a defective structure; the grain size is approximately 1 nm and the crystalline structure is not the normal body centered cubic (bcc) but is a face centered cubic (fcc). By comparing with the vacuum-deposited molybdenum in various conditions, it is concluded that co-deposited oxygen plays an essential role for the defective structure. The vacuum-deposited molybdenum, which has the same structure as the depositions in TRIAM-1M, shows very large and strong hydrogen retention. Implanted deuteriums are desorbed as D2, DH, D2O and DHO. The present work indicates that re-deposition of sputtered atoms on plasma facing surfaces will be a serious problem even in a metallic tokamak. More attention should be paid to hydrogen retention in the deposited material.