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

Microstructure and chemical composition of depositions in TRIAM-1M under lo ng pulse discharges have been examined. It is found that the deposition has a defective structure; the grain size is approximately 1 nm and the crysta lline structure is not the normal body centered cubic (bcc) but is a face c entered cubic (fcc). By comparing with the vacuum-deposited molybdenum in v arious conditions, it is concluded that co-deposited oxygen plays an essent ial role for the defective structure. The vacuum-deposited molybdenum, whic h has the same structure as the depositions in TRIAM-1M, shows very large a nd strong hydrogen retention. Implanted deuteriums are desorbed as D-2, DH, D2O and DHO. The present work indicates that re-deposition of sputtered at oms on plasma facing surfaces will be a serious problem even in a metallic tokamak. More attention should be paid to hydrogen retention in the deposit ed material. (C) 2000 Elsevier Science B.V. All rights reserved.