Helium re-emission and surface deformation in niobium during multiple temperature helium implantation

In previously published work, it was shown that helium implantation at high doses at temperatures of 400--7000C gave rise to relatively low helium re-emission and that the dominant mode of surface deformation consisted of flaking. On the other hand, during implantation at 10000C and above, the helium re-emission approached 100 percent and a very stable mode of surface deformation consisting of holes was created. For CTR first wall applications, the high temperature case appears to be very favorable whereas the low temperature case is unacceptable. Helium re-emission and surface deformation in the low temperature range after high temperature exposure was studied. It was found that essentially no additional surface deformation occurred during low temperature helium implantation when the sample had been pre-exposed at 12000C. Apparently the porous structure created during helium implantation at 12000C gives rise to a significantly different helium sink distribution and this accounts for the radically different low temperature implantation behavior