Electrical and optical surface degradation of silica due to superficial He implantation

Different oxides will be used in ITER and future fusion reactors for electrical insulation and optical components. The vacuum face of these materials will be subjected not only to neutron and gamma irradiation, but also to particle bombardment, due mainly to ionization of the residual gas and acceleration of the resulting ions by local electric fields. Previous work showed that silica suffers electrical and optical degradation when subjected to He bombardment with energies from 300 keV down to 27 keV. As the He ion energy may extend down to some few keV, or less, further work has been performed to study possible degradation for energies from 21 keV down to 5 keV. The results show that both surface and optical degradation occur at these low energies, more rapidly for the lowest energy (5 keV) ions. They also suggest that the superficial narrow implanted He profile plays an important role in the surface degradation.