Nanostructure formation on tungsten exposed to low-pressure rf helium plasmas: A study of ion energy threshold and early stage growth

Nanostructure formation on W targets is explored as a function of He+ impact energy, εi, in the range 20 ≤ εi ≤ 57 eV. Six targets are exposed at 1120 K for 6 h to pure He plasmas, generated by a low-pressure rf Helicon source. One target is additionally exposed to Ar plasma pre-treatment. It is found that He ions of impact energy 32-37 eV are necessary for nanostructure formation to be observed in this type of plasma, as determined by SEM. At 57 eV a nanostructured surface forms readily (similar to that observed by Baldwin and Doerner, Nuclear Fusion 48 (2008) 035001 in the PISCES-B device), but growth is retarded in the Ar pre-treatment case. Thermal desorption shows that nanostructuring is accompanied by increased trapping of He in degenerate vacancies and clusters, but is reduced by a factor of 8 as a result of Ar pre-treatment.