High dose He+ bombardment of niobium at 800° tO 1400°C

In proposed fusion reactors, the first wall will be bombarded by low energy D⁺, T⁺ and He⁺ ions to very high doses. Room temperature irradiations with 9 keV He⁺ (Roth et al. 1975) showed an initial phase of blistering between 0.05 and 0.25 C/ cm², and a final state of considerable surface roughness at 8 to 112 C/cm²(7 × 10²⁰He⁺/cm²). The present irradiation experiments with 6 keV He⁺ are carried out with polycrystalline Nb foils at temperatures between 800 and 1400°C in order to study the influence of He mobility and of Nb surface diffusion during irradiation. The applied doses range from 2.5 × 10¹⁷He⁺/cm² (blistering) to 5 × 10²⁰He⁺/cm² which corresponds to at least several weeks of reactor operation. The resulting changes of the niobium surface structures are observed by scanning electron microscopy and are pictorially presented in this paper. Mainly, sponge-like open structures are seen to develop at high doses, with increasing physical dimensions at higher temperatures.