Deuterium plasma sputtering of mixed Be-W layers

In ITER, the first mirrors would be vulnerable to deposition from the first wall materials, namely beryllium (Be) and tungsten (W) in mixed compositions. In this study we investigate the capactively coupled RF plasma sputtering of such mixed Be-W deposits on rhodium-coated substrates, using deuterium as the process gas, and track the enrichment of W during the sputtering process. Experiments were conducted on Be-W deposits with W concentration varying from 2 to 8.3 at.%, and maximal deuterium ion energies in the plasma of 70 and 220 eV. The evolution of the W concentration in the deposits during the plasma sputtering depended considerably on both deuterium ion energy as well as the initial concentration of W in the films. With 220 eV sputtering, a rapid enrichment of W was observed, with all the W changing from BeW before sputtering to metallic W and its oxides after sputtering. With 70 eV sputtering, there was no net change in the W concentration, as long as the initial concentration of W in the film was below 4 at.%. Moreover, the W remained considerably in the state of BeW after the sputtering as well. However, a W enrichment was also observed with 70 eV sputtering, when the W concentration in the films was higher (8.3 at.%). The rate of W enrichment was also observed to increase monotonically with the increase in the initial concentration of W in the deposits. SDTrimSP simulations performed with experimental parameters indicate that sputtering yield of Be increases with W concentrations in the Be-W layers due to backscattering of D projectiles off W atoms. These studies show that low energy deuterium sputtering of mixed Be-W layers with low W concentrations, allows for removal of Be without rapidly enriching W in the films, making it a promising option for ITER.