SPUTTERING OF LOW-Z MATERIALS

Plasma facing materials are generally made from low-Z materials due to their relatively high tolerable concentration in the plasma. As pure graphite shows regimes of enhanced erosion at elevated temperatures, other low-Z materials, such as Be, B and their compounds and carbides are examined, as well as graphite doped with various elements. For the choice of a plasma facing material in future fusion reactors crucial points are certainly its stability against thermal shocks and neutron irradiation. However, in this study materials are compared only in view of their erosion due to sputtering under ion bombardment. Firstly, low-Z elements are compared with regard to physical sputtering with D+ and selfions, taking the dependence on the angle of incidence into account. Secondly, sputtering data for low-Z compounds, such as BeO, Be2C, Be4B, B4C and TiC are presented. Difficulties in the theoretical treatment of these compounds arising from preferential sputtering and surface segregation are discussed. Finally, the behaviour of low-Z materials at elevated temperatures is compared to pure and boron-doped graphite.