SPUTTERING OF LIMITER AND DIVERTOR 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. However, high-Z materials show a high threshold energy for light ion sputtering and have been proposed as divertor material for the case of a high density, low temperature divertor plasma. Starting from a short introduction into the sputtering mechanism the dependence of sputtering yield, threshold energy and yield enhancement at grazing incidence on target mass is discussed for deuterium ion and self sputtering. The theoretical expectations are compared to experimental results as well as results from computer simulations. The dependence on surface topography is demonstrated both for normal and for oblique angle of incidence. For compounds the special problem is the change of surface composition, either by preferential sputtering of one compound or by implantation and accumulation of ions such as carbon or oxygen. These changes may significantly alter the thermophysical properties of the surface layers. The temperature dependence of the erosion yield is compared for different materials including chemical effects, radiation enhanced and thermal sublimation. With these results the materials are compared concerning their net erosion/deposition at divertor plates, their figure of merit, which relates the maximum tolerable impurity concentration to the sputtering yield, and, finally, concerning their surface temperature limits where run-away self sputtering occurs.