This section describes the present understanding of PWIs for beryllium-containing surfaces. First, it focuses on the erosion properties of ‘clean’ beryl­lium surfaces at different temperatures. Retention of plasma fuel species in both bulk and codeposited layers of beryllium is then described. As beryllium will not be used as the exclusive plasma-facing mate­rial in future confinement devices, issues associated with mixed, beryllium-containing surfaces are also addressed.

4.19.3.1 Beryllium Erosion Properties

The term erosion is used to describe a group of processes that remove material from a surface sub­jected to energetic particle bombardment. Included under the general classification of erosion are pro­cesses such as physical sputtering, chemically assisted physical sputtering, chemical sputtering, and thermally activated release from surfaces. Of these processes, only chemical sputtering, where volatile molecular species are formed on the surface, appears to be inactive in beryllium.