Hydrogen bombardment of the oxide layer on beryllium

Abstract

Beryllium is being considered as a plasma-side material of low atomic number Z for use in fusion reactors. Untreated beryllium surfaces are usually covered by a thin oxide layer which forms during air exposure or from residual gas adsorption in vacuum. Secondary ion mass spectrometry measurements have been made to investigate the durability of this oxide layer during hydrogen ion bombardment at an energy of 1 keV, which is similar to that expected at the plasma edge in a fusion reactor. The surface condition of machined beryllium samples undergoing hydrogen bombardment was monitored by observing secondary ion emission of O -, OH - and BeO -. On the basis of sputtering yield data for BeO, the oxide layer appears to be up to 10 2 monolayers in thickness. At the metal interface it was possible to determine the cross section for the removal of adsorbed oxygen. The measured value is 1.5 × 10 -17 cm 2, which is in agreement with bulk oxide sputtering data. This suggests that adsorbed oxygen forms a surface oxide with a binding energy that is similar to that of the bulk oxide. These results are used to predict the conditions in which an oxide layer will exist on exposed beryllium surfaces during fusion reactor operation.