Effect of impurities on the erosion behavior of beryllium under steady-state deuterium plasma bombardment

As a candidate plasma-facing material for ITER, beryllium has been evaluated with respect to deuterium plasma erosion characteristics using the PISCES-B Mod facility at ion fluxes around 5 x 10(21) ions s(-1) m(-2) in the ion bombarding energy range from 100 to 300 eV. It is found that at elevated temperatures beryllium tends to be contaminated with trace amounts of plasma impurities such as carbon, hydrocarbon, oxygen and nitrogen even under the energetic deuterium plasma bombardment. Among these plasma impurities, carbon and hydrocarbon are observed to form thin films. This carbon deposition effect has made it difficult to interpret the weight loss data in evaluating the erosion yield. A first-order model has been developed to account for the dynamics of this carbon deposition process and has suggested that some surface chemistry effect plays an important role. This surface chemistry effect has been experimentally proved from the observation that carbon deposition can be avoided only at room temperature. Without impurity deposition, the beryllium surface after plasma bombardment is found to be cleaner in oxygen content and smoother in surface topography than the as-received material. Interestingly, however, even without impurity effects, the erosion yield data still tend to agree with beryllium oxide data.