Effects of plasma impurities on materials erosion and mixing in a steady state magnetic fusion device - A comparison between beryllium and tungsten

Data review and modeling on the effects of plasma impurities on materials erosion and mixing under steady-state plasma bombardment are presented in this paper. Non-condensible plasma impurities such as oxygen will interact with surfaces under recycling conditions to enhance materials erosion. In contrast, condensible impurities such as carbon can either recycle or be redeposited over the host materials surface. Due to the masking effect, redeposited impurity films can reduce or completely eliminate host materials erosion. The critical condition to turnover from recycling to redeposition has been described in terms of impurity-to-fuel plasma flux ratio. Because of their high surface reflectivities, high-Z materials such as tungsten are found to require substantially more impurity contents for redeposition than low-Z materials such as beryllium. Enhanced erosion of tungsten and beryllium due to oxygen and carbon impurities has been compared under recycling conditions. Interestingly, trace amounts of recycling impurities can significantly enhance tungsten erosion whereas this effect on beryllium erosion is barely noticeable. (C) 1998 Elsevier Science B.V. All rights reserved.