Defect evolution in tungsten exposed to helium plasma and deuterium plasmas studied by slow positron beam oleh Yi-Ming Lyu, Yu-Ping Xu, Xiao-Chun Li, Xin Shen, Bo-Yu Wu, Chi-Shung Yip, Xiang Gao, Xiao-Dong Lin, Zhong-Shi Yang, Xing-Zhong Cao, Hai-Shan Zhou, Guang-Nan Luo