1. Institute of Plasma Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, P. R. China
2. University of Science and Technology of China, Hefei 230026, P. R. China
3. Institute of Materials, China Academy of Engineering Physics, Mianyang 621907, P. R. China
4. Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, P. R. China
5. Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, P. R. China