1. a Institute of Plasma Physics , Chinese Academy of Sciences , Hefei, P. R. China
2. b Hefei National Laboratory for Physical Sciences at Microscale , University of Science and Technology of China , Hefei, P. R. China
3. c National Synchrotron Radiation Laboratory , University of Science and Technology of China , Hefei, P. R. China
4. d School of Materials Science and Chemical Engineering , Anhui University of Architecture , Hefei, P. R. China