1. Institute of Plasma Physics, HFIPS, Chinese Academy of Science, Hefei 230031, China
2. Science Island Branch, Graduate School of University of Science and Technology of China, Hefei 230031, China
3. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543-0451, USA
4. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China