1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China
2. Science Island Branch of Graduate School, University of Science and Technology of China, Hefei, Anhui 230031, China
3. Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA
4. School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China