1. School of Nuclear Science and Technology and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
2. Key Laboratory of Geospace Environment, CAS, Hefei, Anhui 230026, China
3. Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA
4. Department of Physics and Institute for Fusion Studies, University of Texas at Austin, Austin, Texas 78741, USA
5. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China