1. KTX Laboratory and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
2. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
3. Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA
4. Plasma Physics Laboratory, University of Saskatchewan, Saskatoon, Saskatchewan 7N 5E2, Canada