1. Department of Physics, Nanchang University, Nanchang 330031, China
2. KTX Laboratory and Department of Modern Physics, University of Science and Technology of China, Hefei 230026, Anhui, China
3. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230021, Anhui, China
4. Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, California 90095, USA