1. School of Physics, Nankai University, Tianjin 300071, People's Republic of China
2. FSC and State Key Lab of Nuclear Physics and Technology, Department of Physics, Peking University, Beijing 100871, People's Republic of China
3. Southwestern Institute of Physics, Chengdu 610041, People's Republic of China
4. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of China
5. Department of Physics, University of Colorado Boulder, Boulder, Colorado 80309, USA