1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
2. University of Science and Technology of China,
Hefei 230026, China
3. Department of Physics, Technical University of Denmark, Fysikvej, DK-2800 Kgs.-Lyngby, Denmark
4. School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China