1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
2. Istituto per la Scienza e Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche, Milano 20125, Italy
3. Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Milano 20126, Italy
4. State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
5. University of Chinese Academy of Sciences, Beijing 100049, China
6. University of Science and Technology of China, Hefei 230026, China