1. Key Laboratory for Laser Plasmas (MOE) and Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
2. College of Science, National University of Defense Technology, Changsha 410073, China
3. Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4. SUPA, Department of Physics, University of Strathclyde, Glasgow G4 0NG, United Kingdom