1. Center for Free-Electron Laser Science, DESY, Notkestrasse 85, 22607 Hamburg, Germany
2. School of Physics, Peking University, Beijing 100871, China
3. Max Planck Institute for the Structure and Dynamics of Matter, D-22761 Hamburg, Germany
4. Stanford PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
5. Institute of Physics Czech Academy of Science, Na Slovance 2, 182 21 Prague 8, Czech Republic
6. Institute of Plasma Physics, Czech Academy of Science, Za Slovankou 4, 182 00 Prague 8, Czech Republic