1. SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
2. Institute for Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf e.V., 01328 Dresden, Germany
3. Institut für Laser und Optik, Hochschule Emden/Leer-University of Applied Sciences, 26723 Emden, Germany
4. Institut für Physik, Carl von Ossietzky Universität, 26111 Oldenburg, Germany
5. Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany
6. Institute of Physics, Czech Academy of Sciences, Na Slovance 2, Prague 8 18221, Czechia
7. Institute of Plasma Physics, Czech Academy of Sciences, Za Slovankou 3, Prague 8 18200, Czechia
8. Department of Physics, University of Hamburg, 20355 Hamburg, Germany
9. Institute of Nuclear Physics, Polish Academy of Sciences, Krakow 31-342, Poland
10. Queen's University, Kingston, Ontario K7L 3N6, Canada
11. Imperial College London, SW7 2AZ London, United Kingdom
12. European XFEL, 22869 Schenefeld, Germany
13. Institute of Experimental Physics, University of Hamburg, 22761 Hamburg, Germany
14. Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany