1. SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
2. European XFEL, Schenefeld, Germany
3. Laboratoire pour l’Utilisation des Lasers Intenses, Palaiseau Cedex, France
4. University of Michigan, Ann Arbor, Michigan 48109, USA
5. University of Alberta, Edmonton, Alberta T6G 2R3, Canada
6. Helmholtz Zentrum Dresden-Rossendorf, Dresden, Germany
7. Department of Physics, University of California Berkeley, Berkeley, California 94720, USA
8. Lawrence Livermore National Laboratory, Livermore, California 94550, USA