1. Lawrence Livermore National Laboratory, Livermore, California 94550, USA
2. Deutsches Elektronen-Synchrotron DESY, Hamburg D-22607, Germany
3. Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
4. European XFEL, Schenefeld D-22869, Germany
5. Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 61005, South Korea