1. Department of Applied Physics, KTH Royal Institute of Technology, S-106 91 Stockholm, Sweden
2. Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany
3. Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States