1. Department of Physics, University of Wisconsin Madison 1 , Madison, Wisconsin 53706, USA
2. Department of Physics, University of California Los Angeles 2 , Los Angeles, California 90095, USA
3. RadiaBeam Technologies 3 , Santa Monica, California 90404, USA
4. SLAC National Accelerator Laboratory 4 , Menlo Park, California 94025, USA
5. Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY 5 , Notkestr. 85, 22607 Hamburg, Germany