1. Department of Applied Physics, Stanford University, Stanford, California 94305, USA
2. Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA
3. SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
4. School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
5. European XFEL, Holzkoppel 4, 22869 Schenefeld, Germany
6. Department of Physics, Stanford University, Stanford, California 94305, USA