1. Max Planck Institute of Quantum Optics 1 , 85748 Garching, Germany
2. Department of Physics, Ludwig-Maximilians-Universität 2 , 85748 Garching, Germany
3. LCLS, SLAC National Accelerator Laboratory 3 , Menlo Park, California 94025, USA
4. The Stanford PULSE Institute, SLAC National Accelerator Laboratory 4 , Menlo Park, California 94025, USA
5. Applied Physics Department, Stanford University 5 , Stanford, California 94305, USA