1. Institute for Quantum Electronics, ETH Zurich, CH-8093 Zurich, Switzerland
2. Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
3. LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
4. SwissFEL, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland