1. Department of Physics, University of Alberta, Edmonton, Alberta T6G 2G7, Canada
2. Lawrence Livermore National Laboratory, Livermore, California 94551, USA
3. P.N. Lebedev Physics Institute, Russian Academy of Sciences, Moscow 119991 Russia and Center for Fundamental and Applied Research, VNIIA, ROSATOM, 127055 Moscow, Russia
4. Department of Electrical Engineering, UCLA, Los Angeles, California 90095, USA
5. Association EURATOM-Confederation Suisse, EPFL, 1015 Lausanne, Switzerland
6. Department of Physics and Astronomy, UCLA, Los Angeles, California 90095, USA
7. LCLS, Stanford, California 94025, USA