1. General Atomics, San Diego, California 92121, USA
2. Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109, USA
3. Central Laser Facility, STFC, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom
4. Department of Physics, University of Strathclyde, Glasgow G1 1XQ, United Kingdom
5. Lawrence Livermore National Laboratory, Livermore, California 94550, USA