1. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
2. Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14623, USA
3. Center for Laser Experimental Astrophysical Research, University of Michigan, Ann Arbor, Michigan 48105, USA
4. Instituto de Física, Pontificia Universidad Católica de Chile, Macul, Santiago, Chile