[1] Johns Hopkins Univ, Dept Phys & Astron, Baltimore, MD 21218 USA
[2] Univ Rochester, Laser Energet Lab, Rochester, NY 14623 USA
[3] Univ Michigan, Ctr Laser Expt Astrophys Res, Ann Arbor, MI 48105 USA
[4] Pontificia Univ Catolica Chile, Inst Fis, Santiago, Chile