1. The Ohio State University, College of Mathematical and Physical Sciences, Columbus, Ohio 43210-1123 and University of California, Davis, Davis, California 95616
2. Università degli Studi di Milano and Istituto Nazionale per la Fisica della Materia (INFM), Dipartimento di Fisica, Milan, Italy
3. Ecole Polytechnique, Laboratoire pour l’Utilisation des Lasers Intenses (LULI), Palaiseau, France
4. Lawrence Livermore National Laboratory, University of California, Livermore, California 94550
5. General Atomics, Inc., San Diego, California 92186