1. The Queen’s University, School of Mathematics and Physics, Belfast BT7 1NN, United Kingdom
2. Laboratoire pour l’Utilisation des Lasers Intenses, UMR 7605 CNRS-CEA-École Polytechnique-Université Paris VI, 91128 Palaiseau 3, France
3. University of Nevada, Physics Department, MS-220, Reno, Nevada 89557
4. Kansai Research Establishment, APRC-JAERI, Kizu, Japan
5. Lawrence Livermore National Laboratory, Livermore, California
6. Technical University Darmstadt, Darmstadt, Germany