1. Lawrence Livermore National Laboratory P.O.Box 808, Livermore, California 94550
2. Dipartimento di Energetica, Università di Roma “La Sapienza,” Via A. Scarpa, 14 00161 Roma, Italy and INFM, Roma, Italy
3. General Atomics P.O. Box 85608, San Diego, California 92121-1122
4. Institute of Laser Engineering Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan