1. John Adams Institute, Blackett Laboratory, Imperial College London, London SW7 2AZ, UK
2. Central Laser Facility, Rutherford-Appleton Laboratory, Chilton, Oxon, UK
3. Department of Physics and Astronomy, University College London, London, UK
4. Grupo de Lasers e Plasmas, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Lisboa, Portugal