1. Department of Physics, Lund University, P.O. Box 118, 22100 Lund, Sweden
2. Institut für Laser- und Plasmaphysik, Heinrich-Heine Universität, 40225 Düsseldorf, Germany
3. SUPA Department of Physics, University of Strathclyde, Glasgow G4 0NG, United Kingdom
4. Department of Applied Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden
5. Central Laser Facility, STFC, Rutherford Appleton Laboratory, OX11 0QX Didcot, United Kingdom