1. Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, United Kingdom
2. Department of Physics and Astronomy, University of Canterbury, P.O. Box 4800, Christchurch 8020, New Zealand
3. FELIX Free Electron Laser Facility, FOM – Institute for Plasmaphysics ‘Rijnhuizen’, Postbus 1207, 3430 BE Nieuwegein, The Netherlands