1. Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
2. Center for Ultrafast Optical Science, University of Michigan, 2200 Bonisteel Boulevard, Ann Arbor, Michigan 48109, USA
3. Department of Physics, York Plasma Institute, University of York, York YO10 5DD, United Kingdom
4. Blackett Laboratory, Imperial College London, SW7 2BZ, United Kingdom
5. Central Laser Facility, STFC Rutherford-Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom