1. Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109-2099, USA
2. The John Adams Institute for Accelerator Science, Imperial College London, London SW7 2AZ, United Kingdom
3. Physics and Astronomy, University of California, Los Angeles, Los Angeles, California 90095, USA
4. Electrical Engineering, University of California, Los Angeles, Los Angeles, California 90095, USA
5. York Plasma Institute, Department of Physics, University of York, York YO10 5DD, United Kingdom
6. Department of Physics, Chalmers University of Technology, SE-41296 Gothenburg, Sweden
7. SUPA Department of Physics, University of Strathclyde, Glasgow G4 0NG, United Kingdom
8. Centre for Mathematical Sciences, Plymouth University, Plymouth, United Kingdom
9. Institut für Optik und Quantenelektronik, Friedrich-Schiller-Universität, 07743 Jena, Germany
10. School of Mathematics and Physics, The Queen’s University of Belfast, BT7 1NN Belfast, United Kingdom
11. Central Laser Facility, Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom
12. Physics Department, Lancaster University, Bailrigg, Lancaster LA1 4YW, United Kingdom