1. Department of Materials and Thomas Young Centre, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
2. Department of Physics and Thomas Young Centre, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
3. Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
4. UK Atomic Energy Authority, Culham Center for Fusion Energy, Oxfordshire OX14 3DB, United Kingdom