1. Department of Physics, Durham University, South Road, Durham, DH1 3LE, United Kingdom
2. United Kingdom Atomic Energy Authority, Culham Centre for Fusion Energy, Culham Science Centre, Abingdon, Oxon, OX14 3DB, United Kingdom
3. Plasma Science and Fusion Centre, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
4. Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA