1. York Plasma Institute, Department of Physics, University of York, York YO10 5DD, United Kingdom
2. Centre for Advanced Instrumentation, Department of Physics, Durham University, Durham DH1 3LE, United Kingdom
3. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
4. Culham Centre for Fusion Energy, Culham Science Centre, Abingdon OX14 3DB, United Kingdom