1. Department of Physics–Durham University, South Road, Durham DH1 3LE, United Kingdom
2. Culham Centre for Fusion Energy, Culham Science Centre, Abingdon OX14 3DB, United Kingdom
3. Princeton Plasma Physics Laboratory, 100 Stellarator Road, Princeton, New Jersey 08540, USA
4. York Plasma Institute–University of York, Heslington, York YO10 5DQ, United Kingdom