1. Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom
2. CCFE, Culham Science Centre, Abingdon OX14 3DB, United Kingdom
3. CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France
4. DTU, Department of Physics, DK-2800 Kgs. Lyngby, Denmark