1. Department of Physics, Technical University of Denmark, Kgs. Lyngby, Denmark
2. Institute of Plasma Physics, Chinese Academy of Sciences, P.O. 1126, 230031 Hefei, Anhui, China
3. Max-Planck-Institut für Plasmaphysik, Garching, Germany
4. CCFE, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, United Kingdom