1. Southwestern Institute for Physics, CNNC, C-610200 Chengdu, China
2. Institute for Magnetic Fusion Research, CEA, F-13115 Saint-Paul-lez-Durance, France
3. Department of Applied Physics, Ghent University, B-9000 Ghent, Belgium
4. Max-Planck-Institut für Plasmaphysik, D-17491 Greifswald, Germany
5. Laboratory for Plasma Physics, Royal Military Academy (LPP-ERM/KMS), B-1000 Brussels, Belgium