1. Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany
2. Ghent University, Department of Applied Physics, B-9000 Gent, Belgium
3. LPP-ERM/KMS, B-1000 Brussels, Belgium
4. National Science Center “Kharkov Institute of Physics and Technology,” Institute of Plasma Physics, 61108, Kharkov, Ukraine
5. Institute Jean Lamour, F-54011 Nancy, France