1. Physik-Department E28, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany
2. Max-Planck-Institut für Plasmaphysik, Boltzmannstr.2, 85748 Garching, Germany
3. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of China