1. School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026, People's Republic of China
2. Max Planck Institut für Plasmaphysik, 85748 Garching, Germany
3. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of China
4. Center for Integrated Plasma Studies, University of Colorado at Boulder, Boulder, Colorado 80309, USA