1. Institute for Fusion Theory and Simulation and Department of Physics, Zhejiang University, Hangzhou 310027, China
2. Southwest Institute of Physics, Chengdu 610041, China
3. Institute for Theoretical Physics I, Ruhr University, Bochum D-44780, Germany
4. Princeton Plasma Physics Laboratory, Princeton University, P. O. Box 451, Princeton, New Jersey 08543, USA