1. State Key Laboratory of Nuclear Physics and Technology, School of Physics, Fusion Simulation Center, Peking University, Beijing 100871, China
2. Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, China
3. Center for Fusion Science of Southwestern Institute of Physics, P. O. Box 432, Chengdu 610041, China
4. School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026, China