1. College of Science, Donghua University, Shanghai 201620, China
2. CCFE, Culham Science Centre, Abingdon OX14 3DB, United Kingdom
3. Southwestern Institute of Physics, PO Box 432, Chengdu 610041, China
4. Department of Earth and Space Science, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
5. School of Mathematical and Physical Sciences, Dalian University of Technology, Panjin 124221, China