1. Department of Physics, Sichuan University of Science & Engineering, Zigong, Sichuan 643000, China
2. Center for Magnetic Fusion Theory, CAS, Hefei, Anhui 230026, China
3. Institute for Fusion Studies, University of Texas at Austin, Austin, Texas 78712, USA
4. Southwestern Institute of Physics, P.O. Box 432, Chengdu, Sichuan 610041, China
5. Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China