1. Southwestern Institute of Physics, P.O. Box 432, Chengdu 610041, China
2. Department of Engineering Physics, Tsinghua University, Beijing 10084, China
3. State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei 230026, China
4. National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi, Gifu 509-5292, Japan