1. Southwestern Institute of Physics, P.O. Box 432, Chengdu 610041, People's Republic of China
2. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
3. CCFE, Culham Science Centre, Abingdon OX14 3DB, United Kingdom
4. Department of Engineering Physics, Tsinghua University, Beijing 100084, People's Republic of China