1. Southwestern Institute of Physics, PO Box 432, Chengdu 610041, China
2. Department of Engineering Physics, Tsinghua University, Beijing 100084, China
3. General Atomics, PO Box 85608, San Diego, California 92186-5608, USA
4. CCFE, Culham Science Centre, Abingdon OX14 3DB, United Kingdom
5. Key Laboratory of Materials Modification by Beams of the Ministry of Education, School of Physics, Dalian University of Technology, Dalian 116024, China