1. College of Science, Donghua University, Shanghai 201620, China
2. Member of Magnetic Confinement Fusion Research Centre, Ministry of Education, Shanghai 201620, China
3. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
4. CCFE, Culham Science Centre, Abingdon OX14 3DB, United Kingdom of Great Britain and Northern Ireland
5. Faculty of Electronic information and Electrical Engineering, Dalian University of Technology, Dalian 116024, China