1. Institute of Plasma Physics Chinese Academy of Sciences, PO Box 1126, Hefei, Anhui, China
2. University of Chinese Academy of Sciences, No. 19(A) Yuquan Road, Shijingshan District, Beijing, 100049, China
3. General Atomic, PO Box 85608, San Diego, California 92186-5608, USA
4. Max Planck Institute for Plasma Physics, D-85748 Garching, Germany
5. School of Physics and Electrical Engineering, Anqing Normal University, Anqing 246011, China