1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China
2. Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA
3. Department of Applied Physics, Hunan University, Changsha 410082, China
4. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
5. Johns Hopkins University, Baltimore, Maryland 21211, USA