1. Institute of Plasma Physics and Technology, School of Physical Science and Technology, Jiangsu Key Laboratory of Thin Films, Soochow University 1 , Suzhou 215006, China
2. Princeton Plasma Physics Laboratory 2 , 100 Stellarator Road, Princeton, New Jersey 08540, USA
3. Institute of Plasma Physics, Chinese Academy of Sciences 3 , Hefei, Anhui 230031, China