1. Princeton Plasma Physics Laboratory, 100 Stellarator Road, Princeton, New Jersey 08540, USA
2. Institute of Energy, Hefei Comprehensive National Science Center, Hefei 230031, China
3. Department of Physics, Nanchang University, Nanchang 330031, China
4. Johns Hopkins University, Baltimore, Maryland 21211, USA
5. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China
6. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
7. School of Mechanical Engineering, Anhui University of Science and Technology, Huainan 232001, China
8. Key Laboratory of Photovoltaic and Energy Conservation Materials, CAS, Hefei 230031, China