1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China
2. University of Science and Technology of China, Hefei, Anhui 230026, China
3. Center for Magnetic Fusion Theory, Chinese Academy of Sciences, Hefei, Anhui 230031, China
4. National Institute for Fusion Science, Toki, Gifu 509-5292, Japan