1. Institute of Plasma Physics Chinese Academy of Sciences, Hefei 230026, Anhui, China
2. National Institute for Fusion Science, Toki 509-5292, Gifu, Japan
3. Department of Fusion Science, Graduate University for Advanced Studies, Toki 509-5292, Gifu, Japan
4. Southwestern Institute of Physics, Chengdu 610041, Sichuan, China