1. Institute of Plasma Physics, Chinese Academic Sciences, Hefei, China
2. Science Island Branch of Graduate School, University of Science and Technology of China, Hefei, China
3. National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
4. SOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi-cho, Toki 509-5292, Japan