1. National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
2. Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
3. Department of Material Science, Shimane University, Matsue, Shimane 690-8504, Japan
4. Laboratory for Plasma Physics and Engineering, Hokkaido University, Sapporo 060-8628, Japan