1. Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
2. Plasma Research Center, Tsukuba University, Ibaraki 305-8577, Japan
3. National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
4. Department of Material Science, Shimane University, Matsue, Shimane 690-8504, Japan