1. Interdisciplinary Graduate School of Engineering Science, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan, E-mail: kadzu@nucl.kyushu-u.ac.jp
2. National Institute for Fusion Science, 322-6, Oroshi-cho, Toki, Gifu 509-5292, Japan
3. Department of Material Science, Shimane University, Matue, 1060, Nishikawatsu-cho, Shimane 690-8504, Japan
4. Research Institute for Applied Mechanics, Kyushu University, 6-1, Kasugakoen, Kasuga, Fukuoka 816-8580, Japan