1. National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
2. Japan Atomic Energy Agency, 2-4 Shirakata-shirane, Tokai-Mura, Ibaraki 319-1195, Japan
3. Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
4. Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan