1. National Institute for Fusion Science, Toki, Gifu 509-5292
2. Graduate University for Advanced Studies, Oroshi-cho 322-6, Toki, Gifu 509-5292
3. Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany
4. Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580