1. Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
2. Research Center for Plasma Turbulence, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
3. National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
4. Japan Atomic Energy Agency, Naka, Ibaraki 311-0193, Japan