1. Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan e-mail:
2. Department of Marine Engineering, Kobe University, 5-1-1, Fukaeminami, Higashinada, Kobe 658-0022, Japan e-mail:
3. National Institute for Fusion Science, Oroshi-cho, Toki, Gifu 509-5292, Japan e-mail: