1. National Institute for Fusion Science, Toki 509-5292, Japan
2. Kyoto University, Department of Nuclear Engineering, Kyoto 606-8501, Japan
3. Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan
4. RIKEN, Tera-Photonics Laboratory, Aoba Sendai-City, Miyagi 980-0845, Japan