1. Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8583 Japan
2. School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan
3. High Temperature Plasma Center, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8568 Japan
4. Central Research Institute of Electric Power Industry, 2-11-1 Iwadokita, Komae, Tokyo, 201-8511 Japan