1. Naka Fusion Research Establishment, Naka-shi, Ibaraki-ken 311-0193, Japan
2. Faculty of Science and Engineering, Bunkyo-ku, Tokyo-to 112-8551, Japan
3. Joetsu-shi, Niigata-ken 943-8512, Japan
4. Research Institute for Applied Mechanics, Kasuga-shi, Fukuoka-ken 816-8580, Japan