1. Tritium Technology Group, Fusion Research and Development Directorate, Japan Atomic Energy Agency, 2-4 Shirane Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
2. Department of Advanced Energy Engineering Science, Kyushu University, 6-1 Kasuga-kouen Kasuga, Fukuoka 816-8580, Japan