1. Plasma Research Center, University of Tsukuba, Ibaraki 305-8577, Japan
2. Naka Fusion Research Est., Japan Atomic Energy Agency, Naka,Ibaraki, Japan
3. Toshiba Electron Tubes & Devices Co. Ltd, Otawara, Tochigi, Japan
4. Research Center for Dev. of Far Infrared Region, Univ. of Fukui, Fukui, Japan