1. Toshiba Electron Tubes & Devices Co. Ltd, Otawara, Tochigi 324-8550, Japan
2. Plasma Research Center, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
3. Research Center for Dev. of Far Infrared Region, Univ. of Fukui, Bumkyo, Fukui 910-8507, Japan
4. Naka Fusion Research Est., Japan Atomic Energy Agency, Naka,Ibaraki 311-0193, Japan