1. Plasma Research Center, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
2. Research Center for Developement of Far-Infrared Region, University of Fukui, Fukui 910-8507, Japan
3. National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
4. Faculty of Engineering, Ibaraki University, Hitachi, Ibaraki 316-8511, Japan