1. National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
2. University of Fukui, Research Center for Development of Far-Infrared Region, 3-9-1 Bunkyo, Fukui 910-8507, Japan
3. Max-Plank-Institut für Plasmaphysik, EURATOM-Association, Wendelsteinstr. 1, D-17491 Greifswald, Germany