1. National Institute for Fusion Science Nagoya 464-01 Japan
2. Department of Physics Faculty of Science, University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113 Japan
3. Interdisciplinary Graduate School of Engineering Sciences Kyushyu University Kasuga Fukuoka 816 Japan
4. Department of Applied Physics Faculty of Engineering, Seikei University Musashino-shi Tokyo 180 Japan