1. National Institute for Fusion Science, Toki, Gifu 509-5292
2. Institute for Applied Mechanics, Kyushu University 87, Kasuga, Fukuoka 816-8580
3. Princeton University, Plasma Physics Laboratory, Princeton, NJ 08543, U.S.A.
4. University of California San Diego, La Jolla, CA 92093-0319, U.S.A.