1. Department of Nuclear Engineering, Kyoto University, Nishikyo, Kyoto 615-8540, Japan
2. National Institute for Fusion Science, 322-6 Oroshi, Toki 509-5292, Japan
3. Institute for Fusion Studies, The University of Texas at Austin, Austin, Texas 78712-0262, USA
4. Plasma Science and Fusion Center, MIT, Cambridge, Massachusetts 02139, USA