1. High-Temperature Plasma Physics Research Division, National Institute for Fusion Science, Toki 509-5292, Japan
2. Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
3. Plasma Research Center, University of Tsukuba, Tsukuba 305-8577, Japan
4. KSTAR Research Center, National Fusion Research Institute, Daejeon 305-806, South Korea