1. The Graduate University for Advanced Studies, Toki 509-5292, Japan
2. National Institute for Fusion Science, Toki 509-5292, Japan
3. Department of Nuclear Engineering, Kyoto University, Kyoto 606-8501, Japan
4. Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, D-52425 Juelich, Germany