1. National Institute for Fusion Science, National Institutes of Natural Sciences, Toki 509-5292, Japan
2. Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543-0451, USA
3. University of California Los Angeles, Los Angeles, California 90095, USA
4. Institut Jean Lamour UMR 7198 CNRS, Université de Lorraine, Vandoeuvre-l'es-Nancy 54506, France