1. Southwestern Institute of Physics, Chengdu 610041, People’s Republic of China
2. CEA, IRFM, Saint-Paul-lez-Durance F-13108, France
3. National Institute for Fusion Science, Oroshi-cho 322-6, Toki 509-5259, Japan
4. SOKENDAI (The Graduate University for Advanced Studies), Oroshi-cho 322-6, Toki 509-5292, Japan