1. Department of Fusion Science, Graduate University for Advanced Studies, Toki 509-5292, Gifu, Japan
2. National Institute for Fusion Science, Toki
509-5292, Gifu, Japan
3. Key Laboratory of Materials Modification by Laser, Ion and Electron Beams
(Ministry of Education), School of Physics and Optoelectronic Technology,
Dalian University of Technology, Dalian 116024,
People's Republic of China