1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
2. Hefei Science Center, Chinese Academy of Sciences, Hefei 230031, China
3. School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026, China
4. School of Physics and Electronic Engineering, Anqing Normal University, Anqing 246011, China
5. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
6. National Fusion Research Institute, Daejeon 305-333, South Korea
7. Department of Nuclear Engineering, Seoul National University, Seoul 151-742, South Korea