1. School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026, China
2. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
3. School of Physics and Materials Science, Anhui University, Hefei 230601, China
4. Hefei Science Center, Chinese Academy of Sciences, Hefei 230031, China
5. Department of Nuclear Engineering, Seoul National University, Seoul 151-742, South Korea