1. School of Physical Sciences, University of Science and Technology of China, Hefei 230026, China
2. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
3. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
4. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
5. National Fusion Research Institute, Daejeon 305-333, South Korea
6. Department of Nuclear Engineering, Seoul National University, Seoul 151-742, South Korea