1. Southwestern Institute of Physics, P.O. Box 432, Chengdu 610041, China
2. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
3. School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026, China
4. University of California Irvine, Irvine, California 92697, USA
5. National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
6. Department of Fusion Science, SOKENDAI (The Graduate University for Advanced Studies), Toki, Gifu 509-5292, Japan
7. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China