1. Department of Engineering Physics, Tsinghua University, Beijing 100084, China
2. Center for Fusion Science, Southwestern Institute of Physics, Chengdu, Sichuan 610041, China
3. Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706-1687, USA
4. School of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China