1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
2. University of Science and Technology of China,
Hefei 230026, China
3. General Atomics, PO Box 85608, San Diego,
California 92186-5608, USA
4. State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China