1. Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, People's Republic of China
2. University of Science and Technology of China, Hefei 230026, People's Republic of China
3. MIT Plasma Science and Fusion Center, Cambridge, Massachusetts 02139, USA
4. College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China