1. Institute of Plasma Physics, Chinese Academy of Sciences, 230031 Hefei, China
2. University of Science and Technology of China, 230026 Hefei, China
3. Institute of Energy, Hefei Comprehensive National Science Center, 230051 Hefei, China
4. Faculty of Science and Technology, University of Twente, Energy, Materials and Systems, 7500AE Enschede, The Netherlands