1. School of Mechano-Electronic Engineering, Xidian University, Xi’an 710071, P. R. China
2. Chengdu Development Center of Science and Technology, China Academy of Engineering Physics, Chengdu 610200, P. R. China
3. School of Mechanical and Electrical Engineering, Soochow University, Suzhou 215123, P. R. China
4. Science and Technology on Vacuum Technology and Physics Laboratory, Lanzhou 730000, P. R. China