1. Institute of Electrical Engineering and Plasma Technology, Ruhr University Bochum, Universitaetsstraße 150, D-44780 Bochum, Germany
2. Chair for Experimental Physics II, Ruhr University Bochum, Universitaetsstraße 150, D-44780 Bochum, Germany
3. Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Ministry of Education), School of Physics, Dalian University of Technology, Dalian 116024, People’s Republic of China