1. College of Science, Civil Aviation University of China 1 , Tianjin 300300, People's Republic of China
2. Physics Department, Auburn University 2 , Auburn, Alabama 36849, USA
3. Institute for Fusion Theory and Simulation and School of Physics, Zhejiang University 3 , Hangzhou 310027, People's Republic of China
4. Department of Physics and Astronomy, University of California 4 , Irvine, California 92697-4575, USA
5. Center for Nonlinear Plasma Science, ENEA Frascati 5 , Frascati 00044, Italy