1. Southwestern Institute of Physics 1 , P.O. Box 432, Chengdu 610041, China
2. Center for Nonlinear Plasma Science and C.R. ENEA Frascati 2 , C.P. 65, 00044 Frascati, Italy
3. Department of Physics and Astronomy, University of California 3 , Irvine, California 92697-4574, USA
4. Institute for Fusion Theory and Simulation, School of Physics, Zhejiang University 4 , Hangzhou 310027, People's Republic of China