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