1. Institute for Fusion Theory and Simulation, School of Physics, Zhejiang University 1 , Hangzhou 310027, China
2. PPPL, Princeton University 2 , P. O. Box 451, Princeton, New Jersey 08543, USA
3. Center for Energy Research, University of California San Diego 3 , La Jolla, California 92093, USA
4. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 4 , Anhui 230031, China