1. School of Physics and Astronomy, Institute of Natural Sciences, and MOE-LSC, Shanghai Jiao Tong University 1 , Shanghai 200240, China
2. Max-Planck-Institut für Plasmaphysik 2 , 17491 Greifswald, Germany
3. Max Planck/Princeton Research Center for Plasma Physics 3 , Princeton, New Jersey 08544, USA
4. Princeton Plasma Physics Laboratory 4 , Princeton, New Jersey 08543, USA