1. Department of Astrophysical Sciences, Princeton Plasma Physics Laboratory 1 , Princeton, New Jersey 08543, USA
2. Princeton Plasma Physics Laboratory 2 , Princeton, New Jersey 08543, USA
3. Swiss Plasma Center, École Polytechnique Fédérale de Lausanne 3 , CH-1015 Lausanne, Switzerland
4. Institute of Natural Sciences, School of Physics and Astronomy, and MOE-LSC, Shanghai Jiao Tong University 4 , Shanghai 200240, China