1. Princeton Plasma Physics Laboratory 1 , Princeton, New Jersey 08543, USA
2. Department of Plasma Physics and Fusion Engineering, University of Science and Technology of China 2 , Hefei 230026, China
3. Department of Astrophysical Sciences, Princeton University 3 , Princeton, New Jersey 08544, USA
4. Space Science Institute 4 , Boulder, Colorado 80301, USA
5. Department of Physics and Astronomy, University of California 5 , Los Angeles, California 90095, USA