1. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
2. Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA
3. Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
4. Harvard-Smithsonian Center for Astrophysics, The Institute for Theory and Computation, Cambridge, Massachusetts 02138, USA