1. Department of Astrophysical Science, Princeton University, Princeton, New Jersey 08544, USA
2. General Atomics, PO Box 85608, San Diego, California 92186-5608, USA
3. Princeton Plasma Physics Laboratory, Princeton, New Jersey, 08543-0451, USA
4. Eindhoven University of Technology, 5612 AZ Eindhoven, The Netherlands
5. Department of Physics, Yale University, New Haven, Connecticut 06516, USA
6. Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, USA