1. Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, Tennessee 37831, USA
2. Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA
3. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
4. Princeton Plasma Physics Laboratory, Princeton, New Jersey 05764, USA
5. General Atomics, San Diego, California 92121, USA
6. Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA