1. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543-0451, USA
2. Department of Physics and Astronomy, University of California-Los Angeles, Los Angeles, California 90095, USA
3. General Atomics, P.O. Box 85608, San Diego, California 92186, USA
4. University of California-Los Angeles, Los Angeles, California 90095, USA
5. Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA