1. Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
2. General Atomics, P.O. Box, 85608, San Diego, California 92186, USA
3. Department of Physics and Astronomy, University of California-Los Angeles, Los Angeles, California 90095, USA
4. Center for Energy Research, University of California-San Diego, 9500 Gilman Dr., La Jolla, California 92093, USA
5. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
6. Southwestern Institute of Physics, P.O. Box 432, Chengdu 610041, China