1. Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA
2. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
3. Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA
4. Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53796, USA
5. Associacao do Instituto Superior Tecnico para a Investigacao e Desenvolvimento, P1049-003 Lisbon, Portugal and Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico, Universidade de Lisboa, P1049-001 Lisbon, Portugal
6. University of California San Diego, 9500 Gilman Dr., La Jolla, California 92093-0417, USA