1. MIT Plasma Science and Fusion Center, Cambridge, Massachusetts 02139, USA
2. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08540, USA
3. University of California, San Diego (UCSD) San Diego, California 92093, USA
4. General Atomics, P.O. Box 85608, San Diego, California 92186, USA
5. York University, Heslington, York YO10 5DD, United Kingdom
6. Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Federale de Lausanne, Lausanne 1015, Switzerland