1. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
2. Plasma Science and Fusion Center, MIT, Cambridge, Massachusetts 02139, USA
3. Max-Planck Institute for Plasma Physics, EURATOM Association, Boltzmannstr. 2, 85748 Garching, Germany
4. Department of Applied Physics, Gent University, 9000 Gent, Belgium