1. Plasma Science and Fusion Center, MIT, Cambridge, Massachusetts 02139, USA
2. York Plasma Institute, Department of Physics, University of York, Heslington, York, United Kingdom
3. Ecole Polytechnique Federale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas (CRPP), CH-1015 Lausanne, Switzerland
4. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA