1. Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
3. Centre de Recherches en Physique des Plasmas École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
4. Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom
5. Department of Physics, University of Maryland, College Park, Maryland 20742, USA