1. University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
2. Institute for Fusion Studies, University of Texas at Austin, Austin, Texas 78712, USA
3. DIFFER–Dutch Institute for Fundamental Energy Research, 5612 AJ Eindhoven, The Netherlands
4. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
5. Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany