1. Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
2. Max-Planck-Institut für Plasmaphysik, 17491 Greifswald, Germany
3. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
4. Laboratory for Plasma Physics, Ecole Royale Militaire - Koninklijke Militaire School, 1000 Brussels, Belgium
5. Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung—Plasmaphysik, Partner of the Trilateral Euregio Cluster (TEC), 52425 Jülich, Germany