1. Department of Engineering Physics, University of Wisconsin–Madison, Madison, Wisconsin, 53706, USA
2. Max-Planck-Institut für Plasmaphysik, Association EURATOM-IPP, 17491 Greifswald, Germany
3. ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St Paul Lez Durance Cedex, France
4. Institute for Laser and Plasma Physics, Heinrich-Heine-University, D-40225 Duesseldorf, Germany