1. Department of Engineering Physics, University of Wisconsin - Madison, Madison, Wisconsin 53706, USA
2. General Atomics, San Diego, California 92186-5608, USA
3. Max-Planck Institute for Plasma Physics, Greifswald, Germany
4. Institute of Energy and Climate Research - Plasma Physics, Forschungszentrum Jülich GmbH, Association EURATOM-FZJ, Partner in the Trilateral Euregio Cluster, Jülich, Germany