1. CEA, IRFM, F-13108 Saint-Paul-Lez-Durance, France
2. York Plasma Institute, Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom
3. École Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas (CRPP), CH-1015 Lausanne, Switzerland