1. LAPLACE, Université de Toulouse, CNRS, INPT, UPS, 31062 Toulouse, France
2. Service Commun d’ELectronique, SCEL-UPS, 31062 Toulouse, France
3. Leibniz Institute for Plasma Science and Technology (INP), 17489 Greifswald, Germany
4. Institute of Physics, University of Rostock, 18059 Rostock, Germany