1. Department of Electrical and Electronic Engineering, University of Cagliari, Via Marengo 2, Cagliari 09123, Italy
2. Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, Wendelsteinstraße 1, Greifswald D-17491, Germany
3. University of Szczecin, Institute of Physics, Szczecin 70-451, Poland
4. Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA