1. LECAD Laboratory, Faculty of Mech. Eng., University of Ljubljana, SI-1000 Ljubljana, Slovenia
2. Plasma and Energy Physics Group, Institute for Theoretical Physics, University of Innsbruck, A-6020 Innsbruck, Austria
3. University of Ljubljana, Faculty of Electrical Engineering, SI-1000 Ljubljana, Slovenia and Jozef Stefan Institute, Ljubljana, Slovenia
4. Institute of Physics, Georgian Academy of Sciences, 0177 Tbilisi, Georgia