1. 1 University of Ljubljana, Faculty of Electrical Engineering, Tržaška 25, 1000 Ljubljana, Slovenia
2. Jožef Stefan Institute 2 , Jamova 39, POB 100, 1000 Ljubljana, Slovenia
3. 3 University of Ljubljana, Faculty of Mechanical Engineering, Aškerčeva 6, 1000 Ljubljana, Slovenia
4. Institute of Plasma Physics of the Czech Academy of Sciences 4 , Za Slovankou 3, 18200 Prague 8, Czech Republic
5. Emil Djakov Institute of Electronics, Bulgarian Academy of Sciences 5 , 72, Tsarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria