1. Department of Low Temperature Physics, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, CZ-18000 Prague 8, Czech Republic
2. Department of Materials Engineering, Institute of Plasma Physics of CAS, Za Slovankou 1782, CZ-18200 Prague 8, Czech Republic
3. UJP PRAHA a.s., Nad Kamínkou 1345, CZ-15610 Prague 5 - Zbraslav, Czech Republic