1. Institute of Plasma Physics of the CAS, Za Slovankou 3, Prague 182 00, Czech Republic
2. Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, Prague 121 16, Czech Republic
3. Consorzio RFX (CNR, ENEA, INFN, Università di Padova, Acciaierie Venete SpA), Corso Stati Uniti 4, 35127 Padova, Italy