1. Institute of Plasma Physics of the Czech Academy of Sciences 1 , Za Slovankou 1782/3, 182 00 Prague, Czech Republic
2. Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague 2 , Břehová 78/7, 115 19 Prague, Czech Republic
3. GoLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa 3 , 1049-001 Lisbon, Portugal