1. Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA
2. Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
3. Institute of Plasma Physics, Czech Academy of Sciences, Za Slovankou 1782/3, 182 00 Praha 8, Czech Republic
4. Institute of Physics ASCR, v.v.i. (FZU), ELI BEAMLINES, Na Slovance 1999/2, 182 21 Prague 8, Czech Republic