1. Institute of Physics, University of Greifswald, Felix-Hausdorff-Str. 6, 17489 Greifswald, Germany
2. Max-Planck-Institute for Plasma Physics, Boltzmannstr. 2, 85748 Garching, Germany
3. Institute of Plasma Physics, The Czech Academy of Sciences, 182 00 Prague, Czech Republic
4. Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, 115 19 Prague, Czech Republic