1. Institute of Plasma Physics and Laser Microfusion 1 , Warsaw 01 497, Poland
2. Institute of Plasma Physics AS CR 2 , Prague 182 00, Czech Republic
3. Department of Radiation and Chemical Physics, Institute of Physics, CAS 3 , Prague 182 21, Czech Republic
4. Faculty of Nuclear Science and Physical Engineering of the Czech Technical University in Prague 4 , Prague 115 19, Czech Republic
5. Faculty of Mathematics and Physics, Charles University 5 , Prague 121 16, Czech Republic