[1] IPP CR, Associat EURATOM, Prague 18221 8, Czech Republic
[2] Czech Tech Univ, CR-16635 Prague, Czech Republic
[3] Univ Innsbruck, Inst Theoret Phys, A-6020 Innsbruck, Austria
[4] Associat EURATOM, Royal Inst Technol, Alfven Lab, Stockholm, Sweden