[1] Inst Phys, YU-11001 Belgrade, Yugoslavia
[2] Ruhr Univ Bochum, Fak Phys & Astron, Inst Theoret Phys 4, D-44780 Bochum, Germany
[3] Umea Univ, Dept Plasma Phys, S-90187 Umea, Sweden
[4] Univ Pisa, Dept Phys, I-56100 Pisa, Italy
[5] INFM, I-56100 Pisa, Italy