1. Department of Physics, St. Petersburg State University, St. Petersburg 198504, Russia
2. Department of General and Technical Physics, St. Petersburg Mining University, St. Petersburg 199106, Russia
3. Universite Toulouse III - Paul Sabatier, LAPLACE - Laboratoire Plasma et Conversion d'Energie, 118 route de Narbonne, Bat 3R3, 31062 Toulouse cedex 9, France