1. Department of Physics, Uppsala University, Box 530, SE-751 21 Uppsala, Sweden Moscow Institute of Physics and Technology, 141 700, Dolgoprudny, Moscow Region, Russia
2. Department of Plasma Physics, Umeå University, SE-901 87 Umeå, Sweden
3. Department of Physics, Uppsala University, Box 530, SE-751 21 Uppsala, Sweden P. Kapitsa Institute for Physical Problems, 117 334, Moscow, Russia