1. J.P. Borra et al.: J. Phys. D: Appl. Phys.30 (1997) 84.
2. L. Aubrecht et al.: inProc. Int. Conf. Plasma Phys. & 25th Conf. Contr. Fus. Plasma Phys., Prague, 1998., (Ed. P. Pavlo), ECA22C (1998) 2431.
3. J.E. Jones et al.: J. Phys. D: Appl. Phys23 (1990) 1652.
4. L. Aubrecht et al.: inProc. 14th Int. Symp. Plasma Chem., Prague 1999 (Ed. M. Hrabovsky et al.), Institut of Plasma Physics, Prague, 1999, Vol. V, p. 2601.