Demixing effect induced by the radial electric field in the cylindrically shaped dc plasma. Modeling and experimental investigation

Sample introduction and a demixing effect in the plasma of the d.c. are in argon with aerosol supply are studied using spectroscopic techniques. The observed deviations from the uniform distribution of the partially ionized plasma gas components are correlated to the radial electric field in the are column plasma. A mathematical model is proposed for the quantitative treatment of the radial field effect. The radial electric field is assessed from the radial distribution of the electron pressure. The proposed model is used in two ways: (a) starting from the close-to-real are temperature distribution, other parameters are calculated assuming that plasma is in local thermodynamic equilibrium. The calculations are performed for mixtures of two gases: majority gas with high ionization energy and minority gas with low ionization energy at low concentration. The results obtained show that the partial pressure of an easily ionizable component decreases from the periphery towards the are column axis. The size of the decrease ranges from a negligible value up to four orders of magnitude, depending on the ionization energy of the minority gas. (b) For the experimental verification of the proposed model, all the variables governing the sample entry into the plasma, including the initial partial pressure of the minority gas, are determined by independent spectroscopic techniques. Vapors of Ba, Ca, Fe and Zn are utilized as a minority gas. These values are then used to calculate the radial distribution of the minority gas partial pressure according to the proposed model. Good agreement between the experimental and the calculated values is observed. (C) 1998 Elsevier Science B.V