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. ar e in argon with aerosol supply are studied using spectroscopic techniq ues. The observed deviations from the uniform distribution of the part ially ionized plasma gas components are correlated to the radial elect ric field in the are column plasma. A mathematical model is proposed f or the quantitative treatment of the radial field effect. The radial e lectric field is assessed from the radial distribution of the electron pressure. The proposed model is used in two ways: (a) starting from t he close-to-real are temperature distribution, other parameters are ca lculated assuming that plasma is in local thermodynamic equilibrium. T he calculations are performed for mixtures of two gases: majority gas with high ionization energy and minority gas with low ionization energ y at low concentration. The results obtained show that the partial pre ssure of an easily ionizable component decreases from the periphery to wards the are column axis. The size of the decrease ranges from a negl igible value up to four orders of magnitude, depending on the ionizati on energy of the minority gas. (b) For the experimental verification o f the proposed model, all the variables governing the sample entry int o the plasma, including the initial partial pressure of the minority g as, are determined by independent spectroscopic techniques. Vapors of Ba, Ca, Fe and Zn are utilized as a minority gas. These values are the n used to calculate the radial distribution of the minority gas partia l pressure according to the proposed model. Good agreement between the experimental and the calculated values is observed. (C) 1998 Elsevier Science B.V.