Experimental and theoretical study of the CF4 DC glow discharge positive column

Abstract

An experimental and theoretical study of the positive column of the CF4 DC glow discharge is presented. It includes the experimental determination of the reduced electric field Ez /N and the electron impact excitation rate constant KAr of the line ( lambda =750.3 nm, epsilon th=13.5 eV) of the actinometric addition (Ar) in CF4 plasma for a wide range of parameters (pressure p=0.2-2.3 Torr, current density j=4-35 mA cm-2). All experimental values of the reduced electric field can be described within the limits of experimental error by the universal dependence Ez/N on r0N (r0=0.9 cm, the discharge tube radius). Also the experimental dependence of kAr on Ez/N has been obtained. These results have been used to test different theoretical models of the CF4 discharge positive column. Comparison of the experimental and calculated values of Ez /N and KAr has shown the validity of the theoretical model developed in this work. This model takes into account the large negative ion concentration in the positive column and predicts its division into regions of ion-ion plasma (in the centre) and electro-ion plasma (near the tube wall). The model also accounts for the effect of non-locality of the electron energy distribution function. It has been shown that this effect can be accounted for by solving the Boltzmann equation for the EEDF in the spatially non-uniform case. The incorrectness of the notion of ambipolar diffusion in the plasma of a strongly electronegative gas with high negative ion concentrations has been reaffirmed.