Experimental and theoretical study of the CF₄DC 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.

Concepts, Capabilities, and Limitations of Global Models: A Review

“…The environment of electronegative plasmas poses a particular challenge for empirically describing the edge to center density ratio, as they can exhibit a number of different structures depending on the operating conditions . A variety of expressions, based mainly on heuristic descriptions, have been developed over the years, but are often only valid for a very small subset of plasmas .…”

“…The environment of electronegative plasmas poses a particular challenge for empirically describing the edge to center density ratio, as they can exhibit a number of different structures depending on the operating conditions. [127][128][129][130][131][132] A variety of expressions, based mainly on heuristic descriptions, have been developed over the years, but are often only valid for a very small subset of plasmas. [8,[133][134][135] General expressions have been developed more recently that attempt to provide a more broad description, applicable to a wider range of systems.…”

The Boltzmann relation in electronegative plasmas: When is it permissible to use it?

“…3. So far as recombination-dominated discharges are concerned, the most comprehensive measurements are those of Volynets et al (1993) and Feoktistov et al (1997), where it is shown that the discharge is structured with an ion-ion plasma core surrounded by an electron-ion plasma, and it is for this reason that there is no region where ε eff is approximately constant. Volynets et al in their computational work followed essentially the approach given above, but only for specific parameters related to their experiments.…”

Negative ion density fronts

“…In each of these regions different physical processes dominate. The proposed methodology is based on a fundamental phenomenon, which appears in multi-component plasma that the plasma stratifies into regions with different ion composition [17][18][19][20][21]. For example electronegative plasmas with hot electrons often separate into core region, containing the negative ions (electronegative region), and an electron-ion edge region, near the walls, which contains essentially no negative ions (electropositive region).…”