Populations in the metastable and the resonance levels of argon and stepwise ionization effects in a low-pressure argon positive column

Abstract

The contribution of the ionization from the two metastable and the two resonance levels of argon to the total ionization rate in a low-pressure argon positive column is investigated theoretically and experimentally. A simple yet self-consistent discharge model is developed by coupling the balance between the electron total production and loss rates to the steady-state rate balance equations for the excited states. The predictions of this model, i.e., the maintenance field for the positive column and the populations in the individual excited states as a function of the gas pressure and the discharge current are compared with the experiment (0.05<p<1 Torr; 0.2<I<20 mA; φ=2.6-cm-i.d. discharge tube). It is shown that stepwise ionization becomes predominant for pressures larger than a few tenths of a Torr even for the lowest currents of this experiment. The predicted values of the maintenance field are considerably lower than those that would be required to sustain the discharge by ground-state ionization and compare favorably with the measurements. The calculated populations in the metastable states also compare reasonably well with the experiment. The discrepancies are, however, quite large in the case of the resonance levels owing, presumably, to the lack of accuracy of the theoretical escape factors for the resonance radiation.

Magnetic filter operation in hydrogen plasmas

“…The rate coefficients of the corresponding processes are obtained as in Ref. 47 by using the data for the cross sections and for the diffusion coefficient of the neutrals from [48][49][50][51][52]. The comparison of the results in figures 9(a) and 9(b) with the corresponding results (figures 3(b) and 4(b)) obtained in hydrogen discharges shows that all the effects discussed regarding the magnetic filter operation in hydrogen discharges appear also in argon discharges.…”

Theoretical insight into Ar–O2surface-wave microwave discharges

“…The collisional-radiative module that describes the excitation and de-excitation of the 3p 5 4s levels in a pure Ar discharge has been developed according to the collisional-radiative model of Ferreira et al [29,42]. A list of the Ar collisional and radiative processes taken into account is given in Table 1.…”

“…The binary diffusion coefficients are calculated from Equation (3), but renormalized whenever possible to fit the data reported in [10,42,61,62,63]. The values of the binary diffusion coefficients used are given for p = 1 Torr and T = 273 K in Table 4 Under the formulation presented here, the model provides the species densities, the electric field sustaining the discharge, the Electron Energy Distribution Function and all the information concerning the electron excitation rate coefficients and transport parameters, as well as all the details concerning the chemical kinetics of each species.…”

Emission and afterglow properties of an expanding RF plasma with nonuniform neutral gas density

“…34 The rate coefficient for the metastable-to-resonant transition is K mr % 2 Â 10 À13 m 3 =s, approximately independent of T e above 0.2 eV, 35,36 which implies that when the electron density in the afterglow was n e % 5 Â 10 17 m À3 , the mean lifetime for metastables was À1 mr ¼ ðK mr n e Þ À1 % 10 ls. This observation appears to be problematic, given that the ion density rose over a period of $100 ls in the afterglow (Figs.…”