Modeling of Plasma‐Chemical Reactions in Gas Mixture of CO₂ lasers. II. Theoretical Model and its Verification

Abstract

In the paper we have modelled plasma-chemical reactions in the C 0 2 low pressure, DC excited lasers. A good agreement of theoretical and experimental results has been achieved. It has been proved that neglect of reactions with electronic excited species or heterogeneous recombination leads to almost 50% overestimation of GO2 equilibrium conversion. The relation of C 0 2 equilibrium conversion to the reduced field E / N , pressure and current density depends on discharge conditions and mainly on the role played in discharge by ambipolar diffusion. This role decreases with an increase of the discharge diameter and of the mixture convection velocity. The CO, equilibrium conversion increases with growth of E / N and j and with decrease of pressure for discharges in small, sealed-off laser systems. The CO, equilibrium conversion is not always a monotone function of p in large, convection cooled lasers. It does not depend so much on E / N as the electron temperature alone if conversions in different mixtures are compared.

Influence of nitrogen on thermodynamic properties and plasma composition in discharge tube of CO-laser

“…The rates of dissociation in processes (2) and (3) do not change when nitrogen is added to the mixture, i.e., decrease in the CO dissociation level can be explained by rates for heterogeneous recombination (4). Such change of rates for process (4) could result from change of conditions on discharge-tube-wall surface under nitrogen addition.…”

“…There is a lack of comprehensive models for plasma-chemistry of He-CO-N 2 mixtures (except some common feature with plasma of CO 2 laser [3][4][5]), which would help to estimate the level of degradation of the initial laser mixture. However, it has been shown in [5] that a CO 2 laser plasma with a high nitrogen content, 30%<[N 2 ]< 50%, ensuring proper storage of the vibrational energy in CO 2 lasers, is characterized by a low electron temperature and thus, small chemical reactivity, i.e., a small level of CO 2 dissociation.…”

Spectroscopic Study of CO2 Decomposition in Sealed-off CO2 Lasers

“…Consequently, the phenomenon of CO 2 laser mixture decomposition was extensively studied both theoretically and experimentally in different discharge configurations (e.g. TEA lasers [3,6,10,11], RF-excited lasers [12][13][14], flowing systems [15,16] and sealedoff lasers [17][18][19][20]). This work focuses on sealed-off DC-excited CO 2 lasers, a category where experimental data are relatively scarce.…”

“…and at lower pressures also the heterogeneous recombination on the discharge tube walls will play a significant role [17,22]:…”

“…15,16,21,23] have found that the CO 2 equilibrium conversion into CO increases with decreasing pressure and increasing discharge current. A detailed theoretical study of different CO 2 laser mixtures by Cenian et al [17,22] made clear that the pressure dependence of the CO 2 conversion is linked to the pressure dependence of the reduced electrical field strength. In small diameter discharge tubes, where ambipolar diffusion plays a key role, the CO 2 conversion rapidly falls with pressure, along with the E/N .…”