A. CENIAN (a), A. CHERNUKHO (b), V. BORODIN (b) and G. SLIWINSKI (a) (a) Instiiute of Fluid-Flow Machines, Polish Academy of Sci., 80-952 Gdansk, ul. Fiszera 14. Poland. (b) A. V. Luikov Heat and MassTransfer Institute, Belarus AcademyofSci.. 220072 Minsk, 15 P. Brovki str.. Belarus. AbstractThe chemistry of a pure CO, glow discharge is modelled as a first step in description of C 0 2 lasers plasma-chemical reactions. The equilibrium conversion of CO, into CO strongly depends on E/N ratio. It grows from few percent for E/N = 2 x V cmz to more then 70% for E/N -9x V cm2. The X, values are only slightly influenced by the change of electron density and temperature.,The role of ions and of the CO(a3n) excited electronic state in recombination of CO as well as of heterogeneous recornbination is studied. The time required to reach the steady-state concentrations of chemical species in the system varies from few seconds for high E / N values up to hours in the other end of investigated region; it increases with the fall of discharge current and with the fall of E/N. Satisfactory agreement with experimental data is achieved.

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.

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Degradation of gas mixtures and methods of its reduction in lasers were studied with reference to the DC-excited, transverse-flow CW laser of MLT 1200 type. The long-term output characteristics and their dependence on the mixture's composition determined in the experiment gave a hint concerning the thorough theoretical analysis of decomposition in laser mixtures. The observed optimal concentration of lies within the range and that of is about 2 - 3% for the laser under consideration. In the theoretical analyses of available data we focus our attention on the roles played by the gas pressure, the mixture composition and the growth of the afterglow region in the reduction of working gas degradation. The equilibrium conversion decreases with increasing gas pressure. A 50% reduction in is obtained when the pressure increases from 13.33 to 46.66 hPa. The observed optimal composition corresponds to low reactivity of the laser plasma with a small oxygen content. The optimal period for the laser gas mixture to reside in the afterglow region is larger by a factor of 1000 than the discharge residence time. The relatively small value of the equilibrium conversion in the MLT 1200 laser results from relatively high concentrations of atomic and electronically excited species. The formation of atomic oxygen is inhibited by reactions with electronically excited molecules, especially . Only very small concentrations of and , which are the most harmful reaction products, result according to our calculations for the MLT 1200 system.

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