Improvement of self-regeneration of gas mixtures in a convection-cooled 1.2 kW CO2 laser

Degradation of gas mixtures and methods of its reduction in CO2 lasers were studied with reference to the DC-excited, transverse-flow CW CO2 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 CO2:N-2:He laser mixtures. The observed optimal concentration of N-2 lies within the range 30% less than or equal to [N-2] less than or equal to 50% and that of CO2 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 CO2 equilibrium conversion X(theta) decreases with increasing gas pressure. A 50% reduction in X(theta) 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 CO2 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 N-2(A(3) Sigma). Only very small concentrations of NxOy and O-3, which are the most harmful reaction products, result according to our calculations for the MLT 1200 system.