Theoretical operational life study of the closed-cycle transversely excited atmospheric CO2 laser

Abstract

By using a comprehensive theoretical model that assumes a stable excitation discharge and homogeneous plasma chemical reactions in the discharge plasma, the laser output performance and the variations of the laser gas components during the sealed-off operation of the high-power, closed-cycle transversely excited atmospheric CO2 laser have been investigated. The fractional CO2/N2, molecules decomposition, and the concentration of the various minor impurities accumulated in the laser gas mixture have been theoretically calculated as a function of shots and number of repetitive discharge pulses. According to the results, the gradual reduction of the laser output energy with the successive excitation pulses was mainly due to the depletion of the CO2 molecules and the reduction of the excitation efficiency; the excitation efficiency was decreased in consequence of the increased operational E/N (E is the discharge field strength, N is the total laser gas number density) caused by the accumulation of highly electronegative impurities such as O2 and O3. The nitrogen oxides were found to show little effect on the operational E/N in spite of their large electron attachment cross sections, because these molecules were much less accumulated in the laser gas mixture than O2 or O3. The theoretical model has clarified for the first time that a trace of water (H2O) vapor in the laser chamber effectively acts as a gaseous catalyst to enhance the CO2 reforming reaction in the discharge plasma. Furthermore, this CO2 reforming reaction by H2O, rather than the other backward reactions, predominantly determines the equilibrium CO2 decomposition level in the actual laser chamber. Finally, with regard to the ultraviolet (UV) preionization, it was theoretically shown that the UV absorption depth of the laser gas mixture steeply decreased as the CO2 decomposition increased owing to the contamination of strong UV absorbing species such as O2 and O3.