Kinetic analysis of an optically pumped rare gas lasing medium with a nanosecond repetitively pulsed discharge in Ar-He mixture

A kinetic model of a lasing medium for an optically pumped rare gas laser (OPRGL) was developed that accounts for the production of excited argon atoms Ar* in a nanosecond repetitively pulsed discharge (NRPD). Analytical formulas were derived for the frequency of Ar* radiation losses, population of the energy levels involved in the laser cycle and specific powers for pump absorption and lasing in the limit of bleaching of pump and lasing transitions. The formulas use the kinetic constants of the model as parameters and Ar* number density as the independent variable. Periodic solutions for number density of plasma components, pump absorption and small signal gain, specific pump absorption and lasing were obtained numerically. Mean over the NRPD period values of Ar* number density, specific pump absorption and lasing were calculated as the functions of the preset peak values of the reduced electric field E/N. Triangular electric field pulses of 80 ns FWHM duration and 200 kHz frequency were considered. Optical pumping increases Ar* loss by more than a factor of 10, due to excess spontaneous emission from Ar* levels that populates the 1s4 state of argon, resulting in a reduced NRPD plasma ionization and Ar* production. As a result, without auxiliary optical pumping from the 1s4 state that compensates this loss, the averaged across the discharge period Ar* number density decreases by a factor of 30. Moreover, the average specific heat release in plasma becomes almost equal to the specific lasing power in the acceptable operating modes, making auxiliary optical pumping mandatory for scaling of an OPRGL. The sensitivity of the results to the values of kinetic constants in the model is analyzed.