Gas-dynamic perturbations in an electric-discharge repetitively pulsed DF laser and the role of He in their suppression

The gas-dynamic perturbations in a repetitively pulsed DF laser are studied using a Michelson interferometer. Based on the analysis of experimental data obtained in two experimental sets (working medium without buffer gas and with up to 90% of He), it is concluded that such phenomena as isentropic expansion of a thermal plug, gas heating by shock waves and resonance acoustic waves do not considerably decrease the upper limit of the pulse repetition rate below a value determined by the time of the thermal plug flush out of the discharge gap. It is suggested that this decrease for a DF laser with the SF₆–D₂ working mixture is caused by the development of overheat instability due to an increased energy deposition into the near-electrode regions and to the formation of electrode shock waves. Addition of He to the active media of the DF laser changes the discharge structure and improves its homogeneity over the discharge gape cross section, thus eliminating the reason for the development of this instability. A signification dilution of the active medium of a DF laser with helium up to the atmospheric pressure allowed us to achieve the limiting discharge initiation frequencies with the active medium replacement ratio K∼1.