Investigation of formation of an optical breakdown plasma near a solid surface subjected to microsecond 10.6 µ radiation

Theoretical and experimental investigations were made of the dependences of the delay time of plasma formation in a gas near the surface of a solid and of the energy density released at the target up to the moment of breakdown on the intensity of microsecond CO₂ laser radiation, subject to suitable allowance for the pulse profile. The appearance time of the optical breakdown was found to be determined both by the time needed to heat the target surface to the temperature of advanced evaporation and by the time needed to form a plasma in the gas. The plasma formation times were determined experimentally for the optical breakdown near the surfaces of targets made of graphite and tungsten grids. A study was made of attenuation of radiation by a breakdown plasma as a function of the intensity of laser radiation, as well as of the nature and pressure of the gas.