Breakdown of molecular gases by CO2 laser radiation near a metal surface

An experimental investigation was made of the breakdown of N₂–O₂ mixtures near metal surfaces illuminated with CO₂ laser radiation. The factors which were varied in these experiments included the laser radiation intensity (0.2–10 MW/cm²), gas pressure (0.15–100 kPa), oxygen content (10^–4–0.2), and radius of the focusing spot (0.12–0.45 mm). A relationship between the parameters which govern breakdown was found and dependences controlling the breakdown threshold, irrespective of the method of heating of the metal surface and of the adjoining gas, were obtained. The results confirmed an earlier model of an ionization–thermal explosion of a gas in a low-intensity laser radiation field and they established the dominant role of chemical ionization of nitrogen–oxygen mixtures in such breakdown.