Breakdown of atomic gases by CO2 laser radiation near metal surfaces

An investigation was made of the breakdown of the rare gases He, Ar, and Xe at metal surfaces illuminated with quasi-cw CO₂ laser radiation. The laser radiation intensity (1–10 MW/cm²), the gas pressure (0.15–150 kPa), and the radius of the focusing spot (0.12–0.3 mm) were varied in these experiments. It was found that breakdown occurred when the metal target surface reached a threshold temperature T₊ dependent on the laser radiation intensity S₊, pressure and nature of the gas, and target material. The dependences of the threshold quantities were recorded S₊ (T₊). An analysis of the experimental results showed that, in the investigated range of the parameters, the breakdown of gases is associated with the presence near the heated surface of a precursor plasma of the target vapor whose degree of ionization is governed by the thermionic emission of ions from the target material. A mechanism of the influence of this precursor plamsa on the gas breakdown process is proposed.