Initiation of directional electric breakdown in a gas by the third harmonic of neodymium laser radiation

The optical initiation of directional electric breakdown in air at atmospheric pressure was achieved for the first time. The breakdown was initiated by a collimated ultraviolet beam of 3.5 eV photons, representing the third harmonic of the radiation emitted by a neodymium laser. The initial plasma concentration was estimated and the thirdharmonic power necessary for such initiation was determined. The power density needed to observe the same effect at wavelengths of 1.06 and 0.53 μ was calculated. At the former wavelength, it was considerably higher than the threshold power density in optical breakdown, whereas at the latter wavelength it was comparable to that threshold density. These results were in agreement with the experimental data obtained using a stabilized high-power high-efficiency source of coherent ultraviolet radiation.