Effect of spherical particles on laser-induced breakdown of gases

The laser-induced breakdown of a gas is studied. It is observed that small liquid or solid particles decrease the breakdown threshold intensity by factors of up to the order of 100; reasons for this decrease are discussed. The location of the plasma, either inside or outside the droplet, is investigated using the radiation of a Nd:YAG Q-switched laser at wavelength 0.532 micron focused using a 30 cm focal length plano-convex inside a gas chamber containing He, Ar, and Xe gases at atmospheric pressure. The laser output and breakdown intensities of spherical droplets in the gases are measured. The data reveal that the breakdown threshold depends on the nature of the surrounding gas; the plasma is first created in the gas outside that particle and then propagates back into the droplet prior to the ejection of the plume. (I.F.)