Pressure dependence of the laser-induced breakdown thresholds of gases and droplets.

Laser-induced breakdown threshold intensities for helium, argon, xenon and clean air were measured as a function of pressure (p < 900 Torr) at wavelength lambda = 0.532 microm using the Nd:YAG laser with 6.5-ns pulse duration. Pressure dependence of the breakdown of a 50-microm diam water droplet in these gases was also investigated. For pure gases, different free electron generation processes and electron loss processes dominate in different pressure regions. The water droplets decrease the breakdown thresholds up to 3 orders of magnitude depending on the pressure of the particular gas surrounding the droplet. For the droplet in He, Ar, and clean air for p < 800 Torr, the breakdown at the threshold intensity occurs inside the droplet and is independent of pressure. For the droplet in Xe, the breakdown occurs inside the droplet for p < 140 Torr; however, for p < 140 Torr, the breakdown occurs outside the droplet and is dependent on pressure. Transition from the breakdown inside to outside the droplet takes place in the pressure region where the breakdown thresholds of the bulk liquid and the pure gas are approximately equal.