Conditions for laser spark formation

In order to clarify the physical nature of long laser sparks (LLS), experiments were carried out to study LLS initiation and the effects of solid aerosol particles present in air. A Q-switched neodymium glass laser system with a wavelength of 1.06 microns was employed in the experiments, and the laser pulse energy was varied from 30 to 140 J. The experiments revealed that the spark density in the LLS increases with the concentration of the aerosol particles if the laser light energy initiating the LLS is held constant. Analysis of numerous sparks produced under various conditions revealed that particles with diameters greater than about 1-2 microns play a dominant role in LLS formation. The laser pulse energy expended in exciting LLS plasma foci was determined and additional information regarding the processes in LLS formation was obtained by analyzing the plasma emission spectrum for individual sparks in the LLS.