DF-laser pulse breakdown induced by maritime aerosols

Thresholds for breakdown induced by liquid and solid aerosols in room air have been measured for a 1-jjisduration pulsed D2-F2 laser of 3.58-4.78 p,m bandwidth. The DF laser beam was directed into an aerosol chamber that simulated maritime atmospheres on the open sea. For a focussed beam in which the largest encountered aerosol particles were of 1-4 jim diameter, pulsed DF breakdown thresholds were measured to lie in the range of 0.6-1.8 GW/cm. Salt-water aerosol breakdown thresholds for micron-size particles were found to be 15-30% higher than the corresponding thresholds for fresh-water particles. For a collimated beam that encountered particle diameters as large as 100 p,m, breakdown could not be induced using 0.5 jis (FWHM) pulses at peak intensities of 59 MW/cm. Image converter camera measurements of plasma growth rates were obtained that compare favorably with earlier CO2 measurements of Reilly et al. Pulsed DF breakdown thresholds of 32 MW/cm for 30-|im-diam A12O3 particles were also measured; these data agree with recent Lencioni high-frequency measurements if one assumes that the threshold for microsecond-duration pulses scales as X~ *. An approximate theoretical model of the water particle breakdown process is presented that is found to be in satisfactory agreement with the experimental data and that allows scaling of the present results to other laser pulse durations, aerosol distributions, and transmission path lengths.