<title>High Flux Propagation Through The Atmosphere</title>

Abstract

Discussions are presented on laser-induced plasma generation in free air propagation (no target surface present). Plasmas can be generated with very high flux levels (for clean air) or with much lower flux levels (for dust-laden air). Pure water aerosols have not proven to substantially reduce air breakdown thresholds below those for clean air. Clean air breakdown at laser wavelengths has been predicted successfully for many years using theories developed for microwave wavelengths. By directly extending those theories to laser wavelengths, the dependencies on ambient pressure, spot size, wavelength and intensities have been predicted with favorable comparison to experimental data. Pure water aerosols (fogs, rain, clouds) have been investigated as to the phenomenology to be expected when irradiated by pulsed and CW laser devices at CO2 and DF laser wave-lengths. These aerosols are shown to heat, vaporize and/or shatter at various incident flux and fluence levels. The data base appears to substantiate this phenomology, but no substantive reduction of clean air breakdown thresholds has been observed when water aerosols are present. Dry solid single-material aerosols (e.g., Si0₂ dust) have been examined, both analytically and experimentally, for verification of phenomenology at various flux and fluence levels. Air breakdown induced by the onset of substantial vaporization rates of the irradiated particulate has been shown to be coincident with the onset of dirty air break-down. Real-world and man-made aerosol clouds have not been studied extensively, if at all, to the author's knowledge. Of particular importance are the effects on high energy laser beams of dust, vehicle exhaust, smoke, road dust and maritime aerosols (e.g., sea spray, sea fogs, etc.). For these important components of the HEL propagation story, no substantial HEL propagation data base exists.