An experimental investigation was conducted to determine the auenuation by aerosols of near-ultraviolet laser radiation propagating through a simulated and controlled medium. The propagation of laser beams in the atmosphere is affected by linear and non linear phenomena such as Rayleigh and aerosol scattering, Raman scattering, molecule and particle absorption, atmospheric wrbulence, thermal gradients or optical breakdown depending on wavelength and the intensity of the laser beamLk Experiments were performed using radiation from an excimer laser -KrF A = 0.25 jim; XeF X = 0.35 pm -in an air-sea interaction simulation tunnel. The tunnel water tank was 40 m-long and 1 m-deep. The air test section was 1.5 ptm-high and 3 m-wide. Air and water were recirculated and large heat exchangers allowed regulation of water, air and dew point temperature. Aerosol concentration and size spectra were obtained by means of a PMS 100-HV Classical Scattering Aerosol Spectrometer measuring a diameter of 0.5 to 47 jim in four overlapping ranges. Mean air velocity (O-lOm/s), temperature (10-35°C) and relative humidity (40-100%) were obtained from a Pitot tube, thermocouples and dew point sensors.The current results concentrate on the measurement of aerosol profiles as a function of the z-axis above the water surface, water density in air, mean wind velocity in the tunnel, the difference between air and water temperature corresponding to various fog densities as well as the preliminary differential measurements of the total attenuation of the U.V. radiation passing through this medium. LJNTRODUCTIONIn order to study the propagation of short wavelength laser beams in a uniform field of water droplets, experiments were conducted in association with the Institut Mecanique Statistique de la Turbulence in Marseilles3 in a large air-sea interaction simulation tunnel (Fig.!). Radiation was obtained from a SOPRA 1250 E 20 exciplex laser operating in pulse mode with an active medium of rare gas halides with pumping done by transverse electric discharge. This makes it possible to transmit short wavelength radiation (157 nm F2; 193 nm ArF ; 249 nm KrF ; 308 nm XeC1 ; 351 nm XeF). The repeat rate is adjustable to 20 Hz. Two wavelengths were selected for transmission studies.: A. =249 nm and = 351 nm. The shots were implemented by multiple passes in the same horizontal plane according to the transverse direction of the tunnel at a fetch of x @30 m from the attack edge. The experimentation section was 40 m-long, 3.5 rn-wide and 1.5 rn-high. Below there was an identical length water tank 2.6 m wide and 0.7-im deep depending on the fetch. Air that is circulated in closed circuit can reach a velocity of 10 m/s. The water and air ternperawres in the canal can be modulatedbyaheatexchanger. They can varybetween 5 and 35° C. Thedewpointtemperatureoftheairatthe entrance to the test section can be controlled by condensation drying on cold batteries.
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