Radiation-Induced Acoustic Waves in Water

Abstract

Introduction T momentum transfer caused by a pulse laser upon a surface has been studied both in vacuum and under atmospheric conditions.' These studies were concerned primarily with surfaces of solids, i.e., aluminum, tungsten, etc. When a water surface is irradiated by a powerful pulse laser, an acoustic wave has been observed to propagate into the water.' These experimental observations were made with CO2 lasers and with small spot size (diameter d<H 1 cm). Here we will consider d~~3Q cm. The phenomenology of laser induced acoustic waves in water is rather complex. For very high intensity laser beams, a plasma will be generated above the surface and will alter the characteristics of the beams. For simplicity, when the water is vaporized, the droplets in the water vapor above the surface are assumed to be of the same size as the average aerosols, i.e., 3-5 #m. From the summary of the observed breakdown data in air, the plasma formation threshold for 3-5 /mi particles is of the order of 10 W/cm. Since the formation of plasma is an inefficient mechanism in the process of radiation induced sound, we consider only laser intensities < 10 W/cm. In order to assess the acoustic signals in water, we must first establish the pressure history on the surface. Here, we will divide the acoustic signal generation into two regimes according to laser intensity. For high intensities, i.e., 10 10 W/cm, the blast wave theory will be used to give the surface pressure history. In applying this theory it is assumed that the laser energy is completely absorbed in the blast wave, and in addition, we can ignore the presence of water vapor which may be significant. For low intensities, i.e., less than 10 W/cm, a previously developed thermal conduction code will be used to predict the pressure history on the surface. Finally, knowing the surface pressure history, we can proceed to estimate the far-field signatures.