Measurement and calculation of recoil pressure produced during CO₂ laser interaction with Ice

Evaporation is a classical physics problem, which, because of its significant importance for many engineering applications, has drawn considerable attention by previous researchers. Classical theoretical models [Ya. I. Frenkel, Kinetic Theory of Liquids, Clarendon Press, Oxford, 1946] represent evaporation in a simplistic way as the escape of atoms with highest velocities from a potential well with the depth determined by the atomic binding energy. The processes taking place in the gas phase above the rapidly evaporating surface have also been studied in great detail [So I. Anisimov and V. A. Khokhlov, Instabilities in Laser-Matter Interaction, CRC Press, Boca Raton, 1995]. The description of evaporation utilizing these models is known to adequately characterize drilling with high beam intensity, e.g. >10⁷ W/cm². However, the interaction regimes when beam intensity is relatively low, such as during welding or cutting, lack both theoretical and experimental consideration of the evaporation. It was shown recently that if the evaporation is treated in accordance with Anisimov et.al.’ s approach, then predicted evaporation recoil should be a substantial factor influencing melt flow and related heat transfer during laser beam welding and cutting. To verify the applicability of this model for low beam intensity interaction, we compared the results of measurements and calculations of recoil pressure generated during laser beam irradiation of a target. The target material used was water ice @–10°C. The displacement of a target supported in a nearly frictionless air bearing under irradiation by a defocused laser beam from a 14 kW CO₂ laser was recorded and Newton’s laws of motion used to derive the recoil pressure.