Study of physical processes involved in laser shock processing of materials

Abstract

The generation of shock waves by laser-plasma in Water Confinement Regime (WCR) has been investigated for the first, second and third harmonics of Nd:Glass laser (1,064, 0,532, 0,355 μn) with 0.6, 10 and 25 ns laser pulse durations. Pressure measurements have been mainly performed using a Velocimetry Interferometer System for Any Reflector (VTSAR). It appears that, depending on laser parameters, above a given laser intensity threshold, the peak pressure is saturated and the pressure duration is reduced due to laser-induced breakdown plasma in the confining water. The observation of the interaction zone (confined plasma and confining water) with a fast intensified camera shows that this parasitic breakdown occurs exclusively at the surface of water and limits the efficiency of the process. The time-resolved transitivity of this plasma has been measured with a continuous Argon laser beam for a 1.064 μm/25 ns incident laser irradiation. Above 10 GW/cm2, the peak power density transmitted through the breakdown plasma saturates and the laser pulse transmitted is reduced in agreement with pressure measurements. The relative influence of main physical mechanisms occurring during the generation of the laser breakdown at the surface of water have been discussed. According to the wavelength effect which tends to authorize higher pressure with longer wavelengths, the influence of multiphotoionic processes seem to dominate the effect of avalanche ionization. The protective coatings and laser spot size influences have been also investigated regarding the stress levels induced in the targets. About 30 % stresses increase in the material is shown to occur with adapted coating. On the contrary, the laser spot size has no specific effects on the pressure induced in WCR. Residual Stresses (RS) field have been determined for different laser conditions.The generation of shock waves by laser-plasma in Water Confinement Regime (WCR) has been investigated for the first, second and third harmonics of Nd:Glass laser (1,064, 0,532, 0,355 μn) with 0.6, 10 and 25 ns laser pulse durations. Pressure measurements have been mainly performed using a Velocimetry Interferometer System for Any Reflector (VTSAR). It appears that, depending on laser parameters, above a given laser intensity threshold, the peak pressure is saturated and the pressure duration is reduced due to laser-induced breakdown plasma in the confining water. The observation of the interaction zone (confined plasma and confining water) with a fast intensified camera shows that this parasitic breakdown occurs exclusively at the surface of water and limits the efficiency of the process. The time-resolved transitivity of this plasma has been measured with a continuous Argon laser beam for a 1.064 μm/25 ns incident laser irradiation. Above 10 GW/cm2, the peak power density transmitted through the breakdow...