Dynamics simulation of evaporation process of substance ablated by non-normal incidence laser

Abstract A two-dimensional model was developed to investigate the interaction between the nanosecond pulse and aluminum at non-normal incidence, focusing on the dynamic evolution of ablation characteristics influenced by varying incident angles, i.e., the angle between the laser beam and the normal of the target surface. As the angle increases, the shielding effect becomes weakened and its onset is delayed, due to decreased plasma optical thickness from the reduction in the plasma density and plasma length. Furthermore, the decrease of laser irradiance with the incident angle allows for the gradual reduction of temperature, evaporation velocity and depth at the target center, because of the reduced coupling efficiency between the laser and the target. Notably, a larger area with relatively high temperatures is observed along the major axis (the elongation direction of elliptical spot), owing to the expanded heat conduction area resulting from the elongated laser spot. Moreover, at smaller incident angles, the crater typically exhibits a conical shape with a deep, narrow center, whereas at larger angles, it resembles the energy distribution profile of a Gaussian laser, characterized by a deeper center and shallower sides. These variations arise from the enhanced energy absorption at small angles and high laser fluence, facilitating thermal diffusion into deeper regions, with only the center region retaining sufficient energy for deep ablation. A comparative analysis of the normal and non-normal incidence modes indicates that when the incident angle exceeds 30°, it is preferable to employ the non-normal incidence mode to characterize the laser ablation process because of the nonnegligible impact of the spatial distribution of laser intensity, laser absorptivity, and plasma optical thickness on the ablation characteristics. These theoretical findings will gain a deep insight into ablation characteristics under non-normal incidence, and provide an optimization strategy for adjusting laser parameters to improve the performance of LIBS