Numerical simulation of the effect of laser wavelength on nanosecond laser ablation and plasma characteristic

Based on the heat conduction equation, hydrodynamics equations, and radiation transport equation, a two-dimensional axisymmetric radiation hydrodynamics model is developed. The charge state distribution and energy level population in the plasma are solved by the collisional-radiative model using screened hydrogenic levels. The model is used to study the effect of excitation laser wavelength at 1064 and 266 nm on aluminum target evolution, plasma generation, laser absorption in the plasma, and the plasma characteristic during laser ablation in the presence of atmospheric pressure. For 1064 nm radiation, the evaporation of the target surface stops earlier and the plasma formation time is later. The plasma has higher temperature as well as density and the hottest region is at the forefront of the plasma. The plasma shielding effect resulted in a sharp decrease in the laser transmissivity of 1064 nm radiation to about 0.1%, while the transmissivity of 266 nm radiation only decreased to about 30%. The inverse bremsstrahlung is the most important laser absorption mechanism for 1064 nm, whereas photoionization dominates the entire absorption process in the case of 266 nm radiation. The effect of the plasma model on optical breakdown has been present. The results show that neither breakdown nor plasma formation is encountered if the local thermodynamic equilibrium model is used in 266 nm radiation.