Modelling of laser-plasma in laser microspectral analysis
The behaviour of the laser-produced plasma plume plays an important part in understanding and controlling the laser material processing involved in laser microspectral analysis. A multi-dimensional time-dependent model has been developed to simulate the transient material behaviour after an intense laser pulse irradiating a target surface. In this model, the heating and phase transition of the target material have been studied using the heat transfer theory, and the formation and evolution of the plasma plume were modelled by the Euler equations of fluid dynamics, which include the conservation of mass, momentum and energy. The energy absorption in the plasma by inverse Bremsstrahlung was considered, and the interface between the plasma plume and surrounding air was tracked by a fractional volume method. Moreover, the temperature-dependent optical and thermal properties of the target material and the spatial and temporal characteristics of the laser irradiance have been taken into account. The equations in the model were solved numerically by means of a finite-volume based time marching algorithm. Finally, experiments were carried out to support this model. This model is capable of predicting the onset of melting and evaporation, the crater development, the transient temperature distribution in the target, spatial and temporal behaviour of the plasma flow and, finally, the formation and propagation of a shock wave in the surrounding air. The transient spatial distributions of the plasma parameters, i.e., the temperature, density, pressure and velocity are presented in two-dimensional colour contour diagrams as well as the transient temperature profiles in the target. All these outcomes enable the optimisation of the process of laser-material interaction and provide valuable information in interpretation of the recorded spectrum and the consequent quantitative analysis of target composition in laser microspectral analysis.