Development of a numerical model to characterize laser-induced plasmas in aqueous media

Abstract

A spatio-temporal model for investigating the characteristics of laser-induced plasmas in aqueous media is developed by modifying the general form of the well-known rate equation and simultaneously accounting for the influences of multiphoton and cascade ionization on the propagation of short laser pulses. In this model, the nonlinear absorption of laser pulse energy is considered to be time and space dependent inside the computational volume. The model is verified by comparing the results of three-dimensional axisymmetric numerical simulations with existing experimental data for laser pulses of 30 ps, 1064 nm at focusing angles between 4° and 28° with energies in the wide range between 0.1 to 6000 μJ. This model could reasonably predict the various characteristics of a laser-induced plasma, such as breakdown threshold, size, shape and energy transmitted through the plasma. Also the transmitted energy data obtained from the model is within 10% of the experimental data at the largest focusing angle and 20% at the smallest angle. To compare the calculations with plasma photographs, electron density values are transformed into a gray scale. The simulated plasma shapes correlate well with the existing experimental observations. The outcomes of the model, such as spatial distribution of plasma energy density, could be used as input for a hydrodynamic model to estimate the strength of the mechanical effects associated with plasma formation.