Experimental and theoretical study of the expansion of a metallic vapour plasma produced by laser

Abstract

The interaction between a metallic plasma produced by laser ablation and an ambient gas (argon, air and nitrogen) at atmospheric pressure is studied. The experimental results are compared with theoretical ones given by numerical simulation. Aluminium and copper targets are used. The uniform repartition of iron impurities included in the target (1.6% for Al and 2% for Cu) is warranted by the manufacturer. The Nd : YAG laser delivers pulses of 8 ns FWHM duration with an energy ranging from 70 to 100 mJ at a rate of 10 Hz. The temperature measurements have been performed by the Boltzmann diagram method using iron lines. The influence of ambient gas and target material is studied. After an Abel transform, we observe a maximum of the line intensities off the plasma axis. This shows the formation of gas plasma around the metallic one before complete diffusion of the ambient gas in the metallic plasma. The hydrodynamic model built in one dimension uses the continuity equations. This code is based on the control volume method of Patankar. A temperature and mass fraction profiles are needed as initial conditions. This model is able to describe the temporal evolution of the temperature and of the diffusion in the plasma. It allows us to study the mixture of the metallic plasma and the ambient gas as soon as the plasma is thermalized and local thermodynamic equilibrium is established. There is good agreement between the results of the simulation and the experimental results.