Laser-Matter Interaction Above the Plasma Ignition Threshold Intensity

Abstract

In this chapter we present the process of laser-matter interaction above the plasma ignition threshold intensity. The physics of the pulsed laser ablation process at high intensities is very complex since it involves, besides direct laser-solid interactions, the process of plasma formation and expansion, and the laser-plasma interaction. Inverse Bremsstrahlung and photoionization processes is considered to be the main absorption mechanisms of the laser light within the ablation plumes produced on metallic targets. Plasma kinetics including electron impact excitation/ionization and recombination processes, as well as the energy transfer from electrons to ions and neutral species are considered. Section 4.1 presents the main phenomena involved in production of the ablation plasma and in laser-plasma interaction during PLA: plasma formation and evolution. In this section, plasma heating, self focusing, critical density, shielding, and plume expansion is discussed. Interaction of plasma plume with obstacles is also treated in Sect. 4.1.3. Experimental methods for analyzing the main phenomena involved in laser-plasma interaction (i.e. optical and mass spectroscopy, high speed imaging) are presented in Sect. 4.2. The most important parameters which characterize the laser-ablated plumes (density and the temperature) are usually determined by optical techniques (i.e. interferometry, Thomson-scattering and plasma spectroscopy) which can be used to reveal the characteristic features of plasma, as well as to estimate and describe qualitatively and quantitatively its properties. The theoretical models for describing the laser-plasma interaction allow one to estimate the spatial–temporal distribution of the plasma parameters such as temperature, density and pressure. Among the models describing the dynamics of the expanding ablation vapour/plasma plume, Monte Carlo simulations and hydrodynamic equations approaches have been widely used. The numerical results on the ablation plasma were validated by comparison to the experimental data obtained by using optical emission and absorption spectroscopy, mass spectrometry, time-of-flight and charge collection measurements. Section 4.3 presents in more detail theoretical results obtained within the photo-thermal model on the characteristics of the ablation plasma in relation to the ablation rate in nanosecond irradiation regime.