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 themodels 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 plasmawere

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 inmore 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.