Modeling of laser discharges in noble gases by means of collisional-radiative models*

The interaction of an intense laser beam with a medium which is normally transparent to optical radiation (e.g., air) may result in the formation of a highly conductive and temperature plasma. This phenomenon, often referred to as laser induced breakdown (LIB), was observed for the first time in the 1960s, 

and has since then been investigated from the computational, theoretical and experimental point of views. 

Despite the progresses made in understanding the basic physics of the problem, there still remain some open questions regarding the plasma formation mechanism. One of these is the interplay between multi-photon ionization, plasma expansion and beam refraction in the development of the two-lobed plasma kernel often observed in experiments.

    

Motivated by the above scenario, a computational LIB model accounting for beam refraction and attenuation, cascade and multi-photon ionization as well as post-discharge hydrodynamics is developed here. The plasma is treated as a fluid by assigning distinct temperatures to heavy particles (e.g., atoms) and free electrons. Rate processes such as excitation and line emission are taken into account based on collisional-radiative models constructed using the most accurate cross-section data from the literature. Radiation losses from the plasma are incorporated via escape factors. The propagation of the laser beam is modeled via an envelope equation which is coupled to the Navier-Stokes equations governing the plasma hydrodynamics. Numerical solutions are obtained based on an implicit finite volume method. Applications consider laser discharges in noble gases such as Argon.