Computational Modeling of Laser Induced Breakdown in Gases

The first observations of Laser Induced Breakdown (LIB) in gases go back were reported by Maker in the 1960s. In those experiments it was observed that, gases which are normally transparent to optical radiation, could be transformed in plasmas by focusing a laser beam onto a small volume. When the operating conditions (e.g. pressure) ensure that the process is collision-dominated, plasma formation occurs in two-steps: creation of priming electrons via Multi Photon Ionization (MPI) and cascade ionization initiated and sustained by energy absorption in inverse Bremsstrahlung interactions.

The purpose of this work is to develop a self-consistent model for LIB in gases. The interaction between the laser beam and the plasma is described via a fluid approach based on the Navier-Stokes equations. Non-Local Thermodynamic Equilibrium effects are taken into account using either multi-temperature or State-to-State models. The propagation and attenuation of the beam are modeled based on the Radiative Transfer Equation. Extending beyond conventional approaches in the literature, the proposed model accounts for the creation of priming electrons via MPI. This avoids to start the simulation with an initial artificial plasma. To obtain numerical solutions, the governing equations are discretized in space using the Finite Volume method. Time integration is carried out by an implicit dual-time-stepping method to handle the stiffness resulting from kinetics and diffusion [1]. Applications consider the breakdown and early post-breakdown evolution in various gases (e.g. Air). Results show that MPI, often considered important only creating priming electrons, affects the shape and evolution of the plasma kernel [2]. The proposed model is also able to reproduce key features observed in experiments.