Speaker
Description
The first observations of Laser Induced Breakdown (LIB) in gases were made upon the days of the invention of lasers and were initially reported by Maker. In those experiments it was observed that, gases which are normally transparent to optical radiation (e.g., Air), could be transformed in high-temperature plasmas by focusing a laser beam onto a small volume. When the operating conditions (e.g., ambient pressure) are such that the process is collision-dominated, the plasma formation occurs in two-steps: (i) creation of priming electrons via Multi-Photon Ionization (MPI) and (ii) cascade ionization initiated and sustained by energy absorption in free-free electron-heavy interactions (i.e., inverse Bremsstrahlung).
The purpose of this work is to develop a self-consistent physico-chemical description of LIB in gases. The interaction between the laser and the plasma is described via a fluid model based on the Navier-Stokes equations. Non-Local Thermodynamic Equilibrium (NLTE) effects are taken into account using a two-temperature model. The radiation field is modeled based on the Radiative Transfer Equation (i.e., Kinetic Theory of Photons). Extending beyond many works 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 cold plasma. The inclusion of MPI, which is often thought to be important only in initiating the plasma, affects the shape and evolution of the plasma kernel as discussed in detail in a companion work. To obtain numerical solutions, the governing equations are discretized in space using a second-order finite-volume method. The system of equations is time-integrated by an implicit dual-time-stepping method. Applications consider the breakdown stage and the early post-breakdown evolution in air and oxygen plasmas.
Summary
This work proposes a computational model to describe laser-plasma interactions under non-equilibrium conditions. Its innovative aspect consists in the self-consistent kinetic model for plasma formation which accounts for production of priming electrons via multi-photon ionization, energy absorption in free-free transitions and cascade ionization.
