Speaker
Description
Laser breakdown is observed when a high-intensity laser beam is focused into a small region of gas. The discharge is composed of two stages: (a) creation of the priming electrons, and (b) formation of a highly conductive and absorbing plasma. This initial phase is then followed by a post-discharge, characterized by the formation and propagation of a shock wave from the focal region. The breakdown threshold intensity for laser-generated plasmas depends on specific gas, the background pressure, the laser wavelength, and any presence of impurities. In this work, we present validation against experiments of a non-equilibrium model for laser induced-breakdown and the investigation of plasma kernel dynamics. The application considers laser generated air plasma at atmospheric background conditions. The hydrodynamics are described with the chemically reactive Navier-Stokes equations and the non-equilibrium effects are accounted for with a three temperatures model. Laser-plasma interaction over nanosecond time scales has been modeled with the Radiative Transfer Equation (RTE), including both multiphoton ionization (MPI) and inverse Bremsstrahlung (IB). For model validation, the absorbed energy has been compared against experiments. Preliminary results show that the laser generated plasma exhibits a two-lobe structure, developing both in the forward and backward directions. The extent of the rear lobe is both axially and radially larger than the forward lobe. As the laser is turned on, the first electrons are created via MPI. After those electrons are formed, the incident laser radiation is absorbed through IB. The electrons become in turn very energetic and an avalanche process dominated by electron impact ionization (IE) is initiated. While the reactants for MPI are depleted in the plasma core, MPI is still responsible of the formation of the new electrons at the kernel's boundary. Because of these newly generated electrons the mixture becomes opaque at the plasma boundary and the location of laser energy deposition moves from the inner to the outer plasma layer. This, in turn, guides the dynamics of the radiation sustained plasma waves.
Summary
Laser breakdown is observed when a high-intensity laser beam is focused into a small region of gas. The discharge is composed of two stages: (a) creation of the priming electrons, and (b) formation of a highly conductive and absorbing plasma. The application considers laser generated air plasma at atmospheric background conditions. The hydrodynamics are described with the chemically reactive Navier-Stokes equations and the non-equilibrium effects are accounted for with a three temperatures model. For model validation, the absorbed energy has been compared against experiments. Results show that Multiphoton ionization and inverse Bremsstrahlung guide the dynamics of the radiation sustained plasma waves.
