Dynamics of planar gas expansion during nanosecond laser evaporation into a low-pressure background gas

Abstract

A numerical study in a one-dimensional planar formulation of the dynamics of the neutral gas expansion during nanosecond laser evaporation into a low-pressure background gas is carried out using two different kinetic approaches: the direct simulation Monte Carlo method and direct numerical solution of the Bhatnagar–Gross–Krook equation. Results were obtained for a wide range of parameters: the background gas pressure, masses of evaporated and background particles, temperature and pressure of saturated vapor on the evaporation surface, and evaporation duration. They are in good agreement with the analytical continuum solution for unsteady evaporation into the background gas. The dynamics of the expansion is analyzed, and the characteristic times and distances that determine the main stages of the expansion process are established. General regularities are obtained that describe the dynamics of the motion of external and internal shock waves and the contact surface as well as the maximum density of evaporated particles and the characteristic temperatures of evaporated and background particles in the compressed layer. The obtained results are important for understanding and describing the change in the mixing layer during nanosecond laser deposition in a low-pressure background gas.

Planar Gas Expansion under Intensive Nanosecond Laser Evaporation into Vacuum as Applied to Time-of-Flight Analysis

“…The outlined computational DSMC algorithm was implemented by the first author in the parallel FORTRAN code LasInEx (Laser-Induced-Expansion) and has been successfully used in various studies, e.g., [ 15 , 20 , 26 , 34 ]. In the current work the computational domain in physical space is initially divided into cells of equal size.…”

“…The test calculations, using the code Nesvetay developed by the second author [ 46 , 47 , 48 ], showed the ALE-DVM scheme to be up to 100 times more efficient as compared to the conventional DVM methods. It has been recently successfully used for studying gas expansion into background gas [ 34 ]. For flow into a vacuum, some modifications of the baseline scheme were made in order to improve its robustness.…”

“…Numerically, the formulated problem is quite difficult as the considered flow is time-dependent, contains discontinuities in boundary conditions and involves large variations of local Knudsen numbers as well as steep gradients of the VDF. To make our results more reliable and credible, in addition to the DSMC method, it is worth adding another modeling approach—the direct numerical solution of the model Bhatnagar–Gross–Krook (BGK) kinetic equation computed by the Nesvetay code [ 33 , 34 ]. Previously, good agreement has been obtained using this code both with the DSMC method and with the numerical solution of the exact Boltzmann equation for moderate evaporation (for the number of monolayers Θ < 100) [ 33 ].…”