Thermal blooming with laser-induced convection: radial basis function simulation.

The propagation of a high energy laser through a nearly stagnant absorbing medium is studied. The absorption values and time scale of the problem are such that the laser induces convective heat currents transverse to the beam. These currents couple to the laser via the refractive index, causing time dependent thermal blooming. A numerical method is developed and applied to the model in [J. Electromagn. Waves Appl.33, 96 (2019)JEWAE50920-507110.1080/09205071.2018.1528183], using radial basis functions for spatial differencing, which allows for irregular point spacings and a wide class of geometries. Both the beam and laser-induced fluid dynamics are numerically simulated. These simulations are compared to a historical experiment of a 300 W laser in a smoke-filled chamber with good agreement; both cases include a crescent shaped spot at the target.

Numerical simulation of steady-state thermal blooming with natural convection.

This work investigates steady-state thermal blooming of a high-energy laser in the presence of laser-driven convection. While thermal blooming has historically been simulated with prescribed fluid velocities, the model introduced here solves for the fluid dynamics along the propagation path using a Boussinesq approximation to the incompressible Navier-Stokes equations. The resultant temperature fluctuations were coupled to refractive index fluctuations, and the beam propagation was modeled using the paraxial wave equation. Fixed-point methods were used to solve the fluid equations as well as to couple the beam propagation to the steady-state flow. The simulated results are discussed relative to recent experimental thermal blooming results [Opt. Laser Technol.146, 107568 (2022) OLTCAS0030-399210.1016/j.optlastec.2021.107568], with half-moon irradiance patterns matching for a laser wavelength at moderate absorption. Higher energy lasers were simulated within an atmospheric transmission window, with the laser irradiance exhibiting crescent profiles.

Propagation of high energy lasers through clouds: modeling and simulation.

A model for 10.6 µm high energy laser beam interaction with a uniform, monodisperse cloud of water droplets is developed. The model includes droplet and vapor heating as well as droplet shattering in the "fast regime" as defined in Appl. Opt.28, 3671 (1989)APOPAI0003-693510.1364/AO.28.003671. The cloud dynamics feed back on the laser via changes in the complex refractive index. In one space dimension, the model is solved exactly, including an explicit formula for the front of the cleared channel. Numerical simulations are conducted for the axisymmetric three-dimensional case. Model predictions and limitations are discussed.