Modeling laser-gas interactions for aerospace applications

Abstract

In certain circumstances, laser pulses can propagate kilometers through atmospheric gases relatively untouched. In other conditions, the propagation distances are measured in centimeters and large amounts of laser energy are deposited into the gas resulting in blast waves and plasma regions. This thesis explores the modeling of laser-gas interactions across this spectrum of length scales and intensities. In kilometer scale atmospheric laser propagation, large scale density gradients result in beam deflection. These deflections were studied using a ray tracing algorithm in a look-down LiDAR system to analyze the resulting altitude errors. The altitude errors generated from thermal plumes, atmospheric ducts and vehicle shock waves are also investigated. If the peak intensity of the pulse is increased such that the beam can generate localized breakdown, we can couple significant amounts of laser energy into the gas. For femtosecond laser pulses, this highly localized energy deposition can be considered instantaneous. The rapid temperature and pressure increase generates a set of radially expanding blast waves. We show that the dynamics of these blast waves can be described by quasi-similarity theory using a Navier-Stokes solver with a novel flux correction technique. By analyzing the motion of these weak blast waves in experimental data we can calculate the energy deposited into the gas during the laser breakdown process. The radial dynamics of low density ‘hot channels’ which form along the laser axis and the gas parameters along this laser axis are also characterized. When the pulse length is on the order of nanoseconds, the plasma chemistry can no longer be neglected as the plasma generated in the first half of the laser pulse dramatically affects the propagation of the latter portions of the pulse. Using an integrated chemical-optical solver to track energy deposition from single and dual nanosecond laser pulses, we show that self-defocusing results in a biased growth of the plasma region towards the focusing lens. We also show that this defocusing effect can be harnessed for use in a novel laser machining technique we call Dynamic Masking.