High-Fluence and Radiation-Resistant Gaseous Optics for High-Power Lasers and Inertial Fusion Energy Applications

Pierre Michel | 23-FS-004

Project Overview

The goal of this project was to investigate gas-based optics, where a refractive index modulation is imprinted in a gas in order to turn it into a transient, diffractive optical element. Gas optics were proposed and demonstrated in proof-of-principle experiments in 2020, with measured damage thresholds that are two to three orders of magnitude beyond what traditional (solid-based) optics can sustain. This makes them ideally suited for applications to high power lasers, and in particular to inertial fusion energy (IFE) power plants where they could be used as the final optics element.

We developed a new, comprehensive theoretical model describing the entire gas optics formation and operation process. Gas optics work by absorbing a spatially-modulated, low-energy "writing" UV laser in a gas containing oxygen and ozone in order to create the index modulation: the modulated UV light gets absorbed by the ozone molecules via photo-dissociation, which creates fast oxygen products (O and O2) which will rapidly heat the surrounding gas via collisions. The spatially-modulated heat deposition launches a pressure wave (comprising an acoustic mode and an entropy mode), with a density modulation and an associated refractive index modulation. Caught at the right time (when the oscillation amplitude is largest), the index modulation turns the gas into an optical grating, which can be used as a dielectric mirror. Our theory encompasses all aspects of the process, including the physical chemistry of the ozone photodissociation, the saturated (nonlinear) absorption of the UV imprint beam, the physics of the acoustic/entropy modes, and finally, the diffraction properties of the resulting gas grating for an incident high-power laser. This model will allow us to pursue the development of gas optics for demonstrations with high-power lasers, in conditions relevant to future IFE facilities.

In parallel, we initiated the development of a new "offline" experimental testbed at Stanford University with our collaborator Prof. Matthew Edwards. The Stanford team achieved their first demonstration of a gas optics in the summer (2023).

Mission Impact

This project supports LLNL's Core Compentencies in lasers and optical science and technology as well as high energy density science. Gas optics constitute a transformational technology for high-power lasers, and could address open problems for future IFE concepts such as the final optics issue (the last elements directly in the line of sight of the target—and thus directly irradiated by the intense flux of neutrons and x-rays from the target). With this project, we paved the way for future experimental demonstration of this new technology at full scale for high-power lasers, in conditions relevant to IFE.