Theoretical and experimental investigations of localized CO2 laser-fused silica interactions and thermo-mechanical properties of mitigated sites

Local re-fusion of damaged sites on fused silica surface by using CO2 laser is an effective and commonly used method for mitigating their growth to resist subsequent UV laser irradiations. However, residual stress will be introduced in or around the mitigated sites during or after a rapid cooling process of a heated material, which is detrimental for the re-use of the fused silica element. Thus, controlling the temperature during the heating process is a critical issue. Unfortunately, the temperature and the corresponding stress evolution within irradiation time are still unclear at present. In this article, a thermo-mechanical model based on finite element was developed to investigate these issues, and experiments were performed to validate the simulated results. The surface morphology evolution of damaged sites during the re-fusion processes was elaborated. The residual stresses were quantified by using photoelastic technique and chemical etching methods, which were also compared with the simulated data. Results showed that the theoretical simulation data fits well with experimental results, which will provide a significant contribution on understanding the melting process and explaining the mitigation process of damaged sites irradiated by CO2 laser.