The potassium dihydrogen phosphate (KDP) crystals suffer from nanosecond pulse laser irradiation and are susceptible to damage during the operation of ICF system. In particular, the microcracks on the surface of KDP crystals caused by the single-point diamond fly-cutting (SPDF) process are more likely to cause serious damage under the subsequent laser irradiation. However, the mechanism of laser damage is still unclear. A model that can well represents the laser damage response is very important to reveal the mechanism of laser-induced damage. In this work, the electromagnetic field, stress field and temperature field are coupled, the mechanical characteristics of KDP material are considered, and the reasonable strength equation is applied to model the laser damage response of KDP crystal. Then, the conical crack is taken as an example to explore the laser damage response process of KDP crystal caused by surface defects under laser irradiation. It is found that the surface conical cracks have a great influence on the response process and the morphological characteristics of the laser damage. The existence of surface conical crack defects would lead to the extension of the longitudinal cracks beneath the damage crater, which has great disadvantages for the repairing of the laser damage sites. This work is of great guidance for avoiding the defects-induced damage and improving the service life of the crystal applied in ICF systems.
During the grinding and polishing processes of hard-brittle fused silica optics, the defects would be inevitably formed on the finished surface. Fused silica has a high absorption coefficient for far-infrared lasers, which makes the CO2 laser processing to be the potential repairing technology for machining-induced defects on fused silica surfaces. In this work, using a low-power CO2 laser, a new repairing method to heal the machining-induced micro-defects on the surface of fused silica is proposed. Then, based on the nonlinear thermodynamic parameters of fused silica material, a thermal transfer model under CO2 laser irradiation and a dynamic defect healing model were established. On basis of that, the influence of CO2 laser parameters on the maximum surface temperature and the temperature distribution inside the silica material was investigated. It is found that, under the low-power and near-continuous CO2 laser irradiation, the maximum melting depth can be obtained under the non-evaporative condition. The defect healing process under various laser powers was explored as well. It is found that the defects would be more difficult to be healed under a laser with higher-power, smaller beam size or shorter pulse width. This work can provide theoretical guidance for the determination of the optimal parameters in the laser healing process and the optical manufacturing strategies of fused silica optics.
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