Metal Pollution Induced Fused Silica Surface Damage Mechanism

In the high power laser driven inertial confinement fusion (ICF) system, the resistance of optics to laser damage is the bottleneck problem to limit the high average power output. The defects in the surface of the optics are mainly responsible for the low laser induced damage threshold (LIDT). However, during the operation of the large laser system, the surface contaminations are known to lower the LIDT and shorten the lifetime of optics. In order to enhance the resistance to damage and prolong the serviceable lifetime, it is important to study the mechanism of the laser damage induced by the surface contaminations of fused silica.In this thesis, the initial damage and damage growth induced by several usual metallic contaminations are studied to understand the effects of surface contaminations on the functional damage of fused silica. The research of laser damage mechanism focuses on the film and particle contaminations. In addition, the damage craters are modified by the CO₂ laser conditioning technique to enhance the LIDT and mitigate the damage growth. The mainly contents and conclusions are as follows:1. A model is constructed on damage mechanism induced by contaminations on the surface of fused silica. The finite element analysis code is used to simulate the surface temperature and the stress distributions on the surface of the laser irradiated fused silica. The contamination temperature increases during the laser irradiation. After irradiation, the surface temperature of fused silica quickly increases to the melting point. The maximum stress in the damage area is about 30.73 MPa. The center of the damage area is compressive stress and the margin is tension stress. The simulated results is consistent with the experimental results.2. The contamination films are deposited on the surface of clean fused silica substrates by sputtering. The damage behaviors of contaminated fused silica are studied by the statistical measurement of the LIDT and analysis of the damage morphologies. Damage spots always occur in the output surface of the contaminated fused silica. The samples with contaminations on the input surfaces have the lower LIDT than that with contaminations on the output surfaces. Thermal absorption and surface morphologies of films were investigated by the Stanford photo-thermal technique, ellipsometer, atomic force microscopy. The experimental results are analyzed combined with the thermal shock and plasmaμμμshock models. The different damage morphologies of films are related to the different absorption and microstructure. 3. Three test methods, "1-on-F, '10-on-1", "20-on-1", are used to characterize the LIDTs of the contaminated fused silica. The damage morphologies are obtained by optical microscopy. The damage spot size induced by the input surface contaminations is about 2.5 times of the contamination size after 1 shot. The damage spot size increases with the increasing laser shots. The damage spot size is 6 times of the contamination size after 20 shots. The damage spot size induced by the output surface contaminations doesn't depend on the number of laser shot. The damage spot size is twice of the original contamination size.4. The growth behaviors of the initial damage is investigated on the clean and contaminated fused silica. The experimental results show that the lateral sizes of damage spots located in the output surface exponentially grow with the increasing shot number, and the lateral sizes of damage spots located in the input surface linearly grow with the increasing shot number. The damage pits in the input surface can also result in the damage crater in the output surface. The growth coefficient increases about 11.5% for the contaminated fused silica. After CO₂ laser conditioning, the LIDT of the damage area recovers to that of the clean fused silica.