Radiation Effects And Micro Defects Detection Of KDP Crystals

Laser-induced damage in KDP crystal at low fluence remains a significant factor to limit the increase of laser fluence in high power laser project. The formation of damage may be due to the interaction between the lattice defects and the laser beam. The mechanism about the interaction between laser and defects is not clear, which will influence the application prospects of KDP crystals in the large-aperture high power laser devices such as inertial confinement fusion system. The laser-defects interaction will result in the initial damage of KDP crystals. However, the overall damage performance can be improved with pre-exposure to sub-damage laser fluence. To understand the process involved the damage initiation and laser conditioning in depth would help to develop the protocols to optimally improve the damage performance. It is difficult to identify the intrinsic defects in KDP crystals due to extremely small size and sparse distribution. These defects are modified by exposure with γ-ray and laser beam, and the new radiation-induced defect appears. So it is more difficult to identify these defects. Therefore, it is important and interesting to find the appropriate characterization techniques for the defects and understand the defects evolution under radiation conditions. In this work, various techniques have been selected to characterize the defects evolution for the large size of KDP crystals and the performance of crystals has been studied. The detection and characterization of the defects using done before and after irradiation. The research content and results are as follows:1. The interaction of the KDP crystals with the sub-damage laser is investigated. The influence of the different laser parameters on the defects evolution is studied.(1) Optical absorption spectra of KDP crystals are investigated after irradiation with different laser parameters. The optical absorption intensity in UV-Vis band reduces in KDP single crystals after laser irradiation at sub-damage fluence.(2) There is no change for lattice constant and Raman active modes after laser conditioning with various parameters.(3) Photoluminescence spectra are also investigated after irradiation with different laser parameters. The emission peak shifts after irradiation.(4) There is no correlation between laser conditioning efficiency and the impurity type and concentration.2. The effects of γ-ray irradiation on KDP crystals containing arsenic impurities and excluding the arsenic impurities are investigated with different irradiation doses.(1) After γ-ray irradiation, the samples show an overall absorption increase within 200~360 nm spectral range. It is found that the irradiation-induced optical absorption at 260 nm depends on the irradiation dose and arsenic impurity concentration. A widely and weaker absorption band is observed in the range of 300~360 nm after irradiation at 400 kGy. The samples without arsenic show an overall absorption increase within overall spectral range after γ-ray irradiation, and absorption intensity increases with the increasing irradiation dose. However, there is no new absorption band appeared afterγ-ray irradiation.(2) The defect model was generated via an arsenic ion substituting for a phosphorus ion and formed an AsO4 unit with an interstitial oxygen ion, and optical absorption calculated using an ab initio method. The simulated results confirm that optical absorption at 260 nm is attributed to the formation of defect centers related to arsenic impurities. Similarly, two apparent absorption bands are observed at 277 nm and 300~600 nm for defect model which are attributes to the Al and Mg ions substituting for potassium ions, respectively. The simulated results accords with experimental results in this work.(3) The variation of vacancy-type defects is also performed by using the positron annihilation lifetime spectroscopy(PALS) and Doppler broadening spectroscopy with high detection sensitivity. Positron annihilation lifetime analysis shows that the large vacancy-like defects recover during irradiation process with low doses, and new large vacancy-like defects generated at the highest dose. The Doppler broadening measurements also give testimony that new vacancy-type defects are formed during γ-irradiation process.(4) To characterize the influence of irradiation-induced defects on dielectric properties, the dielectric spectrum is investigated. The increase of the electrical conductivity of irradiated crystals at low dose associates with migration of proton vacancies. After irradiation at high dose, the constant conductivity may associate with migration of the intrinsic defects.3. The effects of electron beam irradiation on KDP crystals containing arsenic impurities and excluding the arsenic impurities are investigated with different irradiation doses. The surface structure and macro properties of samples are studied before and after irradiation.(1) The samples show an overall absorption increase within overall spectral range after electron beam irradiation, and absorption intensity increases with the increasing irradiation dose. The optical absorption at 250 nm is observed after irradiation at 6.2×1016 e/cm2 and 1.0×1017 e/cm2.(2) The influence of irradiation dose on the lattice constant is not observed.(3) The surface morphology of KDP samples is analyzed by atomic force microscopy before and after irradiation. The micro craters in the surface are observed after irradiation, and surface roughness increases with the increasing irradiation dose.4. The effect of thermal annealing is investigated with the different parameters. The correlation between various annealing parameters and crystal performance is also investigated. The optical absorption at 215 nm shifts after annealing with different parameters. It does not affect lattice constant and molecule active modes after thermal annealing with various parameters. There is no correlation between laser conditioning efficiency and the impurity type and concentration.Based on the experimental investigations together with simulation calculation, the influence on the optical absorption and laser-induced bulk damage performances of the KDP crystals containing low concentration of arsenic impurities are successfully obtained and confirm that optical absorption at 260 nm is attributed to the formation of defect centers related to arsenic impurities. The variation of vacancy-type defects is also obtained during γ-irradiation process, but further investigation is needed to validate the correlation between laser damage and vacancy defects.