Study On The Effect Of Machining Micro/Nano Defects On Laser Damage And Novel Micro-repairing Method Of KDP Crystal

Due to its unique photoelectric properties,Potassium Dihydrogen Phosphate(KDP)crystals are indispensable optical components in inertial confinement controlled nuclear fusion devices.However,KDP crystal materials are prone to suffer from laser-induced damage and destroy the whole optical element in the process of laser targeting,which restricts the improvement of the output energy of the laser fusion device.At present,it is generally believed that the surface defects generated by the crystal during processing are the main reason for the low resistance of the crystal to laser damage,but the actual laser damage threshold of the crystal is still much lower than the theoretical value considering the single defect.In this thesis,the micro/nano defects on the crystal surface after single point diamond flying cutting are taken as the research object,and the effects of various defect coupling on the laser damage ability of the crystal are considered by means of simulation and experiment.The laser induced damage mechanism of KDP crystal will be further revealed,and a new method for repairing micro/nano defects on crystal surface is proposed.By classifying and counting the micro/nano defects on the surface of the crystal after flying cutting,the defect feature topography information is obtained.Then the two-dimensional simulation models of multiple scratches,scratches and pits,scratches and impurities on the crystal surface are established,and the comprehensive modulation mechanism of the light field in the crystal under the coupling of various defects is analyzed.The effects of defect size parameters,spacing and relative position on the comprehensive modulation effect are studied in detail.It is found that the internal light intensity of the crystal can reach 4 times of that of a single defect when there are many defects.The effect of coupling of various defects on the laser damage threshold of KDP crystal was studied experimentally.the scratches and pits with different spacing and relative position were preset on the crystal surface.After the laser damage test,it was found that the laser damage threshold of the crystal decreased obviously when multiple defects existed at the same time,and was affected by the spacing and relative position between the defects.The laser damage threshold of multiple scratches is only 45% of that of single scratches,which verifies the correctness of the theoretical simulation results.In order to reduce the influence of surface processing micro-nano-defect on the laser-induced damage of the crystal,a new method for repairing the surface micro-defect of the crystal by using water-solvent is proposed in this thesis.According to the tidal decomposition characteristics of KDP crystal materials,the water meniscus formed by probe and sample surface is used to dissolve,fill and remove the local defect area of KDP crystal under certain humidity and temperature conditions,so as to achievethe purpose of repair.Through the comparison of the surface morphology before and after the repair of the defect area,it is proved that the repair method has obvious effect of "removing protruding and filling concave",and the main factors affecting the repair rate are studied experimentally.Through the measurement of the surface quality of the repaired area,it is found that the surface roughness Ra of the crystal is still about 5nm,or even lower,and the feasibility and effectiveness of the repair method are verified by the analysis of light intensity enhancement before and after defect repair.