Study On The Theory And Technology Of Ion Beam Figuring For KDP Crystals

KH2PO4(KDP) crystal is a kind of excellent nonlinear optical crystal, is the only optical crystal that can be used for laser frequency conversion unit and electro-optic swith in the optical system such as Intertial Confinement Fusion(ICF) and high-power laser weapon system. However, KDP crystal is regarded as one of the most difficulty maching materials due to its material properties of anisotropy, softness, brittle, easy deliquescence and high heat expandence. Currently, the single-point diamond fly-cutting and turning are widely used in the ultra-precision machining of KDP crystals. However, grains of cutting are inevitably appeared on the machined surfacre, which will constrain further improvement of the quality of machined surface. Especially, the micro-scale ripplres left on the finished surface will degrade the quality of laser beam in optical systems. Cutting grains and micro-scale ripplres can be erased by Magnetorheological finishing(MRF) technique based on water-dissolution principle. And surface errors can be corrected by spot rater-scaning the whole surface with variable and controlled velocity. Thus, a new ultra-precision machining route emplying ultra-precision turning and Magnetorheological finishing is established to efficiently fabricate ultra-smooth and ultra-precision KDP workpieces. However, iron podwers in Magnetorheological fluid are easily embedded into soft KDP crystal, which influences the optical property of KDP crystal. So, it is very urgent to clean the embedded iron powders. In this thesis, ion beam finishing(IBF) technique is used to clean the embedded iron powders, which perfect the ultra-precise machining for KDP crystal. Faceing the new problems and challenges in the process of ion beam figuring for KDP crystal, The major research efforts of the thesis include the following points:(1) Ion beam finishing system for ultra-precision optical components is manufactured. Neutralizer was alternated and placed far away from optical component, which would decrease the thermal influence to KDP crystal. Vacuum system with two vacuum chambers and conveyance system of workpiece were manufactured. Workpieces can be exchanged with the work vacuum chamber of operational pressure, enhancing machining efficiency and deferring oxygenization of ion source. Beam removal function is excellent rotationally symmetrical Gaussian figure and stable for a long time. Machining capability of ion beam figuring system KDIBF650-5V was verified by two sample mirrors. The research built the foundation for ion beam figuring of KDP crystal.(2) Temperature and stress fields of optical component in ion beam figuring process are studied. KDP crystal with high heat expansion coefficient and brittleness is easily cracked due to ununiform heating in ion beam figuring process. So, a thermal model is constructed to estime the temperature and stress of optical in IBF process: Software named SRIM is used to simulate the interaction between incidence ions and optical surface. And the energy disposited into optical element by one incidence ion will be calculated. Beam density distribution on the optical surface is analyzed in theory on the basis of ion source optical geometry. Thus, heat source acting on optical surface caused by ion beam bombardment is built up. Based on Fourier law, Thermal conduction differential equation of anisotropic materials is obtained by theoretical derivation. And the model of temperature field and thermal stress field are established. The results of temperature field simulated by ANSYS are compared with that obtained by experimentation, proving the correction of temperature field model.(3) The optimization of technological parameters based on temperature field and thermal stress field are studied. To decrease the thermal effect caused by IBF and keep KDP crystal from being broken, several concrete and feasible methords are put forward: firstly, technological parameters are optimized based on the basis of analyzing the influence of technological parameters on the surface roughness. Secondly, to optimize the moving paths of ion beam, the methord of steping over adjacent paths is applied. Thirdly, beam with smaller radius is favourable toward correcting surface form errors, so, a novel methord based on low-pass filtering theory is put forward and the cut-off frequency is determined based on capability of correcting surface form errors.(4) Surfaces of KDP crystal polished by IBF are studied. Crystalline structure is not transformed after IBF based on the results of Raman spectrum analysis. Influence laws of technological parameters and material property to surface roughness of KDP crystal are researched based on which ultra-smooth surface is obtained by IBF with appropriate technological parameters. Distribution of impurities in depth is analyzed using secondary ion mass spectrometer(SIMS) technology; iron powsers embedded into KDP crystal surface in MRF process are cleaned away by IBF.(5) The theory and experiment of IBF for KDP crystal were studied. Selecting of technological parameters, optimizing of ion beam moving paths and technology of low-pass filter are added to the typical technological chain of IBF, and a novel technological chain of IBF for KDP crystal is proposed. The theory and technology in this thesis are both applied in experimentation of IBF for KDP crystal, and the surface form accuracy is qualified finally, which provides an effective support for the study achievement of this thesis. Laser induced damage threshold of KDP crystal machined by SPDT, MRF and IBF are measured respectively, which reveals higher laser induced damage threshold of KDP crystal polished by IBF.