Study On Mechanism And Process Of Polishing KDP Crystal

Potassium Dihydrogen Phosphate(hereinafter referred to as KDP)is a nonlinear optical crystal with excellent performance.In the high energy laser devices such as nuclear fusion systems and high energy laser weapons,it is the unique non-linear optical crystal used as a laser frequency doubling,electro-optic modulation,and photoelectric switching device.Because of the limitations of material characteristics of KDP crystal,such as high deliquescent,high heat sensitivity,it is considered to be one of the most difficult materials to process.As a non-contact processing technology,ion beam polishing is widely used in ultra-precision surface processing because of its feature of not causing damage to the processed surface.In this dissertation,we combine the ion beam etching polishing and ion beam deposition correction polishing to polish the KDP crystal.In addition,we study the influence of surface impurities on the crystal's laser damage threshold through finite element simulation technology.According to the results from the simulation,we optimize the ultra-precision processing technology of KDP crystal.For the problems and challenges faced in the polishing process,we have done the following research:(1)Research on the characteristics of ion beam etching polishing KDP crystal.Firstly,the effects of polishing angle,ion beam energy,polishing time,and argon flow on the surface roughness after polishing were studied.In addition,the evolution characteristics of KDP polishing results with the changes of the above parameters were obtained.Finally,according to the above evolution characteristics,an orthogonal experiment was designed to obtain the optimal etching polishing parameters with the ion beam energy of 400 e V,an argon flow rate of25 sccm,and the polishing time of 44 min.The surface roughness of the KDP crystal was reduced from 2.50 nm to 1.68 nm.(2)Research about the influence of impurities on the surface of KDP crystal on its laser damage threshold.Using finite element simulation technology,a thermodynamic model of the KDP crystal under laser irradiation was developed to analyze the temperature field and stress field distribution on the surface of the KDP crystal when impurities are present on the crystal surface.The results show that: when there are no impurities on the surface,the probability of bulk damage of the KDP crystal is the largest;On the contrary,when the surface impurities are present,the probability of the crystal surface damage is the largest;the presence of surface impurities will reduce the crystal's laser damage threshold,and the larger the impurity size,the lower laser damage threshold.According to the simulation results,it is determined that in the ion beam deposition correction process,the planarization layer needs to be removed completely by using the over-etching method.(3)Study on the ion beam deposition correction polishing KDP crystal characteristics.Firstly,the evolution of crystal surface roughness under different materials,different initial roughnesses,different deposition angles,and different deposition and etching processes are studied.Then,the following optimal process parameters were finally obtained: the sputtering target is a silicon target,the deposition angle is 90 degrees,the Ar flow rate is 35 sccm,the ion beam energy of the deposition process is 600 e V,the deposition time is 176 min,the ion beam energy in the etching process is 500 e V,and the etching time is 154 min.Finally,the optimal parameters are used to conduct the ion beam cyclic deposition correction polishing experiments on the KDP crystal after ion beam etching and polishing.After the third process,the roughness of the crystal surface is reduced to about 1.40 nm.The polishing effect meets the expected goal of the subject.