Research On Key Technology In Diamond Turning Of KDP Crystal

Potassium dihydrogen phosphate(KDP) crystals, which can be used as the only material for frequency conversion and electro-optical switch in high power laser systems, play an important role in the current inertial confinement fusion(ICF) and laser weapon devices. Single point diamond face flycutting technique is the only method to stably fabricate KDP crystals for high power laser systems considering their properties of anisotropy, softness, brittleness, sensitive to temperature, deliquescence and so on. However, it is disadvantageous for popularization of the technique considering the special machine tool needed. Meanwhile, the fact that only plane profile can be got by face flycutting technique limits the application of KDP crystals. Refered to the demands of National Ignition Facility(NIF, US) Plan and started with the material features of KDP crystals, this thesis seeks for new technique to cut the KDP crystals and achieves the goal to fabricate KDP crystals efficiently, accurately and stably. Based on the researches on brittle ductile transition and anisotropic characteristic during cutting process, turning technique is proposed as a new idea for finishing technique of KDP crystals. The new technique is studied and optimized according to the features of common ultra precision lathe and the requirements of NIF. The major research efforts of the thesis include the following points:(1) The affects of tool geometry as well as cutting parameters on brittle ductile transition have been studied and special phenomenon that cutting speed has a significant impact on brittle ductile transition of KDP crystals is observed. Scratch experiments are adopted to study the affects of tool geometry and cutting parameters on brittle ductile transition. The phenomenon that the material removal of some crystal orientations turns from ductile to brittle as the cutting speed exceeds a special value is observed and the reason is given according to the features of the crystal. The study deepens the theory in cutting KDP-type crystals.(2) The affects of anisotropy of KDP crystals on cutting process have been studied and turning technique is proposed as a new finishing procedure for fabrication of KDP crystals. Spiral scratch is adopted to get the critical depth of cut in various crystal orientations and the cutting forces as well as fabricated surface are observed with various cutting parameters. The study shows that, full aperture ductile material removal can be got if the cutting parameters are appropriate. Based on the researches, turning technique is proposed as the finishing procedure for fabrication of KDP crystals. The new finishing procedure can not only get plane profile, but also generate aspherics and freeform surfaces.(3) The elements to affect low spatial frequency wavefront error of KDP crystal are studied and an error compensation cutting process is proposed to deal with such error. As a key affect element of low spatial frequency wavefront error, vacuum suction deformation is studied and an in-situ measurement system is achieved to measure it. The compensation cutting process eliminates the affect of datum plane of vacuum chuck and one side of crystal face on the final wavefront accuracy and improves certainty of fabrication and convergence speed.(4) The affect of static characteristics of aerostatic spindle on mid spatical frequency periodical waviness error is studied and a generation mechanism of periodical waviness error on fabricated KDP surface is proposed. The relationship among pressure fluctuation, spindle axial movement and surface waviness profile is discussed by analysising the equivalent stiffness of the aerostatic bearing and the kinematic rule of the spindle. The mechanism can provide guidance for the design of the aerostatic spindle and suppression of mid spatical frequency error of KDP crystals.(5) The relationship between rough cutting parameters and the depth of subsurface defects is studied and a rapid procedure for rough cutting is established to achieve no visible subsurface defects of KDP crystal. A water-solution based MRF method is applied to observe and detect the depth of subsurface defects after turning process and the relationship between rough cutting parameters and the depth of subsurface defects has been discussed. Based on the study, a rapid procedure for rough cutting is got to improve the cutting efficiency.Drawn by the application needs, a new cutting process of KDP crystals is established and optimized through theoretical analysis and experimental investigation. After studying on the cutting mechanisim of soft, brittle, anisotropic material, the compensation technique of low spatial frequency wavefront error, the optimization of mid and high spatial frequency wavefront error and the control of subsurface defects, the capacity is formed to fabricate KDP crystals efficiently and accurately. The research results have important theoretical significance and engineering value for fabrication of KDP crystals.