Study On Influence Of KDP Crystal Ultra-precision Fly-cutting Micronano-topography On Its Laser Induced Damage Threshold

The lower laser induced damage threshold （LIDT） of KH₂PO₄（KDP） crystals isa bottleneck link which extremely limits the power output of inertial confinementFusion （ICF） laser. The middle and high frequency topography information and themicro-defects on KDP surface processed by ultra-precision fly-cutting, haveimportant effects on the LIDT of KDP element. However, until now, there are fewworks which are focused on the influence mechanism of KDP fly-cutting surfacequality on its optical performance. For this reason, this project is established. In thispaper, the characterization of fly-cutting surface micro-topography is carried outfirstly, then the influence regulation of surface topography elements and theircharacterizing parameters on the LIDT of KDP element are analyzed. Base on thework above, a typical parameter which has the most important effect on the LIDT ofKDP crystal is found and considered as the optimal aim in order to guide themachining of KDP fly-cutting. In the end, a theoretical model is established tosimulate the fly-cutting spindle, and the reasonability of this model is proved by lotsof experiments. The work of this paper provides fundamental suggestions forguiding the ultraprecision processing of KDP crystal..For analyzing the surface roughness and waviness induced by fly-cutting, theMotif method, PSD, wavelet transform, and fractal theory are introduced to separateand characterize the special elements. Research results indicate that, the meanamplitude of waviness obtained by Motif method can characterize the surfaceamplitude directly without calculating the weightingfactor. The1-d and2-d PSDcurves, angular frequency spectrum, and radius frequency spectrum can characterizein detail the surface frequency components and texture structures, which covers theshortage of Motif parameters. The multi-scale decomposition function of waveletcan separate the3-D topography information and reconstruct the surface of certainfrequency, this ability of handling the3D surface covers the shortage of PSD andMotif methods. The fractal dimension is a good measurement of roughness whichcan characterize the frequency and amplitude at the same time, and it also can avoidthe error of detecting equipments. It has important meaning to use jointly themethods above in characterizing the KDP fly-cutting topography.For study the influence of mid-frequency waviness on the optical performanceof KDP crystal, the PSD method and Fourier model theory are used to analyze themodulation properties of waviness period and amplitude on the incident laser.Research results show that, the fly-cutting surface is consisted of different wavinesswith various spatial periods and amplitudes, and the dangerous waviness periods mostly distribute between80μm and180μm. In this range, the amplitude is moredangerous than period. The mean depth of waviness Motifs can be considered as aneffective parameter to reflect the LIDT of KDP fly-cutting surface. In addition,based on the coupled wave equations of harmonic generation, we pointed out that itis most important to decrease the waviness amplitude for increasing the harmonicefficiency and improving the outgoing wave shape.For study the influence of surface roughness and subsurface cracks on theLIDT of KDP element, the fractal theory and Fourier model method are used toanalyze the modulation properties of roughness profile and subsurface crack,Research results indicate that, compared with the waviness, the modulation effect ofroughness is weaker, and the roughness decreases the LDIT of KDP crystal manilyby enhancing the heat absorption of fly-cutting surface. The cuicumstantial profilefractal dimension distribution can reflect this absortion capacity well. Themodulation effect of subsurface crack is another mechanism which damages KDPelement, andthe changing trend of the subsurface crack depth is inverse with that offly-cutting surface LIDT, but it is consistent with that of the waviness meanamplitude, this conclusion is extremely benefit for the fly-cutting machining of KDPcrystal. It indicates that the subsurface crack depth will be improved when thewaviness amplitude is decreased.For analyzing the vibration factors of fly-cutting machine which introduces thewaviness, this paper use the two freedom degrees gyro technology equation toestablish the dynamic model of the fly-cutting machine air spindle, study theinfluence of fly-cutting parameters on the frequency and amplitude of waviness.Research results indicate that, the multiple and difference relationships commonlyexist between the time frequencies.The waviness frequency can be forecasted bysetting reasonably the spindle speed and the ratio of rotational inertia, and whichprovides the probability for eliminating the dangerous frequency. The increasingfeed and cutting depth will all lead to the rise of waviness amplitude, and theinfluence of feed is more obvious than that of cutting depth. When the spindle speedis low, the material is removed mainly based on the brittle mechanism, the wavinessamplitude are smaller than the high speed situation. But lots of defects exist on suchsurfaces and which will enhance the heat absorption and decrease the LIDT of KDPat last. When the rotation speed is400r/min, feed is around10μm/r, cutting depth isabout10μm, and the tool rake angle is-45, it can obtain better fly cutting surfaces.