Mechanism of micro-waviness induced KH_2PO_4 crystal laser damage and the corresponding vibration source

The low laser induced damage threshold of the KH2PO4 crystal seriously restricts the output power of inertial confinement fusion.The micro-waviness on the KH2PO4 surface processed by single point diamond turning has a significant influence on the damage threshold.In this paper,the influence of micro-waviness on the damage threshold of the KH2PO4 crystal and the chief sources introducing the micro-waviness are analysed based on the combination of the Fourier modal theory and the power spectrum density method.Research results indicate that among the sub-wavinesses with different characteristic spatial frequencies there exists the most dangerous frequency which greatly reduces the damage threshold,although it may not occupy the largest proportion in the original surface.The experimental damage threshold is basically consistent with the theoretical calculation.For the processing parameters used,the leading frequency of micro-waviness which causes the damage threshold to decrease is between 350-1 μm-1 and 30-1 μm-1,especially between 90-1 μm-1 and 200-1 μm-1.Based on the classification study of the time frequencies of microwaviness,we find that the axial vibration of the spindle is the chief source introducing the micro-waviness,nearly all the leading frequencies are related to the practical spindle frequency（about 6.68 Hz,400 r/min） and a special middle frequency（between 1.029 Hz and 1.143 Hz）.

Suppression Strategy of Micro-waviness error in Ultra-precision Parallel Grinding

Ultra-precision parallel grinding is widely used in the machining of complex optic components with high tolerance and excellent surface finish.However,the micro-waviness raised from the relative motion error of the grinding tool is frequently involved in the grinding process despite the fine dressing and dynamic balance work carried out,which posed a remarkable impact on the surface quality and form accuracy.Therefore,it is essential to investigate the evolution mechanism of the micro-waviness error and determine a relevant strategy to suppress this kind of error.In this paper,a model of the distribution of grinding points corresponding to the relative motion error of the grinding wheel is developed by considering the phase effect.A close relationship is found between the micro-waviness geometry and the distribution of grinding points.This indicates that the phase shift is a significant parameter to determine the waviness pattern,and the uniform distribution of grinding points is beneficial to suppress the micro-waviness in parallel grinding.Finally,an error-suppression strategy is proposed by adjusting the wheel speed to maintain an appropriate phase shift to suppress the micro-waviness error.This work provides a new method to control the micro-waviness and reduce the effect of the waviness error on the surface generation.