KDP晶体微纳米加工表层缺陷对其激光损伤阈值的影响

使用有限元方法分析了在激光辐照条件下,KDP晶体已加工表面存在的残余内应力、微裂纹及微孔等多种微纳米加工表层缺陷对晶体激光损伤阈值的影响。通过分析发现:KDP晶体微纳米加工表层缺陷的存在,会影响晶体表面的温度场及热应力场的分布,使入射激光的能量积聚在缺陷附近的很小范围内,造成晶体缺陷处产生局部熔融现象,从而使KDP晶体产生损伤,降低KDP晶体的激光损伤阈值。针对微纳米表层的微裂纹进行了损伤阈值测试实验,结果表明微裂纹的存在会降低KDP晶体的激光损伤阈值(约降低3J/cm2),实验结果与仿真结果符合得很好。     

Mechanism of micro-waviness induced KH2PO4 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 μ^-1 and 30^-1 μ^-1, especially between 90^-1 μ^-1 and 200^-1 μ^-1. Based on the classification study of the time frequencies of micro-waviness, 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).     

水基契伦可夫量能器模型的研究

为了研究极长基线中微子振荡,构造了一个大小为,1m×1m×13m,水基切伦科夫量能器模型.测量得到的水箱的有效衰减长度为(5.74±0.29)m,并且研究了光的收集能力随入射粒子角度变化的关系.同时发展了基于,GEANT4,软件包,包含有详细的光学过程的模拟程序, 所得到的模拟结果与实验测量有很好的一致性. 说明水箱可以作为水基切伦科夫量能器的可行性的方案.     

切削参数对KDP晶体加工表面实际频率特征的影响

针对采用单点金刚石超精加工的KDP晶体光学表面,研究了切削参数对微观形貌频率特征的影响。通过功率谱密度获得表面轮廓频率分布,并用连续小波重构加工过程中随切削用量变化的微观轮廓频率特征。结果表明:切削参数对微观形貌的影响具体表现在实际频率特征上,中频特征波长及幅值反映了切深及转速变化,随切深及转速增加,幅值变大;低频特征反映了进给量变化,随着进给量变小,频率及幅值变小;高频特征是加工过程中振动及材料各向异性的具体表现。     

Study of the near-field modulation property of microwaviness on a KH2PO4 crystal surface

The KH 2 PO 4 crystal is a key component in optical systems of inertial confinement fusion (ICF).The microwaviness on a KH 2 PO 4 crystal surface is strongly related to its damage threshold which is a key parameter for application.To study the laser induced damage mechanism caused by microwaviness,in this paper the near-field modulation properties of microwaviness to the incident wave are discussed by the Fourier modal method.Research results indicate that the microwaviness on the machined surface will distort the incident wave and thus lead to non-uniform distribution of the light intensity inside the crystal;in a common range of microwaviness amplitude,the light intensity modulation degree increases about 0.03 whenever the microwaviness amplitude increases 10 nm;1 order diffraction efficiencies are the key factors responsible for light intensity modulation inside the crystal;the light intensity modulation is just around the microwaviness in the form of an evanescent wave,not inside the crystal when the microwaviness period is below 0.712 μm;light intensity modulation degree has two extreme points in microwaviness periods of 1.064 μm and 1.6 μm,remains unchanged between periods of 3 μm and 150 μm,and descends above the period of 150 μm to 920 μm.     

Molecular dynamics simulation on mechanicalproperty of carbon nanotube torsional deformation

In this paper torsional deformation of the carbon nanotubes is simulated by molecular dynamics method. The Brenner potential is used to set up thesimulation system. Simulation results show that the carbon nanotubes can bear larger torsional deformation, for the armchair type (10,10) single wall carbon nanotubes, with a yielding phenomenon taking place when the torsional angle is up to 63$^{\circ}$(1.1rad). The influence of carbon nanotube helicity in torsional deformation is very small. The shear modulus of single wall carbon nanotubes should be several hundred GPa, not 1\,GPa as others reports.     

Effect of structural parameters of Gaussian repaired pit on light intensity distribution inside KH_2PO_4 crystal

KH2PO4 （KDP） crystal with excellent optical properties is a very important element of inertial confinement fusion （ICF） device. However, KDP crystal surface micro-defects severely reduce the crystal laser damage threshold, affecting the crystal service life. In this paper, Gaussian repaired pit is used to replace the crystal surface micro-defects, in order to improve the laser damage resistance of the KDP crystal with surface micro-defects. At first, the physical model of Gaussian repaired pit is built by Fourier model method, and the accuracy of the method is analyzed. It is found that the calculation error can be reduced by increasing the product of the width-period ratio and the truncation constant of the repaired pit. The calculation results about the physical model of Gaussian repaired pit show that the light intensity distribution within the crystal is symmetrical, and there are evidently enhanced light intensity regions in the crystal. Meanwhile, the maximum relative intensity inside the KDP crystal decreases gradually with the increase of the width of the Gaussian repaired pit. Secondly, the Gaussian repaired pits with different widths and the same depth of 20 μm are processed by micro-milling. Their surfaces are very smooth and present the ductile cutting state under the microscope. Finally, the laser damage threshold of the Gaussian repaired pits on the surface of the KDP crystal sample is measured by a 3 ω, 6-ns laser. The results showthat the maximum threshold of the Gaussian repaired pits is 3.12 J/cm2, which is 60% higher than the threshold of initial damage point, and the laser damage threshold increases with the increase of the width of the Gaussian repaired pit.     

Study on incident laser modulation using surface micro-defects on KH2PO4 crystal

KH2PO4 crystal is a crucial optical component of inertial confinement fusion. Modulation of an incident laser by surface micro-defects will induce the growth of surface damage, which largely restricts the enhancement of the laser induced damage threshold. The modulation of an incident laser by using different kinds of surface defects are simulated by employing the three-dimensional finite-difference time-domain method. The results indicate that after the modulation of surface defects, the light intensity distribution inside the crystal is badly distorted, with the light intensity enhanced symmetrically. The relations between modulation properties and defect geometries （e.g., width, morphology, and depth of defects） are quite different for different defects. The modulation action is most obvious when the width of surface defects reaches 1.064 p-m. For defects with smooth morphology, such as spherical pits, the degree of modulation is the smallest and the light intensity distribution seems relatively uniform. The degree of modulation increases rapidly with the increase of the depth of surface defects and becomes stable when the depth reaches a critical value. The critical depth is 1.064 μm for cuboid pits and radial cracks, while for ellipsoidal pits the value depends on both the width and the length of the defects.