Double Frequency Laser Irradiation Damage Of Kdp Crystals Finite Element Method (fem)

It is well known that the limitation of load ability on optical elements is the keybottleneck of outputting the effective high power laser which is used in ICF（inertialconfinement fusion）. This dissertation, which based on the thermal conductionequations and the thermal-elastic equations, does some researches regarding thedamage threshold of KDP frequency conversion crystal in theory by a finite elementanalysis method.Firstly, the distribution of temperature field and thermal stress field of the idealKDP crystals irradiated by pulse lasers are simulated on the software ANSYS, thenonlinear property of the KDP material and the alteration of elastic module、thermalconductivity、specific heat、thermal expansion coefficient and etc. with the differenttemperature is taken into account. The results indicate that the temperature of thesurface of KDP crystals doesn’t reach the melting point of the material and the thermalstress of it doesn’t get to the requirement for fracture strength of the optical elementwhen irradiated by1Hz and10Hz incidence laser, however, it would go beyond thedamage threshold of the material after more than100pulses when irradiated by100Hzincidence laser. The theoretic results have been supported by the related experiments.In addition, the influence of the surface micro-crack defect on the temperature andthermal stress field distributions of KDP frequency conversion crystal, especiallyaround the neighborhood of the defects, has been simulated. It shows that the existenceof micro-crack extremely influences the distribution and makes the power of theinjected laser beams centralize near the area of the defect, therefore, reduce the laserdamage threshold dramatically.At last, the impurities such as Al³⁺, Fe³⁺and Pb²⁺of the KDP crystal are takeninto account. The results indicate the machined superficial layer impurities are theprimary factor affecting the damage threshold. It is found that there exists a strongtemperature field and thermal stress field in the vicinity of the inclusions owing to theexistence of the strong absorbing ions which would absorb much more percentage ofpower of the laser. It is also found that different contents of inclusions have different effect. What’s more, the effect of the number of laser pulse on the damage threshold ofKDP crystal containing impurities is calculated by the applications because noneresearchers at home has done this work. The results show that the temperature and thethermal stress of the surface of KDP crystals with inclusions nearly have no changewhen irradiated by1Hz incidence laser and increase slightly when irradiated by10Hzincidence laser. By contrast, the temperature would reach the melting point of thematerial and the thermal stress of it would get to the requirement for fracture strengthof the optical element when irradiated by after only several pulses when irradiated by100Hz incidence laser. The theoretic results have been approved by the relatedexperiments.