Numerical Simulation Study On Stress State During KDP Growth And Cooling Process With Translational Method

As an inorganic nonlinear optical material with excellent optical properties, KDP single crystal with high quality and ample size can be grown from solution and it is the only frequency-doubling material used in Inertial Confinement Fusion till now. Besides the strict requirement of size and quality, the quantity of KDP crystal needed in Inertial Confinement Fusion is quite large. Research has been focused on exploring means of growing KDP with high quality and ample size at a high growth rate for many years. Growing KDP crystal in solution by translational motion of crystal is a new method and it has been proved that the value and homogeneity of supersaturation on the surface of KDP crystal can be improved. However, the crack problem during crystal growth and cooling process after drawing out from crystallizer will reduce the production efficiency. The stresses of KDP during translational motion crystal growth and cooling process have been calculated by finite element analysis software ANSYS in order to obtain ways of reducing maximum principle stress value and cracking risk in KDP crystal. The main contents and conclusions are as follows:â‘  Finite element model of KDP crystal which does three-dimensional translation driven by seed rod has been built and the stress distributions have been calculated. The results show that the seed rod causes great stress concentration in KDP crystal and the principle stress value is very high. The KDP crystal is carried by the seed rod and solution buoyancy. The bigger the crystal size is, the heavier load the seed rod bears and the larger principle stress occurs. The maximum principle stress in KDP crystal varies periodically during three-dimensional translation process, which may induce fatigue crack. Maximum principle stress in crystal can be reduced by using seed rod with a spherical apex, increasing seed rod diameter or decreasing the acceleration value during crystal growth, which results in a much smaller risk of crystal crack.â‘¡ Finite element model of KDP crystal which is driven by pallet or plate doing two-dimensional translation has been built and the stress distributions have been calculated. The principle stress in KDP crystal can be significantly decreased by removing the seed rod which causes stress concentration. When the KDP crystal is driven by pallet, both the tensile area and the tensile stress are small and the crack problem during KDP growth can be ignored.â‘¢ Finite element model of KDP crystal has been built and the stress distributions during the cooling process have been calculated. The maximum principle stress value increases at first then decreases during cooling process after drawing out from crystallizer. Without considering the effect of defect and other factors, the inside region of KDP is under a compressive stress state while the outside is under a tensile stress state and the boundary remains relatively unchanged. The tensile stress and compressive stress increase or decrease simultaneously. The inhomogeneity of temperature distribution is the main reason of large principle stress in KDP and the principle stress increases with the difference in crystal temperature. The principle stress and crack risk of KDP during cooling process can be reduced by reducing the temperature difference between the crystal temperature and ambient temperature.