高功率激光装置中长悬臂靶架结构稳定性研究

Study on Structural Stability of Long Cantilever Target Assembly in High Power Laser Facility

在高功率激光装置冷冻靶系统中,靶架的细长悬臂梁结构对冷冻靶内部振源有着较大的振动响应,从而会影响装置的束靶耦合精度。针对该问题,研究了长悬臂梁的附加式减振结构,在不增加整体外形尺寸和质量的前提下,通过增加悬臂梁支撑点的方式增大结构比刚度,从而优化其振动响应特性。以美国国家点火装置(NIF)中的靶架作为研究设计主体,通过建立数学模型明确了响应函数的主要、次要影响参数;利用ANSYS有限元模拟的方法确定了重要工程参数的取值范围;通过自主搭建的实验台模拟了不同工况下的振源并对减振靶架进行测试,最佳方案中的振幅优化率为91.7%,冲击收敛时间优化率为77.1%,固有频率达到183 Hz,证明了所设计的减振结构对长悬臂梁的振动响应特性有较好的优化作用。

Objective High-power laser facilities require a high-precision beam-target coupling,and one of its important error sources is the stability of the target system.There are inevitable internal vibration sources in a cryogenic target system that cause the vibration response of the slender cantilever structure,thus reducing the stability of the target system.The cryogenic target assembly is a slender cantilever-beam structure located at the head of the target system.Therefore,the stability of the suspension end of the target assembly significantly affects the accuracy of the beam-target coupling.There are two common methods of vibration control for cantilever beams,namely,active control and structural optimization.The active control method requires additional control circuits and driving mechanisms that can easily fail and have poor reliability in the low temperature and strong magnetic field environment of the vacuum target chamber.The latest Laser Megajoule(LMJ)device adopts the structural optimization method of reducing the length-to-diameter ratio of the target assembly.However,this method increases the mass of the cryogenic target head,and the stability optimization effect of the target system is not obvious.Compared to the less reliable active control method,the structural optimization method is worth further discussion.The structure specific-stiffness of the target assembly is improved,and the vibration response characteristics are optimized without changing the shape and mass of the original target cantilever beam.This method can effectively improve the stability of the target assembly and provide a reference for the design of target assemblies in future high-power laser facilities.Methods The National Ignition Facility(NIF)cryogenic target system is used as a reference.First,the vibration source is analyzed using relevant literature,and a 1∶1 target assembly model is built according to the data.The preliminary scheme design establishes a damping structure consisting of multiple sets of tensioning wires that contains two damping structure forms.In this scheme,a steel wire with an elastic damping property is attached to the middle of the cantilever beam of the target assembly,and the middle supporting point of the target assembly is added to change the vibration response characteristics of the target assembly.According to the design,the support for the spring damping Bernoulli-Euler equation of the cantilever beam is set up,the modal matrix is sorted based on the theory of mechanics of materials to establish formulas for calculating the stiffness of tensile steel wire vibration components,and the target assembly response function of the installation position of the vibration reduction component,diameter of the wire section,and three key research series damping size parameters are determined.The three theoretical parameters correspond to five parameters in an actual engineering structure:the number of components,installation position,diameter of the steel wire,value of the preload,and damping.ANSYS finite element analysis software simulates and optimizes these five structural parameters.The optimal value is used to design the vibration damping target assembly in detail,and a model of the assembly is constructed and assembled(Fig.9).Two vibration sources can be accurately simulated:1)an eccentric mass block and a direct current(DC)motor can simulate the vibration source of a refrigerator operation,and 2)a fixed track and heavy objects are used as the source of impact vibration when the insulation cover is opened.To characterize the optimization effect for the response amplitude and impact convergence time of the damping structure,the vibration damping target assembly with different parameters and design schemes is tested on a vibration test bench.Results and Discussions In the simulation,the total mass of the target assembly,natural frequency,and amplitude control effect are considered comprehensively(Fig.6),and the installation position of the vibration damping structure is finally determined at the pressure plate.After the installation position is determined,the influence of the steel-wire diameter is simulated.We find that when the diameter is less than 0.8 mm,the vibration amplitude of the suspension end of the target assembly decreases uniformly as the diameter increases.When the diameter is greater than 0.8 mm,the vibration amplitude increases with the increase in diameter in an oscillatory manner(Fig.7).This may be because when the diameter increases,the energy generated by the vibration of the vibration damping component also increases.When the critical point of 0.8 mm is reached,the vibration coupling effect between the vibration damping component and target assembly becomes evident,and the final amplitude increases in an oscillatory manner.The lowest point in the 0.8 mm diameter stationary zone,the point with a large amplitude in the 1.0 mm diameter oscillation zone,and the point with a small amplitude in the 1.2 mm diameter oscillation zone are selected for simulation.The theoretical optimal optimization rate of the damping target assembly is obtained as follows:the amplitude is 90%and the impact convergence time is 55%(Table 6).In the simulation experiment,the variation trend of the amplitude control effect of the two vibration damping structures agrees with the simulation results in Fig.6,which proves that an oscillation zone exists and the minimum amplitude point of the oscillation zone can be realized by customizing the overall mass ratio of the steel wire.The optimal optimization rate of the damping target assembly measured in the simulation experiment is as follows:the amplitude optimization rate is 91.7%and the impact convergence time optimization rate is 77.1%(Table 8 and Fig.13).Comparisons show that the integrated control effect of the designed series structure of the steel wire and damping material is superior(Tables 7 and 8).Conclusions In this study,a structural stability optimization design of the long cantilever structure of a cryogenic target assembly in the NIF is carried out,and a vibration-damping structure in the form of a vibration-damping component is designed according to the characteristics of lightweight and adjustable damping.Mathematical modeling shows that the control effect of the damping structure is mainly related to the installation position,diameter of the steel wire,and series damping.After the simulation experiment,the parameters are optimized,and the initial structural design is modified.In the simulation experiment,the experimental data on the vibration response characteristics of the target assembly are consistent with the simulation results.The results prove that the optimal control point of the damping structure in the oscillation region is achievable.The existing simulation experiments achieve the amplitude optimization rate of 91.7%and the impact convergence time optimization rate of 77.1%.Experiments show that the vibration-damping structure designed in this study has a good optimization effect on the vibration response amplitude and convergence time,and the wire and damping material series design scheme has a better comprehensive effect.