多孔注入冷冻靶快速降温过程瞬态特性研究

Transient Thermal Characteristics of Cryogenic Target with Multiple Laser Entrance Holes during Quick-freezing Process

高度均匀光滑的燃料冰层是惯性约束聚变冷冻靶成功点火的物质前提,其制备关键是在靶丸外建立均匀的球形温度场并进行精确控制。本文针对多孔注入冷冻靶系统,建立了三维仿真模型,数值研究了冷冻靶温度场稳态分布与瞬态降温特性,并分析了接触热阻、氦气压力等因素的影响。结果表明:冷臂温度恒定时,靶丸与充气管接触位置为低温区,激光入射口正对处为高温区,最大温差为0.03 mK;硅臂加热块功率突降后,靶丸表面最大温差在0.25 s内急剧上升至87.88 mK,温度场均匀性显著恶化;与硅爪-套筒完美接触相比,低温胶层的存在可有效改善降温过程中温度场的恶化,但降温响应时间明显增加;1~10 kPa氦气压力范围内,快速降温过程中靶丸温度响应迅速,且最大温差峰值较小,有利于维持靶丸表面的温度均匀性。

Cryogenic target is one of the most widely used technologies in the inertial confinement fusion(ICF)research.The formation of a uniform and smooth ice layer inside the fuel capsule is a prerequisite for the successful ignition of an ICF cryogenic target.The ice layer homogenization process requires precise temperature control of the capsule,so the temperature control of the cryogenic target system is an important part of the capsule preparation and ignition experiments.The smoothness of the ice layer is mainly affected by the crystal growth during the solidification process,and the ice layer experiments show that single crystal growth is conducive to reduce the layer defects and improve the final ice roughness.Since the fast cooling of fuel capsule often results in a polycrystalline ice layer with many defects,further warming and melting follow the freezing process to form the single crystal seed,which requires a precise temperature control of the cryogenic target.In the present study,a full-scale three-dimensional model was established for the cryogenic target with multiple laser entrance holes and cold energy transfer components such as silicon arms and cooling rods.The simulation was based on the DO radiation model using Fluent simulation software,where the precision structure in the model was processed using shell conduction,considering both radial and axial heat transfer.The steady-state distribution and the transient quick-freezing characteristics of the cryogenic target temperature field were numerically studied by controlling the power of the heating blocks on the silicon arms.Moreover,the effects of contact thermal resistance and helium pressure on the temperature stability of the cryogenic target during the fast cooling process were analyzed.The results show that the temperature on the capsule surface reaches its minimum value in the vicinity of the fuel filling tube and peaks at the regions facing the laser entrance holes under constant cooling conditions with a maximum temperature difference of 0.03 mK.When the heaters on the silicon cooling arms reduce their power,the maximum temperature difference on the capsule surface sharply increases to 87.88 mK in 0.25 s,and the uniformity of the temperature field on the capsule surface deteriorates significantly.The temperature difference gradually falls back and stabilizes with time.Compared with the zero thermal contact resistance condition,the presence of ultra-low temperature glue between the silicon cooling arm and the thermal mechanical packages(TMP)can weaken the propagation of temperature fluctuation on the heat conduction path and thus reduce the maximum temperature difference peak.This can prevent the uniform temperature environment from deteriorating,but is unfavourable for the real time control of capsule temperature.From the viewpoints of the rapid capsule temperature response and the small maximum temperature difference,the helium pressure ranging from 1 kPa to 10 kPa is conducive to maintaining the temperature uniformity of the capsule surface.