大型太阳望远镜热光阑的关键技术研究

Thermal control of the heat stop is one of the core issues of large solar telescopes. CGST's diaphragm has high heat load power and power density, thermal control must focus on improving the heat exchange capacity of the diaphragm. In the design of the diaphragm sample, we reduce the thickness of the metal wall between the heated area and the coolant to the extreme, while used a special flow guiding structure to increase the coolant flow rate in the critical area. The simulation results of the coolant fluid velocity field and the diaphragm temperature field shows that the heat exchange capacity is significantly improved. But the quality of the thermal control is not enough to judge by simulation, the temperature field must be measured..The temperature of the heat stop cannot be directly measured and the radiation temperature measurement may be the only available measurement method. Even if we have a radiation thermal imaging camera, the temperature measurement is very difficult. We should use the modulation method to control the sunlight which keep the diaphragm in the warming & cooling process. Then, in the cooling period, we should use continuous multi-frame temperature measurement to master the cooling process, and fit the cooling curve to obtain the initial temperature and the temperature constant, in order to achieve the purpose of temperature measurement..In the above research, we have completed partial verification on the heat stop of 2-meter annular solar telescope which build by Yunnan Observatory of Chinese Academy of Sciences. The "2-meter annulus " is similar to the "F-number" of the CGST also the energy density entering the diaphragm. Therefore, the design idea of the diaphragm can be used for reference, and the temperature measurement method can also transplant. However, due to the huge difference in the concentrating area, the heat load power is far from the same, and the CGST’s diaphragm thermal control needs to be carefully supplemented by automatic control.

焦点光阑热控是大型太阳望远镜的核心问题之一。CGST光阑热负载功率大、功率密度高；热控须专注于提高光阑的热交换能力。在光阑样品设计中我们把受热区域和冷却液间的金属壁厚度减薄到极致，同时采用特殊导流结构提高关键区域的冷却液流速。流体速度场以及光阑温度场的仿真结果显示光阑热交换能力明显提升。但热控的优劣仅靠仿真不足以评判，必须实测温度场。.焦点光阑无法直接测温；辐射测温或是唯一手段。即便拥有测温仪，测温亦非常困难。我们采用调制的方法对阳光进行控制，让光阑处于升&降温过程；在降温期间连续多帧测温，拟合降温曲线获得初始温度和温度常数，达到测温目的。.以上研究，我们已经在“云台2米环形望远镜”光阑上完成了部分验证。CGST与“2米环”F数一致，热负载功率密度相差不多；光阑设计思路可以推广，测温技术也可以移植。但由于聚光面积差异巨大，热负载功率相差甚远，CGST光阑热控需谨慎地辅以自动控制手段。

随着太阳物理学研究的发展，对下一代大型地基太阳望远镜提出了高时空分辨率、高磁场测量精度的要求。中国科学院云南天文台正筹划建造8米级CGST望远镜。大型太阳望远镜需要突破的其中一项关键技术是望远镜热控技术。本研究聚焦于望远镜热控最重要的部件热光阑，通过仿真给出了热光阑温控技术指标。接下来，设计了一种高效冷却结构的热光阑，该结构具有温升小、表面温度场分布均匀的优点；同时，对热光阑进行光学优化设计，抑制了杂散光的产生。最后，攻克了光阑测温技术难点，实现了光阑测温系统闭环控制，保证了控温精度。本文对大型地基太阳望远镜热控技术进行了全面而又深入的研究，把热致视宁度效应抑制在合理范围，保证望远镜观测成像质量。.热光阑的热致视宁度仿真基于CFD理论，选用大涡模型（LES）进行高时空分辨率的三维瞬态仿真，获得研究对象周围温度场的时空分布状况。之后，将温度场转化为折射率场，进行渐变折射率介质中的光线追迹仿真研究，评估热致视宁度。最终，热光阑温控指标为温升控制在10℃以内。.分析了现有热光阑存在的技术缺陷。设计了一种具备倒圆锥通光孔外形，并且采用导流翅片式高效冷却结构的热光阑；在冷却液压降损耗较低的情况下，获得了低温升，温度场分布均匀的温控效果。同时，对光阑进行了光学设计，反光面通光孔附近区域优化了倒圆锥夹角，倒圆锥以外区域采用对称式平面反射镜结构。仿真表明，光阑的热控设计与光学设计均符合指标要求。.光阑温度场测量具有重要意义，是实现温控系统闭环控制的关键。本文提出了两种测温方法，一种是调制法，对加热光源进行调制遮挡后用热像仪获得光阑表面降温曲线，再通过理论分析获得降温曲线的数学模型，拟合得到初始温度场。另一种是温差法，通过测量流经光阑的冷却液温升，依据冷却液温升与光阑温度场之间的映射关系，实时获得光阑温度场。