超导铌腔表面均匀化学蚀刻工艺的优化及实验研究

Superconducting niobium cavities, which have several advantages including small power dissipation compared to copper cavities, are important component of the advanced accelerators. To ensure the success of the niobium cavities, they are subject to chemical etching process, and researchers have concluded that buffered chemical polishing on the inner surface of the cavity improves its performance. But the distribution of removal thickness is not uniform. The iris section of the cavity received more etching than the equator regions due to the geometry limitation of the elliptical cavities, especially in multi-cell cavities. In order to eliminate this problem, a finite element computational fluid dynamics model was developed to simulate the fluid flow characteristics of chemical etching process inside the 500MHz 5-cell cavity. To improve the performance of the etching process, the baffle, which directs flow towards the walls of the cavity, is created in terms of all its possible variables and a parametric study will be done to identify the variables that play a vital role in the etching process. Then the baffle was designed using optimization techniques. The real etching process in the practical experiments will be studied with the optimized baffle using circulating fluid, and the results would be compared with the results of the initial without baffle structure results.

超导铌腔作为核心部件广泛应用于世界新型加速器上，超导铌腔内表面的化学蚀刻处理工艺是超导腔研制过程中极为重要的环节，获得洁净平滑的超导腔内表面是保证超导腔在加速器中长期稳定运行的先决条件。但由于受到腔体的几何形状限制，超导腔各部分内表面的蚀刻厚度并不均匀一致，往往在腔体Iris处出现较大的蚀刻，而腔体Equator部分的蚀刻厚度不足，尤其对于多cell超导腔，该问题更加严重。为了解决蚀刻厚度不均匀的问题，本项目将开展500MHz 5-cell超导铌腔内表面均匀化学蚀刻的基础研究，通过利用流体动力学模拟软件对超导腔内酸液蚀刻过程进行模拟，并通过优化设计折流板的形状尺寸以使得腔内表面酸液流速趋于一致，合理设计折流板在超导腔中的实际安装结构，最后完成5-cell超导铌腔的均匀化学蚀刻的实验研究。

超导高频铌腔是先进加速器中用于加速粒子及补充粒子能量的核心部件，而表面处理工艺是超导腔研制过程中最为关键的环节，往往决定了超导腔最终性能的高低。由于受到超导腔体几何形状的限制，使得在化学抛光过程中腔体内表面蚀刻厚度不均匀，特别是影响腔体性能的赤道处蚀刻厚度严重不足，如何获得均匀蚀刻厚度成为本项目研究的主要内容。本课题首先采用ANSYS FLUENT软件对500MHz 5-cell超导腔体酸液蚀刻过程进行了流体模拟仿真计算，得出添加折流板设计的必要性；其次对折流板尺寸进行了变量参数化研究，并通过参数优化确立了获得最佳腔壁流速的折流板尺寸，通过采用优化后的折流板相比无折流板情况下腔体内边界平均流速提高约5倍，平均流速的标准偏差降低约2倍；最后根据模拟仿真优化结果设计了折流板安装于超导腔胞内的机械结构，并通过1.5GHz单cell超导腔化学抛光实验进行了验证，腔体赤道处的蚀刻厚度比无折流板情况下提高约3.5倍，同时腔胞内的蚀刻厚度均有提高。此外，基于本课题，额外还完成了超导腔化学抛光封闭式循环系统装置的升级改造工作以及单cell超导腔化学抛光磁搅拌装置的升级改造工作，同时也还开展了金属铌试样的电解化学抛光实验研究工作。. 该项目的研究成果一方面可为我国超导铌腔的化学表面处理工艺形成工业标准化提供一定的技术参考，同时所开展的金属铌试样电解化学抛光基础研究成果与该项目中的腔体折流板优化设计技术相结合，可对超导腔的电抛光技术提供一种新的思路。