基于有限元分析法的激光退火铜基铌薄膜温度场模拟

Simulation on Temperature Field of Laser-annealed Copper-based Niobium Thin Film by Finite Element Analysis Method

铜基铌薄膜腔是一种基于纯铌腔发展而来的射频超导腔。新型铜基铌薄膜腔具有与纯铌腔相似的射频超导性能,同时具备经济性、机械稳定性和热稳定性等方面的优势,因此在射频超导领域具有广泛的应用前景。然而,铜基铌薄膜腔的表面缺陷会引起额外的射频损耗,需要在投入使用前进行表面后处理以消除表面缺陷。纳秒级高功率脉冲激光浅退火是一种新兴的腔体表面处理技术,使用峰值功率流密度超过100 MW/cm2的高功率、短脉冲激光局部再结晶表面,可以减小材料表面粗糙度,增大晶粒尺寸并消除部分表面缺陷,以期获得更好的射频性能。目前,在这方面选择的激光器主要是具有高峰值功率的固体激光器,其平均功率非常低,在实际内壁表面积为m2级的超导腔体上不具有实际应用意义。为解决此问题,中国科学院近代物理研究所表面处理研究团队基于一台功率达kW级的纳秒脉冲光纤激光器建立了实用化的整腔激光处理系统。根据该系统的参数,利用有限元分析法模拟研究了接近实际工况下材料表层与内部的温度分布,并结合材料性质模拟了纯铌材料激光表面处理的效果。这项研究初步证实了使用纳秒级脉冲激光对超导腔内壁进行完整的退火处理是可行的,为实现更好的射频超导性能提供了有效的方法。

Copper-based niobium thin film cavity is one kind of superconducting radio frequency(SRF)cavities developed from bulk niobium cavity.This new type of cavity has similar SRF properties to the pure niobium cavities but is more economical,mechanically stable,and thermally stable,making it promising for industrial SRF applications.However,the inevitable surface defects of niobium film on the surface of Nb/Cu cavities induce extra power dissipation,and require post-treatment before cavity operation.Near-surface annealing of SRF cavity by nanosecond high-intensity pulsed laser is an emerging surface treatment technology for SRF cavity.Using high power and short pulsed laser with peak power flux exceeding 100 MW/cm2 to locally recrystallize the surface can reduce the surface roughness,increase the grain size and eliminates some surface defects,so as to obtain better surface RF properties.At present,the type of laser selected in this direction is mainly the solid-state laser with high peak power,which has a very low average power,not practical for the actual superconducting cavity with an m2 inner wall surface area.To solve this issue,a whole-cavity laser processing system based on a kW-level power nanosecond pulse fiber laser has been established by the Surface Treatment Research Team of the Institute of Modern Physics.In this study,finite element simulation was conducted with the parameters of the system to investigate the temperature distribution on the surface and inside the material under near-actual working conditions.Combined with the material properties,the laser surface treatment effect of pure niobium material was simulated.This study preliminarily confirms the feasibility of using nanosecond pulse laser for complete annealing of the inner wall of superconducting cavities,and provides an effective method for achieving better RF superconducting performance.