X射线自由电子激光与金属钌相互作用双温模型的数值分析

Numerical analysis of X-ray free-electron laser interaction with metal ruthenium using the two-temperature model

金属钌(Ru)具有优异的力学性能和化学稳定性,同时具有高反射率、较大临界角等优异性能,是自由电子激光装置中光学元件的候选镀层材料之一。本文对X射线自由电子激光(X-ray Free-electron Lasers,XFEL)脉冲与金属钌薄膜相互作用过程进行了模拟,采用数值方法求解双温模型(Two-temperature Model,TTM),得到了电子和晶格温度随时间的演化规律。获得激光的脉冲宽度、能量密度、穿透深度、材料的电声子耦合系数等对激光与金属钌相互作用过程中热效应的影响规律。通过对X射线自由电子激光脉冲与金属相互作用过程的双温模型数值解,可以获得材料中电子和晶格温度随时间的演化规律,并探究光源的参数对其热传输过程的影响。研究发现,金属钌晶格平衡温度与能量密度呈正相关;金属钌电子最高温度随脉冲宽度的增加而降低;金属钌的晶格平衡温度随穿透深度的减小而增加,最终趋于稳定;电子-声子平衡时间与电声子耦合系数呈负相关。本文获得X射线自由电子激光脉冲与金属钌相互作用的相关数据及基本演化规律,有助于进一步理解金属钌在激光辐照作用下材料的热效应演化机制,为自由电子激光装置光学元件薄膜材料的选取及辐照性能分析提供理论依据。

[Background]Ruthenium(Ru)exhibits outstanding mechanical properties and chemical stability,along with excellent optical properties such as high reflectivity and a larger critical angle.These attributes make it a good candidate coating material for optical components in X-ray free electron laser facilities.[Purpose]This study aims to analyze the fundamental thermal evolutions of the Ru electron and lattice subsystems under irradiation of X-ray free electron laser pulses for better understanding of the laser-optics interaction mechanism under operating conditions.[Methods]The numerical method was adopted to solve the two-temperature model(TTM)in terms of simulating the interaction process between X-ray free electron laser pulses and thin ruthenium films.The laser energy incident on the Ru target material was the actual energy absorbed by the material and the pulse laser incidence was in the x-direction with a film thickness of 50 nm,spatial discretization step size of 0.2 nm and the time step size of 0.02 fs.Simutanously,the influence of different laser source parameters,such as pulse width,penetration depth,energy density,etc.,on the thermal effects during the interaction between laser and metal Ru was explored,and the temperature evolutions of the Ru electron and lattice subsystems were obtained,as well as the corresponding dependency relationships of the heat transfer processes on source parameters.[Results]Numerical solution of the TTM indicate that the system equilibrium temperature of Ru is identified to be positively correlated with energy density.The peak electron temperature of Ru decreases with the increasing of pulse width.Besides,as penetration depth increases,the equilibrium temperature of Ru will decrease until a stable level.As for the front surface,the time taken to achieve electron-lattice equilibrium decrease with the increasing of electron-phonon coupling coefficient.[Conclusions]Simulation data and theoretical analyses presented in this study throw an insight into the thermal response of the optical material(Ru)to laser irradiation.