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

BackgroundRuthenium (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.PurposeThis 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.MethodsThe 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.ResultsNumerical 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.ConclusionsSimulation data and theoretical analyses presented in this study throw an insight into the thermal response of the optical material (Ru) to laser irradiation.