The temperature dependence of optical properties of tungsten in the visible and near-infrared domains: an experimental and theoretical study

Abstract

The knowledge of optical properties of tungsten at high temperatures is of crucial importance in fields such as nuclear fusion or aerospace applications. The optical properties of tungsten are well known at room temperature, but little has been done at temperatures comprised between 300 K and 1000 K in the visible and near-infrared domains. Here we investigate the temperature dependence of tungsten reflectivity from the ambient to high temperatures (<1000 K) in the 500-1050 nm spectral range, a region where interband transitions have a strong contribution. Experimental measurements, performed via a spectroscopic system coupled with a laser remote heating, show that the tungsten reflectivity increases with temperature and wavelength. We have described these dependences through a Fresnel and two Lorentz-Drude models. The Fresnel model reproduces accurately the experimental curve at a given temperature, but it is able to simulate the temperature dependency of reflectivity only thanks to an ad hoc choice of temperature formulae for the refractive indexes. Thus, a less empirical approach is preferred based on Lorentz-Drude models to describe the interaction of light and charge carriers in the solid. The first Lorentz-Drude model, which includes a temperature dependency on intraband transitions, fits experimental results only qualitatively. The second Lorentz-Drude model includes in addition a temperature dependency on interband transitions. It is able to reproduce quantitatively the experimental results, highlighting a non-trivial dependence of interband transitions as a function of temperature. Eventually, we use these temperature dependent Lorentz-Drude models to evaluate the total emissivity of tungsten from 300 K to 3500 K and we compare our experimental and theoretical findings with previous results.

The effect of surface temperature on optical properties of molybdenum mirrors in the visible and near-infrared domains

“…In particular the most evident effect of the temperature increase is to "flatten" the reflectivity spectrum. This effect has been already observed in the case of tungsten [21] and it has been explained through a different response of each Lorentz oscillators of the material to a temperature increase. 4.…”

Emissivity measurement of tungsten plasma facing components of the WEST tokamak

“…Tungsten and its emissivity have been studied from the beginning of the 20 th century [13][14][15][16][17][18] up to now [19][20][21][22][23][24][25]. All these studies have been performed on different kinds of samples depending on manufacturing process (roughness, grade…) and for different ranges of temperatures and wavelengths not always relevant for a direct use in fusion devices.…”

Emission of Fast Hydrogen Atoms in a Low Density Gas Discharge—The Most “Natural” Mirror Laboratory

“…We should point out that for some elements, the theoretical values of reflectance differed on the order of 10% or more. For instance, the most recent values for W demonstrated the reflectance of about 0.55, whereas the previous measurements showed values of 0.5 [28]. For Rh or Cu, the deviations between the different models of reflectivity were even more pronounced.…”