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

Abstract

Molybdenum mirrors will be used in several optical diagnostics to control the plasma in the ITER tokamak. In this harsh environment, mirrors can undergo transient temperature rises. Thus the knowledge of the temperature dependence of optical properties of molybdenum is necessary for a good operation of optical systems in ITER. Molybdenum optical properties have been extensively studied at room temperature, but little has been done at high temperatures in the visible and near-infrared domains. We investigate here the temperature dependence of molybdenum reflectivity from the ambient to high temperatures (<800 K) in the 500-1050 nm spectral range. Experimental measurements of reflectivity, performed via a spectroscopic system coupled with laser remote heating, show a maximum increase of 2.5 % at 800 K in the 850-900 nm wavelength range and a non-linear temperature dependency as a function of wavelength. We describe these dependencies through a Fresnel and a Lorentz-Drude model. The Fresnel model accurately reproduces the experimental curve at a given temperature by using a parabolic temperature dependency for the refractive index, n, and a linear dependency for the extinction coefficient, k. We develop a Lorentz-Drude model to describe the interaction of light with charge carriers in the solid. This model includes temperature dependency on both intraband (Drude) and interband (Lorentz) transitions. It is able to reproduce the experimental results quantitatively, highlighting a non-trivial dependency of interband transitions on temperature. Eventually, we use the Lorentz-Drude model to evaluate the total emissivity of molybdenum from 300 K to 2800 K, and we compare our experimental and theoretical findings with previous results.

Impact of Different Metals on the Performance of Slab Tamm Plasmon Resonators

“…The high melting point of Mo facilitates the application of the STP resonator as emitter. It has to be pointed out that the optical properties of Mo are based on the data of Ordal et al [34], which are in agreement with Minissale et al [44]. However, the values for the negative real part of the permittivity reported in [5] are in sharp disagreement with the previous references, which would result in much lower values for the reflectance and FOM Q .…”

“…Notably, the value for the reflectance is nearly 20% and FOM Q is approximately an order of magnitude lower compared to the metals (at 0 = 4.26 µ m). Despite the significantly impaired optical properties and the additional airgap, similar values of ( 0 ) compared to the W-based STP structures of up to 0.82 (depending on It has to be pointed out that the optical properties of Mo are based on the data of Ordal et al [34], which are in agreement with Minissale et al [44]. However, the values for the negative real part of the permittivity reported in [5] are in sharp disagreement with the previous references, which would result in much lower values for the reflectance and FOM Q .…”

“…However, the values for the negative real part of the permittivity reported in [5] are in sharp disagreement with the previous references, which would result in much lower values for the reflectance and FOM Q . It can be assumed that this discrepancy is related to the characteristics of the surface profile of the Mo film, as carefully processed molybdenum mirrors are proven to show very high reflectance [45], in agreement with [34,44] for the mid-IR region. As these references specifically focus on studying the optical properties of Mo, we choose to use a Lorentz-Drude model with the optical parameters of [34].…”

“…Notably, the high values for the reflectance and FOM Q of Mo, enabling resonances with Q factors of~0.75, together with the excellent mechanical properties and high melting point, suggest Mo to be a promising compromise between Ag and W. For the case of a resonant absorber (where Mo is at room temperature), an additional layer may be beneficial in order to maximize the values for a(λ 0 ). In the case of an application as a thermal emitter (where Mo is at elevated temperatures), the reflectance and FOM Q decrease according to various temperature models ( [44,46]) and, subsequently, an STP structure composed of four layers is sufficient for a good impedance match.…”

Исследование Объемного Радиационного Переноса Тепла В Кристалле И Расплаве С Помощью Численного Моделирования При Выращивании Кристаллов Сапфира Методом Степанова

“…Значение коэффициента черноты границы расплав−формообразователь берется равным 0.4, так как при расчетах предполагалось, что формообразователь выполнен из молибдена. Коэффициент черноты молибдена для зеркальной поверхности при температуре плавления сапфира в газовой атмосфере составляет величину 0.25 [19]. По оценкам радиационных свойств материалов с помощью электромагнитной теории для границы с оптически более плотной средой (расплав−сапфир), а также для шероховатой поверхности коэффициент черноты должен быть больше.…”