Resonant Atmospheric Plasma-Sprayed Ceramic Layers Effectively absorb Microwaves at 170 GHz

Abstract

Microwave absorbing layer materials (MALMs) are extremely important for many components in fusion reactors to absorb microwave radiation in a controlled manner and with predictable power density. Therefore, a detailed knowledge of absorption properties of absorber coating materials used is necessary. Plasma-sprayed mixed oxide coatings are most commonly used in those applications where moderate power density is expected. In this paper, a plane wave absorption model is presented using refractive index and absorption coefficient as internal parameters and incidence angle, polarization, and layer thickness as external parameters. The model has been calculated assuming radiation of 170 GHz, as envisaged for the ITER research facility. Three atmospheric plasma-sprayed coating materials were considered in this work: titanium dioxide (TiO2), chromium oxide (Cr2O3), and a mixed aluminum-titanium oxide Al2O3-TiO2 (40/60). Theoretical results are compared with free wave measurements with two antennas. Different coating thicknesses have been prepared and measured in different polarization and incidence angles. Results are discussed regarding polarization, incidence angle, layer thickness, absorption coefficient, and refractive index.

MISTRAL campaign in support of W7-X long pulse operation

“…These modelling results together with absorbed fractions of stray radiation power measured (section 4.3) are consistent with a scan over fixed angles by Hentrich [12]. This allows to conclude that the absorbed fraction of power using stray radiation exposure is not significantly affected by the angular distribution of the stray radiation.…”

“…This shows, besides the effect of the Fresnell equations, little variation with incidence angle. This is explained, as also noted by Hentrich [12], by only small differences in optical path due to the small refracted angels given the high relative permittivity.…”

“…of a microwave absorbing coating on metal surfaces [10][11][12]. This is practical as a thin layer can applied with Atmospheric Plasma Spraying onto walls or pipes, providing a heat sink at the same time.…”

“…For instruction only normal incidence is shown but the full model includes oblique incidence. An additional change, following Hentrich [12], is the using the complex permittivity in computation of the refracted angle in the layer as opposed to taking only the real part. This has been applied too for computation of the layer impedance.…”

“…This is required as, contrary to low-loss vacuum windows, the imaginary part of the complex permittivity may no longer be much smaller as the real part. The coating used in this work is an Al 2 O 3 /TiO 2 coating [10][11][12], mixed to ratio 87% / 13%. For this coating Hentrich [14] extracted a 0.25λ resonance thickness of 165 µm at 140 GHz.…”