Power absorption by thin films on microwave windows

A recent theory for power absorption by continuous thin surface films on microwave windows is compared with experimental data, focusing on diamond gyrotron windows. Two measurement techniques, the dc method by Jory and the radio-frequency (RF) method by Heidinger et al., are used to estimate the fraction of the average power absorbed by the thin films, assuming that these films are spread uniformly over the window surface. Both measurements indicate that, on the average, about a fraction of 1% of the incident power is absorbed by the thin films, even though the local absorption coefficient may reach 50%. Discontinuous surface films, where the surface contaminants have a patchy or island structure, are modeled with an equivalent transmission line circuit. The discontinuous film model is also applied to alumina windows with TiN coatings, and the results are compared with the continuous film model.