At higher frequencies it is not easy to find a suitable material for output window applications in a high power gyrotron. The room temperature application of sapphire as a window material at higher frequencies is not feasible since its absorption coefficient increases almost linearly with increasing frequency in the millimeter wavelength region. At cryogenic temperature the absorption coefficient value decreases only by a few factors (factor of 2 to 3) in the 90-200 GHz region. The earlier reported temperature squared dependence (decrease) in the absorption coefficient of the loss tangent value is totally absent in our broad band continuous wave data we are reporting here (at 6.5 K, 35 K, 77 K, and 300 K). We utilize our precision millimeter wave dispersive Fourier transform spectroscopic techniques at room temperature and at cryogenic temperatures. The extra high resistivity single crystal compensated silicon is no doubt the lowest loss material available at room temperature in the entire millimeter wavelength region. At higher millimeter wave frequencies an extra high resistivity silicon window or a window made with extra high resistivity silicon coated with diamond film would certainly make a better candidate in the future. A single free standing synthetic diamond window seems to have higher absorption coefficient values at millimeter wavelength region at this time although it is claimed that it possesses good mechanical strength and higher thermal conductivity characteristics. It certainly does not rule out the use of diamond film on a single crystal high resistivity silicon to improve its mechanical strength and thermal conductivity.
Mohammed N. Afsar,
"Window materials for 110-GHz and 280-GHz gyrotron", Proc. SPIE 2211, Millimeter and Submillimeter Waves, (19 August 1994); doi: 10.1117/12.183036; https://doi.org/10.1117/12.183036