Systems that generate and/or transmit high-frequency electromagnetic radiation often require a window that is transparent over a particular frequency range. To accommodate high power levels, the window may be highly transparent to the passing radiation, absorb and/or reflect little of the transmitted power, and present a low thermal resistance path to heat generated within the window by any absorbed radiation. At millimeter-wave frequencies, the loss tangents of many materials commonly used for windows at lower frequencies become much higher, reducing the effectiveness of such materials at millimeter-wave frequencies.
Synthetic diamond has emerged as a preferred window dielectric material in millimeter-wave applications. This is especially true in instances where there is an extremely high power density millimeter wave, such as at the output windows of gyrotron oscillators that produce outputs in excess of 1 MW. Although synthetic diamond has a low loss tangent at millimeter-wave frequencies and a thermal conductivity higher than copper, it is expensive and often available only in limited sizes. In applications where the size of the window needs to be greater than a few inches across, synthetic diamond becomes cost prohibitive.
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