RF and Thermo-Mechanical Examinations of Output Windows for W-Band Gyrotron

The reflection and transmission characteristics of edge-cooled single-disk output radio frequency (RF) windows of 95-GHz 100-kW gyrotron were studied. RF characteristics of output windows made off sapphire (rhombohedral, single crystalline), silicon nitride composite, and pyrolytic boron nitride (poly crystalline) (PBN) were compared. At the operating frequency, the return losses of 64.0, 60.0, and 50.0 dB were obtained, respectively, for sapphire, silicon nitride composite, and PBN while the related transmission losses turned to be, respectively, better than -0.01, -0.02, and -0.04 dB. The heat load due to RF power absorption in the disk was least for sapphire and maximum for PBN window. The structural analyses of these RF windows predicted the least deflection and least tensile stress for silicon nitride and PBN windows, respectively. Comparable bandwidths were offered by silicon nitride window (790 MHz) of thickness 1.13 mm and PBN (773 MHz) of thickness 2.19 mm. However, a major safety margin and thus structural integrity was offered by the silicon nitride (factor of safety 30.6) window in comparison to PBN (factor of safety 10.6).