Boron Nitride and Sapphire Windows for 95-GHz Gaussian RF Beam

This paper presents the reflectivity and insertion loss characteristics of Boron Nitride (BN) and Sapphire RF windows for a 95-GHz Gaussian RF beam of a 100-kW gyrotron. The design of an RF window has been carried out using Fresnel's formula for reflection and transmission of Gaussian electromagnetic wave at the boundary between two media. The return loss of the RF windows of 95 GHz was found to be less than -46.8 dB for BN and less than -57.8 dB for Sapphire, and the transmission loss of the RF windows was better than -0.06 dB for BN and better than -0.01 dB for Sapphire. Furthermore, the heat load due to RF power absorption over the disk surface has been calculated. The Sapphire RF windows have been found with lower heat load and deflection, and higher tensile stress than that of BN. The allowable stress level for Sapphire is higher than that of BN; therefore, using Sapphire is structurally safer to use.