A Broadband Sapphire Disc Pillbox Window for Terahertz Traveling-Wave Tubes

In this paper, to develop a terahertz traveling-wave tube (TWT), a broadband sapphire disc pillbox window is investigated. The transmission line theory is used to analyze its transmission characteristics. To avoid a lot of simulation calculations and improve the design efficiency, the simulated annealing algorithm is employed to provide a rapid design scheme of the pillbox window. On the basis of the initial design scheme, the CST software is used for further optimization and the final design scheme is determined quickly. The simulated results show that the sapphire disc pillbox window has good transmission and low reflection in a broad terahertz band. The pillbox window is fabricated and tested. The preliminary experiment results show that the amplitude of $\mathbf{S}_{21}$ is better than −4.5 dB from 200–240 GHz, and the amplitude of $\mathbf{S_{11}}$ is less than −10 dB. This broadband pillbox window studied here will be applied to the terahertz TWTs.