Theoretical and Experimental Study of Over-Mode Windows for High-Frequency TWTs

In the terahertz (THz) frequency range, the dimensions of microwave windows in traveling-wave tubes (TWTs) decrease significantly, leading to increased transmission loss and processing difficulties. The reduced size of the microwave window poses challenges in terms of fabrication, assembly, and alignment, while the small thickness of the diamond window disk adds complexity to the metallization and vacuum sealing. To address these issues, in this article, the over-mode window has been proposed, which consists of two transmission parts and one resonant cavity. This over-mode window can enlarge the window disk, enhance power capacity, and improve sealing reliability together with the small transmission loss. Unlike the pill-box window, the over-mode window utilizes oversized waveguides that have the capacity to transmit high-order modes. In this configuration, the number of modes in the window disk typically reaches several hundreds. The high-order modes in the oversized waveguides are analyzed using the theory of coupling through small apertures, and the theory has been modified to calculate the coupling strength between different modes of square coupling holes. Test results indicate that at 0.22 THz, the over-mode window matches well within the 12.7-GHz bandwidth, with a lower total loss of 0.8 dB compared to the pill-box window. Furthermore, the test results of the over-mode window together with the high-frequency structure of the TWT are satisfactory, and the over-mode window operating at the 1-THz frequency was designed.