Analytical Modeling, Simulation, and Cold Testing of a Radial SWS for THz Applications

This article presents the analytical design, simulation, and cold-test validation of a radial slow-wave structure for terahertz applications, addressing the limitations of conventional axial designs. Radial vacuum electron devices enhance the interaction area, reduce space charge effects, and enable compact, magnetic field-free operation, making them well-suited for high-power THz sources. A mathematical framework is developed to derive dispersion equations for the radial configuration, and numerical simulations to analyze the dispersion characteristics, external quality factor, and interaction impedance. A radial backward wave oscillator (BWO) is designed to validate the analytical model at 0.65 THz. Its performance is evaluated through particle-in-cell simulations, which model the interaction between the diverging radial sheet electron beam and the electromagnetic wave. The particle simulation confirms that the radial BWO operates without an external magnetic field, achieving a peak output power of 46.4 W at 0.651 THz using a 20.2-kV, 400-mA electron beam. A matched TEM-TE$_{10}$ mode converter is designed to extract the RF power efficiently for practical applications. The proposed BWO, comprising 40 periods and an integrated mode converter, is fabricated with precise surface finishing, dimensional accuracy, and compatibility with THz operational requirements. The cold test of the BWO with the mode converter shows an S$_{11}$ parameter of approximately −13 dB at the desired frequencies, closely matching the simulated results. These findings highlight the potential of radial BWO as compact, high-power THz sources, enabling stable and efficient operation for advanced applications.