Dumbbell-Shaped Slot Resonator Slow Wave Structure With Metamaterial Properties for a W-Band Traveling-Wave Tube Amplifier

This article proposes a novel metamaterial-inspired slow wave structure (SWS) implemented as a periodical array of dumbbell-shaped slots inside a metal plate located in a rectangular waveguide. Analytical formulas for calculating the resonance frequency of the dumbbell-shaped slot are derived. It is demonstrated that when the resonance frequency is below the cut-off frequency of the waveguide, the SWS exhibits the properties of a double-negative metamaterial. Theoretical analysis of the dispersion characteristics of the proposed SWS is performed on the basis of the Drude–Lorentz formulas for effective permittivity and permeability of the metamaterial. Theoretical considerations are verified by 3-D finite-element numerical calculations of the electromagnetic parameters. Through the special selection of the dimensions of the dumbbell-shaped slot, the SWS can demonstrate a high slow-down factor, high interaction impedance, and low ohmic loss. The proposed SWS is designed for application in a W-band traveling-wave tube amplifier with 8.5-kV double sheet electron beam. 3-D particle-in-cell (PIC) simulation of the beam–wave interaction in the SWS driven with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 75$</tex-math> </inline-formula> mA electron beam predicts gain higher than 30 dB in the 92–96 GHz frequency range. At saturation, the output power can exceed 50 W at input signal power of 50–100 mW.