Abstract:
Background The confocal waveguide structure can effectively suppress mode competition due to its characteristic of reducing mode density through diffraction loss, thereby facilitating stable operation of gyro-traveling-wave-tube (gyro-TWT) amplifiers in the terahertz (more than 100 GHz) frequency range.
Purpose This study aims to conduct a comprehensive analysis of the diffraction loss rate (DLR) in a 220 GHz confocal waveguide gyro-TWT, employing a combination of theoretical analysis and three-dimensional particle-in-cell (3D-PIC) simulations.
Methods The research integrates field distribution theory with 3D-PIC simulations to investigate the DLR of the confocal waveguide. A non-ideal waveguide model incorporating the mirror width angle was utilized, and simulations were performed to evaluate beam-wave interaction dynamics under varying DLR conditions.
Results The study reveals that a low DLR induces gyro-backward-wave oscillation (GBWO) in low-order competing modes, while a high DLR significantly reduces beam-wave interaction efficiency, gain, and bandwidth, and lowers tolerance to electron beam velocity spread.
Conclusions For stable single-mode operation of the HE07 mode in the designed gyro-TWT, the DLR should not be less than 0.38 dB/cm, with the corresponding mirror-surface width angle not exceeding 47°. These findings provide crucial design guidelines for terahertz gyro-TWTs.
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