A Low-Loss Flatted-Grating Folded Waveguide TWT Based on High-Phase Operating Strategy

A low-loss flatted-grating folded waveguide (FGFW) slow wave structure (SWS) for terahertz (THz) traveling wave tubes (TWTs) based on a novel high-phase (HP) operating strategy is proposed in this article. Compared to the conventional low-phase (LP) operation of the fundamental mode, the HP strategy significantly enlarges structural dimensions while leveraging the advantages of FGFW. High-frequency analysis demonstrates that the HP-FGFW achieves weaker dispersion characteristics, improved beam-wave synchronization, and lower ohmic loss compared to LP-FGFW. The transmission characteristics verify the effectiveness of the low-loss characteristic of HP-FGFW for the TWT. Particle-in-cell (PIC) simulations predict improvements in the amplification performance and an increase in the 3-dB bandwidth of the HP-FGFW TWT. Meanwhile, the fabrication feasibility has been verified by the nano-CNC technique. A good agreement between simulated and measured transmission characteristics was achieved, with a conductivity of 2.0 S/m. Fabrication quality evaluation showed high-dimensional accuracy with deviations within $\pm 2~\mu $ m and low local surface roughness with a value of 0.105 nm. Overall, HP-FGFW provides a promising solution for high-power, broadband TWTs, effectively addressing the issues of fabrication complexity and ohmic losses.