Small-Signal Model for Inhomogeneous Helix Traveling-Wave Tubes using Transfer Matrices

TL;DR

This paper presents a transfer matrix-based small-signal model for inhomogeneous helix TWTs that incorporates realistic dispersive SWS characteristics and multi-stage configurations, validated against PIC and LATTE simulations.

Contribution

It generalizes the Pierce model to account for inhomogeneity and dispersion in helix TWTs, including multi-stage modeling with simplified lumped element circuits.

Findings

Model accurately predicts gain versus frequency.

Reproduces gain ripple effects due to port mismatches.

Validated against PIC simulations and LATTE code.

Abstract

Abstract We introduce a practical method for modeling the small-signal behavior of frequency-dispersive and inhomogeneous helix-type traveling-wave tube (TWT) amplifiers based on a generalization of the one-dimensional Pierce model. Our model is applicable to both single-stage and multi-stage TWTs. Like the Pierce model, we assume that electrons flow linearly in one direction, parallel and in proximity to a slow-wave structure (SWS) which guides a single dominant electromagnetic mode. Realistic helix TWTs are modeled with position-dependent and frequency-dependent SWS characteristics, such as loss, phase velocity, plasma frequency reduction factor, interaction impedance, and the coupling factor that relates the SWS modal characteristic impedance to the interaction impedance. For the multi-stage helix TWT, we provide a simple lumped element circuit model for combining the stages separated by a sever, or gap, which attenuates the guided circuit mode while allowing the space-charge wave on the beam to pass freely to the next stage. The dispersive SWS characteristics are accounted for using full-wave eigenmode simulations for a realistic helix SWS supported by dielectric rods in a metal barrel, all of which contribute to the distributed circuit loss. We compare our computed gain vs frequency, computed using transfer matrices, to results found through particle-in-cell (PIC) simulations and the 1D TWT code LATTE to demonstrate the accuracy of our model. Furthermore, we demonstrate the ability of our model to reproduce gain ripple due to mismatches at the input and output ports of the TWT.