Traveling Wave Tube Eigenmode Solver for Interacting Hot Slow Wave Structure Based on Particle-In-Cell Simulations

TL;DR

This paper introduces a novel eigenmode solver for traveling wave tubes that uses particle-in-cell simulations to accurately characterize the interaction between the electron beam and electromagnetic fields in hot slow-wave structures.

Contribution

The method combines transfer matrix analysis with Floquet theory to determine complex eigenmodes in hot SWS, applicable to various geometries and validated against Pierce theory.

Findings

Accurately computes complex wavenumbers of hybrid modes in TWTs.

Shows dispersion relations consistent with Pierce theory.

Demonstrates applicability to different SWS geometries.

Abstract

A scheme to characterize the dynamics of the electron beam-electromagnetic power exchange along a traveling wave tube (TWT) is proposed. The method is based on defining a state vector at discrete periodic locations along the TWT and determining the transfer matrix of the unit-cell of the "hot" slow-wave structure (SWS) that takes into account the interaction between the electromagnetic guided field and the electron beam via particle-in-cell (PIC) simulations. Once the estimate of the unit-cell transfer matrix is obtained, we show how to find the hybrid, beam-electromagnetic, eigenmodes in the hot SWS, i.e., where the electromagnetic guided field interacts with an electron beam, by using Floquet theory. In particular, we show how do determine the complex-valued wavenumbers of the hybrid modes and the eigenvectors associated to them. The method is applied to find the hot modes with complex wavenumber that can be supported in a TWT amplifier with a helix SWS. We show dispersion relations of the modal complex wavenumbers of the hybrid modes when varying frequency and beam voltage; the results are in agreement with Pierce theory. The method is also applied to find the complex-wavenumber modes in a hot SWS of a millimeter wave TWT amplifier based on a serpentine waveguide. The technique is general and can be applied to any SWS geometry where electromagnetic modes interact with an electron beam.