Theory and New Amplification Regime in Periodic Multi Modal Slow Wave Structures with Degeneracy Interacting with an Electron Beam

TL;DR

This paper introduces a novel amplification regime in multimodal slow-wave structures for TWTs, leveraging mode degeneracy near band edges to enhance gain and reduce starting current.

Contribution

It develops a theoretical framework for multimodal TWTs using a Multi Transmission Line approach and identifies a new super synchronism condition involving four degenerate modes.

Findings

Identification of a new super synchronism condition with four modes

Observation of multiple cooperative growing modes near a band edge

Potential for higher gain and lower starting current in TWTs

Abstract

We present the theory of a new amplification regime in Travelling Wave Tubes (TWTs) composed of a slow-wave periodic structure that supports multiple electromagnetic modes that can all be synchronized with the electron beam. The interaction between the multimodal slow-wave structure and the electron beam is quantified using a Multi Transmission Line approach (MTL) based on a generalized Pierce model and transfer matrix analysis leading to the identification of modes with complex Bloch wavenumber. In particular, we address a new possible operation condition for TWTs based on the super synchronism between an electron beam and four modes exhibiting a degeneracy condition near a band edge of the periodic slowwave MTL. We show a phenomenological change in the band structure of periodic MTL where we observe at least two growing modal cooperating solutions as opposed to a uniform MTL interacting with an electron beam where there is rigorously only one growing modal solution. We discuss the advantage of using such a degeneracy condition in TWTs that leads to larger gain conditions in amplifier regimes and also to very lowstarting beam current in high power oscillators.