Wave-particle interactions in a long traveling wave tube with upgraded helix

TL;DR

This study explores wave-electron beam interactions in a 4-meter traveling wave tube with an upgraded helix, combining theoretical modeling and experiments to understand linear and nonlinear effects, including wave growth, saturation, and electron trapping.

Contribution

It introduces a new theoretical model validated by experiments and analyzes nonlinear effects in a long TWT with an upgraded slow wave structure.

Findings

Linear theory accurately predicts wave growth at low beam currents.

Nonlinear space charge effects dominate at high beam currents, altering wave parameters.

Electron trapping occurs after saturation, affecting wave amplitude.

Abstract

We investigate the interaction of electromagnetic waves and electron beams in a 4 meters long traveling wave tube (TWT). The device is specially designed to simulate beam-plasma experiments without appreciable noise. This TWT presents an upgraded slow wave structure (SWS) that results in more precise measurements and makes new experiments possible. We introduce a theoretical model describing wave propagation through the SWS and validated by the experimental dispersion relation, impedance, phase and group velocities. We analyze nonlinear effects arising from the beam-wave interaction, such as the modulation of the electron beam and the wave growth and saturation process. When the beam current is low, the wave growth coefficient and saturation amplitude follow the linear theory predictions. However, for high values of current, nonlinear space charge effects become important and these parameters deviate from the linear predictions, tending to a constant value. After saturation, we also observe trapping of the beam electrons, which alters the wave amplitude along the TWT.