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

Abstract

We investigate the interaction of electromagnetic waves and electron beams in a 4 m long traveling wave tube (TWT). The device is specifically 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, and 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.

Time simulation of the nonlinear wave–particle interaction in meters-long traveling-wave tubes

“…More complete models exist where the helix is surrounded by dielectric layers 38,72,73 with relative permittivities r = 1 in the r ∈ [a, ∞[ region.…”

“…For this purpose, a huge 3 meter long TWT was built [32][33][34] at the University of California, San Diego in 1976, and 4 meter one at Aix-Marseille. [35][36][37][38] TWTs are also widely used for space telecommunication. 31 For example, two helix TWTs equip NASA's probe New Horizons (flyby of Pluto and Kuiper belt objects) to amplify signals before sending them to Earth.…”

An Upgraded 4 Meters Long Traveling Wave Tube for Plasma Physics Research

“…In [8], we introduced a theoretical model describing wave propagation through the different components of the SWS. The parameters estimated with this model (dispersion relation, phase velocity, and impedance) present excellent agreement with the experimental data.…”

“…In this paper, we work with a 4 meters long TWT [6]- [8] specially designed to simulate beam-plasma systems with low noise. Such device is presented in Fig.…”

“…Its helix is now rigidly wrapped in a dielectric polyimide tape to guarantee a nearly constant helix pitch all along the TWT. The upgraded SWS enables [8] much more precise measurements, the execution of new experiments, and the benchmarking of numerical models [9], [10].…”