  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.
