Wave-particle interaction investigated in a travelling wave tube

For wave-particle interaction studies, the one-dimensional beam-plasma system can be advantageously replaced by a travelling wave tube (TWT). This allowed us to carefully check the saturation mechanisms by the trapping of a cold electron beam, or by diffusion in the broad spectrum excited by a warm beam. While the former case is well understood, the latter case raises the question of the unsolved quasilinear paradox. When the beam intensity is low enough to ensure that beam-plasma instabilities are ruled out, a beam of test electrons is propagated in the TWT. By recording the test beam energy distribution at the output of the tube, we report the first experimental observation of the resonant domain of a single wave and of the overlap of the resonance domains responsible for the transition to large scale chaos common to many Hamiltonian systems. The fractal structure of phase space associated with this transition can also be explored.