Studies of coherent velocity phase-space structures in traveling-wave tubes and their impact on transmitter performance

A study has been undertaken to investigate the real-time evolution of cold electron beam velocities from the small-signal regime to well beyond saturation in a vacuum microwave radiation source. A classic and readily available Lagrangian 1-dimensional particle-in-cell code has been modified to allow for the study of multi-carrier excitation and feedback oscillation conditions. Examples from amplifier and forced non-linear oscillator configurations are presented. The generation of coherent structures in the electron-beam transport is studied. These structures show similar properties to those investigated in other classes of beam-plasma systems. The relationship between these structures, the signal voltage fidelity, electron beam density, energy distribution function and space charge field are explored in the context of vacuum amplifier/transmitter applications. By analysis and trending of the electron velocity phase space diagrams we have observed the onset of wave-breaking, followed by turbulent behavior and chaotic transport in the various source configurations of interest.