Competition of longitudinal modes and the scenario of single-mode regime build-up for the FOM-Fusion-FEM project

The temporal dynamics of excitation of a free-electron maser with a prolonged and low-Q microwave system is studied taking into account the feedback frequency dispersion, the HF space charge, and the undulator tapering. It is shown that in spite of a great many longitudinal modes excited at the small-signal stage, even a rather broadband feedback dispersion significantly shortens the transient time and provides a stable single-frequency operating regime.