Mode selection in high power microwave sources

In designing a microwave generator or an amplifier of enhanced power, we always risk encountering self-excitation of unwanted oscillations. To suppress parasitic oscillation modes, we may implement a selective rf absorber, organize a selective feedback, taper the rf structure profile to admit only the main operation mode into the wave-electron synchronism band, etc. At a proper switching-on scenario, the main operation mode of the device can suppress its rivals at the nonlinear interaction stage. This brief review is aimed to systematize numerous methods efficiently used to keep the coherence in high power vacuum devices based on the stimulated radiation by electron beams.