Mode selection in high power microwave sources

For microwave sources based on the stimulated radiation of electron beams, an attractive method to enhance the output power is expansion of the electron/RF-field interaction space. However the expansion densifies the spectrum of the system oscillation modes, originates their competition, thus aggravating the problem to keep the RF radiation coherent, In the RF electronics, the multi-mode problem was thoroughly studied, beginning with the papers of Van der Pol, for almost a century. An outstanding example of success based on a spurious mode suppression method is implementation of straps into the multi-cavity magnetron. For the same purpose, most of the classical electron microwave generators and amplifiers (TWTs, klystrons, EIAs etc) have a sectioned interaction space partially filled with absorbers. The innovations appearing in the field during recent decades are reviewed.