Experimental measurements of coherent stimulated radiation from intense, subnanosecond electron bunches moving through a periodic waveguide and interacting with a backward propagating TM01 male are presented. The ultra-short microwave pulses in Ka, W, and G band were generated with repetition frequencies of up to 25 Hz, Observation of RF breakdown of ambient air, as well as direct measurements by hot-carrier germanium detectors, gives an estimate of the peak power up to 140 MW for the 300-400 ps pulses at 38 GHz. The initial observation of 75 GHz 10-15 MW radiation pulses with duration less than 150 ps, and of 150 GHz microwave spikes with a risetime of 75 ps are also reported
We study locking of the oscillation frequency of the operating TE 28,12 mode by an external monochromatic signal in a multimode gyrotron operated at a frequency of 170 GHz in the switchon regime close to the real one. Locking zones, i.e., regions of single-mode generation at the external-signal frequency are found on the "current-detuning" plane of parameters. It is shown that as the number of competing modes increases, the maximum achievable current decreases, and the locking zones contract at sufficiently high currents.
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