Excitation of backward waves in forward wave amplifiers

The sources of coherent electromagnetic radiation based on amplification of traveling waves by electron beams have unique capabilities of wide band, high efficiency operation. The most critical issue in the development of such forward wave amplifiers is the self-excitation of parasitic backward waves in the interaction region. The developed theory allows one to analyze the nonlinear effect of suppression of parasitic backward waves by forward waves operating in the large-signal regime. The results of the analysis of traveling wave tubes (TWTs) and twystrons driven by linear electron beams and the results of the study of gyrodevices (gyro-TWTs and gyrotwystrons) driven by beams of gyrating electrons are presented. The gyrodevices are considered in the cases when the forward and backward waves are in resonance with electrons either at the first or second cyclotron harmonic. The results obtained allow one to estimate to what extent the coupling of an electron beam to the parasitic wave should be decreased in order to provide a stable, high efficiency amplification of the forward wave.