Excitation of parasitic waves near cutoff in forward-wave amplifiers

There is a strong interest in developing high-power amplifiers in the W-band and at shorter wavelengths. To realize high-power operation at very short wavelengths the radiation sources should operate at high-order modes/waves. Such waves have rather dense spectrum of eigenfrequencies. Therefore the operation stability becomes a serious issue. In forward-wave amplifiers (such as traveling-wave tubes) the waves which can be excited near cutoff frequencies are among the most dangerous competitors. The excitation of such waves in the absence of the signal wave was analyzed elsewhere.