One of a family of microwave generators, also called cyclotron resonance masers, in which cyclotron resonance coupling between microwave fields and an electron beam in vacuum is the basis of operation. This type of coupling has the advantage that both the electron beam and the associated microwave structures can have dimensions which are large compared with a wavelength. Thus, cyclotron resonance masers are potentially greatly superior to conventional microwave tubes with respect to power capability at short wavelengths.