Cherenkov radiation in vacuum and plasma-filled microwave sources in the absence of guiding magnetic fields

A large number of sources of coherent electromagnetic (EM) radiation are based on the interaction of electrons with EM waves whose phase velocity is close to the electron velocity. Such an interaction may occur either when the waves propagate in the medium with a large enough dielectric constant or when the waves are excited in a periodic slow-wave structure. Very often the radiation in both these cases is called Cherenkov radiation. Usually, the sources of coherent Cherenkov radiation [traveling-wave tubes (TWTs) and backward-wave oscillators (BWOs)] are driven by linear electron beams. For guiding these beams, the external magnetic fields produced by either heavy and bulky solenoids or permanent magnets are used. In our paper we present the theoretical studies of vacuum and plasma-filled sources of coherent Cherenkov radiation (TWTs and BWOs) for the cases when the electrons exhibit a 3-D motion, either in the absence of external magnetic field (B/sub 0/ = 0) or in the case of weak B/sub 0/´s.