Frequency-tunable relativistic coaxial backward-wave oscillator with a modulating reflector

The theoretical and numerical analyses of a relativistic backward-wave tube with a coaxial slow-wave structure and modulating reflector are carried out. The incident working TEM mode reflects because of a coaxial TM$_{01}$ mode excited in the reflector at a near-cutoff frequency. It is shown that the longitudinal component of the electric field present near the reflector provides the energy premodulation of an electron beam and conditions for oscillation frequency tuning. The mechanical tuning of the oscillation frequency of a relativistic coaxial backward-wave oscillator with a modulating reflector within 10% is demonstrated in numerical simulation and experiment. Tuning is accomplished by varying the drift length between the reflector and the entrance to the slow-wave structure of the oscillator.