We demonstrate the possibility of generating periodical trains of ultrashort microwave pulses based on the effect of passive mode-locking in a scheme with two parallel coaxial electron beams formed by a single cathode and transported in a single vacuum volume. The external tubular beam provides amplification of radiation during rectilinear motion along a periodic slow-wave structure, while the internal paraxial beam provides nonlinear cyclotron absorption.

We theoretically study the acceleration of electrons by a 38-GHz subnanosecond superradiance pulse with a 3-GW peak power in a cylindrical waveguide with an annular dielectric insert. It is shown that the energy gain and the accelerating gradient depend on the dielectric constant of the insert, namely, the energy gain decreases and the maximum accelerating gradient increases with increasing the dielectric constant. For an electron with an initial energy of 300 keV, the maximum accelerating gradient reaches 120 MV / m with an energy gain of 7.2 MeV in a waveguide with a polystyrene insert ( = 2.53) and 220 MV / m with an energy gain of 4.5 MeV in a waveguide with a quartz insert ( = 3.81).

Selective unidirectional excitation of a cylindrical waveguide through the side wall has been studied in application to the signal entry of a millimeter-wave high-current relativistic cherenkov amplifier. The found ratio of the input structure parameters ensures maximum transfer from the fundamental mode of the rectangular waveguide to the unidirectional lowest circular mode of the cylindrical waveguide. The transfer efficiency in absence of ohmic loss reaches 97.5%. The input structure adapted to the 35.5 GHz-amplifier construction has been realized with an efficiency of 75%.