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).