Within the framework of a quasi-optical approach, the nonlinear theory of relativistic surface-wave oscillators is developed. By presenting the radiation field as a sum of two counter-propagating wave-beams which are coupled on a shallow corrugated surface, we describe formation of an evanescent slow wave. Taking into account the excitation of a slow wave by a sheet electron beam, we simulate linear and nonlinear stages of interaction that allows us to define the threshold conditions, the electron efficiency, and the output coupling. It is shown that the considered type of an oscillator can be used for generation of powerful sub-THz radiation.

We propose planar relativistic surface-wave oscillators with two-dimensional periodic gratings. Additional transverse propagating waves emerging on these gratings synchronize the emission from the wide sheet rectilinear electron beam which allows realizing a Cherenkov millimeter wave oscillator with gigawatt output power.

We present the results of experiments on generation of high-power superradiant (SR) pulses by an extended electron bunch moving in a cylindrical surface-wave structure with two-dimensional periodical corrugation. In such a structure, azimuthally propagating wave fluxes are excited, which synchronize radiation from different azimuthal segments of a tubular electron bunch with a diameter greatly exceeding the wavelength. This facilitates a drastic increase in total electron current and, correspondingly, in SR pulses peak power while maintaining the azimuthal coherence of the radiation. In experiments carried out, subnanosecond 90 GHz SR pulses with a record-level peak power of up to 150 MW and an axially symmetric directional pattern were generated. The results of experiments are in line with the theoretical predictions based on the quasi-optical approach and direct particle-in-cell simulations.

We study, both theoretically and experimentally, a free-electron maser (FEM) with a two-mirror resonator that includes the advanced Bragg structure based on coupling of propagating and cutoff waves as an upstream reflector. Similar to gyrotrons, the presence of a cutoff wave in the feedback loop improves significantly the selectivity of the resonator at large values of the oversize factor and facilitates a stable single-mode oscillation regime in short wavelength bands. Proof-of-principle experiments conducted at the induction linac LIU-3000 (Dubna) demonstrate the operability of the FEM with the proposed modification of the Bragg resonator. At the transverse oversize factor of about 5, stable narrow-band generation was achieved with an output power of up to 7 MW and the oscillation frequency of about 80 GHz corresponding to the frequency of the cutoff mode excited in the advanced Bragg reflector. These investigations encourage the application of the advanced Bragg resonators for the development of high-power long-pulse FELs driven by induction linacs and operating in the subterahertz to terahertz frequency range.

We propose the method of providing megawatt (MW)-level output power in continuous wave (CW) or long-pulse second-harmonic (SH) gyrotrons using phase-locking by a weak external monochromatic signal. Typically, the power of harmonic gyrotrons is hindered due to excitation of spurious fundamental-harmonic modes at the front of the accelerating voltage and the beam current. Injection of a seeding signal at the operating SH mode (priming) can suppress the spurious generation during the startup process. Within the frame of a self-consistent multimode model, we demonstrate the applicability of this approach to provide ~1-MW radiation in a 230-GHz SH gyrotron with the high-order TE_34,14 operating mode which is standardly unattainable for harmonic operation.

It is shown that a periodic series of ultrashort pulses can be formed in electron microwave generators with a bleachable absorber in the feedback loop. The peak power of such radiation is considerably