R. M. Rozental

Nonstationary processes in an X-band relativistic gyrotron with delayed feedback

This paper describes experimental and theoretical studies of nonstationary processes in a relativistic X-band gyrotron with delayed feedback. For the first time, the chaotic self-modulation regimes were realized with the power level exceeding 2 MW and efficiency up to 17%. The full particle-in-cell simulations of the self-modulation regimes have been carried out. The period-doubling pattern of the transition from the periodic to chaotic self-modulation regimes have been observed both in experiment and simulations.

On the synthesis of radiation spectrum in a sectioned relativistic backward wave tube

We have studied the possibility of controlling the radiation spectrum of a high-power relativistic backward wave tube (BWT) by varying the period of its delay system. For a BWT comprising several sections with different corrugation periods, two-and three-mode generation regimes with commensurate amplitudes of spectral components have been found, the separation of which can be controlled by changing the corrugation period. It is shown that, for a sufficiently large detuning, such a configuration of the interaction space makes possible the generation of noiselike chaotic signals with separated or overlapping spectra. In the latter case, the width of the radiation spectrum is almost twice that of a regular BWT of the same length.

Amplification of ultrashort electromagnetic pulses propagating along quasi-continuous electron beams

Specific features of amplification of short electromagnetic pulses propagating along steady-state nonequilibrium electron flows with a group velocity differing from the translational velocity of particles are analyzed. It is shown that an amplitude level substantially higher than the saturation level in amplification of quasi-continuous signals can be attained by permanent injection of electrons without initial modulation to one of the pulse fronts. The effective duration of the pulse being amplified is reduced simultaneously. The Cherenkov and undulator interaction mechanisms are considered. Analysis is carried out using a simple 1D model based on the averaged description of the electron-wave interaction, as well as direct numerical simulation based on the KARAT code taking into account the parameters of planned experiments on observation of this effect.

Improvement of Stability of High Cyclotron Harmonic Operation in the Double-Beam THz Gyrotrons

A double-beam scheme of a short-wavelength gyrotron operating at the second cyclotron harmonic is studied both within the framework of the averaged self-consistent approach and using 3-D particle in cell simulations. The analysis shows that the introduction of an additional generating electron beam allows drastically increasing the operating current of a second-harmonic gyrotron with the simultaneous suppression of self-excitation of spurious modes at the fundamental harmonic. As a result, the radiated power of a double-beam gyrotron exceeds the power of a single-beam gyrotron by a factor of four. The developed concept makes it possible to realize high-power (several hundred watts) single-mode gyrotrons in the 0.7-1.0-THz frequency range.

Dual band operation of the relativistic BWO

The possibility of dual-band signal using relativistic backward wave oscillator with a slow-wave system excited by a single beam was described. We demonstrate that a regime of generation at several close frequencies with the same transverse field structure can be rather simply realized in a sectioned system with a stepwise variation of mismatch between the beam and synchronous wave. In this case of high-power relativistic BWO with the microwave system representing sections of a corrugated waveguide, a change of synchronism conditions can provided by variation of the corrugation period.

Self-modulated generation observed in a delayed feedback relativistic microwave gyrotron

Self-modulated generation regimes were studied in a delayed feedback relativistic gyrotron operating on the H01 mode with a central frequency of 9.2 GHz. At a fixed electron beam energy of 230 keV, an increase in the electron beam current from 10 to 45 A led to the transition from a stationary to periodic self-modulated generation regime. The modulation period was about 16 ns, while the relative amplitude of the modulation increased in proportion to the beam current, reaching up to 90%. The microwave pulse duration exceeded 6 μs at an average power of up to 1 MW. The experimental data obtained agree well with the results of simulation using the PIC code KARAT.

Chaotic millimeter wave generation in a helical-waveguide gyro-TWT with delayed feedback

We demonstrate the possibility of chaotic millimeter wave generation in broadband helical-waveguide gyrotron travelling wave tubes (gyro-TWTs) by introducing external delayed feedback. It is shown that for the realization of “developed” chaos the amplitude characteristic of the amplifier should have the maximum slope in the overdrive regime upon saturation. This can be achieved by proper choosing of cyclotron resonance detuning. According to the time-domain averaged model and 3D particle-in-cell simulations with the parameters of the experimentally realized 35 GHz gyro-TWT, the power of chaotic generation can achieve 50 kW for an electron mean efficiency of about 7% and a spectrum width of 3–4 GHz.

Using two-dimensional Bragg structures for the synchronization of radiation in planar backward wave oscillators

We propose to use two-dimensional Bragg resonance structures for the spatial synchronization of emission in planar backward wave oscillators (BWOs) pumped by wide sheet electron beams. It is demonstrated that the synchronization regime is stable with respect to changes in the detuning between the free-running BWO generation frequency and the Bragg frequency.

Controllable spectrum of an axial-mode gyrotron with external reflections

The feasibility of controlling the spectrum of multifrequency oscillations in a gyrotron by means of external reflections is studied. It is shown that, in self-modulation oscillation modes of operation, the radiation spectrum lines may come close to the resonance frequencies of a combined electromagnetic system including the reflector-limited part of the output waveguide. Under these conditions, the frequency separation of modes and, accordingly, the self-modulation period can be controlled by varying the position of the reflector. Theoretical results are corroborated by experiments with a 30-GHz relativistic gyrotron with an external reflector.

Self-modulation spectrum variation in gyrotrons with Bragg type output reflector

A new approach to the line spectrum control in the gyrotrons is discussed. This method is based on introduction of the additional reflector in the output waveguide to form quasi-equidistant mode spectrum in the hybrid cavity which includes interaction region and a section of the output waveguide limited by a such reflector. Under sufficiently large exceeding operating current over threshold in the self-modulation regime a spectrum of generation could be close to the set of eigenfrequencies of the hybrid cavity. In this situation the distance between spectrum lines could be controlled by varying the reflector position. The operability of suggested method was demonstrated in the experimental studies of relativistic Ka-band gyrotron where for the first time the selective Bragg-type reflector was used.