E. V. Ilyakov

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.

X-band high-efficiency relativistic gyrotron

As a result of the theoretical and experimental studies, a relativistic gyrotron with efficiency 50% at power 7 MW or with power 11 MW at efficiency 30% was developed. The full microwave pulse duration was about 6 /spl mu/s, which corresponded to the duration of the voltage-pulse flat top. The regime of periodical, almost 100% depth self-modulation with period 17 ns was observed in the gyrotron with the delayed feedback formed by an additional reflector in the output waveguide.

Relativistic carcinotron with a thermionic injector of electrons

An experimental study directed to the determination of a mechanism for microwave breakdown in a relativistic X-band carcinatron is presented. An electron beam was generated using a thermionic cathode, which provided a stable beam geometry. The use of this cathode decreased the probability of breakdown caused by electron bombardment of the slow-wave structure. An important part played in microwave breakdown are molecules absorbed on the slow-wave structure surface. It is shown that the presence of these molecules, an implementation of conditions for secondary-electron resonant discharge (SERD), can result in a very fast (during 10-20 ns) limitation of the radiation pulsewidth. To remove the adsorbed molecules, heat degassing of the slow-wave structure and a collector of electrons was applied, going on continuously, during a working day of the device. By degassing and choice of slow-wave structure material, output radiation power of the device was increased by a factor of ten (up to 5 MW at the pulsewidth of 10 /spl mu/s).

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.

Experimental investigation of an X-band relativistic backward-wave oscillator based on a thermionic electron injector

The results of the experimental investigation of a relativistic backward-wave oscillator with an electron beam formed by a thermionic injector, are presented. The influence of vacuum conditions on the device operation was studied. Maximum radiation power 5 MW (efficiency not less than 10%) at pulse width 8 microsecond(s) that corresponded to electron current pulse duration was obtained in the X-band experiments. The microwave radiation band was near 10 MHz that corresponded to frequency change depending on voltage.

Powerful Cherenkov oscillators with 2D distributed feedback

The feasibility of using 2D distributed feedback based on 2D planar and coaxial Bragg structures for generating spatially coherent radiation from rectilinear ribbon and tubular electron beams is studied. One-section and sectional Cherenkov masers are analyzed. In the former design, a 2D Bragg structure acts as a resonator and a periodic slow-wave system simultaneously. In the latter (sectional) design, radiation is synchronized in a 2D Bragg structure that is placed at the cathode end of the interaction space and couples longitudinal and transverse (azimuthal) wave flows. The wave is amplified by the electron beam mainly in the fairly long middle section. The output (collector) part contains a standard 1D Bragg structure that partially reflects the amplified radiation toward the cathode and closes the feedback circuit. It is shown that dissipation introduced into the 2D Bragg structure of the sectional design makes it possible to increase one of the transverse sizes of the system to ∼103 wavelengths with the energy exchange efficiency and one-frequency masing mode stability remaining the same. With such an overdimension, the millimeter-wave radiation integral power may reach a gigawatt level.

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.

10 MW, X- and Ka-band gyrotrons

Summary form only given, as follows. The results of the development of moderately relativistic X and Ka-band gyrotrons are presented. For the first time the high efficiency (about 50%) single-mode generation at the power level of about 5-10 MW was realized without recuperation at the beam energy of about 300 keV. The microwave pulse duration at the X-band gyrotron achieved 7 mcs, and at the Ka-band it was varied from 200 to 600 ns. Operation of the relativistic gyrotron was simulated by the method based on the stationary model with the selfconsistent, not-fixed structure of the field. It was found that additional optimization of the cavity profile and parameters of magnetron injection gun might increase relativistic gyrotron efficiency up to 50 %.

400-kV 400-A microsecond electron accelerator with a hot cathode

The description of an electron accelerator of microsecond pulse duration and the results of experiments with a relativistic carcinotron and a gyrotron, carried out on its base, is submitted in the report.