Alexander S. Sergeev

Generation of ultrashort microwave pulses based on cyclotron superradiance

We have theoretically and experimentally investigated the superradiance from a bunch of electrons rotating in a homogeneous magnetic field. A RADAN-303B modulator equipped with a subnanosecond pulse slicer has been used to generate high current subnanosecond electron bunches (250 kV, 0.1-1 kA, 0.3-0.5 ns). Transverse momentum was imparted to the electrons by a kicker. It is shown that for the experimental observation of cyclotron superradiance from high current electron bunches the optimum conditions are the conditions of group synchronism, when the translational velocity of the bunch coincides with the group velocity of the radiation propagating in the waveguide. In the 35 GHz range microwave pulses with record short duration, down to 0.4 ns, with a peak power level up to 200 kW, have been obtained.

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.

Čerenkov superradiance from a subnanosecond electron bunch in a sectional decelerating system

Induced coherent radiation (superradiance) from a subnanosecond electron bunch in a combined decelerating system has been investigated experimentally. In a first section formed by a periodically modulated waveguide, the density of the bunch is modulated and it then radiates in a second section formed by a waveguide partially filled with a dielectric. At an electron energy of 250 keV and a peak current of 800 A, millimeter radiation pulses with powers up to 2 MW and lengths up to 800 ps were obtained.

Super radiance of ensembles of classical electron oscillators as a method for generation of ultrashort electromagnetic pulses

We considered the superradiance of ensembles of electrons rotating in the uniform magnetic field. It is demonstrated that such processes may be used for generation of powerful ultrashort microwave pulses.

Powerful short wavelength fems operated at harmonics of bounce frequency: Recent results and prospects

Summary form only given. High-efficiency narrow-band 30-GHz Bragg FEM-oscillator was elaborated during the last few years in JINR-IAP collaboration based on 0.8 MeV / 200 A / 250 ns LINAC LIU-3000 (JINR). An attractive solution to prospect FEM into short wave-lengths keeping beam energy and geometry of electron-optical system is operation at harmonics of bounce-frequency. Two FEM schemes exploiting this idea are under development at LIU-3000.

Cherenkov oscillators with two-dimensional distributed feedback

Possibility to use a 2D distributed feedback for generation of spatially coherent radiation by a rectilinear electron beams in Cherenkov type devises is discussed. Theoretical and experimental studies of a Ka-band coaxial BWO with external 2D Bragg structure which operates as a synchronizer are presented.

Photonic-crystal superradiant laser: effects of homogeneous and inhomogeneous broadening of an active medium

We investigate superradiant lasing in 1D photonic crystal taking into account both homogeneous and inhomogeneous broadening in a two-level active medium. The latter may be based on, e.g., the quantum-well heterostructures and the fibers activated by color centers where Bragg structure can be fabricated artificially by means of a periodic modulation of the cladding layers. The truncated Maxwell-Bloch equations for the counter-propagating waves, the polarization and inversion of an inhomogeneously broadened two-level medium are used to analyze numerically the interplay between the back-scattering, amplification, dispersion, and de-phasing of waves. It is found that, if the photonic band gap is less than the coherent amplification bandwidth, there is a wide range of the pumping and relaxation parameters of a two-level medium where superradiant lasing exists and results in generation of a quasi-periodic and/or chaotic series of powerful and short pulses similar to those of Dicke superfluorescence. It is the cavity mode selection due to distributed feedback caused by the resonant Bragg structure which is responsible for the existence of superradiant lasing. Typical parameters of superradiant photonic-crystal lasers, including lengths, photonic band gaps and relaxation rates, are indicated.

Optimization of superradiant photonic-crystal lasers

We show that a two-level active sample of one-dimensional photonic crystal can generate chaotic sequence of extremely short pulses in the absence of mirrors. Numerical analysis and optimization of this promising superradiant laser is presented.

Novel source of the chaotic microwave radiation based on the gyro-backward-wave oscillator

First experimental studies of the nonstationary phenomena in a powerful K-band gyro-backward-wave oscillator with an external reflector are presented. Variation of the self-modulation frequency from 69 to 112 MHz and transition to the chaotic oscillations with spectrum width of 100 MHz were obtained due to the tuning of the guiding magnetic field. These results are in a good agreement with simulations, which demonstrate a strong dependence of the system dynamics on the strength of the magnetic field

Quasi-optical theory of surface-wave oscillators with one- and two-dimensional periodic structures

Summary form only given. Within the frame of quasi-optical approach we develop a nonlinear theory of relativistic surface wave oscillators. Our theory provides a possibility to describe both linear and nonlinear stages of formation of evanescent slow-wave structure and include evaluation of threshold conditions, electron efficiency and output power.