Extended Feedback System for Coupled Sub-THz Gyro-Devices to Provide New Regimes of Operation

A new type of high-power pulsed source in the millimeter and submillimeter frequency range, utilizing the method of passive mode locking, was proposed in 2015 by the Institute of Applied Physics (IAP-RAS) in Nizhny Novgorod. This principle, well known from laser physics, allows the generation of a periodic series of powerful, coherent, ultrashort pulses. In the millimeter and submillimeter wavelength range, this can be realized using an amplifier and a saturable absorber coupled in a feedback loop. For the coupling of the two devices, a sophisticated feedback system is required. Such a system, based on simple overmoded waveguide components, was previously proposed by the authors. The present article shows how the proposed feedback system can be extended, allowing for a wide range of possible operation regimes for two coupled gyro-devices. Particularly noteworthy is the application of the modified feedback system for the realization of a two-stage amplifier in the subterahertz (sub-THz) range. Furthermore, it seems to be possible to use two helical gyro-devices coupled in the proposed way as a source of coherent pulses, as a free-running or locked continuous wave (CW) source, and as a two-stage amplifier. In all cases, no design changes of the feedback system are required

Simple Feedback System for Passive Mode Locked Gyro-Devices at 263 GHz

A promising new source to generate a periodic series of coherent, ultra-short pulses in the millimeter and submillimeter frequency range is based on the method of passive mode locking [1]. The basic principle is well known from laser physics [2]. A realization for millimeter and sub-millimeter waves consists of an amplifier and a saturable absorber coupled in a feedback loop. In this paper, a coupling system for two single-window vacuum electron tubes in a mode-locked microwave oscillator is presented. Based on full-wave simulations, the key components of the proposed feedback system at 263 GHz (typical DNP-NMR frequency) are designed

Coherent summation of emission from relativistic Cherenkov sources as a way of production of extremely high-intensity microwave pulses

For relativistic Cherenkov devices, we investigate the process of high-power microwave pulse generation with its phase correlating to the sharp edge of an e-beam current pulse. Our theoretical consideration is referred to quasi-stationary and superradiative (SR) generation regimes when spontaneous emission of the e-beam edge serves as the seed for the development of further coherent oscillations. Phase correlation of the excited microwave pulses with the characteristics of the current pulse front and/or an initial external electromagnetic pulse has been additionally confirmed by particle-in-cell simulations. Pulse-to-pulse stability of the radiation phase within several percents of the oscillation period makes it possible to arrange multichannel schemes producing mutually coherent microwave pulses. In the experiments that have been carried out, the cathodes of independent generators were powered by identical accelerating pulses from strictly synchronized voltage modulators, or by splitting the pulse from a single powerful modulator. For the 2-ns regime with the power of each Ka-band backward-wave oscillator about 100 MW, we demonstrate quadratic growth of the power density in the interference maximum of the directional diagram. In a short pulse SR regime, with the peak power of 600 MW in a single channel, for a four-channel 2-D array, we attained a 16-fold radiation intensity gain

Radiation output system for planar gyrotron

Gyrotrons with planar resonator cavity offer some advantages over conventional cylindrical ones for submillimetre wavelengths. The radiation in the planar gyrotron should be converted from the mixture of several high modes of the rectangular waveguide into the Gaussian beam with high efficiency. For the experimental 145 GHz gyrotron, a two- channel output is considered. Both channels consist of waveguide converter and two convex mirrors and direct the radiation through the single output window. Calculated total efficiency of the system exceeds 98%

Quasi-Optical Theory of Relativistic Cherenkov Oscillators and Amplifiers with Oversized Electrodynamic Structures

Using the quasi-optical approach, we investigate wave propagation along the periodically corrugated surfaces and their interaction with rectilinear relativistic electron beams (REBs). At the periodical structure, the field can be expanded into a series of spatial harmonics, which, in the case of shallow corrugations, represent paraxial wavebeams with mutual coupling described within the method of effective surface magnetic currents. We present the dispersion equation for the normal waves. Two limit cases can be recognized: in the first one, the frequency is far from the Bragg resonance and the wave propagation can be described within the impedance approximation with the field presented as a sum of the fundamental slow wave and its spatial harmonics. In the interaction with a rectilinear REB, this corresponds to the convective instability of particles’ synchronism with the fundamental (0th) or higher spatial harmonics (TWT regime), or the absolute instability in the case of synchronism with the −1st harmonic of the backward wave (BWO regime). In the latter case, at the frequencies close to the Bragg resonance, the field is presented as two antiparallel quasi-optical wavebeams, leading to the absolute instability used in the surface-wave oscillators operating in the π-mode regime. Based on the developed theory, we determine the main characteristics of relativistic Cherenkov amplifiers and oscillators with oversized electrodynamical systems. We demonstrate the prospects for the practical implementation of relativistic surface-wave devices in submillimeter wavebands

Diffraction mode selection in planar lasers with Bragg resonators

Using the coupled-wave approach supplemented by the quasioptical approximation, we investigate the possibilities of diffraction mode selection with respect to the transverse index in planar distributed-feedback lasers. Quality factors and spatial structures of planar Bragg resonators with finite width and length of the corrugate area were found. Allowable values of the Fresnel parameter were determined at which the diffraction losses at the resonator edges lead to effective discrimination of modes with large numbers of transverse variations and thus provide conditions for the onset of a stationary single-mode laser generation regime