Quasi-optical Approach Research Articles

Terahertz super-radiance from picosecond electron bunches moving through a micro-undulator

We suggest using emission from the photoinjector-formed electron bunches moving through micro-undulators for generation of powerful super-radiant pulses in the terahertz/far infrared frequency range. Within the time-domain quasi-optical approach, we demonstrate the spatial coherence of emission with narrow angular spectrum from the electron bunches with transverse size limited by the Fresnel parameter NF ∼ 1, when the diffraction effects together with slippage provide synchronization of radiation from the entire volume of the extended bunch. For picosecond-duration, 5 MeV, 250 pC electron bunches, and a micro-undulator with a period of 3 mm, the peak power of 15 THz SR pulses can be about of 20 MW.

Theoretical and experimental studies of W-band relativistic surface-wave oscillator of planar geometry

This paper presents the results of theoretical and experimental studies of a surface-wave oscillator (SWO) of planar geometry excited by a ribbon high-current relativistic electron beam. Within the framework of the quasi-optical approach and direct three-dimensional particle-in-cell modeling, we demonstrate the advantages of open transverse edged configuration against the closed one for effective mode selection at a fairly large oversize factor. In the experiments carried out on the basis of the SINUKI accelerator (Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, 1 kA/650 keV/17 ns), we form a magnetically guided ribbon electron beam with a cross-section of 0.3 × 20 mm2, which moves parallel to a slow-wave structure with a period of 1.75 mm. A planar W-band SWO both of open and closed edged configuration has been experimentally tested. In full agreement with modeling, the open configuration exhibits much more stable operation, where at a frequency of 75 GHz, we observe pulse generation with duration of about 7 ns. The output power measured by the calorimetric method reaches 25 MW.

Development of the Multi-Beam Transmission Line for DTT ECRH system

The DTT tokamak, whose construction is starting in Frascati (Italy), will be equipped with an ECRH system of 16 MW for the first operation phase and with a total of 32 gyrotrons (170 GHz, ≥ 1 MW, 100 s), organized in 4 clusters of 8 units each in the final design stage. To transmit this large number of power beams from the gyrotron hall to the torus hall building a Quasi-Optical (QO) approach has been chosen by a multi-beam transmission line (MBTL) similar to the one installed at W7-X Stellarator. This compact solution, mainly composed of mirrors in “square arrangement” shared by 8 different beams, minimizes the mode conversion losses. The single-beam QOTL is used to connect each gyrotron MOU output to a beam-combiner mirror unit and, after the MBTL, from a beam-splitter mirror unit to the exvessel and launchers sections located in the equatorial and upper ports of 4 DTT sectors. A novelty introduced is that the mirrors of the TLs are embodied in a vacuum enclosure, using metal gaskets, to avoid atmospheric absorption losses and microwave leaks. The TL, designed for up to 1.5 MW per single power beam, will have a total optical path length between 84 m and 138 m from the gyrotrons to the launchers. The main straight section will travel along an elevated corridor ~10 m above the ground level. The development of the optical design reflects the constraints due to existing buildings and expected neutron flux during plasma operation. In addition, the power throughput of at least 90% should be achieved.

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.

Quasi-optical theory of relativistic Cherenkov surface-wave oscillators with oversized cylindrical waveguides

Within the quasi-optical approach, we investigate the propagation of azimuthally symmetric TM waves in periodically corrugated cylindrical waveguides and their excitation by relativistic electron beams. Presenting the field as two, forward and backward, quasi-optical wavebeams coupled at the shallow corrugation, we obtain a dispersion equation for normal waves and thus a criterion of existence of the surface wave. For a finite-length corrugation section, the spectrum of axial evanescent eigenmodes is estimated; the spatial structure and the quality factor of the fundamental mode are found at an eigenfrequency close to the Bragg frequency. A self-consistent system of equations describing the interaction of electromagnetic waves with a rectilinear electron beam injected into the system is derived. Based on this model, we recognize two oscillation regimes, namely, the π-mode excitation regime and the regime of backward surface wave oscillator. We demonstrate the viability of practical implementation of relativistic surface wave oscillators with a power level of up to 140 MW in the sub-millimeter wavelength band.

Spatiotemporal localizations of light in quadratically nonlinear media with transversal inhomogeneity

In the current study with the help of mathematical modeling we investigate the generation of second optical harmonic provided various group velocity dispersion (GVD) and third order dispersion effects taken into account. Varying values and signs of the third-order dispersion at quasi-zero values of GVD we reveal the conditions of light bullet formation and stable propagation. To this end we use a generalization of the well-known system of the second harmonic generation in quasi-optical approach. Particular attention is paid to the case when GVD at the second harmonic is close to zero. In addition, we investigate these cases using both focusing and defocusing waveguide which means the presence of transversal inhomogeneity.

Generation of intense spatially coherent superradiant pulses in strongly oversized 2D periodical surface-wave structure

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.

Unified quasi-optical theory of short-wavelength radiation amplification by relativistic electron beams moving near the impedance surfaces

We combine impedance approximation with a quasi-optical approach to describe the amplification of short-wavelength radiation by rectilinear relativistic electron beams (REBs) moving near the impedance surfaces. We consider a number of physical systems in which wave propagation and amplification by REBs under certain conditions can be described within the developed unified approach. These include metal surfaces with shallow periodical corrugations, the surface of the isotropic plasma, and metals with finite conductivity. In the latter case, resistive instability arises. For the specified class of systems, universal (differing only in the definition of impedance) linear and nonlinear equations are obtained, which allow for finding the instability increments, the spatial profiles of excited fields, and the efficiency of energy extraction.

Diffraction-Mode Selection in Heterolasers with Planar Bragg Structures

Within the coupled wave method complemented by the quasi-optical approach, the feasibility of diffraction-mode selection over the transverse index in lasers with planar Bragg resonators is studied. Acceptable Fresnel parameters at which the diffraction losses from the Bragg-structure edges provide steady-state single-mode lasing with a narrow angular spectrum are determined. It is shown that the such stable lasing at high Fresnel parameters is achieved by shifting the operating transition frequency to the frequency of the longitudinal mode of a Bragg resonator with the highest Q-factor.

Gyro-devices – natural sources of high-power high-order angular momentum millimeter-wave beams

The Orbital Angular Momentum (OAM) carried by light beams with helical phasefront (vortex beams) has been widely employed in many applications such as optical tweezers, optical drives of micro-machines, atom trapping, and optical communication. OAM provides an additional dimension (diversity) to multiplexing techniques, which can be utilized in addition to conventional multiplexing methods to achieve higher data rates in wireless communication. OAM beams have been thoroughly studied and used in the optical regime but in the mm-wave and THz-wave region, they are still under investigation. In these frequency bands, there are difficulties associated with beam-splitting and beam-combining processes as well as with the use of spiral phase plates and other methods for OAM generation, since the wavelength is much larger compared to those at optical frequencies, leading to higher diffraction losses. The present paper describes the natural generation of high-power OAM modes by gyro-type vacuum electron devices with cylindrical interaction circuit and axial output of the generated rotating higher-order transverse electric mode TEm,n, where m > 1 and n are the azimuthal and radial mode index, respectively. The ratio between the total angular momentum (TAM) JN and total energy WN of N photons is given by m/ω, where ω is the angular frequency of the operating mode, which in a gyrotron oscillator is close to the TEm,n-mode cutoff frequency in the cavity. Therefore, m/ω = Rc/c, where Rc is the caustic radius and c the velocity of light in vacuum. This means that the OAM is proportional to the caustic radius and at a given frequency the same for all modes with the same azimuthal index m. Right-hand rotation (co-rotation with the electrons) corresponds to a positive value of m and left-hand rotation to negative m. The corresponding OAM mode number (topological charge) is l = m – 1. Circularly polarized TE1n modes only possess a Spin Angular Momentum (SAM: s = ±1). TE0n modes have neither SAM nor OAM. This is the result of the photonic (quasi-optical) approach to derive the TAM of modes generated in gyrotrons. The same result follows from the electromagnetic (EM) wave approach for the TAM within a given waveguide volume per total energy of the EM wave in the same volume. Such high-power output beams with very pure higher-order OAM, generated by gyrotron oscillators or amplifiers (broadband) could be used for multiplexing in long-range wireless communications. The corresponding mode and helical wavefront sensitive detectors for selective OAM-mode sorting are available and described in the present paper.

Дифракционная селекция мод в гетеролазерах с планарными брэгговскими структурами

Within the coupled waves method complemented by quasi-optical approach, we investigate a possibility of diffraction mode selection with respect to the transverse mode index in lasers with planar Bragg resonators. Allowable values of the Fresnel parameter are determined which provide for establishment of stationary single-mode oscillation regime with narrow angle spectrum due to diffraction losses from the edges of the Bragg structure. We show that at larger values of the Fresnel parameter, oscillation stability can be improved by shifting the operating transition frequency to the eigenfrequency of the highest-Q longitudinal mode of the Bragg resonator.

Potential of Liquid-Crystal Materials for Millimeter-Wave Application

In this study, we reviewed three topics regarding the application of liquid-crystal (LC) materials to millimeter-wave (MMW) devices. It is essential to develop useful measurement methods for refractive indices of LC materials in the MMW region. Herein, a novel measurement method using optical short is demonstrated using a Si semiconductor substrate. There are two approaches to develop MMW LC devices. One is the quasi-optical approach, which involves scaling up the optical components, and the other approach involves integrating the LC materials into high-frequency electric circuits. A three-dimensional (3D) printer is used to fabricate the Fresnel lens, which is a typical quasi-optical device useful in the MMW region, where we can develop the tunable lens by introducing LC materials. A planar-type MMW waveguide is advantageous for integrating the LC materials to develop LC MMW devices using the second approach. We investigated a useful microstrip-line-type LC phase shifter by developing a novel conversion circuit to introduce the LC material onto the dielectric substrate surface. A phase shifter is an important MMW component that is used to attain a phased array antenna system, and a minimal twin antenna array is demonstrated using the microstrip-line-type LC phase shifters.

Theoretical and experimental studies of relativistic oversized Ka-band surface-wave oscillator based on 2D periodical corrugated structure

Based on a quasioptical approach and direct particle-in-cell simulations, we study dynamics of oversized relativistic surface-wave oscillators (SWOs) of the Cherenkov type with 2D periodical corrugated structures of cylindrical geometry. Such corrugation allows significant rarefication of the spectrum of modes with different azimuthal indices. As a result, selective excitation of a mode with a given azimuthal index is possible. Azimuthal index of the generated mode depends on the voltage rise time. For short (nanosecond scale) rise time, generation of an azimuthally symmetric mode can be realized. For longer (hundreds nanoseconds to microseconds) rise time, the modes with high azimuthal indexes would be excited. These conclusions are supported by the experiments where Ka-band SWOs with 2D corrugated structures were realized based on the $300\text{ }\text{ }\mathrm{keV}/100\text{ }\text{ }\mathrm{A}/4\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ thermionic accelerator SATURN. For an oversize factor of 16, stable narrow-band generation with output power of 1.5--2 MW was obtained at the frequency of 32.5 GHz corresponding to the mode with an azimuthal index of $m=3$. The project of Ka-band subgigawatt power SWOs operating at the azimuthally symmetric mode based on $500\text{ }\text{ }\mathrm{keV}/4\text{ }\text{ }\mathrm{kA}/20\text{ }\text{ }\mathrm{ns}$ high current explosive-emission accelerator SINUS-6 is under development.

Highly selective surface-wave resonators for terahertz frequency range formed by metallic Bragg gratings

In the frame of the quasi-optical approach we solve the diffraction problem and describe surface modes confined at a metallic plate with a shallow grating of finite length. We prove that such planar grating can form a highly selective surface-wave Bragg resonator. For a given material conductivity and grating length, we find the optimum corrugation depth that provides the maximum value of Q factor. These results are applicable for developing resonators for terahertz frequency bands.

Theory of Linear and Nonlinear Gain in a Gyroamplifier using a Confocal Waveguide.

The linear and nonlinear theory of a gyroamplifier using a confocal waveguide is presented. A quasi-optical approach to describing the modes of a confocal waveguide is derived. Both the equations of motion and the mode excitation equation are derived in detail. The confocal waveguide circuit has the advantage of reducing mode competition but the lack of azimuthal symmetry presents challenges in calculating the gain. In the linear regime, the gain calculated using the exact form factor for the confocal waveguide agrees with an azimuthally averaged form factor. A beamlet code including velocity spread effects has been written to calculate the linear and nonlinear (saturated) gain. It has been successfully benchmarked against the MAGY code for azimuthally symmetric cases. For the confocal waveguide, the beamlet code shows that the saturated gain is reduced when compared with results obtained using an azimuthally averaged form factor. The beamlet code derived here extends the capabilities of nonlinear gyroamplifier theory to configurations that lack azimuthal symmetry.

Amplification of short-wave radiation based on the resistive instability of a relativistic electron beam (Quasi-optical theory)

The quasi-optical approach has been used to study the linear and nonlinear stages of instability of an electron beam propagating near a flat boundary of an absorbing medium with dissipative characteristics that can be described by the Leontovich impedance boundary conditions. It has been shown that resistive instability can be used to amplify radiation in the short-wave ranges, including the terahertz range. The instability increments and the efficiency of energy transfer have been determined.

Generation of high-power Cherenkov superradiance pulses using 2D periodic slow-wave structures based on oversized cylindrical waveguides

We propose to stabilize the azimuthal structure of superradiance pulses using two-dimensional (2D) periodic Bragg structures that simultaneously play the role of a slow-wave system. Analysis performed in the framework of a quasi-optical approach is confirmed by the results of 3D particle-in-cell modeling that demonstrates the possibility of using the proposed system for generating multi-GW nanosecond radiation pulses in the millimeter waveband.

Quasi-optical approach for inhomogeneous dissipative media with high-order spatial dispersion

Quasi-optical approach for inhomogeneous dissipative media with high-order spatial dispersion

Quasi-optical approach to reconstruction of plasma fluctuations using amplitude distribution of transmitted microwave beam

Quasi-optical approach to reconstruction of plasma fluctuations using amplitude distribution of transmitted microwave beam

Generation of Subterahertz Superradiance Pulses Based on Excitation of a Surface Wave by Relativistic Electron Bunches Moving in Oversized Corrugated Waveguides.

The first experiments on the observation of short pulsed superradiant (SR) emission with the excitation of a surface wave by a relativistic electron bunch moving in an oversized corrugated waveguide were performed. Subterahertz SR pulses with a central frequency of 0.14THz, an ultrashort duration of 150ps, and an extremely high peak power of 50-70MW were generated. The experiments were based on a theoretical consideration including the quasioptical approach and direct particle-in-cell simulations.