High Power Backward Wave Oscillator Research Articles

An S-band metamaterial based high power backward-wave oscillator

An S-band metamaterial based high power backward-wave oscillator

High-Power Backward-Wave Oscillator Using Folded Waveguide With Distributed Power Extraction Operating at an Exceptional Point

The concept of exceptional point of degeneracy (EPD) is used to conceive a degenerate synchronization regime that is able to enhance the level of output power and power conversion efficiency for backward wave oscillators (BWOs) operating at millimeter-wave and Terahertz frequencies. Standard BWOs operating at such high frequency ranges typically generate output power not exceeding tens of watts with very poor power conversion efficiency in the order of 1%. The novel concept of degenerate synchronization for the BWO based on a folded waveguide is implemented by engineering distributed gain and power extraction along the slow-wave waveguide. The distributed power extraction along the folded waveguide is useful to satisfy the necessary conditions to have an EPD at the synchronization point. Particle-in-cell (PIC) simulation results shows that BWO operating at an EPD regime is capable of generating output power exceeding 3 kwatts with conversion efficiency of exceeding 20% at frequency of 88.5 GHz.

Finite element time-domain body-of-revolution Maxwell solver based on discrete exterior calculus

We present a finite-element time-domain (FETD) Maxwell solver for the analysis of body-of-revolution (BOR) geometries based on discrete exterior calculus (DEC) of differential forms and transformation optics (TO) concepts. We explore TO principles to map the original 3-D BOR problem to a 2-D one in the meridian ρz-plane based on a Cartesian coordinate system where the cylindrical metric is fully embedded into the constitutive properties of an effective inhomogeneous and anisotropic medium that fills the domain. The proposed solver uses a (TEϕ,TMϕ) field decomposition and an appropriate set of DEC-based basis functions on an irregular grid discretizing the meridian plane. A symplectic time discretization based on a leap-frog scheme is applied to obtain the full-discrete marching-on-time algorithm. We validate the algorithm by comparing the numerical results against analytical solutions for resonant fields in cylindrical cavities and against pseudo-analytical solutions for fields radiated by cylindrically symmetric antennas in layered media. We also illustrate the application of the algorithm for a particle-in-cell (PIC) simulation of beam-wave interactions inside a high-power backward-wave oscillator.

Tunable ferromagnetic resonance in La-Co substituted barium hexaferrites at millimeter wave frequencies

Transmittance measurements have been performed on La-Co substituted barium hexaferrites in millimeter waves. Broadband millimeter-wave measurements have been carried out using the free space quasi-optical spectrometer, equipped with a set of high power backward wave oscillators covering the frequency range of 30 – 120 GHz. Strong absorption zones have been observed in the millimeter-wave transmittance spectra of all La-Co substituted barium hexaferrites due to the ferromagnetic resonance. Linear shift of ferromagnetic resonance frequency as functions of La-Co substitutions have been found. Real and imaginary parts of dielectric permittivity of La-Co substituted barium hexaferrites have been calculated using the analysis of recorded high precision transmittance spectra. Frequency dependences of magnetic permeability of La-Co substituted barium hexaferrites, as well as saturation magnetization and anisotropy field have been determined based on Schlömann’s theory for partially magnetized ferrites. La-Co substituted barium hexaferrites have been further investigated by DC magnetization to assess magnetic behavior and compare with millimeter wave data. Consistency of saturation magnetization determined independently by both millimeter wave absorption and DC magnetization have been found for all La-Co substituted barium hexaferrites. These materials seem to be quite promising as tunable millimeter wave absorbers, filters, circulators, based on the adjusting of their substitution parameters.

Axisymmetric Resonance Reflector for Relativistic Gigawatt Backward-Wave Oscillators

We discuss the properties of the resonance reflectors applied in high-power relativistic backward-wave oscillators. The obtained results were used to develop a resonance reflector in the form of a section of a coaxial waveguide for a relativistic backward-wave oscillator operated in the ten-centimeter wavelength range. The results of preliminary experiments agree well with the calculations.

Millimeter wave transmittance/absorption measurements on micro and nano hexaferrites

Millimeter wave transmittance measurements have been successfully performed on commercial samples of micro- and nano-sized particles of BaFe12O19 and SrFe12O19 hexaferrite powders and nano-sized particles of BaFeO2 and SrFeO2 powders. Broadband millimeter wave transmittance measurements have been performed using free space quasi-optical spectrometer, equipped with a set of high power backward wave oscillators covering the frequency range of 30 – 120 GHz. Real and imaginary parts of dielectric permittivity for both types of micro- and nanoferrites have been calculated using analysis of recorded high precision transmittance spectra. Frequency dependences of magnetic permeability of ferrite powders, as well as saturation magnetization and anisotropy field have been determined based on Schlöemann’s theory for partially magnetized ferrites. Micro- and nano-sized ferrite powders have been further investigated by DC magnetization to assess magnetic behavior and compare with millimeter wave data. Consistency of saturation magnetization determined independently by both millimeter wave absorption and DC magnetization have been found for all ferrite powders. These materials seem to be quite promising as tunable millimeter wave absorbers and filters, based on their size-dependent absorption.

A Novel Slow-Wave Structure for High-Power <inline-formula> <tex-math notation="LaTeX">$K_{a}$ </tex-math></inline-formula>-Band Backward Wave Oscillators With Mode Control

We present a novel slow-wave structure (SWS) to significantly enhance the performance of high-power backward wave oscillators (BWOs). The design features a periodic metallic ring insertion and a deeply corrugated cylindrical waveguide. Both serving to improve interaction impedance and flexibility in dispersion curve engineering. A new technique for mode control in waveguides is also introduced. In addition to demonstrating mode control in SWSs, the key aspects of the presented design are mode dominance reversal and a 100% improvement in interaction impedance that can be exploited to achieve greater power conversion efficiency and output mode purity. Performance comparisons on group velocity, phase velocity and interaction impedance of the new SWS versus the conventional corrugated waveguide are provided. We extend the concept of inhomogeneous SWSs by designing a three-section inhomogeneous SWS. Further simulations using a Particle in Cell code of a highly efficient three-section inhomogeneous K a -band BWO generates a peak output power of a 5.92 MW at 27 GHz with a 58% peak efficiency.

Temporal growth rate study of a high power backward wave oscillator with semi-circularly corrugated slow wave structure

The dispersion properties and the temporal growth rate (TGR) of a high power backward wave oscillator (BWO) with a cylindrical metallic slow wave structure with semi-circular corrugation (SCCSWS) driven by an intense relativistic electron beam (IREB) are studied numerically. The IREB is assumed to be guided axially by an infinitely strong magnetic field. The semi-circular axial profile of the SWS is approximated by Fourier series and the study is carried out utilizing linear Rayleigh-Fourier (R-F) theory. The Fourier constants of the axial profile are determined numerically. The dispersion equation is solved numerically for the beam energy of 80–660 kV, beam current of 0.1–1.0 kA. When an electron beam with sufficient energy and current to produce instability propagates through the SWS, microwave radiation is generated. TGR which is obtained from the imaginary value of frequency and wavenumber can be used to qualitatively estimate the strength of the microwave radiation. The periodicity of the axial profile of SCCSWS is varied and the TGR for each case has been calculated by varying the beam parameters for TM01 mode. The proposed structure is comparatively easy to be fabricated and expected to be useful in BWO devices for generating high power microwaves for different applications.

Stable 0.3-THz Gyrotron Backward-Wave Oscillator With a Tapered Coaxial Interaction Waveguide

Mode competition is a severe problem in the development of a high-power gyrotron backward-wave oscillator (gyro-BWO) in terahertz (THz) region. To improve the stability of this device, this paper investigates the possibility of using a tapered coaxial waveguide as the interaction structure of a 0.3-THz gyro-BWO. The inner cylinder rarefies the mode density near the operating mode. Moreover, tapering the wall of the outer cylinder suppresses almost all the competing modes and shifts the optimum magnetic tuning range of the operating mode under stable operating conditions. The gyro-BWO is predicted to stably generate a peak power of 17.2 kW with 11.5% efficiency at 0.307 THz, and a continuous tuning bandwidth of 3.4 GHz $({\approx}{1.1\%})$ for a 30-kV, 5-A electron beam with $\alpha=1.0$ and an axial velocity spread of 5%. Compared with the uniform case, the tapered coaxial interaction waveguide enhances both the stability and the frequency tunability of the THz gyro-BWO.

Theoretical study on high power backward-wave oscillator

A high power backward-wave oscillator (BWO) with a hot cathode is theoretically studied, which has a long life and good stability. Firstly, a single anode electron gun with a hot cathode is simulated by PIC software. The emission current density and the area of the hot cathode are 20 kA/cm2 and 100 cm2, respectively. The optimal design parameters of the electron gun are obtained. The PIC simulation results of BWO show that the output power of BWO can reach 290 MW with a frequency of 10.5 GHz, and the beam-wave interaction efficiency is 29%.

一种高功率返波管理论

一种高功率返波管理论

Size dependent ferromagnetic resonance and magnetic anisotropy of hexagonal barium and strontium ferrite powders

Ferrite powders with different particle diameters were characterized in millimeter wave frequency range. Quasi-optical spectrometer in transmission mode powered by high power backward wave oscillator was employed to measure the transmittance of the powder samples. Strong ferromagnetic absorptions were acquired in the millimeter wave frequency range. The absorption frequencies exhibit distinct shift related to the size of ferrite particles. The complex dielectric permittivity and magnetic permeability are determined from the transmittance spectra. The size dependence of ferromagnetic resonance is clearly observed.

Ferromagnetic resonance of micro- and nano-sized hexagonal ferrite powders at millimeter waves

Complex magnetic permeability and dielectric permittivity of micro- and nano-sized powdered barium (BaFe12O19) and strontium (SrFe12O19) hexaferrites have been studied in a broadband millimeter wave frequency range (30–120 GHz). Transmittance measurements have been performed using a free-space quasi-optical millimeter wave spectrometer, equipped with a set of high-power backward wave oscillators. Real and imaginary parts of dielectric permittivity for both types of micro- and nanoferrites have been calculated using analysis of recorded high-precision transmittance spectra. Frequency dependences of the magnetic permeability have been obtained from Schlömann’s equation for partially magnetized ferrites. These materials show promise as tunable millimeter wave absorbers, based on their size-dependent absorption.

Magnetic Properties of Nanoferrites Near Ferromagnetic Resonance in Millimeter Waves

Complex magnetic permeability and dielectric permittivity of nano-sized pure powdered and solid diluted barium and strontium ferrites have been studied in a broadband millimeter wave frequency range for the first time. Transmittance measurements have been performed using a free space quasi-optical millimeter wave spectrometer, equipped with a set of high power backward wave oscillators. Backward wave oscillators have been used as sources of tunable coherent radiation at each individual <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> - and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W</i> -frequency bands. Real and imaginary parts of dielectric permittivity for both types of pure and diluted nanoferrites have been calculated using analysis of recorded high precision transmittance spectra. Frequency dependences of the magnetic permeability have been calculated from Schlömann's equation for partially magnetized ferrites. Tunable millimeter wave absorber, based on manipulating physical properties of nano-sized powdered ferrite materials and external magnetic field has been presented.

Millimeter-wave dielectric properties of single-crystal ferroelectric and dielectric materials

Transmittance measurements on various single crystal ferroelectric and dielectric materials, BaTiO(3), SrTiO(3), LiNbO(3), LiTaO(3), (PbMg(1/3)Nb(2/3)O(3))0.73-(PbTiO(3))0.27, LaAlO(3), and Bi(4)Ge(3)O(12), over a broad millimeter-wave (MMW) frequency range have been performed. Frequency dependence of the complex dielectric permittivity has been measured in the MMW region using high-power sources for the first time, using a free-space, quasi-optical MMW spectrometer equipped with high-power backward wave oscillators (BWOs) as sources of coherent radiation, tunable in the range from 30 to 120 and 180 to 260 GHz. These results are compared with MMW permittivity of these materials obtained by other methods as well as to RF, microwave, and optical frequency permittivities for all the materials tested. The effects of both crystallographic orientation and quality of the surface polishing of the crystals have been examined. Uncertainties and possible sources of instrumentation and measurement errors related to the freespace MMW technique are discussed. This work demonstrates that precise MMW permittivity data can be obtained even on relatively small and thin crystals of different surface conditions and orientations using the high-power BWO-based quasioptical approach.

Terahertz ellipsometry and terahertz optical-Hall effect

Ellipsometry has been proven as an excellent tool for the precise and accurate determination of material optical properties in the spectral range from the far infrared to the vacuum ultraviolet. In the terahertz frequency domain, however, ellipsometry is still in its infancy. Here we report on our recent development of rotating optical element frequency domain terahertz ellipsometry using electron-beam based, quasi-optical light sources. We demonstrate that high power backward wave oscillator type sources are readily available for the use in spectroscopic ellipsometry instrumentation for the terahertz spectral range. We review recent results on the application of terahertz ellipsometry. Exemplarily, the contact-free optical determination of free-charge carrier properties for very small doping concentrations and doping profiles in iso- and aniso-type Si homojunctions will be discussed. Furthermore, terahertz optical-Hall effect measurements on high-mobility epitaxial graphene on SiC and very low-doped Si are presented.

Millimeter-Wave Transmittance and Reflectance Measurement on Pure and Diluted Carbonyl Iron

Transmittance and reflectance measurements on highly absorbing carbonyl iron materials over a broad millimeter-wave frequency range have been performed. The frequency dependence of the complex dielectric permittivity of a carbonyl iron diluted composite and pure powdered materials has been determined in the millimeter waves for the first time. The measurements have been employed using a free-space quasi-optical millimeter-wave spectrometer equipped with a set of high-power backward-wave oscillators as sources of coherent radiation, which are tunable in the range from 30 to 120 GHz. A significant transmission zone of millimeter-wave radiation at a frequency around 60 GHz has been observed in the transmittance spectra for the carbonyl iron materials.

Diffraction selection of modes in planar backward-wave oscillators

We study the possibility of obtaining spatially coherent radiation in a planar backward-wave oscillator fed with a wide sheet electron beam by using natural diffraction divergence of the radiation. This method is efficient for Fresnel parameters being approximately equal to unity. To solve the two-dimensional boundary problem, the spatio-temporal approach is used, which allows one to determine both the self-excitation conditions and the parameters of the stationary generation regime and the boundaries of its stability. The developed theory is used to evaluate the possibility of achieving diffraction synchronization of the radiation in a high-power planar backward-wave oscillator operated in the 8-mm wavelength band and based on a sheet relativistic electron beam produced by the U-3 accelerator of the Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia.

Dielectric-Parameter Measurements of SiC at Millimeter and Submillimeter Wavelengths

High-precision continuous spectra of the absorption coefficient, refractive index, complex dielectric permittivity, and loss tangent for several silicon carbide (SiC) specimens are reported in this paper over a broad millimeter- and submillimeter-wave range for the first time. Measurements have been successfully performed using three different types of specially constructed spectrometer systems: a dispersive Fourier transform spectroscopy, an automated 60-GHz open resonator, and a free-space quasi-optical millimeter-wave spectrometer equipped with high-power backward-wave oscillator sources and their associated state-of-the-art dielectric-measurement techniques. Data are presented as continuous functions of frequency from 30 to 600 GHz. The employment of various measurement systems and techniques ensured the measurement of polycrystalline SiC specimens with various degrees of absorption and dispersion characteristics over an extended frequency range with high precision. Results presented here provide comprehensive information of SiC on its optical and dielectric behavior as a function of frequency and purity for its potential application in semiconductor and radio-frequency devices and circuits. An error analysis of measured dielectric-parameter results is also provided.

Complex Magnetic Permeability and Dielectric Permittivity of Ferrites in Millimeter Waves

We report a first-time study of complex magnetic permeability and dielectric permittivity of 99% pure powdered barium cobalt ferrite (Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sub> ) and pure solid barium hexaferrite ceramics (BaFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sub> ) in a broadband millimeter-wave frequency range. We performed transmittance measurements using a free-space quasi-optical millimeter-wave spectrometer, equipped with a set of high-power backward-wave oscillators as sources of tunable coherent radiation at each Q-, V-, and W-frequency band. We calculated frequency dependences of complex permittivity for both types of ferrites using analysis of recorded high-precision transmittance spectra and obtained frequency dependences of the magnetic permeability from Schlomann's equation for partially magnetized ferrites.