Surface Wave Oscillator Research Articles

A comparative study of measurements and numerical simulations of acoustically excited non-spherical bubbles oscillation

A detailed GPU-based parameter study is performed on the nonlinear shape deformation of acoustically excited bubbles using a second-order perturbation theory developed by Shaw (2006). The parameter scan is utilized for an extensive validation study based on a detailed measurement study conducted by Cleve et al. (2019). Out of the ten measurements, six exhibit an excellent match with the simulation results. For the discrepancies of the remaining four, the authors identified two causes. First, the parameter identification of the measurement is prone to significant error; second, the dynamics of the surface wave oscillation is out of the validity limit of the employed model.

Sub-Gigawatt W-Band Oversized Surface-Wave Oscillator With 2D-Periodical Slow-Wave Structure of Cylindrical Geometry

Sub-Gigawatt W-Band Oversized Surface-Wave Oscillator With 2D-Periodical Slow-Wave Structure of Cylindrical Geometry

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.

75 GHz Relativistic Surface-Wave Oscillator of Planar Geometry

We present the results of simulations and experimental studies of a W-band surface-wave oscillator powered by a ribbon high-current relativistic electron beam produced by SINUKI accelerator (IAP RAS, Nizhny Novgorod, 1 kA / 650 keV / 17 ns). Planar geometry of the interaction space facilitates efficient mode selection over wide transverse coordinate by using open waveguide at fairly large oversize factor (the width of the waveguide is ~8 wavelengths). Stable oscillation regime is obtained at frequency of 75 GHz with a pulse duration of about 5 ns. The output power measured by the calorimetric method reaches 25 MW.

Enhanced Frequency Stability of SAW Yarn Tension Sensor by Using the Dual Differential Channel Surface Acoustic Wave Oscillator.

This paper presents a 60 MHz surface acoustic wave (SAW) yarn tension sensor incorporating a novel SAW oscillator with high-frequency stability. A SAW delay line was fabricated on ST-X quartz substrate using the unbalanced-split electrode and bi-directional engraving slots. The dual differential channel delay linear acoustic surface wave oscillator is designed and implemented to test yarn tension, which can effectively remove the interference of temperature, humidity, and other peripheral factors through differential design. The yarn tension sensor using the surface acoustic wave has high-precision characteristics, and the SAW delay line oscillator is designed to ensure the test system’s stable operation. The effect of time and tension on oscillator frequency stability is studied in detail, and the single oscillator and the dual differential channel system were tested, respectively. After using the dual differential channel system, the short-term frequency stability from is reduced from 1.0163 ppm to 0.17726 ppm, the frequency accuracy of the tension sensor is improved from 134 Hz to 27 Hz, and the max frequency jump steady is reduced from 2.2395 ppm to 0.45123 ppm.

Effect of Oversized Factor on 0.1 THz Surface Wave Oscillator

Surface wave oscillator (SWO) is an intense subterahertz wave source based on the interaction of an electron beam with a surface wave in a cylindrical corrugated waveguide (CCW). The oversized factor is a key parameter of SWO and is the ratio of the CCW diameter to the wavelength of a generated wave. In this study, we experimentally examine 0.1 THz SWO with different oversized factors of 10 and 7. For an oversized factor of 7, a kilowatt class radiation with a maximum efficiency of 8% is observed. SWO operation can be improved by decreasing the factor from 10 to 7.

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.

Experimental study on performance change in F-band surface-wave oscillator due to slow-wave structure length

A surface-wave oscillator (SWO) is a high-power terahertz-wave source based on a slow-wave structure (SWS). A cylindrical corrugated waveguide is used as the SWS. The length of SWS is an important factor in the oscillation. Experiments of the SWO with various length structures are conducted. An instability responsible for the SWO changes depending on the SWS length. For the 200-period SWS, the radiation is due to the absolute instability and depends on the beam current like the temporal growth rate. By contrast, for the 80-period SWS, the radiation is due to the global instability having the current dependence like the spatial growth rate. Compared to the 80-period SWS, the 200-period SWS can improve the radiation power more than seven times in the backward wave oscillation (BWO) region. The maximum BWO power reaches up to about 10 kW and the maximum electric efficiency of 1.5% is obtained.

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.

Relativistic Sub-THz Surface-Wave Oscillators With Transverse Gaussian-Like Radiation Output

Surface-wave oscillators driven by intense relativistic sheet electron beams can produce sub-terahertz waves at sub-gigawatt level of output power; however, evanescent nature of the operating wave hampers further radiation transmission and application. For these devices, we propose a method of efficient conversion of the surface operating wave into the output Gaussian-like wavebeam radiated in transverse direction with respect to the grating by applying an additional corrugation with a period twice larger than the main period. The analysis based both on the quasi-optical model and on the PIC simulations shows feasibility of 150-GHz surface-wave oscillator with 35 % efficiency and 100 MW output power. The transverse energy extraction significantly reduces the Ohmic losses down to 10% of the radiated power, which is essential for sub-THz range.

Experimental Study on Microwave Generation due to Merged Instability in F-Band Surface Wave Oscillator

A surface wave oscillator (SWO) is driven by an electron beam to generate intense microwaves. The electron beam possesses slow space-charge (SSC) and slow cyclotron (SC) modes that interact with the surface wave leading to microwave generation. The beam current and external magnetic field affect the relationship between SSC and SC modes. The SSC mode gradually approaches the SC mode when the beam current increases. Meanwhile the SC mode gradually approaches the SSC mode when the magnetic field decreases. The two modes merge in a low magnetic field and high beam current. In this work, we experimentally examine the operation of an F-band SWO in the low magnetic field region. The output power decreases with low beam current when magnetic field decreases. Meanwhile, the SWO maintains its power level with high beam current even though the magnetic field decreases to around 0.4 T. The merged instability enables a sustained power level in the low magnetic field region.

Observing molten pool surface oscillations during keyhole processing in laser powder bed fusion as a novel method to estimate the penetration depth

Various in-situ monitoring techniques have been developed for the detection of process drifts in the Laser Powder Bed Fusion (LPBF) process. Currently, optical emission monitoring can retrieve information regarding molten pool characteristics, such as temperature, width, length and area which provide substantial process signatures. Nonetheless, a fundamental indicator for the retrieval of a complete set of spatially distributed information is missing: the molten pool depth. Within the present investigation, a system for the estimation of the penetration depth based on the detection of molten pool surface oscillations is reported. Initially, the fundamentals of the monitoring technique are presented. The principle relies upon the observation of molten pool surface ripples through the measurement of probe light reflections in the melt area. Proof of concept testing of the sensing principle was conducted through an experimental investigation on a prototypal platform. A monitoring system (consisting of a high-speed camera and a secondary illumination light) was employed to view the process while realising both bead-on-plate material remelting and single track powder bed fusion depositions of AISI316L at different levels of laser emission power. Oscillation frequencies were extracted from the high-speed imaging acquisitions after image processing and signal analysis. The surface wave oscillations were measured to be in the range of 3.5–5.5 kHz in keyhole conditions. Metallographic cross-sections allowed to observe the effective molten pool penetration depth and cross-sectional area and were correlated to oscillation frequencies. Higher values of oscillation indicated shallower penetration and consequently a smaller mass of molten material.

Design and Fabrication of 200-GHz Oversized Surface-Wave Oscillator

A surface-wave oscillator (SWO) is one of the promising terahertz-wave sources based on excitation of a surface wave using an electron beam. The surface wave is formed by a metal periodic structure, and the frequency is determined by the size parameter of the structure and the parameters of the electron beam. In this article, we design and fabricate a 200-GHz cylindrical corrugated waveguide which has the periodic structure on the inner wall. The field intensity of the surface wave rapidly decreases to zero from the structured wall. Due to a manufacturing error of the corrugated waveguide, the frequency of the surface wave in the fabricated structure decreases to 189 GHz at $\pi $ -point in comparison with the frequency of 204 GHz in the designed one. The effect of the error is evaluated by calculating the intensity distribution of the electric field of the surface wave and its growth rate due to the beam interaction. We also conduct the SWO experiment using the fabricated waveguide. Intense radiation with a total power is roughly estimated to 0.2 kW with a 45-keV and 60-A electron beam.

Experimental study of surface-wave oscillator with different electron beam forms

We experimentally studied intense terahertz-wave generation based on a W-band (0.1 THz) surface-wave oscillator (SWO) using annular electron beams with different beam forms. In order to generate different beam forms, we prepare two-type anodes with open beam limiter and close beam limiter. The electron beam is shaped by the close beam limiter, and the thickness becomes thinner than that of a non-shaped beam with the open limiter. The electronic efficiency of SWO with the shaped beam is improved in comparison to that with non-shaped beam, and maximum efficiency is about 1% for 0.1 THz radiation.

Enhancement-limit of Smith–Purcell radiation with self-bunched keV-order electron beam

We studied enhanced Smith–Purcell (SP) radiation produced by a self-bunched keV-order electron beam based on a W-band surface-wave oscillator. The self-bunched beam is generated in the process of the beam interaction with the surface-wave mode. The bunched beam contains the second harmonic of the surface wave that enhances the SP radiation. The observed radiation shows that the enhancement is limited when the interaction transitions from the Compton regime to the Raman regime.

Relativistic Surface Wave Oscillator in Y-Band with Large Oversized Structures Modulated by Dual Reflectors

To increase the generation efficiency of the terahertz wave in the Y band, the idea of dual-reflector is introduced in the relativistic surface wave oscillator (SWO) with large oversized structures. The dual-reflector and the slow-wave structure (SWS) construct a resonator where the field strength of TM01 mode inside is intensively enhanced and then the efficiency is increased. The pre-modulation on electron beam caused by the reflector is also helpful in improving the output power. Meanwhile, the reflector can reduce the loss of negatively going electrons. Through the particle-in-cell (PIC) simulations, the optimized structure is tested to be stable and little power is transmitting back to the diode area. The output power reaches 138 MW in the perfectly electrical conductivity condition and the frequency is 337.7 GHz with a pure spectrum. The device’s efficiency is increased from 10.7% to 16.2%, compared with the device without any reflectors. The performance of device with lossy material is also focused on. In the situation of copper device, the output power is about 41 MW under the same input conditions and the corresponding efficiency is about 4.8%.

Effects of frequency-dependent surface impedance on the vacuum electronic terahertz sources

When the working frequency of vacuum electronic device reaches the terahertz frequency, the ohmic loss has a significant impact on the vacuum electronic device. To study the effect of the ohmic loss on the working characteristic of the vacuum electronic terahertz devices, this paper implements the frequency-dependent surface impedance boundary condition (SIBC) in the 3 dimensional particle in cell code UNIPIC-3D. Conformal mesh is adopted in the code to overcome the staircase error in traditional particle in cell method. By using the surface impedance boundary, we eliminate the need to study the field inside the lossy dielectric objects which require extremely small grid cells for numerical stability. In comparison with constant parameter SIBC, the dispersive SIBC is applicable over a very large frequency bandwidth and over a large range of conductivities. The correctness of the implementation is verified by simulating the lossy resonant cavity and circular waveguide, the simulated power loss is comparable with the theoretical predication. High power vacuum electronic devices of terahertz regime are attracting extensive interests due to their potential applications in science and technologies. The impulse-wave relativistic surface wave oscillator (SWO) and low-voltage continuous-wave planar grating backward wave oscillator (BWO) both made of copper are numerically studied by using UNIPIC-3D and dispersive surface impedance boundary condition. Numerical results show that the strongest field is very close to the slow wave structure where the beam-wave interaction occurs and that terahertz wave generates both in these two devices. The distributed wall loss has a considerable effect on the devices: the output power has a significant decrease and the startup time becomes longer, but the working frequencies of the two devices keep unchanged. To improve the efficiency of relativistic SWO, a resonant reflector is proposed between the diode and the slow wave structure. Numerical results show that the working frequency of the device with a resonant reflector keeps unchanged as the original one, but the output power increases to 60 MW from 41 MW of the original one when the ohmic loss is considered.

Pumping Systems for Compton Free-Electron Lasers: Microwave Undulators and Powering Sources

The concept of Compton-type free-electron lasers (FELs) operating in short wavelength ranges with a high efficiency and power level is currently underway at the IAP RAS (Nizhny Novgorod). This concept is aimed at reducing the energy of a driving relativistic electron beam and thereby increasing the efficiency of the electron–wave interaction in FELs, as well as making the oscillator relatively compact. The basis of this concept is microwave undulators of a new type—the so-called “flying” undulators. This paper is devoted to the results of the current studies of these undulators, their simulation, and “cold” electrodynamic tests in the Ka band. For powering microwave undulators, a spatially extended narrow-band ˇ Cerenkov surface-wave oscillators (SWOs) are developed in the specified frequency range driven by Sinus-6, a high-current accelerator, with a particle energy of 0.5 MeV, a current of 5 kA, and a pulse duration of 25 ns. The required sub-gigawatt power level of output radiation combined with a high stability of the narrow-band oscillation regime is achieved under conditions of a strongly oversized oscillator by using two-dimensional distributed feedback provided in a 2D doubly-periodic slow-wave structure. The design parameters of a 32 GHz/0.5 GW SWO intended for powering microwave undulators are presented and the results of its simulation and reported.

Acoustic cavitation and ultrasound-assisted nitration process in ultrasonic microreactors: The effects of channel dimension, solvent properties and temperature

Experimental studies on acoustic cavitation and ultrasound-assisted nitration reaction were systematically investigated in two laboratory-built ultrasonic microreactors by tuning the microchannel dimension, solvent properties and temperature. Under ultrasound irradiation, acoustic cavitation microbubbles were generated and underwent violent oscillation in microchannel. With the decrease of channel size, acoustic cavitation was largely confined, and channel size 1 × 1 mm2 was recognized as the critical size to eliminate the confinement effect. Acoustic cavitation was also highly dependent on the properties of sonicated liquids. The onset of surface wave oscillation on gas bubble was obviously promoted with decreasing solvent viscosity and surface tension. Additionally, ultrasound-assisted nitration process of toluene was studied in a temperature-controlled ultrasonic microreactor. The effects of channel size as well as liquid properties on ultrasound intensification agreed well with the finding in cavitation research. Under ultrasound power 50 W, toluene conversion was enhanced by 9.9%–36.3% utilizing 50 vol.% ethylene glycol aqueous solution as ultrasound propagation medium, exhibiting ultrasound applicability on intensifying fast reaction processes in microreactors.

Non-Axisymmetric Radiation from Surface-Wave Oscillator Driven by Weakly Relativistic Electron Beam

Non-Axisymmetric Radiation from Surface-Wave Oscillator Driven by Weakly Relativistic Electron Beam