Surface Wave Oscillator Research Articles

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.

A megawatt-level surface wave oscillator in Y-band with large oversized structure driven by annular relativistic electron beam

High power vacuum electronic devices of millimeter wave to terahertz regime are attracting extensive interests due to their potential applications in science and technologies. In this paper, the design and experimental results of a powerful compact oversized surface wave oscillator (SWO) in Y-band are presented. The cylindrical slow wave structure (SWS) with rectangular corrugations and large diameter about 6.8 times the radiation wavelength is proposed to support the surface wave interacting with annular relativistic electron beam. By choosing appropriate beam parameters, the beam-wave interaction takes place near the π-point of TM01 mode dispersion curve, giving high coupling impedance and temporal growth rate compared with higher TM0n modes. The fundamental mode operation of the device is verified by the particle-in-cell (PIC) simulation results, which also indicate its capability of tens of megawatts power output in the Y-band. Finally, a compact experimental setup is completed to validate our design. Measurement results show that a terahertz pulse with frequency in the range of 0.319–0.349 THz, duration of about 2 ns and radiation power of about 2.1 MW has been generated.

Intensification of liquid-liquid two-phase mass transfer by oscillating bubbles in ultrasonic microreactor

The synergistic effects of gas agitation and ultrasound on mass transfer between immiscible liquids were investigated in an in-house made ultrasonic microreactor. With the introduction of inert gas (N2), a three-phase slug flow with slug bubbles either dispersed in continuous aqueous phase or encapsulated in oil plugs was observed. Under ultrasound irradiation, slug bubbles underwent surface wave oscillation and induced agitation in microchannel. In addition, microbubbles were generated by acoustic cavitation, oscillating intensely and resulting in the formation of O/W emulsion. Bubble oscillation (i.e., slug bubbles and microbubbles) as well as emulsification promoted liquid-liquid mass transfer significantly. Extraction of vanillin from aqueous solution to toluene was employed to demonstrate the mass transfer enhancement. Compared with silent operation, both mass transfer coefficient and extraction efficiency were largely improved by the combined use of gas agitation and ultrasound. With gas flow velocity being 0.005–0.083 m/s at fixed ultrasound power of 30 W, the overall mass transfer coefficients ranged from 0.047 s−1 to 0.429 s−1, which was 2.33–17.20 times larger than the corresponding liquid-liquid two-phase process without ultrasound irradiation.

Powerful surface-wave oscillators with one-dimensional and two-dimensional periodic planar structures

Powerful surface-wave oscillators with one-dimensional and two-dimensional periodic planar structures

Efficiency improvement of THz overmoded surface wave oscillator by circular spoof surface plasmon polaritons coupler

The terahertz surface wave oscillator (SWO) faces the problem of the low energy conversion efficiency due to the fact that working mode is the surface wave but the output mode is the volume wave, and it is very difficult to extract the terahertz wave efficiently. In the meantime, the possibility of the RF breakdown increases as a result of the intense electric field on the surface of slow wave structures (SWSs). To overcome these problems, the field distribution in the SWSs of the 0.147 THz relativistic SWO is analyzed in this paper, and the results indicate that the electric field distribution in the SWSs is similar to that in spoof surface plasmon polaritons (SSPPs). In order to improve the energy conversion efficiency of the SWO, we propose to adopt the circular SSPP coupler with tapered gratings at the end of the uniform SWSs, which is implemented to smoothly bridge the surface wave and guided wave. Second, the SWO with the coupler is simulated using particle-in-cell code UNIPIC. The simulation results indicate that the output terahertz power enhances 52% after the SSPP coupler is used. More importantly, the numerical results demonstrate that the intensity of the electric field decreases 25% on the corrugation surface of the SWSs, so the RF breakdown can be avoided in the terahertz relativistic SWO.

Stable lossless polaritons on non-Hermitian optical interfaces

Surface polaritons, e.g., surface plasmon polaritons, are invaluable tools in nanophotonics. However, considerable plasmon loss narrows the application regime of plasmonic devices. Here we reveal some general conditions for lossless surface polaritons to emerge at the interface of a gain and a loss media. The gain medium does not only compensate the energy loss, but also modifies surface wave oscillation mechanisms. A new type of surface polaritons induced by the sign switch of the imaginary part of the permittivity across the interface is discovered. The surface polaritons exhibit spin Hall effect due to spin-momentum locking and unique Berry phase. The spin Hall coefficient changes the sign across the parity-time symmetric limit and becomes quantized for perfect metal-dielectric interface and for dielectric-dielectric interface with very large permittivity contrast, carrying opposite topological numbers. Our study opens a new direction for manipulating light with surface polaritons in non-Hermitian optical systems.

Study on Operation of a Surface-Wave Oscillator Around the $\pi$ -Point Region

We numerically and experimentally studied the operation of a W-band (75–110 GHz) surface-wave oscillator driven by the electron beam energy of less than 100 keV. Two numerical methods of the real wavenumber analysis and the saddle point analysis are studied to examine the operation of the oscillator. We analyzed the Cherenkov instability for the interaction of the space charge modes and the structure modes EH01. We showed that the instabilities are of absolute instabilities when the oscillator operates around the $\pi $ -point region. In the experiment, radiations due to the absolute instability are detected around the $\pi $ -point region when the length of the slow-wave structure (SWS) is short enough. On the other hand, the absolute instability generates no meaningful radiation in the $\pi $ -point region for the long length of SWS.

弱い相対論的電子ビームを用いた大口径W-band表面波発振器による大強度マイクロ波の発振特性

弱い相対論的電子ビームを用いた大口径W-band表面波発振器による大強度マイクロ波の発振特性

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

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

A high-power subterahertz surface wave oscillator with separated overmoded slow wave structures**Project supported by the National Natural Science Foundation of China (Grant No. 61231003).

A megawatt-level subterahertz surface wave oscillator (SWO) is proposed to obtain high conversion efficiency by using separated overmoded slow wave structures (SWSs). Aiming at the repetitive operation and practical applications, the device driven by electron beam with modest energy and current is theoretically analyzed and verified. Then, the functions of the two SWS sections and the effect of the drift tube are investigated by using a particle-in-cell code to reveal how the proposed device achieves high efficiency. The mode analysis of the beam-wave interaction region in the device is also carried out, and the results indicate that multi-modes participate in the premodulation of the electron beam in the first SWS section, while the TM01 mode surface wave is successfully and dominantly excited and amplified in the second SWS section. Finally, a typical simulation result demonstrates that at a beam energy of 313 keV, beam current of 1.13 kA, and guiding magnetic field of above 3.5 T, a high-power subterahertz wave is obtained with an output power of about 70 MW at frequency 146.3 GHz, corresponding to the conversion efficiency of 20%. Compared with the results of the previous subterahertz overmoded SWOs with integral SWS and similar beam parameters, the efficiency increases almost 50% in the proposed device.

Mode competition and selection in overmoded surface wave oscillator

The overmoded surface wave oscillator (SWO) is one of the promising devices to generate high-power millimeter and subterahertz waves for its merits of high efficiency and easy fabrication. But the employed slow wave structure with large diameter may introduce mode competition as the adverse effects. Therefore, the mode competition and selection in the overmoded surface wave oscillator are investigated in detail in this paper. By using the theoretical analysis and particle-in-cell simulation, the potential transverse mode and axial mode competition is pointed out, and the physical mechanisms and methods for mode selection are investigated. At last, the results are verified in the design of a 0.14 THz overmoded SWO without mode competition, which can generate the output power up to 70 MW at the frequency of 146.3 GHz with conversion efficiency almost 20% when beam voltage and current are, respectively, about 313 kV and 1.13 kA.

Overmoded subterahertz surface wave oscillator with pure TM01 mode output

Overmoded O-type Cerenkov generators using annular electron beams are facing the problem of multi-modes output due to the inevitable structural discontinuities. A simple but effective method to achieve the pure TM01 mode output is applied on the 0.14 THz overmoded surface wave oscillator (SWO) in this paper. In spite of still using an overmoded slow wave structure to ensure the easy fabrication, the followed smooth circular waveguide is shrinkingly tapered to the output waveguide with appropriate radius that it cuts off other higher modes except TM01 mode. Moreover, the modified device here has the same power capacity as the previous one according to the numerical analysis. By optimized lengths of the transition waveguide and tapered waveguide, particle-in-cell simulation results indicate that the subterahertz wave with output power increased 14.2% at the same frequency is obtained from the proposed SWO under the previous input conditions, and importantly, the output power is all carried by TM01 mode as expected. Further simulation results in the pulse regime confirm the feasibility of the optimized structure in the actual experiments. This simple and viable design is also applicable to overmoded devices in the lower frequency band of subterahertz wave.

Hydrodynamics and mass transfer of oscillating gas‐liquid flow in ultrasonic microreactors

Ultrasonic microreactors were used to intensify gas‐liquid mass‐transfer process and study the intensification mechanism. Fierce surface wave oscillation with different modes was excited on the bubble. It was found that for slug bubbles confined in smaller microchannel, surface wave oscillations require more ultrasound energy to excite due to the confinement effect. Cavitation microstreaming with two toroidal vortices was observed near the oscillating bubble by a streak photography experiment. Surface wave oscillation at the gas‐liquid interface increases the specific surface area, while cavitation microstreaming accelerates the interface renewal and thus improves the individual mass‐transfer coefficient. With these two reasons, the overall mass‐transfer coefficient was enhanced by 3–20 times under ultrasonication. As for gas‐liquid flow hydrodynamics, ultrasound oscillation disturbs the bubble formation process and changes the initial bubble length and pressure drop. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1294–1307, 2016

Small-signal theory of subterahertz overmoded surface wave oscillator with distributed wall loss

A small-signal theory of the overmoded surface wave oscillator (SWO) with distributed wall loss is presented in this letter. The wall loss considered here includes the surface resistance and surface roughness. The cold and hot characteristics of 0.14 THz SWO are studied by the small-signal theory. Numerical results show that as the increase of wall loss, the working frequency decreases slightly, the rise time and startup time of oscillation increase significantly, and the output power decreases dramatically. Particle-in-cell (PIC) simulation confirms the prediction by the small-signal theory.

Novel high-power subterahertz-range radial surface wave oscillator

A novel high-power subterahertz-range radial surface wave oscillator (SWO), in which the electron beam is emitted radially and interacts with the slow wave structure (SWS) machined on a planar plate, is presented in this paper. Compared to the axial SWO where the electron beam is emitted axially and interacts with the SWS machined on the inner wall of a cylindrical waveguide, the radial SWO has two advantages. One is that fabrication of the radial SWS is much easier than that of the axial SWO. The other is that the radial SWO is a low-impedance device, it can produce much higher current than the axial SWO when they are driven by the same driven voltage, and hence, it may generate much higher output power. Particle-in-cell simulation results demonstrate that the proposed radial SWO driven by the voltage of 312 kV can produce the terahertz wave with the mean output power of 680 MW at the frequency of 0.142 THz, it has a very pure TM01 mode and the higher modes can be effectively suppressed.

Quantum ion-acoustic wave oscillations in metallic nanowires

The low-frequency electrostatic waves in metallic nanowires are studied using the quantum hydrodynamic model, in which the electron and ion components of the system are regarded as a two-species quantum plasma system. The Poisson equation as well as appropriate quantum boundary conditions give the analytical expressions of dispersion relations of the surface and bulk quantum ion-acoustic wave oscillations.

Influence of slow wave structure explosive emission on high-power surface wave oscillator

As the working frequency of a vacuum electron device reaches the terahertz frequency band, the cross section of the surface wave oscillators (SWO) becomes very small, and the micro-fabrication precision of the device cannot be guaranteed, at the same time, because the electromagnetic field of SWO concentrates on the inner surface in slow wave structure, when the working voltage of surface wave oscillator is very high, the explosive emission probability of the slow wave structure increases greatly, and the explosive emission can influence the working characteristic of the device. This paper analyses the distributing property of the electrical field in the slow wave structure of 0.14 THz SWO. Parameters of the SWO under study are as follows: working voltage is 312 kV, explosive emitted current is 1.67 kA, periodic length of the slow wave structure is 0.7 mm, width of the slot is 0.4 mm, and the height of it is 0.3 mm; cold-test results indicate that the amplitude of the electrical field in the slow wave structure varies sinusoidally; the amplitude of the electrical field reaches a maximum value in the middle of the slow wave structure near its inner surface, and the explosive electron emission can occur most possibly in this position, because the electrical field in the slow wave structure varies with very high working frequency. The explosive emitted electron may bombard back the slow wave structure, and the secondary electrons will be emitted at a certain probability, for which the formula proposed by Vaughan is used to compute the secondary emission yield, and this formula is implemented in the self-developed particle-in-cell code UNIPIC; while the code is used to simulate 0.14 THz SWO with explosive emission in the slow wave structure. In the simulation, the slow wave structure multipactor discharge induced by electrons is also considered; the phase space of the electrons emitted from the slow wave structure shows that the energy of secondary electrons is below 5 keV, so the validity for secondary electron yield is affirmed. Numerical simulation results indicate that because the emitted electrons from the slow wave structure change the distribution character of the electrical field in the slow wave structure, especially the amplitude of the electrical field in the middle of the slow wave structure, the beam-wave interaction is weakened, and as a result, output power decreases from about 22.6 megawatts to only 1.89 megawatts.

Effect of Ohmic loss on coaxial surface wave oscillator in terahertz band

When the working frequencies of vacuum electronic devices reach the terahertz frequency (0.1–10 THz), the Ohmic loss has a great impact on the vacuum electronic devices. To study the effect of the Ohmic loss on the working characteristic of the vacuum electronic devices in the terahertz band, this paper presents the boundary condition of surface impedance used in the 2.5-dimensional fully electromagnetic particle simulation code UNIPIC, which is verified by simulating the terahertz wave in the circular copper waveguide; the simulation result indicates that the code can correctly simulate the propagation of terahertz waves in the waveguide with an Ohmic loss. Then, the coaxial surface wave oscillators (SWO) with slow wave structures (SWS) made of different metals are numerically studied by using the above code, and the dependences of output power on the SWOs with different metal SWSs are analyzed. Numerical results show that the metal conductivity has a considerable effect on the output power of the device: When the conductance of the metal decreases, the quality factor of the device becomes smaller, the start-up time becomes longer, also the output power of the device decreases also. For the coaxial SWOs operating at 0.14 THz, the output powers from the copper and stainless steel SWSs are reduced by 13.4% and 63.9%, the start-up times of the devices are delayed by 0.4 ns and 15 ns, respectively. Meanwhile, the working frequencies of the devices with the SWSs made of different metals keep unchanged.

Numerical studies on a 0.14 THz coaxial surface wave oscillator with double-ring metamaterial slow wave structure

This paper presents a relativistic coaxial overmoded surface wave oscillator (SWO) working at the terahertz band in the double-ring metamaterial slow wave structure (SWS). A relativistic electron beam passes through the SWS between the inner and outer rings. A coaxial overmoded SWS made up of metal metamaterial is designed to generate the high-power terahertz wave by increasing the beam-wave interaction efficiency and enlarging the transverse size of the terahertz device. It consists of double rings periodically arrayed along the z-direction, and a coaxial conductor with a radius of 2.4 mm. By its dispersive relation the proposed device is studied, from which we choose the 0.14 THz as the operating frequency of the device. Then the parameters of the geometric structure and the electron beam are optimized; the transitional section for extracting the terahertz signal is designed of the largest propagation coefficient. Particle simulation code UNIPIC is employed to verify the initial expectation and potential advantages. When the beam voltage and current are increasing, the operating frequency of the device remains almost constant, and this is the typical characteristic of the SWO. Particle simulation results show that the coaxial inner conductor has a stable operating mode of double-ring metamaterial SWS and can increase the beam-wave interaction efficiency of the SWO at the terahertz band. For a guiding magnetic field of 2.0 T, with the electron beam of 600 kV and a current of 1.0 kA, a 0.141 THz wave output power of 316.8 MW is obtained.

Study on 0.34 THz overmoded surface wave oscillator

The simulation design and preliminary experiment on a 0.34 THz large-power overmoded surface wave oscillator are presented in this paper. For the slow wave structure (SWS) with large overmoded ratio (D/λ ≈ 6.8), a small signal theory is derived for appropriate dimensions of SWS and gap between electron beam and SWS, and makes the device oscillate near the π point of surface wave at TM01 mode. PIC (particle in cell) simulation results show that this SWO (small wave oscillation) can genetate the terahertz wave with frequency and output power of 0.34 THz and 22.8 MW, respectively. SWS with stainless steel is integrally and precisely fabricated by employing mirco-EDM technology, and the experimental setup is built based on a miniaturized pulse power driving source. Results of preliminary experiment and diagnostics show that a terahertz pulse is radiated at a frequency range of 0.319–0.349 THz, a power of no less than 250 kW, and a pulse duration of about 2 ns at beam voltage of about 420 kV and beam current of about 3.1 kA. Finally, the reason for discrepancy between the measured power and the simulation result is analyzed and discussed, laying the foundation for the performance improvement of terahertz surface wave oscillator.