Backward Wave Oscillator Research Articles

An S-band klystron-like relativistic backward wave oscillator operating at low magnetic field with high conversion efficiency

In order to overcome the problem of efficiency decrease in the relativistic backward wave oscillator (RBWO) operating at low magnetic field, an S-band klystron-like RBWO with a three-period slow-wave cavity-chain and a dual-cavity extractor is proposed and investigated in detail in this paper. In the device, several measures are taken to enhance the conversion efficiency at low magnetic field, which include increasing the beam bunching depth by a two-stage modulation structure comprised of a resonant reflector upstream and a three-period cavity-chain modulator downstream, strengthening the energy exchange process from the electron beam to the microwave by a dual-cavity extractor, and optimizing the envelope distribution of the electron beam in the interaction region by adjusting the drift length before the resonant reflector. Under the condition of the guiding magnetic field of 0.28 T, the device could generate a microwave with a power of 4.3 GW, a frequency of 3.79 GHz, and the conversion efficiency of 43.3% in the simulation.

Experimental study on the influence of thermal accumulation on breakdown in slow-wave structures

Radio frequency (RF) breakdown in the slow-wave structures (SWSs) is a crucial bottleneck restricting relativistic backward wave oscillator (RBWO) to pursuing higher output power and longer pulse width. This paper has experimentally studied the influence of thermal accumulation during repetitive operation on RF breakdown in an X-band RBWO. A method for cooling the SWSs using water flow has been proposed to restrain the temperature rise to some extent. Under different heat dissipation conditions, the operating states of the RBWOs exhibit great differences. The greater the distance between the water-cooling heat transfer channel and the SWSs, the more serious the pulse shortening of high-power microwave. Moreover, the breakdown traces appearing in the SWSs become more obvious with the worse convective heat transfer capacity. The observed experimental phenomena provide a new guideline that helps to enrich the mechanism of RF breakdown and to explore corresponding suppression methods in RBWO.

Investigation of High-Power Ring-Bar Traveling Wave Tube With Metamaterial Absorber

A rectangular split resonant ring (RSRR) metamaterial absorber (MMA) was proposed to suppress backward wave oscillation (BWO) in a ring-bar traveling wave tube (RB-TWT). The unit structure parameters of the RSRR were optimized to maximize the absorption of backward waves. The amplification performance of the proposed slow wave structure (SWS) was also carried out by using particle-in-cell (PIC) simulations. With an input power of 0.125 W, the operating bandwidth of the RB-TWT loaded with RSRR-MMA was 8–13 GHz, which was higher than that loaded with centralized attenuators (CAs; 8–11.5 GHz). The output power exceeded 5 kW with a gain of 46 dB in the range of 9.1–12.7 GHz. Furthermore, its saturated output power exceeded 5 kW, the saturated gain was over 41 dB, and the saturated electronic efficiency was more than 14.5% over the entire working frequency range of 8–13 GHz. Therefore, the RB-TWT with RSRR-MMA achieved a wider bandwidth compared with that of the RB-TWT with CA, where exhibits the potential application of RSRR-MMA in high-power vacuum electronic devices.

A Technique for Planar Integration of Components of W-Band Backward-Wave Oscillator to Eliminate Misalignment

The alignment within the required tolerance between components like electron beam source, beam-wave interaction structure and spent beam collector is a major challenge in the realization of the high-power vacuum-tube sub-THz source. To overcome this major issue, a novel integrated W-band beam-wave interaction circuit along with spent beam collector and beam adapter region has been designed and developed on two planar metallic plates itself for the first time. It accomplished the level of required alignment of different components of a sub-THz source. This interaction circuit along with beam adapter region and spent beam collector has been developed on a single plate. These structures have been developed using optimized micro-milling techniques with achieved dimensional deviation less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ &lt; 7~\mu \text{m}$ </tex-math></inline-formula> . The average surface roughness is found to be around 100 nm, which is much less than the skin depth in the W band.

Tunable THz flat zone plate based on stretchable single-walled carbon nanotube thin film

Tunable optoelectronics have attracted a lot of attention in recent years because of their variety of applications in next-generation devices. Among the potential uses for tuning optical elements, those allowing consistent parameter control stand out. We present an approach for the creation of mechanically tunable zone plate lenses in the THz range. Our devices comprise single-walled carbon nanotube (SWCNT) thin films of predetermined design integrated with stretchable polymer films. These offer high-performance and in situ tunability of focal length up to 50%. We studied the focusing properties of our lenses using the backward-wave oscillator THz imaging technique, supported by numerical simulations based on the finite element frequency domain method. Our approach may further enable the integration of SWCNT films into photonic and optoelectronic applications and could be of use for the creation of a variety of flexible and stretchable THz optical elements.

Square- and Rectangular-Ring Vertex-Bar Slow Wave Structures for High-Efficiency Wide Bandwidth TWTs

In this article, novel square-ring vertex-bar (SRVB) and rectangular-ring vertex-bar (RRVB) slow wave structures (SWSs) are proposed to develop high-efficiency wide bandwidth traveling wave tubes (TWTs). High-frequency characteristics including dispersion, interaction impedance, and transmission of these novel slow wave circuits are analyzed. It is shown that both the SRVB and RRVB possess the advantages of flatter dispersion and higher interaction impedance of the fundamental space harmonic of the forward wave in comparison with the traditional circular ring-bar (RB) SWS. Beam–wave interaction study of SRVB TWT with a pencil beam and RRVB TWT with a sheet beam is carried out. The particle-in-cell (PIC) simulations predict that with the beam voltage of 15.7 kV and beam current of 0.23 A, the maximum output power of the SRVB TWT and the RRVB TWT can reach 616 and 695.6 W, respectively. Both the SRVB TWT and RRVB TWT can produce larger output power, higher differential small-signal gain, and wider 3-dB bandwidth than the RB counterpart. The SRVB TWT is superior to the RRVB TWT for its stability against backward-wave oscillations but with relatively small output power.

The impact of accelerating diode resonances on L-band high-power long-pulse relativistic backward wave oscillator operation

An L-band high-power relativistic backward wave oscillator is designed. In the simulation, microwaves centered at 1.6 GHz are generated, with the power of 3.6 GW and the efficiency of 40%. In the preliminary experiment, the pulse duration of the device was only 45 ns, presenting a pulse-shortening phenomenon. Through the 3D particle-in-cell simulation analysis, it was found that the accelerating diode resonances significantly impact the operation of the L-band high power relativistic backward wave oscillator, and the resonance of a TE11 mode in the accelerating diode played the primary role in the pulse shortening. Moreover, we found that choosing the appropriate distance between the cathode baffle and the end of the annular cathode is beneficial to effectively suppress the starting oscillation of the parasitic TE11 mode. In the improved experiment, we changed the distance between the cathode baffle and the end of the annular cathode from previous 5.4 to 4.6 cm. Eventually, when the diode voltage is 650 kV and the diode current is 14 kA, microwaves centered at 1.58 GHz are generated with the power of 3.3 GW, the efficiency of 36%, and the pulse duration above 104 ns.

Nonlinear Dynamics of Radiation in Multiple-Beam Vacuum Electronic Devices

The article is devoted to overview of different types of vacuum electronic devices with two or more charged particle beams. There are travelling wave and backward wave tubes, free electron lasers and masers, volume free electron lasers. Two different cases take place in such situation: multiple-beam instability in such devices and multiple-stream instability. In the first case some charged particle beams moves in the system with different velocities. In the second one there are beams with almost equal velocities (streams). Two systems of equations for volume free electron laser with two electron beams are proposed. Some numerical results of VFEL numerical simulation are given and discussed.

GEANT4 simulation of parasitic X-ray radiation for backward wave oscillator SINUS-350

Parasitic X-ray radiation for the backward wave oscillator SINUS-350 is investigated. Using the GEANT4 simulation toolkit we have made a model of this oscillator which allows calculating the radiation dose and energy spectrum around the accelerator. The results of these calculations have been compared with measurements for 2 s accelerator operation time. The obtained values of radiation doses at a distance of 1–2 m from the generator are in the range (10–300) μSv. These values significantly exceed the maximum permissible amount because the personal maximum permissible dose per year is 1000 μSv. Therefore, the question of how to achieve a significant reduction is very important. We have also calculated the energy spectrum of the X-rays for different places. The largest part of the spectrum is in the range 50–350 keV.

Gain and Bandwidth Improvement Studies of Millimeter Wave Periodically Dielectric Loaded Gyro-Twystron Amplifier

In this article, the spurious oscillations in the output section of gyro-twystron have been analyzed and the design and modeling of its various subassemblies also been presented. The periodic dielectric loading (PDL) technique has been adapted in the output waveguide section to improve the device gain by suppressing the backward wave oscillations (BWOs). Further, to improve the performance of the amplifier all the cavities were staggered tuned. The particle-in-cell (PIC) simulation of the present PDL gyro-twystron predicted an RF output power of ~120 kW in the fundamental TE01 mode at 94 GHz. The conversion efficiency, 3-dB bandwidth, and saturated gain were calculated as ~34%, ~2.2 GHz, and ~51 dB, respectively. A triode magnetron injection gun (MIG) was designed for 60 kV and 6 A gyrating electron beam and optimized for the velocity spread of ~2.2% with a velocity ratio of 1.6. A single-stage depressed (SSD) collector has also been designed to enhance the efficiency of the amplifier to ~57%. A double-disk window was designed and optimized for the broader bandwidth of ~8 GHz.

A compact S-band relativistic backward wave oscillator with a 2.5-period slow-wave structure

Compact is a significant research direction of low-frequency high-power microwave devices. In this paper, a compact S-band relativistic backward wave oscillator is presented. The radial dimension of the device is reduced by utilizing the characteristic that the cutoff frequency of the fundamental mode (quasi-TEM mode) of the coaxial Slow-Wave Structure (SWS) is zero. Moreover, a coaxial extraction structure is designed to enhance its resonance characteristics to reduce the number of slow-wave structures. In addition, a reflective structure with the phase difference between inner and outer conductors is used, which replaces the pre-reflection cavity and shortens the axial size of the device. A relativistic backward oscillator with a 2.5-period coaxial slow-wave structure is proposed through theoretical analysis, simulation calculation, and experiment. Its radial dimension is 160 mm (∼1.28λ), and its axial dimension is 320 mm (∼2.56λ). In the experiment, the results show that the output microwave has a center frequency of 2.41 GHz, a power of 1.5 GW, a pulse width of 92 ns, and continuous operation for 20 s at a repetition of 10 Hz.

A Multi-Constrained Optimization Method for THz Backward Wave Oscillators

The current design period for various backward wave oscillators (BWOs) is still at least several months. How to find the best structure parameters with an efficient and stable optimization method is a problem facing researchers in both scientific research and engineering work. In this paper, a non-randomized iterative optimization method is proposed. It applies orthogonal design methods to find local solutions that can provide optimal ‘gradient direction’ for several successive next iteration steps. An evaluation function is designed to distinguish the better ones from the local solutions in the multi-constrained optimization of such BWOs. Optimizations from different starting points are performed separately for a global optimal solution. Two BWOs at different frequency ranges are optimized using the proposed method. The validity and stability of the method are verified. It is believed that the method can provide the global optimum and shorten the design period of THz BWOs.

Study on temperature distribution and response of electron collector in high-power microwave generator

Relativistic backward wave oscillator is a high-power microwave generator with great application potential. Since a large amount of waste electron are deposited on the collector, the temperature of the collector rises, which reduces the safety and stability of relativistic backward wave oscillator. The collector temperature characteristics are calculated by using the Fourier thermal conduction model and the phase change heat transfer model. A method to reduce the collector temperature and shorten the retardation time is obtained. The results show that the collector temperature, water temperature, and vapor volume fraction show periodic fluctuation. The collector temperature can be reduced by increasing the working pressure, increasing the volume flow rate, decreasing the channel width, and increasing the thermal conductivity of the collector material. The temperature response of the collector thermal conduction is synchronized with the thermal interaction in the nanosecond's order. There is a retardation time for the convection and phase change heat transfer. The convection heat transfer retardation time is about 5 ms, and the phase change heat transfer retardation time is about 10 ms. The thermal conductivity, channel diameter, and volume flow rate have great effects on the heat dissipation, so they are the major influence factors of retardation time.

Design and Simulation Investigations of Dual-Band RBWO Using Sectional Slow Wave Structures for Long Pulse Generation

In this article, a relativistic backward wave oscillator (RBWO) with sectional slow wave structures (SWSs) has been designed to generate a long high-power microwave (HPM) pulse at two different frequencies. Two individual SWSs were cascaded using a drift section (drift section-II) that separated them at a sufficient distance to generate dual microwave frequencies. The first section of the SWS (SWS-I) has been designed to generate S-band frequency and the second section of the SWS (SWS-II) to generate C-band frequency. A rectangular resonant reflector (RR) has been used to reflect the backward traveling wave into a forward wave toward the collector. The drift section-II also acted as an RR for the C-band frequency in addition to make the phase adjustment between the backward and forward microwaves and end reflections of SWS-I. The performance of the dual-band RBWO with sectional SWSs has been studied through the particle-in-cell (PIC) simulation by a finite difference time domain (FDTD)-based numerical code. The present simulation predicted a total RF output power of ~600 MW in TM01 mode at ~3.6 and ~4.5 GHz with a power conversion efficiency of ~20% for an annular electron beam with voltage ~550 kV, developed current ~5.4 kA, and the guiding magnetic field of ~1.3 T. A clear and more stable RF output power up to 100 ns of simulation time at both operating frequencies was observed with a frequency difference (between S- and C-band) of ~0.9 GHz.

Design, Simulation, and Cold Test of a W-Band Double Nonparallel Staggered Grating Backward Wave Oscillator

A novel double nonparallel staggered grating (DNPSG) slow wave structure (SWS) is proposed to enhance the coupling impedance in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula> -band backward wave oscillator (BWO) driven by a pseudospark-sourced sheet electron beam. The DNPSG SWS has been shown a broadband of 72–125 GHz and a higher coupling impedance compared with the traditional double staggered grating (DSG) SWS in simulation. The DNPSG BWO structure consisting of ten SWS units and a broadband output structure is designed and fabricated. In the cold test of the DNPSG BWO, the measured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{11}$ </tex-math></inline-formula> (double of the losses in the DNPSG BWO) is above −10 dB in most of the band, which is satisfactory in the pseudospark-driven high-power device. The hot-test performance of the DNPSG BWO is analyzed by particle-in-cell (PIC) simulation in a beam voltage range of 14–90 kV and a current density range of 1.5– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\times10$ </tex-math></inline-formula> 7 A/m2, obtaining a high output power (max. 190 kW) over an ultrawide tuning band of 38 GHz (75–113 GHz) due to enhanced coupling impedance.

Microfabrication, Characterization, and Cold-Test Study of the Slow-Wave Structure of a Millimeter-Band Backward-Wave Oscillator with a Sheet Electron Beam

In this paper, the results of the microfabrication, characterization, and cold-test study of the previously proposed truncated sine-waveguide interaction structure with wideband-matched output couplers for the millimeter-band backward-wave oscillator (BWO) driven by a high-current-density sheet electron beam are presented. Computer-numerical-control (CNC) micromilling was used to fabricate the designed interaction structure. The first sample was microfabricated from an aluminum alloy to test the milling process. The final sample was made from oxygen-free copper. Scanning electron microscopy (SEM) and optical microscopy were used to investigate the morphology of the microfabricated samples, and stylus profilometry was used to estimate the level of the surface roughness. Cold S-parameters were measured in Q- and V-bands (40–70 GHz), using a vector network analyzer (VNA). Using the experimentally measured phase data of the transmitted signal, the dispersion of the fabricated interaction structure was evaluated. The experimentally measured dispersion characteristic is in good agreement with the numerically calculated.

A Novel Helical Slow-Wave Structure of a TWT With Increased Number of Helix-Support Rods for the Suppression of BWO Caused by Thermal Deformation

Backward-wave oscillation (BWO) is one of the important factors affecting the output power and operating stability of helix traveling-wave tubes (TWTs). A novel helical slow-wave structure (SWS) with six helix-support rods at the end of SWS of TWT is proposed to suppress BWO caused by thermal deformation. An electrical–thermal–mechanical finite element coupling analysis model for the TWT is established. The simulation analysis shows that the maximum temperature at the end of the helix is reduced by 49 °C, the maximum thermal deformation is reduced by 56%, and the saturated output power at 11 GHz is increased from 464 to 519 W. The analysis confirms that the novel SWS can effectively suppress BWO caused by thermal deformation in a high-power TWT and improve the TWT’s output power and operating stability.

Design, simulation, and development of bipolar pulse forming network based Marx generator for S band backward wave oscillator.

Pulse power systems have a wide range of applications, one of which is microwave generation. Microwave emission is associated with a certain time delay between the application of a high voltage pulse and the generation of the microwave signal. This time delay is known as microwave delay time, and it depends on the time period of the microwave signal being generated. As the time period of the microwave signal increases, the required input electrical pulse duration also goes up. To achieve this, a pulse forming network (PFN) based Marx generator is proposed. The Bipolar Marx generator is preferred over the uni-polar Marx generator to obtain the high voltage high current pulse. This also helps in maintaining the impedance requirement for Backward Wave Oscillator (BWO) devices that generate the microwave pulse. To the best of our knowledge, PFN based Marx generators have been developed up to 400kV. Here, a bipolar Marx generator has been designed with ratings of 800kV peak voltage, 10kA peak current, and 150ns flattop pulse duration. The design includes analytical calculations and numerical analysis by electromagnetic simulation. The triggering method to get a wide triggering range has also been discussed. The design values have also been experimentally verified, and the resulting parameters were applied to a BWO to simulate the microwave power that it can produce. A peak microwave power of ∼1 GW has been observed in the particle-in-cell simulation.

A Sub-THz High-Order Mode Backward Wave Oscillator Driven by Pseudospark Sourced Multiple Sheet Electron Beams

A novel concept of combining the advantages of pseudospark (PS)-sourced electron beam (high beam current density), multiple-sheet-electron-beams (large total beam cross-sectional area), and high-order mode (HOM) slow wave structure (SWS) (high power capacity) to produce high-power terahertz signals is presented in this article. As an example, a sub-terahertz backward wave oscillator (BEO) driven by PS-sourced dual-sheet-electron-beams is designed. Measured results of the interaction circuit, including transmission/reflection coefficients and the dispersion relation, were in good agreement with simulation predictions. Beam-wave interaction simulations having considered the plasma effect and conductor loss predicted stable output signals with radiation power of 167.4–255.4 W over a tunable bandwidth of 234.4–241.0 GHz.

NON-LINEAR ANALYSIS OF CHAOTIC SELF-OSCILLATIONS IN BACKWARD-WAVE TUBE

The paper presents the results of the analysis and modelling of topological and dynamic invariants for the regimes of chaotic self-oscillations in the backward-wave oscillator, in particular, the analysis of chaotic time series for the amplitude of the output signal, which is the solution of the equations of the non-stationary nonlinear theory for the O-type backward-wave oscillator (without taking into account space charge, relativistic effects, energy losses, etc.). The main attention is paid to the calculation and analysis of the spectrum of Lyapunov exponents based on the Sano-Savada algorithm. Numerical data of the Lyapunov backward-wave oscillator calculated for the time series of the amplitudes of the output signal are given, which definitely indicate the presence of elements of advanced chaos in the dynamics of the system.