Relativistic Backward Wave Oscillator Research Articles

Deep learning assisted optimization of Ka-band relativistic backward wave oscillator operating in TM03 mode with low guiding magnetic field

To accelerate the design of a high-power microwave device, a deep learning assisted multi-objective optimization method is used to optimize a Ka-band relativistic backward-wave oscillator (RBWO) operating with a low magnetic field. Particle-in-cell simulation results show that the optimized RBWO with a tooth-shaped slow wave structure (SWS) can generate microwave pulses with an output power of 1.24 GW and an operating frequency of 26.8 GHz under a diode voltage of 623.3 kV, and the diode current is 6.56 kA at a guiding magnetic field of 0.8 T. Compared with the original RBWO, the output power of the optimized RBWO has been increased by 201.2%, and the beam-to-microwave conversion efficiency has increased from 10.0% to 30.3%. The detailed analysis reveals that in an overmoded RBWO with low guiding magnetic fields, the introduction of a tooth-shaped SWS is beneficial to mode competition, improves output power, and decreases microwave starting time.

Advancing electromagnetic field modeling in axially anisotropic media with conformal method in 3D cylindrical coordinate systems

A conformal methodology for solving electromagnetic fields in axially anisotropic media in 3D cylindrical coordinate systems is proposed for the investigation of high-power microwave devices filled with such media. This method leverages the material properties of axially anisotropic media and employs the finite integral technique for solution. A simplified conformal algorithm is utilized for the boundaries of the computational region, extending the traditional conformal method for axially anisotropic media in 3D cylindrical coordinates. The proposed methodology is validated through simulations of TM01 and TE01 mode propagation in a circular waveguide filled with the medium. Subsequently, the method is integrated into particle-in-cell simulations, and an X-band relativistic backward wave oscillator filled with an axially anisotropic medium exhibiting azimuthal conductivity is numerically modeled. The results demonstrate that the medium with azimuthal conductivity effectively mitigates asymmetric modes in the relativistic backward wave oscillator, aligning with published literature. This study offers technical insights for research on high-power microwave devices incorporating axially anisotropic media, potentially facilitating the development of innovative microwave devices with enhanced performance characteristics.

Multi-objective design method for high-power microwave source devices based on preferred PIC/gNSGA-II

To solve the challenging problem of multi-objective optimization for high-power microwave sources, a specialized preference-based multi-objective optimization method, namely PIC/gNSGA-II, is proposed. Additionally, to achieve the quantitative optimization of the frequency spectrum and power of high-power microwave source devices, objective functions for both the spectrum and power are designed. PIC/gNSGA-II is employed to carry out structural optimization tests on relativistic backward wave oscillators (RBWO). The optimized results produced novel device structures that meet the specified frequency bands: in the X-band, the average output power is 845.85 MW, and the operating frequency is 9.53 GHz; in the Ku-band, the average output power is 167.25 MW, and the operating frequency is 14.73 GHz. This demonstrates that by controlling the parameters of such objective functions, the code can simultaneously optimize the spectrum and power of RBWOs. Comparing the optimization results of PIC/gNSGA-II with those of NSGA-II, PIC/gNSGA-II demonstrates better optimization efficiency and the ability to design device structures that meet various performance criteria.

Influence of quality factor on operating mode of TM<sub>02</sub> mode relativistic backwave oscillator

<sec>The mode competition in an overmoded relativistic backward wave oscillator is studied through theoretical analysis and three-dimensional particle-in-cell simulation in this work. Based on the quality factor and coupling impedance, the mode selection for a TM<sub>02</sub> mode backward wave oscillator is achieved, and its output power and magnetic field strength are optimized in the simulation.</sec><sec>The quality factor is related to the group velocity and end reflection of each mode. The dispersion curves of some non-axisymmetric modes are very close, and the group velocities are basically equal. Therefore, the end reflection needs considering to distinguish between the quality factors of different modes. In frequency domain simulation, analyzing the quality factor of each mode by using the <i>S</i><sub>11</sub> parameter curve can avoid calculating the end reflection.</sec><sec>The three-dimensional simulation results show that the coupling impedance and quality factor jointly affect the operating mode. When the coupling impedance advantage of the working mode is not obvious, changing the resonant frequency of the high-frequency structure can affect the beam-wave interaction process, thereby changing the excitation mode. When the advantage is obvious, the beam-wave interaction of the excitation mode will not be destroyed by the resonant mode, and other modes of microwave output mainly come from the conversion of the same frequency modes. Due to the constant dispersion curve, the effect of resonance on the mode is essentially the effect of the quality factor on the mode dominated by the end reflection.</sec><sec>The insensitive parameters and the electron beam radius obtained from the simulation are used as the optimal parameters, and the automatic optimization algorithm is used in combination with the two-dimensional simulation to perform multi-objective optimization design in the above device. The final output power of the backward wave oscillator reaches 534 MW, with an efficiency of 23.64%, an increase of 221.7% compared with the efficiency of the original device. The device operating mode remains stable, with a power ratio of TM<sub>02</sub> mode reaching 94.95%.</sec>

Highly efficient superradiant oscillator with front-extraction output in TE11 mode operating at low magnetic field

With a low guiding magnetic field, a highly efficient X-band coaxial superradiant relativistic backward wave oscillator (SR-RBWO) with a front-extraction cavity and a built-in mode converter is proposed in this paper. The coaxial structure is used to reduce the guiding magnetic field of the intense relativistic electron beam. To decrease the transmission loss of microwave power in the slow wave structure (SWS), a front-extraction cavity is introduced. The inner conductor of the coaxial SWS is hollowed out as the output waveguide, and then, the microwave is extracted by the front-extraction cavity and enters the hollow waveguide rather than re-enter the SWS to output. With this improvement, the power conversion factor K can be increased by 0.44 even more. Making use of the feature of longer SWSs of SR-RBWOs, a built-in mode converter is designed, which can convert the circular TM01 mode propagating in the output waveguide into the circular TE11 radiation mode that can be radiated into free space directly. Therefore, the length of the radiation system can be reduced significantly, which is beneficial to the miniaturization of the entire high-power microwave system. Particle-in-cell simulation demonstrates that with a diode voltage of 180 kV and a guiding magnetic field of 0.37 T, the output SR pulse with an average power of 1.00 GW corresponding to power conversion factor K = 1.8 can be obtained.

"Design, development and qualification of permanent magnet based focusing lens for X-band relativistic backward wave oscillator"

"Design, development and qualification of permanent magnet based focusing lens for X-band relativistic backward wave oscillator"

Experimental study on the influence of grain boundary on breakdown in relativistic backward wave oscillator

The presence of radio frequency (RF) breakdown seriously degrades the performance of relativistic backward wave oscillator (RBWO) and affects its lifetime. In this paper, the influence of grain boundary on RF breakdown in an X-band RBWO has been experimentally investigated. Firstly, titanium with different grain sizes has been prepared using a high-temperature annealing method before its field emission characteristics are studied by a field emission test system. The experimental results indicate that the segregation effect of the impurities at the position of the grain boundary may become more pronounced as the grain size increases, which results in a significant improvement in the field emission performance of titanium. A high-power microwave experimental platform is then set up, and the influence of the grain boundary of titanium on RF breakdown has been revealed. The shortening of the microwave pulses is more serious with the increase of the grain size, and the corresponding breakdown traces appearing in the high-frequency structures become more apparent. Therefore, developing fine-grained and single-crystal metallic materials may be an important method for suppressing RF breakdown in RBWOs.

Suppression of high order mode oscillation in a C-band long pulse high efficiency relativistic backward wave oscillator

In order to realize mode control and long pulse operation of the proposed C-band high efficiency relativistic backward wave oscillator, the asymmetric high order mode oscillation and its influences on the working mode operation are investigated in the 3D particle-in-cell simulation. The simulation shows that the quasi-TM21 mode is the dominant competing mode, which is mainly located in the two-period extraction cavity of the device. Accordingly, a simplified model of a two-period extraction cavity is established to investigate the mechanism of the high order mode excitation. It is indicated that the distance between two gaps of the two-period extraction cavity has great impact on the startup time of the competing mode. By optimizing the gap distance to delay the startup of the competing mode, as well as shortening the rising time of the input pulsed power pulse, the competing quasi-TM21 mode can be suppressed sufficiently.

A high-efficiency C-band coaxial transit time oscillator with a dual-cavity extractor under low-magnetic fields

A high-efficiency C-band coaxial transit time oscillator with a dual-cavity extractor under low-magnetic fields is designed and studied through small-signal theory and particle-in-cell simulation. Small-signal theory analysis indicates that a dual-cavity extractor is superior to a single-cavity extractor in terms of the beam-coupling coefficient, the resonant frequency, and the external quality factor, which are good for high efficiency. Typical simulation results of the proposed device show that an output power of 1.73 GW and a frequency of 6.37 GHz can be obtained with a diode voltage of 455 kV and current of 9.75 kA. The corresponding power efficiency reaches 39%, and the guiding magnetic field is 0.8 T. Further simulation demonstrates that the power efficiency exceeds 34% in a rather large range of diode voltage from 385 kV to 470 kV and can reach higher than 35% with a low guiding magnetic field of 0.4 T. Then this coaxial transit time oscillator is compared with the typical relativistic backward wave oscillators from the magnetic field, efficiency and power saturation time.

Toward 80% Efficiency in a Super Klystron-Like Relativistic Backward Wave Oscillator With Second and Third Harmonic Coaxial-Premodulation Cavity

To further increase the beam-wave conversion efficiency, a super klystron-like relativistic backward wave oscillator (RBWO) with coaxial-premodulation (CPM) cavity is proposed. Compared with previous dual-premodulation (DPM) cavities, the CPM cavity intensifies the second and the third harmonic components, resulting in a larger fundamental harmonic current both in the slow-wave structure (SWS) and the extraction cavity, and, consequently, leads to an enhanced microwave power generation by the Cherenkov radiation and transition radiation. In the particle-in-cell simulation, an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{X}$</tex-math> </inline-formula> -band microwave with a power of 3.44 GW and record efficiency of up to 80% has been achieved.

Experimental Demonstration of Dual-Mode Relativistic Backward Wave Oscillator With a Beam Filtering Ring Packaged With Permanent Magnet

It is reported on the design and experiment of a dual-mode (TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> -TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> ) relativistic backward wave oscillator (RBWO) with a beam filtering ring (BFR) packaged with permanent magnet. The BFR makes use of the large transverse oscillation of electrons at low magnetic field. By absorbing part of the electron beam periodically, the amount of the electrons that would absorb power from the microwave in the slow-wave structure (SWS) is greatly reduced, so the beam-wave conversion efficiency is improved. Experimental results demonstrate that a 3.2 GW mixed-modes microwave at 9.87 GHz with an efficiency of 40% is obtained with a 675 kV, 11.8 kA beam guided by a 0.68 T magnetic field. The BFR has a considerable lifetime and can effectively improve the efficiency of the device with stable operation.

Design and Efficiency Enhancement of Klystron-Like Relativistic Backward Wave Oscillator With Triple Cavity Extractor

In this article, the design and investigation of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{X}$</tex-math> </inline-formula> -band klystron-like relativistic backward wave oscillator (KL-RBWO) are presented. The present design includes three extractor cavities at the output section of KL-RBWO that has been studied for their beam-wave interaction behavior using a finite difference time domain (FDTD) based 3-D electromagnetic (EM) particle-in-cell (PIC) code. The three extractor cavities used in the downstream of KL-RBWO before the collector increased the overall efficiency of the device with a sharp increase in the axial electric field and current in the region. The PIC simulation of the present KL-RBWO predicted an RF power of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 2.3 GW at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 9.55 GHz with electronic efficiency of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 60% for the developed electron beam current of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 6.72 kA. A constant microwave generation was observed up to 270 ns under the external guiding magnetic field of 2.9 T.

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.

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.

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.

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.

SIMULATION CHAOTIC DYNAMICS OF RELATIVISTIC BACKWARD-WAVE OSCILLATOR WITH USING CHAOS THEORY AND QUANTUM NEURAL NETWORKS

Nonlinear simulation and forecasting chaotic evolutionary dynamics of such complex system as relativistic backward-wave oscillator is treated using the new combined method, based on the chaos theory algorithms, concept of geometric attractors, and algorithms for quantum neural network simulation. It has been performed modelling the dynamics of multilayer photon echo neural network for the case of noisy input sequence. It has been performed analysis, modelling and processing the temporal dependence of the output amplitude for the backward-wave oscillator, described by system of the nonstationary nonlinear theory equations for the amplitude of electromagnetic field and motion of a beam. The data on the Lyapunov’s exponents, Kolmogorov entropy, correlation coefficient between the actual and neural networks prognostic rows, referred to a temporal dependence of the output signal amplitude of the nonrelativistic (relativistic) backward-wave oscillator are listed. The combining the advanced algorithms of the modern chaos theory, concept of a compact geometric attractors and one of the effective neural network algorithms, or, in a more general using an effective model of artificial intelligence etc, could provide very adequate and quantitatively correct description of temporal evolutionary dynamics of most complicated systems, in particular, in the field of modern ultrahigh-frequency electronics

Effect of Microwave Leakage on Backward Current in an X-Band Dual-Mode RBWO Packaged With Permanent Magnet

In this article, the effect of microwave leakage into the diode region on the suppression of backward current in a relativistic backward wave oscillator (RBWO) operating at low magnetic field is investigated by the theoretical analysis, particle-in-cell (PIC) simulation, and experiments. The theoretical results show that some backward electrons are absorbed by the cathode holder due to microwave leakage, and as the microwave power increases, the microwave frequency increases, and the guiding magnetic field decreases, it will promote the absorption of electrons. The PIC simulations based on the first principle reveal that the leakage microwave partially suppresses the backward current and the suppression is enhanced when the incidence TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> is dominant. In the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${X}$ </tex-math></inline-formula> -band dual-mode RBWO packaged with permanent magnet experiments, a microwave pulse with the power of 3.9 GW, the frequency of 9.93 GHz, and the efficiency of 33% was measured when the diode voltage was 820 kV and diode current was 14.5 kA.