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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 TM02 is dominant. In the ${X}$ -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. [ABSTRACT FROM AUTHOR]

A super klystron-like relativistic backward wave oscillator (RBWO) is presented. In the device, the distributed Cherenkov radiation generated in the slow wave structure (SWS) is absorbed by the electron bunch in the last corrugation and re-accelerated, creating high-energy electrons that overcome the space-charge repulsion force to further compress the bunch length and maximize the fundamental harmonic current, contributing to the extremely efficient microwave generation in the extraction cavity through concentrated transition radiation. A novel elliptical extraction cavity is used to improve the power capacity. A soft magnet is inserted to precisely control the beam path, which provides a higher beam kinetic energy to be extracted and a stronger electric field that interacts with the tighter bunch. In the experiment, as the diode voltage was 585 kV and diode current was 7.5 kA, a microwave pulse with a power of 2.9 ± 0.3 GW measured in the radiation field, a frequency of 8.23 GHz, and a pulse duration of 22 ns was obtained. The corresponding efficiency is up to 66 ± 8%, which is an experimental record for GW-class relativistic vacuum electronic devices. [ABSTRACT FROM AUTHOR]

High power microwave (HPM) sources without guiding magnetic field have the advantages of compactness and miniaturization. However, the efficiency is generally limited due to the poor beam quality and low beam transmission rate. Here, we demonstrate a scheme that actually makes use of the large oscillation of electrons in the absence of a magnetic field. In a novel radial three-cavity transit-time oscillator (TTO) with two output ports, about half of the diode current is deposited on the walls of the first two cavities. On the one hand, the truncated electrons are with lower energy, which has delivered some of their kinetic energy to the standing-wave field in the first two cavities. Other electrons then gain energy from the standing-wave field and are accelerated to a much higher beam voltage than the diode voltage. On the other hand, after part electrons are truncated by the first two cavities, the remained electrons are characterized with smaller energy spread, higher current modulation coefficient, which results in a very efficient energy exchange between the beam current and the electromagnetic field in the third cavity with two extraction gaps. Particle-in-cell (PIC) simulation shows that when the diode voltage is 496 kV, beam current is 43.4 kA, the total output microwave powers are 11.4 GW, corresponding to an unprecedented beam-wave conversion efficiency of 53%. [ABSTRACT FROM AUTHOR]

In this article, the effect of second harmonic (SH) on the microwave conversion efficiency in a klystron-like relativistic backward wave oscillator (RBWO) is investigated by the nonlinear theory, particle-in-cell (PIC) simulations, and experiments. The nonlinear theory of beam-wave interaction is developed with both the fundamental frequency and SH taken into account. The calculation results show that the optimized conversion efficiency is increased from 50% to 73% as the SH is included. The PIC simulations reveal that with the introduction of a dual-premodulation (DPM) cavity to increase the SH current and then the fundamental current modulation coefficient in a klystron-like RBWO, the conversion efficiency can be enhanced from 59% to 72%. In the klystron-like RBWO experiments, a significant efficiency increase from 58% for the device without the DPM cavity to 66% with the DPM cavity has been realized. As the electron beam power is 4 GW, the out powers of the fundamental frequency and SH are measured to be 2.6 GW and 100 MW, respectively. This provides helpful guidance to further increase the microwave conversion efficiency in high-power microwave sources driven by intense relativistic electron beams. [ABSTRACT FROM AUTHOR]

In this letter, a dual-cavity TM01 and TM02 mixed modes to TM01 mode converter is proposed for a dual-mode relativistic backward-wave oscillator (RBWO) application. This converter is more compact than the conventional converters and the TM02 mode is cutoff in the output waveguide, which forces the purity of the output TM01 mode to 100%. Especially, the converter is adjustable, and by simply replacing the waveguides in it, the converter can be well-matched to different proportional mixed modes. Electromagnetic simulation results show that the conversion efficiency of the converter is higher than 99%. The converter is validated in the high-power microwave (HPM) generation experiment of the dual-mode RBWO and a conversion efficiency of 98% is realized. A pure TM01 microwave with power of 4.6 GW and pulse duration of 42 ns at 10 GHz was obtained. [ABSTRACT FROM AUTHOR]

An investigation of a super klystron-like relativistic backward wave oscillator is presented in this article. To obtain high power and high efficiency, an additional anode cavity is used to promote electron-beam modulation. Additionally, two waveguide cavities at the end of the electron collector are employed to enhance the constructive resonance, which intensifies energy conversion from the modulated electron beam to the electromagnetic field. Because of the enhancement of centralized transition radiation, the power generated in the extractor constitutes over 95% of the total power. Consistent with the results from numerical simulations, microwaves with a power of 5.0 GW, a frequency of 4.4 GHz, and a pulsewidth of 36 ns were obtained in high-power microwave generation experiments with an external magnetic field of 0.42 T, in which the diode voltage and beam current were configured to be 815 kV and 15.5 kA, respectively, indicating a beam–wave conversion efficiency of 40%. The diode insulation was investigated experimentally, and an unfortunate shunting current of over 3 kA was observed when the working voltage was beyond 800 kV, which indicates an actual conversion efficiency of 33% in the diode region. [ABSTRACT FROM AUTHOR]

We propose a novel dual-frequency high-power microwave generator consisting of both an axial generator and a radial one stretching from the middle of the tube head of the former. As a high-voltage pulse is injected into the diode system, a cathode with two blades generates two relativistic electron beams that separately travel through the axial and radial diodes. Therefore, both the subsources are driven by the relativistic beams and generate intense microwave pulses simultaneously. An axial relativistic backward-wave oscillator and a radial transit-time oscillator are designed and combined to realize this purpose. Particle-in-cell simulation shows that when the diode voltage is 505 kV, microwaves with powers of 0.95 and 1 GW are generated in the axial and radial subsources, with frequencies of 15.2 and 9.5 GHz, respectively. An equivalent efficiency of over 35% is achieved with an axial magnetic field of 0.86 T and a radial magnetic field of 0.5 T. [ABSTRACT FROM AUTHOR]

In this paper, we study an X-band klystron-like relativistic backward wave oscillator (RBWO) with a pre-modulation cavity, modulation ridge, drift tube in the slow wave structure (SWS) and extraction cavity in both theoretical and experiment. PIC simulations show that the conversion efficiency can be up to 62% with output power of 3.0 GW. In experiment, when the diode voltage is 660 kV, and the diode current is 6.4 kA, the output power is 1.9 GW, and the conversion efficiency is 45%(±5%). And we find that the conversion efficiency mainly suffers from the breakdown in the extraction cavity and the plasma from the cylindrical collector. To suppress the breakdown and plasma effect, the breakdown electric field threshold in RF structures(especially in the extraction cavity) had better be larger than 700 kV/cm. Moreover, the distance between the position where the electrons are collected and the end of extraction cavity should be larger than 20 mm in case of the cylindrical collector. [ABSTRACT FROM AUTHOR]

The two-way power combiner consisting of two TM01–TE11 serpentine mode converters with a common output has demonstrated high-power capacity and high combination efficiency for two phase-locked relativistic backward wave oscillators (RBWOs). To realize channel power combination for four phase-locked X-band RBWOs, we propose an all circular waveguide four-way power combiner based on the previous two-way power combiner. Four TM01 modes are first combined into two TE11 modes with two TM01–TE11 mode two-way power combiners; then, through two separate 90° bending waveguides and tapered waveguides, the two TE11 modes are combined into one TE11 mode with a TE11–TE11 mode two-way power combiner. Separate calculations show that at 9.22 GHz, the conversion efficiency of the TM01–TE11 mode two-way power combiner is 95.4%, the transmission efficiencies of 90° bending waveguide and tapered waveguides, and the combination efficiency of the TE11–TE11 mode two-way power combiner are close to 100%. Integrated simulations indicate that as four incident ports are fed with TM01 modes with power of 2.5 GW at each port and phase of 0°, 180°, 0°, and 180°, respectively, the total combination efficiency is 94.5%, and the maximum electric field in the four-way power combiner is 774 kV/cm. [ABSTRACT FROM AUTHOR]

This paper demonstrates the effect of voltage rise time on phase synchronization in two parallel relativistic backward-wave oscillators (RBWOs) through 3-D particle-in-cell simulations. With a short rise time, the axially symmetrically mode is excited, the frequency spectrum is pure for both RBWOs, and phase synchronization between the two microwaves is realized. As the rise time increases, asymmetric modes appear in one or two channels, mode competition occurs, and phase synchronization cannot be achieved. This can be explained as the transient excitations in the RBWOs initiated by the current variation in beam head, whose magnitudes are inversely proportional to the rise time. It is suggested that phase synchronization can be obtained at a larger voltage rise time in the RBWOs with stronger resonant characteristics. Furthermore, an external signal can suppress the asymmetric mode and control the relative phase of output microwaves in the two RBWOs. [ABSTRACT FROM PUBLISHER]

A klystron-type relativistic Cerenkov generator, which combines the frequency and phase control properties associated with an X-band overmoded relativistic klystron amplifiers with the high-power and high-efficiency associated with a klystron-like relativistic backward wave oscillator, is presented. After the beam current is modulated by the input cavity and buncher cavity, it achieves a large enough modulation depth to induce the produced microwave frequency of the backward wave output section. Moreover, the lossy materials inserted in the device can isolate the input cavity, buncher cavity, and backward wave output section effectively, and thus prevent the generated microwave from locking the input signal. Particle-in-cell simulations show that for a 1 MW injected microwave with a frequency of 9.30 GHz, the generated microwave power is 2.8 GW, and the beam–wave interaction efficiency is 45%. As the injected power increases from 100 kW to 2 MW, the frequency control width extends from 20 to 125 MHz. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive understanding of the influences of the modulation cavity and extraction cavity on microwave generation and starting oscillation in a klystron-like relativistic backward wave oscillator. Near the region of the modulation cavity, the axial electric field is small, which is favorable to the reduction of the energy spread of the modulated beam electrons. In addition, the presence of the modulation cavity increases the fundamental harmonic current in the extraction cavity, where the beam-wave interaction is predominant due to the larger axial electric field. The introduction of the extraction cavity leads to a large increase in the amplitude and a sudden change in the phase of the axial electric field, which is beneficial for decelerating the bunched electron beam that has fallen into the acceleration phase. Furthermore, the peak electric field interval in the second sectional slow wave structure (SWS) and extraction cavity is decrescent, making them similar to the role of nonuniform SWS. The extraction cavity enhances the fundamental harmonic current significantly. The modulation cavity acts like a localized wave absorber during the process of starting oscillation and enhances the start current, and the extraction cavity decreases the start current due to the increased coupling impedance. [ABSTRACT FROM PUBLISHER]

The klystron-like relativistic backward wave oscillator (RBWO) combines the transition radiation with Cerenkov radiation and has demonstrated microwave output of high power and high efficiency. The coaxial slow wave structure device can produce microwave with a lower frequency in a smaller cross section. For the purpose of high efficiency, low frequency, and miniaturization, a coaxial klystron-like RBWO with a premodulation cavity is presented. Particle-in-cell simulations show that a microwave with power of 1.15 GW and frequency of 2.1 GHz is generated with conversion efficiency of 48%, whereas for the device with a reflector, the efficiency is 38%. [ABSTRACT FROM AUTHOR]

A klystronlike relativistic backward wave oscillator (RBWO) with a dual-cavity extractor is presented. The phase of the axial electric field component Ez varies by 0° in the dual-cavity extractor, and a larger electric field appears in the second extraction cavity, in comparison with a single-cavity extractor. Such an improved field distribution is beneficial for converting modulated beam power into microwave power. The particle in cell simulation results reveal that microwaves with power of 10 GW, frequency of 4.3 GHz are generated, and conversion efficiency is 48% when diode voltage is 1.2 MV and beam current 17.3 kA. [ABSTRACT FROM AUTHOR]