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An efficient Cherenkov oscillator with gigawatt phase-controlled super-radiance (SR) pulses is studied for the application of coherent summation systems. To obtain phase-controlled SR pulses, an ultra-short seed pulse is required to be injected into the interaction space from the direction of the collector, which substitutes the impact of the spontaneous emission from the front edge of the electron beam. It means that, for a conventional Cherenkov oscillator, the injection seed pulse and output gigawatt SR pulse need to share the same channel. Therefore, an additional quasi-optical reflection system is needed to separate these two signals. To optimize such a scheme, we introduce a front extractor near the reflector and an injection channel at the side of the collector, allowing the output and injection channels to be independent of each other. Particle-in-cell simulations reveal that as the diode voltage is 260 kV, the beam current is 3.5 kA, and the magnetic field is 0.42 T, a short SR pulse with peak power of 1.93 GW is obtained. The corresponding conversion factor (ratio of average output power and input DC power) is up to 2.12. When the seed pulse has a rise time of 0.3 ns and a width of 0.2 ns injection, the phase of the seed pulse and the initiated SR pulse are closely correlated with the accuracy of 0.17 rad as the power ratio is down to −25 dB. The advantages of high efficiency and phase control make the oscillator a promising device used for the miniaturization and practicability of coherent summation systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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]

Increasing the dimensions of high power microwave devices is an efficient method to improve the power capacity. However, an overmoded structure usually results in mode competition and a low beam-wave conversion efficiency. In this paper, a multi-mode operation mechanism is used to avoid mode competition and increase the efficiency. The calculation results of nonlinear theory of beam-multimode interaction show that the optimized conversion efficiency is up to 48% when TM01 mode, TM02 mode, and TM03 mode are all considered. As only the TM01 mode, TM02 mode, or TM03 mode is taken into account independently, the corresponding efficiency is 38%, 22%, or 20%. Based on this, a multi-mode relativistic backward wave oscillator is proposed with the ratio of the mean diameter of the slow wave structure (SWS) to the wavelength of the output microwave to be 3.5. The non-uniform SWS is used to increase the beam-wave conversion efficiency, and a combined reflector is adopted to reflect partial of the mixed microwave modes and make the device compact. The particle-in-cell simulations show that as the diode voltage is 1.1 MV, the beam current is 22.8 kA, and the external magnetic field is 0.76 T, the conversion efficiency is 45%, and the output microwave of 11.3 GW is the mixed modes of TM01 mode, TM02 mode, and TM03 modes with the corresponding power ratio of 74%, 7%, and 19%, respectively. [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]

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]

An annular cathode is generally fixed on a cylindrical cathode holder in a magnetically insulated coaxial foilless diode. Electrons emitted from cathode plasmas or the cathode holder may easily move backward under the action of the quasi-static electric field and axial magnetic field, leading to backward current loss, which will seriously reduce the efficiency of high power microwave sources, especially under the condition of a low guiding magnetic field. A novel storage electrode immersed in the uniform magnetic field is proposed to suppress the backward current. Reverse electrons will be decelerated and blocked by the storage electrode. Under the guiding magnetic field of 0.66 T, over 66% of the backward current is suppressed (from 3.6 kA to 1.2 kA) when the storage electrode is used, and furthermore, the waveforms of the diode voltage and current are improved significantly. [ABSTRACT FROM AUTHOR]

In a relativistic backward-wave oscillator operating at a low magnetic field, forward intense relativistic electron beams propagate with large transverse velocities and form a non-uniform beam-density distribution. This paper first investigates periodical density bunching by bombarding targets with electron beams in a relativistic drift tube. Then, the dependence of the density-bunching phase on interaction efficiency is studied experimentally for the first time with a C-band relativistic backward-wave oscillator. The results show a reduction in microwave power of over 40%, arising from a mismatch between the density-bunching phase and the standing-wave field in the resonant reflector, which is in reasonable agreement with particle-in-cell simulation results. [ABSTRACT FROM AUTHOR]

An X-band dual-mode relativistic backward wave oscillator (RBWO) operating at low magnetic field is presented in this paper. Three new design principles are introduced in the device. First, the electron beam interacts with TM01 mode and TM02 mode simultaneously, rather than with a fixed single mode. Second, the device outputs with mixed modes, rather than with a pure mode. Third, an internal reflector inserted into the annular cathode, rather than a long resonant reflector before the slow-wave structure, is adopted to reflect part of the backward wave. Accordingly, the beam–wave interaction efficiency is increased significantly and the whole device is very compact. The particle in cell simulation results reveal that at a magnetic field of 0.64 T, the output microwave power is 4.8 GW and the conversion efficiency is up to 44%. In the experiment, at a guiding magnetic field of 0.66 T, a microwave pulse with power of 4.6 GW, frequency of 9.96 GHz, pulse duration of 42 ns, and efficiency of 42% was generated when the diode voltage was 880 kV and beam current was 12.5 kA, which agree well with the simulation results. Furthermore, as the diode voltage was 1.17 MV, a highest microwave power of 7.6 GW was achieved. This is a record of high efficiency and high power of microwave generation in an X-band RBWO operating at low magnetic field. [ABSTRACT FROM AUTHOR]

The design and preliminary results for a C-band relativistic backward wave oscillator (RBWO), which is magnetically well insulated, are presented. Under an external magnetic field of 0.36 T, the RBWO generated high power microwave radiation with a power of 3.3 GW and a frequency of 4.37 GHz for a diode voltage of 870 kV and a beam current of 13.5 kA. The electric field on the surface of the cathode holder was below the emission threshold, and an inlaid graphite cathode was designed to suppress the shunting current in the diode area. The device, operating with a low magnetic field and diode insulation, is a promising candidate for use as a permanent magnet package in high power microwave systems. [ABSTRACT FROM AUTHOR]

We found that the electron drift length, L1, between a diode area and a resonant reflector in a relativistic backward wave oscillator operating at a low guiding magnetic field has a periodical influence on device power, which is a special characteristic due to periodical transverse motions of electron beams. In this paper, electron motion characteristics under both TM02 and TM01 fields are investigated using single-particle simulation, and the result shows that the condition of beam expansion is dependent on the beam state that spatially meets the TM02 standing-wave field. A larger beam expansion will lead to a larger axial energy spread in the modulation area, which is unfavorable for efficient beam-wave interaction. Furthermore, rapid transverse expansion of electron beams can be observed under a TM01 travelling-wave field, which is the main cause for the obvious decline in the current collected by the collector as the output power reaches saturation. [ABSTRACT FROM AUTHOR]

We combine the relativistic backward-wave oscillator (RBWO) with an electron beam that encircles the structure axis, and present a high-current large-orbit gyro-like RBWO. In the device, both Cherenkov radiation and cyclotron radiation play significant roles in mode selection and beam–wave interaction. Particle-in-cell simulations show that when the beam energy is 900 keV, the beam current is 1.2 kA, and the guiding magnetic field is 0.23 T, the generated microwave power is 320 MW, the frequency is 13.3 GHz, and the efficiency is 30%. The operation mode is HE22 mode and EH22 mode, and the output mode is the EH21 mode. [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]

A novel coaxial relativistic Cerenkov generator (CRCG) operating in quasi-transverse electromagnetic mode is proposed for generating Ku-band high-power microwave radiation. It is found that using a cathode with a shielding electrode can decrease the diffusing electric field in the diode, thereby allowing a CRCG with the proposed diode structure to operate in a low guiding magnetic field. Taking advantage of the two-sectional slow wave structure, the output power and conversion efficiency are improved. Furthermore, a simple optimization rule is proposed for the length of the drift cavity and is verified by numerical simulation. In a particle-in-cell simulation, when the diode voltage and beam current are 550 kV and 4.2 kA, respectively, the microwave with an output power of 1 GW at 15.3 GHz is achieved in a guiding magnetic field of 0.35 T, yielding a conversion efficiency of 43%. [ABSTRACT FROM AUTHOR]

A klystron-like relativistic backward wave oscillator (RBWO) with a local decompression magnetic field is presented. There is a local non-uniform magnetic field near the extractor. As the electron beam travels through the RBWO, it will transport upwards and approach the extractor wall, then move downwards, and enter a coaxial collector. Owing to a higher kinetic energy of the electron beam which can be converted into microwave energy efficiently and a stronger electric field that can interact with the electron beam during the beam decompression in the extractor, the efficiency of the klystron-like RBWO is significantly increased to about 65%, compared to 55% with the conventional magnetic field. [ABSTRACT FROM AUTHOR]

A method on electromagnetic mode analysis in overmoded structure terahertz source is studied in this paper. This mode analysis method is based on Fourier-Bessel series and the particle-in-cell simulation. The result of mode analysis shows that there are TM02 and TM03 modes present in the waveguide motivated by mode change in the discontinuity. The mode content keeps stable when the device starts to work. The magnetic field and the voltage of beam do not change the mode content to some extent. The parameters of diode affect the mode content significantly. [ABSTRACT FROM AUTHOR]

A klystron-like relativistic backward wave oscillator with a ratio of transverse dimension to free-space wavelength being about four is presented. In the beam-wave interaction region, the electron beam interacts with surface wave and volume wave simultaneously. The cathode holder plays an important role in the reflection of backward waves. A guard electrode, an electron collector ring, and a reflection ring are used to optimize the beam-wave interaction. The particle in cell simulation results reveal that microwaves with a power of 2 GW and a frequency of 12.3 GHz are generated with an efficiency of 42% when the diode voltage is 400 kV, the beam current 12 kA, and the magnetic field 0.48 T. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the factors that affect the microwave pulse duration in a klystron-like relativistic backward wave oscillator (RBWO), including the diode voltage, the guiding magnetic field, the electron beam collector, the extraction cavity, and the gap between the electron beam and the slow wave structure (SWS). The results show that the microwave pulse duration increases with the diode voltage until breakdown occurs on the surface of the extraction cavity. The pulse duration at low guiding magnetic field is generally 5-10 ns smaller than that at high magnetic field due to the asymmetric electron emission and the larger energy spread of the electron beam. The electron beam collector can affect the microwave pulse duration significantly because of the anode plasma generated by bombardment of the electron beam on the collector surface. The introduction of the extraction cavity only slightly changes the pulse duration. The decrease of the gap between the electron beam and the SWS can increase the microwave pulse duration greatly. [ABSTRACT FROM AUTHOR]