Klystron-like Relativistic Backward Wave Oscillator Research Articles

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.

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.

Conversion of Cherenkov Radiation to Transition Radiation by Electron Bunch Post-Acceleration for Extremely Efficient Beam–Wave Interaction

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.

Role of Second Harmonic in the Optimization of Microwave Conversion Efficiency From an Intense Relativistic Electron Beam

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.

Experimental Investigation of a Super Klystron-Like Relativistic Backward Wave Oscillator Operating With Low Magnetic Field

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.

Investigation of an X band high efficiency klystron-like relativistic backward wave oscillator

This paper investigates an X band high efficiency klystron-like relativistic backward wave oscillator (RBWO) in detail. The klystron-like RBWO consists of a pre-modulation cavity, a resonant reflector with a ridge, a sectional slow wave structure, and an extraction cavity. First, this paper gives some theoretical studies about beam modulation and energy extraction. For beam modulation, the optimized distance between the pre-modulation cavity and the resonant reflector is studied theoretically, and theoretical results agree well with simulation results. For energy extraction, an ellipse extraction cavity with high power capacity is come up with, and the electric field on the inner surface of the ellipse extraction cavity decreases by 25% in PIC simulation. Also, the paper analyzes the effect of the position of dumped electron on conversion efficiency. Interestingly, it’s found that the efficiency dramatically decreases with the increase of the distance between the extraction cavity and the position of dumped electron, which is caused by the increase of potential energy of electron and the decrease of electric field. Fortunately, we find that the use of guiding magnet with special magnetic field distribution almost eliminate this unfavorable effect. Besides, effects of the distance between the cathode and anode &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;ak&lt;/sub&gt; are investigated. It’s shown that the optimized diode voltage decrease with the increase of the distance &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;ak&lt;/sub&gt;, and the conversion efficiency is higher at larger &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;ak&lt;/sub&gt;. The experimental studies are also given. The power capacity of ellipse extraction cavity is verified, also we find that the efficiency is enhanced by 10% and the width of microwave pulse increases by 7 ns when the roughness of RF structure surface is improved from Ra 0.4 μm to Ra 0.05 μm. Typically, the klystron-like RBWO outputs X band high power microwave with power of 2.15 GW, with pulse duration of 25 ns, and with conversion efficiency of 50%(± 5%). Experimental results agree well with theoretical and PIC simulation results.

Efficiency enhancement of a klystron-like relativistic backward wave oscillator with local decompression magnetic field

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.

Theoretical and experimental research on a high efficiency X-band klystron-like RBWO

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.

Mechanism of phase control in a klystron-like relativistic backward wave oscillator by an input signal

Theoretical analyses and particle-in-cell (PIC) simulations are carried out to understand the mechanism of microwave phase control realized by the external RF signal in a klystron-like relativistic backward wave oscillator (RBWO). Theoretical calculations show that a modulated electron beam can lead the microwave field with an arbitrary initial phase to the same equilibrium phase, which is determined by the phase factor of the modulated current, and the difference between them is fixed. Furthermore, PIC simulations demonstrate that the phase of input signal has a close relation to that of modulated current, which initiates the phase of the irregularly microwave during the build-up of oscillation. Since the microwave field is weak during the early time of starting oscillation, it is easy to be induced, and a small input signal is sufficient to control the phase of output microwave. For the klystron-like RBWO with two pre-modulation cavities and a reentrant input cavity, an input signal with 100 kW power and 4.21 GHz frequency can control the phase of 5 GW output microwave with relative phase difference less than 6% when the diode voltage is 760 kV, and beam current is 9.8 kA, corresponding to a power ratio of output microwave to input signal of 47 dB.

Influences of the Modulation Cavity and Extraction Cavity on Microwave Generation and Starting Oscillation in a Klystron-Like Relativistic Backward Wave Oscillator

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.

Improved power capacity in a high efficiency klystron-like relativistic backward wave oscillator by distributed energy extraction

With the efficiency increase of a klystron-like relativistic backward wave oscillator, the maximum axial electric field and harmonic current simultaneously appear at the end of the beam-wave interaction region, leading to a highly centralized energy exchange in the dual-cavity extractor and a very high electric field on the cavity surface. Thus, we present a method of distributed energy extraction in this kind of devices. Particle-in-cell simulations show that with the microwave power of 5.1 GW and efficiency of 70%, the maximum axial electric field is decreased from 2.26 MV/cm to 1.28 MV/cm, indicating a threefold increase in the power capacity.

Role of dc space charge field in the optimization of microwave conversion efficiency from a modulated intense relativistic electron beam

We demonstrate an efficiency of 70% with 5.1 GW microwave power for a diode voltage of 770 kV and a current modulation coefficient of 1.67 in a klystron-like relativistic backward wave oscillator. The device combines the advantages of reducing electron beam radius, adopting dual-cavity extractor, and introducing two pre-modulation cavities. A large dc space charge field is present due to the conversion of considerable potential energy to kinetic energy at the end of beam-wave interaction region. A nonlinear theory is developed to show that the dc space charge field can increase the current modulation coefficient and microwave conversion efficiency significantly.

Improved fundamental harmonic current distribution in a klystron-like relativistic backward wave oscillator by two pre-modulation cavities

In a klystron-like relativistic backward wave oscillator, the velocity modulation is mainly obtained from the resonant reflector. By introducing two pre-modulation cavities between the input cavity and the resonant reflector, the amplitude and phase of density modulation can be adjusted relatively independently, to ensure an improved fundamental harmonic current distribution. Two peaks of harmonic current with high modulation coefficient of 1.2 appear in the second slow wave structure and the dual-cavity extractor and result in large beam energy losses in both regions. Particle-in-cell simulations show that a microwave with power of 11.5 GW and efficiency of 57% can be obtained.

A high-efficiency overmoded klystron-like relativistic backward wave oscillator with low guiding magnetic field

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.

Factors influencing the microwave pulse duration in a klystron-like relativistic backward wave oscillator

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.

速调型相对论返波管理论研究

速调型相对论返波管理论研究

High efficiency coaxial klystron-like relativistic backward wave oscillator with a premodulation cavity

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%.

RF phase control in a high-power high-efficiency klystron-like relativistic backward wave oscillator

The klystron-like relativistic backward wave oscillator (RBWO) with a dual-cavity extractor has demonstrated 10 GW output power with 48% efficiency in recent simulations. To realize radio-frequency (RF) phase control for the purpose of coherent power combining, a high-power high-efficiency klystron-like RBWO with an input signal is presented. In the device, an independent input cavity is introduced before the resonant reflector to reduce the leakage power from the input port, and the applied RF signal is injected into the input cavity through a mode convertor and a coaxial waveguide along the magnet, which avoids separating the magnet by the injection waveguide, and suppresses the asymmetric modes excited in the input cavity. For a frequency difference of 15 MHz between the input signal and free-running klystron-like RBWO, an input power of 10 MW is sufficient to control the phase of 10 GW output microwave.

A high-power high-efficiency klystronlike relativistic backward wave oscillator with a dual-cavity extractor

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.