Relativistic Backward Wave Oscillator Research Articles

Stable Operation of a Repetitively Pulsed X-Band Relativistic Backward Wave Oscillator

This letter describes a recent activity in the development of a high-power microwave source based on a modified relativistic backward wave oscillator that is driven by an advanced version of a SINUS-family accelerator using a double-width pulse-forming line insulated with a modern synthetic oil. The relatively compact source of microwave pulses (0.6 GW, 20 ns, 9.4 GHz) can operate at a 100 pps repetition rate in a batch mode with batches of duration up to 15 s. The long-term stability of the oscillator performances during $10^{{6}}$ pulses was achieved due to the improved conditions for explosive emission of the graphite cathode and the use of a titanium slow wave structure. Both factors prevented the shortening of the microwave pulses. With a proper choice of the magnetic field strength and longitudinal distribution and of the position of the resonant reflector, the pulse-to-pulse deviation of the microwave power was reduced with sacrifice in efficiency, which decreased from a simulated maximum efficiency of 44% to 40% in the simulation and to 35 ± 4% in the experiment. At an accelerating voltage of 418 kV and a magnetic induction of 0.64 T at the cathode, the standard deviation of the microwave pulsed power could be close to the voltage deviation.

Studies of a high-efficiency, long-pulse relativistic backward wave oscillator

A high-efficiency long-pulse relativistic backward wave oscillator (RBWO) operating in the C band is described. The electrodynamical structure of the RBWO consists of a resonance reflector, a slow-wave structure (SWS), an extractor, and a coaxial collector. To achieve high electron beam-electromagnetic wave interaction efficiency, a SWS for the RBWO is designed comprising of two structural segments with different corrugations that enables the gradually decreased electron beam axial velocity to better match the spatially constant electromagnetic wave phase velocity of the operating mode of the RBWO. To suppress electron emission and plasma formation on the inner surface of the electrodynamical structure, which is caused by a strong RF electric field, the RF electric field magnitude is optimized in computer simulations to be less than 700 kV/cm. A coaxial collector is designed to slow down the expansion of the collector plasma. The design of the RBWO is optimized in numerical simulations and its operation is validated in practical microwave-generation experiments. An output power of 3.3 GW is obtained in a practical experiment with a 796-kV, 10.3-kA electron beam guided by an external magnetic field of 2.2 T. The RBWO efficiency and microwave pulse width are measured in the experiment and found to be about 40% and 100 ns, respectively.

Mechanism of radio frequency breakdown on metal surfaces in relativistic backward wave oscillator

Radio frequency (RF) breakdown can result in pulse shortening and severely limits the performance of the relativistic backward wave oscillator (RBWO). In this study, segmented slow-wave structure (SWS) rings, which can be observed directly using a scanning electron microscope without any post-processing, are used to study the breakdown in a RBWO. After undergoing high-power microwave experiments, the working surface of the SWS rings was found to exhibit corrugated morphologies at the sub-mm to mm scale, and a large number of craters in the vicinity of the corrugated morphologies and a mass of scattered droplets were visible on the surface. It was deduced that the unusual morphologies resulted from electron bombardment of the working surface. The observed structures and droplets provide new evidence that helps to explain the mechanism for RF breakdown in the RBWO.

Generation of a Sequence of Powerful Ultrashort Microwave Pulses Based on the Effect of Superradiance of a Relativistic Backward-Wave Oscillator with Reflectors at the Ends of the Interaction Space

Generation of a Sequence of Powerful Ultrashort Microwave Pulses Based on the Effect of Superradiance of a Relativistic Backward-Wave Oscillator with Reflectors at the Ends of the Interaction Space

Direct generation of a circularly polarized TE11 mode in a relativistic backward wave oscillator by loading anisotropic medium

A relativistic backward wave oscillator (RBWO) for direct generation of a circularly polarized TE11 mode is proposed. Using a coaxially loaded anisotropic medium with an appropriate longitudinal conductivity, the TM01 mode is effectively suppressed and the TE11 mode successfully stimulated. A dual-resonant reflector is incorporated in the RBWO to effectively reflect the TE11 mode. An isotropic absorber is deployed in front of the dual-resonant reflector to reduce the external Q factor of the TM01 mode and avoid undesired modulation. These distinctive features of the RBWO, allow a circularly polarized TE11 mode output to be obtained. Particle-in-cell simulations show that for a beam voltage of 1.12 MeV and a beam current of 12.3 kA, circularly polarized TE11 mode microwaves with power 3.9 GW and frequency 6.4 GHz can be obtained.

Генерация последовательности мощных ультракоротких микроволновых импульсов на основе эффекта сверхизлучения релятивистской лампы обратной волны с отражателями на концах пространства взаимодействия

In a non-stationary numerical experiment, it is shown that introducing the reflections at the ends of the slow wave structure of a relativistic backward-wave oscillator operating in the superradiance mode provides the conditions for the formation of a sequence powerful ultrashort microwave pulses during a single pulse of the current. The conversion factor, defined as the ratio of the peak power of an ultrashort pulse to the power of an electron beam, is about 3 for the first (main) pulse and decreases to ≈ 2 for subsequent pulses

Non-uniform guiding magnetic field for efficiency enhancement of RBWO

Relativistic backward wave oscillator (RBWO) having a resonant reflector and slow wave structure (SWS) combination has been simulated for a 400 kV peak accelerating voltage and a 4.0 kA peak electron beam current. The particle in cell simulations predict an electron beam to microwave conversion efficiency of 44% at a 3.22 GHz operating frequency. Phase space plot analysis reveals that the backstreaming of electrons from the end reflector and the end part of SWS affects the device efficiency. It was demonstrated using particles in cell simulations that with non-uniform guiding magnetic field, these backstreaming electrons do not attain synchronism with the forward wave while drifting toward the anode end and eventually rebound from the anode–cathode gap. Backstreaming of electrons reduces with increasing non-uniformity in the magnetic field. These two phenomena result in a significant enhancement in the output microwave power at a 3.22 GHz frequency and device efficiency enhances to a level of 62%. With the proposed guiding magnetic field profile, RBWO retains a larger than 52% electron beam to microwave conversion efficiency with the nominal accelerating voltage anywhere in between 400 and 600 kV and an electron beam current of 3.9–7.2 kA.

Research on 3-GW Repetitively Operating Relativistic Backward Wave Oscillator

Relativistic backward wave oscillator (RBWO) is a promising high-power microwave (HPM) generator. In past decades, many efforts have been devoted to improving its practicality, in which the repetitively operating RBWO has drawn much attention. However, two bottlenecks once limited its development. One is the generation of massive anode plasmas or even damage of the collector, and the other is the RF breakdown of electrodynamic structures. This article describes an RBWO which can operate repetitively with 3-GW microwave power by mitigating these two problems. First, a titanium collector, which shows better performance in resisting electron beam bombardment than the conventional stainless steel collector, is adopted. Meanwhile, water-cooling technology is applied to expedite heat dissipation of the collector. Second, since our investigations demonstrate that an anodic mechanism, not the conventional cathodic mechanism, is the main cause for RF breakdown of electrodynamic structures under strong guiding magnetic field, the titanium material is also used to manufacture the electrodynamic structures of the RBWO in order to increase its power capacity. The advantage of the titanium RBWO is verified in experiments. The application of these techniques, as well as some important surface treatment techniques such as polishing, ultrasonic washing, and vacuum baking, leads to the stable repetitive operation of the RBWO with 3-GW output power and 25-ns microwave duration under a repetition rate of 50 Hz. The successive operation time of the device is up to 20 s.

Research on effects of space charge field in relativistic backward wave oscillator

The breakdown problem in the relativistic backward wave oscillator (RBWO) limits the microwave pulse energy and should be solved. The slow wave structure (SWS) is the most key component of the RBWO. The breakdown of SWS may be triggered by field-induced electron emission and electron bombardment on the wall. This paper demonstrates that the space charge field of the relativistic electron beam (REB) can enhance the electric field forbidding field-induced electron emission and decrease the electric field igniting the RF breakdown, and therefore, the RF breakdown can be significantly suppressed. However, the enhanced electric field will enlarge the radial displacements of electrons, especially the high-energy electrons. When the RBWO operates with a low guiding magnetic field or a long-pulse REB, this situation will worsen and some electrons may possess large enough radial displacements and bombard on the SWS surface with the help of radial oscillation of REB and radial expansion of cathode plasmas. This may aggravate the breakdown of SWS.

Experimental investigations on density bunching and its power influence in a relativistic backward-wave oscillator with low-magnetic-field operation

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.

Influences of plasma on a relativistic backward wave oscillator with a resonant reflector and an extracting cavity

Using the particle-in-cell method, the influences of the plasma produced in the resonant reflector and extracting cavity of an X-band relativistic backward oscillator are investigated. For the reflector case, the dominant rule of the microwave emission termination of the TM01 mode is the disturbance of the initial phase relationship between the RF fields of the modulated electron and the synchronous harmonic, which increases the generation frequency, excites a high-order mode competition, and then leads to microwave pulse shortening. For the extracting cavity case, the influence of plasma on the beam–wave interaction process is relatively negligible. A notable reduction in the output power is principally caused by the absorption of plasma electrons, which has been preliminarily validated in experiments.

Preliminary investigation of a magnetically insulated relativistic backward wave oscillator operating in the C-band with low magnetic field

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.

Design and Simulation of Dual-Band Nonuniform Relativistic Backward Wave Oscillator Using a Bragg Structure as Its RF Circuit and Reflector-Cum-Mode Converter

A relativistic backward wave oscillator (RBWO) operating in dual (C and X) band is designed and studied using finite-difference time domain (FDTD) particle-in-cell (PIC) simulations implemented in MAGIC. It consists of a sinusoidally corrugated, moderately overmoded nonuniform slow wave structure (SWS) in the downstream section and a single fold two-way helically corrugated Bragg structure in the upstream section. A single intense relativistic electron beam (IREB) having ~5.1 kA of current is predicted for 600 kV of diode voltage. An RF output power ~720 MW in X-band and ~510 MW in C-band are predicted in TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode with a guiding magnetic field of ~2.5 T.

Efficient generation of multi-gigawatt power by an X-band dual-mode relativistic backward wave oscillator operating at low magnetic field

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.

A Multifunctional Long-Lifetime High-Voltage Vacuum Insulator for HPM Generation

A multifunctional, long-lifetime, compact, and coaxial high-voltage (HV) vacuum insulator for Tesla-type generators is proposed for high-power microwave (HPM) generation. This vacuum insulator has two features. First, it is multifunctional since it can not only support the two conductors of the transmission line (TL), supply a vacuum environment for load, but also can eliminate a prepulse from the main pulse, which is realized via a grounded inductor. Second, it has a long lifetime. This feature is realized by optimizing the grounded inductor on the switch side and by grooving on the vacuum side. A 50-cm 700-kV compact coaxial HV vacuum insulator of such kind was designed, which was applied to a 1-MV Tesla-type generator to drive the relativistic backward-wave oscillator (RBWO) for HPM generation. Significantly, it has operated for a lifetime longer than 3 00 000 pulses at a repetition rate of 50 Hz without failure, which verifies the design.

Studies of Pulse Shortening Phenomena and Their Effects on the Beam–Wave Interaction in an RBWO Operating at Low Magnetic Field

Particle-in-cell (PIC) simulations using the MAGIC-3D code are performed to illustrate the beam-wave interaction behavior and the effects of pulse shortening in relativistic backward wave oscillator (RBWO) operating at a low guiding magnetic field. The causes of pulse-shortening phenomena such as beam degradation, high RF electric field breakdown, and collector plasma formation are studied in simulations. The PIC simulations of the RBWO operating at a low guiding magnetic field ~0.61 T including all the effects of pulse shortening predicted that an RF output power ~0.70 GW with operating frequency ~9.5 GHz and the power conversion efficiency ~21.3% will be shortened after ~63 ns of simulation time.

Effects of transverse electron beam motion in a relativistic backward wave oscillator operating at low guiding magnetic field

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.

Investigation of asymmetric mode competition in a relativistic backward wave oscillator

The relativistic backward wave oscillator (RBWO) is one of the most important high-power microwave generators. Its stability and reliability must be further improved to meet the demands of key applications. The factors that limit its performance are varied but include radio frequency breakdown and mode competition. Based on a C-band RBWO, the mode competition was carefully analyzed and the effects on the performance of the RBWO were revealed. As shown in the studies, for an axisymmetric system, asymmetric mode competition probably still exists. Therefore, a three-dimensional particle-in-cell model was configured to investigate the mode competition. The theoretical and experimental results confirmed that the competing mode for a C-band RBWO was EH21. Changing the gap between the cathode and anode was found to be effective for suppressing the asymmetric mode.

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.

Developments of Pulsed Electron Beam Sources for High-Power Microwave Applications

High-current pulsed electron beam sources are the core components of high-power microwave systems. In order to meet the requirements of future applications, one needs to improve the performance of electron beam sources in terms of vacuum insulation, beam transportation, and thermal management. In this paper, we report about our recent progress in the development of high-current vacuum electron beam sources. In order to meet the vacuum maintenance requirements of high-power microwave tubes, a high-electric field ceramic vacuum interface is designed and fabricated based on the ceramic metal brazing technique. In our experiments, a stable operation of the ceramic vacuum interface is demonstrated in the 10 Hz repetition mode with withstand voltage of larger than 600 kV and pulse width of about 100 ns. Besides, a cold cathode is developed using SiC nanowires, and an average beam current density of 1.2 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is achieved under the electric field strength of 90 kV/cm. Compared with traditional velvet cathodes, the characteristics of the SiC nanowire cathode, such as macro-electrical stability, emission uniformity, and operating life have been significantly improved. Furthermore, a high-current electron beam collector has been developed for relativistic backward wave oscillator tubes. A spiral flume is designed in the collector to meet the requirement of both high specific energy and low flow rate. It shows that the withstand heat flow density is in the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is suitable for the long pulse and repetitive operation of the system. These results represent a significant step towards the practical application of long-life high-power microwave systems.