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The electromagnetic characteristics of the input cavity with electron beams loaded are measured successfully in an X-band triaxial relativistic klystron amplifier (TKA) with a high frequency electrical probe. To solve the frequency response issue in the X band, a coaxial electrical probe is optimized and inserted obliquely into the input cavity at the designed angle so that the microwaves in the cavity can be sampled and extracted with the required coupling coefficient while the probe does not interfere with the solenoid coil, which is tightly integrated outside the TKA. Besides, the TKA is redesigned as a structure integrally sealed by the solenoid coil, and the secondary sealing induced by probe insertion is implemented on the flange of the solenoid coils rather than on the wall of the TKA, which is conducive to simplifying the coupling structure while maintaining the high vacuum level. The design is examined in a GW level TKA experiment, indicating that microwaves with a power of about 670 MW are generated in the input cavity at a frequency of 8.40 GHz, which is completely consistent with the injected and output microwave frequencies. [ABSTRACT FROM AUTHOR]

Ka-band frequencies are becoming increasingly popular due to their application potential in high-data-rate wireless communications relevant to 5G applications, satellite link establishment, etc. High-power amplifiers in this frequency band, offering several tens of watts of output RF power, are one of the main enabling components of these communication systems. This article reports the design studies and analysis of the radio frequency (RF) section of a multiple beam-extended interaction klystron (MB-EIK). The proposed multiple beam RF section with a ladder-type EIK structure offers several crucial features, such as a low-voltage operation, moderate operational bandwidth, and high output power. Starting from the design of the intermediate cavities, the input and output sections and the overall RF section are presented. The proposed RF section supports the operation at 28.5 GHz center frequency with about a 500 MHz 3 dB bandwidth employing four electron beams with a 4 kV DC accelerating field. [ABSTRACT FROM AUTHOR]

This article reports about the design approach, electromagnetic simulation and analysis results of high-frequency ladder-type input, output, and intermediate RF cavaties for Ka-band multi-beam extended interaction klystron. Several parameters of the cavity, such as quality factor, shunt impedance, etc., have been investigated by the assistance of electromagnetic software CST microwave studio. [ABSTRACT FROM AUTHOR]

The multimode beam–wave interaction behavior in the RF interaction structure of a second-harmonic, 32.3 GHz gyroklystron amplifier operating in the TE02 mode has been studied by simulating the device using a commercial 3-D particle-in-cell code, “CST Studio Suite.” The effect of the variation of numerous parameters, such as driver frequency, RF power, magnetic-field strength, and beam current, has been investigated to realize the sensitivity of the device performance to these parameters. The effect of varying the position of the buncher cavity with respect to input and output cavity on the 3 dB gain and efficiency has also been investigated. The simulation results show that the designed gyroklystron amplifier produces a constant RF output power of ~315 kW at 32.3 GHz center frequency for a 70 kV, 20 A electron beam. The 3 dB gain of the device is obtained as ~26 dB with ~22.5% electronic efficiency and ~11 MHz bandwidth. The velocity spread is assumed as 0% for the present work. For validation, the simulated results have been compared with the previously reported experimental results and the previously reported analytical results, which are found to be in good agreement (i.e., less than 5% variation). [ABSTRACT FROM AUTHOR]

A miniaturized, high electronic efficiency, S-band metamaterial extended interaction klystron (MEIK) is proposed. The metamaterial-based extended interaction resonant cavities were designed, fabricated, and tested, the assembled and cold-tested results confirm the feasibility of miniaturization in MEIK. Furthermore, the particle-in-cell (PIC) simulation predicts that, typically, when the beam voltage and current are 30 kV and 3 A, respectively, the focusing magnetic field is 1000 G, a 3-cavity MEIK yields the output power of 56 kW at 2.453 GHz, with the saturated gain of 47 dB and the electronic efficiency of 62%, and, importantly, with the cavity diameter of its interaction structure being ~1/2 of a conventional extended interaction klystron. As a power amplifier, the emerging MEIK with its miniaturization and high electronic efficiency has potential applications such as in future accelerators and microwave heating systems. [ABSTRACT FROM AUTHOR]

In this article, the design, characterization, and manufacture of a circuit for a ${W}$ -band, ultracompact continuous wave sheet beam klystron are described. This klystron, aiming to produce more than 1.5-kW output power, operates at a comparatively low voltage benefiting the device size and extending its application space. However, a lower operation voltage adversely affects the beam-gap interaction efficiency. Two multigap output cavities have been employed to achieve a higher beam interaction efficiency without introducing additional competing modes or shrinking the interval between the operation mode and competing mode. Inspired by the resynchronization technology, the second output cavity can be optimized for the spent beam from the first output cavity. Comprising a single cavity output structure, the circuit efficiency of this double output cavity circuit is increased from 2.37% to 4.96%. In this circuit, electromagnetic modeling and design have been optimized to avoid complicated structures which are constrained by current mechanical design and manufacturing techniques. The MAGIC simulation shows that, driven by a 1-W RF signal, this circuit can extract more than 1.7-kW output power from a 35-kV, 1-A beam. A prototype single output cavity circuit has been designed and manufactured to determine the required machining tolerances for the final double output cavity circuit. [ABSTRACT FROM AUTHOR]

The Ka-band extended interaction klystrons (EIK) have been developed at the Institute of Electronics, Chinese Academy of Sciences (IECAS), to satisfy the requirement for the high-power millimeter-wave sources in scientific researches. In this work, two kinds of high compression electron gun with the solid or hollow beams, the derivative electron optics systems using the compact permanent magnet uniform focusing and the shared efficient interaction circuit based on the multigap ladder cavity, have been studied in detail and well designed through a mass of calculations. After the mechanical design, finely manufacturing, and a series of technical processes, the Ka-band EIK prototype tubes have been successfully built and tested. For the solid beam scheme, the maximum output power achieves 20 kW in the bandwidth of 60 MHz, and, correspondingly, the gain of 53 dB and the efficiency of 24% are observed. The measured −1 and −3 dB bandwidths are about 220 and 350 MHz, respectively. Meanwhile, the beam transmission is excellent, that is over 97% in dc state and over 90% in RF operation. For the hollow beam case, an over 91% dc transmission test has finished. [ABSTRACT FROM AUTHOR]

Energy consumption is a major concern for future large-scale high-energy accelerator projects such as the compact linear collider (CLIC), future circular collider (FCC), and international linear collider (ILC). Thus, efficient generation of microwave power recently gained new attention by the accelerator community. In response, we propose an innovative two-stage multibeam klystron (TS-MBK) concept. It combines a very high RF power generation efficiency (84%) with a compact and cost effective electro-mechanical configuration. This article presents the concept and design study of a 1.0-GHz, 24-MW klystron. The technological novelties and mitigation of issues specific to this concept are described. TS-MBK performance has been validated using particle-in-cell computer simulations. [ABSTRACT FROM AUTHOR]

In an effort to enhance the bandwidth of a multiple-beam klystron having rectangular double reentrant cavity with circular cylindrical ferrule, reduction of the unloaded quality factor of the cavity was proposed by placing circular cylindrical posts extending from one of the cavity side-walls to the opposite wall in an orientation perpendicular to the electric field. A typical cavity was analyzed using 3D electromagnetic field solver CST Studio and the post-loading was found to reduce the unloaded quality factor by around 90% compared to that for no-post-loading. The proposed method was found to be the only method other than the use of passive loss to achieve this order of reduction in the quality factor with a minor increase in the cavity transverse dimensions. Post made of highly resistive material was found to have pronounced effect in reducing the quality factor. The 3D electromagnetic analysis was validated against measurements on a typical S-band cavity. [ABSTRACT FROM AUTHOR]

A pair of ladder-type multigap cavities, namely, wide-coupling cavity and narrow-coupling cavity, were comparatively studied to improve the performance of $G$ -band extended interaction klystrons (EIKs). It is found that the narrow-coupling cavity has higher effective characteristic impedance and stronger electric field distribution which lead to relatively stronger beam–wave interaction strength and obviously better output characteristics. Particle-in-cell (PIC) simulation confirmed this conclusion. Under an working voltage of 16.5 kV, current of 0.3 A, and operating mode of $\pi $ mode, the narrow-coupling cavity structure has greater output power and wider bandwidth. Specifically, when the ratio of long and short slots is 1.13, 1.21, and 1.26, compared with the wide coupling cavity structure, the output power of the narrow coupling cavity structure is increased by 40, 90, and 50 W, and the bandwidth is significantly increased by 50, 100, and 200 MHz, respectively. Then a maximum output power of 400 W, a gain of 39 dB, a bandwidth of 500 MHz can be achieved with our narrow-coupling cavity structure EIK in $G$ -band. Thus, such structures show potential performance improvement of millimeter-wave and terahertz EIK. [ABSTRACT FROM AUTHOR]

In Klystron amplifiers, monotron oscillations may cause unacceptable beam instabilities. For facilitating fast and accurate analysis of such processes, rather than performing time-consuming particle-in-cell simulations, the theory of the monotron oscillations has been further developed for the small- and large-signal regimes and implemented into the klystron computer code KlyC. This development includes full considerations of the space-charge effects, relativistic effects and can operate with arbitrary field distributions of the resonant mode. The effectiveness of these methods, have been demonstrated through benchmarking against PIC codes and has shown good (at 1% level) agreement, while KlyC computation time is significantly (at least 100 times) faster than in the PIC simulations. [ABSTRACT FROM AUTHOR]

High-frequency, high-power klystrons proved to be significant for application in compact particle accelerators, high-resolution radars, and fast data communication systems. As most of the linear beam devices, the performance of the klystron deteriorates progressively with an increasing operating frequency due to the reduction of individual RF cavity impedances. Coupled cell structures can alleviate such degradation by extending the beam–wave interaction region. The accurate simulation of the klystron with the coupled cell structure relies on the resources and the time-consuming particle-in-cell (PIC) codes. To speed up the klystron design process, the coupled-mode theory was carefully reviewed and extended to facilitate the calculation of the complex spectra of the arbitrarily coupled cavity system and its interaction with the bunched electron beam. These new methods were implemented in the large signal klystron simulation code KlyC. The design and optimization of the Ku-band, low-voltage klystron is explained as a demonstration of the new technique’s effectiveness. The KlyC simulations have been thoroughly benchmarked with the CST PIC code and both codes showed a good (within 1%) agreement, while KlyC is significantly (about 100 times) faster. [ABSTRACT FROM AUTHOR]

Equivalent circuit analysis of a rectangular double-reentrant cavity of klystron with circular cylindrical ferrule operating in the fundamental TM110 mode was developed for obtaining the lumped parameters such as capacitance and inductance by considering the cavity as a parallel resonant circuit. Resonant frequency and characteristic shunt impedance of the cavity were then obtained using these lumped parameters. Analysis was validated against 3-D electromagnetic simulations using CST Studio and measurements. [ABSTRACT FROM AUTHOR]

We propose a novel system of input and output couplers, excited in azimuthal symmetric and stable mode of operation, for an oversized coaxial relativistic klystron amplifier (OCRKA) at Ka-band. A mode converter is introduced in the output coaxial waveguide to provide a tunnel for the input waveguide and fix the inner and outer conductors of the OCRKA. After characterizing the proposed input and output couplers, the high-frequency structure of the OCRKA is designed and then optimized by Computer Simulation Technology. The simulated frequency and loaded quality factor of the input, output, and idler cavities are validated against measurement. Also, the beam–wave interaction of the OCRKA using the proposed input and output couplers is simulated by particle-in-cell software, the average saturated output power of 880 MW of the OCRKA at Ka-band, with 29.3% electronic efficiency and 58-dB saturated gain are obtained. Moreover, no pulse shortening of the output power waveform is observed during the simulation time of 100 ns, which indicates that the proposed input and output couplers can suppress the higher order mode self-oscillation in the Ka-band OCRKA studied here. [ABSTRACT FROM AUTHOR]

The High Efficiency International Klystron Activity (HEIKA) was initiated at CERN in 2014 to evaluate and develop new klystron bunching technologies for high-efficiency klystrons in application to the large-scale scientific projects such as the Compact Linear Collider (CLIC) and the Future Circular Collider (FCC). The success of such development strongly depends on the availability of the specialized klystron computer codes. Unfortunately, the accurate and efficient 2-D large-signal codes are proprietary and are not freely available to the wide klystron community. The new 1.5-D klystron code called KlyC has been developed as an attempt to bridge the gap between fast, but approximate 1-D models and time/resources consuming particle-in-cell (PIC) codes. KlyC simulates the specific physical processes such as the current emission from the metal surface, the space charge depression along the klystron, and the residual RF current modulation of the spent beam. KlyC internal eigenmode solver allows fast simulations of the 2-D complex electromagnetic (EM) field in the axis-symmetrical cavity with the arbitrary cavity profile. The internal KlyC optimizer module can help the designer to speed up the klystron development process. In this paper, the theoretical model, general review of the code performance, and benchmarking between KlyC, AJDisk/1-D, PIC code MAGIC/2-D, and CST/3-D are presented using FCC klystrons as an example. [ABSTRACT FROM AUTHOR]

Increasing the output power of the microwave is one of the most important development directions for high-power microwave (HPM) technology. However, when the output power is increased, the radio frequency breakdown would occur owing to superhigh electric field strength. This problem limits the power handling capability of a single-HPM generator, and this problem becomes more serious for high-frequency bands and long pulse. One solution to overcome this problem is coherent power combining, and coherent power combining can achieve an equivalent power of the order of mymargin TW. The relativistic klystron amplifier (RKA) is a promising candidate for coherent power combing owing to its character of controllable frequency and steady phase. However, the radiation power of RKA quickly decreases when the source is scaled up to higher frequencies such as ${X}$ -band, and moreover, the multigigawatts long pulse radiation from RKA at the ${X}$ -band has not yet been obtained experimentally. In this paper, we present investigations into an ${X}$ -band coaxial multibeam RKA, The power handling capability was increased by introducing 20 electron drift tubes and a four gaps extended interaction output cavity. The amplitude uniformity of the multiple electron beams was improved by optimized the anode and cathode configuration. Then, a radiation power of 2.2 GW with pulsewidth of 120 ns is generated experimentally; the working frequency and amplifier gain are 9.4GHz and 50 dB, respectively. [ABSTRACT FROM AUTHOR]

The microchannel cooling structure, as an efficient heat dissipation technology, is applied to improve the power capacity of a W-band continuous-wave (CW) sheet beam (SB) extended interaction oscillator. Its configuration is designed and optimized to further enhance the heat flux handling. The results show that microchannel cooling technology can greatly enhance the heat transfer efficiency. Specifically, the maximum temperature of the cavity drops from 308.9 °C to 144.9 °C. The power capacity of CW increases to 7 kW with 2% (49-kV and 2-A SB) electronic interception, and the tolerable electronic interception rate increases to 4% with 3.6-kW CW output power and 2.3-kW ohmic power loss. The microchannel cooling technology enables compact devices with higher frequencies and higher power to work under CW conditions. [ABSTRACT FROM AUTHOR]

Rectangular reentrant cavities are most widely used in the high frequency compact klystrons. However, an analytical approach for calculation of cavity parameters of a rectangular re-entrant cavity with cylindrical ferrule is not available in the published literature. A simple approach for calculation of cavity parameters, namely, the resonant frequency (f0), the unloaded quality factor (Q0) and the characteristic impedance (R/Q0) of a rectangular reentrant cavity with cylindrical ferrule was developed, by way of transforming the rectangular reentrant cavity geometry to that of an equivalent cylindrical reentrant cavity. The equivalent lumped circuit parameters were then obtained using the procedure available for cylindrical reentrant cavity. Proposed analysis was benchmarked against the results obtained from the 3D electromagnetic simulation and measurement results. [ABSTRACT FROM AUTHOR]

A beam-wave interaction model for G-band rectangular beam extended-interaction klystron (EIK) with unequal-length slots is proposed and analyzed to achieve high output power and broad bandwidth. The length ratio of long slot with short slot is carefully optimized to obtain a large effective characteristic impedance and wide mode separation. The influence of beam tunnel size on the effective characteristic impedance is studied with the beam aspect ratio of 2.3:1. The beam conductance for 5-gap, 7-gap, and 11-gap cavities are analyzed theoretically to optimize the beam-wave interaction stability. Furthermore, beam current and input power are investigated to obtain high performance. The analysis of fabrication tolerance and alignment tolerance indicates that less than 5- $\mu \text{m}$ machining precision is required. With beam current of 0.40 A and voltage of 16.5 kV, 3-D PIC simulation predicts peak power of 1139 W and 3-dB bandwidth of 1.15 GHz, with efficiency of 17.3% and gain of 37.5 dB. In addition, PIC simulations are compared with 1-D AJDISK calculations, the approximate results with similar tendencies indicate that our proposed beam-wave interaction model for G-band EIK is relatively reliable. [ABSTRACT FROM AUTHOR]

We demonstrate the possibility of chaotic generation in a gyroklystron by introducing an external delayed feedback. It is shown that for the realization of “developed” chaos regimes with improved spectral uniformity, the region of the phase characteristic of a gyroklystron with rapid phase shift should be used. According to the time-domain-averaged model and direct particle-in-cell simulations with the parameters of the experimentally realized 93-GHz gyroklystron, the chaotic generation power can reach tens of kilowatts with a spectrum width of about 1 GHz. [ABSTRACT FROM AUTHOR]

We present the results of our comprehensive study of the stability of a two-gap input cavity used in an experimental broadband multiple-beam klystron (MBK) with 18 beams and 7 cavities, which was designed for projected >500 kW maximum output power and ~13% bandwidth (so-called NRL MBK-3 design). The device was experimentally observed to be prone to self-excitation at all beam voltages below its designed operational value of ~42 kV. Postexperiment examination of the two-gap input cavity revealed traces of breakdown on its face. Detailed simulations performed using a 3-D computational electromagnetic code HFSS have shown presence of a few high-Q higher order modes (HOMs), which could explain self-excitation of the two-gap input cavity in a wide range of beam voltages. To explore HOM instability of the two-gap input cavity of the MBK in greater details, we applied the code TESLA-MB, which represents a parallel extension of the 2-D large-signal klystron code TESLA and which is suitable for an accurate modeling of multiple-beam devices with unique distributions of R/Qs over gaps in all its beam-tunnels. [ABSTRACT FROM AUTHOR]

We report a design of a new quasi-optical input coupler developed for the 35-GHz two-cavity gyroklystron. The coupler is a section of longitudinally corrugated waveguide placed between two cutoff irises. It operates with an eigenmode being a superposition of the rotating on azimuth TE7,4 and TE13,2 modes in the input cavity. The corrugated cavity is designed with holes in the walls to ensure necessary mode selection, because in a broad frequency band the operating eigenmode only does not have fields at the holes. In experiments, the gyroklystron equipped with this coupler has shown a peak output power of 4.2 MW with a nearly 1- \mu \texts pulse duration, a power gain of 24 dB, and an efficiency of about 15%. [ABSTRACT FROM PUBLISHER]

A high-gain X -band overmoded relativistic klystron is presented in this paper. The device mainly consists of an input cavity, two buncher cavities, and an output cavity. Moreover, two reflectors are added before and after the input cavity, therefore a standing wave is formed between the two reflectors, improving the initial modulation of the electron beam. With two sectional RF lossy materials inserted, the input cavity and the two buncher cavities are effectively isolated with each other. An additional reflector located just before the output cavity further reflects the backward-flowing power. Particle-in-cell simulations show that for a 10-kW injected microwave with a frequency of 9.37 GHz, the generated microwave power is 1.2 GW, corresponding to a beam-wave interaction efficiency of 30% and a gain of 50.8 dB. As the injected power increases from 10 to 100 kW, the frequency control range extends from 30 to 130 MHz. [ABSTRACT FROM PUBLISHER]

The performance of electronic switching between multiple backward-wave modes is studied in a designed extended interaction oscillator (EIO) based on a ladder circuit with finite number of periods to overcome electronic tuning range limits of EIOs operated in standing-wave mode. The dispersion characteristic of the circuit with finite number of periods, which is constructed by a series of discrete modes, is investigated. The mode separation is analyzed and reduced to support continuous switching between multiple different modes by increasing the number of periods as compared with the standing-wave EIO approach. An output circuit is designed to extract the power of backward wave. The electronic switching between nine backward-wave modes has been achieved by changing the beam voltage from 4.1 to 10.5 kV, where the maximum output power over 58 W is obtained at 5.3 kV from the simulation prediction. The EIO can operate over an electronic tuning range of 3.53 GHz from 89.65 to 93.18 GHz in ensuring the output power no less than 20 W. This technique can be extensively applied to increase operating band for extended interaction klystrons (EIKs) and electronic tuning range for EIOs, making them more suitable for many potential applications. [ABSTRACT FROM AUTHOR]

Manufacturing of klystrons in the millimeter-wave frequency range is challenging due to the small size of the cavities and the ratio of the maximum gap voltage to the beam energy. The small dimensions also make difficult to produce devices with the output power required by a number of applications at millimeter wave, such as communications and spectroscopy. Operating with a higher order mode can be a potential solution, as a larger transverse size structure can be used. Unfortunately, high-order mode cavities have a lower impedance than in fundamental mode. In this paper is proposed a novel solution to overcome the reduced impedance by utilizing an upconverter, where all cavities except the output cavity are designed to work in high-order mode. To demonstrate the effectiveness of the approach, two klystron upconverters were designed. One has six cavities aiming to achieve a maximum output power of ~90 W at 105 GHz. The second klystron upconverter was a simpler three-cavity structure designed for quick prototype. Millimeter-wave measurements of the three-cavity klystron upconverter are presented. [ABSTRACT FROM PUBLISHER]

The driving engine of the superconducting accelerator of the European X-ray free electron laser (XFEL) is a set of 27 radio frequency (RF) stations. Each of the underground RF stations consists of a multibeam horizontal klystron that can provide up to 10 MW of power at 1.3 GHz. Klystrons are sensitive devices with a limited lifetime and a high mean time between failures. In real operation, the lifetime of the tube can be significantly reduced because of failures. The special fast protection klystron lifetime management (KLM) system has been developed to minimize the influence of service conditions on the lifetime of klystrons. The main task of this system is to detect all events which can destroy the tube as quickly as possible, and switch off the driving RF signal or the high voltage. Detection of events is based on a comparison of the value of the real signal obtained at the system output with the value estimated on the basis of a high-power RF amplifier model and input signals. The KLM system has been realized in field-programmable gate array (FPGA) and implemented in XFEL. Implementation is based on the standard low-level RF micro telecommunications computing architecture (MTCA.4 or xTCA). The main part of the paper focuses on an estimation of the klystron model and the implementation of KLM in FPGA. The results of the performance of the KLM system will also be presented. [ABSTRACT FROM AUTHOR]

Development of high frequency, high-average power linear beam devices is hindered by the slow progress in advancement of suitable mechanical design and manufacturing techniques. To compliment the significant progress in electromagnetic modeling and design of high-frequency electronics, the mechanical design and manufacturing approach employed for the development of a 10 kW continuous wave W-band sheet beam klystron is reported. The reported design enabled demonstration of 10-kW, 20-ms duration pulses with 17-dB gain and 97% (hot) beam transmission. This high-average power design was based on the demonstration of a 56 kW short pulse ( 1~\mu \texts ) device with 54 dB of gain. The high-average power design features a multilayer strengthened material structure with hybrid microcooling and macrocooling circuits for enhanced heat flux handling. Microcooling channels fit between circuit cavities providing heat removal as well as thermal-stress relief for reduced cavity deformation. The design achieves a 1.5-safety factor on yield and less than 2- \mu \textm deformation at the cavities. The presented design and manufacturing approach can be applied to the development of other high-average power high-frequency linear beam devices. [ABSTRACT FROM AUTHOR]

Large-scale particle-in-cell (PIC) simulations of C-band multibeam klystron (MBK) on the Milky Way high-performance computing platform are conducted in this paper. First, the parameters of the electron beams, as well as the magnetic field distribution of the focusing system, resonant cavity, input cavity, output cavity, and collector are analyzed. Based on these structure parameters, large-scale parallel PIC simulations on C-band MBK are performed on the Milky Way high-performance computing platform. The simulation results show that the maximum speedup reaches 28 with an input of 50 W/5.6 GHz, a peak output power of 760 kW, a gain of 40 dB, and a bandwidth of 300 MHz. [ABSTRACT FROM AUTHOR]

The design methodology of a gyroklystron amplifier has been discussed and subsequently used for the design and optimization of a second-harmonic D -band four-cavity gyroklystron amplifier. The multimode beam–wave interaction behavior of the device operating in the TE02 mode has been analyzed using the time-dependent multimode analysis and also simulated using a commercial 3-D particle-in-cell code “CST Particle Studio.” The effect of various parameters, such as driver frequency, driver power, beam voltage, and beam current, has been studied to understand its sensitivity on the device performance. The simulation results predict that the designed second-harmonic gyroklystron amplifier produces a stable RF output power of $\sim 1$ kW (assuming 0% velocity spread) at 140 GHz center frequency, with $\sim 2.5$ % electronic efficiency, $\sim 30$ dB gain, and $\sim 1$ GHz bandwidth for a 40-kV, 1-A electron beam. [ABSTRACT FROM AUTHOR]

In this paper, the analytical and particle-in-cell (PIC) simulation studies of an X -band megawatt class gyrotwystron have been presented. The device employs a tapered output waveguide to achieve stability against self-start oscillations due to azimuthally asymmetric modes, including the fundamental harmonic TE11 and TE21. A full nonlinear self-consistent, single particle, multimode, and multiharmonic, transient code has been developed to study the beam–wave interaction behavior. Furthermore, the device is simulated for studying its transient behavior using a commercial PIC code “CST Particle Studio.” This predicts the saturated RF power of $\sim 20$ MW in the fundamental TE01 mode and $\sim 1$ MW in the TE02 ( $s = 2$ ) mode, which is a self-generated spurious oscillation in the output section. The saturated gain is calculated as $\sim 24$ dB with a power conversion efficiency of $\sim 20$ % for a relativistic electron beam having 7% axial velocity spread. [ABSTRACT FROM PUBLISHER]

In this paper, high-efficiency low-filling-factor sheet beam extended interaction oscillator (SB-EIO) with an improved ladder structure is discussed. The Ka-band cavity with nine gaps is designed to operate at $2\pi $ mode. A sheet beam (8 mm \times $ 0.6 mm) of 34 kV and 3.5 A is injected into SB-EIO through a beam tunnel of 12~\text {mm}\times 1.2 mm. The axial electric field distribution and the synchronization conditions are optimized to improve the beam-wave interaction efficiency. An effective method is explored and utilized to suppress the mode competition. A stable output power of 35.9 kW and 30% efficiency are obtained for a 0.56 filling factor in narrow direction and 0.67 filling factor in wide direction. An output power of 22.8 kW and 19.1% efficiency are also attained even with a 0.4 filling factor in narrow direction and 0.67 filling factor in wide direction. [ABSTRACT FROM PUBLISHER]

In this paper, an analytical study of the high-power microwave oscillator—reltron, has been presented. The basic device operating principle has been briefly discussed considering the multiple resonating modes, i.e., 0, $\pi $ /2, and $\pi $ , in the side-coupled modulating cavity. The device oscillation condition has been analyzed and used to define the range of RF phase variation in the modulating cavity for sustained oscillation. The analysis has been extended further to obtain the electronic efficiency of the reltron. The analytical device efficiency has been found in agreement of $\sim 7$ %–10% with the previously reported experimental results. [ABSTRACT FROM AUTHOR]

In this paper, a brief analytical study of start oscillation current of reltron source has been discussed. The device design methodology of the reltron is described in detail and subsequently used for the design of a reltron using commercial PIC code CST Particle Studio. RF cavity simulation is performed in the absence of electron beam to show the electric field patterns in the three different modes, i.e., 0, $\pi $ /2, and $\pi $ , in the modulating cavity. In order to evaluate the overall performance of the device, beam present simulation is performed using explosive electron emission model and obtained an RF output power of $\sim 225$ MW with $\sim 36$ % efficiency. The simulated results have been found in agreement of $\sim 5$ % with the previously reported experimental value. [ABSTRACT FROM PUBLISHER]

The increase in efficiency of RF power generation for future large accelerators is considered a high priority issue. The vast majority of the existing commercial high-power RF klystrons operates in the electronic efficiency range between 40% and 55%. Only a few klystrons available on the market are capable of operating with 65% efficiency or above. In this paper, a new method to achieve 90% RF power conversion efficiency in a klystron amplifier is presented. The essential part of this method is a new bunching technique—bunching with bunch core oscillations. Computer simulations confirm that the RF production efficiency above 90% can be reached with this new bunching method. The results of a preliminary study of an $L$ -band, 20-MW peak RF power multibeam klystron for Compact Linear Collider with the efficiency above 85% are presented. [ABSTRACT FROM PUBLISHER]

Several spectroscopy applications of high-frequency gyrotrons require relatively broadband ( $\sim 1$ %) frequency tuning. In this paper, the use of a sectioned klystronlike cavity with mechanically changing parameters is proposed as a way to solve this problem. [ABSTRACT FROM PUBLISHER]

A 1-MW, 352.2-MHz continuous wave (CW) radio frequency (RF) test stand based on TH 2089 klystron amplifier is developed and tested up to its rated power. Klystron amplifier demands stringent performances from its beam bias power supply for its proper operation. A novel crowbar less solid-state modular topology is adopted for −100-kV, 25-A dc beam bias power supply and its achieved performance parameters are presented in this paper. Wire burn test, which is crucial for CW klystron amplifier, was conducted on this power supply to ensure that its stored energy during klystron arcing is less than 20-J. The criterion for the selection of test wire is also presented. A real-time control system is developed for proper sequence of operation of various power supplies as well as for control and monitoring of crucial parameters of this test stand. The testing of this high-power klystron amplifier is quite involved in nature and various tests have been carried out under several operating conditions before it is employed in the high-power RF test stand. This paper studies and analyzes the effect of variations of various parameters on the performance of this high-power klystron amplifier. Variation of output power and RF gain against RF input power as well as variation of RF phase over beam voltage is presented. Variations of RF output power against frequency, beam voltage, beam current, and mod-anode voltage are also presented. Heat run test was conducted on this klystron amplifier for 24 h to ensure its reliability. [ABSTRACT FROM PUBLISHER]

Circuit analysis of a multiple-beam version of a high-power extended-interaction klystron is performed. The circuit is based on the barbell cavity with five coplanar electron beams across the transverse section of the cavity. Although the basic circuit design is straightforward, stable operation is challenging due to the mode competition and the self-oscillation in an oversized multiple-gap cavity driven by multiple beam sources. The 3-D particle-in-cell (PIC) simulation technology and the space-charge wave theory were exploited to analyze the stability. The physical design of the interaction system was accomplished with the beam parameters of voltage 19 kV and of overall current 4 A (0.8 A $\times 5$ ). PIC results show that a power of 11.28 kW can be achieved at a frequency of 94.48 GHz with an instantaneous 3-dB bandwidth of 160 MHz. The corresponding gain and electric efficiency are 55.75 dB and 14.84%, respectively. [ABSTRACT FROM AUTHOR]

High-harmonic terahertz-frequency-range gyrotrons require long-length operating cavities due to both weak electron-wave coupling and relatively low electron currents. Since diffraction $Q$ -factors of such cavities are very high, a great share of the radiated RF power is spent to ohmic losses. A sectioned klystron-like cavity was proposed as a way to combine a long electron-wave interaction region with a relatively low diffraction $Q$ -factor. In this brief, results of the first experiment under the high-harmonic gyrotron with the klystron-like cavity are reported. In this experiment, selective operation of the gyrotron at the second (0.55 THz) and the third (0.74 THz) cyclotron harmonics has been observed. [ABSTRACT FROM PUBLISHER]

To improve the output power and electron efficiency and broaden the operating bandwidth, the three-gap extended output cavity was used in the relativistic klystron to replace the single-gap output cavity. The high-frequency characteristics of the three-gap extended cavity were studied. The electron load conductance was derived and corrected based on the theory of relativity, by which a more accurate relation of electron load conductance versus transmit angle can be obtained. The characteristic impedance is three times bigger than that of the single-gap cavity under the case of the $2\pi $ mode, which can improve the M^{2}(R/Q) and the beam–wave interaction efficiency. The model of sheet beam klystron (SBK) is comprised of three-gap input cavity, two-gap first idler, three-gap second idler cavity, and three-gap output cavity. The 3-D particle-in-cell simulation results show that for a 500 kV, 1 kA, 40.134 GHz, and 20 W input power, the SBK is capable of generating an output power higher than 142 MW with the gain of 68.5 dB, maximum interaction efficiency of 28.4%, and 3-dB bandwidth of 240 MHz. [ABSTRACT FROM PUBLISHER]

The influences of the eigenfrequency and the intrinsic quality factor of a cavity on the phase shift in the relativistic klystron amplifier (RKA) are investigated. An analytical expression is derived to describe the phase of the gap voltage in a cavity. The theoretical and simulation results indicate that the eigenfrequency of the buncher cavity can significantly affect the phase shift of the output microwave in the RKA. In contrast, the influences of the eigenfrequency and the intrinsic quality factor of the output cavity on the phase shift of the RKA are small. The analytical expression can effectively estimate the phase difference among different RKAs resulting from the machining and assemblage inconformity in the experiment. It is suggested that the differences among the eigenfrequencies of the buncher cavities should be managed in 0.1% of the operation frequency to achieve phase locking among different RKAs. [ABSTRACT FROM AUTHOR]