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This article presents a demonstration of the improved performance of an X‐ray free‐electron laser (FEL) using the optical klystron mechanism and helical undulator configuration, in comparison with the common planar undulator configuration without optical klystron. The demonstration was carried out at Athos, the soft X‐ray beamline of SwissFEL. Athos has variable‐polarization undulators, and small magnetic chicanes placed between every two undulators to fully exploit the optical klystron. It was found that, for wavelengths of 1.24 nm and 3.10 nm, the required length to achieve FEL saturation is reduced by about 35% when using both the optical klystron and helical undulators, with each effect accounting for about half of the improvement. Moreover, it is shown that a helical undulator configuration provides a 20% to 50% higher pulse energy than planar undulators. This work represents an important step towards more compact and high‐power FELs, rendering this key technology more efficient, affordable and accessible to the scientific community. [ABSTRACT FROM AUTHOR]

This paper presents an extended interaction klystron (EIK) operating at 0.22 THz based on a four-sheet-beam orthogonal interconnection structure (OIS). The proposed OIS comprises four vertically positioned trapezoidal line structures, significantly enhancing the coupling efficiency between cavities and achieving optimal interactions with the TM81 mode. This study investigates the dispersion characteristics and field distribution numerically, while the optimal working parameters and output structure were analyzed using the particle-in-cell (PIC) studio. The parameters of each cavity and the selection of five intermediate cavities were finalized by calculating the beam conductance, external quality factor, and stagger-tuning. The EIK generated a continuous 480 W output power with a four-sheet-beam at 17.5 kV and a current density of 200 A/cm2 from an input drive signal of 20 mW. The gain was approximately 43.8 dB. [ABSTRACT FROM AUTHOR]

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]

For most applications in the millimeter wave band, corresponding to Ka and higher-frequency bands, relatively high atmospheric absorption necessitates the use of high-power sources. Here, a new approach for projecting an oversized beam tunnel in an overmoded structure by concentrating the axial field is demonstrated to meet the high-frequency and high-power demands of compact devices. Due to the enhanced intense beam loading capability of the interaction circuit, a six-cavity Ka-band extended interaction klystron with a four-coupling-hole disk-loaded structure is designed that can stably obtain high output power. An analysis of optimization tradeoffs from introducing high order modes for allowing the application of more powerful beams to improving high order mode field distribution for enhancing the electron-wave coupling and suppressing mode competition is reported. 3D particle-in-cell simulations show attainable output powers of 1.11 MW at 32.94 GHz with a saturated gain of 57 dB by injecting a 3.3 mm diameter electron beam with a current of 24 A. [ABSTRACT FROM AUTHOR]

Future linear accelerators require klystrons with higher radio frequency (RF) to drive higher gradient accelerating structure. An x-band accelerator structure was used to accelerate electrons at the Shanghai Soft X-ray Free Electron Laser Facility (SXFEL) in Shanghai Advanced Research Institute, Chinese Academy of Sciences (SARI-CAS). A pulse transformer is a crucial device in an RF system. This study presents a highvoltage pulse transformer used for a 50 MW x-band pulsed klystron in SXFEL. Typical specifications of the pulse transformer are peak pulse voltage 420 kV, peak pulse current 300 A, 50 Hz repetition rate and 1.5 µs flattop pulse width. Design and optimization of pulse transformer are achieved by using equivalent circuit analytic methods and computational aided simulation. The relevant experiments show that this pulse transformer can meet the requirements of 50MW x-band klystron. [ABSTRACT FROM AUTHOR]

The V-band frequencies are becoming popular due to their application potential towards secure high data rate communications. This article reports bandwidth enhancement of an 11-cavity V-band Klystron amplifier employing staggered tuning. A systematic approach is presented to stagger-tune the periodically allocated multiple cavities of the Klystron operating at 60.1 GHz. Using the three-dimensional particle-in-cell (PIC) simulation, it is shown that, employing the proposed approach, the −3 dB bandwidth of the device (with peak tuned configuration) has been increased from 165 MHz to 540 MHz, demonstrating a 260% increment. The −1 dB bandwidth of the device is estimated to be 270 MHz. The proposed approach of stagger tuning may be employed for similar devices employing multiple RF cavities to meet the requirement of wide bandwidth. [ABSTRACT FROM AUTHOR]

Increasing klystron tubes' efficiency is essential in developing new-generation colliders and free-electron lasers. Different factors can affect the efficiency of a multi-beam klystron. One of the significant factors is the electric field symmetry inside cavities, especially in the output section. In this research, two different types of couplers are investigated in the extraction cavity of a 40-beam klystron. The first approach is a single slot coupler, which is frequently utilized and easier to fabricate but disturbs the electric field symmetry inside the extraction cavity. The second method has a more complex structure with symmetric electric fields. In this design, the coupler consists of 28 mini slots on the inner wall of the coaxial extraction cavity. Both designs are evaluated via particle in cell simulations, and the results exhibit around 30% more power extracted from the structure with a symmetric field distribution. The symmetric structures can also decrease the number of back-streamed particles by up to 70%. [ABSTRACT FROM AUTHOR]

Based on steady-state experiments, a three-dimensional (3D) simulation study under pulsation conditions was carried out. This was performed to study the effect of pulsation of the supply system on the atomization characteristics of a gas-centered swirl coaxial injector. The results indicated that the spray shape resembled a pagoda when the Klystron effect occurred. When the low-frequency pulsation (200 Hz) was applied to the liquid inlet, the peak of the exit pressure was close to the trough of the inlet mass flow rate. Meanwhile, the exit mass flow rate and pressure were approximately sinusoidal. There was no significant lag effect in the response of the exit mass flow rate to the inlet mass flow rate but with a small sawtooth-like oscillation. Increasing the amplitude of the liquid inlet disturbance amplified the exit mass flow rate fluctuation, which was greater than the set amplitude of the inlet pulsation. The dominant frequency of the injector exit mass flow rate and pressure variation was significantly less than the inlet pulsation frequency when the frequency of the liquid inlet pulsation was increased to 1000 Hz. This was due to the low-pass filtering effect of the annular slit. There was no significant change in the overall spray morphology for the gas inlet pulsation. By applying the same amplitude and frequency of pulsation to the gas and liquid inlets of the injector, the pulsation of the liquid inlet had a stronger effect on the injector exit spray than that of the gas inlet. [ABSTRACT FROM AUTHOR]

This paper presents the design, prototype engineering, and initial testing of a relatively low-voltage, high aspect ratio electron gun for versatile pulsed power applications. The sheet beam, diode electron gun is designed for (23–25) kV voltage and 6 µA/V1.5, as is required for x-band klystron, as a part of a compact Klylac delivering electron beam energy in the MeV range. The gun prototype was engineered, built, and tested using a Scandinova M1-0.5 modulator. Beam loss on the anode aperture is evaluated from transient waveforms and equivalent circuit modeling. [ABSTRACT FROM AUTHOR]

In this paper, an angular radial extended interaction amplifier (AREIA) that consists of a pair of angular extended interaction cavities is proposed. Both the convergence angle cavity and the divergence angle cavity, which are designed for the converging beam and diverging beam, respectively, are investigated to present the potential of the proposed AREIA. They are proposed and explored to improve the beam–wave interaction capability of W-band extended interaction klystrons (EIKs). Compared to conventional radial cavities, the angular cavities have greatly decreased the ohmic loss area and increased the characteristic impedance. Compared to the sheet beam (0°) cavity, it has been found that the convergence angle cavity has a higher effective impedance and the diverging beam has a weaker space-charge effect under the same ideal electron beam area; the advantages become more obvious as the propagation distance increases. Particle-in-cell (PIC) results have shown that the diverging beam (8°) EIA performs better at an output power of 94 GHz under the condition of lossless, while the converging beam (−2°) EIA has a higher output power of 6.24 kW under the conditions of ohmic loss, an input power of 0.5 W, and an ideal electron beam of 20.5 kV and 1.5 A. When the loss increases and the beam current decreases, the output power of the −2° EIA can be improved by nearly 30% compared to the 0° EIA, and the −2° EIA has a greatly improved beam–wave interaction capacity than conventional EIAs under those conditions. In addition, an angular radial electron gun is designed. [ABSTRACT FROM AUTHOR]

The feasibility of a photonic band-gap (PBG)-based interaction structure was analytically explored for a multiple-beam extended interaction klystron amplifier. This configuration accrues the advantages of the multiple-defect PBG-based extended interaction structure and multiple-beam operation. The mode configurations were analysed through 3D electromagnetic simulation and the applicability of the photonic band-gap cavity was studied for the multiple-beam operation of an extended interaction klystron amplifier. A typical six-defect cavity operating at around 83 GHz was designed for electron–wave interaction at 2π mode with 6 electron beams each carrying 300 mA current at the accelerating potential of 16.5 kV. A particle-in-cell simulation shows that an output power of ∼2 kW is possible with electronic efficiency of around 6.7%. A frequency-scaled-down interaction structure at Ku-band was fabricated and cold measurements were carried out to ascertain the feasibility. The measured values of the frequency for various modes, loaded quality factor and 3 dB bandwidth were found to be within 0.36%, 5.3% and 4.07%, respectively, against those from the simulation. [ABSTRACT FROM AUTHOR]

To realize online measurement of power, frequency, and phase for a modularized relativistic triaxial klystron amplifier in a narrow space, a compact directional coupler with high directivity, high power handling capacity, and large bandwidth has been investigated numerically and experimentally. The method with coupling holes distributed in both the longitudinal and lateral directions is proposed, and the simulation results indicate that when two holes are employed, directivity larger than 20 dB is achieved over a 300 MHz bandwidth in the X-band and the maximal electric field strength is controlled to be 198.5 kV/cm with 1 GW power injection, while the whole coupler longitude is optimized to be as short as 50 mm. High power experiments have been performed with a 10 GHz relativistic triaxial klystron amplifier, and the results show that the measured power, frequency, and phase shift of the directional coupler have good agreement with the results obtained in the far-field. [ABSTRACT FROM AUTHOR]

We report the high gradient testing results of two single-cell off-axis coupled standing wave accelerating structures. Two brazed standing wave off-axis coupled structures with the same geometry were tested: one made of pure copper (Cu) and one made of a copper–silver (CuAg) alloy with a silver concentration of 0.08%. A peak surface electric field of 450 MV/m was achieved in the CuAg structure for a klystron input power of 14.5 MW and a 1 μs pulse length, which was 25% higher than the peak surface electric field achieved in the Cu structure. The superb high gradient performance was achieved because of the two major optimizations in the cavity's geometry: (1) the shunt impedance of the cavity was maximized for a peak surface electric field to accelerating gradient ratio of ∼2 for a fully relativistic particle, and (2) the peak magnetic field enhancement due to the input coupler was minimized to limit pulse heating. These tests allow us to conclude that C-band accelerating structures can operate at peak fields similar to those at higher frequencies while providing a larger beam iris for improved beam transport. [ABSTRACT FROM AUTHOR]

Developing a robust and reliable high-voltage (HV) pulse power system (PPS) is essential for the characterization and testing of microwave tubes and industrial applications. This paper presents the design, simulation, and implementation of a modular and versatile high-voltage pulse power supply used in microwave device characterization and testing. A microcontroller-based digitally controlled pulse generation unit is also developed to generate the trigger pulses for high-voltage switching modules to control the turn-ON and turn-OFF of the switching devices. The digitally controlled pulse generation unit provides better resolution of pulse width, pulse repetition frequency, and system protection over the entire range of operations. The pulse generation and control circuit controls the pulse repetition frequency (PRF) and pulse wide (PW). The PRF and PW can be varied from 100 to 1000 Hz and 10 to 100 µs, respectively. The experimental prototype with ten series-connected high-voltage modules is connected to achieve a 25 kV pulsed output voltage. The experimental results of HV-PPS on a resistive dummy load and 5 GHz klystron tube are presented in this work. [ABSTRACT FROM AUTHOR]

To solve the problem of mode competition in high-order working modes, this paper presents a fast design method based on the TM31-2π mode for the dumbbell shaped extended interaction cavity. Based on the equivalent circuit theory, the high-frequency characteristics of the structure are studied. In addition, we propose a W-band continuous wave sheet beam extended interaction klystron high-frequency circuit. The circuit was driven by a 20 kV and 0.85 A sheet electron beam, with a 3.2 × 0.3 mm2 section. The beam was focused by a 0.8 T permanent magnet system. The high-frequency system adopts five dumbbell shaped five-gap cavities, and the output system adopts a symmetrical output waveguide. The 3D particle-in-cell simulation shows that more than 1.5 kW power can be obtained at an input power of 0.12 W, and the electron efficiency and gain are 9.2% and 41.2 dB, respectively. [ABSTRACT FROM AUTHOR]

The excitation and suppression of asymmetric modes in a coaxial relativistic klystron amplifier (RKA) with cascaded single-gap bunching cavities are analyzed in this paper. Through theoretical analysis and simulated verification, we find that the asymmetric modes of cascaded single-gap bunching cavities have low external quality factors, the same resonant frequencies, and negative beam-loading conductance ratios so that they are easily excited by the electron beam. To solve this issue, a feasible method is proposed in this paper to suppress the asymmetric modes of cascaded single-gap bunching cavities, under the conditions of decreasing their coupling coefficients and increasing the frequency separation, choosing an appropriate drift tube length between them. These improved bunching cavities are further examined in an X-band coaxial RKA by 3D particle-in-cell simulation, which shows that high power microwaves with a power of 0.8 GW are generated corresponding to an efficiency of 40%. Furthermore, there is no asymmetric mode competition during 150 ns of simulation time. [ABSTRACT FROM AUTHOR]

This article presents the design, analysis, and simulation for the klystron coaxial radio frequency (RF) output windows. Three new RF windows (Type-2, 3, and 4) are proposed based on the coaxial RF window with a doorknob structure (Type-1) to improve its performances. The proposed RF windows are simulated in high frequency simulator structure. At the operating frequency, the return loss are obtained as −49, −60, −56, and −52 dB, and the insert loss as −0.003 62, −0.002 51, −0.002 74, and −0.003 dB, respectively, for Type1, 2, 3, and 4. Moreover, for Type-1, the E-field near the short-circuit surface is significantly greater than that near the output port side due to the asymmetrical structure of RF window. However, this phenomenon has been greatly improved in the proposed RF windows. The heat load because of RF power absorption over the ceramic has been calculated. The distribution of heat load in three new windows is more uniform and less than that in Type-1, and the heat load in Type-2 is about half of that in Type-1. The thermal and structural analyses of RF windows predict the lower temperature and less mechanical deformation for the three new RF windows. Multipactor analysis is carried out to determine the safety at the operating power level. [ABSTRACT FROM AUTHOR]

A compact coaxial relativistic klystron amplifier (RKA) with three cascaded bunching cavities is investigated to obtain high efficiency output in this paper. When the injection power and drift tube length are both decreased for the compactness of the initial coaxial RKA, the conversion efficiency of this device decreases to only 20%, which is mainly due to the insufficient modulation of intense relativistic electron beam (IREB). To solve this issue, three cascaded bunching cavities are designed to strengthen the modulation of IREB, and each bunching cavity is designed to a non-uniform single-gap cavity with a high external quality factor. Moreover, a specific drift tube length is chosen for minimizing the TEM mode leakage between every two adjacent cavities. With these methods, the self-oscillation between input and three cascaded bunching cavities can be successfully suppressed without loading any reflectors. The proposed three cascaded single-gap bunching cavities are further examined in a Ku-band coaxial RKA, and the output efficiency of this device increases to 44%, which is more than twice that of the initial coaxial RKA. Furthermore, the injection power and axial length of the propagating IREB in the improved coaxial RKA are reduced to only 5 kW and 10 wavelengths, respectively. [ABSTRACT FROM AUTHOR]

Self-oscillation of three cascaded reflectors easily occurs in a triaxial klystron amplifier (TKA) since the TEM mode leakage cannot be cut off by a coaxial waveguide. To solve this issue, we propose a non-uniform reflector to obtain an optimal external quality factor, which indicates its minimum TEM mode leakage. The self-oscillation of three non-uniform reflectors is then suppressed by selecting a specific drift tube length between them. Moreover, different eigenfrequencies are chosen for the three improved reflectors to enhance the tolerance of the drift tube length between them. With these methods, the self-oscillation of three cascaded reflectors can be successfully eliminated, and a TKA with high output efficiency is capable of achieving a long pulse output. [ABSTRACT FROM AUTHOR]

Because of the scaling invariance, the over-mode ratio of the coaxial resonator can be increased to increase the power handling capability. However, as the over-mode ratio increases, the characteristic impedance and external quality factor decrease, which causes the modulation of the electron beam to be weakened. Moreover, when the output microwave power increases, the double-gap output cavity will suffer from severe radio frequency breakdown. Therefore, an X-band high-power and high-efficiency coaxial relativistic klystron oscillator with a four-gap modulation cavity and a three-gap extraction cavity is proposed. First, a four-gap modulation cavity can increase the modulation depth of the electron beam to improve the beam-wave conversion efficiency. The operating mode of the modulation cavity is the 3π/4 mode of the coaxial TM01 mode. Second, a three-gap extraction cavity is adopted to enhance the microwave extraction energy and reduce the RF field strength. The simulation results show that when the diode voltage is 650 kV, the beam current is 15.4 kA, and the guiding magnetic field is 0.48 T, the device outputs a microwave power of 4.2 GW, a frequency of 8.4 GHz, and an efficiency of 42%. [ABSTRACT FROM AUTHOR]

This paper introduces the detailed design scheme of a multi-beam electron gun applied to an S-band high power multi-beam klystron. The multi-beam gun adopts a coaxial cavity arrangement, and the total number of electron beams are 40 (19 in the inner layer and 21 in the outer layer). The working voltage of the electron gun is 60 kV, the total current is 300 A (evenly distributed on 40 electron beams), and the single beam perveance is 0.51 μP. The total perveance is 20.4 μP. The cathode loading of the gun is 12 A/cm2, which can meet the requirements of cathode operational lifetime. The beam focusing system uses a periodic reversal permanent magnet to realize miniaturization of the klystron. [ABSTRACT FROM AUTHOR]

This article studies and summarizes the operating characteristics of a ${G}$ -band folded waveguide traveling wave tube (TWT) amplifier enhanced by the $\pi $ -mode extended interaction cavities. Compared with conventional ${G}$ -band TWTs, the proposed amplifier can at once obtain a higher gain in a shorter interaction circuit length and ensure a proper operating bandwidth. The key of the design is introducing the $\pi $ -mode multigap resonant cavity with alternating wide and narrow slots, which will improve the working performance of the whole device by shortening the length of the interaction circuit, enhancing its interaction effect, and improving its gain of unit length. In this design, the operation bandwidth is expanded by stagger tuning technique. In addition, the influence of cavity loss is examined so that the optimal performance is achieved. The PIC simulation results show that when the operating voltage is 21 kV and the operating current 80 mA, the maximum average output power of the designed TWT amplifier is 65.78 W at 217.4 GHz, which is corresponding to the maximum gain and efficiency of 37.38 dB and 3.9%, respectively. The gain per unit length is 12.64 dB/cm and the 3-dB bandwidth is 3.5 GHz. [ABSTRACT FROM AUTHOR]

A ${G}$ -band multigap extended interaction klystron (EIK) adopting mode overlap method with ladder-type structure is proposed to achieve high power, high efficiency, and broad bandwidth. The gap number is set larger than the conventional EIK to achieve its potential. The high-frequency characteristics of the modes in output cavity are studied. The length ratio of long and short slots is optimized to obtain appropriate mode frequency interval and effective characteristic impedance. In our scheme, the gap number of output cavity is determined as 17 with the mode overlap of $\pi $ mode and $15/ 16\pi $ mode. The circuit structure and working conditions are carefully designed and optimized to solve the problem of instability under large gap number. With a beam current of 0.3 A, a voltage of 15.8 kV, and a magnetic field of 0.75 T, the particle-in-cell simulation shows that with the increase of frequency, the output cavity can work with two modes overlap and expand the 3-dB bandwidth to 1.32 GHz, which is more than two times of our previous work. Also, a maximum output power of 870 W, an efficiency of 18.4%, and a gain of 46.4 dB are finally obtained. The numerical simulation results definitely demonstrate the feasibility of mode overlap and its ability to the expansion of EIK bandwidth in the terahertz band. [ABSTRACT FROM AUTHOR]

In this article, the design and multimode analysis of a Ka -band, single cavity followed by a periodically dielectric-loaded (PDL) waveguide-based gyro-twystron operating in the fundamental TE01 mode have been presented. The present amplifier has been studied for its stability against the backward wave oscillations (BWOs) and the beam–wave interaction by using the linear and nonlinear multimode theory, respectively. Furthermore, the design of the present gyro-twystron has been validated by using a commercially available 3-D particle-in-cell (PIC) code. Moreover, a few key subassemblies, including a ${Y}$ -shaped input coupler, triode-type magnetron injection gun (MIG) with $\sim 4$ % velocity spread, a curved collector, and a double-disk RF window, have been designed and studied for their performance behavior. The 3-D PIC simulation of the present gyro-twystron predicted $\sim 260$ -kW RF output power in the TE01 mode at $\sim 35$ GHz. The gain, efficiency, and bandwidth are calculated as $\sim 54$ dB, 40%, and 1.3 GHz, respectively. [ABSTRACT FROM AUTHOR]

The two-stage (TS) multibeam klystron (MBK) technology has recently attracted significant research attention due to its compactness and high-efficiency (HE) performance. However, there is still a lack of scientific research on the beam optics for such microwave power sources integrated with a postacceleration (PA) gap. In this article, a comprehensive optics study based on the newly developed 2-D optics code CGUN is conducted for the first time to demonstrate the most critical steps in the optics design process, by adopting the 400-MHz TS MBK for the future circular collider (FCC) as an example. Two specific challenges arise in this TS MBK, which are studied in this article, and solutions are given. First, due to the combination of slow electrons, impedance change from individual beamtubes into common volume, and the mild decay of the magnetic field, there are possible reflected electrons at the collector entrance. This requires an increase in the beam voltage to 80 kV, beyond the requirements from considering the output gap alone, as well as tighter control on bouncing electrons. The beam scalloping is also found to be highly sensitive to the position of the PA gap and magnetic field, which later demonstrates that large gap length and magnetic field are required. Final all-in-one particle-in-cell (PIC) simulations of this klystron equipped with this special optics design demonstrate that the specification of 1.2-MW continuous wave (CW) power is practically attainable with an efficiency of 77.5% and without the presence of reflected electrons at any point in the whole circuit. [ABSTRACT FROM AUTHOR]

The klystron-based Compact Linear Collider (CLIC) was initially proposed with a low center-of-mass energy of 380 GeV, primarily due to potential cost-effectiveness. To enhance overall cost-efficiency, reliability, and stability, a novel RF module for klystron-based CLIC main linac has been designed and studied. This RF module utilizes two X-band klystrons to feed eight traveling wave accelerating structures, resulting in a beam energy increase of 138 MeV. A key innovation of the new RF module is the integration of double-height waveguides, which contributes to a 30% reduction in surface fields and a 40% decrease in RF loss. To meet the double-height requirement, a majority of the RF components responsible for transmitting the high RF power underwent a redesign. Two solutions, based on the choke mode flange and the L-shape waveguide, were proposed to facilitate the adjustment of the accelerating structures for beam-based alignment. Additionally, bent damping waveguides and HOM loads and a high order Magic-T, were designed for the accelerating structure named CLIC-K. The paper will comprehensively present and describe both the RF design and the integration of the klystron-based CLIC module. [ABSTRACT FROM AUTHOR]

A novel transverse-mode overlapping technology is proposed to enable significant advancement in the development of wideband extended interaction klystrons (EIKs). To obtain the individual beams with comparatively low perveance and low current density, the double sheet beams are used to drive the circuit. For the purpose of allowing the circuit to operate in the fundamental mode, two innovative extended structures, ridge-loaded barbell structure (RLBS) and ridge-loaded ladder structure (RLLS), are proposed. It is found that both the two structures are capable of transverse-mode overlapping (superposition of TM11 mode and TM21 mode) by adjusting the dimensions of the ridges. On the basis of such ideas, a wideband six-cavity circuit at 0.22 THz is designed. The input and output cavities are with the RLBS, while the idler cavities are with the RLLS. The bandwidth of the output cavity and the frequency tuning of cavities are carefully considered to preserve good gain uniformity across the continuous frequency band. The particle-in-cell (PIC) simulation results show that the maximum output power reaches 138 W with the operating voltage of 15.8 kV and each beam current of 0.3 A, and the corresponding gain is 34.3 dB. The 1-dB bandwidth is 1.95 GHz, approximately 1% of the operating frequency. The relatively small in-band ripple indicates the excellent bandwidth characteristics of the circuit. [ABSTRACT FROM AUTHOR]

Multicavity Klystron (MCK) is a high power microwave vacuum electronic device used to amplify radio frequency (RF) signals. MCKs have numerous applications, including radar, radio navigation, space communication, television, radio repeaters, and charged particle accelerators. The microwave-generating interactions in klystrons take place mostly in coupled resonant cavities positioned periodically along the electron beam axis. Importantly, there is no electromagnetic coupling between cavities. The cavities are coupled only by the flow of bunched electrons drifting from one cavity to the next. We advance here a Lagrangian field theory of MCKs with the space being represented by a one-dimensional continuum. The theory integrates into it the space-charge effects, including the so-called debunching (electron-to-electron repulsion). The corresponding Euler–Lagrange equations are ordinary differential equations with coefficients varying periodically in the space. Utilizing the system periodicity, we develop the instrumental features of the Floquet theory, including the monodromy matrix and its Floquet multipliers. We use them to derive closed form expressions for a number of physically significant quantities. Those include, in particular, the dispersion relations and the frequency dependent gain foundational to the RF signal amplification. We assume that MCKs operate in the voltage amplification mode associated with the maximal gain. [ABSTRACT FROM AUTHOR]

In this article, the design study of a V-band klystron with an internal coupling cavity is presented. The effective characteristic impedance of the internal coupling cavity is approximately 25% higher than that of the external coupling cavity when both operate at $\pi $ mode. A V-band RF circuit of 35% efficiency has been designed based on the proposed internal coupling cavity. An RF circuit based on single-gap resonators was designed first with the help of genetic algorithms (GAs). Then, a five-gap internal coupling cavity replaced the last several resonators to improve circuit bandwidth and efficiency. Simulation results have shown that the proposed internal coupling cavity can significantly improve the efficiency and bandwidth of an RF circuit based on single-gap resonators. Driven by a 60-kV and 9-A pencil beam, the designed RF circuit can achieve a peak power of 188.5 kW, a gain of 47.8 dB, and a 1-dB bandwidth of 180 MHz. The matching high compression ratio beam optics is also elaborated in this article. [ABSTRACT FROM AUTHOR]

The efficiency of the RF power sources is one of the major concerns in the large-scale high-energy accelerator projects such as the future circular collider (FCC) and the compact linear collider (CLIC). The new bunching method, the core stabilization method (CSM), can achieve high efficiency in the case of shorter interactive length. The multibeam klystron (MBK) could accommodate high RF power with a low operating voltage, compared with a single-beam klystron. This article presents the design and simulation of a 650-MHz, 0.9-MW continuous wave (CW) CSM MBK. The detailed design of a six-beam optics system is also elaborated in this article. The performance of such a device was validated by the 3-D particle-in-cell (PIC)-code computer simulation. With an operating voltage of 50 kV, PIC simulation predicts an efficiency of 81.8%. [ABSTRACT FROM AUTHOR]

In this article, the design and experiment of a hollow electron beam (HEB) electron optics system (EOS) for Ka-band extended interaction klystrons (EIKs) are presented. To realize the fast switch of the current emission at a low cutoff voltage, an electron gun with two focus electrodes (FEs) is chosen. The emitting surface of cathode is a ringed spherical surface. The first focus electrode (FE1) is around the cathode, and the second focus electrode (FE2) is set in the central hole of the cathode. The current emitted from the cathode surface is about 2A, when the operating voltage is 25 kV. Three simulation codes, Egun, Superfish, and CST, are used for designing the electron gun and the permanent magnet focusing system (PMFS). Some sensitivity analyses with respect to the PMFS’s critical parameters are presented. The simulation shows the dc beam transmission is 100%. The electron beam, confined by 6000 Gauss uniform field, propagates through the drift tube of 37 mm with good laminarity and small ripple. The emission current can be suppressed when the voltage of the FEs is −3.4 kV. According to the results of simulation, the Ka-band EIK had been constructed and the hot-test experiments had been fulfilled in succession. The experimental results exhibit that the dc beam transmission is about 99% and the beam can sustain 98% transmission at high frequency (HF) operation. In addition, the output power is higher than 8 kW in the bandwidth of over 100 MHz. [ABSTRACT FROM AUTHOR]

In this article, the design and thermal analysis of a novel dual-emitter magnetron injection gun (MIG) for dual-band gyroklystron amplifiers have been completed. First, we determine the operating mode, frequency, current, and other parameters of the dual-band gyroklystron. Then, the geometrical structure of MIG was preliminarily calculated by the trade-off equation and optimized by using the 3-D code CST particle studio and 2-D code EGUN, and the corresponding magnetic guidance system was designed. Finally, the temperature and thermal deformation of the cathode were performed. This dual-emitter MIG can successfully work in the $X$ / Ku-band, laying the foundation for the development of dual-band gyroklystrons. [ABSTRACT FROM AUTHOR]

The design approaches of the overlapping-mode extended interaction klystrons (EIKs) are presented to satisfy the requirement for broadband terahertz power amplifiers. Since all the cavities are designed and tuned to operate in multiple-cavity modes, the overlapping-mode EIKs can provide high output power over a wider frequency band. The circuit characteristics of the overlapping-mode EIKs are studied, including the dispersion curve, the coupling coefficient, and the frequency interval between cavity modes. Moreover, a six-cavity EIK operating at 0.34 THz is designed to demonstrate the broadband output capability of the overlapping-mode EIKs. The $2\pi $ -mode and its adjacent axial mode, the $\pi $ /13-mode, are chosen as the operating modes of the designed EIK. The 3-D particle-in-cell (PIC) simulation predicts a 3-dB bandwidth of 2.9 GHz for the designed power amplifier at a small-signal level or in saturation. Driven by an input power of 10 mW, the amplifier can provide the output power of 3.9 W, and the corresponding gain is 25.9 dB. When the input power is 150 mW, the amplifier is partially saturated and can provide the output power of 33 W with a corresponding electronic efficiency of 1.23%. [ABSTRACT FROM AUTHOR]

A four-cavity, large-sized beam tunnel, G-band extended interaction Klystron (EIK) is here proposed. A three-hole coupling cavity has been optimized to ensure a sufficient beam–wave interaction. The beam conductance and the total quality factor of each cavity have been analyzed to evaluate the stability of the beam–wave interaction for a 0.2-A beam current and 14.5-kV operating voltage. The particle-in-cell (PIC) simulation results show that the maximum output power is 134.5 W, with a gain of 30.3 dB and an efficiency of 4.6%, with a 3-dB bandwidth of 1 GHz. [ABSTRACT FROM AUTHOR]

A novel broadband multimode extended interaction klystron (EIK) circuit is proposed to overcome the bandwidth limitation of the conventional single-mode EIK. The uniform-gap and stagger-gap extended interaction circuits are comparatively studied; the multimode operation is demonstrated; stability and feasibility of this multimode circuit are analyzed. A −3-dB bandwidth of 860 MHz (2.51% relative bandwidth) in Ka-band is predicted by a particle-in-cell simulation. Meanwhile, comparative study on the uniform-gap and stagger-gap circuits also explains the working mechanism of the conventional single-mode (${\pi } $ mode) stagger-gap circuit. [ABSTRACT FROM AUTHOR]

We propose a high-power extended interaction klystron (EIK) amplifier exploiting open gratings as surface-wave cavities driven by a relativistic high-current sheet electron beam. For in- and out-coupling of the radiation, an additional subharmonic corrugation with a period twice larger than the period of the grating is proposed. Simulations based on both averaged quasi-optical model and particle-in-cell (PIC) code demonstrate the feasibility of the 150-GHz amplifier with 20–40-MW output power and 20–30-dB linear gain in 1% bandwidth. [ABSTRACT FROM AUTHOR]

This article presents a higher-order mode (HOM) multiple-beam klystron (MBK) with a multiple-gap output cavity that operates at 9.3 GHz. There are a total of 12 beams in the klystron, and each beamlet has a current of 12 A. The klystron has a beam voltage lower than 60 kV. The disadvantage of the low beam voltage on efficiency is analyzed, and the efficiency of 40% only can be obtained by using a single-gap output cavity. The relationships between the optimal efficiency and the electron transit angle of $\pi $ -mode double-gap and triple-gap output cavities are studied in this article. Based on these relationships, a single-beam klystron with a triple-gap output cavity is designed to reach an efficiency of 60%. The radio frequency (RF) circuit for an MBK with a triple-gap is designed to obtain an efficiency higher than 55% and an output power of 4.5 MW. Specific 3-D effects with asymmetric input power and phase and one absent beam are studied in this article. KlyC is used for 1-D and 1.5-D calculations, and CST is used for 3-D calculations. [ABSTRACT FROM AUTHOR]

To predict the stability of a multi-gap extended interaction cavity used in the klystrons, a general electron conductance expression has been rigorously derived from the small-signal space charge wave theory, where the field shape at the gap edge is described by a uniform distribution or a hyperbolic cosine function. Moreover, it is also pointed out that a simplified expression is acceptable in applications. This formula can be readily applied to the solid and annular beams, and its validity has been demonstrated through the stability analysis of a triple-gap extended interaction cavity, where the effect of adopting different gap field shapes is presented. This general expression is very useful for the parameter choice during the cavity design of extended interaction klystrons. [ABSTRACT FROM AUTHOR]

A small-signal theory describing the electron beam modulation in the radial-line relativistic klystron amplifier (RL-RKA) is developed in this paper. We first derive theoretically the space-charge limited current, the dispersion relation of space-charge waves on the radial-radiated electron beam, and the beam coupling coefficient for the radial-line electromagnetic structure. Then, a smallsignal theory is established to analyze the beam current bunching during the initial modulation period. With particle-in-cell simulations, the theoretical analysis is verified through observing the fundamental harmonic current of an input cavity in a previous Ku-band RL-RKA. More importantly, the presented small-signal theory can provide a theoretical guidance for the RL-RKA design. [ABSTRACT FROM AUTHOR]

The power required by each cavity of the ESS superconducting linac varies along the length of the accelerator, which results in a wide range of power levels to be provided by the RF power sources. For the medium and high beta sections of the linac, the selected power sources are klystrons with a nominal maximum output power of 1.5 MW. Operating these klystrons as efficiently as possible is among the goals of the ESS objectives to approach a carbon-neutral facility, but the ESS power profile presents specific challenges. This paper will discuss some of the solutions that will be implemented to increase the operational efficiency of the RF systems, focusing on klystron operation. • Energy efficiency is crucial for operating large particle accelerators. • Energy savings can be achieved by focusing on efficient operation of the klystrons, by optimizing output match and focusing. • The proposed optimization does not require large investment upfront and can also lead to increased klystron lifetime. [ABSTRACT FROM AUTHOR]

The Goldstone Solar System Radar (GSSR) facility is the largest fully steerable ground-based radar in the world for nonclassified high-resolution ranging and imaging of planetary and small-body targets. Over the years, the use of the GSSR to detect and characterize near-Earth objects (NEOs) has become critical to keep track of potential Earth-impacting hazardous NEO. This article relates the specific modifications made to the GSSR hardware and procedures in the last few years, as well as the new capabilities derived from those upgrades: reduced complexity in remote operations, increased experimental design versatility, and increased performance on bistatic radar experiments from GSSR to other complexes. In addition, we dedicate a section to provide an update on the current GSSR power capabilities as the new klystrons are installed. The work detailed in this article is intended to reach the broader science community in order to communicate how those modifications and the derived new capabilities can make science experiments more successful. [ABSTRACT FROM AUTHOR]

To improve the output performance of 0.22 THz extended-interaction klystrons (EIKs), we utilized a 20:1-aspect-ratio sheet beam to drive the circuit. The applicable high-frequency structure is discussed to obtain effective characteristic impedance, which is conducive to stronger beam-wave interaction. In addition, the loaded waveguide and the tuning scheme in the input cavity are proposed to provide good electric field uniformity over the whole beamwidth. Moreover, the nonuniform period output circuit based on the phase dynamic synchronization technology is optimized to further enhance power. Meanwhile, considering the feasibility of cathode performance, the beam current density in the tunnel is limited to 330 A/cm2. The designed four-cavity interaction circuit is comprised of two 9-gap idler cavities and 11-gap input and output cavities. With the operating parameters of 16.2 kV beam voltage and 0.66 A beam current, the particle-in-cell (PIC) simulation predicts a stable output power of 700 W with a 39 mW input excitation. The corresponding gain and 3 dB bandwidth are 42.5 dB and 500 MHz, respectively. An asymmetric single-stage depressed collector is also designed to recover the spent beam energy from the interaction circuit. The collector efficiency is 81.1% with zero electron reflux rate while taking secondary electrons into consideration, and the overall tube efficiency is 26.9%. [ABSTRACT FROM AUTHOR]

In this paper, a W-band multi-beam extended interaction klystron operating in the coaxial high order mode (HOM) is proposed. The HOM coaxial cavity with large transverse dimensions is suitable for accommodating multiple electron beams, promising to achieve high output power in the millimeter-wave frequency band. However, the operating method also brings lower characteristic impedance and a higher risk of mode competition. Based on the angular boundary condition of the coaxial HOM electric field, we propose a coaxial multi-gap cavity with a ridge structure to solve these problems. The TM51-2π mode is selected as the operating mode, of which the characteristic impedance can overwhelm that of the competing mode. The corresponding five-beam gun with a total current of 7 A (5 × 1.4 A) at a voltage of 25 kV is focused by a 0.65 T permanent magnet system. To demonstrate the capability of the device, the 3D particle-in-cell simulation predicts an output power of up to 30 kW at 94.4 GHz. The gain and electric efficiency are 54.8 dB and 17.2%, respectively. [ABSTRACT FROM AUTHOR]

A narrow coupling ladder-type extended interaction klystron (EIK) operating at 0.34 THz is proposed and analyzed. Compared with the traditional THz EIKs, the narrow coupling EIK can offer higher power capability and increased efficiency because such arrangement has higher characteristic impedance and a more uniform electric field in the beam tunnel. This article studies the electric field distribution of the narrow coupling circuits and traditional circuits. Moreover, the coefficient of variation is introduced to quantify the uniformity of the axial electric field in the beam tunnel. The operating stability is also ensured by adjusting the period length and number of cells. The 3-D particle-in-cell (PIC) simulation predicts the ideal EIK, driven by a 0.15-A, 17.5-kV electron beam confined in a 0.22-mm diameter beam tunnel, and generates an output power of 41 W. The corresponding gain, electronic efficiency, and 3-dB bandwidth are 28.03 dB, 1.56%, and 600 MHz, respectively. Finally, in order to verify the feasibility of the designed structure, we perform a sensitivity study on the EIK design for an eight-micron fabrication tolerance assuming normal distributions for all critical dimensions per each cell with PIC simulations. [ABSTRACT FROM AUTHOR]

The power conversion efficiency of relativistic klystron amplifiers (RKAs) is severely limited by the strong space-charge repulsion force caused by the intense electron beam. A radial-line RKA (RL-RKA) driven by the disk beam may provide great potentials to alleviate this issue due to its radially propagating characteristic of the electron beam. However, such potential advantage of high efficiency has not been fully exploited in previous research due to the insufficient beam modulation and relatively low beam-wave coupling strength. In this article, an initial RL-RKA is improved significantly by utilizing a three-stage cascaded bunching link, a four-gap output cavity, and a lengthened drift tube. The three-stage cascaded bunching link plays a role in increasing the fundamental harmonic current and decreasing the velocity spread of the electron beam simultaneously. The four-gap output cavity is designed to improve the coupling impedance and strengthen the beam-wave power conversion. Moreover, a drift tube is lengthened suitably to boost the fundamental harmonic current further. Eventually, the power conversion efficiency of the RL-RKA is enhanced up to 67% by these measures, which indicates a significant advance over the initial state of 38%. [ABSTRACT FROM AUTHOR]

The continued development of linear accelerators at higher frequencies poses several technological challenges. One such challenge is the requirement for high-frequency amplifiers to drive linearization systems. Presented in this article is the design of a 48 GHz gyroklystron amplifier appropriate for application in a harmonic linearizer for a 6 GHz or 12 GHz drive frequency. The beam used in the gyroklystron is provided by a magnetron injection gun (MIG), which has been designed using genetic optimization. The gyroklystron interaction circuit was designed with the aid of a Particle-in-Cell (PIC) simulation study which predicted 2 MW of output power at 48 GHz with a gain of 35 dB and an efficiency of 38%. This article summarizes the preceding work on the interaction circuit and MIG with additional phase stability analysis as well as presenting the design and analysis of vacuum windows, an input coupler, and the collector. [ABSTRACT FROM AUTHOR]

We demonstrate the operation of a compact x-ray free-electron laser (FEL) using the optical klystron mechanism. This effect speeds up the FEL process using the dispersion of magnetic chicanes placed between the undulator modules of the FEL beamline. The demonstration was performed at the soft x-ray beamline of SwissFEL, called Athos, which is, as far as we are aware, the only x-ray FEL to date with magnetic chicanes between every two undulator modules. In our measurements, we show that, compared to standard operation without chicanes, the required undulator length to achieve FEL saturation is reduced between 15% and 30% for radiation wavelengths between 1 and 2 nm. Fully exploiting the optical klystron effect represents an important step toward more compact FEL designs, rendering this key technology more affordable and hence accessible to a larger science community. [ABSTRACT FROM AUTHOR]

In order to reduce the size of high impedance diodes and magnetic energy consumptions, a high impedance intense current diode is designed in this paper. By reducing the center radius of the cathode and utilizing the gradient magnetic field, the electron beam is emitted in the cathode with a small radius and transmitted in the drift tube with a large radius. The influence of magnetic field distribution on the electron beam transmission is analyzed by particle-in-cell simulation. When the diode voltage is 680 kV, the impedance is 77 Ω, and the cathode radius is 60 mm, the anode radius of the traditional diode is 116 mm and the cathode–anode axial gap is 57 mm. At the same voltage and impedance, the cathode radius of the novel diode is 38 mm, the anode radius is reduced to 80 mm, and the cathode–anode axial gap is shortened to 36 mm. Further application of the novel diode to a relativistic triaxial klystron amplifier device shows that the output power of the novel diode is the same as that of the traditional diode. Through simple quantitative analysis, it is found that the magnetic energy consumption of the novel diode is less than that of the traditional diode. To sum up, the novel diode can achieve good performances in a smaller volume than the traditional diodes. [ABSTRACT FROM AUTHOR]

The beam power of the CEPC Collider is about 60 MW, so an efficiency of an RF power source is very important for cost of project implementation. The most popular source for an accelerator is a klystron, which has the advantage that it can be operated at high power with a reasonable high efficiency. IHEP is developing 650 MHz klystron with 800 kW CW output power and 80% efficiency. To reach this goal, a couple of klystron prototypes will be manufactured in the near future. The first prototype is completely manufactured by Institute of Electronics (IE) and GLVAC Company and the first step of high-power conditioning and commissioning is also completed in IHEP. The design schemes of high-efficiency klystron are also in progress. [ABSTRACT FROM AUTHOR]

Solid-state klystron modulators are typically based on oil-immersed high-voltage (HV) pulse transformers because of their high performance, robustness, simplicity, and straightforward design. However, pulse transformer size is fundamentally linked to application pulse length, pulse power, and pulse rise time. For high-power applications transformer size quickly becomes very problematic when approaching pulse lengths on the order of one millisecond. This article presents a systematic study of the applicability of HV pulse transformers for such long-pulse high-power applications. Both the single-layer and pancake winding techniques are evaluated, keeping reduction of transformer volume as the main design objective. First, design models and efficient optimization procedures are developed. The proposed models are validated through circuit simulation, 3-D finite element analysis, and comparison with a commercial HV long-pulse transformer. Then, the developed optimization procedure is used in studying the evolution of pulse transformer size when pulse length is varied from 500 $\mu \text{s}$ to 5 ms assuming peak pulse power requirements corresponding to that of the European Spallation Source (ESS) klystron modulators (115 kV, 100 A, 14 Hz). Finally, the developed trends are used to derive general analytical equations expressing maximum attainable pulse length as a function of application parameters and system constraints. [ABSTRACT FROM AUTHOR]