Klystron Output Research Articles

Simulation study for efficiency optimization of multi-beam klystron output cavity

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%.

Analysis the influence of pulse-to-pulse stability of modulator on high-power microwave output of pulsed klystron

X-ray free electron laser (XFEL) facility based on electron linear accelerator (LINAC) is regarded as one kind of the fourth-generation light source with the characteristics of high intensity, exceptional brightness, ultrashort pulse duration, and spatial coherence. In electron linear accelerator, energy of beam bunches is provided by high-power electromagnetic microwaves which are generated by a microwave tube called klystron. The stability of beam voltage of klystron occupies a key position in both the stability of output RF (Radio Frequency) power and the jitter of output RF phase, furthermore, it plays an extremely important role in beam energy stability of electron linear accelerator. In this paper, high power RF fluctuation and phase jitter of klystron output caused by beam voltage instability of klystron are analyzed and calculated. Influence of klystron beam voltage instability on beam energy gain in linear accelerator have also been further analyzed and calculated. The calculating procedure is particularly valuable for us to understand the relationship between pulse modulator stability and beam energy gain fluctuations. Finally, relevant experimental results measured by Shanghai Soft X-ray Free Electron Laser Test Facility (SXFEL-TF) is presented.

Arcing-based protection system for the klystron output window of the BEPCII Linac

PurposeThe Beijing Electron–Positron Collider II (BEPCII) Linac is powered by 20 klystrons, and the output windows of each klystron need to be protected from being broken, which are currently protected by a system based on reflected radiofrequency (RF) power and vacuum levels. The primary weak point of reflection-based protection is the difficulty of accurately measuring the value caused by an uneven waveform. Moreover, vacuum protection is restricted by the long response time, which results in delayed protection. Both of the weak points may damage the output window of the klystron. An arcing-based protection system is then introduced into the BEPCII Linac, considering the constant presence of light during arcing and the experiences of other laboratories and colleagues in the RF group.MethodsThe system development mainly consists of the design and test of the E-bend waveguide observation window, selection of the arcing detection module, offline joint test, system assembling and online operation.ResultsThe system was assembled with a new klystron at the K6 position on the summer of 2018. During the following annual operation year, the light signal was captured, and the interlock was successfully realized.ConclusionThe arcing-based protection system assembled online was found to work well. Moreover, the light signal was successfully captured, and the interlock function worked fine. The system is planned to be applied in other klystron locations in the future.

Influence of External Electric Circuits on the Static and Dynamic Mode of Operation of Multipath Collectors of Powerful Klystrons

Abstract—The article considers the influence of external electric circuits on the work of the collector zone with an nonstationary intense electron beam. It is shown that the presence of capacitance of the collector and inductance of the external circuit under certain conditions can lead to the appearance of large inhibitory voltage values and deterioration of the klystron output parameters. The results of experimental studies are presented.

Validation of 500-kW/1000-s operation of 5-GHz klystron for KSTAR LHCD system

In this study, a klystron for the KSTAR lower hybrid current drive (LHCD) system was developed as a prototype for a 5 GHz, 500 kW CW operation that is aimed to meet the requirement of the ITER LHCD system as well. Before the 2012 KSTAR campaign, the prototype klystron was validated only for 350 kW CW operation. The weakest part of the klystron for 500 kW CW operation was the RF output window failure due to a thermal stress caused by the temperature gradient. The klystron was equipped with two RF windows, each of which should transmit 250 kW power. Prior to the full performance test of the klystron, the performance of the test RF windows that were identical to those mounted on the klystron was tested with half of the klystron’s full power. The temperature increase in the test RF windows was monitored using an IR camera. The temperature difference, ΔTce, between the center and edge of the test window at 250-kW 1000-s transmitting power was predicted to be 23 °C, which is smaller than the safe ΔTce, which is 50 °C according to the manufacturer. Based on the result of the test window measurement, 500-kW 1000-s operation of the klystron was conducted successfully. The klystron output power characteristics depending on the phase of load reflection VSWR 1.4:1 were investigated. The klystron generated stable power on a load VSWR of 1.4:1 at various phases. The output power at the worst phase was 380 kW. Details of the IR-imaging setup are discussed and the results obtained are presented.

Design and development of radio frequency output window for circular electron–positron collider klystron**Project supported by Yifang Wangʼs Science Studio of the Ten Thousand Talents Project, China.

This paper presents the first phase of design, analysis, and simulation for the klystron coaxial radio frequency (RF) output window. This study is motivated by 800 kW continuous wave (CW), 650 MHz klystrons for the future plan of circular electron–positron collider (CEPC) project. The RF window which is used in the klystron output section has a function to separate the klystron from the inner vacuum side to the outside, and high RF power propagates through the window with small power dissipation. Therefore, the window is a key component for the high power klystron. However, it is vulnerable to the high thermal stress and multipacting, so this paper presents the window design and analysis for these problems. The microwave design has been performed by using the computer simulation technology (CST) microwave studio and the return loss of the window has been established to be less than −90 dB. The multipacting simulation of the window has been carried out using MultiPac and CST particles studio. Through the multipacting analysis, it is shown that with thin coating of TiN, the multipacting effect has been suppressed effectively on the ceramic surface. The thermal analysis is carried out using ANSYS code and the temperature of alumina ceramic is lower than 310 K with water cooling. The design result successfully meets the requirement of the CEPC 650 MHz klystron. The manufacturing and high power test plan are also described in this paper.

Development of new S-band SLED for PAL-XFEL Linac

In order to achieve beam acceleration to the beam energy of 10GeV at the end of its 716m-long linear accelerator (Linac), the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) is going to operate the Stanford Linear Accelerator Energy Doubler (SLED) at the maximum klystron output peak power of 80MW, with a pulse length of 4μs, and at a repetition rate of 60Hz. The original SLED that had been used in Pohang Light Source-II (PLS-II) can no longer sustain such a high-power operation because excessive radiation caused by RF breakdown has been frequently detected even at the lower klystron peak power during the PLS-II operation. Therefore, a new SLED is designed by modifying both the 3-dB power hybrid and the waveguide-cavity coupling structure of the original SLED where the excessive radiation has been mainly detected. The finite-difference time-domain (FDTD) simulation in the CST Microwave Studio shows that the new SLED has a peak electric field and a surface current lower than those of the original SLED at the same level of the RF input peak power, which would secure stable high-power operation. All of the 42 SLEDs in the PAL-XFEL Linac are newly fabricated and installed. During the RF conditioning of the PAL-XFEL Linac, no significant vacuum and radiation issue was found in the new SLEDs. Finally, the accelerated electron beam energy of 10GeV obtained at the end of the PAL-XFEL Linac verified that the RF performance of the new SLED is stable.

A new method for compensation of the effect of charging transformer's leakage inductance on PFN voltage regulation in Klystron pulse modulators

The Line type modulators have been widely used to generate high voltage rectangular pulses to power the klystron for high power RF generation. In Line type modulator, the Pulse Forming Network (PFN) which is a cascade combination of lumped capacitors and inductors is used to store the electrical energy. The charged PFN is then discharged into a klystron by firing a high voltage Thyratron switch. This discharge generates a high voltage rectangular pulse across the klystron electrodes. The amplitude and phase of Klystron's RF output is governed by the high voltage pulse amplitude. The undesired RF amplitude and phase stability issues arises at the klystron's output due to inter-pulse and during the pulse amplitude variations. To reduce inter-pulse voltage variations, the PFN is required to be charged at the same voltage after every discharge cycle. At present, the combination of widely used resonant charging and deQing method is used to regulate the pulse to pulse PFN voltage variations but the charging transformer's leakage inductance puts an upper bound on the regulation achievable by this method. Here we have developed few insights of the deQing process and devised a new compensation method to compensate this undesired effect of charging transformer's leakage inductance on the pulse to pulse PFN voltage stability. This compensation is accomplished by the controlled partial discharging of the split PFN capacitor using a low voltage MOSFET switch. Theoretically, very high values of pulse to pulse voltage stability may be achieved using this method. This method may be used in deQing based existing modulators or in new modulators, to increase the pulse to pulse voltage stability, without having a very tight bound on charging transformer's leakage inductance. Given a stable charging power supply, this method may be used to further enhance the inter-pulse voltage stability of modulators which employ the direct charging, after replacing the direct charging with the resonant charging.

Different Solutions for the Generator-accelerator Module

The most important part of the particle accelerators [1] – is the power generator together with the whole feeding system [2]. All types of generators, such as klystrons, magnetrons, solid state generators cover their own field of power and pulse length values. For the last couple of year the Inductive Output Tubes (IOT) becomes very popular because of their comparative construction simplicity: it represents the klystron output cavity with the grid modulated electron beam injected in it. Now such IOTs are used with the superconductive particle accelerators at 700MHz operating frequency with around 1MW output power. Higher frequencies problem – is the inability to apply high frequency modulated voltage to the grid. Thus we need to figure out some kind of RF gun. But this article is about the first steps of the geometry and beam dynamics simulation in the six beam S-band IOT, which will be used with the compact biperiodic accelerating structure.

Inside view

inside view

Monotron Oscillation in Double-Gap Coupling Output Cavities of Multiple-Beam Klystrons

This paper presents a method to calculate the admittance of electron beam and of circuits in a multiple-beam klystron output circuit of double-gap coupling cavity. With the conditions of monotron oscillation, the possibility of forming monotron oscillations in a double-gap coupling output cavity is studied based on analytical calculations, and a method for calculating the initial oscillation current of monotron oscillation is derived. To further verify whether monotron oscillations form in the coupling double-gap output cavity and to analyze the characteristics of monotron oscillation, a particle simulation is performed to a particular coupling double-gap output cavity loaded by a cutoff waveguide filter, and the results show that monotron oscillations form in the particular coupling double-gap output cavity with an initial current of about 22 A under the condition of 18 kV accelerating voltage of the beam.

Klystron Output Circuit of High Order Transverse Magnetic Mode in Coaxial Cavity Loaded with Waveguide Filter

摘要: 该文设计了工作于高阶横磁TM310模式单间隙封闭圆柱同轴腔。在输出矩形波导内设计了横向电感滤波膜片的新型结构。用模拟场分析法计算了腔内6个漂移管中心处间隙阻抗平均值的频率特性。模拟和计算发现，该结构模式输出回路的间隙阻抗平均值随频率的变化出现双峰曲线。这表明，采用滤波膜片波导的输出带宽比采用空波导情形的带宽有明显的增加。 关键词: 多注速调管 / 同轴谐振腔 / 高阶横磁模 / 间隙阻抗 / 输出带宽 / 滤波器加载输出回路

Design of W-Band Extended Interaction Output Cavity

In this paper, a novel extended interaction klystron output cavity is presented. With the help of a three-dimensional (3D) particle-in-cell (PIC) simulation, design and optimization geometry of the cavity is performed, which is composed of ladder loaded rectangular waveguide. In present work, the output continue wave (CW) power 724W is achieved, the efficiency and bandwidth reaches 7.5% and 1300MHz, which is operating at beam voltage and beam current is 16 kV and 0.6A , respectively.

Impact of Nonlinear Memory Effects on Digital Communications in a Klystron

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Nonlinear memory effects in a klystron and their impact on digital communications are investigated using a time-domain physics-based model. The simulation results are compared to an idealized block model based on the frequency response and amplitude/phase drive curves typically used in system design with vacuum electronic amplifiers. Significant departures in transient behavior are noted in the physics-based model in comparison to the block model when the klystron is at or near saturation, provided that the signal bandwidth is simultaneously large ( <formula formulatype="inline"><tex Notation="TeX">$\geq\ \sim$</tex></formula>65% of the klystron output cavity bandwidth). Such nonlinear memory effects exist for pure amplitude, pure phase, and mixed transients of both the step and ramp variety. The effects of these nonlinear phenomena on 16-state phase-shift keying (16-PSK) digital communications waveforms (with preequalized symbols) are examined using symbol constellation diagrams and symbol error rate (SER) plots. Operation at saturation with signal bandwidths of 32%, 65%, and 93% of the klystron output cavity bandwidth, for a bit-energy-based signal-to-noise ratio of 18 dB, yields SER values of <formula formulatype="inline"><tex Notation="TeX">$ \hbox{2.0}\times \hbox{10}^{-5}$</tex></formula>, <formula formulatype="inline"><tex Notation="TeX">$\hbox{2.3} \times \hbox{10}^{-4}$</tex></formula>, and <formula formulatype="inline"><tex Notation="TeX">$\hbox{2.7} \times \hbox{10}^{-3}$</tex> </formula>, respectively, in comparison to the ideal 16-PSK value of <formula formulatype="inline"><tex Notation="TeX">$\hbox{1.1} \times \hbox{10}^{-5}$</tex></formula>. </para>

Status of Infrared Free Electron Laser at Nihon University

A free electron laser (FEL) system has been developed for the experiments of material, life, resource sciences and others at the Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University. The beam from a 125MeV electron linac was first introduced to the FEL system in 1998. We had encountered several difficulties especially in the electron beam stability for the long pulse operation of the linac which was required for a relatively short wavelength FEL system. In 2001 the first lasing of 1.5µm FEL was achieved at the electron energy of 86MeV using an undulator designed for the near infrared FEL. We could suppress the beam current fluctuations quite effectively by improving the stability of the phase of the klystron output RF and have succeeded in lasing. The current important subject is to improve the stability of the FEL lasing for the applications of the LEBRA FEL to various studies.

Beam-cavity interaction circuit at W-band

In this paper, we describe the design, fabrication, and bench study of a millimeter-wave cavity employed as a relativistic klystron output structure. The oxygen-free electronic-grade copper cavity was prepared by electro-discharge machining and diffusion bonding, and cleaned and tuned to 91.4 GHz. Measured cavity characteristics are presented and compared with theory, including quality factor Q, coupling parameter /spl beta/, scattering matrix S/sub 11/, and axial electric field profile E/sub z/. This paper provides the basis for an understanding of the cavity as a transfer structure.

The multiplet cavity: A buncher for broad-bandwidth klystron amplifiers

The bandwidth of a klystron output cavity scales (approximately) as p/sup 0.8/P/sup 0.2/, where p is the beam perveance and P is the beam power. For high-perveance (p>10 /spl mu/pervs) high-power (P>10 kW) electron beams, it is relatively straightforward to design a broad-band output cavity. However, the design of the input cavity of the broad-band klystron is in some ways more difficult. The purpose of the input cavity is to produce a velocity-modulated electron beam with a frequency-dependent modulation amplitude that will optimize the bandwidth of the entire klystron system, while providing a magnitude of velocity modulation large enough to minimize the length of the klystron. This paper shows how a multiplet (multiple cavity) buncher cavity can be designed to provide broad-band (>20%) operation while keeping short the drift section length of the klystron.

Stabilization of the output power of a 20-MW klystron to within 0.1%

For operation of a free electron laser, requirements on electron energy stability are extremely tight. When a radio frequency field is used to accelerate the electrons, this leads to stringent requirements on the stability of the klystron output power. We present results obtained with a 20-MW klystron which is energized by a pulse-forming network. The achieved stability of the klystron power is better than 0.1% during 20 μs.

A 50 MW pulse-forming network with a voltage stability within 0.03%

Requirements on electron energy stability are extremely tight for operation of a free-electron laser. In case a radio-frequency field is used to accelerate the electrons, this leads to stringent requirements on the stability of the klystron output power and, hence, on the stability of the source producing the cathode voltage. A pulse-forming network (PFN) capable of delivering a 19 kV pulse with a stability of 0.03%, during 20 μs, is presented. The PFN consists of four parallel lines, each containing 19 cells of a 20 nF capacitor and a 20 μH solenoid. The solenoids are equipped with taps and a plunge tuner. An innovative line switch is used, namely a stack of 32 thyristors in series. Numerical simulations of the pulse shape show that, in order to obtain the best possible pulse flatness, it is essential to arrange the capacitors such that the difference between adjacent capacitances is minimized.

A self-adaptive feedforward rf control system for linacs

The design and performance of a self-adaptive feedforward rf control system are reported. The system was built for the linac of the Accelerator Test Facility (ATF) at Brookhaven National Laboratory. Variables of time along the linac macropulse, such as field or phase are discretized and represented as vectors. Upon turn-on or after a large change in the operating-point, the control system acquires the response of the system to test signal vectors and generates a linearized system response matrix. During operation an error vector is generated by comparing the linac variable vectors and a target vector. The error vector is multiplied by the inverse of the system's matrix to generate a correction vector is added to an operating point vector. This control system can be used to control a klystron to produce flat rf amplitude and phase pulses, to control a rf cavity to reduce the rf field fluctuation, and to compensate the energy spread among bunches in a rf linac. Beam loading effects can be corrected and a programmed ramp can be produced. The performance of the control system has been evaluated on the control of a klystron's output as well as an rf cavity. Both amplitude and phase have been regulated simultaneously. In initial tests, the rf output from a klystron has been regulated to an amplitude fluctuation of less than ±0.3% and phase variation of less than ±0.6°. The rf field of the ATF's photo-cathode microwave gun cavity has been regulated to ±0.5% in amplitude and simultaneously to ±1° in phase. Regulating just the rf field amplitude in the rf gun cavity, we have achieved amplitude fluctuation of less than ±0.2%.