Relativistic Klystron Research Articles

A non-uniform three-gap buncher cavity with suppression of transverse-electromagnetic mode leakage in the triaxial klystron amplifier

The triaxial klystron amplifier is an efficient high power relativistic klystron amplifier operating at high frequencies due to its coaxial structure with large radius. However, the coaxial structures result in coupling problems among the cavities as the TEM mode is not cut-off in the coaxial tube. Therefore, the suppression of the TEM mode leakage, especially the leakage from the buncher cavity to the input cavity, is crucial in the design of a triaxial klystron amplifier. In this paper, a non-uniform three-gap buncher cavity is proposed to suppress the TEM mode leakage. The cold cavity analysis shows that the non-uniform three-gap buncher cavity can significantly suppress the TEM mode generation compared to a uniform three-gap buncher cavity. Particle-in-cell simulation shows that the power leakage to the input cavity is less than 1.5‰ of the negative power in the buncher cavity and the buncher cavity can efficiently modulate an intense relativistic electron beam free of self-oscillations. A fundamental current modulation depth of 117% is achieved by employing the proposed non-uniform buncher cavity into an X-band triaxial amplifier, which results in the high efficiency generation of high power microwave.

相对论速调管放大器一维非线性理论的数值分析

相对论速调管放大器一维非线性理论的数值分析

Investigation and suppression of pulse shortening problem caused by non-working mode self-excitation in an S-band long pulse relativistic klystron amplifier

We have investigated the pulse shortening problem of relativistic klystron amplifier. It is shown that non-working mode self-excitation is an important cause of pulse shortening. Theoretical and numerical analyses show the mechanism of non-working mode self-excitation caused by the coupling between cavities. By the use of radio frequency absorbers, the problem of non-working mode self-excitation can be solved. In the experiment, the RF output power is 920 MW, the output power pulse width of the device can be increased from 77 ns to 137 ns.

衰减材料对高增益相对论速调管自激振荡的抑制

衰减材料对高增益相对论速调管自激振荡的抑制

Experimental researches of a four-cavity intense relativistic klystron amplifier

This paper described a high-order mode oscillation phenomenon, in the simulation of four-cavity intense relativistic klystron amplifier(IRKA). And we have analyzed the cause of this kind of oscillation, and explored the suppression methods for the whole tube. Two-dimensional and three-dimensional electromagnetism simulation software are used to study the influence of parameter’s changes (such as the oscillation frequency of cavities, the cavity Q value, the length of drift tube, and the characteristic impedance) on the high-order mode oscillation. The effectiveness of the high-order mode oscillation suppression methods is verified by simulation. Then, by optimizing the structure of IRKA, we have achieved an output microwave with a power of 2.13 GW, a gain of 60 dB and an efficiency of 26% in simulation, and the high-mode oscillation is avoided effectively. Finally, an output microwave with a power of 1.9 GW, a gain of 61 dB, and an efficiency of 24% is gained in the experiment.

Mode control in a high gain relativistic klystron amplifier with 3 GW output power

Higher mode excitation is very serious in the relativistic klystron amplifier, especially for the high gain relativistic amplifier working at tens of kilo-amperes. The mechanism of higher mode excitation is explored in the PIC simulation and it is shown that insufficient separation of adjacent cavities is the main cause of higher mode excitation. So RF lossy material mounted on the drift tube wall is adopted to suppress higher mode excitation. A high gain S-band relativistic klystron amplifier is designed for the beam current of 13 kA and the voltage of 1 MV. PIC simulation shows that the output power is 3.2 GW when the input power is only 2.8 kW.

X-band high power microwave combination technology based on the off-axis relativistic klystron amplifiers

In this paper, in order to overcome the self-excited oscillation in the high-gain relativistic klystron amplifier, an X-band high-gain relativistic klystron amplifier (RKA) is designed driven by the relativistic beam with its current at a kA-level. The corresponding diode impedance is 800 Ω. Its output power reaches 284 MW at the frequency of 9.47GHz, and the gain and efficiency are 50.6 dB and of 37.4% respectively when the beam voltage is of 800 kV. And for the obtained GW-level RF power in low-impedance pulsed power sources, the power microwave combination technology of off-axis eight-tube high-gain RKA is used. In the three-dimensional model, the microwave output of this kind of RKA is almost constant when the device is at off-axis 54 mm. Simulation is based on the 4.5 T superconducting magnet which is 4.5 m long in the laboratory, and the eight-tube high-gain RKA is simulated using a magnet with the microwave output of 284 MW. Then an eight to one power combiner is designed using HFSS software, and the combiner with eight-tube high-gain RKAs is simulated by a three-dimensional model. Its output power is 1.84 GW, with a gain of 50.7 dB and efficiency of 28.8% respectively.

High power operation of an X-band coaxial multi-beam relativistic klystron amplifier

An X-band coaxial multi-beam relativistic klystron amplifier is designed in order to increase output microwave power and operating frequency of the amplifier tube. The experiment is performed on a Tesla-type accelerator. The amplifier is driven by an electron beam of 2.8 kA at 720 kV, and a microwave power of 30 kW and frequency of 9.384 GHz is injected into an input cavity by means of an external source, then a microwave power of over 800 MW is extracted, the amplifier gain is about 44 dB, and conversion efficiency is 40%. The experiment proves that output power of nearly GWs can be generated with the X-band coaxial multi-beam relativistic klystron amplifier driven by a kW-level input power.

Gigawatt peak power generation in a relativistic klystron amplifier driven by 1 kW seed-power

An S-band high gain relativistic klystron amplifier driven by kW-level RF power is proposed and studied experimentally. In the device, the RF lossy material is introduced to suppress higher mode excitation. An output power of 1.95 GW with a gain of 62.8 dB is obtained in the simulation. Under conditions of an input RF power of 1.38 kW, a microwave pulse with power of 1.9 GW, frequency of 2.86 GHz, and duration of 105 ns is generated in the experiment, and the corresponding gain is 61.4 dB.

Influence of the strong beam impedance on injection and bunching features of the relativistic klystron amplifier

With numerical calculation and particle simulation program, the influences of the intense electron beam impedance, voltage and current characteristics on the beam modulation and bunching characteristics in relativistic klystron amplifier (RKA) are analyzed. Within the particle-in-cell simulation program, the beam emission method is used to accurately control the impedance of the electron beam. The results show that the electron beam of low-impedance can reduce the bunching distance and shorten the overall length of the RKA devices but degrade the injected modulation of the electron beam. Electron beam of high impedance is just opposite. When the electron beam impedance is constant, the increase of the electron beam accelerating voltage is similar to the increase of the impedance of the electron beam. In addition, with the particle simulation method, the beam loaded conductance of a specific input cavity loaded by a different impedance of the electron beam is determined, thereby meeting the demand of the power level of the seed, and the requirements for the externally loaded quality factor of the input cavity.

电子回流对相对论速调管稳定工作的影响

电子回流对相对论速调管稳定工作的影响

Nonlinear interaction between modulated electron beam and the intermediate cavity of RKA

In the relativistic klystron driven by intense relativistic electron beam, due to the influences of intense current and high electric field, especially the high Q value for the intermediate cavity, the nonlinear interaction between the intermediate cavity and the electron beam is very strong. It will significantly affect the performance of the device. According to the Maxwell equations and one-dimensional motion equation of electron, the self-consistent equation of the beam-wave interaction is obtained in the intermediate cavity. Based on these equations, the influences of the modulation depth and the modulation frequency on the amplitude and phase of gap voltage are studied respectively. The voltage amplitude obtained by the self-consistent equation is close to the voltage amplitude of particle in cell simulation, especially under the higher modulation depth compared with that obtained from the equivalent circuit model of conventional klystron. Meanwhile, the bandwidth of device becomes wide with the increase of the modulation depth. Finally, an S-band high-gain RKA is designed, and the corresponding experiments are done on the LTD accelerator. The output power with 1.1 GW is obtained, the gain is 49 dB.

Matching Conditions of the RKA Input Cavity Based on the Cavity Absorbing Property Under Intense Beam Loading

Based on the equivalent circuit model, expressions for the power absorbed by an externally coupled input cavity of the relativistic klystron amplifier in both cold and hot cavities are derived. The matching conditions are clearly concluded from the expressions for the two cases. Under the guide of the matching conditions, the expressions are applied in the determination of the beam-loaded resonant frequency and the beam-loaded quality factor of the cavity loaded by the intense relativistic electron beam in a particle-in-cell simulation-aided approach presented in this paper.

A Long Pulse Relativistic Klystron Amplifier Driven by Low RF Power

Higher order modes self-excitation can seriously degrade klystron performance and cause pulse shortening in the high-gain relativistic klystron amplifier. In order to suppress higher order modes self-oscillation in the device, some special measures are taken to increase the threshold current of such modes. An output power of 1.02 GW with an efficiency of 24.3% and a gain of 53.7 dB is obtained in the simulation with an input power of 4.3 kW and a frequency of 2.88 GHz. The experiment is performed on the linear transformer driver. Under conditions of an RF power of 30 kW, the peak output power has achieved 1.2 GW, with an efficiency of 28.6% and a gain of 46 dB; the corresponding pulse length is 120 ns.

Design and analysis of a radio frequency extractor in an S-band relativistic klystron amplifier

A radio frequency (RF) extractor converts the energy of a strongly modulated intense relativistic electron beam (IREB) into the energy of high power microwave in relativistic klystron amplifier (RKA). In the aim of efficiently extracting the energy of the modulated IREB, a RF extractor with all round coupling structure is proposed. Due to the all round structure, the operating transverse magnetic mode can be established easily and its resonant property can be investigated with an approach of group delay time. Furthermore, the external quality factor can be low enough. The design and analysis of the extractor applied in an S-band RKA are carried out, and the performance of the extractor is validated with three-dimensional (3D) particle-in-cell simulations. The extraction efficiency reaches 27% in the simulation with a totally 3D model of the whole RKA. The primary experiments are also carried out and the results show that the RF extractor with the external quality factor of 7.9 extracted 22% of the beam power and transformed it into the high power microwave. Better results are expected after the parasitic mode between the input and middle cavities is suppressed.

Design and 3D simulation of a two-cavity wide-gap relativistic klystron amplifier with high power injection

By using an electromagnetic particle-in-cell (PIC) code, an S-band two-cavity wide-gap klystron amplifier (WKA) loaded with washers/rods structure is designed and investigated for high power injection application. Influences of the washers/rods structure on the high frequency characteristics and the basic operation of the amplifier are presented. Generally, the rod structure has great impacts on the space-charge potential depression and the resonant frequency of the cavities. Nevertheless, if only the resonant frequency is tuned to the desired operation frequency, effects of the rod size on the basic operation of the amplifier are expected to be very weak. The 3-dimension (3-D) PIC simulation results show an output power of 0.98 GW corresponding to an efficiency of 33% for the WKA, with a 594 keV, 5 kA electron beam guided by an external magnetic field of 1.5 Tesla. Moreover, if a conductive plane is placed near the output gap, such as the electron collector, the beam potential energy can be further released, and the RF power can be increased to about 1.07 GW with the conversion efficiency of about 36%.

Brief Introduction and Recent Applications of a Large-Scale Parallel Three-Dimensional PIC Code Named NEPTUNE3D

This paper introduces a large-scale parallel 3-D fully electromagnetic particle-in-cell code named Nonlinear Evolution of Plasma-interaction Technology Under Nonneutral Environment-Three Dimensional (NEPTUNE3D) developed with the support of a parallel adaptive structured mesh application infrastructure JASMIN. NEPTUNE3D can be used to simulate beam-wave interactions in high-power microwave (HPM) generators. Some novel parallelization techniques are adopted in the NEPTUNE3D code for high parallel efficiency and good scalability. Some real-world HPM devices have been successfully simulated by NEPTUNE3D, such as the relativistic backward wave oscillator (RBWO), the magnetically insulated line oscillator (MILO), the virtual cathode oscillator, and the relativistic klystron amplifier. In this paper, two recent simulation examples are presented. The first is simulations of a knife-edge cathode RBWO, using 128 processors very efficiently. The second example is simulations of a MILO with a plate-inserted mode-transducing antenna, modeled using 27 million cells, 3 million particles, and 1024 processors with relatively high parallel efficiency.

Study of nonlinear interaction between bunched beam and intermediate cavities in a relativistic klystron amplifier

In intermediate cavities of a relativistic klystron amplifier (RKA) driven by intense relativistic electron beam, the equivalent circuit model, which is widely adopted to investigate the interaction between bunched beam and the intermediate cavity in a conventional klystron design, is invalid due to the high gap voltage and the nonlinear beam loading in a RKA. According to Maxwell equations and Lorentz equation, the self-consistent equations for beam-wave interaction in the intermediate cavity are introduced to study the nonlinear interaction between bunched beam and the intermediate cavity in a RKA. Based on the equations, the effects of modulation depth and modulation frequency of the beam on the gap voltage amplitude and its phase are obtained. It is shown that the gap voltage is significantly lower than that estimated by the equivalent circuit model when the beam modulation is high. And the bandwidth becomes wider as the beam modulation depth increases. An S-band high gain relativistic klystron amplifier is designed based on the result. And the corresponding experiment is carried out on the linear transformer driver accelerator. The peak output power has achieved 1.2 GW with an efficiency of 28.6% and a gain of 46 dB in the corresponding experiment.

Suppression of higher mode excitation in a high gain relativistic klystron amplifier

Suppressing higher mode excitation is very important in the high gain relativistic klystron amplifier because higher mode can seriously degrade klystron performance and cause pulse shortening. The mechanism of higher mode self-excitation is explored in the PIC simulation, and it is shown the coupling between cavities is the main cause of higher mode self-excitation. The coupling forms the positive feedback loop for higher mode to be excited just like that in the oscillator circuit. The formula for startup current of higher mode self-excitation is developed based on the coupling between cavities. And the corresponding methods are taken to avoid higher mode self-excitation. Finally, mode control is realized in the RKA with output power up to 1.02 GW when driven power is only few kilowatts.

The generation and transmission research of the fan-shaped multi-beam intense relativistic electron beams

Compared with the beam of conventional relativistic klystron, each beam of the multi-beam relativistic klystron has a low perveance and low space charge force, but it has a high conversion efficiency of beam-wave. According to these requirements, in this paper we investigate the generation and transmission of fan-shaped multi-beam intense relativistic electron beams by the experiment and the simulation with using the three-dimensional software, and analyse the electrostatic field distribution of the cathode end and the influence on the generation of the electron beams by establishing a three-dimensional model of electron gun. The emission currents by the particle-in-cell simulation, then the beam spot pictures of electron beam transmission in a hollow drift tube and multiple fan-shaped hole drift tube by the particle tracking solver are obtained. The theoretical analysis and explanation with the aid of the sheet beam theory are presented. The simulation and experimental results show that the beams rotate not only around their own center, but also around the center of the system in the transmission process of the electron beams in the hollow drift tube. Thus we can increase the transmission efficiency by rotating multiple fan-shaped hole drift tube to align the beams.