Klystron Oscillator Research Articles

Design of an X-band high-efficiency coaxial relativistic klystron oscillator packaged with permanent magnet

Design of an X-band high-efficiency coaxial relativistic klystron oscillator packaged with permanent magnet

An X-band coaxial relativistic klystron oscillator with a novel diode structure packaged with permanent magnet

To achieve compactness and miniaturization for high-power microwave devices, this paper proposes an X-band permanent magnet-packaged coaxial relativistic klystron oscillator (RKO) with a novel diode structure featuring a stair-step cathode. First, by adopting a large-radius anode structure, the emission of beam currents from the side of the cathode surface is diminished, thereby reducing the proportion of the backward current in the beam current from 10% to 7%. Second, a stair-step cathode is used to inhibit the return flow of electron beams from striking the anode and generating plasma. Concurrently, a radially non-uniform coaxial resonant cavity structure is employed to enhance the fundamental wave modulation depth of the electron beam and improve the beam–wave interaction's efficiency. The experimental results show that the novel device outputs a microwave with a frequency of 9.46 GHz and a pulse width of 55 ns. It generates a power output of 1.5 GW with an efficiency of approximately 30.5%, in the case in which the diode voltage is 505 kV, the beam current is 9.7 kA, and the guiding magnetic flux density is 0.4 T. This novel structure surpasses traditional diode configurations by enhancing the beam-to-wave conversion efficiency by roughly 10%, thereby offering substantial support for the compactness and miniaturization of RKO.

Analysis and Suppression of Mode Competition in a Relativistic Klystron Oscillator

Analysis and Suppression of Mode Competition in a Relativistic Klystron Oscillator

Research on Mechanism of the Pulse-Shortening in an X-Band Relativistic Klystron Oscillator Caused by the Asymmetric Mode Competition

The phenomenon of pulse shortening caused by asymmetrical mode competition is observed in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{X}$</tex-math> </inline-formula> -band relativistic klystron oscillator (RKO). Based on theoretical and simulation analysis of the electronic conductivity, external quality factor, and resonance characteristics of the TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{013}}$</tex-math> </inline-formula> mode and asymmetric TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{512}}$</tex-math> </inline-formula> mode, the reasons for the excitation of the asymmetric mode are obtained. First, the asymmetric TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{512}}$</tex-math> </inline-formula> mode has negative electronic conductors and great external quality factors in the buncher, which provides a starting condition. Second, asymmetric TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{512}}$</tex-math> </inline-formula> mode has higher resonance in the extraction structure to provide growth conditions. Therefore, strong positive feedback establishes between the buncher and the extraction structure, which makes the asymmetrical TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{512}}$</tex-math> </inline-formula> mode effectively excited. Finally, by optimizing the radius of the extraction structure, the resonance characteristics of the TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{013}}$</tex-math> </inline-formula> mode are enhanced, while the asymmetrical mode is weakened. So the TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{013}}$</tex-math> </inline-formula> mode establishes stronger positive feedback between the buncher and the extraction structure, effectively suppressing the excitation of the asymmetric mode. The simulation and experimental results show that this method can effectively suppress the excitation of the asymmetrical mode, and solve the problem of pulse shortening.

Preliminary Experimental Investigation of an -Band Low Magnetic Field Coaxial Relativistic Klystron Oscillator

An <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula> -band coaxial relativistic klystron oscillator (RKO) is proposed to improve the beam–wave interaction efficiency in the low magnetic field and low impedance. The device adopts a four-gap modulation cavity to increase the fundamental current, and a three-gap extraction cavity is used to improve the microwave extraction capability. The simulation results show that when the guiding magnetic field and impedance are 0.55 T and 40 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> , respectively, the output microwave power is 3.65 GW and the corresponding efficiency is 40%. Subsequently, a preliminary experimental investigation was carried out to verify the feasibility of this technical route. In the experiment, the output microwave with a power of 2.3 GW, a pulsewidth of 69 ns, and a center frequency of 9.39 GHz was generated when the magnetic field and impedance are 0.62 T and 41 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> , respectively. Moreover, the explosive electron emission model of the extraction cavity is established, and the simulation results show that the explosive electron emission generated by the radio frequency (RF) breakdown at the extraction cavity is the main reason for the difference between the experimental and simulated microwave power.

An X-band high-power and high-efficiency coaxial relativistic klystron oscillator with four-gap buncher and three-gap extractor

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%.

A Carbon-Nanotube Cold-Cathode Reflex Klystron Oscillator: Fabrication @ X-Band and Returning Electron Beam Realization

This paper presents the design and fabrication of a reflex klystron oscillator based on a carbon nanotube (CNT) cold-cathode. An X-band klystron oscillator structure is assembled with a CNT cold-cathode electron gun with an electrostatic focusing, a re-entrant cavity as anode, and a repeller. The electron gun adopts a convex CNT film emitter as the cathode. A re-entrant cavity resonating at 8.376 GHz is fabricated. The study mainly focuses on the returning electron beam in the klystron oscillator structure. The experimental results of variations of the anode current and returning electron beam amplitude with repeller voltage are presented. It is demonstrated that a higher extracting voltage of the cold-cathode has an important influence on the returning electron beam. To decelerate electron velocity from the extracting voltage, increasing negative focusing voltage and focusing electrode height in the electron gun can improve the returning electron beam characteristics.

A Ku-Band Compact Disk-Beam Relativistic Klystron Oscillator Operating at Low Guiding Magnetic Field

The disk-beam relativistic klystron oscillator (DB-RKO) with the radial-line high frequency structure is one of the attractive high power microwave (HPM) sources due to its specific virtues of weak space-charge effect, high power handling capacity and strong electron collection ability. However, such a device generally exhibits a bulky volume in the radial direction, retarding its actual applications in some compact occasions. Besides, it usually saturates slowly because of the insufficient beam-wave coupling strength. For this purpose, a compact DB-RKO with a rapid saturation process is proposed and physically designed in this paper. By compressing the axial width of the electron beam drift tube suitably, the beam-wave coupling strength is enhanced clearly and the saturation process is accelerated significantly. The whole beam-wave interaction structure is shortened with a radial length of about 5 cm. Besides, in order to increase the device compactness further, an improved magnetic-excited method is presented to enhance the magnetic field in the diode area by introducing a pair of soft-magnets. Furthermore, with the guiding of a low magnetic field of 0.4 T, the compact DB-RKO is experimentally demonstrated to be capable of generating Ku-band HPM radiations with a peak power of 810 MW, a power conversion efficiency of 28%, a center frequency of 14.19 GHz and a pulse duration of about 27 ns. The proposed DB-RKO is promising for the package of the permanent magnets which are preliminarily designed in this paper.

Operating characteristics of a clamp klystron oscillator with a sloping cavity at W-band

We present the design of an oscillator that is an evolution of our previously proposed “clamp klystron.” In this new embodiment, we utilize sloping cavities and increase the operating frequency to 94 GHz and 100 GHz (W-band). In this study, we utilized the UNIPIC particle-in-cell code. Simulations confirmed that this new oscillator with one sloping “buncher” cavity and one sloping “catcher” cavity with an inclination angle of 5° and shaped like a “clamp” produces 700 kW output power with an efficiency of 7.1% at 94 GHz when powered using an applied voltage U = 90 kV and beam current 110 A. When one sloping “buncher” cavity and one sloping “catcher” cavity are utilized with an inclination angle of 15°, the oscillator produces 270 kW output power with 3.0% efficiency at 100 GHz when powered using the same electron beam parameters. If two oscillators with sloping cavities along the z-axis are combined serially, then the output power can be as high as 800 kW with 8.0% efficiency at 100 GHz. This new oscillator design can be a promising approach to high power millimeter wave source designs at W-band and higher frequencies.

Operating characteristics of a clamp klystron oscillator at E-band

We describe a klystron-like oscillator operating at a frequency of 77.2 GHz (E-band) using particle-in-cell (PIC) simulations with the code MAGIC. It is shown that the klystron oscillator with one “buncher” cavity and one “catcher cavity” shaped like a “clamp” driven with an applied voltage U=20 kV−100 kV can operate with an electronic efficiency of above 11% with MW output power. To increase the electronic efficiency, we combined two clamp klystrons along the z-axis in a hybrid configuration between a klystron and the slow wave structure of a coupled-cavity TWT. PIC simulations demonstrate that when this hybrid design operates in the TE30 mode, its electronic efficiency can be as high as 28% with an output power as high as 1.35 MW. This article describes in detail this approach to the high power millimeter-wave source design.

Experimental demonstration of a Ku-band radial-line relativistic klystron oscillator based on transition radiation

We report on a radial-line relativistic klystron oscillator (RL-RKO), which is physically designed to generate gigawatt-level high power microwaves (HPMs) at Ku-band. The 3π/4 mode of a four-gap buncher is selected to highly modulate the radially propagating intense relativistic electron beam (IREB). A three-gap extractor operating at the π mode is employed to extract the radio-frequency energy efficiently. The Ku-band RL-RKO is investigated experimentally on an intense-current electron beam accelerator. The radially propagating IREB is well focused with an axial-width of 2 mm by a radial magnetic field of 0.4 T. Microwaves with a frequency of 14.86 GHz and a power of 1.5 GW are generated, corresponding to an efficiency of 24%, which indicates a significant advance for the research of radial-line HPM sources.

The mutual synchronization of coupled delayed feedback klystron oscillators

We report on the results of a numerical investigation of the synchronization of two coupled klystron oscillators with an external feedback circuit. Simulation has been carried out using the particle-in-cell method. We have also considered the results of a numerical analysis of an amplifier klystron and an isolated klystron oscillator, which make it possible to choose the optimal values of parameters of coupled klystrons. The structure of the synchronization domain for various parameters has been analyzed. The possibility of increasing the total output power with an appropriate choice of parameter of coupling between the oscillators has been revealed.

Computer simulation of a two-stage millimeter-wave klystron oscillator

The results of computer simulation of a two-stage millimeter-wave klystron oscillator are reported. The oscillator consists of two closed-loop floating-drift-tube klystrons with the output cavity of one klystron connected to the input cavity of the other and vice versa. It is shown that 200-W oscillations at a frequency of 95 GHz can be reached by optimizing the coupling coefficient between the cavities and the loaded Q factor.

Experimental and theoretical investigations of the influence of the external noise on dynamics of a klystron oscillator

Experimental and theoretical investigations of the influence of an external noise source on a klystron oscillator with delayed feedback are performed. It is shown that the action of noise leads to suppression of regular, self-modulation, and chaotic signals of the klystron oscillator. The physical mechanism of generation suppression, which is related to the influence of noise on the value of the bunched beam current in the output klystron cavity is revealed. The possibility of the appearance of synchronization induced by noise in a klystron chaotic oscillator subjected to an external noise source is shown theoreticallyk.

Coherent resonance in klystron oscillator at self-excitation threshold

The phenomenon of coherent resonance in a nonlinear dynamical system based on a delayed-feedback klystron (DFK) oscillator has been experimentally studied. The application of an external white noise signal to the DFK oscillator at the self-excitation threshold leads to the appearance of a resonant response in the system, the maximum of which is attained at a definite power of the external noise. It is demonstrated for the first time that the coherent resonance in response to an external harmonic signal in the microwave range allows the signal-to-noise ratio at the output of a regenerative ring amplifier to be significantly increased.

High order mode oscillation in a terahertz photonic-band-gap multibeam reflex klystron

TM 330 -like higher order mode was excited in a multibeam reflex klystron oscillator employing a hybrid photonic-band-gap (PBG) cavity using a three-dimensional particle-in-cell simulation. One side of a conventional metal cavity was replaced with a dielectric photonic crystal lattice to form a hybrid PBG resonator that uses lattice band-gap effects resulting in a more uniform field of a higher order mode as well as the exclusion of some conventional-cavity-type modes, thereby reducing mode competition. Simulated reflex klystron in the hybrid PBG cavity produced an output power much higher than could be delivered in a conventional metal cavity.

Investigation on an S-band relativistic klystron oscillator

The physics of modulation and rf extraction of an S-band relativistic klystron oscillator is studied in this paper via experiment, theory, and simulation. It is found that the intense relativistic electron beams (IREBs) can be intensely current-modulated when the IREBs drift through three pillboxes with high coupling. After bunching in the downstream the modulated IREBs can excite high-power microwave in the triaxial cavity. These properties, which have short oscillating time, compact geometry and high beam-wave conversion efficiency, were encouraging. Using a 1 MV, 13 kA, 40 ns electron beam and a 0.9 kGs leading magnetic field, 3.5 GW radiated power was extracted in 20 ns FWHM pulses at 2.86 GHz. The efficiency was 27%, and the instantaneous bandwidth was 2%. The radiated power was 3.4 GW when the repetition rate IREBs was 20 Hz. The experimental results agree well with the simulations.

Theoretical analysis of cross-talking signals between counter-streaming electron beams in a vacuum tube oscillator

The basic theory of cross-talking signals between counter-streaming electron beams in a vacuum tube oscillator consisting of two two-cavity klystron amplifiers reversely coupled through input/output slots is theoretically investigated. Application of Kirchhoff’s laws to the coupled equivalent RLC circuit model of the device provides four nonlinear coupled equations, which are the first-order time-delayed differential equations. Analytical solutions obtained through linearization of the equations provide oscillation frequencies and thresholds of four fundamental eigenstates, symmetric/antisymmetric 0∕π modes. Time-dependent output signals are numerically analyzed with variation of the beam current, and a self-modulation mechanism and transition to chaos scenario are examined. The oscillator shows a much stronger multistability compared to a delayed feedback klystron oscillator owing to the competitions among more diverse eigenmodes. A fully developed chaos region also appears at a relatively lower beam current, ∼3.5Ist, compared to typical vacuum tube oscillators (10-100Ist), where Ist is a start-oscillation current.

Complex dynamics of a double-cavity delayed feedback Klystron oscillator

The complex dynamics of a model double-cavity delayed feedback klystron oscillator is considered. The self-oscillation and stationary oscillation conditions are analyzed theoretically. The results of numerical simulation of the self-modulation and chaotic regimes are presented, and routes to chaos at the center and boundaries of the oscillation zone are studied in detail. The effect of space charge forces on the oscillator dynamics is discussed.

Chaotic Dynamics of Delayed Feedback Klystron Oscillator and its Control by External Signal

Complex nonlinear dynamics and applications of a microwave klystron oscillator with delayed feedback are studied in this paper on the basis of a system of delayed differential equations. Results of numerical simulation of self-modulation and chaotic regimes are presented, unveiling a complex bifurcation pattern in a wide range of parameters. Various types of chaotic attractors and transition to chaos scenarios are described. Experimental results for the five-cavity klystron oscillator are presented as well, showing good qualitative agreement with numerical simulations. Special attention is paid to application of the oscillator in direct chaotic communications. Information transmission using chaos shift keying (CSK) modulation and coherent reception is demonstrated via numerical simulation. The driving by an external signal is used to control the chaotic states of the transmitter oscillator for the purpose of generation of CSK basis functions.