Output Cavity Research Articles

Design and experiment of X-band high-repetition rate high-power multi-beam relativistic klystron amplifier

The relativistic klystron amplifier (RKA) is a very important kind of high power microwave device, which has the advantages of high power, high efficiency, stable output phase and amplitude. The development of multi-injection RKA toward engineering and practical application needs to further improve operating frequency and output power of klystron amplifier, while the RKA of conventional circular waveguide drift tubes is restricted by the physical factors such as geometric size, space charge force and high-voltage breakdown. The multi-beam RKA based on the technology of multiple electron beams can work at low voltage and guiding magnetic field, and can also possess high electron beam current and diversion coefficient. The physical limitation of conventional structure RKA is overcome, and the working frequency and the output power are improved. In the experiment, the X-band GW level power of microwave is obtained. The multi-beam RKA needs to further improve its working frequency and working life. In the experiment, the power conversion efficiency of multi-beam RKA is about 35%, and most of the remaining electron energy will accumulate on the collection pole at the end. If the heat dissipation of the collector is not designed appropriately, the collection will be seriously ablated when working at high heavy frequency. Thus a large quantity of plasma and secondary electrons are generated, which affects the stability of the device. To solve the problem of electron reflux bombarding the output cavity after electron beam exchanging energy in the gap of output cavity, the reflux process of relativistic electron beam in the device is analyzed in this paper. On this basis, a coaxial extraction structure with four-gap extension interaction is designed to avoid electron reflux and reduce the gap electric field, thus improving the working life of the device. At the same time, in order to solve the problem that the conventional water cooling channel can affect the output microwave mode in a high-frequency over-mode device, a mode transformation structure of coaxial TEM mode-fan-shaped TE<sub>10</sub> mode-coaxial TEM mode-circular waveguide TM<sub>01</sub> mode is designed. The mode conversion efficiency is greater than 99.9%, and the influence of collecting polar water cooling channel on the output microwave mode is avoided. The stable operation of multi-beam RKA in the X-band with a repetition rate of 45 Hz is realized experimentally, while the output power is over 1 GW and the microwave pulse width is over 100 ns. At present, the multi-beam RKA runs about 10000 times in total, and the output microwave parameters do not decrease significantly.

Input and Output Couplers for an Oversized Coaxial Relativistic Klystron Amplifier atKa-Band

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

Oversized coaxial relativistic extended interaction oscillator with gigawatt-level output at Ka-band

In this paper, an oversized coaxial relativistic extended interaction oscillator (OCREIO) with gigawatt-level output at the Ka-band is proposed. We adopt a four-gap buncher cavity and a three-gap output cavity to increase the power capability and improve the electronic efficiency of the OCREIO. The beam-loading conductance for each mode existing in the buncher cavity is derived based on a small-signal theory. The beam-wave interaction and power capability for the 2π/3 and π modes exciting in the buncher cavity are analyzed. The operation mode 2π/3 is chosen by considering the beam-loading conductance and power capability. In addition, an improved method is proposed to mitigate the electric field intensity of the buncher cavity so as to moderate velocity modulation. The weak velocity modulation offers enough distance to make density modulation, thereby obtaining a relatively high fundamental harmonic current amplitude. Our study on the device performance of the proposed OCREIO, using the particle-in-cell simulation code CHIPIC, predicts an output power of 922.7 MW and a high electronic efficiency of 32.9% at 30.5 GHz, as well as the relatively less time of frequency-lock of self-oscillation <20 ns. The simulated results of the proposed OCREIO motivate us to practically develop the device in the near future.

Strong quadrature squeezing and quantum amplification in a coupled Bose–Einstein condensate—optomechanical cavity based on parametric modulation

We consider an optomechanical cavity with a movable end-mirror as a quantum mechanical oscillator (MO) containing an interacting cigar-shaped Bose–Eisenstein condensate (BEC). It is assumed that both the MO and the BEC interact with the radiation pressure of the cavity field in the red-detuned and weak coupling regimes while the two-body atomic collisions frequency of the BEC and the mechanical spring coefficient of the MO are coherently modulated. By analyzing the scattering matrix, we show that in the largely different cooperativities regime together with strong modulations, the mechanical mode of the MO and the Bogoliubov mode of the BEC exhibit quadrature squeezing which can surpass the so-called 3dB limit (up to 75 dB) with high robustness to the thermal noises. Surprisingly, in this regime by controlling the system and modulation parameters, a very high degree of squeezing (up to 16 dB) together with high purity of quantum state for the output cavity field is achievable. Furthermore, one can attain simultaneous strong quantum amplification, added-noise suppression, and controllable gain-bandwidth for the complementary quadratures of squeezed ones in the subsystems.

Investigation on pulse-shortening of S-band, long pulse, four-cavity, high power relativistic klystron amplifier

In order to improve the power and gain of a long-pulse relativistic klystron amplifier (RKA), the simulation and experimental studies of an S-band RKA driven by long pulse intense relativistic electron beams (IREBs) were carried out. We investigated the phenomena of self-excitation and pulse shortening, their roots, and the suppression measures in the process of IREB modulation and microwave extraction. The research results show that the reflection of electrons and the transmission of high order modes in the drifting tube, which are excited at the output cavity, are the main origins of the output microwave pulse-shortening. By employing cavities with different working modes, adding a graphite ring in the drifting tube as a microwave absorber and an electron blocker, the problem of self-excitation and pulse shortening is significantly mitigated. The output microwave with a peak power of 1.73 GW and a pulse width of 110 ns was obtained from the four-cavity RKA driven by an annular beam of voltage 850 kV, current 7.6 kA, and pulse width 190 ns; the gain is 51 dB; and the phase jitter range is about ±20°.

A Study of the generation of laser soliton from spontaneous emission by ring cavity with carbon nanotube

We demonstrate a behavior of laser pulse grows through fiber laser inside and output cavity with a soliton fiber laser based on the multi-wall carbon nanotube saturable absorber (SA), we investigate the effects of a saturable absorber parameter on the mode-locking of a realistic Erbium fiber ring laser. Generalized nonlinear Schrodinger equation including the nonlinear effects as gain dispersion, second anomalous group velocity dispersion (GVD), self phase modulation (SPM), and two photon absorption used to describe pulse evolution. An analytical method has been used to understand and to quantify the role of the SA parameter on the propagation dynamics of pulse laser. We compute the chirp, power, width and phase of the soliton for range of SA parameters and we studied stability against nonlinear effects at different SA parameter ranges.

Directional amplifiers in a hybrid optomechanical system

The unidirectional amplification of signals is of great importance in signal processing and communication. We propose a hybrid optomechanical system to achieve directional amplification between light fields with different frequencies, where three cavities are coupled to a common mechanical resonator. Two cavities function as an input cavity and an output cavity, respectively, while the third cavity is introduced to facilitate the unidirectional transfer between the input and output cavities. In our scheme, three tones are required to realize the directional amplifier without requiring any gain medium or additional mechanical driving. We find that under certain conditions, the amplifier can reach a large photon number gain with the quantum-limited added noise, with no limitation on the gain–bandwidth product. Furthermore, the directional amplification behavior is controllable by modulating the phase differences between the effective optomechanical couplings.

Investigation of an &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${X}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Band Long Pulse High-Power High-Gain Coaxial Multibeam Relativistic Klystron Amplifier

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

Optimization and Improvement of Output Performance in G-Band Extended Interaction Klystron

To improve the output performance of G-band extended interaction klystron (EIK), several practicable methods have been discussed. Firstly, the synchronization characteristics of the electron beam with the operating mode are analyzed in order to obtain large output power for optimized period. The influence of input gap voltage on the beam-wave interaction is studied, and the well-matched condition is established by optimizing the structure of input cavity. The gap length of output cavity is optimized to improve the electric field distribution. By adopting the optimized gap length and non-uniform gap lengths, the output power significantly increases from 360 to 570 W and 660 W, respectively; the corresponding bandwidth reaches to 650 MHz and 470 MHz. Moreover, AJDISK calculation is adopted to compare the results with CST simulation, and the similar gains and tendencies validate the relative reliability of our designed beam-wave interaction circuit. Therefore, our proposed methods for improving the output performance of G-band EIK are feasible and utilizable.

Design of a High-Efficiency S-band Klystron with a Multi-cell Output Cavity

As large-scale scientific facilities develop, the klystron efficiency becomes an important issue to reduce high operating costs. We are developing high-efficiency klystrons for the Pohang Light Source-II (PLS-II) and the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) at the Pohang Accelerator Laboratory (PAL). The newly developed klystrons should have the same specifications, including perveance and overall length, to replace existing ones. Under these limited conditions, we designed the klystrons based on a multi-cell output cavity to improve the power-conversion efficiency significatly. We simulated the beam-dynamics using the Field Charge Interaction (FCI) code with a MATLAB script to find the optimum design parameters for high efficiency. We have determined the design parameters, such as cell tuning frequencies, inter-cell distances, couplings between cells and the Qe of the last cell, of the multi-cell output cavity.

Increasing the bandwidth of an extended interaction klystron at 0.34 THz by staggered tuning of the multi-gap cavities

To increase the bandwidth of EIKs at 0.34 THz, a synthetical method containing several steps is proposed. As the crucial unit determining the performance of EIKs, the output cavity is studied. Based on the theory of space charge wave, the number of gaps in the output cavity is optimized to increase both the interaction efficiency and the instantaneous bandwidth. The approaches of tuning the shape of a barbell gap and the number of gaps in idler cavities are studied, leading to the practicable method of staggered tuning on all the intermediate cavities. As a result, the bandwidth of an EIK is improved step by step. Another approach of adjusting the drift tubes is also discussed and the synthetically tuning strategy for terahertz EIKs is concluded. Finally, the optimized nonuniform structure is verified by PIC simulations and the results show that it can effectively broaden the −3 dB bandwidth up to 500 MHz, which is double that of the uniform structure. The synthetical method is proved to be practicable in tuning the bandwidth and capable to keep the device operating in stability.

Design of a Ka‐band MW‐level high efficiency gyroklystron for accelerators

Design of a three-cavity Ka-band MW-level gyroklystron operating in the TE 02 mode in the output cavity while the input cavity and buncher are operating with the TE 01 mode is presented. The optimised design of the magnetron injection gun (MIG) and interaction circuit has been completed by particle-in-cell simulation and analytically using a MATLAB code. The simulations show the gyroklystron can deliver an output power of more than 1.5 MW with a gain of >35 dB at 36 GHz. The achieved efficiency exceeds 40% when driven by a 95 kV, 45 A beam. The optimised MIG has a transverse velocity spread of <;3% when the velocity ratio is around 1.3.

Extended interaction oversized coaxial relativistic klystron amplifier with gigawatt-level output at Ka band

In this paper, an extended interaction oversized coaxial relativistic klystron amplifier (EIOC-RKA) with Gigawatt-level output at Ka band is proposed. We introduce the oversized coaxial and multi-gap resonant cavities to increase the power capacity and investigate a non-uniform extended interaction output cavity to improve the electronic efficiency of the EIOC-RKA. We develop a high order mode gap in the input and output cavities to easily design and fabricate the input and output couplers. Meanwhile, we design the EIOC-RKA by using the particle-in-cell simulation. In the simulations, we use an electron beam with a current of 6 kA and a voltage of 525 kV, which is focused by a low focusing magnetic flux intensity of 0.5 T. The simulation results demonstrate that the saturated output power is 1.17 GW, the electronic efficiency is 37.1%, and the saturated gain is 57 dB at 30 GHz. The self-oscillation is suppressed by adopting the absorbing materials. The proposed EIOC-RKA has plenty of advantages such as large power capacity, high electronic efficiency, low focusing magnetic, high gain, and simple structure.

Optimum design and measurement analysisof 0.34 THz extended interaction klystron

In order to develop an extended interaction klystron (EIK) with high performance in the terahertz range, the staggered-tuned structure is numerically studied, manufactured, and measured. First, the circuit is optimized to get high interaction strength and avoid the mode overlapping in the output cavity, ensuring the efficiency and stability for the device. Then the clustered cavities are staggered tuned to improve its bandwidth. The particle-in-cell (PIC) code is employed to research the performances of the device under different conditions and accordingly the practicable and reliable conditions are confirmed. The device can effectively amplify the input terahertz signal and its gain reaches around 19.6 dB when the working current is 150 mA. The circuit and window are fabricated and tested, whose results demonstrate their usability. The experiment on the beam’s transmission is conducted and the results show that about 92% of the emitting current can successfully arrive at the collector, ensuring the validity and feasibility for the interaction process.

A High-Gain ${X}$ -Band Overmoded Relativistic Klystron

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

Oversized coaxial output cavity for Ka band relativistic klystron

Oversized coaxial output cavity for Ka band relativistic klystron

Phase control of entanglement and quantum steering in a three-mode optomechanical system

The theory of phase control of coherence, entanglement and quantum steering is developed for an optomechanical system composed of a single mode cavity containing a partially transmitting dielectric membrane and driven by short laser pulses. The membrane divides the cavity into two mutually coupled optomechanical cavities resulting in an effective three-mode closed loop system, two field modes of the two cavities and a mechanical mode representing the oscillating membrane. The closed loop in the coupling creates interfering channels which depend on the relative phase of the coupling strengths of the field modes to the mechanical mode. Populations and correlations of the output modes are calculated analytically and show several interesting phase dependent effects such as reversible population transfer from one field mode to the other, creation of collective modes, and induced coherence without induced emission. We find that these effects result from perfect mutual coherence between the field modes which is preserved even if one of the modes is not populated. The inseparability criterion for the output modes is also investigated and we find that entanglement may occur only between the field modes and the mechanical mode. We show that depending on the phase, the field modes can act on the mechanical mode collectively or individually resulting, respectively, in tripartite or bipartite entanglement. In addition, we examine the phase sensitivity of quantum steering of the mechanical mode by the field modes. Deterministic phase transfer of the steering from bipartite to collective is predicted and optimum steering corresponding to perfect EPR state can be achieved. These different types of quantum steering can be distinguished experimentally by measuring the coincidence rate between two detectors adjusted to collect photons of the output cavity modes. In particular, we find that the minima of the interference pattern of the coincidence rate signal the bipartite steering, while the maxima signal the collective steering.

A Millimeter-Wave Klystron Upconverter With a Higher Order Mode Output Cavity

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

An effective method to increase bandwidth of EIK at 0.34 THz

ABSTRACTTo increase the bandwidth of Extended Interaction Klystron (EIK) at 0.34 THz, the method of staggered tuning on cavities’ configurations is proposed. Based on the analysis of phase relationship between gap voltage and the bunched beam, the buncher cavities in EIK are reasonably staggered-tuned to achieve various resonance frequencies, which is helpful to flat the gain response of the whole device. The characteristics of output cavities with different numbers of gaps are then researched and the issue of start current for the self-oscillation mode is also involved, leading to the optimum number of gaps to enhance the interaction and avoid the instability. By comparing the performances of various typical stagger-tuned models, the final configuration is accordingly confirmed. Particle-in-cell simulation is eventually applied to study performance of the optimised structure, whose gain is 34.8 dB in peak and −3 dB bandwidth reaches about 500 MHz, which is double that of the synchronous-tuned structure.

Experimental demonstration of a 5th harmonic mm-wave frequency multiplying vacuum tube

We report the experimental demonstration of a 5th harmonic mm-wave frequency multiplying vacuum electronic device, which uses an over-moded spherical sector output cavity. In this device, a pencil electron beam is helically deflected in a transverse deflecting cavity before entering the output cavity. No magnetic field is required to focus or guide the beam. We built and tested a proof-of-principle device with an output frequency of 57.12 GHz. The measured peak power was 52.67 W at the 5th harmonic of the drive frequency. Power at the 4th, 6th, and 7th harmonics was 33.28 dB lower than that at the 5th harmonic.