Output Cavity Research Articles

Design and Simulation of High-Aspect-Ratio Sheet Beam EIK at 0.22 THz

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/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . 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%.

Efficiency Enhancement of a High Power Radial-Line Relativistic Klystron Amplifier Driven by Disk Intense Electron Beam

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%.

A Coaxial High Power Output Cavity Operating in Hybrid TM₀₁-TM₀₂ Modes for Repetitive Operation

An output cavity operating in hybrid TM01-TM02 modes is investigated for the sake of producing repetitive high power microwaves (HPMs) in coaxial electromagnetic structures. To suppress anode plasma formation, the generated microwave is changed to be extracted inwards, so that the intense electron beam can be collected on the outer conductor, which makes beam collector cooling feasible practically in this topology. The issue of radio frequency breakdown is addressed by using a large extraction waveguide, and particularly the external quality factor required for optimal beam wave interaction is achieved by adjusting mode coupling. The design is examined in an X-band triaxial klystron amplifier by particle-in-cell simulation, which shows HPM with a power of 2 GW can be generated with an efficiency of about 36%.

A G-Band High Output Power and Wide Bandwidth Sheet Beam Extended Interaction Klystron Design Operating at TM31 with 2π Mode

In this paper, we propose a high-order mode sheet beam extended interaction klystron (EIK) operating at G-band. Through the study of electric field distribution, we choose TM31 2π mode as the operating mode. The eigenmode simulation shows that the resonant frequency of the modes adjacent to the operating mode is far away from the central frequency, so there is almost no mode competition in our high mode EIK. In addition, by studying the sensitivity of the related geometry parameters, we conclude that the height of the coupling cavity has a great influence on the effective characteristic impedance, and the width of the gap mainly affects the working frequency. Therefore, it is necessary to strictly control the fabrication tolerance within 2 μm. Finally, the RF circuit using six barbell multi-gap cavities is determined, with five gaps for the input cavity and idler cavities and seven gaps for the output cavity. To expand the bandwidth, the stagger tuning method is adopted. Under the conditions of a voltage of 16.5 kV, current of 0.5 A and input power of 0.2 W, the peak output power of 650 W and a 3-dB bandwidth of 700 MHz are achieved without any self-oscillation.

Effect of a non-uniform annular electron beam on the microwave characteristics in a coaxial structure Ka-band RKA output cavity

The influence of electron beam uniformity on the microwave characteristics of coaxial structure Ka-band RKA output cavity is investigated with particle-in-cell simulations. The electron beam non-uniformity is simulated in four different electron emission models: (1) continuous non-emission area, (2) spaced emission, (3) enhanced emission and (4) current density variation emission. The simulation results with the first emission model shows that the output power of Ka-band RKA decreases as the non-emission area increases, while the continuous non-emission area has little effect on the frequency and pulse width of Ka-band microwave; The simulation results with the second emission model shows that the output power of Ka-band RKA is related to the distribution of Z-component of electric field and excitation electric field strength in the beam–wave interaction gap. The more uniform the distribution of Z-component of electric field is, the greater the output power is. The stronger the excitation electric field is, the greater the output power is; The simulation results with the third emission model shows that the intense current density in local area will cause the reduction of output power. The higher the current density of the enhanced emission area is, the smaller the output power is; The simulation results with the fourth emission model shows that the fluctuation of current density during the steady-state phase will cause the fluctuation of microwave frequency and output power. The fluctuation amplitude of frequency and output power are positively related to the fluctuation amplitude of the current density. The larger the fluctuation amplitude of current density is, the greater the fluctuation amplitude of frequency and output power are. The faster the current density changes, the greater the fluctuation amplitude of frequency and output power are.

Single-quadrature quantum magnetometry in cavity electromagnonics

A scheme of an ultrasensitive magnetometer in the cavity quantum electromagnonics is proposed, where the intracavity microwave mode is coupled to a magnonic mode via magnetic dipole interaction. It is shown that by driving both magnonic and microwave modes with external classical fields and controlling the system parameters, one can reduce the added noise of magnetic field measurement below the standard quantum limit (SQL). Surprisingly, we show that beyond the rotating wave approximation (RWA), not only can the added noise be suppressed but also the output cavity response to the input signal can be substantially amplified in order to achieve a precise magnetic-field measurement. The estimated theoretical sensitivity of the proposed magnetic amplifier-sensor is approximately on the order of ${10}^{\ensuremath{-}18}\phantom{\rule{0.28em}{0ex}}\mathrm{T}/\sqrt{\mathrm{Hz}}$, which is competitive compared to the current state-of-the-art magnetometers like superconducting quantum interference devices (SQUIDs) and atomic magnetometers. The advantages of the proposed sensor in comparison with the other magnetometers is its high sensitivity at room temperature, sensing in a wide range of frequencies up to MHz, and its capability for signal-response amplification.

A compact AAA-compatible multispectral solar simulator based on spherical cap chamber

Rapidly developing thin-film solar cells prompts the demand for artificial narrow-band spectral illumination. Here, an LED-based compact spherical cap solar simulator is proposed to provide multiple independent color channels, while compatible with the traditional broadband IEC60904-9 standard in terms of illumination non-uniformity, stability and solar-like spectrum. Firstly, a one-dimensional model is established and then validated to analyze locations of maximum power point which may degenerate the spatial homogeneity. Secondly, the LED layout is optimized by Monte Carlo ray tracing, and it is found that a good illumination uniformity can be achieved if they are rotational symmetrically distributed. Thirdly, another two structural parameters, including output slot size and cavity height, are simulated to balance the output uniformity and output power. Finally, an instance simulator based on the structure is developed for performance verification. In the implementation, heat dissipation strategies are introduced to stabilize the system, including the use of heat sinks and flip-connected printed circuit boards to keep heat sources away from the optical chamber, thereby ensuring 0.052% short-term and 1.13% long-term stability. In addition, seven types of LEDs are adopted to simulate the solar spectrum, and its maximum deviation between the AM 1.5G is −8.82%. A control system consisting of a graphical user interface, STM32 microprocessor and constant current sources is developed to configure the current of different color channels. As a result, in predominant area of the entire output slot, six narrow-band and broadband illuminations can achieve 2% non-uniform, which is of interest for testing thin film solar cells.

An optomechanical platform for quantum hypothesis testing for collapse models

Quantum hypothesis testing has shown the advantages that quantum resources can offer in the discrimination of competing hypothesis. Here, we apply this framework to optomechanical systems and fundamental physics questions. In particular, we focus on an optomechanical system composed of two cavities employed to perform quantum channel discrimination. We show that input squeezed optical noise, and feasible measurement schemes on the output cavity modes, allow to obtain an advantage with respect to any comparable classical schemes. We apply these results to the discrimination of models of spontaneous collapse of the wavefunction, highlighting the possibilities offered by this scheme for fundamental physics searches.

Study of a Dual-Mode Multibeam Interaction Circuit With Coaxial Structure for Ka-Band High-Power EIK

A dual-mode five-beam extended interaction klystron (EIK) with a coaxial structure is proposed that can stably obtain high output power. 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">$_{\textit {mn}}$ </tex-math></inline-formula> mode with uniform angular distribution in the coaxial cavity can effectively interact with beams located at the peaks of the axial electric field. However, when the center radius of the cavity is enlarged to meet the cathode area requirements of high-power devices at high frequency, the interval between adjacent modes will rapidly decrease. Especially when combined with a multigap structure, the circuit will suffer from mode competition. This article describes a study of the beam–wave interaction characteristics of the multigap coaxial cavity driven by multiple beams, contributing to a stable circuit design. The results show that the appropriate mode selection for each cavity can increase the stability and efficiency of this device. A <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band EIK with four cavities has been designed to show the circuit capability. The output cavity operates in the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">51</sub> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\pi $ </tex-math></inline-formula> mode, but the other cavities operate in the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\pi $ </tex-math></inline-formula> mode. The 3-D particle-in-cell (PIC) simulation predicts a power of 211.6 kW at 35.19 GHz with the beam parameters of a voltage of 42 kV and a total current of 15 A (3 A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times5$ </tex-math></inline-formula> ). The corresponding gain, electronic efficiency, and 3-dB bandwidth are 53.26 dB, 33.5%, and 50 MHz.

Design of G-band 500 W sheet beam extended-interaction klystron

High power generation in terahertz frequency band is limited by physical mechanism. A G-band sheet beam extended-interaction klystron was designed to demonstrate the power level and the physical factors that affect the performance of the klystron. An elliptical electron beam with a voltage of 24.5 kV, a current of 0.6 A and the dimension of 1 mm×0.15 mm was used. To match the size of the sheet beam and obtain high efficiency and high gain, the transverse-oversized barbell type multi-gap resonant cavity was used as the interaction circuit. The 3D PIC simulation results show that more than 500 W of power output can be obtained with the actual cavity loss considered, and the electron efficiency and gain are 3.65% and 38.2 dB respectively. It is found that the power and efficiency are largely restricted by the mode stability of the multi-gap cavity as well as the ohmic loss. The ohmic loss of the output cavity has a significant effect on the final output power which should be given special consideration in engineering design. The research in this paper has laid a good foundation for the development of high frequency sheet beam extended-interaction devices.

On the Influence of the Electron Velocity Spread on the Passband Properties of Gyroklystrons

We present the results of studying the passband properties of gyroklystrons theoretically and experimentally and analyze the main causes of limitations in the bandwidth of the frequencies that they amplify. It is shown that in the range of low beam currents, where the electron velocity spread is rather low, the amplification bandwidth of a two-cavity gyroklystron is close to the bandwidth of the output cavity loaded with an electron beam. In the range of high currents, the amplification bandwidth depends only weakly on the parameters of the output cavity and is determined mainly by the spread in the times of electron passing through the drift space, which is caused by the difference in their translational velocities. Good qualitative agreement of the calculated and experimental dependences indicates that the proposed method can be applied to the calculation and interpretation of frequency characteristics of gyroklystrons. The possibility to expand the passband of operating frequencies in the high-current region by decreasing the influence of the electron velocity spread on beam bunching in the three-cavity gyroklystron operating in the maximum acceleration regime is demonstrated experimentally. Theoretically, the developed method for calculation of frequency characteristics can be expanded to the case of a three-cavity gyroamplifier with slight modifications.

Optimization of multibeam klystron double gap cavities loaded by metal rods

AbstractIn this paper, the results of numerical investigation of a metal photonic band gap cavity are presented. The investigated resonator is dual‐gap and is considered as an intermediate for the K‐band multibeam klystron. A working mode is a 2n mode with a frequency of 24.15 GHz. Electrodynamic characteristics of the cavity (characteristic impendance, beam coupling factor, relative electronic conductivity, Q‐factor) were calculated and the conditions for achieving the maximum effciency are found. Based on the obtained data, the required number of gaps in the output cavity was estimated, as well as the power and efficiency of the multibeam klystron with this type of cavities, depending on the microperveance of the electron beam.

Systematic Time-Dependent, Multimode Simulation Study of a Ka-Band, Three-Cavity Second-Harmonic Gyroklystron Amplifier

The multimode beam–wave interaction behavior in the RF interaction structure of a second-harmonic, 32.3 GHz gyroklystron amplifier operating in the TE02 mode has been studied by simulating the device using a commercial 3-D particle-in-cell code, “CST Studio Suite.” The effect of the variation of numerous parameters, such as driver frequency, RF power, magnetic-field strength, and beam current, has been investigated to realize the sensitivity of the device performance to these parameters. The effect of varying the position of the buncher cavity with respect to input and output cavity on the 3 dB gain and efficiency has also been investigated. The simulation results show that the designed gyroklystron amplifier produces a constant RF output power of ~315 kW at 32.3 GHz center frequency for a 70 kV, 20 A electron beam. The 3 dB gain of the device is obtained as ~26 dB with ~22.5% electronic efficiency and ~11 MHz bandwidth. The velocity spread is assumed as 0% for the present work. For validation, the simulated results have been compared with the previously reported experimental results and the previously reported analytical results, which are found to be in good agreement (i.e., less than 5% variation).

Multioutput Circuit for Low Voltage Ultracompact W-Band Klystron

In this article, the design, characterization, and manufacture of a circuit for a ${W}$ -band, ultracompact continuous wave sheet beam klystron are described. This klystron, aiming to produce more than 1.5-kW output power, operates at a comparatively low voltage benefiting the device size and extending its application space. However, a lower operation voltage adversely affects the beam-gap interaction efficiency. Two multigap output cavities have been employed to achieve a higher beam interaction efficiency without introducing additional competing modes or shrinking the interval between the operation mode and competing mode. Inspired by the resynchronization technology, the second output cavity can be optimized for the spent beam from the first output cavity. Comprising a single cavity output structure, the circuit efficiency of this double output cavity circuit is increased from 2.37% to 4.96%. In this circuit, electromagnetic modeling and design have been optimized to avoid complicated structures which are constrained by current mechanical design and manufacturing techniques. The MAGIC simulation shows that, driven by a 1-W RF signal, this circuit can extract more than 1.7-kW output power from a 35-kV, 1-A beam. A prototype single output cavity circuit has been designed and manufactured to determine the required machining tolerances for the final double output cavity circuit.

Influence of annular electron beam uniformities on the microwave characteristics in an S-band relativistic klystron amplifier

The non-uniform annular electron beam models are established based on the annular cathode explosive emission luminescence images. The influence of electron beam uniformities on the microwave characteristics of an annular structure S-band relativistic klystron amplifier output cavity is investigated with particle-in-cell simulations. The electron beam non-uniformities are simulated using four different electron emission models: (1) continuous area without emission, (2) spaced emission, (3) enhanced emission, and (4) current density variation emission. The simulation results with the first emission model show that the output power decreases as the continuous area without emission increases, while the continuous area without emission has little effect on the frequency and pulse width. The simulation results with the second emission model show that the output power of the spaced emission is related to the distribution of non-emission areas. The more evenly the areas without emission are distributed on the annular cathode, the greater the output power is. The simulation results with the third emission model show that the higher the current density in the local area is, the smaller the output power is. The simulation results with the fourth emission model show that the fluctuation of current density causes the fluctuation of frequency and output power. The larger the change area of the current density is, the greater the fluctuation amplitudes of the output power and frequency are. The larger the fluctuation amplitude of current density is, the greater the fluctuation amplitudes of the output power and frequency are.

Study on the Efficiency of Klystrons

The limits of beam-to-radio frequency (RF) efficiency, which are achievable using klystrons, are discussed in this article. Interaction between the electron beam and RF field in the output cavity is also investigated. A 100% efficiency could be achieved for certain RF fields and point-like beams. However, efficiency is reduced when fringe field, space-charge effect, and transverse beam distribution are considered. Numerical calculations on these factors are presented to investigate the power extraction mechanism in the output cavity, which will help in the optimization of future klystron designs. Large-signal klystron simulation code KlyC is used for 1-D and 1.5-D calculations.

Demonstration of efficient beam-wave interaction for a MW-level 48 GHz gyroklystron amplifier

The development of high-frequency RF linear accelerators (linacs) requires the consideration of several technological challenges, such as electron bunch linearization. Presented in this paper is the design of the interaction circuit for a 48 GHz MW-level three-cavity gyroklystron amplifier, appropriate for application as a millimeter wave power source in a fourth harmonic linearizing system for an X-band linac. The output cavity is operated at the cylindrical TE0,2,1 mode, while the input and buncher cavities are operated at the TE0,1,1 mode. The interaction circuit has been designed using a combination of analytical calculations and particle-in-cell simulations. The optimized gyroklystron is shown, through simulation, to deliver an output power of up to 2.3 MW with a gain of 36 dB and an efficiency of 44% at 48 GHz, when driven by a 140 kV, 37 A electron beam.

Back-action evading measurement of the collective mode of a Bose–Einstein condensate

In this paper, we investigate theoretically the back-action evading measurement of the collective mode of an interacting atomic Bose–Einstein condensate (BEC) trapped in an optical cavity that is driven coherently by a pump laser with a modulated amplitude. It is shown that, for a specified kind of amplitude modulation of the driving laser, one can measure a generalized quadrature of the collective mode of the BEC indirectly through the output cavity field with a negligible back-action noise in the good-cavity limit. Nevertheless, the on-resonance added noise of measurement is suppressed below the standard quantum limit (SQL), even in the bad cavity limit. Moreover, the measurement precision can be controlled through the s -wave scattering frequency of atomic collisions.

Compact filtering power divider based on corrugated third‐mode circular SIW cavities

AbstractIn this article, a compact single‐layer filtering power divider based on third‐mode circular substrate integrated waveguide (SIW) cavities is proposed. The third‐mode SIW technique is introduced to divide an SIW cavity into three cavities and get 66% size reduction. The proposed power divider is composed of three third‐mode circular SIW cavities within a circuit with the size of a circular SIW cavity. The power is coupled from the input cavity to two output cavities through the electric coupling, separately. To achieve a more size reduction, the designed cavities are corrugated along the cutting planes. By increasing the length of the corrugated edges, the center frequency can be easily shifted to lower frequencies. In order to prove the proposed design procedure, the presented power divider with a second‐order bandpass filtering response is designed and fabricated. Reasonable agreements between the measured and simulated results are found. The measured results show that the power divider achieves an insertion loss of 1.2 dB at 5.61 GHz, return loss of higher than 14 dB, and 3‐dB fractional bandwidth of 7.8%. The results show that this power divider has the advantages of the compact size, high selectivity, and single‐layer structure.

Near-infrared methane sensor system using off-axis integrated cavity output spectroscopy with novel dual-input dual-output coupling scheme for mode noise suppression

Mode noise suppression in off-axis integrated cavity output spectroscopy (OA-ICOS) is a key for improving the signal-to-noise ratio (SNR) of a sensor system, which basically depends on the ability to smooth out the cavity mode structure and to reduce the cavity mode linewidth. We demonstrated a novel dual-input dual-output (DIDO) coupling scheme of OA-ICOS for mode noise suppression. The influences of beam splitting ratio and light reflection number to the output intensity and cavity mode linewidth were theoretically studied. Experimental investigation on the suppression of cavity mode noise was carried out in terms of detection sensitivity and SNR through methane (CH4) measurements. A SNR of 221 and a detection sensitivity of 1.73 × 10−8 cm-1 Hz-1/2 was obtained for the DIDO-based OA-ICOS sensor system. The SNR was improved by a factor of ∼ 2.5 and the sensitivity was improved by a factor of ∼ 2.2 compared to the regular single-input single-output (R-SISO) approach. Multi-input multi-output (MIMO) configuration was further numerically studied to verify the noise suppression ability of this detection concept. This work reveals a new laser-to-cavity coupling method and provides a novel way to exploit OA-ICOS sensors with improved SNR and sensitivity.