Output Cavity Research Articles

Research on a broad-band X-band high power relativistic klystron amplifier

This paper introduces a high-power broadband X-band klystron amplifier that achieves an output power of 157 MW and a 3 dB operating bandwidth of 6.7%. The amplifier employs an explosive emission diode with high impedance, which balances the requirements of high power and broadband. The multi-gap input and output cavities are designed to operate at two different longitudinal modes within the operating frequency band, resulting in a flat absorption rate and output efficiency. Moreover, an eight-stage stagger-tuned bunching section is implemented to achieve a uniform fundamental harmonic current modulation depth across the frequency band. Simulation results indicate that when the diode voltage is 550 kV and the beam current is 550 A, the amplifier can achieve a maximum power of 157 MW with an efficiency of 51.9% at the central frequency of 9.8 GHz. Furthermore, within the 3 dB operating bandwidth of 6.7% (670 MHz), the power output remains higher than 80 MW. This novel klystron amplifier structure exhibits exceptional performance in terms of bandwidth, power, and efficiency, thus validating the effectiveness of the design in enhancing bandwidth and providing a solid foundation for the broadband design of high-power microwave sources.

Design and simulation of a Ka-band frequency doubling gyroklystron amplifier

The design methodology of a Ka-band frequency doubling two cavity gyroklystron has been discussed in this paper. In this study, the amplifier’s design and simulation are accomplished by employing nonlinear analysis and validated using particle-in-cell simulation techniques. The output cavity functions in the TE02 mode at two times the input signal frequency at the second harmonic of the cyclotron frequency, while the input cavity is powered by a 17.5 GHz source and functions in the TE01 mode at the first harmonic of the cyclotron resonance. The device’s performance has been examined both in the presence and in the absence of a beam. A number of factors were taken into account in order to optimize the interaction structure dimension to reach the device’s targeted performance. By adjusting the structural features and electron beam parameters, the device’s RF output power is assessed in an effort to increase efficiency. Simulation results estimate that the device displays a gain of 21.5 dB, an efficiency of 21.6%, a power of 227 kW, and a −3 dB bandwidth of ∼0.3%.

Steady motional entanglement between two distant levitated nanoparticles

Quantum entanglement in macroscopic systems is not only essential for practical quantum information processing, but also valuable for the study of the boundary between quantum and the classical world. However, it is very challenging to achieve the steady remote entanglement between distant macroscopic systems. We consider two distant nanoparticles, both of which are optically trapped in two cavities. Based on the coherent scattering mechanism, we find that the ultrastrong optomechanical coupling between the cavity modes and the motion of the levitated nanoparticles could be achieved. The large and steady entanglement between the filtered output cavity modes and the motion of nanoparticles can be generated if the trapping laser is under the red sideband. Then through entanglement swapping, the steady motional entanglement between the distant nanoparticles can be realized. We numerically simulate and find that the two nanoparticles with 10 km distance can be entangled for the experimentally feasible parameters, even in room temperature environments. The generated continuous variable multipartite entanglement is the key to realizing the quantum enhanced sensor network and the sensitivity beyond the standard quantum limit.

Modeless Raman fiber laser

The discrete multi-longitudinal mode structure and temporal periodic fluctuation are the intrinsic features of conventional lasers. However, longitudinal mode spacing limits the maximum resolution in high-resolution laser sensing systems. In addition, temporal periodic fluctuation reduces the security of secure communication and deteriorates the randomness in fast physical random bit generation. Therefore, it still remains an open challenge to realize a laser source without discrete longitudinal mode and temporal periodic fluctuation. Here, a modeless Raman fiber laser (RFL) with high efficiency was demonstrated. The output cavity mirror adopts a fiber Bragg grating with ultralow reflectance of −27dB. Due to the modulation instability, the discrete multi-longitudinal modes in the RFL gradually broaden with the increase of intracavity Stokes wave power and eventually fully overlap. At high power levels, the RFL no longer has discrete longitudinal modes like a conventional laser but generates a quasi-continuous spectrum. Benefiting from the modeless nature, the RFL can achieve extremely low relative intensity noise, favorable temporal stability, and rather low coherence. Modeless RFLs are expected to be ushered as ideal light sources into secure communication, optical sensing, and optical imaging.

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%.

A 10-kW Output Power Ka- to W-Band Klystron Harmonic Generator

The growing interest in high-power millimeter (mm)-wave amplifiers for various applications, such as high-resolution radar, active mm-wave imaging and atmospheric sensing, near object analysis, and 5G communications leading to novel solutions through which the researchers can overcome difficulty, complexity, and high expenses involved in fabrication technology of conventional vacuum electron devices including gyrotron, klystron, and traveling wave tube at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W</i> -band frequency. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W</i> -band klystron harmonics generator (KHG) can be an ideal solution to overcome these challenges. This article presents the design and simulation of a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W</i> -band KHG, which triples input frequency and amplifies third harmonics. In the design of the beam–wave interaction region, two methods, including the Friedlander method and the core oscillation method, have been used. In the output cavity, TM130 higher mode was applied. Using high-order mode increases the dimensions of the output cavity and decreases mechanical complexity and cost. The results gained from simulation in particle-in-cell computer simulation technology (CST) solver showed that the output power could reach higher than 10.7 kW with 53-dB gain and 24% beam efficiency in TM310 output cavity mode. The electron beam voltage is 45 kV with a 1-A beam current.

Coherence controlled generation of Gaussian quantum discord in a quantum beat laser

Quantum discord, an appropriate measure of quantum correlation, is analyzed in a two-mode Gaussian state of the cavity field evolved by a system of quantum beat laser. In the laser system, two arbitrary single-mode Gaussian states of the cavity fields and an external classical field couple to the atomic lasing medium. We calculate the time-dependent expression for the quantum discord both analytically and numerically, by following the reduced density matrix equation of the resulting cavity field. In this framework, we investigate the generation and control of Gaussian quantum discord in the two-mode Gaussian state of the cavity field at the output in terms of the purity and non-classicality of the two input cavity modes, the Rabi frequency of the classical driving field, the relative phase of the coupling parameters, and the damping rates of the cavity modes. The behaviour of quantum discord appears oscillatory due to the quantum beats in the output cavity field developed by the classical driving field in the medium as time passes. Moreover, we find that quantum discord switches on and off while adjusting the relative phase of the coupling parameters. Analysis of the analytical results agrees well with our numerical simulations.

Stable Operation of a Repetitively Pulsed X-Band Gigawatt Long Pulse Coaxial Multi-Beam Relativistic Klystron Amplifier

In this letter, the stability of an X band gigawatt coaxial multi-beam relativistic klystron amplifier (CMB-RKA) operating in repetitively pulsed mode was investigated by the particle-in-cell (PIC) simulation and experiments. A four-gap extended-interaction output cavity was optimized to avoid electrons reflection. And a water-cooled electron beams collector which was integrated with the output mode converter was designed to reduce the plasma and secondary electrons. The explosive emission conditions were improved by using the C–C composite material cathode, and the titanium beam-wave interaction structure was used to replace the stainless steel structure. All these factors brought greater stability of the CMB-RKA. In the experiment, the X-band CMB-RKA can operate at a 20 pps repetition rate with duration of 120 seconds, when the output power was about 0.95 GW with pulse width of 160 ns. The long-term stability of the X-band CMB-RKA performances during <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{{5}}$ </tex-math></inline-formula> pulses was realized, which is driven by a long pulse compact accelerator.

Advanced Theoretical Models for Broadband Klystron Development

Broadband klystrons (BBKs) offer the potential of both high power and broad-bandwidth performance as microwave amplifiers. Staggered tuned bunching circuit and filter-loaded output cavity are the mainstream techniques adopted for developing such devices. However, in the design and optimization stage, the accurate and fast theoretical models are still missing before final particle-in-cell (PIC) simulations are conducted. In this article, the small-signal theory of klystron has been derived from accurate large-signal models used in KlyC, where complex mode field distribution, relativistic effects, mode coupling, and precise space charge model are fully considered in the final analytic formulas compared with existing simplified models. Furthermore, mode coupling theory with partially beam-loading conditions has been developed in KlyC to analyze the filter-loaded output cavity and overall klystron performance in the large-signal domain. An example of the retrofit design of 100-kW five-cavity <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 klystron is then presented to demonstrate the advanced design methodology based on the above fast theoretical models. The 3-D PIC simulation finally verifies this BBK design, showing −1-dB bandwidth of 200 MHz is attainable with output power over 100 kW, where the maximum discrepancy between theoretical models and PIC is within 2% in the whole bandwidth.

Continuous-wave and passively Q-switched pulsed 1.5 µm Er:Yb:Ba3Gd(PO4)3 lasers.

An eye-safe 1567 nm continuous-wave laser with a maximum output power of 50 mW and a slope efficiency of 21.1% was demonstrated in an Er:Yb:Ba3Gd(PO4)3 crystal. By using a Co2+:MgAl2O4 crystal with an initial transmission of 95% as a saturable absorber, a stable passively Q-switched pulsed laser was also realized in the crystal. The effects of the output coupler transmission and cavity length on pulsed performance were investigated. At an absorbed pump power of 350 mW, a 1541 nm Er:Yb:Ba3Gd(PO4)3 pulsed laser with a repetition frequency of 0.86 kHz, duration of 38 ns, energy of 21.2 µJ, and peak output power of 0.56 kW was obtained.

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 High-Power and Broadband G-Band Extended Interaction Klystron Based on Mode Overlap

A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${G}$ </tex-math></inline-formula> -band multigap extended interaction klystron (EIK) adopting mode overlap method with ladder-type structure is proposed to achieve high power, high efficiency, and broad bandwidth. The gap number is set larger than the conventional EIK to achieve its potential. The high-frequency characteristics of the modes in output cavity are studied. The length ratio of long and short slots is optimized to obtain appropriate mode frequency interval and effective characteristic impedance. In our scheme, the gap number of output cavity is determined as 17 with the mode overlap of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> mode and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$15/ 16\pi $ </tex-math></inline-formula> mode. The circuit structure and working conditions are carefully designed and optimized to solve the problem of instability under large gap number. With a beam current of 0.3 A, a voltage of 15.8 kV, and a magnetic field of 0.75 T, the particle-in-cell simulation shows that with the increase of frequency, the output cavity can work with two modes overlap and expand the 3-dB bandwidth to 1.32 GHz, which is more than two times of our previous work. Also, a maximum output power of 870 W, an efficiency of 18.4%, and a gain of 46.4 dB are finally obtained. The numerical simulation results definitely demonstrate the feasibility of mode overlap and its ability to the expansion of EIK bandwidth in the terahertz band.

Ultra-high resolution mass sensing based on an optomechanical nonlinearity

Ultra-high resolution mass sensing used to be realized by measuring the changed mechanical oscillation frequency by a small mass that should be detected. In this work we present a different approach of mass sensing without directly measuring such mechanical frequency change but relying on the modified light field due to a previously less explored nonlinear mechanism of optomechanical interaction. The concerned optomechanical setup used for the mass sensing is driven by a sufficiently strong two-tone field satisfying a condition that the difference of these two drive frequencies matches the frequency of the mechanical oscillation, so that a nonlinear effect will come into being and lock the mechanical motion under the radiation pressure into a series of fixed orbits. A small mass attached to the mechanical resonator slightly changes the mechanical frequency, thus violating the exact frequency match condition. Such small change can be detected by the amplitude modification on the higher order sidebands of the cavity field. Even given a moderate mechanical quality factor for the setup, the added mass can still be detected to the levels corresponding to a mechanical frequency shift from 5 to 7 order less than the mechanical damping rate. Because the output cavity field difference for very close values of mechanical frequency is not blurred by thermal noise, such mass sensing can be well performed at room temperature. The previous tough requirements for ultra-high resolution mass sensing can be significantly relaxed by the method.

Quantum treatment of cavity-assisted entanglement of three-level atoms and two fields in an electromagnetically-induced-transparency configuration

We examine the field-field and field-atom entanglements generated by \ensuremath{\Lambda}-type three-level atoms placed in an optical cavity driven above threshold by two coherent input optical fields. It is shown that under realistic experimental conditions entanglement between the two output cavity fields as well as between the fields and the atoms can be achieved when the two cavity fields are both one- and two-photon (Raman) resonant with the atomic transitions, as in the configuration for electromagnetically induced transparency (EIT). The entanglement has similar features to pump-signal-idler three-color entanglement in an above-threshold optical parametric oscillator [Villar et al., Phys. Rev. Lett. 97, 140504 (2006)]. This cavity-enhanced atom-field cooperative coupling could enable convenient and efficient generation of bright, nondegenerate, narrow-band entangled fields, which may find potential applications in realistic quantum communications and networking protocols.

Design of the HOM MBK With Multiple-Gap Output Cavity

Design of the HOM MBK With Multiple-Gap Output Cavity

Unconditional measurement-based quantum computation with optomechanical continuous variables

Universal quantum computation encoded over continuous variables can be achieved via Gaussian measurements acting on entangled non-Gaussian states. However, due to the weakness of available nonlinearities, generally these states can only be prepared conditionally, potentially with low probability. Here we show how universal quantum computation could be implemented unconditionally using an integrated platform able to sustain both linear and quadratic optomechanical-like interactions. Specifically, considering cavity opto- and electromechanical systems, we propose a realization of a driven-dissipative dynamics that deterministically prepares the required non-Gaussian cluster states---entangled squeezed states of multiple mechanical oscillators suitably interspersed with cubic-phase states. We next demonstrate how arbitrary Gaussian measurements on the cluster nodes can be performed by continuously monitoring the output cavity field. Finally, the feasibility requirements of this approach are analyzed in detail, suggesting that its building blocks are within reach of current technology.

Klystron-like Cyclotron Amplification of a Transversely Propagating Wave by a Spatially Developed Electron Beam

A klystron-like gyro-amplifier based on the excitation of a wave propagating across a spatially developed (in the transverse direction) electron beam is described within the simplest 2-D model. Such a configuration is attractive as a way of implementation of a short-wavelength source with a relatively high level of output power and with the possibility of quasicontinuous frequency tuning. We study the peculiarities of the 2-D process (developing in both the axial and transverse directions) of electron bunching and “free” wave emission from the electron beam in the open drift space, as well as the excitation of the output cavity used to provide formation of a compact and powerful output wave signal. The main problem of this 2-D process is that different fractions of the electron beam (located at different points of its cross-section) move in different wave fields. In addition, excitation of the parasitic wave propagating in the opposite direction relative to the operating wave is possible. However, we show that it is possible to organize effective electron–wave energy exchange for almost all fractions of the electron beam.

Investigation on High-Efficiency Beam-Wave Interaction for Coaxial Multi-Beam Relativistic Klystron Amplifier

To significantly improve the electronic efficiency of coaxial multi-beam relativistic klystron amplifier (CMB-RKA), the physical process of beam-wave interaction and parameters that affect efficiency was studied. First, the high efficiency of beam-wave interaction was discussed by simulating the efficiency versus the parameters (frequency of cavity, drift tube length between cavities, and external quality factor of output cavity), in the one-dimensional (1-D) large-signal simulation software. Moreover, the further physical process of beam-wave interaction was analyzed through simulating the current modulation factor and the number of particles at the entrance of the output cavity, in the three-dimensional (3-D) particle in cell simulation software. Last, with the optimal parameters in 3-D simulations, the CMB-RKA, which has 14 electron beams with a total current of 4.2 kA (14 × 300 A), can generate an output power of 1.02 GW with a saturation gain of 55.6 dB and an efficiency of 48.7%, when beam voltage is 500 kV, which indicated the CMB-RKA can achieve high efficiency for high-power microwave radiation.

Phase-sensitive seeded modulation instability in passive fiber resonators

Modulation instability is one of the most ubiquitous phenomena in physics. Here we investigate the phase-sensitive properties of modulation instability with harmonic seeding in passive fiber resonators. Theoretical investigations based on the Lugiato−Lefever equation with time dependent pump and a three-wave truncation show that the dynamics of the system is sensitive to the relative phase between input signal, idler, and pump waves. The modulation instability gain can even vanish for a peculiar value of the initial relative phase. An advanced multi-heterodyne measurement technique had been developed to record the real time evolution, round-trip to round-trip, of the power and phase of the output cavity field to confirm these theoretical predictions.

Design study and modeling of multi-beam Klystron for Circular Electron Positron Collider

Aimed at energy-saving for the Circular Electron Positron Collider of China, a multi-beam klystron is designed for a higher efficiency than existing commercial klystrons in a continuous-wave UHF band range. The design considerations for every constituent part of the multi-beam klystron are discussed in detail, including modeling of the realistic structures and results of simulations. The electron gun and beam focusing solenoids, which are the fundamental basis for a high average power klystron, are brought out by establishing a good beam transportation rate. The interaction region is designed with the new bunching method using three-dimensional PIC simulation software. The resulted efficiency from the PIC simulation is higher than 80%, considering realistic RF coupling structures in the input and output cavities. The collector is capable of the full beam power dissipation. In the meanwhile, efforts have been taken to avoid reflecting electrons from the collector due to the space-charge depression. The scheme and considerations of the output windows are also introduced.