Output Cavity Research Articles

Controllable double electromagnetically induced transparency in a closed four-level-loop cavity–atom system**Project supported by the National Natural Science Foundation of China (Grant No. 11174109).

We present a theoretical study of an optical cavity coupled with single four-level atoms in closed loop formed via applied control lasers. The transmitted probe field from the cavity is analyzed. We show that the electromagnetically induced transparency (EIT) in the cavity and the normal mode splitting will be very different with changing the closed interaction phase and the intensity of the free-space control laser. This coupled cavity–atom system presents a variational double-EIT that comes from modulating the splitting of the dark state, which means that we could realize the gradual transfer between one EIT peak and two EIT peaks by adjusting the applied control lasers, and the normal mode splitting sidebands will shift slightly by changing the free-space control laser. This means that we could control the output cavity probe field more freely and it is easer to realize optical switch controlled by more parameters. We also depict the angular dispersion of the intracavity probe field in different free-space control laser. The large phase shift () of the reflected intracavity probe field will be very useful for optical temporal differentiation and quantum phase gate.

Numerical Study of a Multibeam Klystron on the Milky Way High-Performance Computing Platform

Large-scale particle-in-cell (PIC) simulations of C-band multibeam klystron (MBK) on the Milky Way high-performance computing platform are conducted in this paper. First, the parameters of the electron beams, as well as the magnetic field distribution of the focusing system, resonant cavity, input cavity, output cavity, and collector are analyzed. Based on these structure parameters, large-scale parallel PIC simulations on C-band MBK are performed on the Milky Way high-performance computing platform. The simulation results show that the maximum speedup reaches 28 with an input of 50 W/5.6 GHz, a peak output power of 760 kW, a gain of 40 dB, and a bandwidth of 300 MHz.

Dynamically Manipulating Topological Physics and Edge Modes in a Single Degenerate Optical Cavity.

We propose a scheme to simulate topological physics within a single degenerate cavity, whose modes are mapped to lattice sites. A crucial ingredient of the scheme is to construct a sharp boundary so that the open boundary condition can be implemented for this effective lattice system. In doing so, the topological properties of the system can manifest themselves on the edge states, which can be probed from the spectrum of an output cavity field. We demonstrate this with two examples: a static Su-Schrieffer-Heeger chain and a periodically driven Floquet topological insulator. Our work opens up new avenues to explore exotic photonic topological phases inside a single optical cavity.

An Improved X-Band Triaxial Klystron Amplifier for Gigawatt Long-Pulse High-Power Microwave Generation

Triaxial klystron amplifier (TKA) is prom- ising for coherent power combining of high-power microwaves (HPMs) in high-frequency bands. In this letter, we present an improved X-band TKA to generate gigawatt long-pulse HPM radiations. By increasing the intrinsic quality factor of the input cavity, the seed microwave loss is reduced significantly. The beam loaded quality factor of the input cavity is increased to enhance the gap voltage and, moreover, to strengthen the electron beam modulation. In addition, a double-gap output cavity is designed on the one hand to increase the radio-frequency power capacity and on the other hand to improve the beam-wave power conversion efficiency. The improved TKA is demonstrated to be capable of producing the HPM X-band radiation typically with power of 1.1 GW and pulse duration of 105 ns in experiments. Furthermore, the phase of the generated microwave is locked by the seed signal over more than 90 ns.

Stokes-to-Stokes and anti-Stokes-to-Stokes energy transfer in a Raman fibre laser under different cavity configurations

We report an experimental study of the signals delivered by a Raman fibre laser under different output coupling conditions. Our main results allow us to conclude that feedback reflectivity variations influence Stokes energy-transfer mechanisms. We also confirmed that in low-feedback systems high conversion efficiencies imply high thresholds for Stokes generation, whereas the contrary occurs for high-reflective output cavities.

Stability analysis of a C-band 500-kW klystron with a multi-cell output cavity

A progogype 5-GHz 500-kW CW klystron (model E3762 provided by Toshiba Electron Tubes & Devices Co. Ltd.) has been operating as the RF source for the lower hybrid current drive (LHCD) system in the KSTAR tokamak. In order to investigate how the efficiency of the 5-GHz 500-kW CW klystron prototype could be enhanced, the cavity design study is being carried out with simulation code based on the main klystron operation parameters. This is being done by simulating klystron performances for various cavity parameters including the number of cavities, inter-cavity distance, and cavity tuning frequencies. The simulation has been done with the FCI (field charge interaction) code aided by a matlab script for scanning input parameters. Initial set of scan parameters was obtained by benchmarking the E3762 klystron. It was possible to obtain optimized design parameters with better efficiency for a cavity system adopting a multi-cell output cavity. However, the multi-cell output cavity is prone to produce a self-oscillation due to the prolonged (several half RF periods) beam-field interaction along its multiple gaps. We have checked the feasibility of the optimization by evaluating the stability of output cavity system. The stability is given by the ratio of a beam-loading conductance to the circuit conductance.

Limiting amplitude–frequency characteristics of the output cavities of klystrons in the presence of parasitic oscillations

The interaction impedance of the of the output cavity of the klystron generally connected to N passive cavities forming a filter system is analyzed in the presence of parasitic oscillations.

Limiting amplitude–frequency characteristics of the output cavities of klystrons

An analytical ratio determining the maximum possible amplitude of the interaction impedance of the output cavity of the klystron generally connected to N passive cavities forming a filter system is obtained.

A high efficiency Ku-band radial line relativistic klystron amplifier

To achieve the gigawatt-level microwave amplification output at Ku-band, a radial-line relativistic klystron amplifier is proposed and investigated in this paper. Different from the annular electron beam in conventional axial relativistic klystron amplifiers, a radial-radiated electron beam is employed in this proposed klystron. Owing to its radially spreading speciality, the electron density and space charge effect are markedly weakened during the propagation in the radial line drift tube. Additionally, the power capacity, especially in the output cavity, is enhanced significantly because of its large volume, which is profitable for the long pulse operation. Particle-in-cell simulation results demonstrate that a high power microwave with the power of 3 GW and the frequency of 14.25 GHz is generated with a 500 kV, 12 kA electron beam excitation and the 30 kW radio-frequency signal injection. The power conversion efficiency is 50%, and the gain is about 50 dB. Meanwhile, there is insignificant electron beam self-excitation in the proposed structure by the adoption of two transverse electromagnetic reflectors. The relative phase difference between the injected signals and output microwaves keeps stable after the amplifier saturates.

RF characteristics of multiple-gap resonant cavity for sheet beam extended interaction klystron

The sheet beam extended interaction klystron( SBEIK) has the typical and attractive characteristics with the plan structural multiple-gap cavity and extended beam-wave interaction system. A type of novel miniaturization strong-coupling five-gap extended resonant cavity was investigated combined with the design of W-band SBEIK,which was compared with the traditional weak-coupling five-gap resonant and output cavity. The obtained characteristics shows that the strong-coupling five-gap cavity may have many advantages of resonant mode separation,radio frequency( RF) fields coupling in the gap and RF energy exporting,etc.Then,the degeneracy mode competition for the strong-coupling five-gap cavity was studied due to its axial period structures. The result shows that the frequency interval between the work mode and vicinal non-working mode is above 600 MHz,which is higher enough than the required 100 MHz bandwidth for the SBEIK. Finally,the 3D particle in cell( PIC) simulation software was used to calculate the beam-wave interaction for the strong-coupling five-gap extended output cavity,the expected high output power was observed with the corresponding performance and favorable stability. Thus,the research is very important for the physical design and engineering development of the SBEIK in the millimeter-wave and terahertz band with high power output radiation later.

Design of a C-band relativistic extended interaction klystron with coaxial output cavity

In order to overcome the disadvantages of conventional high frequency relativistic klystron amplifiers in power capability and RF conversion efficiency, a C-band relativistic extended interaction klystron amplifier with coaxial output cavity is designed with the aid of PIC code MAGIC. In the device, disk-loaded cavities are introduced in the input and intermediate cavity to increase the beam modulation depth, and a coaxial disk-loaded cavity is employed in the output cavity to enhance the RF conversion efficiency. In PIC simulation, when the beam voltage is 680 kV and current is 4 kA, the device can generate 1.11 GW output power at 5.64 GHz with an efficiency of 40.8%.

Quantum cooling and squeezing of a levitating nanosphere via time-continuous measurements

With the purpose of controlling the steady state of a dielectric nanosphere levitated within an optical cavity, we study its conditional dynamics under simultaneous sideband cooling and additional time-continuous measurement of either the output cavity mode or the nanosphere’s position. We find that the average phonon number, purity and quantum squeezing of the steady-states can all be made more non-classical through the addition of time-continuous measurement. We predict that the continuous monitoring of the system, together with Markovian feedback, allows one to stabilize the dynamics for any value of the laser frequency driving the cavity. By considering state of the art values of the experimental parameters, we prove that one can in principle obtain a non-classical (squeezed) steady-state with an average phonon number .

Analysis and Simulation of a Multigap Sheet Beam Extended Interaction Relativistic Klystron Amplifier

To improve the output power and electron efficiency and broaden the operating bandwidth, the three-gap extended output cavity was used in the relativistic klystron to replace the single-gap output cavity. The high-frequency characteristics of the three-gap extended cavity were studied. The electron load conductance was derived and corrected based on the theory of relativity, by which a more accurate relation of electron load conductance versus transmit angle can be obtained. The characteristic impedance is three times bigger than that of the single-gap cavity under the case of the 2π mode, which can improve the M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (R/Q) and the beam-wave interaction efficiency. The model of sheet beam klystron (SBK) is comprised of three-gap input cavity, two-gap first idler, three-gap second idler cavity, and three-gap output cavity. The 3-D particle-in-cell simulation results show that for a 500 kV, 1 kA, 40.134 GHz, and 20 W input power, the SBK is capable of generating an output power higher than 142 MW with the gain of 68.5 dB, maximum interaction efficiency of 28.4%, and 3-dB bandwidth of 240 MHz.

Influence of High-Frequency Characteristic of a Cavity on the Phase Shift in the Relativistic Klystron Amplifier

The influences of the eigenfrequency and the intrinsic quality factor of a cavity on the phase shift in the relativistic klystron amplifier (RKA) are investigated. An analytical expression is derived to describe the phase of the gap voltage in a cavity. The theoretical and simulation results indicate that the eigenfrequency of the buncher cavity can significantly affect the phase shift of the output microwave in the RKA. In contrast, the influences of the eigenfrequency and the intrinsic quality factor of the output cavity on the phase shift of the RKA are small. The analytical expression can effectively estimate the phase difference among different RKAs resulting from the machining and assemblage inconformity in the experiment. It is suggested that the differences among the eigenfrequencies of the buncher cavities should be managed in 0.1% of the operation frequency to achieve phase locking among different RKAs.

Analysis and design of a Ka-band sheet beam relativistic extended interaction klystron amplifier

The sheet beam klystron is a kind of novel powerful microwave and millimeter-wave vacuum electron device, in which used is a thin rectangular sheet beam with high aspect ratio in order to improve beam-wave interaction efficiency by improving space-charge-limiting current of electron beam and obtaining big electric current, and it has many actual and potential applications. Based on the motion of the single electron under the small signal condition, the transit-time effect of electron beam in 2π-mode standing wave electric field in a multiple-cavity resonator is investigated, the expression of electron load conductance in a multiple-cavity resonator is presented, and the influence of the cavity number N on transit-time effect in a multiple-cavity resonator is discussed. The high frequency characteristics of the three-gap extended cavity are studied. The abilities for the single-gap cavity and three-gap cavity to modulate the sheet beam are compared by 3D PIC simulation. The simulation result shows that the three-gap extended interaction cavity operating at 2πmode is better than the single-gap cavity. The electron load conductance is derived and corrected based on the theory of relativity, by which a more accurate relation of electron load conductance to transmit angle can be obtained. In order to improve the output power and electron efficiency, the three-gap extended output cavity is used in the relativistic klystron to replace the single gap output cavity. By using the electromagnetic simulation software and 3D PIC code, a Ka-band sheet beam relativistic extended interaction klystron amplifie is designed. A sheet electron beam with a width-to-height ratio of 30 is adopted to reduce the space charge effect. In the PIC simulation, when the beam voltage is 500 kV and current is 1 kA, the device can generate a 190 MW output power at 40 GHz with an efficiency of 38% and a gain of 69 dB. The 3 dB bandwidth of the EISBK is about 150 MHz. Meanwhile, the output microwave is without the clutter jamming, which makes the contribution avoid the shrinkage of output microwave impulse. This study is of great importance for the physical design and process in engineering of the Ka-band sheet beam extended interaction relativistic klystron amplifier.

Novel Slow-light Reflector Composed of a Fiber Ring Resonator and Low-reflectivity Fiber Bragg Grating

Slow-light components are attractive for many practical applications, such as narrow linewidth lasers and optical delay line. Characteristics of a novel slow-light reflector, which is composed of a fiber resonator and an extremely low reflectivity fiber Bragg grating (LRFBG), are proposed and analyzed theoretically and experimentally. Due to the light recirculating propagation in the fiber resonator by using a low split ratio coupler, the LRFBG inserted ring provides not only high reflectivity, but also an enlarged group delay, which shows a good slow-light property. In our research, the effective reflectance up to unity can be obtained when the FBG reflectivity and coupler split ratio reach to the critical values for lossless cases. Double peak resonant effects are also observed when the reflectivity of FBG is larger than the critical value. A supernarrow-linewidth fiber laser using the proposed slow-light reflector as its output cavity mirror is built experimentally to verify the analyzed results. It is believed that the proposed slow-light component will have some good applications due to the merits of compactness and low cost.

Different Solutions for the Generator-accelerator Module

The most important part of the particle accelerators [1] – is the power generator together with the whole feeding system [2]. All types of generators, such as klystrons, magnetrons, solid state generators cover their own field of power and pulse length values. For the last couple of year the Inductive Output Tubes (IOT) becomes very popular because of their comparative construction simplicity: it represents the klystron output cavity with the grid modulated electron beam injected in it. Now such IOTs are used with the superconductive particle accelerators at 700MHz operating frequency with around 1MW output power. Higher frequencies problem – is the inability to apply high frequency modulated voltage to the grid. Thus we need to figure out some kind of RF gun. But this article is about the first steps of the geometry and beam dynamics simulation in the six beam S-band IOT, which will be used with the compact biperiodic accelerating structure.

Cavity-output-field control via interference effects

We show how interference effects are responsible for manipulating the output electromagnetic field of an optical micro-resonator in the good-cavity limit. The system of interest consists in a moderately strongly pumped two-level emitter embedded in the optical cavity. When an additional weaker laser of the same frequency is pumping the combined system through one of the resonator's mirrors then the output cavity electromagnetic field can be almost completely suppressed or enhanced. This is due to the interference among the scattered light by the strongly pumped atom into the cavity mode and the incident weaker laser field. The result applies to photonic crystal environments as well.

An improved suppression method of the transverse-electromagnetic mode leakage with two reflectors in the triaxial klystron amplifier

Suppression of the transverse-electromagnetic (TEM) mode leakage is crucial in the design of a triaxial klystron amplifier with high gain, because a small microwave leakage from the buncher or the output cavity could overwhelm the input signal with low power. In this paper, a specially designed reflector is proposed to suppress the TEM mode leakage, whose axial electric field is approximately zero at the beam radial position. Theoretical analysis indicates that the reflector introduces little influence on the normal modulation of the beam while keeping a high reflection coefficient. By using two such reflectors with different eigen frequencies located in front of the buncher cavity and the output cavity, respectively, an improved triaxial klystron amplifier is presented. The simulation results show that the reflectors substantially decrease the TEM mode leakage power and achieve very good isolation among the cavities. The improved triaxial klystron amplifier can operate normally with 10's kW microwave injection without self-oscillations.

Monotron Oscillation in Double-Gap Coupling Output Cavities of Multiple-Beam Klystrons

This paper presents a method to calculate the admittance of electron beam and of circuits in a multiple-beam klystron output circuit of double-gap coupling cavity. With the conditions of monotron oscillation, the possibility of forming monotron oscillations in a double-gap coupling output cavity is studied based on analytical calculations, and a method for calculating the initial oscillation current of monotron oscillation is derived. To further verify whether monotron oscillations form in the coupling double-gap output cavity and to analyze the characteristics of monotron oscillation, a particle simulation is performed to a particular coupling double-gap output cavity loaded by a cutoff waveguide filter, and the results show that monotron oscillations form in the particular coupling double-gap output cavity with an initial current of about 22 A under the condition of 18 kV accelerating voltage of the beam.