Beam-wave Interaction Research Articles

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.

Preliminary Study of a G-Band Extended Interaction Oscillator Operating in the TM31-3π Mode Driven by Pseudospark-Sourced Multiple Electron Beams

This paper presents the first design that combines pseudospark-sourced (PS) electron beams with a multiple-beam extended interaction oscillator (EIO). The PS electron beam is an excellent choice for driving EIOs because it has high current density and does not require a focusing magnetic field. The EIO with coaxial structure adopts the method of multiple electron beams, which plays a crucial role in improving the average output power. At the same frequency, the EIO operating in the high-order TM31-3π mode has a larger cavity size than the EIO operating in the traditional TM01-2π mode. The high-order TM31-3π mode solves the problem of the EIO’s manufacture at high frequency. In order to verify the above points, a G-band PS multiple-beam EIO operating in TM31-3π mode has been designed. The beam–wave interaction particle-in-cell simulation results show that the EIO’s peak output power is 39.2 kW at 217 GHz, and that its efficiency is around 6.1%. The EIO with six pencil beams operates at a voltage of 43 kV. The total current of the six electron beams is 15 A (equally distributed among the six beams), and the corresponding current density is about 5000 A/cm2. Considering the ohmic loss and the effect of skin depth, the conductivity used in these simulations is 2 × 107 S/m. The design is an excellent way to improve the output power of EIO operating at high frequency.

Metamaterial-Inspired Interaction Structure for MW-Level Klystron at 714 MHz

A metamaterial (MTM)-inspired interaction structure for MW-level klystron at 714 MHz is proposed. The electromagnetic characteristics of MTM-inspired resonant cavities with single-bridge complementary electric split ring resonators (CeSRRs) are studied, and the cold-tested results confirm the feasibility of miniaturizing the resonant cavity. Furthermore, the beam–wave interaction simulation for the interaction structure constructed by MTM-inspired resonant cavities with single-bridge CeSRRs is carried out, demonstrating 2.28 MW of output power at 714 MHz for a 100 kV, 40 A electron beam. The radius and length of the MTM-inspired interaction structure are 1/2 and 2/3 of the conventional counterparts, respectively. The promising performance makes it attractive for future applications such as linacs.

Electromagnetic Particle Algorithm for Beam–Wave Interaction in Traveling Wave Tube of Symmetry

In many fields, such as space astrophysics, plasma and vacuum electronics, there are many nonlinear strong coupling physical problems. In order to solve the problem of electron beam–wave interaction in cylindrical Traveling wave tube (TWT) with symmetrical structure, a multi particle simulation algorithm for beam circuit is designed. The algorithm allows aperiodic time input, nonuniform linearity and large space diagnosis for different particles. In this algorithm, the field of coupled slow-wave transmission line is simulated by finite difference method. Assuming that there is strong coupling between the beam and the circuit, the space center equation of transmission along the line is obtained, and the space charge field is modeled considering the space charge effect, which can easily be ignored. The Particle In Cell (PIC) method of frog leaping step scheme is adopted to evaluate the electric field of each particle center, determine the circuit and space charge field, and design the termination part to compensate for the loss in order to avoid self-excited agitation. Finally, a simple numerical simulation is carried out for the electromagnetic problem and the accuracy of the code is checked against the electromagnetic simulator CHPIC. Therefore, the algorithm can be used to solve the problem of beam–wave interactions in a fixed structure (cylindrical) in multiple fields and can accurately record the data in the process.

Experimental Study on Positronium Detection under Millimeter Waves Generated from Plasma Wakefield Acceleration

Positronium (Ps) is an unstable system created by the temporary combination of electrons and negative electrons, and Ps generation technology under resonance conditions at millimeter waves is emerging as a new research topic. In general, Ps can be observed when an unstable separate state remains after electron and positron pair annihilation, as in positron emission tomography (PET). However, in this study, a plasma wakefield accelerator based on vacuum electronics devices (VEDs) was designed in the ponderomotive force generating electrons and positrons simultaneously using annular relativistic electron beams. It can induce Cherenkov radiation from beam–wave interaction by using dielectric materials. According to the size of dielectric materials, the frequency of oscillation is approximately 203 GHz at the range of millimeter waves. At this time, the output power is about 109 watts-levels. Meanwhile, modes of millimeter waves polarized by a three-stepped axicon lens are used to apply the photoconversion technology. Thus, it is possible to confirm light emission in the form of a light-converted Bessel beam.

An Efficient Vircator With High Output Power and Less Drifting Electron Loss by Forming Multivirtual Cathodes

Multi virtual cathodes (MVCs) were created in an axial vircator utilizing two dielectric reflectors (DRs). The wall charge of drifting electrons on the surface of DRs led to the creation of VC2 and VC3 in the downstream area of drift tube, according to phase space simulations. The microwave generated by VC1 interacted with the charge of VC2 and VC3 to raise the power amplitude, resulting in an increase in output power from 284 MW to 1028 MW. The use of two dielectric reflectors was proven to have a remarkable impact in reducing electron loss, increasing output power, and improving efficiency. The suggested vircator’s efficiency with MVCs improved from 7.9 % to 28.6 %. The findings indicate important signs of progress in vircator research to decrease electron losses and boost beam–wave interaction, which might lead to the advancement of high-efficiency vircators.

Effect of Electron Beam Properties on a Second Harmonic Gyrotron

In harmonic gyrotrons, the narrow operating region of the harmonic mode is significantly restricted by the severe mode competition from the fundamental parasitic modes, which makes it important to study the nonlinear effects of beam-wave interaction in the case of a nonideal electron beam. In this article, the effect of the electron velocity spread and the beam radius spread on the mode excitation, interaction efficiency as well as mode competition are analyzed for the desired harmonic mode and the parasitic fundamental mode operating far from the cut-off frequency. The effect of velocity spread is interpreted via the electron cyclotron resonance condition as well as the electron beam susceptibility with respect to the resonator field. The results show that the velocity spread can greatly broaden the effective operating region of the harmonic mode and the efficiency degradation can be contained at a reasonable level by proper design consideration of the interaction circuit. Whereas, the beam radius spread is the main factor that accounts for the efficiency degradation and can aggregate the mode competition. The results are referential to the design and analysis of harmonic gyrotrons as well as continuously frequency tunable gyrotrons.

Adjoint Equations for Beam-Wave Interaction and Optimization of TWT Design

By formulating and solving the adjoint equations governing the beam–wave interaction in a traveling-wave tube (TWT), we show the partial derivatives with respect to design parameters of various TWT figures of merit (FoMs) may be efficiently calculated. FoMs include average gain, gain flatness, and gain–bandwidth product, and design parameters include beam voltage and circuit geometry. We use these derivatives in an optimization algorithm that finds parameter values that minimize or maximize the desired FoM. We show that a 1-D large-signal simulation code, such as CHRISTINE, may be easily modified to compute the adjoint solutions. We further show that only three runs of the modified code suffice to compute the partial derivatives of the output power and phase at a specified frequency with respect to an arbitrary number of parameters, resulting in potentially large savings in computing time compared with direct, finite difference calculation of the partial derivatives. We illustrate the method by optimizing the beam voltage and gap spacing of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula> -band folded-waveguide (FWG) TWT. The formulation given here applies only to TWT slow wave structures, such as coupled-cavity and FWGs, and to klystrons, composed of discrete gaps followed by drift spaces; it does not apply to helix structures, which may be the subject of a future paper.

A Sub-THz High-Order Mode Backward Wave Oscillator Driven by Pseudospark Sourced Multiple Sheet Electron Beams

A novel concept of combining the advantages of pseudospark (PS)-sourced electron beam (high beam current density), multiple-sheet-electron-beams (large total beam cross-sectional area), and high-order mode (HOM) slow wave structure (SWS) (high power capacity) to produce high-power terahertz signals is presented in this article. As an example, a sub-terahertz backward wave oscillator (BEO) driven by PS-sourced dual-sheet-electron-beams is designed. Measured results of the interaction circuit, including transmission/reflection coefficients and the dispersion relation, were in good agreement with simulation predictions. Beam-wave interaction simulations having considered the plasma effect and conductor loss predicted stable output signals with radiation power of 167.4–255.4 W over a tunable bandwidth of 234.4–241.0 GHz.

Amplification of THz waves by beam-wave interaction in self-assembled helical slow-wave structures with single and double chirality

We investigate the interaction between an electron beam and a THz guided electromagnetic wave in a helical slow-wave structure formed by self-assembly of a conductive ribbon. We have previously shown the controlled fabrication of this slow-wave structure and its potential to form the basis for widely deployable millimeter-through-THz traveling-wave tube amplifiers. The process allows the fabrication of helical slow-wave structures with single and double chirality. Here, we use three-dimensional simulations to perform a comparative analysis of beam–wave interaction in self-assembled gold helices with single and double chirality. First, the structures are modeled without the electron beam (cold helices) to calculate the distribution of the electric field generated by the high-frequency wave. We perform simulations of cold helices by using Computer Simulation Technology Microwave Studio. Second, we evaluate the interaction between an electron beam and the THz travelingwave by using a particle in cell simulator in Computer Simulation Technology Particle Studio. Simulation studies show that a switch in chirality in the middle of self-assembled helices generates a reflected wave that boosts beam–wave interaction. We demonstrate that this efficient energy exchange will potentially provide high gain in THz traveling-wave tube amplifiers based on self-assembled helices.

Using Phase Jumping Method to Enhance the Beam–Wave Interaction Efficiency in Terahertz Folded-Waveguide Traveling-Wave Tube

The folded-waveguide (FWG) traveling-wave tube (TWT) is a promising high-power terahertz (THz) amplifier. However, it is challenging to develop THz FWG-TWT with high beam–wave interaction efficiency. In this work, a phase jumping technology, which originates from free-electron lasers, is proposed to boost the beam–wave interaction efficiency of THz FWG-TWT. A modified slow wave structure acts as the phase shifter to adjust the phase gap between electron beam and electromagnetic wave. By jumping the phase gap to an appropriate value, the bunching center of electron beam keeps for a long time in the deceleration region. Thus, more energy is transferred to electromagnetic wave from bunching electron beam. To verify such proposition, a THz FWG-TWT example utilizing the phase jumping is presented in this article. Compared with conventional one, its efficiency within the operation bandwidth from 212 to 224 GHz is improved by 60%–140%.

Experimental Investigation of a Shape-Optimized Staggered Double-Vane Slow-Wave Structure for Terahertz Traveling-Wave Tubes

Enormous concerns focus on the high-power and wide bandwidth traveling-wave tube (TWT) for its outstanding performance. In this article, a shape-optimized staggered double-vane slow-wave structure (SWS) for terahertz (THz) sheet beam TWT is proposed. The shape-optimized staggered double-vane SWS takes the advantages of higher interaction impedance, lower transmission loss, and lower phase velocity than conventional staggered double-vane SWSs. The shape-optimized staggered double-vane SWS with 85 periods is designed, fabricated, and cold tested. The measured transmission loss of the shape-optimized staggered double-vane SWS is less than 0.79 dB/cm in the frequency range of 0.211–0.26 THz, which is in good agreement with the simulation results. Furthermore, the beam–wave interaction analysis of a sheet beam TWT with this SWS is given. The output power is predicted to be >100 W with 3-dB bandwidth of >50 GHz. The results show that the shape-optimized staggered double-vane SWS is a very promising scheme to construct a high-power and wideband THz TWT for future applications such as communication and imaging.

Time-Domain Simulation of Helical Gyro-TWTs With Coupled Modes Method and 3-D Particle Beam

A new self-consistent time-domain model for the simulation of gyrotron traveling-wave tubes with a helically corrugated interaction space (helical gyro-TWTs) is presented. The new model links classical methods using the approach of slowly varying variables together with an expansion of the electromagnetic field in eigenmodes and advanced full-wave particle-in-cell (PIC) solvers. The aim is to significantly reduce the required calculation time compared to full-wave PIC solvers, while less strict assumptions are introduced as in the classical approaches of slowly varying variables. For the first time, the classical theory of coupled circular waveguide modes for the description of the operating electromagnetic eigenmode in the helical interaction space is combined with a 3-D PIC representation of the electron beam. This allows the simulation of the beam–wave interaction over a broad bandwidth and at arbitrary harmonics of the cyclotron frequency. In addition, arbitrary electron beams (with spreads, offsets of the guiding center from the symmetry axis, and so on) can be investigated. The new approach is compared with the full-wave 3-D PIC code CST Microwave Studio. A good agreement of the simulation results is achieved, while the computing time is significantly reduced.

Design and simulation investigations of a high gain millimeter wave gyro-twystron amplifier

In this paper, the design and multimode analysis of a single cavity Ka-band gyro-twystron have been presented. The present amplifier has been studied for its beam-wave interaction by using a nonlinear multimode analysis, which has also been verified with a 3D Particle-In-Cell (PIC) simulation. A single anode magnetron injection gun (MIG) has been designed with a ∼4% velocity spread using electron optics code. The PIC simulation of the current amplifier predicted an RF output power of ∼240 kW in TE01 mode at 35 GHz. The power gain, electronic efficiency, and 3-dB bandwidth were calculated as ∼53 dB, ∼37%, and ∼2 GHz, respectively, using the electron beam of 65 kV, 10 A with the velocity ratio of 1.4. Also, a collector is designed with a heat wall loading 0.116 kW/cm2 for the proposed design.

Double-mode and double-beam staggered double-vane traveling-wave tube with high-power and broadband at terahertz band

In this paper, a 220 GHz broadband and high-power staggered double-vane traveling-wave tube has been designed and verified. Firstly, a planar double-beam staggered double-vane slow-wave structure is proposed. By using the double-mode operation scheme, the transmission performance and bandwidth have been almost increased with twice as the single mode. Second, to satisfy the high output power requirement and improve the stability of the traveling-wave tube, a double pencil-beam electron optical system has been designed, the 20–21 kV driven voltage and the 2 × 80 mA current are set as the design target. By using the mask portion and the control electrode in the double beam gun, the two pencil beams can be focused with a compression ratio of 7 along their respective centers with narrow distances of about 0.18 mm with good stability. The uniform magnetic focusing system has also been optimized. The stable transmission distance of planar double electron beams could reach 45 mm with the focusing magnetic field of 0.6 T, which was long enough to cover the whole high-frequency system (HFS). Then, to verify the availability of the electron optical system and the performance of the slow-wave structure, particle-in-cell (PIC) simulation has also been carried out with the whole HFS. Results demonstrate that the beam-wave interaction system can get a nearly 310 W peak output power at 220 GHz with a 20.6 kV optimized beam voltage and beam current of 2 × 80 mA, the gain is of 38 dB with the 3-dB bandwidth over 35 dB about 70 GHz. Finally, the high precision microstructures fabrication has been carried out to verify the performance of the HFS, the results show that the bandwidth and the transmission properties are in good agreement with simulation result. Thus, the proposed scheme in this paper is expected to develop the high-power and ultra-wideband terahertz band radiation source with potential applications in the future.

Transverse magnetic electromagnetic mode analysis with electron beam effect in overmoded waveguide of coaxial magnetic wiggler

The coaxial wiggler with periodic permanent magnet is one of the ways to realize portable and practical high power microwave, which has a relatively small volume and low weight. However, the electromagnetic mode competition will significantly affect the output power at a high electron current. Based on the dispersion curve considering the effect of the electron beam, we propose a method of judging whether the mode can oscillate as the main mode in the coaxial magnetic wiggler. We analyze the wave–beam interaction by the first-order perturbation and predict some modes that seem impossible in traditional methods. It is verified by 3D particle-in-cell simulation that these electromagnetic modes will be the operating modes.

Analysis and calculation of beam-wave interaction in 220GHz gyrotron resonator

In this paper, the beam-wave interaction of the 220GHz gyrotron resonator is analyzed. The design scheme of the physical structure of the TE36,17 cylindrical open resonator is given. According to the beam-wave coupling coefficient, the center radius of the electron beam guidance of the resonator is obtained, and the working parameters of the resonator are determined by the start current. The single-mode self-consistent nonlinear theory in the frequency domain of the resonator is deduced, and the output characteristics of the resonator are studied by using MATLAB.

An oversized Ku-band Cerenkov oscillator with pure TM01 mode output

An oversized Ku-band Cerenkov oscillator with pure TM01 mode output is proposed. By utilizing a separated slow-wave structure (SSWS), the resonant characteristic of the operating mode is preserved, whereas the resonant characteristics of the high order electromagnetic modes are destroyed. As a result, only the expected mode can be stimulated, and undesired beam–wave interactions are suppressed effectively. In terms of an oversized Cerenkov oscillator, the usage of SSWS shows great potential for obtaining pure operating mode output. By utilizing particle-in-cell simulation, microwaves with an output power of 4.5 GW and frequency of 14.1 GHz are obtained, when the beam voltage is 0.9 MeV, and the beam current is 12.9 kA. The percentage of the operating mode is up to 99.5% and exceeds 99% in a wide range of the beam voltage.

Design and Cold Test of a G-Band 10-kW-Level Pulse TE01-Mode Gyrotron Traveling-Wave Tube

Overall design and cold test of a G-band gyrotron traveling-wave tube (gyro-TWT) amplifier are presented. This gyro-TWT aims at achieving a goal of 10-kW pulse output power in the range of 210–220 GHz. It is operated in a circular TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode at the fundamental cyclotron harmonics, which is driven by a 50-kV, 3-A gyrating electron beam. Low velocity spread diode-type magnetron injection gun (MIG), multichannel TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode input coupler, broadband metasurface output window, and lossy material loaded beam–wave interaction circuit are simulated and partially measured. Particle-in-cell (PIC) simulation shows that the designed gyro-TWT can achieve a saturated output power over 10 kW in the range of 210–228 GHz with a beam velocity spread of 2.29%.

Design and Experiment of 1 THz Slow Wave Structure Fabricated by Nano-CNC Technology

This article reports a slow-wave structure (SWS) working at 1 THz which can be fabricated through nano-computer numerical control (nano-CNC) technology for the first time. First, a 1 THz deformed quasi sine waveguide (D-QSWG) SWS has been designed and simulated in this article. To reduce the metal loss of input and output waveguide, a coupler which transit the waveguide to over mode rectangular waveguide has been designed. The simulation result shows that input and output waveguides with this coupler have lower metal loss than that of WR1 rectangular waveguide. To fabricate the designed D-QSWG SWS, tungsten steel endmills have been used. The fabricated SWS has the surface roughness around 58&#x2013;74 nm, the corresponding effective conductivity is around <inline-formula> <tex-math notation="LaTeX">$2\times 10^{{7}}$ </tex-math></inline-formula>&#x2013;1.85 <inline-formula> <tex-math notation="LaTeX">$\times 10^{{7}}$ </tex-math></inline-formula> S/m. And the cold test results show that the insertion loss of the fabricated model is less than 30 dB while the reflection loss is less than &#x2212;15 dB in the frequency range of 0.998&#x2013;1.016 THz, which has a good consistency with the simulated results. The beam-wave interaction simulation results based on the cold test results show that the designed D-QSWG traveling wave tube (TWT) has the output power of 300 mW, and the 3 dB bandwidth is around 3 GHz.