Sheet Beam Research Articles

Research on a 220-GHz modified staggered double corrugated waveguide backward wave oscillator

The output power of a backward wave oscillator (BWO) is limited by the low interaction impedance of a slow wave structure (SWS) when the frequency is in the THz band. A BWO based on modified staggered double corrugated waveguide (MSDCW) SWS is researched as a high-power source operating at THz band to break through the coupling impedance limitation in this paper. The MSDCW-SWS designed for BWO with wide electronic tunable band is optimized by numerical Eigenmode simulation to obtain higher coupling impedance. The simulation results show that the coupling impedance of MSDCW-SWS at 211.5 GHz is 42% higher than that of the original staggered double corrugated waveguide (SDCW) SWS when the dispersion conditions are the same for both SWSs. The sheet beam MSDCW-BWO with a broadband output structure is designed to operate under voltage between 23 and 49 kV and an electron beam current of 160 mA. The 3-D interaction simulation results reveal that more than 80.6 W of average output power can be produced from 210 to 238 GHz and the highest electron efficiency of 2.5% can be obtained at 211.5 GHz. The MSDCW-BWO can be considered a promising high-power THz source.

High-efficiency terahertz-wave generation based on extended interaction oscillator with strongly-coupled 2π mode operation

A slow-wave structure improvement for enhancing the 2π-mode electronic efficiency is embodied in the validation of an extended interaction oscillator (EIO), which has an electronic efficiency of 6.52% at 0.22 THz from particle-in-cell (PIC) calculations. A 2π-mode bi-periodic slow-wave structure (BPSWS) with staggered long and short slots is utilized for optimizing the circuit performance. The proposed BPSWS has the capability of combining the respective advantages for both π and 2π-mode in terms of coupling performance and output performance, thus supporting a strongly-coupled 2π-mode with higher coupling capability. Compared with the typical mono-periodic SWS (MPSWS), the adopted strongly-coupled 2π-mode effectively improves the characteristic impedance M2R/Q by 103% to 66.79 Ω, the coupling coefficient by 66% to 0.497, and the normalized wave-amplitude by 22%. Accordingly, 503 W of average output power can be derived for the BPSWS-EIO with a 25.7 kV and 0.3 A sheet beam injected. Cold-test experiments were conducted, confirming that the 0.22 THz structure exhibits commendable fabrication precision and consistency and thus demonstrates the expected frequency response.

An E-plane loaded sine waveguide for G-band traveling wave tube

A new slow wave structure (SWS), named E-plane loaded sine waveguide (EPLSW) SWS is proposed for the development of G-band traveling wave tubes (TWTs). The structure, derived from the sine waveguide (SW) SWS, features staggered rectangular gratings on the two E-planes, intersecting with the sine-shaped gratings. The natural electron beam tunnel lies between these gratings. The EPLSW-SWS offers improved characteristics compared to the conventional SW-SWS. High-frequency simulation studies reveal that the passband of the EPLSW-SWS extends by more than 33 GHz over the SW-SWS under identical dispersion conditions. Additionally, the interaction impedance of EPLSW-SWS surpasses that of the SW-SWS in the frequency range of 195–255 GHz. Further analysis through particle-in-cell (PIC) results presents that when operating at 19.5 kV and 145 mA for the sheet beam current, it attains a maximum saturated power of 132.7W and a 3 dB bandwidth exceeding 50 GHz.

Two-dimensional space charge limited current in regime between accelerating diode and drift space for sheet and circular beam

We develop a general electrostatic steady-state model to calculate the two-dimensional (2D) space-charge-limited current (SCLC) with an initial velocity v0 (i.e., kinetic energy eKV=mv02/2) injected from a cathode into a vacuum diode with a spacing D and a potential difference of Vg. We consider two types of beam cross section: (a) a sheet beam with a width W and (b) a circular beam with a radius R, where both W and R are larger than D in the calculation. By introducing a parameter ε≡KV/Vg≥0, we can tune the operating regime to be in between the accelerating diode (ε≪1) and drift space (ε≫1). Hence, we verify the 2D SCLC model by converging to some prior analytical findings such as the 2D Child–Langmuir law at ε = 0, 2D SCLC at drift space at ε≫1, and their 1D limits at W/D≫1 or R/D≫1. Our 2D SCLC model indicates that there is a maximal value at ε∼1/60, which is independent of W/D or R/D. Furthermore, the hysteresis behavior is also studied when the injected current density is beyond the SCLC threshold and its dynamics are mainly influenced by ε instead of geometrical effect. This model is useful in high current electron beam emission with a finite emission area and with a non-negligible arbitrary injection kinetic energy.

Terahertz fan-beam computed tomography.

A terahertz (THz) fan-beam computed tomography (CT) system using a 0.3 THz continuous-wave sheet beam is proposed. The diffraction-free sheet beam expands in a fan shape in only one direction and provides propagation-invariant focal lines and extended the depth-of-field. The fan-beam CT based on this beam is the second-generation THz CT. It breaks the conventional 4-f symmetric structure of THz CT using the parallel beam. The fan-beam THz CT allows for use with a linear array detector, which reduces the time required to collect data. To demonstrate its feasibility for three-dimensional (3D) imaging, the 3D structure of a metal rod packed in a carton is reconstructed with the support of the system. The results show that the object's internal structure can be obtained by this new THz CT system while retaining the geometrically magnified features of the cross-sectional structure. The results of our research provide a template for the second-generation THz CT system, which provides an additional method for nondestructive testing.

Demonstration of an Even Symmetric Overmoded Coupler for Power Measurement in Sheet Beam Traveling-Wave Tubes

Demonstration of an Even Symmetric Overmoded Coupler for Power Measurement in Sheet Beam Traveling-Wave Tubes

Design and Experiment of a Sheet Beam Gun for Extended Interaction Oscillator

Design and Experiment of a Sheet Beam Gun for Extended Interaction Oscillator

Accurate Space Charge Modeling for 1-D Large Signal Simulation of Sheet Beam TWTs

Accurate Space Charge Modeling for 1-D Large Signal Simulation of Sheet Beam TWTs

Gas sheet beam induced fluorescence based beam profile measurement for high intensity proton accelerators

The paper describes the design simulations, developmental details and characterization results of a dual gas sheet generator system for non-invasive beam profile measurement of high-intensity proton beams. The system is being developed for the Low Energy High Intensity Proton Accelerator facility at Bhabha Atomic Research Centre, India. To our knowledge, this is the first time an intense low energy proton beam was used to directly view the beam induced fluorescence of a gas sheet. The system works by generating two thin nitrogen gas sheets which produces fluorescence when a charged particle beam passes through them, thereby providing a visual representation of the beam's intensity profile. The gas sheets are generated at 45° to the beam axis, which allows the fluorescence created by the interaction to be viewed directly through a viewport or camera capture. The beam profile measurements were carried out with a typically 20 keV, 10 mA pulsed proton beam from an ECR ion source under different operating parameters like gas sheet pressure, beam current, beam energy and focusing magnetic field. The Geant4 simulation has been performed to study the scintillation process and optical transport. It may become useful to study methods of reconstructing the beam profile from image obtained.

Design of an ultrashort H‐plane coupler without transition structure for sheet beam traveling‐wave tube amplifier at W‐band

AbstractAn ultrashort H‐plane sheet beam traveling‐wave tube (SB‐TWT) coupler without transition structure is proposed. The coupler adopts a new idea of directly coupling the microwave from H‐plane of the slow‐wave circuit (SWC) and optimizes the matching by introducing an asymmetric circular ridge on the E‐plane of the coupled waveguide. As a result, the length of the SB‐TWT coupler is reduced from more than 3λ to 1λ. At the same time, the beam tunnel area is enlarged to twice that of the SWC beam tunnel. In the working band (90–100 GHz) of the SWC, the simulation results show that the reflection is less than −25 dB, the isolation is greater than −22 dB. And the influence of the dimensions and locations of ridge on the performance of the coupler is studied. Particle‐in‐cell simulation results also show that the new coupler can not only complete the microwave coupling well but also reduce the number of SWC periods.

Investigation on Out-of-Tube Reflection Oscillations and Suppression Technology for Wideband Sheet Beam TWTs

Investigation on Out-of-Tube Reflection Oscillations and Suppression Technology for Wideband Sheet Beam TWTs

Theoretical Study of a Novel G-Band Sheet Beam Slots-Loaded Coupled-Cavity Traveling Wave Tube

In this article, the design of a novel symmetrical double slots coupled-cavity (SDSCC) traveling wave tube (TWT) operating in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{G}$</tex-math> </inline-formula> -band is presented. The novel slow wave structure (SWS) suits sheet electron beam and has a large sheet beam channel and high coupling impedance. The novel structure shows great potential in high-power terahertz amplifier field. The dispersion characteristics are studied by both theory and simulation. An equivalent circuit of SDSCC SWS is built. The coupling impedance and electric field of operating mode are simulated. A suitable input–output structure is designed for this novel SWS. The transition characteristics are exhibited. The beam-wave interaction process is also simulated. The peak output power arrives at 384 W at 218 GHz by the usage of a sheet beam with 26.2 kV and 300 mA. The average dc current density in the sheet beam tunnel is around 160 A/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{2}}$</tex-math> </inline-formula> . The spectrum of the output signal is pure, and the risk of mode competition is very low.

Demonstration of a PCM-Focused Sheet Beam TWT Amplifier at G-Band

A sheet beam traveling-wave tube (TWT) amplifier focused by periodically cusped-magnetic (PCM) fields has been successfully developed. The amplifier produces <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 115 W of peak power at 212.4 GHz with a voltage of 25.8 kV. The output power is more than 100 W from 210.2 to 213.6 GHz. The measured transport current is about 106 mA accounting for 81.5% of the total current. The high-efficiency circuit design, along with the novel coupling structure, makes it possible to greatly reduce the required current, enabling the use of the PCM focusing. As a result, large power and a wide band have been achieved in a compact volume with an overall weight of less than 1.8 kg. The results demonstrate a significant breakthrough in developing the sheet-beam amplifier at terahertz frequencies. Although a single-section circuit without the attenuator is used at this stage, we have verified all the key technologies. The measured results are in good agreement with the design.

Compact E-Band Sheet Beam Folded Waveguide Traveling Wave Tube for High Data Rate Communication

The design of a compact <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{E}$</tex-math> </inline-formula> -band (71–76 GHz) sheet beam (SB) folded waveguide (FWG) traveling wave tube (TWT) for high data rate communications is presented in this article. The round beam tunnel for a pencil FWG slow wave structure (SWS) has been redesigned with a rectangular beam tunnel to be suitable for the SB with increased beam current. A novel periodic permanent magnet (PPM)–periodic quadrupolar magnet (PQM) focusing system has been developed to reduce the beam ripple and improve the transmission ratio. A multistep side-out coupler is designed to import and extract the RF power from the SWSs and be compatible with the planar periodic magnetic focusing system. Particle-in-cell (PIC) simulations are employed to evaluate the performance of the high-frequency circuit. Driven by a 1 W signal, the 30-period circuit is estimated to produce more than 100 W power and achieve a 21 dB gain with a less than 0.4 dB variation over 71–76 GHz. Moreover, phase and linearity analysis show a less than 6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> in-band phase distortion and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 30 dB third-order intermodulation distortion (C/3IM) at 30 W of output power, meeting the requirements of a communications-focused TWT. Over 5% circuit efficiency and 40% overall efficiency when equipped with a single-stage collector can be achieved by the compact TWT over the target band.

Investigation on Self-Excited Oscillation for Nonuniform Periodic Sheet Beam Traveling-Wave Tubes

The utilization of phase velocity tapering or jumping techniques in the design of traveling wave tubes (TWTs) has become a widely accepted practice for enhancing the efficiency of beam–wave interaction. However, these approaches are not without their challenges, as they can lead to a series of self-excited oscillation (SEO) issues that restrict further improvement in power. In this article, a nonlinear numerical method is explored for fast prediction of SEO in nonuniform periodic sheet beam TWTs (NP-SBTWTs). The oscillation thresholds at the upper and lower band edges are analyzed and discussed under different voltage and current conditions to determine the stable operating conditions for NP-SBTWT. In addition, the concept of “sideband absolute self-excited state” is proposed to be avoided in dispersion design, and the impact of actual reflection characteristics on stability is quantified by defining the eigenvalues of the feedback gain of the interaction structure based on the real reflection parameter. The results demonstrate that this approach distinguishes itself from conventional time-domain particle-in-cell (PIC) simulation techniques by its capacity to integrate authentic reflection parameters into the computation process to achieve a fast prediction of SEO in NP-SBTWT.

Study on predictions of spray target position of gasoline direct injection injectors with multi-hole using physical model and machine learning

A lot of effort is being invested in improving the performance of injectors, the core components of gasoline direct injection (GDI) engines, such as injection stability and accuracy. The purpose of this study is to establish models that can predict spray targeting according to the design parameters of GDI injector, to improve the injection accuracy and enhance the engine performance. First, this study used laser sheet beam visualization technology to measure the spray targeting images of injectors with different design parameters in different cross-sections and obtained the spray targeting coordinates through image post-processing. Then, using the experimental data, two different approaches (empirical formula and machine learning), were used to create models for predicting spray targeting, and their applicability was compared. The research results showed that both the physical model and the machine learning model had a prediction accuracy of >0.98 in terms of R2, but the physical model had a lower prediction error in terms of the root mean square error (RMSE). Further, the tendency of the target coordinate to change is proportional to 0.2 power of the injection pressure (Pinj0.2), −0.1 power of ratio of hole length to hole diameter ((L/D)−0.1), and − 1.5 power of the angle between axes of two holes (θij−1.5).

Design and testing of a high perveance sheet beam electron gun

This paper presents the design, prototype engineering, and initial testing of a relatively low-voltage, high aspect ratio electron gun for versatile pulsed power applications. The sheet beam, diode electron gun is designed for (23–25) kV voltage and 6 µA/V1.5, as is required for x-band klystron, as a part of a compact Klylac delivering electron beam energy in the MeV range. The gun prototype was engineered, built, and tested using a Scandinova M1-0.5 modulator. Beam loss on the anode aperture is evaluated from transient waveforms and equivalent circuit modeling.

Novel 0.22-THz Extended Interaction Oscillator Based on the Four-Sheet-Beam Orthogonal Interconnection Structure

Numerous terahertz vacuum electronic devices (TVEDs) have been proposed. Among these devices, extended interaction oscillators (EIOs) exhibit small-volume, lightweight, low-working-voltage, stable-working frequency, and high-power characteristics. The ladder-line slow wave structure exhibits strong coupling and can interact with the sheet beam to considerably improve the efficiency of beam–wave interaction. In this study, a novel high-frequency structure was proposed. Four conventional ladder-line slow wave structures were placed vertically to form a four-sheet-beam orthogonal interconnection structure (OIS) that considerably increased the coupling efficiency between the cavities, improved output power and efficiency, and interacted perfectly with the TM81 mode. In this study, the dispersion characteristics and field distribution were studied through numerical and simulation calculations, and the optimal working parameters and output structure were analyzed using particle-in-cell (PIC) software. Next, a 0.22-THz extended interaction oscillator based on four-sheet-beam OSI was designed. Simulation results have shown the achievement of the output power of 0.22-THz wave over 500 W with four sheet beams at 16.6 kV and each current of 0.8 A. Finally, preliminary processing and cold tests of the structure were performed. This novel structure provided an alternative for the development of terahertz EIOs.

An Angular Radial Extended Interaction Amplifier at the W Band

In this paper, an angular radial extended interaction amplifier (AREIA) that consists of a pair of angular extended interaction cavities is proposed. Both the convergence angle cavity and the divergence angle cavity, which are designed for the converging beam and diverging beam, respectively, are investigated to present the potential of the proposed AREIA. They are proposed and explored to improve the beam-wave interaction capability of W-band extended interaction klystrons (EIKs). Compared to conventional radial cavities, the angular cavities have greatly decreased the ohmic loss area and increased the characteristic impedance. Compared to the sheet beam (0°) cavity, it has been found that the convergence angle cavity has a higher effective impedance and the diverging beam has a weaker space-charge effect under the same ideal electron beam area; the advantages become more obvious as the propagation distance increases. Particle-in-cell (PIC) results have shown that the diverging beam (8°) EIA performs better at an output power of 94 GHz under the condition of lossless, while the converging beam (-2°) EIA has a higher output power of 6.24 kW under the conditions of ohmic loss, an input power of 0.5 W, and an ideal electron beam of 20.5 kV and 1.5 A. When the loss increases and the beam current decreases, the output power of the -2° EIA can be improved by nearly 30% compared to the 0° EIA, and the -2° EIA has a greatly improved beam-wave interaction capacity than conventional EIAs under those conditions. In addition, an angular radial electron gun is designed.

Metamaterial-Inspired 0.22 THz Traveling-Wave Tubes With Double Sheet Beams

To develop high power and compact terahertz (THz) sheet beam traveling-wave tube (TWT), an all-metal metamaterial (MTM)-inspired slow wave structure (SWS) is proposed. This MTM-inspired SWS exhibits obvious advantages such as high interaction impedance, double beam tunnels, and compactness. The MTM-inspired SWS is devised and then simulated. The calculated average interaction impedance of the SWS for the first spatial harmonic of the fundamental mode is over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5 \Omega $ </tex-math></inline-formula> in the frequency range of 0.21–0.226 THz, which is bigger than that of the counterparts. Further, the beam-wave interaction of the MTM-inspired 0.22 THz TWT with double sheet beams is simulated. The maximum output power is predicted to be 400 W with 3-dB bandwidth of 5.4 GHz when the input power is 100 mW. Most importantly, the total length of the interaction structure of the 0.22 THz TWT is only 29.2 mm. The cross section of the proposed SWS and beam tunnels are <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.5\times0.42$ </tex-math></inline-formula> mm and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.5\times0.37$ </tex-math></inline-formula> mm, respectively. The simulation results show that the compact THz radiation source presented here is promising for future applications such as in imaging and material analysis.