Sheet Beam Traveling Wave Tube Research Articles

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 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.

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.

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.

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.

Demonstration of an Ultracompact, High-Isolation Power Coupler for W-Band Sheet Beam Traveling Wave Tubes

The emerging sheet beam devices have become a research hotspot in the field of vacuum electronics due to their excellent performance in the millimeter-wave and even terahertz bands, but there are still many technical bottlenecks in the actual manufacture and the engineering realization. In this article, an ultracompact high-isolation power coupler prototype is demonstrated for high-efficiency injection and extraction of electromagnetic energy from the sheet beam traveling wave tubes (SB-TWTs). The introduction of polarized rotators avoids the coupling of electromagnetic waves into the electron channel, thereby improving isolation performance. The improved structure with multiparameter tunability ensures low reflection and broadband characteristics. The simulation shows that its extreme bandwidth (BW) is 40% and the in-band isolation is less than −50 dB. Moreover, the parasitic resonance mode is analyzed and related suppression technology is also proposed. Then, two prototypes 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 coupler have been processed and tested. The related machining errors have been analyzed and a good consistency is verified between the experiment and the simulation. Compared with the best level of the existing coupler, this ultracompact structure with less than one free-space wavelength (calculated from the central frequency point) relieves the pressure of the sheet beam transmission system, reduces the risk of electronic interception, and provides a feasible solution for engineering realization of sub-millimeter-wave or terahertz SB-TWTs.

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.

Design, Microfabrication, and Experiment of a Compact Mode Selection Filter for Sub-Terahertz Sheet Beam Traveling-Wave Tubes

In this article, a compact mode selection filter and a mode converter for the 0.22-THz sheet beam traveling-wave tube (SB-TWT) has been proposed, fabricated, and tested to realize selective absorption of high-order mode without interference on the working mode. The reasonable structure and placement is explored from theory to manufacturing method in order to minimize reflection and mode distortion by junction discontinuity. Two proof-of-concept prototypes have been fabricated and tested to analyze the error sources in terahertz device. And the corresponding error models are also constructed. Final transmission data exhibit an excellent agreement with the results of computer simulations when conductor loss is taken into consideration. Such −15-dB loss performance for high-order mode is verified allowing to cover the frequency band from 0.3 to 0.315 THz.

Design and Experiments of the Sheet Electron Beam Transport with Periodic Cusped Magnetic Focusing for Terahertz Traveling-Wave Tubes

The successful transport of a sheet electron beam under the periodic cusped magnet (PCM) focusing at the terahertz frequencies is reported. The sheet beam with a current density of 285 A/cm2 is intended for the developing G-band sheet-beam traveling-wave tube (TWT) whose operating voltage is nominally 24.5 kV. A beamstick was developed to validate the design of the electron optics system, which is considered as the most challenging part for developing a sheet-beam device. A beam transmission ratio of 81% is achieved over a distance of 37.5 mm at a cathode voltage of −25.0 kV. The total current and the collector current were measured to be 125 and 102 mA, respectively. The experimental results are promising, demonstrating that the PCM scheme is capable of focusing a high-current-density sheet beam and hence can find use in the terahertz TWTs, offering the advantages of compact size and light weight.

Investigation of High Interaction Efficiency of Sheet Beam TWT Based on All-Period Optimization and Beam Bunching Analysis

This article explores an extremely high beam&#x2013;wave interaction (BWI) efficiency technology of sheet beam traveling wave tubes (SBTWTs). An all-period phase velocity tapering optimization method is proposed to improve BWI efficiency of slow wave structure (SWS). The effects of the special electron bunching state and synchronous phase velocity change on the improvement of BWI efficiency generated by this method are analyzed. To solve the computational burden caused by dozens of parameters, self-developed 1-D nonlinear BWI code is used to calculate the BWI. For the best optimization effect, particle swarm optimization (PSO) algorithm selected from a variety of algorithms is utilized to search for the global optimal solution. The PIC simulation verification results show that by the all-period optimization method, the 80-period staggered double vane (SDV) SWS achieves a stable output power of 18 kW with 31 kV voltage 0.9-A current sheet beam at 30 GHz. The BWI efficiency of all-period phase velocity tapering SWS has reached 64.5&#x0025; at center frequency, the efficiency between 28 and 35 GHz exceeds 40&#x0025;.

Improved Model for Beam–Wave Interaction With Ohmic Losses and Reflections of Sheet Beam Traveling Wave Tubes

In this article, an improved model for the beam-wave interaction of sheet beam in traveling wave tubes (TWTs) considering ohmic losses and reflections is presented. The ohmic losses are obtained by field analysis and equivalent method. The space charge magnetic field is derived from the active Helmholtz's equation. An algorithm to obtain the S-matrix by the equivalent circuit method is presented. The relativistic Boris method is applied to accelerate macroparticles. The exchanged power is computed by the work the electromagnetic field applied to the macroparticles. The theoretical model is applied for validation to a G-band staggered double vane TWT and validated in comparison with CST Particle Studio and simulations without losses and reflections. The convergence of this algorithm is also discussed. The simulation time of the model is substantial faster than 3-D particle-in-cell (PIC) simulations.

Continuous Wave Operation of a Ka-Band Broadband High-Power Sheet Beam Traveling-Wave Tube

To meet the need to transmit increasingly massive volumes of data, a more than 3 kW continuous wave (CW) Ka-band sheet beam traveling-wave tube (SB-TWT) is reported for the first time. Working under unsaturated drive, the maximum gain obtained is 29 dB with a sheet beam of 30.9 kV, 0.82 A, and the efficiency exceeds 24% with a single-stage collector depressed to -15 kV. The 1-dB bandwidth reaches 6 GHz from 26 GHz to 32 GHz and the bandwidth with output power greater than 2 kW can even reach 7GHz from 26 GHz to 33 GHz. The realization of 99.8% static transmission as well as over 97% actual CW transmission of sheet beam is verified successfully, which benefits from adopting periodic cusp magnet (PCM) focusing, and thereby total weight is less than 15 kg. The relevant experimental details are introduced in this letter.

Design, Microfabrication, and Characterization of a Subterahertz-Band High-Order Overmoded Double-Staggered Grating Waveguide for Multiple-Sheet Electron Beam Devices

At present, there is a great interest in increasing the power level of terahertz (THz) radiation sources. Aimed to improve the output power of sheet electron-beam devices, a sub-THz-band rectangular metal column-loaded double-staggered grating waveguide (RMC-DSGW) is proposed in this article, which is suitable for the interaction between the high-order overmoded wave and multiple-sheet electron beams. The RMC is used to suppress adjacent competing modes. To verify the idea, a sub-THz-band TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> -like mode RMC-DSGW joined with two identical rectangular TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> mode converters was designed and manufactured by using nanocomputer numerical control milling. A reflection coefficient lower than ~ -12.6 dB and a transmission coefficient higher than ~ -8.9 dB were measured in the frequency range of 231.3-271.4 GHz. Beam-wave interaction simulations with the ohmic losses being taken into account for an RMC-DSGW-based dual-sheet beam traveling wave tube (TWT) predicted an output power of >332 W and a gain of >30.2 dB over a 3-dB bandwidth of 50 GHz (230-280 GHz). Its output power was approximately twice that of the conventional fundamental mode sheet beam TWT. It proves that the combination of the RMC-DSGW and multiple-sheet electron beams is an effective approach to improve the output power of sheet electron beam devices.

Numerical Simulation and Theoretical Analysis of Terahertz Sheet Beam TWTs Based on a Ridge-Loaded Folded Waveguide SWS

Generation of terahertz (THz) radiation from an interaction between sheet electron beam with the electromagnetic wave in a smooth transition ridge-loaded folded waveguide (RFWG) structure is proposed in this article. It was found that in the structure the electric field density can be increased so that the average interaction impedance of the proposed circuit is higher than that of conventional sheet beam (SB) folded waveguide (FWG) circuit. According to the beam–wave simulation results, at a designed voltage of 13.1 kV and an electron beam current of 100 mA, a maximum output power of 91.13 W can be achieved, which corresponds to a power gain of 32.88 dB and an efficiency factor of 6.96%. Therefore, the smooth transition RFWG traveling wave tube (TWT) should be considered as a promising THz radiation source. Moreover, based on a recently published 3-D nonlinear theory for a SB FWG-TWT, a version of the 3-D large-signal frequency-domain nonlinear theory model representation for the proposed smooth-transition SB RFWG-TWT was developed. The code is orders of magnitude faster than, and nearly as accurate as, full 3-D particle-in cell (PIC) simulations.

A Wideband High-Voltage Longitudinal Output Structure for Ka-Band Sheet Beam Traveling-Wave Tubes

This article proposes a longitudinal output structure in Ka-band sheet beam traveling-wave tube (SBTWT) with an ultrawideband transmission and high breakdown collector voltage. A novel electromagnetic bandgap (EBG) cutoff structure is designed to separate the slow-wave structure (SWS) and the depressed collector. The large stopband property of the EBG structure is applied to achieve not only the high transmission and the low gap leakage but also the long gap distance to provide a high breakdown voltage of the depressed collector. The 3-D simulations of a single longitudinal structure show a full Ka-band (26.5–40 GHz) reflection of less than −20 dB and gap leakage less than −0.02 dB. The entire longitudinal output collector, including two large gaps cutoff structures and the over mode electron collecting parts, is designed and manufactured. The simulations show a bandwidth of 10.4 GHz with a reflection coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} _{\mathbf {11}}$ </tex-math></inline-formula> of lower than −20 dB. The cold test demonstrates the wild bandwidth of ~11.1 GHz with a reflection coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} _{\mathbf {11}}$ </tex-math></inline-formula> of lower than −15 dB. The hot test reveals a stable collector dc voltage of −14.2 kV.

The High-Order Coalesced TM11-Like Mode Operation for 220 GHz Sheet Beam Traveling-Wave Tube

Sheet beam vacuum electron tubes are an attractive solution for high-power sources or amplifiers at millimeter-wave and terahertz range. In this article, a high-order coalesced TM11-like mode operation for 220 GHz sheet beam traveling-wave tube is proposed. The feasibility of coalesced TM11-like mode operation is investigated. The utilization of high-order mode expands the RF circuit power capacity, increases the beam current and reduces the cathode emission density. The self-consistent nonlinear code and three-dimensional particle-in-cell simulations are used to verify the 6.7% beam-wave interaction at 220 GHz without phase matching optimization. The maximum output power reaches 370 W. The results demonstrate this methodology is a good solution to build a high-power device for a wide region of terahertz.

Design and Cold Test of Dual Beam Azimuthal Supported Angular Log-Periodic Strip-Line Slow Wave Structure

A novel Ka-band azimuthal supported angular log-periodic strip-line (ASALS) slow wave structure (SWS) is proposed for a miniature radial sheet beam traveling wave tube (TWT). In the proposed SWS, two dielectric rods are employed to support the ASALS at both azimuthal sides. The strip-line is made of high-melting metal; thus, the ASALS SWS has a higher power capacity than microstrip SWSs. The high-frequency characteristics of that proposed SWS are investigated by using simulation software. The ASALS SWS is then fabricated and assembled for cold test. The measured transmission loss and reflection loss are better than − 2.5 dB and − 10 dB, respectively. The hot performance of the proposed is studied based on particle-in-cell (PIC) simulation. It is shown that when the operating voltage is 5700 V, the ASALS TWT can give a maximum output power of 320 W at the frequency of 39 GHz, corresponding to a gain of 14.9 dB and electron efficiency of 9.12%.

Design and Analysis of the Staggered Double Grating Slow Wave Circuit for 263 GHz Sheet Beam TWT

The complete design process of a high-frequency circuit for a 263 GHz staggered double grating, sheet beam traveling wave tube is presented in this article. This device, for use in a pulsed electron paramagnetic resonance spectrometer instrument, requires >30 dB gain and a 20 GHz bandwidth centered at 263 GHz. In addition to the standard Pierce gain analysis and point by point particle in cell (PIC) simulation, a novel fast pulse response analysis technique was utilized to determine the accurate synchronization voltage and test the ``hot” feature of the high-frequency circuit. Since this new method can evaluate the entire bandwidth in a single simulation, it can significantly reduce the computational resources normally required for the conventional PIC analysis while also illustrating the potential instabilities near the band edge. Good agreement between the fast pulse response and PIC simulation results validate the accuracy of this novel method. According to the analysis results and given the manufacturing limits of nano/micro-computer numerical control (CNC) machining, an appropriate SWS, coupler, and sever designs were chosen to complement the simulated design. Finally, a prototype circuit was manufactured and cold tested to verify the design and inspect the machining tolerances.

Design, Fabrication, and Cold Test of a High Frequency System for an H-Band Sheet Beam Travelling Wave Tube

The design, fabrication and cold test of a high frequency system (HFS) for an H -band (220–325GHz) sheet beam travelling wave tube is presented in this article. The HFS was composed of a 90-period double-staggered grating waveguide and a pair of identical L-shaped couplers with Bragg reflectors and matching steps. The HFS was manufactured by nano-computer numerical control machining and its electromagnetic properties were measured by using a vector network analyzer. The measured S -parameters were in good agreement with the simulated ones, which predicted a 3 dB bandwidth of ∼47.0 GHz. The maximum value of the measured transmission coefficient S 21 was −4.9 dB and the in-band port reflection S 11 was around −15.0 dB. Based on the cold testing-based HFS, simulations of the beam wave interaction predicted a stable output power of over 55.1 W in the frequency range of 230–280 GHz. In addition, the instabilities of the beam wave interaction were investigated in the simulation.

Design and Measurement of a Terahertz Double Staggered Grating Waveguide With an Arc-Shaped Beam Tunnel

A millimeter-waveband double staggered grating waveguide (DSGW) with an arc-shaped beam tunnel was previously proposed for sheet beam traveling wave tubes. In this article, the design, fabrication, and cold test of a terahertz band DSGW with an arc-shaped beam tunnel are presented. Simulations demonstrated that the novel DSGW would have a higher coupling impedance and a better beam wave interaction in comparison to the conventional DSGW with a linear-shaped beam tunnel. Beam wave interaction simulations showed that the output power and the electronic efficiency, respectively, increased from ~420 W to ~580 W and from 7.0% to 9.3% at 240 GHz. These two kinds of DSGWs were fabricated by using the nanocomputer numerical control (CNC) milling and wire cutting techniques, and their performances were cold tested by using a vector network analyzer. The measurement results were in good agreement with the simulations and demonstrated that both the traditional and novel DSGWs achieved a wide frequency bandwidth of ~40 GHz (0.22–0.26 THz) with a transmission coefficient ${S}_{{21}}$ better than −2.7/−3.0 dB and a reflection coefficient ${S}_{{11}}$ lower than −13.0/−15.7 dB, respectively.