Millimeter-wave Traveling-wave Tube Research Articles

A Staggered Vane-Shaped Slot-Line Slow-Wave Structure for W-Band Dual-Sheet Electron-Beam-Traveling Wave Tubes.

A staggered vane-shaped slot-line slow-wave structure (SV-SL SWS) for application in W-band traveling wave tubes (TWTs) is proposed in this article. In contrast to the conventional slot-line SWSs with dielectric substrates, the proposed SWS consists only of a thin metal sheet inscribed with periodic grooves and two half-metal enclosures, which means it can be easily manufactured and assembled and has the potential for mass production. This SWS not only solves the problem of the dielectric loading effect but also improves the heat dissipation capability of such structures. Meanwhile, the SWS design presented here covers a -15 dB S11 frequency range from 87.5 to 95 GHz. The 3-D simulation for a TWT based on the suggested SWS is also investigated. Under dual-electron injection conditions with a total voltage of 17.2 kV and a total current of 0.3 A, the maximum output power at 90 GHz is 200 W, with a 3 dB bandwidth up to 4 GHz. With a good potential for fabrication using microfabrication techniques, this structure can be a good candidate for millimeter-wave TWT applications.

Three-Way Serpentine Slow Wave Structures With Stationary Inflection Point and Enhanced Interaction Impedance

We introduce two novel variants of the serpentine waveguide slow-wave structure (SWS), often utilized in millimeter-wave traveling-wave tubes (TWTs), with an enhanced interaction impedance. Using dispersion engineering in conjunction with transfer matrix methods, we tune the guided wavenumber dispersion relation to exhibit stationary inflection points (SIPs), and also non-stationary, or tilted inflection points (TIPs), within the dominant TE10 mode of a rectangular waveguide. The degeneracy is found below the first upper band-edge associated with the bandgap where neighboring spatial harmonics meet in the dispersion of the serpentine waveguide (SWG) which is threaded by a beam tunnel. The structure geometries are optimized to be able to achieve an SIP which allows for three-mode synchronism with an electron beam over a specified wavenumber interval in the desired Brillouin zone. Full-wave simulations are used to obtain and verify the existence of the SIP in the three-way coupled waveguide and fine-tune the geometry such that a beam would be in synchronism at or near the SIP. The three-way waveguide SWS exhibits a moderately high Pierce impedance in the vicinity of a nearly-stationary inflection point, making the SWS geometry potentially useful for improving the power gain and basic extraction efficiency of millimeter-wave TWTs. Additionally, the introduced SWS geometries have directional coupler-like behavior, which enables distributed power extraction at frequencies near the SIP frequency.

Investigation of Half Rectangular-Ring Helix Slow Wave Structure for W-Band Wide Bandwidth High-Efficiency TWTs

The half rectangular-ring helix (HRRH) slow wave structure (SWS), which evolves from the rectangular helix (RH), is proposed to develop a high-efficiency wide bandwidth millimeter-wave traveling-wave tube (TWT). The comprehensive advantages of the HRRH SWS are lower phase velocity and higher interaction impedance in comparison with the RH and the planar helix with straight-edge connections (PH-SECs) SWS with the same dimensions. The particle-in-cell (PIC) simulation shows that, at a beam voltage of 6450 V, a beam current of 0.03 A, the maximum output power of 42.2 W, corresponding to a differential small-signal gain of 1.43 dB/mm, and an electron efficiency of 21.8% can be obtained for the HRRH TWT at 95 GHz. The output power, differential small-signal gain, and electron efficiency of the HRRH TWT are considerably enhanced compared with the RH TWT in a wide frequency range of 60–115 GHz under the same electrical parameters.

Design and RF Characterization of High-Power Ferrule-Loaded Folded Waveguide SWS for mm-Wave TWT

Design and analysis of ferrule-loaded folded waveguide slow wave structure is carried out for a high-power millimeter-wave traveling-wave tube (TWT). The design of the structure is carried out by numerical simulation. A typical structure has been designed and fabricated at the Ka-band, and cold test measurement is carried out for the cold circuit parameters and the results are compared against numerical analysis. To compare the efficacy of the ferrule-loaded folded waveguide structure, a conventional folded waveguide structure is also designed for the same operating bandwidth, and cold circuit parameters are compared. The comparison results shows that the interaction impedance of the ferrule-loaded folded waveguide structure is double compared with the conventional structure at the cost of reduction in cold bandwidth. Furthermore, particle-in-cell (PIC) simulation is also carried out to estimate the output power and gain for both the structures for the same operating band, and the results are compared. The comparison results show that the cross section is reduced by 22% and the overall interaction length is reduced by 50% for the ferrule-loaded FW-SWS compared with the conventional FW-SWS for the same output power of about 500 W.

Experimental Validation of Phase Velocity and Interaction Impedance of Meander-Line Slow-Wave Structures for Space Traveling-Wave Tubes

Meander lines are promising slow-wave structures (SWSs) for millimeter-wave traveling-wave tubes (TWTs) due to low-cost manufacture, low-voltage operation, and high interaction impedance. However, experimental results on meander lines are rare in the literature. Phase velocity and interaction impedance are the most important parameters for the design and characterization of TWT SWSs. Their experimental determination in meander lines is crucial for validating simulations and developing new topologies. Based on a new theoretical model, this article presents an experimental procedure to determine the phase velocity and the interaction impedance. The method is validated on four different Ka -band (33–37 GHz) meander-line SWSs, including two of a novel topology.

${H}$ - and ${E}$ -Plane Loaded Slow Wave Structure for ${W}$ -Band TWT

Sheet beam vacuum electron tubes are an attractive solution for high-power sources or amplifiers at millimeter-waves. In this article, a novel ${W}$ -band slow wave structure (SWS) for traveling wave tube (TWT) amplifiers supporting a sheet beam is proposed. The SWS is based on a rectangular waveguide with ${H}$ - and ${E}$ -plane loaded (HEL) metal corrugations. A test structure of the proposed HEL SWS with purposely designed input and output couplers was built in the frequency range of 91–98 GHz ( ${W}$ -band). The measured scattering-parameters agree well with the simulations showing ${S}_{11} dB over 10-GHz bandwidth. A TWT was designed and simulated with the HEL SWS. It shows very good gain-bandwidth performance. The SWS is easy to manufacture by low-cost computer numerical controlled (CNC)-milling. The results demonstrated that the HEL SWS is a very good solution to build high-power, wideband millimeter-wave TWTs for a wide range of applications that need high power in a broad frequency range.

Stability and Multimode Simulation Studies of ${W}$ -Band Uniformly Dielectric-Loaded Gyrotron Traveling-Wave Tube Amplifier

In this article, a nonlinear self-consistent, multimode analysis, and particle-in-cell (PIC) simulation of a uniformly dielectric-loaded (UDL) millimeter-wave gyrotron traveling-wave tube (gyro-TWT) amplifier has been discussed. A single-anode magnetron injection electron gun is designed to produce a relativistic electron beam of 100 kV and 5 A with a unity velocity ratio and 5% of axial velocity spread. The desired operating mode TE01 is fed by converting the TE10 rectangular waveguide mode through the Y-shaped power transducer. Both the multimode theory and PIC simulation results are agreed and predicted the saturated RF power of ~140 and ~145 kW at 92 GHz, respectively. A saturated gain of ~51 dB, a 3-dB bandwidth of ~5%, and a power conversion efficiency of ~28% are obtained. To collect the spent electrons, a collector is designed by using electron optics, E-GUN. The maximum heat wall loading at the collector surface due to a spent beam is calculated as 0.255 kW/cm2.

Design and Experimental Study of 250-W ${W}$ -band Pulsed TWT With 8-GHz Bandwidth

To increase output power of W-band pulsed millimeter-wave traveling-wave tubes, a prototype tube with phase velocity step-tapering circuits was designed and fabricated. The simulation methodology involved the optimization of the phase velocity step-tapering, cold characteristics, and particle-in-cell (PIC) simulation. The testing data show that it was capable of delivering an output power of over 250 W in a frequency range of 89.6-97.6 GHz (8 GHz), with a maximum power of ~424 W and electronic efficiency of ~10% at 92.9 GHz, with 189-mA beam current and 22.5-kV beam voltage. Besides, more than 4-GHz bandwidth with output power of over 300 W and electronic efficiency of over 7% was obtained with saturated gain over 36 dB. It was also found that the tested output power from the prototype tube was in an excellent agreement with the PIC simulated result.

U-shaped meander-line slow-wave structure with stub-loading

In this paper, a U-shaped meander-line slow-wave structure (SWS) with stub-loading is proposed for applications in Ka-band traveling-wave tube (TWT). This new slow-wave structure, loaded with a stub at the center of the U-turn section, has higher interaction impedance and lower phase velocity compared with conventional U-shaped meander-line SWSs, indicating that the devices based on this structure may have a lower operating voltage and higher output power. The dispersion characteristic, interaction impedance, transmission characteristics, and beam-wave interaction are simulated by utilizing simulation tool. The simulation result predicts that the millimeter-wave traveling-wave tube design based on this slow-wave structure is capable of delivering over 200 W with a gain of 33 dB and interaction efficiency 14.5% at the center frequency 34 GHz. This design, more compact and powerful in comparison with those based on more conventional vacuum electronic mechanisms, is demonstrated as a prospective option for integrated millimeter-wave power modules (MMPMs) empowering a broad spectrum of fields, from target detection, to imaging and telecommunications, among others.

An Open Rectangular Waveguide Grating for Millimeter-Wave Traveling-Wave Tubes**Supported by the National Natural Science Foundation of China under Grant No 61271029, the National Science Fund for Distinguished Young Scholars of China under Grant No 61125103, and the National Research Foundation of Korea under Grant No MSIP: NRF-2009-0083512.

Millimeter-wave traveling-wave tube (TWT) prevails nowadays as the amplifier for radar, communication and electronic countermeasures. The rectangular waveguide grating is a promising all-metal interaction circuit for the millimeter-wave TWT with advantages of high power capacity, fine heat dissipation, scalability to smaller dimensions for shorter wavelengths, compact structure and robust performance. Compared with the traditional closed structure, the open rectangular waveguide grating (ORWG) has wider bandwidth, lower cut-off frequency, and higher machining precision for higher working frequencies due to the open transverse. It is a potential structure that can work in the millimeter wave and even Terahertz band. The rf characteristics including dispersion and interaction impedance are investigated by both theoretic calculation and software simulation. The influences of the structure parameters are also discussed and compared, and the theoretical results agree well with the simulation results. Based on the study, the ORWG will favor the design of a broadband and high-power millimeter-wave TWT.

Simulation of Rectangular Helix Slow-Wave Structure for 140 GHz Traveling-Wave Tube

The rectangular helix has been proposed to develop a low-voltage, wide-band, and high-power millimeter-wave traveling-wave tube (TWT). This structure, which is evolved from the original circular helix slow-wave structure, is a kind of planar structure, which is suitable for the microfabrication technology. The high-frequency characteristics of the rectangular helix structure, including dispersion properties, interaction impedances, and reflection characteristics are investigated. The interaction between the sheet electron beam and the slow wave in the structure is also obtained by utilizing the particle-in-cell code in CST Particle Studio. A circuit sever is set into the rectangular helix to reduce the backward wave oscillation. Our calculations indicate that, at beam parameters of 9.95 kV and 40 mA, the averaged output power and the electron efficiency can reach 46 W and 11.5% at 140 GHz, respectively, and a transient 3-dB bandwidth of 6 GHz (ranging from 137 to 143 GHz).

A Microfabricated Planar Helix Slow-Wave Structure to Avoid Dielectric Charging in TWTs

Modifications to a slow-wave structure (SWS) consisting of a planar helix with straight-edge connections are proposed so as to avoid dielectric charging problem when it is used in a traveling-wave tube (TWT). The planar helix SWS is suitable for microfabrication using materials such as silicon (Si) and silicon dioxide (SiO2). First, through a simple dielectric slab model, it is shown that the phenomenon of dielectric charging can be accurately simulated using Computer Simulation Technology Particle Studio. Next, a more realistic double-dielectric model is considered to obtain guidelines to avoid dielectric charging. Based on these guidelines, two simple modifications are suggested for the planar helix SWS when it is microfabricated using Si wafers and a layer of SiO2. The modifications consist of partial removal of the SiO2 layer and a careful enhancement of the conductivity of the Si layer. The simulation results are presented for a Ka-band planar helix SWS to demonstrate a very significant reduction in dielectric charging while maintaining a low insertion loss when these modifications are incorporated. The proposed technique is compatible with microfabrication and is expected to be useful in microfabricated millimeter-wave TWTs.

Backward-Wave Suppression Analysis, and Design and Fabrication of a Prototype Millimeter-Wave Ring-Bar Slow-Wave Structure

A challenge for high-power millimeter-wave (mmw) traveling-wave tube (TWT) amplifiers is to realize high-power operation without incurring oscillation from backward-wave (BW) interaction. Conventional wisdom purports that contrawound (including ring bar) helix TWTs are superior to monofilar helix TWTs for stability against BW oscillations due to a zero or at least greatly suppressed BW interaction impedance (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BW</sub> ) compared with the forward-wave (FW) interaction impedance (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FW</sub> ). We use 3-D electromagnetic (EM) field calculations of a ring-bar helix to compare the BW and FW interaction impedances. Using a sine/cosine basis set and a comparison at a fixed phase velocity (rather than fixed frequency assumed in previous analyses), we show that the BW interaction impedance is not always significantly suppressed in the ring-bar/contrawound helix. In spite of the lack of ubiquitous BW stability, we use EM simulations to illustrate that there remain specific, efficient, high-gain mmw ring-bar designs with K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BW</sub> <; K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FW</sub> . The accuracy of the simulations is validated by experimental measurements that confirm simulation predictions of phase velocity and S-parameters.

Studying Characteristics of the Slow-Wave System of the Traveling-Wave Tube with a Sheet Electron Beam

We propose a method for calculation of a periodic slow-wave system designed as a double comb immersed into a rectangular waveguide. The approach is based on solving a system of integral equations obtained by the method of field matching in different regions. The system of integral equations is reduced to an algebraic system of linear homogeneous equations using Galerkin’s method with allowance for the singular behavior of the field on the edges of the comb pins. The results of calculating characteristics of the slow-wave system of the millimeter-wave traveling-wave tube with a sheet electron beam are presented.

Novel Folded Frame Slow-Wave Structure for Millimeter-Wave Traveling-Wave Tube

A new type of folded frame slow-wave structure (SWS) is introduced and used in the design of a low-voltage, high-efficiency, and widebandwidth millimeter-wave traveling-wave tube (TWT). The high-frequency characteristics of the folded frame structure, including dispersion properties, coupling impedances, and reflection characteristics are investigated. The beam-wave interaction of the TWT with the folded frame SWS working at the millimeter-wave frequency range is also calculated using 3-D particle-in-cell algorithms. The simulation results reveal that with sheet electron beam parameters of 6000 V and 0.2 A, the average output power and electron efficiency can reach 196 W and 16.3%, respectively. Compared with the symmetric double V-shaped microstrip meander-line SWS, the folded frame SWS has larger coupling impedances and can generate higher output power.

A novel slotted helix slow-wave structure for high power Ka-band traveling-wave tubes

A novel slotted helix slow-wave structure (SWS) is proposed to develop a high power, wide-bandwidth, and high reliability millimeter-wave traveling-wave tube (TWT). This novel structure, which has higher heat capacity than a conventional helix SWS, evolves from conventional helix SWS with three parallel rows of rectangular slots made in the outside of the helix tape. In this paper, the electromagnetic characteristics and the beam-wave interaction of this novel structure operating in the Ka-band are investigated. From our calculations, when the designed beam voltage and beam current are set to be 18.45 kV and 0.2 A, respectively, this novel circuit can produce over 700-W average output power in a frequency range from 27.5 GHz to 32.5 GHz, and the corresponding conversion efficiency values vary from 19% to 21.3%, and the maximum output power is 787 W at 30 GHz.

A Novel Winding Microstrip Meander-Line Slow-Wave Structure for V-Band TWT

A novel winding microstrip meander-line slow-wave structure (SWS) has been proposed to develop low-voltage and high-power millimeter-wave traveling-wave tube. This structure, which interacts with a round electron beam, evolves from a V-shaped microstrip meander-line SWS. The electromagnetic characteristics and beam-wave interaction of this novel structure are calculated in this letter. As is seen from the results, traveling-wave tubes with this novel circuit can provide over 130-W output power ranging from 58 to 63 GHz. In addition, the maximum electronic efficiency may reach 10% at 61 GHz.

Nonlinear investigation of beam-wave interaction in double-groove loaded folded-waveguide traveling-wave tube

A W-band traveling-wave tube (TWT) with double-groove loaded folded waveguide structure (FWSWS) has been designed and numerically modelled. The nonlinear performance of such a TWT is investigated by a particle-in-cell code MAGIC3D. Simulation results indicate this TWT produces a saturated electromagnetic power of 170.2 W at 90 GHz, corresponding to 36.9 dB gain and 69.6 mm interaction distance. A comparison between the novel folded waveguide traveling-wave tube (FWTWT) and the conventional one is also carried out to verify the effect of groove loading on the large-signal performance of TWT. Within the same working conditions, the double groove-loaded FWTWT could obtain higher saturated output power and gain in a shorter interaction length. The maximum of output power and gain of this novel TWT is 58.6% and 10% higher than those of the conventional FWTWT, while the 3-dB bandwidth of TWT is reduced to 4 GHz. With the additional advantage of ease of fabrication based on micro-electro-mechanical systems (MEMS) technologies, the double-groove loaded FWSWS is suitable for a millimeter-wave TWT with high power capacity and gain.

Analysis of Coupled Planar Helices with Straight-Edge Connections for Application in Millimeter-Wave TWTs

Two different types of coupled planar helix structures with straight-edge connections (PH-SECs), namely, an unconnected pair and a coaxial pair of planar helices, are proposed and analyzed in the context of application in millimeter-wave traveling-wave tubes. It is shown that the nature of the propagating modes, dispersion characteristics, field distribution, and coupling impedance for the proposed structures can be obtained as special cases of a generic structure that consists of four parallel infinite unidirectionally conducting screens. For both the coupled planar helix structures, the results based on simple analytical approximations of the generic structure match well with the simulation results. As an application of these structures, simulation results are presented to show that the unconnected pair of planar helices has a reduced interaction with backward waves compared with that for a corresponding single planar helix. The proposed PH-SECs offer easier fabrication using printed-circuit or microfabrication techniques compared with their circular counterparts.

A Novel Ridge-Vane-Loaded Folded-Waveguide Slow-Wave Structure for 0.22-THz Traveling-Wave Tube

A novel slow-wave structure called ridge-vane-loaded folded waveguide combined with pencil electron beam has been proposed for millimeter-wave traveling-wave tubes. The high-frequency characteristics including the dispersion properties and the interaction impedance of this kind of structure have been analyzed by means of an equivalent circuit based on the transmission-line cascading network. The theoretical results agree well with those obtained by the 3-D electromagnetic high-frequency simulation software. The nonlinear interaction between the electron beam and the electromagnetic field is investigated with the help of the Computer Simulation Technology Particle Studio 3-D particle-in-cell (PIC) code. The PIC simulation results revealed that this kind of structure could produce about 56 W of peak output power at 0.22-THz when the cathode voltage and current of the pencil electron beam are set to 14.65 kV and 50 mA, respectively. Moreover, the maximum gain and interaction efficiency can reach 37.5 dB and 7.7% at 0.22 THz, respectively. A simulated 3-dB bandwidth of ~ 30 GHz is demonstrated.