Terahertz Traveling Wave Tube Research Articles

A Symmetrical Quasi-Synchronous Step-Transition Folded Waveguide Slow Wave Structure for 650 GHz Traveling Wave Tubes.

For the purpose of improving performance and reducing the fabrication difficulty of terahertz traveling wave tubes (TWTs), this paper proposes a novel single-section high-gain slow wave structure (SWS), which is named the symmetrical quasi-synchronous step-transition (SQSST) folded waveguide (FW). The SQSST-FW SWS has an artificially designed quasi-synchronous region (QSR) to suppress self-oscillations for sustaining a high gain in an untruncated circuit. Simultaneously, a symmetrical design can improve the efficiency performance to some extent. A prototype of the SQSST-FW SWS for 650 GHz TWTs is designed based on small-signal analysis and numerical simulation. The simulation results indicate that the maximum saturation gain of the designed 650 GHz SQSST-FW TWT is 39.1 dB in a 34.3 mm slow wave circuit, occurring at the 645 GHz point when a 25.4 kV 15 mA electron beam and a 0.43 mW sinusoidal input signal are applied. In addition, a maximum output power exceeding 4 W is observed at the 648 GHz point using the same beam with an increased input power of around 2.8 mW.

Performing Intelligent Design of Broadband Pillbox Window for a Terahertz Traveling-Wave Tube by Using Physics-Informed Neural Network and Genetic Algorithm

Performing Intelligent Design of Broadband Pillbox Window for a Terahertz Traveling-Wave Tube by Using Physics-Informed Neural Network and Genetic Algorithm

G-band traveling wave tube based on the elliptical pillar defected photonic crystal

In order to reduce the ohmic losses in the slow wave structure (SWS), an elliptical pillar defected photonic crystal waveguide (EPDPCW) is proposed as a SWS for the terahertz traveling wave tube (TWT). Compared with rectangular pillar defected photonic crystal waveguides (RPDPCW), the EPDPCW shows better high-frequency characteristics and lower ohmic losses. According to the particle-in-cell (PIC) results, in the frequency range of 188 GHz to 218 GHz, the saturated output power of EPDPCW is greater than 13.4 W, and the maximum saturated output power is 26.8 W at 196 GHz at a voltage of 9.78 kV and a beam current of 150 mA. Compared to the RPDPCW-TWT, the performance of the EPDPCW-TWT has been significantly improved.

Investigation of Spindt Cold Cathode Electron Guns for Terahertz Traveling Wave Tubes

In this work, a Spindt cold cathode electron gun with a PPM (periodic permanent magnet) focusing system for a terahertz TWT (traveling wave tube) was designed and simulated based on the Pierce electron gun structure. More specifically, a new 3D (three dimensional) emission model was used, where the cathode radius of the electron gun was 1 mm and the cathode current was 30 mA, with an emitting half angle of about 28°. It was demonstrated that the electron beam was well focused with an electron beam radius of 0.3 mm and a filling ratio of 0.5 when the maximum value of the PPM field along with the axis was 0.122T. According to the simulation results, a planar cold cathode electron gun was developed. Measurements demonstrated that the I/V characteristics of the cold cathode gun were consistent with that of a cold cathode, revealing that the electrons emitted from the cathode are not intercepted when passing through the electron gun.

A Wideband 220-GHz Traveling Wave Tube Based on Slotted Piecewise Sine Waveguide

A slotted piecewise sine waveguide (SPWSWG) is suggested as the slow wave structure (SWS) to develop the terahertz traveling wave tube (TWT). This letter is the first to report the SPWSWG SWS with the advantages of flat dispersion and large coupling impedance from our calculation. Based on this novel SWS, a wideband 220-GHz TWT has been designed, fabricated and tested. The cold test results show that the reflection parameters at the input and output port of the TWT are less than -15.2 dB in the frequency range of 209-235 GHz. Moreover, the hot test results demonstrate that the maximum saturated output power reaches 28.6 W at 215 GHz and the 3-dB bandwidth exceeds 15.5 GHz, which are basically consistent with the simulation results.

A Novel Design of G-Band Broadband Low-Gain Fluctuation Slow-Wave Structure With Improved Folded Waveguides

The design method for ultrawideband amplification with low-gain fluctuation is explored for the terahertz folded-waveguide traveling-wave tube (TWT). Although the folded-waveguide circuit has a large cold bandwidth ( <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%), effective amplification with a flat gain characteristic can only be achieved in a limited range due to sharply reduced interaction impedance. The situation further deteriorates in the design of a terahertz TWT where an oversized beam tunnel has to be used to facilitate the transport of the beam. Through careful analysis of the folded-waveguide (FWG) design characteristics, a modification to the geometry of the circuit has been proposed, leading to a significant enhancement in design capability. As a result, the interaction impedance is increased by 26% over a wide operation frequency band in comparison to the normal folded-waveguide circuit. This makes it possible to use the phase velocity tapering technology to increase efficiency and balance the gain at the same time. A design example 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 interaction circuit is capable of producing 18-W output power over 30 GHz from 202 to 232 GHz with a gain fluctuation of less than 1.5 dB.

Digital Fabrication and Reliability Evaluation Method for G-Band TWT Electron Optical System

Terahertz traveling wave tubes (THz-TWTs) electron optical system (EOS) has strict fabrication accuracy requirements. The low reliability of traditional manual assembly always is the direct problem hindering the development of TWT to high frequency, high power, high efficiency, and high reliability. In this article, a digital fabrication and reliability evaluation method for THz-TWTs EOS was presented. First of all, through improving the gun shell structure and sealing molds, developing automatic and semi-automatic assembly methods, writing automatic geometric measure programs, measuring the thermal deformation of cathode and focusing electrode (FE) under working temperature, etc., the implementation of digital fabrication technology was completed, which promoted the fabrication accuracy (1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma \text{)}$</tex-math> </inline-formula> to 3–10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> m and 0.05 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> –0.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> . Then, in order to find the tolerance limit to keep the electronic trajectory stable and the balance between design reliability and fabrication reliability, the CST particle solver single and multi-parameter sweep function was used to analyze the EOS of a 0.22 THz-TWT. Based on the digital fabrication technology process capability, 6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma $</tex-math> </inline-formula> quality management theory, and equal proportion compress single parameter tolerance limit method, we defined four independent sensitive parameters, and calculated the reliability of THz-TWT EOS digital fabrication was not less than 72.47% by MINITAB software. This method can be used as a quantitative calculation of whether the TWT EOS design and fabrication are matched, and give appropriate tolerance and compensation range in the actual TWT fabrication process.

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 a Modified Angular Log-Periodic Folded Waveguide Traveling Wave Tube at Ka-Band

This article demonstrates a Ka -band prototype traveling wave tube (TWT) with a single-stage gain of more than 35 dB, employing our previously proposed modified angular log-periodic folded waveguide (MALP-FW) slow wave structure (SWS). The basic operating principle of the MALP-FW SWS has been thoroughly investigated in our previous work, where its advantage of suppressing oscillations has been demonstrated by particle-in-cell (PIC) simulations. Guided by the principles, a prototype MALP-FW TWT operating at Ka -band is designed, fabricated, and tested. A 1600-G periodic permanent magnetic (PPM) focusing system is designed to focus the 12-kV and 100-mA electron beam. The measured maximum gain of the TWT is more than 35 dB at 32.5 GHz, and the output signal spectrum is noise-free. These results are in good agreement with the simulation results, which show that a maximum gain of 39.1 dB can be obtained at 32.4 GHz with an input power of 7 mW. This article validates the ability of MALP-FW SWS to suppress oscillations at the experimental level and its potential for future applications in terahertz 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.

Optimum Design of Electron Gun for 0.22-THz Traveling Wave Tubes

To improve the performance of a pencil beam electron gun for terahertz traveling wave tubes (TWT), a support vector regression (SVR) method combined with a goal attainment algorithm, which is used to optimize its structural parameters considering the periodic permanent magnetic (PPM) focusing system, is proposed in this article. Moreover, a parameter correction method to reduce the impact of the thermal expansion on the electron gun is proposed. An electron gun for 0.22-THz TWT gun is developed, in which a 30-mA/23.8-kV electron beam emitted by a curved barium–tungsten cathode with a beam loading of 4 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is achieved with the optimized parameters. To the high current density for THz beam–wave interaction, the area compression ratio of electron beam is used in beam tunnel and reaches 144. The experimental results are very similar to the simulation results, which indicate that the methods would be beneficial to the development of TWT.

G-Band High-Power and Ultrawide Band Staggered Double-Vane Slow-Wave Circuit With Double Beams

In this article, the design of a 220 GHz ultrawide band staggered double-vane slow-wave circuit, its fabrication, and cold tests’ results are discussed. The fundamental double modes, including odd and even modes, are adopted to match with the same electron beam to significantly increase the bandwidth. Multiple-beam slow-wave circuit could increase beam current with the enhancement of output power and gain compared with a single-beam structure. Linear taper couplers are designed as input/output structure, and the whole slow-wave circuit with couplers has been fabricated with high precision computer-numerical-control machine. Good agreements with simulation results are found at an ultrawide band about 70 GHz from 207 to 277 GHz with a return loss better than −10 dB and an insertion loss of about −7 dB. Particle-in-cell simulations with a double-mode multibeam two-stage slow-wave circuit within a total length of 45 mm show that it can get a nearly 310 W peak output power at 220 GHz with a 20.6 kV beam voltage and beam current of 2 × 0.08 A. The 3-dB bandwidth over 35 dB reaches about 70 GHz with a driven voltage of 20.6 kV. It agrees well with the cold test results. So, this design could promote the development of ultrawide band with high-power terahertz traveling-wave tubes.

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.

Parallel Arrangement Folded Double-Ridge Groove Waveguide for High-Power Terahertz Traveling-Wave Tube

In this article, a terahertz traveling-wave tube (TWT) with two identical folded double-ridge groove waveguides (DRGWs) operated in parallel is presented. Driven by a sheet electron beam in each DRGW TWT allows a higher beam current, and hence higher output power can be achieved. A magic T based on DRGW is designed as the input and output power couplers for this novel TWT. The performance of the magic T is optimized using Chebyshev impedance transforming structures. The simulated reflection and transmission coefficients of the whole tube with magic T in the operating frequency range of 330–350 GHz are less than −20 and −75 dB, respectively. In addition, a particle in cell (PIC) simulation of the beam–wave interactions involving the two sheet electron beams predicted an output power of 160 W at 340 GHz with a 3-dB bandwidth of 11 GHz when a beam of 27.68 keV in energy and 0.25 A in total current is used.

Focusing of the Sheet Electron Beam With Two-Plane Periodic Cusped Magnetic System for Terahertz TWTs

The focusing and transmission of a sheet electron beam with periodic cusped magnetic (PCM) fields were studied for the terahertz traveling-wave tubes (TWT). Due to the limitation on the width of the sheet beam from the slow wave structure (SWS), the resulting transmission current density is inevitably high and the width of the sheet beam is highly disproportionate to the magnet system. This defines a situation different from before, which challenges the validity of the traditional PCM scheme. Thus, the focusing of a sheet beam in the above scenario was theoretically analyzed to evaluate the focusing conditions. A practical magnetic field system was then designed based on the offset-pole-piece PCM scheme with the shortened pole pieces, which can meet the focusing requirements, especially on the side focusing. The theoretical analysis and the modified PCM scheme lead to a significant improvement in the focusing/transmission of the sheet beam. The 3-D particle simulation showed that a stable long-distance transmission over 100 mm can be achieved with the two-plane PCM focusing where an elliptical-cross section sheet beam is assumed with dimensions of 0.6 mm ×0.11 mm, voltage of 24.5 kV, and current of 0.18 A.

Design of an asymmetric multi-stage depressed collector for terahertz traveling wave tubes

The multi-stage depressed collector (MDC) plays an important role in the terahertz traveling wave tube (TWT) for recovering spent electron beam, which has significant impact on its working total efficiency. For the purpose of improving the efficiency of the whole tube, the designscheme method of a depressed collector with non-axisymmetric structure in terahertz TWT is proposed. The axisymmetric depressed collector for terahertz TWT is designed by analyzing the energy characteristics of spent electron beam after beam-wave interaction, and then on-axisymmetric structure is used for optimization. The results of three-dimensional particle simulation (PIC) show that for the TWT with the three-stage axisymmetric MDC, the optimal recovery efficiency is 79.30%, electron reflux rate is 3.12%, and the working efficiency is 4.47% when considering the secondary electron emission. For the TWT with the three-stage non-axisymmetric MDC, the recovery efficiency increases to 89.50%, the particle reflux rate decreases to 0.53%, and the whole tube working efficiency reaches 8.38%. The working efficiency of the whole tube can be increased effectively by taking the asymmetric structure.

Wideband Output Brewster Window for Terahertz TWT Amplifiers Application

A wideband terahertz (THz) output network consisting of a pyramidal horn and a diamond Brewster window has been developed for 0.7-1.0-THz traveling-wave tube (TWT) amplifier and is measured at 0.325-0.5 THz owing to the limitation of the measurement facility. A pyramidal horn is designed to convert the waveguide mode into a high-purity quasi-plane polarized mode to meet the demand of the Brewster window. A diamond Brewster window plate with low THz transmission loss is fabricated via chemical vapor deposition (CVD). The plate is directly placed on the pyramidal horn with a Brewster angle. A circular window plate supporter is intended to balance the stress distribution during diamond-metal brazing. The measurement shows transmission coefficient is better than 0.85 within a wide bandwidth of 0.325-0.5 THz. This letter provides a practical way for a single diamond window structure for THz TWT amplifiers.

A Novel Scheme for Gain and Power Enhancement of THz TWTs by Extended Interaction Cavities

A novel scheme is proposed for the enhancement of the gain and output power of terahertz (THz) traveling-wave tube (TWT) amplifiers. The scheme combines a slow-wave structure (SWS) and multigap resonant cavities, resulting in a complementary hybrid of traveling-wave and standing-wave elements, thus offering a compact circuit length. The idea has been verified by a specific design of a 220-GHz TWT amplifier by means of particle-in-cell (PIC) simulations. For an electron beam with a voltage of 21 kV and a current of 200 mA, a maximum gain of 42.22 dB and a 3-dB bandwidth of 6.5 GHz has been achieved while the overall circuit length is only 38 mm. For an input power of 11.25 mW, the CW peak power is predicted to be 187.5 W at 220 GHz, corresponding to an electronic efficiency of 4.47%. These simulation results predict a performance that well exceeds that reported for conventional TWTs, along with a significant reduction of the input power and circuit length.

Novel Double Tunnel Staggered Grating Slow Wave Structure for 0.2 THz Traveling Wave Tube

A novel slow wave structure (SWS), called double tunnel staggered grating (DTSG), which originates from the double staggered grating (DSG) and folded waveguide (FW), is proposed to develop a dual-beam traveling wave tube (TWT) with high power and electron efficiency in Terahertz (THz) band. Using two key techniques: (1) the rectangular holes are opened in the grating-ridges, and the operating mode is cut-off at holes; (2) specific grating-height, the DTSG-SWS possesses the wide operating band as DSG-SWS, high interaction impedance as the rectangular grating (RG) SWS, and good transmission characteristics as FW-SWS. In addition, it is not only suitable for the dual-beam TWT, but also is simple and easy of fabrication as DSG-SWS. The particle-in-cell simulation analysis confirms that the saturated power and electron efficiency of TWT using DTSG-SWS have been significantly improved compared with the TWT using DSG-SWS.

Experimental Verification of the Low Transmission Loss of a Flat-Roofed Sine Waveguide Slow-Wave Structure

The sine waveguide (SWG) is presented as a potential slow-wave structure (SWS) for the millimeter-wave (mm-wave) and terahertz (THz) traveling-wave tube (TWT) because of the advantages of wide bandwidth, natural electron beam tunnel, and easy fabrication. In particular, this theoretical study indicates that the transmission loss of the SWG is far less than that of the folded waveguide SWS, which is widely employed in the mm-wave and THz TWT. Here, the flat-roofed SWG slow-wave circuit working in the ${W}$ -band has been machined using nano-computer numerical control technology, which guarantees micron tolerances and surface roughness of tens of nanometers using carbide tooling with 100- ${\mu }\text{m}$ diameter. The cold test results show that the transmission coefficient ${S}_{{{21}}}$ of the entire high-frequency slow-wave circuit is more than −4.1 dB with a total length of 123.84 mm in the frequency range from 90 to 100 GHz, and the transmission loss is less than 0.36 dB/cm.