Phase Velocity Taper Research Articles

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.

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.

High-Efficiency W-Band Pulsed TWT With Three-Stage Depressed Collector

This letter presents the design and experimental study 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 traveling wave tube (TWT) with an overall efficiency of over 40% using multistage depressed collectors (MDCs). The folded waveguide (FWG) with a high coupling impedance and phase velocity tapering (PVT) circuit has been used to achieve the electronic efficiency of over 10%. Furthermore, a three-stage depressed collector with a high recovery efficiency was designed to enhance the overall efficiency in <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 pulsed TWTs. A prototype tube was then fabricated and tested with a beam current of 193 mA and a beam voltage of 20.8 kV. The testing data shows that the overall efficiency of 40.6% was obtained at over 410 W output power within 5 GHz at 20% duty cycle, with the maximum efficiency of 47%. That is the highest efficiency test results of <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 TWTs reported so far.

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;.

Power Enhancement in W-Band Pulsed Folded Waveguide TWT

To obtain higher power in ${W}$ -band traveling-wave tubes (TWTs), a modified folded waveguide (FWG) with non-concentric bends is introduced and demonstrated, which has higher coupling impedance compared with the normal FWG structure and is utilized to eliminate the band-edge instability of TWT. Phase velocity tapering (PVT) is also used to improve the interaction efficiency between the beam and the circuit. Furthermore, a higher beam current is used and the corresponding magnetic focusing system is designed to confine the beam. A prototype tube was then fabricated and tested at a beam current of 220 mA and beam voltage of 21.9 kV. The testing data show that this ${W}$ -band TWT is capable of delivering an output power of over 450 W with a 4.5-GHz bandwidth at a duty cycle of 10%, with a maximum power of 647 W and electronic efficiency of 13.4%. That is the highest output power and electronic efficiency test results of ${W}$ -band TWTs reported so far.

Demonstration of a Pulsed G-Band 50-W Traveling Wave Tube

This letter presents the research progress of a pulsed G-band 50 W traveling wave tube (TWT) with pencil beam focused by periodic permanent magnet (PPM). Three approaches have been combined to improve the tube's operation performances including setting the operating point near the cutoff frequency, using the folded waveguide (FWG) slow wave structure (SWS) with modified circular bends (MCBs), and adopting phase velocity tapering (PVT) technique. The measured output power can be over 50 W at 5% duty cycle with a bandwidth of 3.6 GHz when the beam voltage is 24.25 kV and the beam current is 59 mA. Corresponding electronic efficiency and gain are higher than 3.5% and 35 dB, respectively. The results provide a demonstration of a compact terahertz amplifier with high power, high electronic efficiency and significant bandwidth.

Study of Performance Improvement for a &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${Q}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Band Sheet-Beam Traveling-Wave Tube

The study of performance improvement for a ${Q}$ -band sheet-beam traveling-wave tube (SB-TWT) based on an improved double-staggered grating waveguide slow-wave structure (SWS) is presented. To enhance the tube performance, such as reducing the driving power requirement and increasing the electronic efficiency, three approaches were combined together in the tube design, including making the operating frequency be closer to the cutoff frequency of the SWS, improving the SWS with a curved beam tunnel, and introducing the phase velocity tapering technique. Beam–wave interaction simulations predicted that great improvements have been achieved. The output power, gain, and electronic efficiency at 42.5 GHz have, respectively, increased from 7.5 to 20.2 kW, from 23.7 to 60 dB, and from 13.9% to 30.3%, in comparison with a previously designed SB-TWT.

Design of a 0.22THz, 100W Planar TWT for Ultra Wide Band Wireless Communication

Development activities addressed to 0.22THz centre frequency is of significant importance for ultra wideband communication and remote sensing because of the available atmospheric spectral window over a band from 0.20THz to 0.30THz. 0.22THz planar TWT of output power more than 100W, gain more than 30dB, and bandwidth more than 30GHz is designed for high data rate wireless communication, imaging and remote sensing. Planar TWT is designed using a rectangular sheet electron beam of beam voltage 20kV and beam current 100mA, and of beam size 100m (thickness) x 600m (width). A planar staggered double vane loaded rectangular waveguide slow-wave structure (SDV-SWS) of broad bandwidth, high impedance and low circuit loss is designed for the 0.22THz TWT. Planar structure is used because it is compatible for sheet beam operation and it is also easy to fabricate using MEMS technologies. Large signal analysis code SUNRAY-P, developed to determine RF performance of a planar TWT with sheet beam, is used to design the 0.22THz 100W TWT with sever, couplers and phase velocity taper. WR-3 rectangular waveguide of size 864m x 432m is used for the input and the output couplers.

Design of a high gain and high efficiency W-band folded waveguide TWT using phase-velocity taper

The folded-waveguide (FW) slow-wave structure (SWS) is a very promising beam-wave interaction circuit for high-frequency traveling wave tubes (TWTs). This paper presents the design methodology for enhancing the gain and efficiency of a practical W-band FWTWT using phase-velocity taper. Initially, phase-velocity, interaction impedance, and gain-growth of a single-section FWTWT are predicted theoretically. Further, these parameters are optimized using the eigenmode solver and transient solver of 3-D CST Studio. The simulated results agreed with theoretically predicted results within ~1% discrepancy. Then the single-section SWS along with the input and output couplers consisting of quarter-wave impedance transformers and pillbox windows are simulated using transient solver and a return-loss of around −20 dB is obtained over the desired band. Finally, the output power, gain and electronic efficiency for a two-section TWT with the phase-velocity taper are obtained using particle-in-cell simulation of CST Studio. At an applied beam voltage of 20 kV and a current of 0.1 A, an output power of 130 W, corresponding to a saturated gain of 39.4 dB, an instantaneous 3-dB bandwidth of around 5.3%, and electronic efficiency of around 6.5% at 94 GHz has been obtained. It was found that at 94 GHz the ‘positively and negatively phase-velocity tapered’ sections enhanced the gain by almost 3 dB and doubles the electronic efficiency of the two-section FWTWT design as compared to the two-section FWSWS with the same interaction circuit length with no phase-velocity taper.

Design of W‐band sheet beam travelling wave tubes based on staggered double vane slow wave structure

In this study, a complete and viable plan about a W-band sheet beam travelling wave tube based on a staggered double vane slow wave structure is drawn up. The dispersion characteristics, transposition characteristics of high-frequency system, sheet electron beam gun, vacuum window, periodically cusped magnetic focusing system and beam–wave interaction are analysed by CST Microwave Studio, CST Particle Studio and HFSS. An attenuator is used in this device to suppress oscillation. A phase velocity taper is used to increase output power. The operation voltage is 20.6 kV and the current is 80 mA. With a 10 W input power, the output power is 1100 W versus the gain is 20.41 dB at 95 GHz. The peak amplitude approaches 1200 W versus the gain approaches 20.79 dB at 99 GHz.

Nonlinear analysis of beam-wave interaction in a planar THz travelling-wave tube amplifier

Large-signal analysis is presented for analysing non-linear beam-wave interaction in a planar THz TWT with a sheet electron beam. The sheet beam within one RF beam wavelength is represented by uniform rectangular charged discs for confined flow of the beam, and the discs are tracked in small distance steps of forward integration. For a planar THz TWT, a staggered double-vane loaded rectangular waveguide slow-wave structure (SDVSWS) is used. Frequency-domain analysis is used to define the RF circuit field in the SDVSWS and the interaction of the RF circuit field with the electron beam is carried out for steady-state TWT operation. The RF circuit field is considered to be continuous as in a helix SWS and it is represented by transmission line theory at each integration plane. A large-signal analysis code of a helix TWT with pencil beam (SUNRAY-1D) was modified for a planar TWT with a sheet beam. The accuracy of the modified SUNRAY code was checked against the 3D e.m. field simulator (CST-PS) by comparing the output performance for a 0.22-THz planar TWT. The results for the gain and the output power over the operating band given by the two codes were found to be comparable. A significant advantage of the SUNRAY code is that it is very fast compared to the CST code. It takes less than a minute to simulate the RF performance of a typical THz TWT at a single frequency and drive power on a standard PC. The SUNRAY code can therefore, be used interactively for design a complete SWS with input/output couplers, sever, loss profiles and phase velocity taper for a high efficiency high gain THz TWT.

A Novel Gap-Groove Folded-Waveguide Slow-Wave Structure for G-Band Traveling-Wave Tube

In this paper, a novel gap-groove folded-waveguide slow-wave structure (SWS) for high-efficiency G-band traveling-wave tube (TWT) is presented. In this novel tube, a sheet electron beam passes through the small gap between a bed of nails and a folded groove realized in a metallic plate. The bed of nails and the metallic plate form a high impedance structure-perfect electric conductor parallel plate waveguide, which prevents the fields from leaking transverse to the propagation direction. The phase velocity of the proposed SWS has been analytically calculated and the results show good agreement with those obtained using Eigenmode solver of computer simulation technology (CST). Meanwhile, the simulation results indicate that the interaction impedance of the proposed SWS is considerably higher than the conventional folded-waveguide SWS. Furthermore, employing a proper phase velocity taper in the end section of circuit leads to increasing the efficiency of the proposed TWT. According to Particle-in-cell simulations performed by the CST Particle Studio, the designed TWT can generate a peak power of 225 W at 220 GHz, corresponding to the maximum gain and efficiency of 42.7 dB and 14.9%, respectively.

A 1-kW 32–34-GHz Folded Waveguide Traveling Wave Tube

Folded waveguide (FWG) traveling wave tubes (TWTs) are potential sources of wideband, high-power millimeter and terahertz wave radiation. However, low efficiency limits its application. In this paper, we discuss a design strategy to improve efficiency of the FWG TWT meanwhile how to avoid instability is also discussed. The critical factors for the slow-wave structure design and the sever design are analyzed. A phase velocity taper profile incorporating a positive period followed by a negative period is introduced into the design to improve the efficiency. A Ka-band FWG TWT was developed, which demonstrated the output power up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${&gt;}{\rm 1}~{\rm KW}$</tex></formula> in 2-GHz bandwidth and electronic efficiency up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${&gt;}{9.4\%}$</tex></formula> .

DESIGN OF A HIGH POWER, HIGH EFFICIENCY KA-BAND HELIX TRAVELING-WAVE TUBE

The design and analysis of a high power and high e-ciency helix traveling-wave tube operating in the Ka-band are presented. First, a double-slotted helix slow-wave structure is proposed and employed in the interaction circuit. Then, negative phase-velocity tapering technology is used to improve electronic e-ciency. From our calculations, when the design voltage and beam current are set to be 18.45kV and 0.2A, respectively, this tube can produce average output power over 800W ranging from 28GHz to 31GHz. The corresponding conversion e-ciency varies from 21.83% to 24.16%, and the maximum output power is 892W at 29GHz.

Particle-in-Cell Simulation and Optimization for a 220-GHz Folded-Waveguide Traveling-Wave Tube

The folded-waveguide slow-wave structure (SWS) is a robust beam-wave interaction circuit for millimeter-wave traveling-wave tubes. We applied a particle-in-cell solver (Computer Simulation Technology Particle Studio) to analyze and optimize a 220-GHz folded-waveguide SWS. The beam-wave interaction and the electron beam dynamics were studied based on simulation results. The influence of the beam tunnel, with respect to its transverse shape and size, on the circuit performance was investigated in detail. A beam tunnel with a square cross section exhibits lower gain and efficiency and similar bandwidth compared with a beam tunnel with a circular cross section. With a proper phase-velocity taper employed in the nonlinear region of the circuit, the small-signal gain, peak saturated output power, and efficiency were significantly improved, increasing from 21.5 to 27.8 dB, 41 to 70.5 W, and 4.5% to 8%, respectively, for a 36-mm-long loss-free circuit. A simulated 3-dB bandwidth of ~ 15 GHz is demonstrated.

Design of Helix Pitch Profile for Broadband Traveling-Wave Tubes

To improve the performance of a broadband traveling-wave tube (TWT), a helix pitch profile incorporating a section with reduced phase velocity followed by a positive phase-velocity taper is used. A simple technique to design a pitch profile of this kind is elaborated upon and later applied to an existing 6-18-GHz uniform pitch mini-TWT to improve its interaction efficiency. By using the newly designed helix, a TWT is developed, which shows increases in electronic efficiency and output power up to 8.75% points and 2.6 dB (75 W), respectively. Further application of this method to another uniform pitch baseline tube results in similar improvements.

Analytical Exploration of Ultrawideband Helix Slow-Wave Structures Using Multidispersion Phase Velocity Taper

The concept of broadbanding of a helical slow-wave structure using ldquomultidispersionrdquo phase velocity taper has been analytically explored, following Pierce's small-signal analysis in order to form a physical basis behind this approach. Furthermore, large-signal analysis has been used to make out the efficacy of the physical basis arrived at from the small-signal analysis. Useful inferences are also drawn on the flattening of gain- and power-frequency responses and the reduction of second-harmonic content, using multidispersion phase velocity taper.

Improvements in Performance of Broadband Helix Traveling-Wave Tubes

The effectiveness of positive phase velocity tapering to improve the performance of a broadband helix traveling-wave tube (TWT) has been investigated. A large-signal model of an existing 4.5-18 GHz two-section mini-TWT with uniform pitch has been validated against the experimental results, and used as a starting point to design and develop a number of TWTs with different taper lengths. Comparisons between the experimental results of uniform pitch TWT and positively tapered TWTs show significant improvements in output power, electronic efficiency, and second harmonic content throughout the operating frequency band for the positively tapered TWTs. Increases in electronic efficiency and output power up to 5.2% points and 50 W (1.7 dB), respectively, have been achieved.

Efficiency Improvement of Broadband Helix Traveling Wave Tubes Using Hybrid Phase Velocity Tapering Model

A hybrid phase tapering model is proposed for a wideband (two-octave) vane-loaded helix traveling wave tube, of which the dispersion characteristic is anomalous over the operating frequency band. The small signal gain and electron efficiency is studied by using a novel nonlinear one-dimensional beam-wave interaction theory. Compared with a conventional helix traveling wave tube, of which the helix pitch is held constant in the output section, this hybrid phase velocity tapering model, which can be realized by changing the helix pitch, can provide a wideband efficiency improvement, especially in the lower or higher frequency band. It is concluded from our results that in order to effectively improve the electron efficiency, dispersion shaping and helix pitch tapering should be taken into account simultaneously.

Efficiency Enhancement and Harmonic Reduction of Wideband Helix Traveling-Wave Tubes with Positive Phase Velocity Tapering

Using numerical simulations, a wideband (>1.5 octave) vane loaded helix traveling-wave tube (TWT) was studied, the output section of which is provided with a `positively' phase velocity tapered helix portion, in which the helix pitch is made to linearly increase with distance towards the output end for reasonably high efficiency values. As compared to a device with a conventional 'negatively' phase velocity tapered helix portion, which gives an efficiency hike only in a narrow band of frequency, the positively tapered device provided a wideband efficiency improvement. Also, over the band, it provided a higher efficiency and a lower second harmonic content than a device with a nontapered portion. Factors such as the beginning position of the taper from the input and the amount of taper were considered in the optimum design for improving efficiency and reducing the second harmonic content of the device.