High Interaction Impedance Research Articles

Reversed Cherenkov radiation amplifier with compact structure and high efficiency

Based on the reversed Cherenkov radiation excited in metamaterials, a novel S-band reversed Cherenkov radiation amplifier (RCRA) with two output ports is proposed in this paper. Its metamaterial slow-wave structure (MSWS) consists of a hollow circular waveguide periodically loaded with all-metal double-ridge complementary electric split-ring resonators (CeSRRs). The CeSRR exhibits subwavelength and strong resonance characteristics, which lead to miniaturization and high interaction impedance of the MSWS. The period of the MSWS is optimized using the phase velocity jump technique to further increase the output power and electronic efficiency. The simulation results demonstrate that when the microwave signal of 7.8 W at 2.286 GHz is inputted at port 2, the output power at port 1 is 307 W, and the saturated output power and saturated gain at port 3 are 5.48 kW and 28.47 dB, respectively. Notably, the total electronic efficiency of the RCRA is predicted as 33.84%. The RCRA features high electronic efficiency and miniaturization and may meet special application scenarios that require two output microwave signals at the same frequency.

A Novel Folded Rectangular-Grating Slow Wave Structure for Dual-Beam Traveling Wave Tube

Increasing the input current is an important method to achieve the high-power output for the traveling wave tubes (TWTs) operating in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{W}$</tex-math> </inline-formula> -band and higher terahertz (THz) band. For the folded waveguide (FW) TWT, increasing the height of the straight waveguide section of slow wave structure (SWS) can realize a double tunnel to increase the input current, but its electromagnetic characteristics, such as the operating passband, interaction impedance, and ohmic loss, become worse. Thus, a novel SWS, called folded rectangular-grating (FRG), with two tunnels for more input current, was proposed. Its two beam tunnels, where the electromagnetic wave in the operating band is cutoff, are located on the upper and lower edges of the grating, respectively, which makes the FRG-SWS have a wider operating passband, higher interaction impedance, and lower ohmic loss by freely adjusting its grating height. More importantly, its structural complexity is the same as that of FW-SWS. First, the electromagnetic characteristics, including dispersion, interaction impedance, and ohmic loss, are analyzed. Then, the particle-in-cell (PIC) simulation was carried out to verify that the saturated power, gain, and electron efficiency of TWT using FRG-SWS have been significantly improved compared with the TWT using FW-SWS. In the end, the cold test of transmission characteristics for FRG-SWS was done, and the experimental results are basically in good agreement with the simulation ones when the conductivity is set as 2.25 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{7}}$</tex-math> </inline-formula> S/m.

Ka-Band Low-Voltage Multiple-Beam Mini-TWT

We describe the design, development, and initial testing of a very compact, multiple-beam folded-waveguide traveling wave tube (TWT) operating in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band at <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> 6 kV. A proof-of-principle four-beam, <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> 100 mA/beam device has been fabricated and tested, and a 16-beam version of the device has been designed and is predicted to produce an output power up to 1 kW. Here, we describe the design approach we devised and implemented to overcome two fundamental challenges: 1) generating and transporting the tightly spaced beams required to operate in the millimeter-wave regime and 2) developing folded waveguide circuits that can provide high interaction impedance for each of the beams as well as stable operation in a dense high-order-mode environment.

A Novel Staggered Double-Segmented Grating Slow-Wave Structure for 340 GHz Traveling-Wave Tube

In this paper, a novel staggered double-segmented grating slow-wave structure (SDSG-SWS) is developed for wide-band high-power submillimeter wave traveling-wave tubes (TWTs). The SDSG-SWS can be considered as a combination of the sine waveguide (SW) SWS and the staggered double-grating (SDG) SWS; that is, it is obtained by introducing the rectangular geometric ridges of the SDG-SWS into the SW-SWS. Thus, the SDSG-SWS has the advantages of the wide operating band, high interaction impedance, low ohmic loss, low reflection, and ease of fabrication. The analysis for high-frequency characteristics shows that, compared with the SW-SWS, the SDSG-SWS has higher interaction impedance when their dispersions are at the same level, while the ohmic loss for the two SWSs remains basically unchanged. Furthermore, the calculation results of beam–wave interaction show that the output power is above 16.4 W for the TWT using the SDSG-SWS in the range of 316 GHz–405 GHz with a maximum power of 32.8 W occurring at 340 GHz, whose corresponding maximum electron efficiency is 2.84%, when the operating voltage is 19.2 kV and the current is 60 mA.

An elliptical beam-tunnel sine waveguide slow wave structure for G-band elliptical beam traveling wave tube

In order to improve the performance of sine waveguide (SW) traveling wave tube (TWT), an elliptical beam-tunnel sine waveguide (EBTSW) slow wave structure (SWS) is proposed. EBTSW-SWS not only has high interaction impedance in a relatively wide frequency band but also has an elliptical beam-tunnel. The simulation results of high-frequency characteristics show that, under the same dispersion, the passband of EBTSW-SWS increases by 30% compared to the SW-SWS, and the average interaction impedance of EBTSW-SWS is higher than that of SW-SWS in almost the whole passband. Moreover, the particle-in-cell (PIC) simulation results reveal that under the operating voltage of 20.8 kV and beam current of 150 mA, EBTSW-TWT can provide more than 70 W of saturated output power in the frequency range of 200-260 GHz, reaching the maximum power of 151 W at 220 GHz with the maximum electronic efficiency of 4.85%.

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.

Design and Analysis of Truncated Velocity Tapered Sine-Waveguide Structure for Traveling-Wave Tube in THz Band

A truncated sine-waveguide traveling-wave tube (SWGTWT) is studied to obtain wideband amplification at 600 GHz. The cold test parameters are obtained and finalized through simulation to achieve minimum dispersion coupled with a high interaction impedance. The interaction impedance at various points of the beam tunnel cross section area is evaluated to choose the right shape of the electron beam for the TWT. The effect of velocity tapering on the SWG structure is studied. Particle-in-cell (PIC) simulation is carried out using cylindrical as well as sheet beams. The velocity-tapered structure exhibits higher power and efficiency. The maximum output power and 3-dB bandwidth are estimated to be 7 W and 40 GHz, respectively. The corresponding gain and electronic efficiency of the truncated SWGTWT are 25.4 dB, and 1.88%, respectively.

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 of a High-Efficiency Ka-Band TWT Power Amplifier for Radar Applications

The design of a high-efficiency traveling-wave tube (TWT) based on a nonconventional folded waveguide slow wave structure (FW-SWS) operating over 34–36 GHz for naval and ground Ka-band radars is presented. The ridge-vane-loaded FW structure allows a good compromise between high interaction impedance and wide bandwidth along with a low weight required for aircraft radar applications and a simple planar manufacturing process structure. Small-signal simulations are performed with a custom interaction impedance evaluation program. The CST suite 2021 3-D electromagnetic simulation code is used for the prediction of the dispersion diagram. Starting from a sensitivity analysis of the FW in terms of width, a high electronic efficiency value of 14.6% is obtained with a reduction in the width of 0.1 mm. To further improve electronic efficiency, a negative phase-velocity step tapering technique is applied and a value of 15.4% from 34 to 36 GHz is obtained. Finally, under an optimized beam voltage and a current of 21 kV and 550 mA, respectively, a peak value of the output power of approximately 1.7 kW and an output power of 800 W with a duty cycle of 12% over the frequency band are obtained. Measurement results of the interaction impedance of a ridge-vane-loaded FWG structure composed of a ten-period high-conductivity oxygen-free copper carried out according to the Langerstrom perturbation method confirm the goodness of the simulation results.

Compact reversed Cherenkov radiation oscillator with high efficiency

An all-metal metamaterial slow-wave structure (MSWS), which comprises a hollow circular waveguide loaded by periodically arranged complementary electric split-ring resonators (CeSRRs), is proposed. Due to the CeSRRs with the characteristics of both subwavelength and strong resonance, the MSWS has both compact size and high interaction impedance relative to conventional SWS. Based on the reversed Cherenkov radiation in the MSWS, a compact reversed Cherenkov radiation oscillator (RCRO) with high efficiency is developed. The measured electronic efficiency is 19.54%, and measured output power is 10.16 kW at 2.221 GHz under the conditions that the beam voltage is 26 kV, the beam current is 2.0 A, and axial magnetic flux density is ∼962 G. The experimental results clearly show that the brand-new RCRO can be applied to large-scale scientific facilities, medical imaging, microwave heating, microwave disinfection and sterilization, and so on.

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.

Investigation of Sine Groove Waveguide Slow Wave Structure for Terahertz Traveling Wave Tube

A novel sine groove waveguide slow wave structure (SGW-SWS), originated from the sine waveguide (SW), is proposed for a wideband terahertz (THz) traveling wave tube (TWT). SGW-SWS not only has high interaction impedance in a relatively wide frequency band, but also has a wide beam tunnel. In addition, it can be fabricated using the Nano-Computer Numerical Control (CNC) milling techniques. The high-frequency simulation results show that, under the same dispersion, the passband of SGW-SWS is 23 GHz wider than that of SW-SWS, and the average interaction impedance of SGW-SWS is higher than that of SW-SWS in the whole passband. And the transverse dimension of beam tunnel is independent of the dispersion characteristics of SGW-SWS, so the beam current can be further increased by enlarging the size of beam tunnel. Moreover, the particle-in-cell (PIC) simulation results reveal that the saturated power and electronic efficiency of SGW-TWT in the frequency range of 201–260 GHz are more than 84.9 W and 2.72%, respectively, when the operating voltage is 20.8 kV and the beam current is 150 mA.

A novel H-plane loaded on a double-staggered grating waveguide slow-wave structure for W-band traveling-wave tubes

A novel H-plane loaded on a double-staggered grating waveguide (DSGW) slow-wave structure (SWS) is proposed. The main advantage of this SWS is its high interaction impedance and low phase velocity, based on which higher output power can be expected. Electromagnetic characteristics and particle-in-cell simulations were performed using CST Microwave Studio software. The dispersion diagram of the SWS was optimized for a central frequency of 94 GHz which corresponds to a beam voltage of 13.2 kV in the second spatial harmonic (2 $$\pi$$ –3 $$\pi$$ ). A sheet electron beam with a current of 200 mA is used to interact with the longitudinal electric field. The reflection signal at the input port is below −15 dB for frequencies of 89–98 GHz and a small-signal gain of 35 dB is achieved at the output port for a tube length of 80 mm.

0.2-THz Traveling Wave Tube Based on the Sheet Beam and a Novel Staggered Double Corrugated Waveguide

A novel slow wave structure (SWS), named staggered double corrugated waveguide (SDCW), is proposed for the development of wideband high-power terahertz (THz) traveling wave tubes (TWTs). The SDCW-SWS not only has the wideband characteristic of staggered double grating (SDG) but also has the high interaction impedance characteristic of symmetrical double grating. In addition, its tunnel width is independent of the lower cutoff frequency and can be further increased. Numerical Eigen mode calculations show that, under the same dispersion, the passband of SDCW-SWS is increased by more than 10 GHz compared with SDG-SWS, and the average interaction impedance at 195 GHz is increased by about 62%. A particle-in-cell simulation analysis confirms that the average output power, saturated gain, and electron efficiency of SDCW-TWT at 195 GHz are ~110 W, ~20.34 dB, and ~5.46%, respectively, when the operating voltage is 20 kV and the beam current is 100 mA. Therefore, SDCW is a promising slow wave circuit of wideband high-power THz traveling wave amplifier.

A Novel Chevron-Shape Double-Staggered Grating Waveguide Slow Wave Structure for Terahertz Traveling Wave Tubes

A novel slow wave structure (SWS), called the chevron-shaped double-staggered grating waveguide structure, is proposed for traveling wave tube (TWT) amplifier for the terahertz (THz) region. The potential of this SWS is that it provides higher interaction impedance and has low phase velocity; thus, higher output power can be expected at a lower beam voltage. The electromagnetic characteristics and particle-in-cell (PIC) large-signal simulations were performed with CST Microwave Studio software. The SWS was optimized for the central frequency of 200 GHz, which synchronizes with a beam voltage of 18.8 kV in the second spatial harmonic ( ${2}\pi -{3}\pi $ ). A sheet electron beam with a current density of ${133}~\text {A/cm}^{{2}}$ was used to excite the structure, which interacts with the longitudinal electric field component to provide gain. The reflection coefficient at the port interface is less than −15 dB over the frequency range 181–220 GHz, and a small-signal gain of 30.98 dB is achieved for a tube length of 60 mm.

Development of a planar helix slow-wave structure based on ball-grid array technology

The fabrication process for a planar helix slow-wave structure (SWS) based on ball-grid array (BGA) technology is developed to be used in the traveling wave tube (TWT). The planar helix layers are structured on two printed circuit boards and the solder ball is used to connect each other upside down. The material properties of the used substrate and also the solder ball connection for being used by TWT are described. Additional to previous work, filled via paths are applied inner side of the RF-substrate as an enhancement of thermal management. A typical planar helix is modeled, and the parameters of the cold and hot tests are calculated in CST microwave studio. The simulation results show that the proposed structure has high interaction impedance and wide bandwidth. The fundamental space harmonic mode overlaps with a 5.5 kV electron beam in the dispersion diagram. The scattering parameters are reported for 100 periods of the proposed structure that 1−15 GHz frequency range has a minimum reflection. The maximum gain of 22 dB is achieved at 10 GHz frequency.

A Terahertz Traveling-Wave Tube Based on Defect Photonic Crystal Waveguide

A defect photonic crystal waveguide (DPCW) is presented as a slow-wave structure (SWS) for terahertz (THz) traveling-wave tube (TWT). Due to the characteristics of DPCW, the mode competition and backward wave are greatly depressed, improving the stability of TWT. The DPCW SWS has flat dispersion characterization and high interaction impedance at 220 GHz. Simulation results show that saturated output power of 12.1 W, corresponding to a gain of 24.3 dB, and the 3-dB gain ripple within 20 GHz can be achieved at the center frequency of 220 GHz. The DPCW-TWT has a low working voltage at 9.52 kV compared to that of folded waveguide (FWG) SWS in this frequency band. The proposed DPCW for the THz regime is easy to fabricate and assemble using the modern fabrication processing technology.

Investigation of Double Tunnel Sine Waveguide Slow-Wave Structure for Terahertz Dual-Beam TWT

Traveling wave tube (TWT) is an attractive solution for wideband and high-power sources or amplifiers in the terahertz (THz) band. In this article, a novel slow-wave structure (SWS) for TWT, named double tunnel sine waveguide (DTSW), which is based on sine waveguide (SW), is proposed. The following key technologies are adopted: 1) the rectangular holes, at which the operating mode is cutoff, are alternately opened on the sine-shaped ridges to form two electron beam tunnels and 2) the lower surface of rectangular holes is flush with the top of sine-shaped ridges. Compared with the SW-SWS, DTSW-SWS has the wider operating band and higher interaction impedance under the condition that the phase velocity of the two SWSs is at the same level. In addition, DTSW-SWS is simple in structure and easy to fabricate. The particle-in-cell simulation results reveal that DTSW-TWT has an output power of more than 202 W in the frequency range of 202–255 GHz with the maximum power of ~405 W and electron efficiency of ~9.8% at 215 GHz, under the operating voltage of 23 kV and beam current of 180 mA. Therefore, the DTSW should be considered as a promising SWS for the high-power and wideband THz traveling wave amplifier.

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.

Meander-Line Slow-Wave Structure for High-Power Millimeter-Band Traveling-Wave Tubes With Multiple Sheet Electron Beam

For advanced telecommunications and radar systems, high-power miniaturized millimeter-band traveling-wave tubes (TWTs) are required. In this letter, we propose a meander-line slow-wave structure (SWS) that is suitable for multiple sheet beam interaction that helps achieve high power. The simulation of cold electromagnetic parameters of the V-band SWS demonstrates good transmission characteristics, wide bandwidth, and high interaction impedance. Hot performance of the V-band TWT amplifier with the proposed SWS is also studied. The 3-D particle-in-cell (PIC) simulation predicts that the designed circuit yields a stable gain in the 50 – 70 GHz frequency range with a 25 – 30-dB small-signal gain. 250-W average output power is attained from a 1.5 – 2.0-W input driving signal at 68.1 GHz.