Wide-band Traveling-wave Tube Research Articles

Dispersion Control and Size Enlargement of Helical Slow-Wave Structure by Double-Positive Metamaterial Assistance

A novel dispersion control technique, using I-shaped double-positive metamaterial (MTM) as the helical slow-wave structure (SWS) support, exhibiting low-loss, broadband response, has been proposed for wideband traveling-wave tubes (TWTs). The proposed SWS provided the desired flat-to-negative wideband dispersion characteristics giving higher interaction impedance than that is obtainable by conventional vane-loaded helical SWS. A coaxial transmission line-based simulation model has been used to retrieve the effective relative permittivity and permeability of the I-shaped MTMs. The electromagnetic analysis in the sheath-helix model was validated against the CST Microwave Studio simulation with respect to the dispersion characteristics within 14%–16% and interaction impedance versus frequency characteristics within < 12%. Moreover, the proposed structure can be made to exhibit positive, flat, or negative dispersion characteristics by controlling the dimensions of the I-shaped metal strips without altering the dimensions of the helical SWS and metal envelope of the structure. Furthermore, with the help of the proposed structure, the diameter of the structure could be increased without adversely affecting the shape of the dispersion characteristics and reducing the value of the interaction impedance. The structure will reduce the fabrication complexity at higher frequencies and make the structure suitable for high-power, high-gain, and high-efficiency wideband TWTs.

Multiple Dielectric-Supported Ridge-Loaded Rhombus-Shaped Wideband Meander-Line Slow-Wave Structure for a V-Band TWT

A multiple dielectric-supported ridge-loaded rhombus-shaped meander-line (MDSRL-RSML) slow-wave structure (SWS) is proposed for a V-band wideband traveling wave tube (TWT). The high-frequency and transmission characteristics of the SWS are investigated. The proposed structure can realize stable output via attenuator and special phase-velocity jumping. Particle-in-cell (PIC) results indicate that, for a 7 kV, 0.1 A sheet-beam, the average output power can reach 60 W at 60 GHz and a 3 dB bandwidth of 9 GHz, with the corresponding gain and electron efficiency of 30.8 dB and 17.2%, respectively. Compared with the dielectric-supported rhombus-shape meander-line (DS-RSML) SWS, the proposed structure has a wider bandwidth, higher gain, more stable structure, and better heat dissipation ability, which make it a good candidate source in millimeter-wave communications.

Study on linearization of Ka-band wideband traveling-wave tube amplifer

Linearizer is a key component in the millimeter-wave communication system, it plays an important role in improving the linearity performance of amplifier and communication quality. At present, the development of traveling-wave tube amplifier (TWTA) linearization technology cannot meet the application requirements of communication technology, therefore, the research of linearization technology is very important. In this paper we propose a kind of a new wide-band analog pre-distortion structure used to improve the nonlinear characteristics of Ka-band TWTA. The simulation results show that when the input power changes from −20 to 10 dBm in the frequency range of 26−30 GHz, the gain expansion of the linearizer is greater than 5.08 dB, and the phase expansion exceeds 64.81°. The linearizer and the TWTA are cascaeded for testing. The test results show that the gain compression and phase compression of center frequence is less than 3.12 dB and 2.31° respectively, and the third-order intermodulation (IMD3) improves significantly.

Investigation of angular log-periodic folded groove waveguide slow-wave structure for low voltage Ka-band TWT

In this paper, a novel angular log-periodic folded groove waveguide (ALFGW) slow-wave structure (SWS) has been investigated theoretically and experimentally for application in Ka-band traveling-wave tubes (TWTs). The dispersion relation for the ALFGW is derived analytically, and the dispersion characteristics are calculated for a Ka-band design. The designed SWS is fabricated using oxygen-free-copper that is silver electroplated. The measured cold-test parameters show good agreement with the simulation results, with S21 varying from −2.7 dB to −4.8 dB and S11 better than −13.6 dB over the frequency range of 30–38 GHz. Simulations of beam–wave interactions using a 4850 V and 0.4 A sheet beam with a high aspect ratio of 28:1 indicate an output power of 128 W, corresponding to a maximum gain and electronic efficiency of 18.1 dB and 6.6%, respectively. Due to the log-periodic form, a higher output power, higher efficiency, wider bandwidth, and lower operating voltage are achieved as compared to a TWT based on the conventional folded groove waveguide (FGW) SWS. These results show that the proposed ALFGW SWS has good potential for application in relatively high-power wideband TWTs.

Stacked dual beam electron optical system for THz integrated wideband traveling wave tube

In this paper, a stacked dual beam electron gun and the associated electron optical system are proposed. The stacked dual beam electron gun includes two compact focusing electrodes which help to achieve dual sheet beams. As an application of this dual beam electron gun, a 340 GHz integrated dual beam traveling wave tube (TWT) based on the staggered dual vane slow-wave structure (SWS) is also put forward. In order to reduce the length of the TWT, a novel input/output coupler is introduced. The overall transmission characteristics of the SWS structure together with the input/output couplers show a wide bandwidth covering a frequency range of 306 GHz to 360 GHz. Based on the parameters obtained for the integrated TWT, a stacked dual-beam electron gun with dual focusing electrodes is designed to achieve a beam current of 43 mA, a beam voltage of 21.4 kV, and a cross-sectional size of each beam of 0.3 mm × 0.08 mm. A uniform magnetic field of 0.52 T is utilized to focus the dual electron beams, and a beam transmission efficiency of 97.1% is achieved over a length of 50 mm. Finally, particle in cell simulation results show that the integrated dual-beam TWT can generate an output power of 5 W over the frequency range of 315 GHz to 350 GHz, with the maximum output power of 24.5 W at 330 GHz.

Ring-Double-Bar Slow Wave Structure for High Power TWTs

The electromagnetic properties of a ring-double-bar (RDB) slow wave structure are investigated. Use of two bars to connect neighboring rings instead of one as in a conventional ring-bar (RB) structure leads to the splitting $E$ - and $H$ -waves, with the $H$ -wave concentrated in thin layers near the rings. This effect leads to the flattening of the dispersion characteristic, similar to that in a helix with longitudinal vanes. Prototype RDB structures have been fabricated, measured, and compared with the conventional helix and RB circuits. Analysis, simulation, and cold test measurements demonstrate that the use of an RDB structure may make it possible to increase the maximum stable operating voltage of wideband traveling-wave tubes to 30–40 kV, enabling an order of magnitude increase in output power without the danger of backward wave oscillation.

Experimental Validation of Slow-Wave Phenomena in Curved Ring-Bar Slow-Wave Structure

Curved ring-bar (CRB) slow-wave structure (SWS) has been presented before in order to design an SWS for high-power and wideband traveling-wave tube in S-band. It was analyzed using the coupled transmission line theory and predicted to deliver 1-MW output power across 1.8–2.4-GHz bandwidth. However, the slow-wave characteristics were not experimentally validated through measurements. In this paper, we present $\omega -\beta $ measurement results to experimentally demonstrate that the CRB structure is providing the predicted slow-wave characteristics at the S-band. A novel synthetic technique is used to determine the $\omega -\beta $ relation using a six-period fabricated CRB structure. The measurement results exhibit a phase velocity of $0.7c$ – $0.75c$ across 2–2.5 GHz with a maximum error of less than 5%. In addition, the measured on-axis interaction impedance was $>43\Omega $ across the specified frequencies implying satisfactory agreement with theoretical predictions.

Application of absorbers with frequency-dependent attenuation in high-power wideband traveling-wave tubes

It is proposed to use frequency-dependent absorbers, which have significantly higher attenuation at the low frequency than at the high one for extension of the bandwidth of high-power wideband helical traveling-wave tubes (TWTs) toward lower frequencies. Calculations carried out for a TWT with an operating frequency band of more than two octaves show that, due to this modification, the output power at lower frequencies increases and the level of the second harmonic decreases.

$W$ -Band Multiple Beam Staggered Double-Vane Traveling Wave Tube With Broad Band and High Output Power

A design study for a high-power, high-efficiency, high-growth-rate wideband traveling wave tube (TWT) in $W$ -band using a staggered double-vane slow-wave structure (SWS) combined with three plan alignment pencil beams is described in this paper. The electromagnetic characteristic simulation shows that it has a wide bandwidth, high interaction impedance (about two to three times higher than those of the same structures with the sheet beam scheme), and a more simply designed input/output coupler. 3-D particle-in-cell simulations predict that the TWT can produce over 2000 W of output power from 91 to 95 GHz just using a 52-period two section SWS with a total length of 70.3 mm when the voltage and current of three pencil beams are set to 22 kV and $140\times 3$ mA, respectively. The maximum peak output power is about 2256 W with a corresponding gain of 43.5 dB and an electronic efficiency of 12.2% at 94 GHz. The 3-dB bandwidth can be achieved at about 15 GHz with an instantaneous relative bandwidth of about 15.9%. Finally, the comparisons of sheet beam, multiple beam, and single pencil beam staggered double-vane TWT are presented and analyzed.

TRANSMISSION LINE THEORY AND THERMAL ANALYSIS OF COAXIAL COUPLER FOR TWTS

This paper presents the transmission line analysis, thermal and structural analysis of a multi-section coaxial coupler (MSCC) used in traveling-wave tubes (TWTs) to handle high average power over a wide frequency range. Power transmission through coupler from the device demands very good matching between load and source impedances such that low voltage standing wave ratio (VSWR) is achieved. Modeling of the MSCC for wideband TWTs in commercially software packages takes very long iteration time even in high-end computers. An analytical approach has been developed to model MSCC which takes less iteration time even in ordinary computer. Analytical results have been compared to those obtained from CST microwave studio. Finally, thermal and structural analysis has been carried out to study the thermal aspects for handling high average RF power.

Double Corrugated Waveguide for G-Band Traveling Wave Tubes

A novel wide-band traveling wave tube (TWT) based on the double-corrugated waveguide (DCW) is proposed for operating in IEEE G-band (110-300 GHz). The DCW has been conceived for relaxing critical technological issues in terahertz vacuum tube realization, such as assembly, and to support a cylindrical electron beam. The study of the properties of the DCW in the forward wave regime demonstrates wide band performance suitable for TWTs fabrication at millimeter wave-terahertz frequencies. A traveling wave tube with 18-dB gain and up to 3.7 W output power over a bandwidth of about 30 GHz at 225 GHz as central frequency is demonstrated, assuming 13-kV beam voltage and 30-mA beam current.

Wideband traveling-wave tube for a X/Ku-band vacuum solid-state amplifier

Results of the numerical simulation of the slow-wave structure and the electron-optical system of a wideband continuous-wave traveling-wave tube with an output power of 250 W and gain of 23 dB, which is designed for operation in an X/Ku-band vacuum solid-state amplifying module, are given.

Bi-Helix SWS for High-Power TWTs

We have studied the electromagnetic properties of bi-helix slow-wave structures (SWSs) with added azimuthal conductors in the form of: 1) metal rings connecting the helices and 2) an azimuthally conducting screen formed by metal rings with and without capacitive gaps. We show that these added azimuthal conductors suppress backward-wave oscillation and enable stable high-voltage, high-power operation of traveling-wave tubes (TWTs). Prototype circuits were fabricated and the dependence of phase velocities of the in-phase (fundamental) and opposite-phase (BWO) modes on frequency and on the geometrical parameters of the rings were measured and analyzed. Novel, easily fabricated designs of bi-helix SWSs with circular metallization on diaphragm supports are proposed. These practical designs will make it possible to increase the maximum stable operating voltage of wideband TWTs to 30-40 kV, with a corresponding increase in output power without the danger of BWO.

A method for reducing the second harmonic level in wideband traveling wave tubes

To reduce the second harmonic level and enhance the efficiency and stability against backward-wave self-excitation of wideband helical traveling-wave tubes containing a screen with longitudinally conducting edges, it is suggested that the gap between edge ends and a helix in the output section be gradually increased toward the energy output, the helix pitch being constant or slightly varied for optimizing the efficiency. It is shown that the increase in the gap reduces the second harmonic level and enhances the efficiency and the start current of backward-wave self-excitation.

Investigation of a wideband folded double-ridged waveguide slow-wave system

The folded double-ridged waveguide structure is presented and its properties used for wide-band traveling-wave tube are investigated. Expressions of dispersion characteristics, normalized phase velocity and interaction impedance of this structure are derived and numerically calculated. The calculated results using our theory agree well with those obtained by using the 3D electromagnetic simulation software HFSS. Influences of the ridge-loaded area and broad-wall dimensions on the high frequency characteristics of the novel slow-wave structure are discussed. It is shown that the folded double-ridged waveguide structure has a much wider relative passband than the folded waveguide slow-wave structure and a relative passband of 67% could be obtained, indicating that this structure can operate in broad-band frequency ranges of beam-wave interaction. The small signal gain property is investigated for ensuring the improvement of bandwidth. Meanwhile, with comparable dispersion characteristics, the transverse section dimension of this novel structure is much smaller than that of conventional one, which indicates an available way to reduce the weight of traveling-wave tube.

A Ka-band folded double-ridged waveguide traveling-wave tube

A novel slow-wave structure (SWS)， the folded double-ridged waveguide structure， is presented and its properties used for wide-band traveling-wave tube (TWT) are investigated. A Ka-band folded double-ridged waveguide TWT is modeled by Magic 3D and the beam-wave interaction is simulated and analyzed. Within the same beam condition， it is concluded from the comparison of folded double-ridged waveguide TWT and folded waveguide TWT that: in the condition of similar gain maximum， the folded double-ridged waveguide TWT has wider bandwidth of which is 22%， and higher electron efficiency of which is 9％.

A SIMPLE ANALYSIS OF HELICAL SLOW-WAVE STRUCTURE LOADED BY DIELECTRIC EMBEDDED METAL SEGMENTS FOR WIDEBAND TRAVELING-WAVE TUBES

A simple fleld analysis was developed for helical slow- wave structure symmetrically supported by rectangular shaped discrete dielectric support rods partially embedded in the metal segments projecting radially inward from a metal envelope for wideband traveling-wave tubes. The tape helix model was used for the prediction of the dispersion relation and the interaction impedance characteristics. The closed form simplifled expressions are obtained by combining the tape model dispersion relation for free-space helix and the dielectric loading factor obtained for the loaded helix in the sheath model. The dispersion characteristics and the interaction impedance characteristics obtained by the present analysis were compared with other more

A high-power wideband traveling-wave tube with stepwise changes in the diameter of the drift channel

Results of the design of a high-power wideband (in the band 8–18 GHz) traveling-wave tube (TWT) with stepwise changes in the diameter of the drift channel are presented. The TWT is stable against self-excitation by a backward wave at high values of the accelerating voltage. It is shown that the output section of the device can be increased by a factor of 1.5 and the operating current can be raised by a factor of 1.3. These increases are attained owing to the 1.27-GHz diversity of the π-type frequencies of sections with different values of the channel diameter and owing to growth of the starting current corresponding to self-excitation by the backward wave. Application of the TWT with the interaction space that has steps of the channel diameter improved the electron efficiency by a factor of 1.2 and increased the output power by a factor of 1.6.

Influence of variations in the phase velocity and coupling impedance under the absorbing layer on the output characteristics of a wideband traveling-wave tube

The electrodynamic parameters (coupling impedance, slowing factor, and damping factor) of a wideband traveling-wave tube (TWT) are calculated as functions of the surface resistance of the absorbing layer and of the frequency. The data obtained are approximated by quadratic polynomials within a wide range of values of the parameters, and a PC program is developed for computing the coupling impedance and slowing factor in the absorbing layer as functions of the longitudinal distribution of the damping factor and of frequency. The effect of these dependences on the output and statistical characteristics of a wideband TWT is analyzed. It is shown that the maximum effect is produced at the long-wavelength edge of the operating frequency range.

Spatial and frequency selection of waves in pseudoperiodic slow-wave structures

The principles of the joint selection of spatial harmonics and the extension of passbands of pseudoperiodic slow-wave structures (SWSs) are considered. Relationships for calculating the continuous spectrum of spatial harmonics are presented. Recurrence formulas for determining the input reflection coefficient from an SWS section are derived and used for the analysis of frequency characteristics. Selection of spatial harmonics and transformation of passbands in pseudoperiodic folded SWSs (a folded waveguide, an interdigital structure, etc.) are investigated. The possibility of extraction of either the first or the fundamental spatial harmonic in a frequency band from 30% to one octave and simultaneous suppression of spurious harmonics, including backward waves, is demonstrated. Owing to this feature, pseudoperiodic SWSs can be used in high-power wideband traveling-wave tubes.