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.

In-Orbit Reliability Evaluation of Space TWTA Based on Copula Function and Bivariate Hybrid Stochastic Processes

Currently, it is still a challenge to study the degradation mechanisms of the space traveling wave tube amplifier (TWTA) with no failure and small sample tests. Given that the Copula functions are used to describe the correlation of multiple performance characteristics, this paper develops a bivariate hybrid stochastic degradation model to evaluate the in-orbit reliability of TWTA. Firstly, based on the impact analysis of the life of TWTA, helix current and anode voltage are selected as the performance degradation parameters. Secondly, stochastic processes with random effects based on the one-dimensional Wiener process and Gamma process are applied to describe the degradation of TWTA’s helix current and anode voltage, respectively, and the corresponding marginal distribution function is obtained. Then, the Copula function is utilized to describe the correlation between two different performance parameters of TWTA. Meanwhile, this paper also proposed a two-step method to estimate the reliability level of TWTA based on its in-orbit telemetry data through a two-step method, which contains a Markov Chain Monte Carlo (MCMC) algorithm and a maximum likelihood estimation (MLE) algorithm. Besides, the Bayes-Bootstrap sampling method is also used to improve the evaluation accuracy to overcome the defect of an in-orbit small sample of TWTA. Finally, a TWTA degradation case with a set of telemetry data is carried out, and the results show that the method proposed in this paper is more applicable and more accurate than other methods.

Experiments on a Disk-on-Rod Traveling Wave Tube Amplifier Driven by a Nonlinear Transmission Line Modulated Electron Beam

Experiments have demonstrated high-power amplification (100 MW class) from a traveling wave amplifier driven by a modulated electron beam. The modulated electron beam is generated by applying a modulated voltage pulse from a nonlinear transmission line (NLTL) to the cathode of the electron gun. The system showed that over the designed operating range of the disk-on-rod (DoR) TWT amplifier (1.2–1.4 GHz), the peak frequency of the amplifier output tracked the peak frequency of the NLTL to within ±2%; however, the output was not phase stable. Measured RF energy gain for this frequency regime ranged from 1.5 at 1.2 GHz up to 5.0 at 1.4 GHz).

Circular TE₀ₙMode Content Analysis Technique for High-Power Gyro-TWT System

A circular TE_0<i>n</i> mode content analysis technique based on the multi-mode coupler with high mode selectivity is proposed. To improve the mode selectivity, a deformed waveguide, entitled the quad-polar waveguide, which discriminates the azimuthally symmetric TE-mode on the waveguide wall, is introduced. The influence of deformation on the TE₀₁ and TE₀₂ modes&#x2019; field pattern, the corresponding transmission loss, and the power capacity in the quad-polar waveguide are analyzed. Based on the analysis, a dual-mode coupler of the circular TE₀₁ and TE₀₂ modes, consisting of an oversized deformed circular waveguide coupler and a pair of circular-to-quad-polar waveguide transitions, is designed for a high-power <i>Ka</i>-band gyrotron traveling-wave tube system. To verify the high mode selectivity of a quad-polar waveguide, a simple coupling with equal-radius and equal-interval holes are used. A prototype is fabricated and measured, and the results show that the coupling coefficient is &#x2212;40 dB with the directivity of 27 dB for the TE₀₁ mode, suppressing the unwanted mode TE₀₂ mode with 53 dB. For the TE₀₂ mode, the coupling coefficient is &#x2212;40 dB with the directivity of 25 dB and suppresses the unwanted mode TE₀₁ mode with 75 dB from 30 to 40 GHz.

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.

Broadband-Printed Traveling-Wave Tube Based on a Staggered Rings Microstrip Line Slow-Wave Structure

To increase the output power of microstrip line traveling-wave tubes, a staggered rings microstrip line (SRML) slow-wave structure (SWS) based on a U-shaped mender line (U-shaped ML) SWS and a ring-shaped microstrip line (RML) SWS has been proposed in this paper. Compared with U-shaped ML SWS and RML SWS, SRML SWS has a wider transverse width, which means SRML SWS has a larger area for beam–wave interaction. The simulation results show that SRML SWS has a wider bandwidth than U-shaped ML SWS and a lower phase velocity than RML SWS. Input/output couplers, which consist of microstrip probes and transition sections, have been designed to transmit signals from a rectangular waveguide to the SWS; the simulation results present that the designed input/output structure has good transmission characteristics. Particle-in-cell (PIC) simulation results indicate that the SRML TWT has a maximum output of 322 W at 32.5 GHz under a beam voltage of 9.7 kV and a beam current of 380 mA, and the corresponding electronic efficiency is around 8.74%. The output power is over 100 W in the frequency range of 27 GHz to 38 GHz.

Multiple electron beam generation with different energies and comparable currents from a single cathode potential for high power traveling wave tubes (TWTs)

We present a technique for generating multiple electron beams with different energies and comparable currents from a single cathode stalk at a single potential using nested magnetically insulated coaxial diodes (MICDs). The application is for a multi-stream traveling wave tube. Particle-in-cell simulations are performed for an experimental parameter-based geometry where two thin-walled intense relativistic electron beams immersed in a strong uniform magnetic field propagate through a cylindrical vacuum channel. The analytically derived results are obtained by extending Fedosov’s solution for generating a hollow electron beam from an MICD on a cathode stalk in an infinite magnetic field. Two electron beams are generated and accelerated downstream assuming zero initial kinetic energy of the electrons from the cathodes. Results show both electron beam currents ranging from 66 to 2.8 kA with an energy difference ranging from 6% to 27% depending on voltages applied from 100 to 600 kV and the geometry of the two MICDs. An optimal geometry is a crucial factor in achieving the maximum energy difference between the electron beams for comparable currents. The analytical and numerical simulation results show good agreement. We are currently in the process of planning experiments using the electron beam accelerator (SINUS-6 at UNM) to validate the analytical and simulation results.

Experimental and numerical investigation of traveling wave tube radial heat sink connector thermal stress and deformation with a focus on energy cost management

The present investigation concentrated on cooling high temperature traveling wave tube collectors under steady state conditions. The reference of power (high temperature) is a constant power flow (1.1 A) came from a traveling wave tube electron generator utilizing for a Klystron lamp performance. The purpose of the comprehensive research is to determine a proper microchannel number on the collector's geometry and picking a suitable initial temperature for the working fluid concerning structural stability for energy cost management. The idea of studying TWT collectors thermal stress and deformation is novel and has not investigated yet. Simulation outcomes presented a satisfactory agreement among the experimental results. By considering the number of channels (from 200 to 400) and working fluid temperature at 300 CFM, the length and width of microchannels, Nusselt and Reynolds number, required pumping power, pressure drop, friction factor, thermal deformation, and thermal stress studied. The investigation outcomes depicted that Kovar, Copper, and Alumina can be considered as the collectors and connectors manufacturing materials. Moreover, 25 °C is functional as the coolant temperature at 300 CFM.

Design of a Ka-Band Traveling Wave Tube Using Low Turn-On Field Emission Electron Source Made by Carbon Nanotubes

This article designed a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band traveling wave tube (TWT) with the single wall carbon nanotubes (SWCNTs) field emission cathode electron gun. The designed cold-cathode TWT was miniaturized and could instantly startup at room temperature because of the field emission cathode. First, SWCNT cathode was made by imprinting. The field emission measurement results showed that its turn-on field was as low as 0.57 MV/m, and the maximum emission current was about 10 mA. Then, a grid-controlled electron gun with planar field emission cathode was simulated based on the experiment results, which had low operation voltage of the electron gun and would prevent their grid mesh from burning. The optimized electron beam waist radius of the electron gun was 0.15 mm with the structure of the focusing magnetic field. According to experiment and simulation results, we preliminarily designed a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band helix TWT with an average output power of 8.6 W and a gain of 24.6 dB at 28 GHz. The designed miniaturized TWT with a low-operation-voltage field emission cathode electron gun is expected to be used in low-power applications including 5G communication.

Electron-Optic System With High Compression of a Multiple Elliptic Electron Beam for a Miniaturized THz-Band Vacuum Electron Device

Electron-optic systems (EOSs) with high compression are required for THz-band vacuum-tube electron devices to reduce the cathode load, which is necessary to increase lifetime and enable operation in a continuous-wave (CW) mode. However, it is difficult to achieve high compression of multiple electron beams. In this article, we present the design and simulation of EOS with triple elliptic electron beam for a 0.2-THz traveling-wave tube. The triode electron gun with planar electrodes provides <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times 0.06$ </tex-math></inline-formula> -A electron beam with compression factor of 16. At the cathode, the current density is 26.31 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> while in the beam tunnel it is higher than 400 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The magnetic focusing system with 1.34-T axial magnetic field consisting of Nd–Fe–B permanent magnets and pole pieces is also designed. The numerical simulation predicts stable beam transmission at 25-mm distance.

Performances of 2r16APSK and DVB-S2 16APSK Modulations over a Two-Link Satellite Channel

In this paper, the 2r16APSK modulation is contrasted&nbsp;with the 16APSK modulation adopted in the digital&nbsp;video broadcast standard, the DVB-S2, aiming at verifying if&nbsp;the 2r16APSK can be considered an alternative choice for the&nbsp;DVB-S2 and other alike communication systems when subjected&nbsp;to nonlinear distortions. To this end, the performances of both&nbsp;modulations are assessed in terms of the metrics: bit error&nbsp;rate (BER), constellation figure of merit (CFM), peak-to-average&nbsp;power ratio (PAPR), total degradation (TD) versus input back-off&nbsp;(IBO), and spectral regrowth, when the transmitted signal goes&nbsp;through a two-link satellite channel under memoryless nonlinear&nbsp;distortion produced by a traveling wave tube amplifier (TWTA),&nbsp;which is described by the Saleh model. The results show that the&nbsp;2r16APSK modulation is indeed an alternative choice.

A Novel Analog Predistortion Linearizer Based on a Schottky diode for Communication Applications

Background and Objectives: In order to compensate the nonlinearity, several linearization approaches such as feedback, feed-forward, and predistortion have been proposed. Among these linearization methods, the analog predistortion (APD) method is very suitable for the power amplifier and TWTA linearization, because it has a good compromise between complexity, linearity improvement and bandwidth. On the other hand, analog predistortion circuit can be added to existing amplifiers as a separate circuit. Many of the reported linearizer circuits are capable of linearizing power amplifiers with high gain and phase variations. However, these circuits are complex and bulky due to the use of distortion producer, phase shifter, adjustable attenuator, combiner/divider, and control unit. The objective of this paper is increasing the dynamic range of the predistorter by a simple proposed circuit. Hence, linearization capability of the linearizer is boosted.Methods: A new analog predistortion linearizer based on a coupler and a Schottky diode is presented. The proposed structure satisfies linearization requirements of the solid-state power amplifiers (SSPAs) and traveling wave tube amplifiers (TWTAs). Using a parallel combination of two Schottky diodes along with transmission line in port 2 and 3 of the coupler, the dynamic range of reflection coefficient of each port is increased compared to using a single diode in each branch. By increasing the range of reflection signal, the linearization capability has been increased to linearize power amplifiers and TWTAs. The AM/AM and AM/PM of the linearizer is controlled by changing the bias voltage of power supply.Results: The simulation results show that about 30° phase shift and 5 dB gain expansion can be compensated for power amplifiers and TWTAs. Because of utilizing two diodes, the linearization capability of the new linearizer is augmented compared to the conventional linearizer.Conclusion: A novel analog predistortion circuit based on the hybrid coupler and Schottky diodes is presented. The suggested topology increases the dynamic range of reflected power from each port. Using the proposed idea, AM/AM and AM/PM characteristics are expanded such that this characteristic can linearize the power amplifier with high linearity.

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.

A NEW PRE-DISTORTION APPROACH TO TWTA COMPENSATION FOR WIRELESS OFDM SYSTEMS

Orthogonal Frequency Division Multiplexing (OFDM) has several desirable attributes which makes it a prime candidate for a number of emerging wireless communication standards. However, one of the major problems posed by OFDM is its high Peak-to-Average-Power ratio (PAPR), which seriously limits the power efficiency of the High Power Amplifier (HPA) because of the nonlinear distortion resulting from high PAPR. The present paper presents a new mixed computational/analytical approach for compensation of this nonlinear distortion for the case in which the HPA is a Traveling Wave Tube Amplifier (TWTA) with time-varying characteristic. TWTAs are used in wireless communication systems when high transmission power is required as in the case of the digital satellite channel. Compared to previous pre-distorter techniques based on LUT (Look-Up Table) or adaptive schemes, our approach relies on the analytical inversion of the Saleh TWTA model in combination with a nonlinear parameter estimation algorithm. This leads to a sparse and yet accurate representation of the pre-distorter, with the capability of tracking efficiently any rapidly time-varying behavior of the TWTA. Computer simulations results illustrate and validate the approach presented.

Sources of High-Power Continuous-Wave Multi-Frequency Radiation for Plasma Applications Based on Gyroresonance Traveling-Wave Tubes With a Helically Corrugated Waveguide

Sources of High-Power Continuous-Wave Multi-Frequency Radiation for Plasma Applications Based on Gyroresonance Traveling-Wave Tubes With a Helically Corrugated Waveguide

Radiation Input/Output System in a Ten-Barrel W-Band Gyrotron-Type Traveling-Wave Tube With Helically Corrugated Waveguides

Radiation Input/Output System in a Ten-Barrel W-Band Gyrotron-Type Traveling-Wave Tube With Helically Corrugated Waveguides

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.

A Study of Generation of Broadband Chaotic Radiation in a W-Band Traveling-Wave Tube With Delayed Feedback

A Study of Generation of Broadband Chaotic Radiation in a W-Band Traveling-Wave Tube With Delayed Feedback

Investigation of a Miniaturized E-Band Cosine-Vane Folded Waveguide Traveling-Wave Tube for Wireless Communication

To realize the miniaturization of E-band traveling-wave tubes (TWTs), the size analysis and optimization design were carried out based on an improved cosine-vane folded waveguide (CV-FWG) slow-wave structure (SWS) that operates in a low voltage. In addition, a novel miniaturized T-shaped coupler was proposed to achieve a good voltage standing wave rate (VSWR) in a broad bandwidth. The coupler length was reduced by as much as 77% relative to an original design. With higher coupling impedance, the radius and length of the shortened SWS were optimized as 1.3 mm and 50 mm, respectively. Using microwave tube simulator suit (MTSS) and CST particle studio (PS), 3D beam–wave simulations at 9400 V, 20 mA predicted a gain of 20 dB and a saturated output power of 9 W. The simulation results for CV-FWG TWTs were compared with conventional FWG TWTs from 81 GHz to 86 GHz, showing significant performance advantages with excellent flatness for high-rate wireless communication in the future. The CV-FWG SWS circuit will be fabricated by 3D printing, and this work is underway.

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.