High-power Traveling-wave Tube Research Articles

Design of a Low Voltage and High Power Traveling Wave Tube Based on a Sheet-Beam Rectangular Ring-Bar Slow-Wave Structure

Design of a Low Voltage and High Power Traveling Wave Tube Based on a Sheet-Beam Rectangular Ring-Bar Slow-Wave Structure

A Novel Helical Slow-Wave Structure of a TWT With Increased Number of Helix-Support Rods for the Suppression of BWO Caused by Thermal Deformation

Backward-wave oscillation (BWO) is one of the important factors affecting the output power and operating stability of helix traveling-wave tubes (TWTs). A novel helical slow-wave structure (SWS) with six helix-support rods at the end of SWS of TWT is proposed to suppress BWO caused by thermal deformation. An electrical–thermal–mechanical finite element coupling analysis model for the TWT is established. The simulation analysis shows that the maximum temperature at the end of the helix is reduced by 49 °C, the maximum thermal deformation is reduced by 56%, and the saturated output power at 11 GHz is increased from 464 to 519 W. The analysis confirms that the novel SWS can effectively suppress BWO caused by thermal deformation in a high-power TWT and improve the TWT’s output power and operating stability.

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.

Cold Characteristics of Overmoded Ridge- and Groove-Loaded Folded Waveguides for High-Power Traveling Wave Tubes

The TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> high-order mode operation characteristics of the ridge-loaded (RL) and groove-loaded (GL) folded waveguide slow wave structures (FWG SWSs) with two electron beam channels are studied for high-power traveling wave tubes (TWTs) at the terahertz (THz) frequency regime. Operating in a higher order mode means that the SWSs can have larger structure dimensions to reduce the fabrication difficulty at the THz frequency band and use higher current to increase the output power. For FWG SWSs operating in the high-order mode, it is difficult to suppress backward wave oscillation (BWO) of fundamental mode. In this article, we proposed the RL and GL FWG SWSs to suppress BWO and studied the dispersion, coupling impedance, and transmission loss of them. The results show that the RL structure can substantially increase the start oscillation length of the interaction circuit and avoid using many circuit sections for spurious mode depression which ease the assembly process of the tube. The GL structure can broaden the bandwidth with high-order mode operation but with the ease of BWO of fundamental mode. The straight RL and GL waveguides are used to explain the above results, and the effect of electron beam channel position on coupling impedance is also studied.

Accurate and Efficient Thermal Analysis of Slow Wave Structures for Helix Traveling-Wave Tubes by Finite-Element Method

In slow wave structures (SWSs) for helix traveling-wave tubes, both thermal conductivities (TCs) of materials and thermal contact resistances (TCRs) are temperature-dependent, which have a great influence on the heat dissipation of helix SWSs. This article presents an accurate and efficient thermal analysis method of helix SWSs by finite-element method (FEM). In this method, a novel contact boundary condition (CBC) approach is proposed. This CBC approach can easily integrate various TCR models to simulate temperature-dependent TCRs accurately and conveniently. Specially for double nonlinearities arising from temperature-dependent TCs and TCRs within helix SWSs, a fast nonlinear solver named FMONS is also proposed to significantly improve the efficiency of the thermal simulation for helix SWSs. Based on this method, we further developed a 3-D thermal design tool for helix SWSs called TS2. Different from all the present FEM codes, for example, ANSYS, our TS2 can directly model temperature-dependent TCRs. By simulating various helix SWSs including a global model of the entire helix SWS, it is found that our TS2 has very high accuracy and efficiency in the thermal analysis of helix SWSs, which would be very useful for the design of high-power helix traveling-wave tubes.

Design of a low-gain high-power W-band sheet-beam traveling wave tube using a double-staggered grating slow wave structure

In order to further improve the output power of W-band traveling wave tube (TWT), a low gain TWT was proposed as a secondary amplifier. It uses a double staggered grating slow wave structure (DSG-SWS) and a sheet beam. Firstly, the design of DSG-TWT was given. Through simulation, the DSG-TWT could provide a saturation output power of more than 1000 W in the frequency range of 93–97 GHz, and the maximum output power could reach 1200 W at 95 GHz. No oscillation and current interception were found. Then DSG-SWS was fabricated by E-plane partition method using high-precision CNC milling. The SWS was fed with a high-power input-output coupler which consists of a mode converter and an impedance transformer. Simulation and cold-test results were presented, showing that the S11 is less than −10 dB and the voltage standing wave ratio (VSWR) is less than 2.0 in the frequency range of 93–100 GHz.

A New Amplification Regime for Traveling Wave Tubes With Third-Order Modal Degeneracy

Engineering of the eigenmode dispersion of slow-wave structures (SWSs) to achieve desired modal characteristics, is an effective approach to enhance the performance of high power traveling wave tube (TWT) amplifiers or oscillators. We investigate here for the first time a new synchronization regime in TWTs based on SWSs operating near a third order degeneracy condition in their dispersion. This special three-eigenmode synchronization is associated with a stationary inflection point (SIP) that is manifested by the coalescence of three Floquet-Bloch eigenmodes in the SWS. We demonstrate the special features of "cold" (without electron beam) periodic SWSs with SIP modeled as coupled transmission lines (CTLs) and investigate resonances of SWSs of finite length. We also show that by tuning parameters of a periodic SWS one can achieve an SIP with nearly ideal flat dispersion relationship with zero group velocity or a slightly slanted one with a very small (positive or negative) group velocity leading to different operating schemes. When the SIP structure is synchronized with the electron beam potential benefits for amplification include (i) gain enhancement, (ii) gain-bandwidth product improvement, and (iii) higher power efficiency, when compared to conventional Pierce-like TWTs. The proposed theory paves the way for a new approach for potential improvements in gain, power efficiency and gain-bandwidth product in high power microwave amplifiers.

Design of two stage depressed collector for high-power traveling wave tube

This paper presents a method for design a suitable two stage depressed collector of X band large duty ratio coupled-cavity traveling wave tube (CC-TWT). The axial, radial velocities and radius of 96 electron trajectories in the output potion of this TWT were simulated and analyzed by 2.5 dimensional interaction package owned by 772nd factory. A refocusing segment was designed to improve the flow of electron injection layer with the help of re-simulation of these parameters. Then a two stage depressed collector was designed with optical design, structural design and thermal analysis. Finally, a TWT was assembled for testing. The measured proportion of the current distribution in every collector electrode and the collector efficiency are close to the analytical full-band average results, which means that this analytical method is suitable for design of two stage depressed collector for high-power CC-TWT.

Propagation of Degenerate Band-Edge Modes Using Dual Nonidentical Coupled Transmission Lines

We present a novel propagation medium using a pair of non-identical coupled transmission lines. The medium is referred to as `butterfly' structure and is composed of four coupled transmission lines. These four coupled transmission lines generate higher-order dispersion modes by coupling the modes supported on each of the transmission lines. When these transmission line pairs are placed inside a circular waveguide, they support a strong TM01-like Degenerate Band Edge mode. This property can be used to enhance coupling of the electron beams to RF waves for high-power Traveling Wave Tubes. In this paper, we present the slow wave characteristics and Degenerate Band Edge mode formation using the coupled pair of transmission lines.

Study of three-dimensional nonlinear backward-wave interaction model and numerical simulation for helical traveling wave tube

The wide band high power traveling wave tubes (TWTs) employed in radar, communication systems, etc. are always facing the backward wave oscillation (BWO) problem. However, it takes much time and computer resource to simulate BWO by the large electromagnetic software. Thus, several parametric models are developed to solve the problem faster. Most of those models do not discuss the saturated oscillation power. In this paper, a three-dimensional (3D) nonlinear backward-wave interaction model is presented, by which the BWO phenomenon can be accurately studied in TWTs and the oscillation power is also analyzed. This model is established with the equation of 3D excitation fields combined with 3D motion equations and 3D space charge force. The oscillation frequencies and the start-oscillation lengths are calculated by one-dimensional (1D) and 3D models, respectively, and they are carefully compared in the cases of with and without the space charge force, indicating that the space charge force in 1D model is much weaker than in 3D model. The reason for that is the model of current density for space charge model in 1D model is supposed to be proportional to particle radius, but the one in 3D model is almost uniform, which is indicated by 3D beam trace distribution analysis. The BWO saturated powers and the oscillation frequencies are studied by this nonlinear 3D backward-wave interaction model. The simulation results show that the BWO saturated power increases as the beam-wave interaction length extends before many trajectories intercept the helix. While the oscillation frequencies decrease, the large saturated power supplies more energy to the beam at the very beginning in beam-wave interaction starting region. Then the BWO suppression induced by the magnetic field effect of the beam ripple is also under consideration. As the magnetic force increases, not only some cross area of interaction beam is suppressed, but also the interaction impedance of -1 space harmonic decreases. So increasing magnetic field strength can obviously reduce BWO, while the effect on forward wave interaction should be balanced. Finally, a Ka-band tube is used to validate the 1D and 3D nonlinear backward-wave interaction models. The BWO frequencies at different voltages are compared among the experimental results and the calculations by 1D and 3D models. The results from the 3D model in the test voltage range are 4.8% lower than the experimental data, while the difference from the results of the 1D model is 6.7%. The 3D model seems to be more accurate than the 1D model.

Design of Coating on Helix for High-Power Traveling Wave Tube

Helix as a key component of traveling wave tube (TWT) for amplifying microwave signal is traditionally made of tungsten, because tungsten has high melting point and good mechanical strength. However, high resistivity and poor thermal conductivity of tungsten limit the further development of high-power TWT. Here, we design and synthesize Cu/Au composite film on the surface of helix based on electromagnetic wave transmission mechanism. The output power of coated helix TWT was obviously improved. The thickness and thermal reliability of Cu/Au composite film having important effects on the output power of TWT were investigated.

Curved Ring-Bar Slow-Wave Structure for Wideband MW-Power Traveling Wave Tubes

A coupled transmission line (TL)-based slow-wave structure is presented and characterized. This TL is a modified ring-loop structure referred to as curved ring bar. It inherits the features of the ring-loop structure, but includes elliptic loops to enhance the axial electric field for TM01 mode. The structure demonstrates wideband behavior and is especially designed for high-power traveling wave tube amplifiers. It is characterized using a pair of uniform and identical coupled TLs utilized to achieve desired dispersion characteristics. This characterization explains the dispersion curves and interaction impedance. A simple way to derive dispersion behavior in terms of circuit parameters is also provided. A traveling wave tube (TWT) model is also presented that gives an output power of 1 MW across 1.8–2.4 GHz. It is noted that the designed TWT provides a bandwidth of 20% and delivers an output power of 1 MW, larger than previously reported in S-band.

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.

An Overmoded W-Band Coupled-Cavity TWT

A 94-GHz overmoded traveling-wave tube (TWT) has been designed, fabricated, and successfully tested. The TWT operates in the rectangular TM31 mode of the cavity, while lower order modes are suppressed using selectively placed strips of lossy dielectric. The 87-cavity TWT circuit was directly machined from Glidcop, a dispersion-hardened copper. The TWT was tested in a 0.25 T solenoidal magnetic field in 3- $\mu \text{s}$ pulses. Operating at a voltage of 30.6 kV with 250 mA of collector current, the TWT was zero-drive stable and achieved 21 ± 2 dB linear device gain with 27-W peak output power. Taking into account 3 dB of loss in both the input and output coupling circuits, the gain of the TWT circuit itself is 27 ± 2 dB with 55 W of saturated circuit output power. Using the 3-D particle-in-cell code CST Particle Studio, the linear circuit gain was estimated to be 28 dB and the saturated output power 100 W, in good agreement with the experimental results. The measured bandwidth of 30 MHz was significantly smaller than the predicted value of 250 MHz. The overmoded TWT is a promising approach to high-power TWT operation at W-band and to the extension of the TWT to terahertz frequencies.

Thermal management of helix used in high-power high-efficiency traveling-wave tubes

In this paper, an approach has been presented to estimate thermal contact resistance (TCR) between helix slow-wave structure to support rod and support rod to metal envelope to study the effective heat dissipation from the helix. Based on this method, TCRs are optimized by comparing analytically and experimentally estimated helix temperature with those obtained from ANSYS simulation. Helix temperature, at a given power loss in the helix, is obtained by varying the fraction of effective resistance between helix and support rod for a given fraction of effective resistance between support rod to barrel envelope and vice versa. This helix temperature has been compared with the measured temperature to predict the TCRs.

A novel 140-GHz sheet-beam folded-waveguide traveling-wave tube

A novel folded-wave guide (FWG) slow-wave structure (SWS) with sheet-beam tunnel is presented for the high-power and considerable bandwidth in terahertz (THz) wave band. Its electromagnetic characteristics and nonlinear interaction between the electron beam and the electromagnetic field were investigated. A novel input and output coupler was proposed that could produce good input and output matches. The particle-in-cell simulation results revealed that the tube can be expected to produce over 100 W of average power in the range from 141 to 146 GHz, assuming an RF input signal with an average power of 45 mW. Compared with the conventional folded-waveguide traveling-wave tube (FWGTWT) at 140 GHz, the novel FWGTWT had a 48% higher output power, and the novel FWGTWT can operate at same frequency band with larger dimensional parameters. Meanwhile, the novel FWG was also much shorter than the normal FWG with good performance at the working frequencies. It, therefore, will favor the miniaturized design of a high-power terahertz wave band traveling-wave tubes (TWT).

Simulation of the output characteristics of high-power helical traveling-wave tubes

A numerical simulator of the interaction processes in helical traveling-wave tubes is developed on the basis of the method of large particles in the 1D approximation with the use of the calculation methods developed previously. The output characteristics (frequency dependences of the output power and gain factor) of high-power traveling wave tubes based on helical slow-wave structures for operation in the C and Ku wave bands are studied theoretically and experimentally. It is shown that the results of numerical simulation are in satisfactory agreement with experimental data.

V-Shape Folded Rectangular Groove Waveguide for Millimeter-Wave Traveling-Wave Tube

A novel slow-wave structure called V-shape folded rectangular groove waveguide is proposed to develop high-power, wide-bandwidth, and high-efficiency millimeter-wave traveling-wave tube. This structure evolves from a conventional rectangular groove waveguide bending the groove with V-shape along its longitudinal direction, which can operate with the sheet electron beam. In this paper, the electromagnetic characteristics and the beam-wave interaction of this structure operating in the V-band are obtained. From our calculations, this circuit structure possesses low transmission loss and can produce an output power of over 900 W ranging from 56 to 65 GHz when the cathode voltage and beam current are set to 12.8 kV and 500 mA, respectively. The corresponding gain and electron efficiency can reach over 36.5 dB and 14%, respectively.

Design and simulation of 140 GHz high power staggered double vane traveling-wave tube

Staggered double vane slow wave structure (SWS) and sheet electron beam are employed to investigate a 140 GHz high power traveling wave tube. Numerical calculation of eigenmode shows that the SWS has a good characteristic of dispersion and interaction impedance. The transition structure, input/output coupler and concentrated attenuator are especially proposed for the circuit to ensure that the tube will work well. Particle-in-cell simulation results demonstrate that the traveling wave tube can provide over 300 W of peak power in a frequency range of 132-152 GHz with a maximum of 546 W and a corresponding gain of 37.37 dB at 138 GHz assuming a beam power to be 5.115 kW and input power to be 0.1 W. The output power of the tube can exceed 440 W in a frequency range of 128-152 GHz with a corresponding interaction efficiency of over 8.6% when the input powers range from 0.027 W to 0.46 W. Such a traveling wave tube has a great significance and a potential application in high power short millimeter wave field.

V波段大功率带状注曲折波导行波管

V波段大功率带状注曲折波导行波管