Helical Slow-wave Structure Research Articles

Integrated Couplers of THz Radiation for Helical Slow-Wave Structures.

We have designed an integrated device that couples input and output coaxial horn antennas with a self-assembled helical slow-wave structure cradled in a v-groove on a silicon-on-insulator wafer. These devices will potentially enable the characterization of cold parameters and beam-wave interaction in slow-wave structures on a wafer level, i.e., without having to dice and package individual devices. In addition, such vertically integrated antennas and helical slow-wave structures may form the basis of compact and low-cost traveling-wave tube amplifiers operating at THz frequencies.

Analysis of a New Metamaterial-Loaded Helical Slow-Wave Structure for Vacuum Electron Device Applications

Analysis of a New Metamaterial-Loaded Helical Slow-Wave Structure for Vacuum Electron Device Applications

Electroplating on Unconventional Ultra-Compliant Substrates for Travelling Wave Tube Amplifiers

We report gold electroplating on ultra-compliant substrates comprising helical slow wave structures (SWSs) for traveling wave tube amplifiers (TWTAs) [1]. The novel ultra-compliant substrates are composed of edge-tethered tri-layer metal ribbons with a helical geometry of microscale diameter. After electroplating with gold, we obtain overall thicknesses of a few um. We discuss different controllable electroplating conditions that influence thickness, uniformity, roughness, and related properties of deposited gold films on helical ribbons. In addition to increasing conductance of the electroplated helical ribbons, electroplating stabilizes the helix at its equilibrium diameter, with the pitch predicted by Prakash et al.[1].Our method of fabrication of ultra-compliant helical ribbons starts with defining strips of width 5 µm to 10 µm by optical lithography, metal evaporation, and lift-off, deposited on Si substrates coated with a sacrificial layer of Ge or GeOx,. We use Cr/Au/Cr tri-layers to create an inherent stress gradient that causes the strip to self-assemble into a helix, after etching with XeF2 for selective removal of a sacrificial layer. Diameter and pitch of the released helices are controlled by varying the thickness, the elastic modulus, the residual stress, and the in-plane geometry of the deposited tri-layer metal strips. The gold electroplating process uses a sulphate-based gold solution in a two-electrode electrochemical setup with the helix as the cathode and a platinized mesh as the anode [2]. We deposit gold to a thickness of a few um, using a pulsed current source with variable parameters. Direct current can also be used with smaller deposition times. Our results demonstrate the application of electroplating to unconventional ultra-compliant helix of nanoscale dimensions. Reference [1] Divya J. Prakash,., Matthew M. Dwyer, Marcos Martinez Argudo, Mengistie L. Debasu, Hassan Dibaji, Max G. Lagally, Daniel W. van der Weide, and Francesca Cavallo. 2020. “Self-Winding Helices as Slow-Wave Structures for Sub-Millimeter Traveling-Wave Tubes.” ACS Nano, 2021, 15, 1229-1239. doi:10.1021/acsnano.0c08296.[2] Max G. Lagally, Anjali Chaudhary, Daniel van der Weide, Divya J. Prakash, and Francesca Cavallo, Improved Self-assembly of Helices via Electrodeposition on Freestanding Nanoribbons for TWT Application, provisional US patent application (P220249US01)Work supported by U.S. AFOSR-Award No FA9550-22-1-0086.

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.

Amplification of THz waves by beam-wave interaction in self-assembled helical slow-wave structures with single and double chirality

We investigate the interaction between an electron beam and a THz guided electromagnetic wave in a helical slow-wave structure formed by self-assembly of a conductive ribbon. We have previously shown the controlled fabrication of this slow-wave structure and its potential to form the basis for widely deployable millimeter-through-THz traveling-wave tube amplifiers. The process allows the fabrication of helical slow-wave structures with single and double chirality. Here, we use three-dimensional simulations to perform a comparative analysis of beam–wave interaction in self-assembled gold helices with single and double chirality. First, the structures are modeled without the electron beam (cold helices) to calculate the distribution of the electric field generated by the high-frequency wave. We perform simulations of cold helices by using Computer Simulation Technology Microwave Studio. Second, we evaluate the interaction between an electron beam and the THz travelingwave by using a particle in cell simulator in Computer Simulation Technology Particle Studio. Simulation studies show that a switch in chirality in the middle of self-assembled helices generates a reflected wave that boosts beam–wave interaction. We demonstrate that this efficient energy exchange will potentially provide high gain in THz traveling-wave tube amplifiers based on self-assembled helices.

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.

Thermal Study on the Dependence of Contact Resistances on Helical SWS of TWTs

Broadband operation of traveling wave tubes (TWTs) demands the use of nonresonant helix structure as the slow-wave structure (SWS). But there are certain limitations to the power handling capabilities of helical SWS which require rigorous thermal investigations. Further, thermal contact resistances (TCRs) at the interfaces between various parts serve as a detrimental factor inhibiting the efficient thermal conduction in the SWS. Hence, authors have extensively studied the effect of TCRs at various interfaces on the thermal dissipation along with a developed analytical model. Moreover, the systematic thermal analysis of the SWS has been carried out for two different stuffing techniques-hot and cold stuffing, and the results have been compared with the perfect contact scenario. Different support rod materials have also been explored and their corresponding thermal and structural simulations are performed. Based on those results, hot-stuffed SWS performs thermally efficient resulting in 10% better thermal dissipation as compared to the cold-stuffed SWSs. Further recommendations on the support rod material to be used for efficient thermal dissipation are also provided.

Self-Winding Helices as Slow-Wave Structures for Sub-Millimeter Traveling-Wave Tubes.

We present a transformative route to obtain mass-producible helical slow-wave structures for operation in beam-wave interaction devices at THz frequencies. The approach relies on guided self-assembly of conductive nanomembranes. Our work coordinates simulations of cold helices (i.e., helices with no electron beam) and hot helices (i.e., helices that interact with an electron beam). The theoretical study determines electromagnetic fields, current distributions, and beam-wave interaction in a parameter space that has not been explored before. These parameters include microscale diameter, pitch, tape width, and nanoscale surface finish. Parametric simulations show that beam-wave interaction devices based on self-assembled and electroplated helices will potentially provide gain-bandwidth products higher than 2 dBTHz at 1 THz. Informed by the simulation results, we fabricate prototype helices for operation as slow-wave structures at THz frequencies, using metal nanomembranes. Single and intertwined double helices, as well as helices with one or two chiralities, are obtained by self-assembly of stressed metal bilayers. The nanomembrane stiffness and built-in stress control the diameter of the helices. The in-plane geometry of the nanomembrane determines the pitch, the chirality, and the formation of single vs intertwined double helices.

Influences of Diamond Material on Heat Dissipation Capabilities of Helical Slow Wave Structures

From theoretical calculations, simulations, and emulations, as well as laboratory testing, we evaluated the influences of both diamond and diamond-film-coated support rods, as well as diamond-film-coated helices, on the heat dissipation capabilities of slow wave structures (SWSs). A comparison between the experiment and simulation data shows that the computer simulation results are in good agreement and offer an important reference for the application of diamond in SWSs. An expression for the coefficient of thermal conductivity for the helix material and for the support rod material indicates that the greater their value, the better the heat dissipation capability of the helical SWS. The simulation and emulation analysis results show that when the helix reaches the same temperature, SWSs made of diamond-film-coated support rods and helix are able to consume 20%-60% more heating power than normal SWSs. Similar to a purely diamond support rod, a diamond-film-coated helix and support rod are able to improve the heat dissipation capabilities of SWSs effectively. In experiments, we tested the heat dissipation capabilities of SWSs with their helix surfaces plated with high-performance gold/copper films and BeO/BeN support rods. The experimental results agree well with the ANSYS simulation and emulation results.

Investigations into Helix Slow-Wave Structure Assisted by Double-Negative Metamaterial

A helical slow-wave structure consisting of a metallic helix supported by double-negative metamaterial (DNG-MTM) rods, symmetrically arranged around the helix, in a metal envelope was analyzed, considering the frequency-dependent permeability and the permittivity of the DNG-MTM following Lorentz and Drude models. The analysis was validated against the 3-D electromagnetic simulation using CST Studio and was extended for studying the effects of the DNG-MTM parameters on the dispersion and interaction impedance of the structure. Tunability of the structure over multiple-frequency bands was also demonstrated. The possible means of metal MTM layering of the radial faces of the discrete support rods were suggested in a more realistic structure configuration. The structure exhibited a high value of interaction impedance as compared to its traditional counterpart, promising enhanced device efficiency and miniaturization. The structure has potential applications in backward-wave amplifiers and backward-wave oscillators.

Novel ultra-wideband test fixture and method for attenuation of the attenuator-coated dielectric support rod in a helical slow-wave structure.

In order to measure the attenuation along the dielectric support rod in a helical slow-wave structure, a new test fixture and the corresponding test method are introduced. The proposed fixture is a transmission line that consists of three parts: angle gradient transitions, hyperbolical tapered lines, and parallel strips with a sample hole in the middle. The hyperbolical tapered lines on the top and bottom sides of the substrate were designed to realize an approximate Klopfenstein tapered line. The parallel strips provide a relatively reasonable electric field for measuring the attenuation of the dielectric support rod. Meanwhile, two modified metal blocks on both sides of each connector are designed to suppress unwanted radiation. The test fixture works in the ultra-wideband range from 7 GHz to 40 GHz, and it exhibits an insertion loss of less than 3 dB and a return loss of more than 10 dB over the whole frequency range. The sample hole was regarded as the mutually coupled parallel strip pairs (a four-port network), and the odd-even mode method was used to analyze the scattering parameters of the single parallel strip. The dielectric support rod deposited by the resistive thin-film was measured with the experimental test setup developed in this paper. Compared with the method of the double ridge waveguide, the measured results present a great agreement with the actual distribution of the attenuation in the length ranges from 12 mm to 15 mm, 25 mm to 27 mm, and 46 mm to 52 mm, and the differences in other length ranges are explained.

The new wave-ring helical (WRH) slow-wave structure for traveling wave tube amplifiers

In this paper, the new slow-wave structure called wave-ring helix to enhance the power of the traveling wave tubes is introduced. In this new structure, without increasing the length and radius of the helix, the wave motion path can be increased to radiofrequency wave in phase with the electron beam. The results show that in the special frequency range the output power and gain are greater than conventional helix. In this paper, optimization results are presented in cold and hot tests on the new structure. The software CST is used in S-band frequency range.

Dispersion control of helical slow-wave structure by double-negative metamaterial loading

A helix supported by a double-negative metamaterial (DNG-MMT) surrounding it, in a metal envelope, was field-analyzed to show that the shape of the dispersion is more sensitive to the value of the relative permittivity of the metamaterial (MMT) than that of its relative permeability. The field analysis was validated against equivalent circuit analysis as well as against simulation. The values of the MMT support parameters were adjusted to obtain nearly flat, rather slightly negative, dispersion characteristics together with a high value of the interaction impedance of the structure as required for a wideband traveling-wave amplifier. Unlike in a conventional traveling-wave tube, the flow of power of RF wave supported by DNG-MMT loaded helix was found to be in a direction opposite to that of the RF phase velocity of the wave supported by the structure resulting in a negative value of the interaction impedance of the structure suggesting that the device using a helix with double-negative MMT supports would operate in the backward-wave mode analogously to a reversed Cherenkov amplifier.

Thermal behaviour of a periodic structure supported by dielectric rods in vacuum

In a traveling-wave tube (TWT) the helical periodic slow-wave structure (SWS) is supported by dielectric supports in a metal envelope in high vacuum. The heat generated in the helix during beam-wave interaction, restricts the average power handling capability of a TWT, dissipated by conduction through support rods. Thermal contact resistances, arises at different joints of different materials, affect heat dissipation from the helix which have been studied and presented here.

Analysis of Helical Slow-Wave Structures for Modeling Helix Thickness Using an Improved Helical-Harmonics Approximation—Part II: Simulation Results

With the theory of an improved helical-harmonics approximation (HHA) presented in the first part of this paper, the numerical results of the method for helix slow-wave structures for different rod shapes and frequency bands are presented in this part. It is shown that this improved theory yields a more accurate model for considering the tape thickness than the tape-helix model and also the conventional HHA. Comparison of the results obtained using this improved HHA with those obtained from a commercial 3-D simulator, and also the experimental data further validate the technique.

Analysis of Helical Slow-Wave Structures for Modeling Helix Thickness Using an Improved Helical-Harmonics Approximation—Part I: Theory

In this two-part paper, an improved helical-harmonics approximation (HHA) is presented, and subsequently validated by applying it to the analysis of cold (beam-absent) helix slow-wave structures (SWSs) with smoothed rods. The first part presents the theory and relevant formulations. By starting from Maxwell's equations, we obtain an improved form of supporting equations for field components in the so-called helical coordinate system with the help of which the effect of helix pitch angle on the validity of HHA is revealed. By evaluating the error in solutions caused by small changes in the pitch angle across the tape thickness, we show that HHA holds to be valid in a region whose radial thickness is a fraction of actual helix tape thickness. Furthermore, field components of a helix SWS and its dispersion relation are determined using this modified approximation. Moreover, all helical armonics that are necessary for the convergence of the results are incorporated in our analysis. The present improved HHA-based analysis of helical SWS yields a more accurate modeling of the tape thickness than the analysis based on the conventional HHA as well as that by the tape-helix model.

螺旋线厚度对螺旋线慢波结构高频特性的影响

螺旋线厚度对螺旋线慢波结构高频特性的影响

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.

High Power Microwave Pulse Compression Using a Helical Slow-Wave Structure

One interesting method of microwave pulse compression is passive-frequency-based microwave pulse compression where a dispersive medium is used to compress a frequency swept pulse. Oversized circular waveguide having helical corrugation supports eigenmodes that are suitable for pulse compression for frequency modulated input pulse is as shown by Brat et.al. [IEEE Trans. Plasma Sci., vol. 33, no.2, pp. 661-667, 2005.]. In this paper, theoretical studies on microwave pulse compression in helically corrugated waveguide is presented. The numerical results show a eleven fold peak power compression.

新型自由矩形螺旋线慢波结构高频特性

新型自由矩形螺旋线慢波结构高频特性