Helix Slow-wave Structure Research Articles

Correction of the dispersion characteristic of a helix slow-wave structure in traveling-wave tubes

On the basis of a physical notions of the specific features of the slow-wave field distribution in the helix structure of a traveling-wave tube (TWT), various methods for controlling the amount of wave slowing and the slope of the dispersion characteristic are analyzed. The effect of the dielectric supports and correcting elements placed outside the helix is considered. In addition to the known methods for correcting dispersion through the use of a dielectric shield and longitudinally conducting elements, which weaken the interaction with the electron flow, new methods that provide both an efficient interaction with the electrons and the possibility of operation without backward-wave self-excitation at voltages higher than the voltages used with conventional helices are proposed and analyzed. With an analysis based on the equivalent-line method, it is shown that the slope of the helix dispersion characteristic can be decreased by the appropriate use of ring-shaped supports. Design solutions using the new methods of correction are considered. The history of development of ultrabroadband TWTs, including the work done in the Soviet Union on tubes with centrifugal electrostatic focusing, whose actual amplification band can be as wide as three octaves, is partially presented.

Design and Development of Helix Slow-Wave Structure for Ku-Band TWT

A helix slow-wave structure (SWS) for a high-efficiency (electronic efficiency >; 25%) Ku-band 140-W space traveling-wave tube (TWT) has been designed. The cold circuit parameters of helix SWS, like propagation constant and on-axis interaction impedance, were determined using 3-D electromagnetic field simulators ANSOFT-HFSS and CST-MWS, and those were validated with the experimental results. The in-house developed large-signal code SUNRAY-1D was used to design a complete helix SWS in two sections with sever and taper to achieve desired output power, gain, efficiency, and other linearity parameters like phase shift, AM/PM factor, and I/M components. The simulated RF performance of the Ku-band 140-W helix TWT was validated with the experimental results. A close agreement between the simulated and experimental results has been found.

An Improved Helical Resonator Design for Rubidium Atomic Frequency Standards

The slow-wave structure (SWS) can be used as the resonant cavity for rubidium atomic frequency standards (RAFSs), in which the longitudinal component of the magnetic field can enable the rubidium atoms' participation in the stimulated transitions. In this paper, an improved helix SWS based on good propagation characteristics as a helical resonator for RAFS is designed. The theoretical analysis is performed together with the simulations of the propagation characteristics. Using finite integration technique (FIT), the simulated results, including phase velocity, on-axis interaction impedance, and distribution of the magnetic field, are implemented and compared with the experimented results. The effects of the different helical pitch and dielectric material in the insulator cylinder on the propagation characteristics are analyzed in detail. The analyzed results show that smaller helical pitch and proper usage of dielectric material in the insulator cylinder can make the propagation characteristics of the helix SWS superior. The improved helix SWS for RAFS adapts a smaller helical pitch and is loaded with the dielectric material of Teflon in the insulator cylinder. This structure can reduce the cavity volume to 14.7 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . The short-term stability of RAFS is up to 1.28 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> /¿ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> , and the frequency stability is 2.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> /°C at the temperature range of -20°C to +55°C.

Improvement of the Performance of a Helix Slow-Wave Structure With a Copper-Embedded Barrel

The effect of a copper-embedded barrel on the performance of a helix slow-wave structure (SWS) has been studied theoretically and experimentally. The heat dissipation capability and cold-test characteristics of the SWSs with a monel barrel, a normal copper barrel, and a copper-embedded monel barrel have been analyzed and compared. With its combination of high mechanical strength and good thermal conductivity, the copper-embedded monel barrel enables better heat dissipation capability than the other barrel types and has low RF losses.

DESIGN AND CHARACTERIZATION OF HELIX SLOW WAVE STRUCTURE FOR KU-BAND SPACE TWT

A helix slow-wave structure (SWS) for a high e-ciency Ku-band 140W space TWT has been designed. Cold circuit parameters of helix SWS like propagation constant and on axis interaction impedance were determined using 3-D electromagnetic fleld simulators ANSOFT-HFSS and CST-MWS and validated with the experimental results. In-house developed large-signal code SUNRAY- 1D was used to design the complete helix SWS in 2-section with sever and taper to achieve desired output power, gain, e-ciency, and other linearity parameters such as phase shift, AM/PM factor, I/M components. Simulated RF performance of the Ku-band 140W helix TWT was validated with the experimental results. A close agreement between the simulated and experimental results has been found.

Effect of the nanopariculate metal film on heat radiation

The effect of the nanoparticulate metal film on the heat radiation performance has been studied by evaluating the thermal conduction of different helix slow-wave structures (SWS). The heat dissipation capabilities of the SWSs with normal barrel, with normal Ir film deposited barrel and nano-Ir deposited barrel have been analyzed and compared by experiments and simulations. With the measured data, the radiation emissivities of the employed materials have been estimated, and the accuracy has been verified by computer simulation. The experimental results and the simulation results make perfect agreement, suggesting that the nanoparticulate metal film gives much better heat radiation performance.

English

Equivalent circuit analysis of a helix slow-wave structure was carried out and closed form expressions were derived for the shunt capacitance and shunt conductance per unit length of the transmission-line equivalent circuit of the structure. These equivalent circuit parameters were interpreted for the dielectric attenuation constant of the slow-wave structure. The analysis was computationally simple and showsed excellent agreement with published results. The analysis was furthered for predicting the dielectric loss in typical C-Ku band and Ka band helical slow-wave structures, and variation of dielectric loss with temperature. Defence Science Journal, 2009, 59(5), pp.549-552 , DOI:http://dx.doi.org/10.14429/dsj.59.1558

Small-signal analysis of a rectangular helix structure traveling-wave-tube

This paper investigates the properties of traveling wave-beam interaction in a rectangular helix traveling-wave-tube (TWT) for a solid sheet electron beam. The ‘hot’ dispersion equation is obtained by means of the self-consistent field theory. The small signal analysis, which includes the effects of the beam parameters and slow-wave structure (SWS) parameters, is carried out by theoretical computation. The numerical results show that the bandwidth and the small-signal gain of the rectangular helix TWT increase as the beam current increases; and the beam voltage not obviously influences the small signal gain. Among different rectangular helix structures, the small-signal gain increases as the width of the rectangular helix SWS increases, however, the bandwidth decreases whether structure parameters a and L or Ψ and L are fixed or not. In addition, a comparison of the small-signal gain of this structure with a conventional round helix is made. The presented analysis will be useful for the design of the TWT with a rectangular helix circuit.

Design of Tip Loss Profile on Support Rod for a Helix TWT

This paper describes the design of tip loss profile on support rod for a high gain, high efficiency helix TWT. The tip loss profile at sever ends for the input and the output section of the helix slow wave structure has been designed for return loss at severs ends better than -20 dB. The experimental results have been carried out in the input and the output section of the helix assemblies for the high gain helix TWT to find the return loss in the region of carbon coated tip loss on the support rods. The experimental result has been compared with the simulated performance for the return loss at the sever ends. The design of tip loss profile on the support rod for helix TWT has been carried out in real situation using Ansoft HFSS. A good agreement has been found in the simulated and experimental results.

Effect of Plated Metal Film on the Performance of Helix TWT Slow-Wave Structure

The effect of the plated metal film on the heat dissipation capability and cold-test characteristics of the helix slow-wave structure (SWS) have been studied in several experimental tests and computer simulations. The thermal conduction and the power losses of the helix SWS were improved by using a plated helix. A brazed SWS with end-plated rods exhibits excellent heat dissipation capability. The side-plated rods affect the cold-test characteristics of the SWS.

Study on Effects of Different Metallic Vane-Loaded Helix Slow-Wave Structures in Traveling-Wave Tubes

The effects of different metallic vane-loaded helix slow-wave structures of a traveling-wave tube are proposed based on the analysis of the Fourier expansions of the exterior region with metallic vanes. The influences of the metallic vanes dimensions on the phase velocity and interaction impedance are considered in detail. The computed data is compared with the reference data in the 0−16 GHz frequency range with a good consistency. The analytical results reveal that the method of using Fourier expansions can contribute effectively to the reducing of the error between the theoretical and experimented data (around 1.2%). By analyzing the computed results, the performances of the helix slow-wave structure, with T-shaped metallic vanes are superior to the sector-shaped with the same designed parameters. Adjustments can be made to the outer radius of T-shaped metallic vanes which then control the dispersion relation showing either negative or positive, and it is similar to sector-shaped vanes by adjusting its inner radius. And with increasing the distance between the helix and metallic vanes, the dispersion characteristics and interaction impedance of the helix slow-wave structure with T-shaped/sector-shaped metallic vane are all improved.

Finite Element Electromagnetic Analysis of TWT Slow-Wave Structures in Grid Environment

In this paper, some new finite element procedures are presented to analyze TWT helix slow-wave structures in a distributed computing environment. In particular, very computing time expensive analyses required in TWT SWS simulations are parallelized in order to obtain a remarkable reduction of computing time. The example presented regards the construction of the Brillouin diagram of a helix SWS. The obtained results confirmed us the good performance of the grid environment for the numerical analysis and optimization of TWTs.

Estimation of circuit loss in a broadband segment-loaded helical slow-wave structure of a traveling-wave tube

A simple method of estimating the attenuation constant due to conductivity losses in a helix slow-wave structure was developed that uses the inputs like the quality factor and dispersion characteristics obtained from the eigen-mode solutions of 3D HFSS modeling of the structure. The method was benchmarked against two practical structures published in the literature, and applied in demonstrating the computation of circuit loss in a segment-loaded broadband helical slow-wave structure. It was seen that proximity of segment to the helix increases the circuit loss significantly thereby affecting the device efficiency. A study of different parameters that affect the circuit attenuation was also carried out.

Study on the thermal interface resistance of the helix slow-wave structure

The thermal interface resistance of the helix slow-wave structure （SWS） has been studied theoretically and experimentally. The effect of the thermal resistance at two interfaces on heat dissipation capability of the SWS is analyzed. A novel method to calculate the thermal interface resistance has been developed, and its accuracy and feasibility were validated by experiments. With this method, the thermal interface resistances at two interfaces can be calculated separately, considering the variation of the material performance with the temperature changes. This novel method is used to study the effect of the rod material and the assembling method on the thermal interface resistance.

A Simple and Accurate Analysis of Conductivity Loss in Millimeter-Wave Helical Slow-Wave Structures

Electromagnetic field analysis of a helix slow-wave structure was carried out and a closed form expression was derived for the inductance per unit length of the transmission-line equivalent circuit of the structure, taking into account the actual helix tape dimensions and surface current on the helix over the actual metallic area of the tape. The expression of the inductance per unit length, thus obtained, was used for estimating the increment in the inductance per unit length caused due to penetration of the magnetic flux into the conducting surfaces following Wheeler’s incremental inductance rule, which was subsequently interpreted for the attenuation constant of the propagating structure. The analysis was computationally simple and accurate, and accrues the accuracy of 3D electromagnetic analysis by allowing the use of dispersion characteristics obtainable from any standard electromagnetic modeling. The approach was benchmarked against measurement for two practical structures, and excellent agreement was observed. The analysis was subsequently applied to demonstrate the effects of conductivity on the attenuation constant of a typical broadband millimeter-wave helical slow-wave structure with respect to helix materials and copper plating on the helix, surface finish of the helix, dielectric loading effect and effect of high temperature operation – a comparative study of various such aspects are covered.

Analysis of Helix Slow Wave Structure for High Efficiency Space TWT

This paper describes the analysis of helix slow-wave structure (SWS) for a high efficiency space traveling wave tube that is carried out using Ansoft HFSS and CST microwave studio, which is a 3D electromagnetic field simulators. Two approaches of simulating the dispersion and impedance characteristics of the helix slow wave structure have been discussed and compared with measured results. The dispersion characteristic gives the information about axial propagation constant (Beta). Which in turn yields the phase velocity at a particular frequency. The dispersion and impedance characteristics can be used in finding the pertinent design parameters of the helix slow-wave structure. Therefore a new trend has been initiated at CEERI to use Ansoft HFSS code to analysis of the helix slow wave structure in its real environment. The analysis of the helix SWS for Ku-band 140W space TWT has been carried out and compared with experimental results, and found is close agreement.

Dispersion Characteristics of a Rectangular Helix Slow-Wave Structure

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A special type of helical slow-wave structure encompassing a rectangular geometry is investigated in this paper, and the slow-wave characteristics are studied taking into account the anisotropically conducting helix. By using the electromagnetic integral equations at the boundaries, the dispersion equation and the interaction impedance of transverse antisymmetric modes in this structure are derived. Moreover, the obtained complex dispersion equation is numerically calculated. The calculation results by our theory agree well with the results obtained by the 3-D EM simulation software HFSS. The numerical results reveal that the phase velocity decreases and interaction impedance increases at higher frequencies by flattening (increasing the aspect ratio of) the rectangular helix structure. In addition, a comparison of slow-wave characteristics of this structure with a conventional round helix is made. </para>

Design of Coaxial Couplers for High Efficiency Helix TWT

The main objective of the paper is to make an efficient design of the input and output coaxial coupler for a helix TWTs. An approach has been developed for the efficient design and analysis of the coaxial couplers in the practical situation. Normally multi-section impedance transformer approach is used for any wide band coupler. For a space helix TWT, coupler should be wide bandwidth and small size. In this case coupler is matched with helix slow wave structure and the standard 50-ohm connectors. The simulated return loss (dB) profile for different type of couplers is obtained by using Ansoft HFSS, CST microwave studio and compares those with experimental results. The tip loss design at sever ends for the input and the output section has been also optimized.

Radio-Frequency Characteristics of a Printed Rectangular Helix Slow-Wave Structure

A new type of printed rectangular helix slow-wave structure (SWS) is investigated using the field-matching method and the electromagnetic integral equations at the boundaries. The radio-frequency characteristics including the dispersion equation and the coupling impedance for transverse antisymmetric (odd) modes of this structure are analysed. The numerical results agree well with the results obtained by the EM simulation software HFSS. It is shown that the dispersion of the rectangular helix circuit is weakened, the phase velocity is reduced after filling the dielectric materials in the rectangular helix SWS. As a planar slow-wave structure, this structure has potential applications in compact TWTs.

Improvement of Heat Dissipation Capability of Slow-Wave Structure Using Two Assembling Methods

Two assembling methods, i.e., the sputter brazing method and the diffusion brazing method, have been developed for improving the heat dissipation capability of the helix traveling-wave tube slow-wave structure (SWS). The magnetron sputter plating technology and the pressure diffusion technology are employed in this letter to complete the SWS assembly. The effect of these two methods on the thermal conduction of the SWS was verified by several experimental tests. These two methods being pursued enable better heat dissipation capability of the helix SWS, compared to the conventional nonbrazing methods.