B. N. Basu

Modeling of a vane-loaded helical slow-wave structure for broad-band traveling-wave tubes

A model of a vane-loaded helical slow-wave structure (SWS), in which an infinite number of vanes (INV) of infinitesimal thickness is considered, is improved. The modified INV (MINV) model takes into account the effect of the finite number and angular thickness of the vanes. This is done by suitably modifying the radial vane dimension on the basis of the interpretation of the data on the penetration of the axial electric field beyond the tips of the vanes into the intervane region as a function of the SWS parameters, including the number and angular extent of vanes. The theoretical dispersion characteristics obtained from both the INV and MINV models are compared to theoretical and experimental results published elsewhere. The INV model is found to be suitable for a structure with a relatively larger number of vanes. The study shows the superiority of the MINV over INV model in practical situations, wherein relatively fewer vanes of finite angular thickness are used. Methods to estimate the characteristic as well as interaction impedance of the structure, under the INV model approximations, are indicated. >

Modeling of practical multi-octave-band helical slow-wave structures of a traveling-wave tube for interaction impedance

A theoretical model is developed for evaluating the interaction impedance of practical helical slow-wave structures which are anisotropically and/or inhomogeneously loaded specially for multioctave bandwidths. The discrete supports have been azimuthally smoothed out into a number of dielectric tubes of different permittivity values, while the metal vanes have been modeled by an axially conducting cylinder suitably located in the structure. >

Three-Dimensional Electromagnetic Analysis of Attenuator-Coated Helix Support Rods of a Traveling-Wave Tube

Attenuator-coated helix support rods in a helical slow-wave structure (SWS) were modeled by considering them to be made up of lossy dielectric material having equivalent bulk conductivity varied along the length following the loss of the coating. The same conductivity values were used in 3-D modeling of the helical SWS having the helix supported by three such support rods inside a metal envelope, and the attenuation constant of the structure was found. The approach was used for characterizing the attenuation constants in the sever regions of two typical SWSs, one operating in the X-Ku-band and the other in K-Ka-band. The method is general and would be useful for analyzing sever loss in any type of helical SWS having different geometries as well.

Large-signal analysis for BWO start condition in helix TWTs

Large-signal analysis for backward-wave oscillation (BWO) start condition in a helix travelling-wave tube (TWT) amplifier has been developed. The effects of distributed circuit loss and beam-filling factor on start oscillation condition are investigated, and large-signal analysis results are presented vis-a-vis Eulerian analysis results.

Equivalent circuit analysis of a planar helix slow-wave structure for application in high frequency traveling-wave tubes

A planar helix slow-wave structure comprised of a pair of unidirectionally conducting screens was analyzed for its equivalent circuit parameters. These parameters were interpreted for the structure dispersion characteristics and, subsequently, the structure interaction impedance and the Pierces gain parameter of a planar-helix TWT. For identical situations with respect to the structure parameters, the present analysis of the planar helix predicted a lower phase velocity, higher interaction impedance, and, consequently, a larger Pierces gain parameter and hence a greater promise for a higher device efficiency over a wide frequency range than the conventional circular helix.

Simple Formulas for Stopband Attenuation Characteristics of Asymmetric Helical Slow-Wave Structures of Traveling-Wave Tubes

Simple closed-form formulas for the estimation of the π-mode stopband and the stopband attenuation in an azimuthally asymmetric helical slow-wave structure (SWS) are developed, following the coupled-mode analysis of multiple reflections. The formulas are simple and amenable to easy computation, and also allow the use of the dispersion characteristics of the structure obtainable from any standard electromagnetic modeling, thereby accruing the accuracy of 3-D electromagnetic analysis. The analysis is benchmarked against published results with close agreement. Based on the analysis, SWS configurations that would reduce the deleterious effects of asymmetry and also would improve TWT efficiency are suggested.

Gain-frequency response of a helix traveling-wave tube with T-shaped dielectric support rods in a metal envelope

Abstract The slow-wave structure parameters of a traveling-wave tube with T-shaped dielectric rods in a metal envelope were optimized for flat-to-negative dispersion characteristics obtained by the sheath-helix model of analysis. The analytical results were validated against those obtained by the tape-helix model of analysis and CST Microwave Studio. Such optimized structure parameters also gave wideband gain–frequency response of the device as predicted by Pierce’s small-signal theory. The electron beam voltage was also identified as an additional parameter for widening the device bandwidth.

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

Abstract 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.

Investigation into a metamaterial supported helix slow-wave structure

A helix supported by a double-negative metamaterial in a metal envelope was analyzed in the sheath-helix model to explore its potential as the slow-wave structure of high efficiency backward-wave devices. The transmission line equivalent distributed line parameters of the structure were interpreted for the dispersion relation of the structure. The ratio of the envelope-to-helix radii was found to control the dispersion characteristics and hence the frequency range of backward wave interaction, a critical value of the ratio of radii corresponding to the existence of a non-propagating, leaky wave.

A Simple Equivalent Circuit Analysis of the Dielectric Loss in a Helical Slow-Wave Structure of a Traveling-Wave Tube

The electromagnetic field analysis of a helical slow-wave structure (SWS) is carried out based on a tape-helix model incorporating the effects of space-harmonic propagating modes and the surface current on the helix over the actual metallic area of the tape. Using this analysis, closed-form expressions are derived for the shunt capacitance per unit length and the shunt conductance per unit length of the transmission-line equivalent circuit of the structure. The analysis is interpreted for the circuit attenuation constant contributed by the loss of the dielectric helix-support rods. The analysis is accurate, amenable to easy computation, and validated against published results. The analysis is subsequently used for investigating the dielectric loss in an SWS due to the backward-wave (-1) space-harmonic mode of propagation.