Sheath Helix Research Articles

Quasi-three-dimensional perturbation technique, including dielectrics for TWT's

A perturbation technique is described for finding phase velocities and coupling impedances in a traveling wave tube for an arbitrary distribution of dielectric material. A model of the sheath helix is presented. Tape helix results will be presented in a separate paper. In all cases presented, without adjusting the dielectric constant, the calculated perturbed phase velocity provided a better answer than the homogeneous dielectric solution, or Naval Research Laboratories' Small Signal Gain Program. Deviation from theory versus experiment is reported by stating the average sum of the squares difference between theoretical calculations and a second order least squares fit of the measured data. Phase velocities can be calculated for uniform dielectric support rods where the average sum of the squares /spl les/1.19/spl times/10/sup /spl minus/5/. For cases with notched dielectric support rods phase velocities can be calculated where the average sum of the squares /spl les/1.94/spl times/10/sup /spl minus/5/. For NRL's SSG program the average sum of the squares was /spl les/1.01/spl times/10/sup /spl minus/4/ by comparison. For uniform dielectric support rods the perturbation does not significantly alter the basic shape of the predicted dispersion curve. For notched dielectric support rods applying the perturbation does alter and flatten the shape of the predicted dispersion curve in agreement with experiment.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Excitation of hybrid electrostatic-electromagnetic modes in a plasma-loaded sheath helix

A plasma-loaded sheath helix supports lower hybrid modes that extended to the outside vacuum region. The phase velocity of the mode is significantly retarded by the presence of the sheath helix, making it liable to excitation by a low-energy electron beam. The extent of the mode outside the plasma is also reduced by the helix. An analysis of lower hybrid mode excitation by a beam propagating in the vacuum region reveals that nonlocal effects significantly reduce the growth rate. If the instability is taken to saturate via particle trapping, it acquires a large amplitude, giving oscillatory velocity large enough to lead to nonlinear effects.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A New Approach to Analysis for Loaded Tape Helix

A new dispersion relation for a dielectric loaded tape helix in an anisotropically conducting metal shell has been developed to include the effect of spatial harmonics. It has been established that the number of forbidden zones appearing in dispersion characteristics of a structure is equal to the number of spatial harmonics taken into consideration. Also, a comparison has been made between the dispersion results of tape helix model and those of sheath helix model for typical structures.

Self-consistent field theory of a helix traveling wave tube amplifier

A self-consistent relativistic field theory of a helix traveling wave tube (TWT) is presented for a configuration in which a thin annular beam propagates through a sheath helix enclosed within a loss-free wall. A linear analysis of the interaction is carried out, subject to the boundary conditions imposed by the beam, helix, and wall. A detrimental dispersion equation is obtained which implicitly includes beam space-charge effects without recourse to a heuristic model of the space-charge field. The equation is valid for arbitrary azimuthal mode number and is solved numerically for the azimuthally symmetric case. The coupled-wave Pierce theory is recovered in the near-resonant limit. Numerical comparisons between the complete dispersion equation and the Pierce model are described. A discrepancy is found between the Pierce and the field theory even for low currents in the nominally ballistic regime, owing to the dielectric effect of the beam on the helix modes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Radiation from a sheath helix excited by a circular waveguide: a Wiener-Hopf analysis

The excitation of a semi-infinite sheath helix by the TE11 mode of a circular metallic waveguide with the same diameter is studied using the Wiener-Hopf technique. Expressions are presented for the TE11 reflection coefficient at the waveguide end and for the relative power in the sheath helix discrete modes. Numerical results corresponding to the current density at the sheath helix surface and to the far-field radiation patterns are presented. These are compared to previous results obtained from the modal analysis of an infinite sheath helix that is excited by a current sheet. Experimental patterns corresponding to a finite length multifilar helix show a satisfactory agreement with the theoretical patterns when the scanning mode is dominant.

Complex Wave Radiation from a Sheath Helix Excited by the Circular Waveguide TE 11 Mode

A modal approach is used to analyse the radiation from a dielectric loaded infinite sheath helix, that is excited by a circular sheet of electric and magnetic currents equivalent to the TE11 fields over the cross-section of a circular metallic waveguide of the same diameter. To estimate the far-field radiation pattern of a finite length sheath helix fed by the aperture of a circular waveguide, the near-field solution of the infinite problem, expressed as a sum of discrete modes and a continuous spectrum wave, is integrated over a finite length of the infinite structure using the Kirchhoff-Huygens formulation. The approximations involved are discussed. Measured radiation patterns of multifilar helices fed by the circular waveguide TE11 mode, are compared to computed radiation patterns.

Excitation of whistler modes in a sheath-helix loaded waveguide

The excitation of electromagnetic modes in a cylindrical plasma column by an electron beam is considerably improved by the introduction of a sheath helix inside the vacuum chamber. The helix slows down the whistler mode and relaxes the requirements on the beam energy for the generation of a given frequency.

Finite length helical sheath antenna in a general homogeneous medium

The sheath helix is an anisotropically conducting cylindrical tube that represents a good approximation to an actual helix consisting of many turns per wavelength. In this paper an integral equation for the current distribution along a finite length, axisymmetrically driven helical sheath antenna in a general homogeneous medium is solved by the method of moments to obtain the current distribution and radiation pattern of the antenna. The characteristics of various helical sheath antennas in both dissipative and nondissipative media are presented. Comparisons are made of the current distributions along multifilar thin‐wire helical antennas and helical sheath antennas in free space.

M=0,±1 low frequency fast Alfven modes of a plasma column loaded in a sheath helix

The dispersion relation for a cold, collisionless plasma column loaded in a sheath helix is derived. This is then solved numerically for the m=0, +or-1 fast Alfven waves for different plasma radii and helic parameters; the frequency range is below the ion cyclotron frequency. The results are compared with those of the plasma-waveguide system. There is no m=0 cut-off and instead there are two (m=0) waves which propagate down to low frequencies. While the m=+1 mode retains its general behaviour, the m=-1 mode is altered significantly. Relevance of these results to the coupling of Alfven waves with helices is discussed.

Electromagnetic Instability of a Rotating Electron Layer in a Sheath Helix

A sheath helix supports slow electromagnetic modes with phase velocity considerably Iower than the velocity of light in a vacuum. In the presence of a rotating electron layer, the modes can be resonantly driven unstable via cyclotron maser interaction. Using the perturbation technique, the growth rate of the instability is obtained in the weak-beam approximation and is seen to decrease with the slowing down of the modes. For lower order modes, the growth rate is comparable to the one with a cocentric cylindrical waveguide. However, for higher order modes, the growth rate decreases rapidly, suggesting that a sheath helix may be used to suppress the higher order modes.

Electromagnetic wave propagation through an azimuthally perturbed helix

A field analysis is developed for the helical slow-wave structure of a traveling-wave tube in the anisotropic sheath helix model considering the effects of the azimuthal harmonics generated due to periodical positioning of the support bars around the helix. A practical relevance is added to the problem by way of considering the effect of an overall metal enclosure; the losses of the structure are, however, ignored. The analytical result justifies the usual heuristic approach of treating the support bars as a continuous dielectric of a suitably interpreted permittivity.

Propagation on a Sheath Helix in a Coaxially Layered Lossy Dielectric Medium (Short Paper)

Radial and axial dependence of the azimuthally symmetric fields in each coaxial layer may be expressed in terms of modified Bessel functions and complex exponentials, respectively.

Gyrotron amplifier in a helix loaded waveguide

Stability properties of the cyclotron maser instability in a relativistic annular electron beam propagating through a cylindrical waveguide loaded with a sheath helix are investigated, in connection with application to the gyrotron amplifier. The stability analysis is carried out, assuming that the beam is thin and tenuous. A closed form of the dispersion relation for the cyclotron maser instability is obtained, including the influence of the sheath helix on stability behavior. One of the most important features in this analysis is that the bandwidth of the gyrotron amplifier for a helix mode is very broad. Moreover, it is also shown that for an appropriate choice of the helix radius and pitch angle, the maximum gain of the gyrotron amplifier for a hybrid wave is twice that for an ordinary waveguide.

Theory of gyrotron amplifier in a tape helix loaded waveguide

Stability properties of the cyclotron maser instability in an annular electron beam propagating through a cylindrical waveguide loaded with a tape helix are investigated, in connection with applications on the gyrotron amplifier. Closed form of the dispersion relation for the cyclotron maser instability is obtained, including influence of the tape helix in stability behavior. It is shown that the bandwidth of the gyrotron amplifier for a tape helix is narrow in comparison with results for a sheath helix. However, the growth rate of the tape helix gyrotron is comparable to that of the sheath helix gyrotron.

Modal approach to scattering of electromagnetic waves by a conducting tape helix

Electromagnetic wave scattering by a tape helix of infinite extent is studied by using Floquet wave expansions for the guided modes and scattered fields. The solution reduces to earlier results as a special limiting case for normal incidence on a sheath helix. The current induced on an infinite helix computed by the presented technique bears close resemblance to the current induced on a long but finite helix as computed by Galerkin's method. The spatial frequency spectrum of the induced current is plotted to show the dominance of the spatial harmonics that are phase matched with the guided modes of the helix. Azimuthal patterns of the scattered field are included to illustrate that interference increases as the diameter of the helix is increased.

On the self-capacitance of solenoidal coils

Expressions for the self-capacitance of single-layer solenoidal coils both balanced and with one end grounded are established by considering the electromagnetic field solution along a sheath helix of infinite length. A transmission line equivalent circuit is developed that permits the determination of the driving point admittance of the coil. Adaptation of this expression for the admittance to a coil of finite length leads to a semi-empirical formula for the self-capacitance. Further, it provides a physically illuminating explanation for the experimentally based practice of taking the self-capacitance as a single lumped capacitance connected across the coil terminals and of taking the effective inductance as the direct current inductance of the coil. The results are shown to be in excellent agreement with existing measurements of self-capacitance.

Use of Sheath Heiix Slow-wave Structure as an Applicator in Microwave Heating Systems

This paper concerns a theoretical study of using the sheath helix slow-wave structure, surrounded by a conducting tube, as an applicator in microwave heating systems of dielectric filamentary mater...

On the electromagnetic heating of low loss materials using induction coils

A sheath helix model is used to develop a complete wave solution for the electric and magnetic fields associated with a long solenoid wound on a arbitrary cylindrical core and the solution is used to investigate the induction heating of low loss dielectric materials. It is shown that efficient and uniform heating of low loss cores at low frequencies is possible even through the eddy current heating usually associated with an induction coil is negligible in these cases. The theoretical results ahve been experimentally verified and various samples of wood, concrete and different grades of Athabasca oil sand have been successfully heated. Potential applications in the areas of construction, food processing, medicine and hydrocarbon resource recovery are suggested.

Guided Waves on a Flattened Sheath Helix (Letters)

The "flattened sheath helix" considered in this letter consists of a pair of parallel unidirectionally conducting screens conducting in different directions and having different dielectric media in the sandwiched and outer regions. Special cases are 1) the normal flattened helix in which the inner medium is a solid dielectric and the outer medium is air, and 2) the inverted flattened helix with the two media interchanged. The guiding properties of such structures are studied.

Some aspects of spermiogenesis in a lizard.

AbstractSpermatids of the American chameleon Anolis carolinensis, have been examined with the electron microscope during the period from the first appearance of caudal sheath microtubules to the stage of differentiation just before the caudal sheath disappears. The caudal sheath microtubles are arranged initially in the form of a helix with a very low pitch. As the condensation of the chromatin and the elongation of the nucleus proceed, the caudal sheath helix unwinds. Shortly before the caudal sheath disappears, the sheath microtubles are oriented parallel to the longitudinal axis of the nucleus and the sheath resembles those found in other vertebrate spermatids. Assuming that the caudal sheath is functionally related in some way to the changes that occur in the nucleus, it is proposed that the unusual arrangement of the caudal sheath in A. carolinensis spermatids is a device to avoid a large excess of microtubules clustered around the nucleus at the end of nuclear elongation.