Cylindrical Waveguide Mode Research Articles

Nonlinear theory of beam excitation of azimuthal surface modes in plasma-filled cylindrical metal waveguides

It is shown that electromagnetic surface waves propagating along the azimuthal angle can be excited efficiently by an annular electron beam in a cylindrical metal waveguide partially filled with a magnetoactive plasma. A self-consistent system of differential equations is obtained to describe the nonlinear interaction between the beam particles and an azimuthal surface wave in the single-mode regime. This system of equations is analyzed numerically and the influence of the parameters of this waveguide structure on the development of the resonant beam instability is determined.

Possibility of using a large orbit regime for operation at bounce-frequency harmonics in a free-electron maser with a guiding magnetic field

It is suggested that a free-electron maser based on the LIU-3000 accelerator (Joint Institute for Nuclear Research, Dubna) can be made to emit in the short-wavelength part of the microwave range by using radiation at harmonics of the bounce frequency of an electron beam propagating along a helical trajectory with a large gyration radius on the transverse inhomogeneity scale of the rf field and selectively exciting a cylindrical waveguide mode whose azimuthal index is equal to the number of the harmonic.

Linear Analysis of Axisymmetric Modes in Plasma-Filled Waveguides with Finite Strength Axial Magnetic Fields

Numerical analyses on dispersion relations of the axisymmetric quasi- TM 0 s and quasi- TE 0 s modes in a plasma-filled cylindrical waveguide and a corrugated wall metal slow wave structure are carried out under the condition of finite axial magnetic fields. It is shown that a hybrid mode given by a linear combination of extraordinary and ordinary modes satisfies the boundary conditions of plasma at metal surface. There are two genera in the branches in the dispersion relation. One of them is conventional electromagnetic modes and the other is Trivelpiece-Gould modes. Present analysis will be helpful to understand the various unsolved problems in high power plasma-filled backward wave oscillators.

Trapped modes in cylindrical waveguides

We prove the existence of trapped modes in the presence of two classes of obstacles in cylindrical acoustic waveguides. First we prove that trapped modes exist whenever the obstacle is thin and has a normal which is everywhere perpendicular to the generators of the cylinder. Secondly we prove that for the case of a circular cylindrical guide containing an axisymmetric obstacle, an infinite sequence of trapped modes exists, the frequency of the modes tending to infinity. In each case we consider an example where the trapped mode frequencies can be calculated numerically using the residue calculus method.

Analysis of a cylindrical dielectric waveguide with three regions by use of an invariant mode-definition parameter

Boundary conditions at a dielectric interface are expressed in terms of electrodynamic vector and scalar potentials that are expressed in terms of two-dimensional scalar solutions to the wave equation. Possible field configurations are expressed in terms of linear combinations of the scalar solutions. Their behavior at a dielectric interface and within a region of constant permittivity is investigated, and a parameter that is used to define propagation modes in cylindrical dielectric waveguides is shown to have the same value in regions with different permittivities. Scalar expressions that include this parameter are used to calculate exact expressions for fields and propagation constants in a cylindrical dielectric waveguide with three coaxial regions. The characteristics of propagation modes in these waveguides are discussed.

Cold-cavity vectorial eigenmodes of VCSELs

An analytical approximate solution of Maxwell's equations for the cold-cavity eigenmodes of cylindrical etched air-post vertical-cavity surface-emitting lasers (VCSELs) is presented. In radial and azimuthal directions, the modes correspond to the hybrid modes of cylindrical optical waveguides. A vectorial transform matrix approach is derived which takes into account coupling of bound eigenmodes in VCSEL structures. The method is illustrated for the case of noncoupled modes and the corresponding simplified transform matrix approach is used to calculate the field profiles in longitudinal direction and predict the resonance wavelengths for the VCSEL eigenmodes. Although approximate, the resulting eigenmodes may be viewed as a useful alternative to full numerical solutions, especially with regard to future more comprehensive modeling of VCSELs.

A perturbation method for the modes of cylindrical acoustic waveguides in the presence of temperature gradients

This paper concerns itself with how the cut-off frequencies and modal shapes of cylindrical acoustic waveguides are altered by the presence of temperature gradients induced by an applied temperature distribution along the duct walls. A physical model is first formulated which incorporates an inhomogeneous sound speed as well as a density gradient. The associated mathematical model is then a generalized eigenproblem. This is then discretized via the control region approximation (a finite difference scheme) yielding a generalized matrix eigenproblem. Under the assumption that the boundary temperature distribution is nearly constant, we apply a procedure of Schrödinger to express the propagation constants and modal functions as perturbation series whose successive terms can be generated recursively. The case of a rectangular duct with temperature variations in the cross section is considered in detail. All numerical computations are performed using MATLAB©.

Plasma circular cylindrical waveguide in lossy material

The surface modes in the plasma circular cylindrical waveguide in a lossy material are analyzed, particularly for the variations of their propagation properties with plasma parameters and surrounding material. The characteristic equations of surface modes are derived, and their relevant approximate solutions are given. The limit results of this paper are consistent with that given in literature’s. The analysis show that it can be used to improve the properties of mode suppressors and fabricat cut-off attenuators.

Broad-band microwave characterization of bilayered materials using a coaxial discontinuity with applications for thin conductive films for microelectronics and material in air-tight cell

A new measurement method of complex permittivity of bilayered materials has been developed using a coaxial discontinuity. The electromagnetic analysis is performed according to the "mode-matching" method. The reflection coefficient of the principal transverse electromagnetic (TEM) mode is calculated by matching the fields at the interfaces of the layered material and using the orthogonality properties of modes in cylindrical waveguides. The complex permittivity of several known liquid or solid materials in bilayered structure are measured using this method. The experimental results over a wide frequency band (1 kHz-18 GHz) are consistent with those in previous papers and with dc measurements.

Effects of circuit loss and space charge in gyro-travelling-wave tubes

A modified dispersion relation of a gyro-TWT using a cylindrical waveguide excited in aT E mn -mode is derived incorporating the effect of the transverse space charge forces. The relation is subsequently studied considering the circuit losses and interpreted for the conventional TWT-type gain equation. The second harmonic operation using theT E 02 cylindrical waveguide mode resonated with the second beam-harmonic mode is found to be more sensitive to the Pierce's loss and space-charge parameters for a gyro-TWT than the fundamental operation using theT E 01 cylindrical waveguide mode resonated with the first beam-harmonic mode. It is established that these parameters substantially influence the device gain and must be considered for the prediction of its accurate value. Furthermore, these parameters when properly controlled can also improve the device bandwidth.

Design of a low-voltage annular beam Ku-band free-electron laser

A nonlinear analysis of an annular beam free-electron laser (FEL) with a helical wiggler and axial guide field is presented. An annular beam has the advantage over a solid beam of reduced dc self-fields, facilitating beam transport in short period wigglers. The annular beam geometry also permits use of a central structure to enhance the wiggler field, which tends to decrease as the period is decreased. Two configurations of the waveguide and wiggler were theoretically investigated. First, an annular beam interacting with a TE/sub 11/ cylindrical waveguide mode in the presence of a helical wiggler was studied. Next, the interaction with a coaxial TE/sub 11/ mode in the presence a novel coaxial wiggler with both inner and outer bifilar helical current windings was examined. The beam under consideration is nominally 55 kV and 5 A and the wiggler period is 0.9 cm. An axial guide field up to 4 kG was used. The beam and wiggler parameters correspond to grazing incidence with the TE/sub 13/ mode in Ku-band. Nonlinear slow-time-scale simulations show that for an ideal beam, efficiencies greater than 10% can be achieved with instantaneous bandwidths in excess of 20%. The effect of axial energy spread on interaction efficiency is examined.

Theory and design of a photoinjector-driven chirped pulse free-electron maser

An overview of the design parameters of a compact high-gradient high-luminosity X-band (8.568 GHz) photoinjector is followed by a more detailed description of each of its major subsystems: X-band RF gun, GHz repetition rate synchronously modelocked AlGaAs quantum well laser oscillator, and eight-pass Ti:Al/sub 2/O/sub 3/ chirped pulse laser amplifier. The photoinjector uses a high quantum efficiency (/spl sim/5%) Cs/sub 2/Te photocathode, and is capable of producing high-charge (>1 nC) relativistic (5 MeV) ultrashort (<1 ps) electron bunches at 2.142-GHz repetition rate in burst mode (100 photoelectron bunches). Design studies indicate that a normalized rms transverse emittance /spl epsi//sub n/=0.75 /spl pi/ mm-mrad is possible at 0.1 nC charge, while 2.5 /spl pi/ mm-mrad is obtained at 1 nC. One of the most interesting applications of the photoinjector, namely the generation of ultrashort pulses of coherent synchrotron radiation, is then addressed in detail. The spectral and temporal radiation characteristics of an axially extended (finite-size) transversely accelerated charge distribution propagating on fixed helical trajectories through a wiggler are derived for a cylindrical waveguide. At grazing, where the axial bunch velocity matches the electromagnetic wave group velocity, the single output radiation pulse is extremely short, and chirped over the full interaction bandwidth; the pulse duration is determined by group velocity dispersion. A 5-MeV 1.4-nC 1-ps photoelectron bunch is then considered. The grazing frequency is adjusted to 175 GHz for the TE/sub 12/ cylindrical waveguide mode, corresponding to an 8.5-kG 30-mm-period helical wiggler. The instantaneous bandwidth of the chirped output pulse after 10 wiggler periods extends from 125 to 225 GHz. The corresponding power level is 2.2 MW, with a pulsewidth of 15 ps full width at half maximum.

Simulation of an annular beam free-electron laser

Abstract A nonlinear analysis of an annular beam FEL with a helical wiggler and axial guide field is presented. An annular beam has the advantage of reduced DC self-fields, facilitating beam transport in short period wigglers. A 55 kV/5 A annular beam interacting with the TE11 cylindrical waveguide mode is considered. The inner and outer beam radii are 0.27 and 0.33 cm, respectively. The wiggler amplitude is 250 G and the period is 0.9 cm. Axial guide fields up to 3 kG are studied. The ARACHNE slow-time-scale simulation code shows that efficiencies of 10%, corresponding to gains >40 dB, are possible for grazing incidence with the TE11 mode in Ku-band. In addition, the 3 dB instantaneous bandwidth is found to be greater than 20%.

Stable high-power TE/sub 01/ gyro-TWT amplifiers

The stability of high power gyro-TWT amplifiers operating in the low-loss TE/sub 01/ mode of cylindrical waveguide has been studied, Linear theory has been used to determine the threshold start-oscillation beam current for absolute instability in the operating mode and the critical section lengths for the dominant gyro-BWO interactions occurring at various cyclotron harmonics in other waveguide modes. The performance of the amplifier was evaluated with a nonlinear, self-consistent slow-timescale simulation code. Utilizing interaction sections whose lengths are less than the threshold start-oscillation length and are separated by attenuating severs for isolation, two stable three-section devices have been designed which are predicted to yield: (1) a peak output power of 230 kW at 35 GHz with an efficiency of 23%, a saturated gain of 46 dB and a constant-drive bandwidth of 6% for a 100 kV, 10 A electron beam with an /spl alpha/=/spl nu//sub /spl perp// //spl nu//sub z/=1.0 and an axial velocity spread /spl Delta//spl nu//sub z///spl nu//sub z/=5% and (2) 105 kW at 94 GHz with 21% efficiency, 45 dB saturated gain and 5% constant-drive bandwidth for a similar 5 A electron beam. In addition, the design of the 0 dB input/output couplers and the MIG electron gun are given. Due to the low loss of the TE/sub 01/ mode, both of these amplifiers can be operated continuously. >

Experimental proof-of-principle results on a mode-selective input coupler for gyrotron applications

Proof-of-principle results for a mode selective input coupler are presented. Transmission and reflection measurements for the TE/sub 02/ cylindrical waveguide mode are given along with the output mode pattern. The results show good agreement for the cutoff frequency, mode pattern general behavior and variation with frequency for signals above cutoff. A maximum passband of 1.2 GHz (/spl sim/7%) has been achieved. Comparisons with theory for overall frequency response (from 15 to 18 GHz) and mode pattern characteristics (at 17.5 GHz) are also presented. The design and concept are promising for harmonic gyrotron-traveling-wave-tube amplifier and phase-locked gyrotron oscillator applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Basic ideas of microwave processing of polymers

AbstractThe objective of this effort has been to investigate the relationship between polymer structure and microwave absorptivity. Dielectric loss factor, ε″, loss tangent, tan δ, and oscillator strength, (εS −‐ ε∞), were used to evaluate potential material processability under applied microwave radiation. Numerous polymeric materials varying in chemical and physical structures were irradiated in a low power (≤ 100W) electric field at 2.45 GHz. Electromagnetic radiation was applied as either traveling or resonant wave modes in cylindrical and rectangular waveguides. In general, heatability was found to be a direct function of the dielectric loss dispersion dependence on temperature and frequency. The dielectric loss factor obtained at low frequency measurements was found to be directly proportional to the heatability of polymers. A WLF plot was used to predict the shift of dielectric loss maxima into or out of the microwave frequency range.

Interactive display of vector fields inside waveguides

The educational software package WGVMAP has been developed to enable students to plot interactively the electric and magnetic field lines of TE and TM propagating modes in cylindrical waveguides. The waveguide cross-sections considered are rectangular, circular, sectoral, and circular with a conducting baffle. The vector field display and generation of the field lines can be performed on a very modest configuration of an IBM PC or compatible computer, and plotted on an x-y plotter. Examples are presented to demonstrate the interactive procedure for generating the field lines.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spatiotemporal-mode structure in spherical dielectric waveguides

A novel physical concept termed ‘‘spatiotemporal mode’’ is presented for ultrashort optical pulse-beam propagation in a saturable Kerr-law nonlinear medium. The mathematical structure of the mode with spherical symmetry is described analytically. Dispersion characteristics are calculated and compared with those of the conventional spatial modes in planar and cylindrical dielectric waveguides. Nonvanishing cutoff frequency is obtained even for the fundamental mode.

Effect of a parasitic mode on the characteristics of a cylindrical waveguide mode converter with harmonically varying radius

AbstractA cylindrical waveguide with harmonically varying radiius can be used to convert selectively the TEmq mode to the TEmp mode. The characteristics of this waveguide mode converter in the presence of the parasitic TEmr mode are treated, and a modified design for the improvement of the performance of this mode converter is presented.

Operation of a large-orbit high-harmonic gyro-traveling-wave tube amplifier

The high-harmonics gyro-traveling-wave tube (gyro-TWT) is a high-power (>or=1-kW) millimeter wave amplifier based on the synchronous interaction of a beam of large-orbit axis-encircling electrons with a high-order cylindrical waveguide mode. Since the interaction occurs at a high harmonic of the cyclotron frequency, the intense magnetic fields required for the conventional fundamental-mode gyro-TWT are not required. A proof-of-principle experiment designed to demonstrate the interaction of a 150-mA, 350-keV electron beam with the TE/sub 81/ mode of a cylindrical waveguide is described. Principal results include a small signal gain of 10 dB, an interaction bandwidth of 4.3%, and a saturated power transfer from electron beam to wave of 0.5 kW. Additional measurements include the dependence of gain on electron beam current and the measurement of the beam's gamma , beta /sub 4/F, beta /sub ///, and Delta beta /sub ///. Sufficient agreement between the experimental results, the simulation codes, and an analytic description of the interaction is demonstrated to permit the design of high-performance millimeter wave amplifiers. >