A. Jostingmeier

Application of the GSD technique to the analysis of slot-coupled waveguides

The generalized spectral domain (GSD) technique is applied to the analysis of waveguide coupling by axially uniform slots. For a slotted-circular waveguide, cut off wavenumbers of TE and TM modes as well as the corresponding field distributions in the composite waveguide are determined and compared to that obtained by other methods. Plots of the field lines corresponding to the dominant as well as a number of higher order modes are presented. It is shown that considering the H/sub 00/ modes in the comprising waveguides is necessary. Furthermore, it is demonstrated how the efficiency of the method regarding accuracy and CPU time requirements can considerably be enhanced if infinite sums over eigenmodes which have to be evaluated are replaced by closed-form expressions. >

Analysis of inhomogeneously filled cavities coupled to waveguides using the VIE formulation

A method based on the volume integral equation (VIE) formulation is presented. It can be used to analyze inhomogeneously filled cavities of arbitrary shape to which cylindrical waveguides of arbitrary cross section are coupled. The inhomogeneity inside the cavity is described by a spatially dependent permittivity. The method is applied to the problem of scattering by dielectric bodies inside waveguides. The authors also show how the convergence of the method can be accelerated. >

Application of the eigenmode transformation technique for the analysis of planar transmission lines

The eigenmode transformation technique is suitable for the analysis of inhomogeneously filled shielded waveguides containing metal inserts. The permittivity of the filling medium may be an arbitrary function of the transverse coordinates. The method is based on expanding the electromagnetic field in terms of the eigenmodes of the corresponding empty shielding waveguide. The metal inserts have the effect of linearly transforming these eigenmodes into those of the waveguide containing the metal inserts only. This leads to a proper matrix eigenvalue problem. The method is applied to different types of planar transmission lines and the results are compared with other methods.

Numerically efficient taper analysis with controlled resolution

Circular waveguide tapers, which are frequently used in gyrotrons, are analyzed by subdividing the taper into waveguide steps and uniform waveguide sections. Using a special subdivision and controlling the field resolution lead to a high speed-up factor compared to conventional approaches. Numerical results for a linear taper are presented which show that the speed-up factor becomes larger if the number of steps is increased. The results obtained demonstrate the high numerical efficiency of the method.<<ETX>>

Unified analysis of quasi-TEM and higher order modes in planar transmission lines

TEM modes are shown to be derivable from a scalar magnetic potential /spl psi/ if the supporting transmission line can be considered a slot coupled waveguide with more than one coupling slot. The magnetic potential must jump at the coupling slots in order to give rise to the axial electric current on the strips separating these slots. The advantage of this formulation is its compatibility to the numerically efficient generalized spectral domain (GSD) technique which has already been used for the calculation of TE and TM modes in slot-coupled waveguides. Substrates are taken into account by applying the eigenmode transformation technique. Numerical results are presented for TEM modes in multi-slot lines and for quasi-TEM and higher order modes in coplanar waveguides. Excellent agreement with the results obtained by other methods have been achieved with moderate cpu time and storage requirements.

Mode computation in long tapered multi-cell linear accelerator structures using the GSM method

For a proper design of linear colliders it is important to know the resonant modes corresponding to higher order dipole passbands of long tapered multi-cell structures. Grid-oriented codes, as, for example, the MAFIA program package, cannot be used for the analysis of such structures. In this contribution, the accurate and numerical efficient generalized scattering matrix method is applied to the computation of these modes. The sixth dipole passband of the 180-cell accelerating structure used for the S-band linear collider at DESY is extensively being investigated with the proposed method. The calculations predict that this passband is especially dangerous for a stable operation of the collider which will lead to a change of the current structure design. The validity of the developed code is confirmed by comparing the results which have been obtained from the MAFIA program package for a 36-cell structure with those of our method. Furthermore a special numerical technique is suggested allowing a reliable computation of the so-called trapped modes which often occur in tapered multi-cell structures.

Self-consistent multi-mode time domain analysis of gyrotrons

A self-consistent time domain analysis of gyrotrons is presented which allows studying multi-mode, multi-frequency operation. The electromagnetic field in the gyrotron cavity is expanded with respect to complete sets of eigenfunctions so that space charge effects are included in the analysis. It is demonstrated that the strong numerical requirements of this method can be met by using a vector computer. The simulations show that the assumption of a monofrequent steady state operation of gyrotrons, which is made by the commonly used frequency domain methods is not always fulfilled. For a low Q gyrotron, both oscillation build-up and steady state operation is investigated including mode competition and window reflections.<<ETX>>

Systematic searching for cavity and waveguide modes by making use of Foster's theorem

Resonance and irrotational cavity modes as well as waveguide eigenmodes are usually determined numerically (except for a few structures whose solutions are analytically known). Most of the methods of analysis end up at a determinental equation, the zeros of which characterize these modes. Numerically overlooking some of these zeros is one of the problems which greatly degrade the accuracy of a field expansion in terms of the corresponding modes. In this contribution it is shown that it is always possible to find a reactance (or susceptance) function having the same set of zeros as the original determinental equation. This enables making use of Fosters theorem to systematically determine these zeros.<<ETX>>

Analysis of an efficient TE/sub 0,n/-to-TE/sub 0,n+p/ mode converter in circular waveguides

Mode converters which have recently been proposed for high-power gyrotron applications are analyzed. For the computation of the local modes at each cross section of the converter, a generalized spectral domain technique is applied, while in the axial direction a waveguide taper analysis is performed which is based on a generalized scattering matrix method. The numerical results demonstrate the good accuracy and the high numerical efficiency of the method.<<ETX>>