[Show abstract][Hide abstract] ABSTRACT: In many technical applications the electromagnetic eigenmodes — frequency spectrum and field distributions - of rf-components are to be determined during the design process. There are numerous cases where the
studied component is too complex to allow for a detailed enough simulation on usual servers. One way out of this situation
is domain decomposition and parallelization of the field simulation. Yet, this demands for a parallelized solver. In our approach,
we combine the use of commercial single processor-based software for the field simulation with a tool based on scattering
parameter description. The studied component is decomposed in several sections. The scattering matrices of these sections
are computed in time domain for instance with a FDTD field solver. A linear system is set up to compute the eigenfrequencies
of the complete system and the field amplitudes at the internal ports common to a pair of sections. With the knowledge of
these amplitudes the fields of the eigenmodes can be computed with help of a frequency domain field solver. This approach
is denoted as Coupled S-Parameter Calculation (CSC). Some advantages of this procedure are the possibility of easy exploitation
of symmetries in the studied components and the use of very different granularities in discretization of the single sections.
This paper presents the method, its validation using a standard eigenmode solver and applications in the field of accelerator
physics. Special attention is given to the eigenmodes of structures with slight deviations from rotational symmetry.
[Show abstract][Hide abstract] ABSTRACT: Es wird eine Methode zur Berechnung der Hochfrequenz-Eigenschaften komplexer Strukturen vorgestellt. Das Verfahren beruht auf der Zerlegung der Gesamtstruktur in einzelne einfachere Segmente, deren breitbandige S-Matrizen mit kommerziellen Programmen berechnet werden. Das Gesamtsystem kann von beliebiger Topologie sein, und die Zahl der die Segmente verkoppelnden Hohlleiter- Moden ist nicht begrenzt. Als Ergebnis steht bei offenen Strukturen deren vollständige S-Matrix, bei abgeschlossenen deren Resonanzeigenschaften zur Verfügung. Die theoretischen Grundlagen werden beschrieben und die Anwendung mit Beispielen aus dem Gebiet der Teilchenbeschleuniger und zu Eigenschaften schwach elliptisch geformter Resonatoren illustriert.
A method called Coupled S-Parameter Calculation – CSC is described which is used to calculate the rf properties of complex structures, i.e. either their scattering (devices with ports) or their resonance properties. The method is based on the segmentation of the entire system into sections being less complex, the external calculation of the section’s broadband S-matrices, and a combination scheme, which is applicable to any topology and number of modes. The method’s principle is described. Examples from the field of particle accelerator cavities and about the properties of weakly elliptical resonators are given.
Advances in Radio Science 01/2004; DOI:10.5194/ars-2-45-2004
[Show abstract][Hide abstract] ABSTRACT: Recently a third harmonic structure has been proposed for the injector of the TTF-FEL to avoid nonlinear distortions in the longitudinal phase space. This structure consists of four nine cell TESLA-like cavities. For the use of this structure in combination with the TTF-FEL it might be interesting to investigate higher order modes (HOM) in the structure and their effect on the beam dynamics. In the 5th dipole passband one mode with a frequency around 9.05 Ghz was found to be almost trapped in the cavity with very small fields in the end cells and the beam pipes. CST MicrowaveStudio® (MWS) and Coupled S-Parameter Calculation (CSC) have been applied to investigate this frequency range. The CSC method (1) is based on the scattering parameter description of the rf components found with field solving codes or analytically for components of special symmetry. This paper presents the results of the calculation of frequencies and field distributions of dipole modes in the frequency range around 9.05 GHz.
[Show abstract][Hide abstract] ABSTRACT: This paper starts with a very brief review of FIT on triangular grids. Next, it reports about some recent features in FIT to minimize the geometrical error while keeping the number of grid points as small as possible. Finally, a method is introduced to compute scattering properties and/or resonant fields in complex RF structures by combination of domain decomposition with any available CEM tool. We denote this method as coupled S-parameter calculation. Some examples are presented underlining the power of the methods described.
[Show abstract][Hide abstract] ABSTRACT: Large rf-structures are sometimes too complex to be calculated
entirely in single simulation runs. Usually, if the structure has open
ports, the scattering properties - the so-called S-parameters - are of
primary interest. As a matter of fact, they can be derived from
scattering properties of parts of the entire structure, which are
calculated in the procedure presented here in separate, less expensive
simulations. A very compact representation of the underlying theory was
found, which is presented in the paper. Furthermore, a Mathematica
application called CSC based on this formulation is introduced. CSC
calculates the scattering properties of an object, which are a
combination of an arbitrary structure of segments with previously
calculated S-parameters. To illustrate the use of CSC, three examples
are shown: higher order mode (HOM) coupling properties of components of
the TESLA Test Facility without and with accelerating cavities and the
coupling of polarizations in chains of structures with slight deviations
from circular cross sections
[Show abstract][Hide abstract] ABSTRACT: Numerical calculation of RF-properties of accelerating structures is typically performed by numerical field solving codes such as MAFIA [1] or Microwave Studio™ [1]. Even if components like cavities are of cylindrical symmetry, full 3D modelling is required in order to consider the effects of power-or HOM-couplers. This implies a numerical effort significantly higher than the separate treatment of parts with and without rotational symmetry. Therefore we have developed a method called Coupled S-Parameter Calculation (CSC) which is based on a scattering parameter description. It uses S-parameters of the various components and results in the entire structure's scattering properties. The S-parameters of the single components are computed by customary field solving codes utilizing any component's symmetry or repetition of subsections. The authors want to demonstrate the capabilities of CSC reporting results on the effect of different HOM-coupler geometries in the TESLA channel and compare the effects of different cavity-and coupler arrangements. From the CSC data the frequencies and the Q-factors of the modes in TESLA cavities are calculated.
[Show abstract][Hide abstract] ABSTRACT: The calculation of electromagnetic fields in accelerating
structures is normally done by dedicated numerical solvers such as
MAFIA. Even if components like cavities are of cylindrical symmetry,
full 3D modelling is required in order to consider the effects of power-
or HOM-couplers. This implies a numerical effort significantly higher
than the separate treatment of parts with and without rotational
symmetry. Therefore we have developed a method called coupled
S-parameter calculation (CSC) which is based on a scattering parameter
description. It uses S-parameters of the components found with field
solving codes utilising any component's symmetry or repetition of
subsections. In the paper we present a parameter variations within a
TESLA-9-cell-cavity with couplers and in a 4-cavity-chain in order to
demonstrate the capabilities of CSC
Particle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001; 02/2001
[Show abstract][Hide abstract] ABSTRACT: The TTF free electron laser needs very short bunches to produce self-amplified spontaneous emission. This short bunch length is produced in a magnetic bunch compressor where the trajectories of particles with different energy have different path length in a way that the bunch is longitudinally compressed. As a parasitic effect the wake fields produced by the passing bunch will have the possibility to interact with the bunch itself and cause emittance growth. The high frequency behaviour of the beam pipe in the bunch compressor has to be analysed in order to identify trapped higher order modes (HOM) and to estimate beam distortion. Because of the complexity of the bunch compressor section direct eigenmode calculation is not possible due to lack of available computer power. A technique is presented which allows to compute eigenmodes of rf-structures by using scattering-parameters of subsections of the bunch compressor. This is done numerically based on the computer code MAFIA to model the different sections of the beam pipe
[Show abstract][Hide abstract] ABSTRACT: Large rf-structures are sometimes too complex to be calculated entirely in single simulation runs. If the structure has open ports usually the scattering properties with respect to these ports - the so-called S-parameters - are of primary interest . They can be derived from scattering properties of parts of the entire structure, which may be calculated in separate, less expensive simulations. The paper describes briefly the underlying theory and introduces a Mathematica™ application called CSC, that calculates the scattering properties of an object which is a combination of an arbitrary structure of segments with previously calculated S- parameters. For users who are not familiar with Mathematica, a short introduction is given. The commands available in the CSC package are explained and an example is illustrated in detail.