ICAP 2002 Abstracts and Posters

S-Parameter-Based Computation in Complex Accelerator Structures: Q-Values and Field Orientation of Dipole Modes

Abstract

For structures with very fine details or slight geometrical deviations an approach fully based on discretization frequently causes problem dimensions which are much too big even for powerful computing hardware. In this paper two such situations are studied: 1.) simulation of an entire 9-cell TESLA cavity, fully equipped with HOM- and Input-couplers ? the latter demanding for 3D-computation; 2.) chains of cavity cells being slightly elliptically deformed with different axis orientation. Both simulations were performed using a scheme which we denote as Coupled-S-Parameter-Calculation (CSC).

CSC applies a segmentation in several sections of the entire structure. The scattering properties of every segment needs to be computed with field solving codes, resulting in (multi-dimensional) frequency dependent S-matrices. Then CSC is used both for the concatenation of the segments and - if the whole system is entirely closed - to determine its eigen-resonances.

In our first example we take profit of the rotational symmetry of the inner part - the TESLA cavity, which therefore can be calculated in 2D. The scattering properties of the couplers at both cavity ends are computed separately. Then the CSC result of the overall system S-matrix is used to derive the Q-values of each single resonance.

The study of dipole modes in deformed cavities uses the ability of CSC to calculate for eigenmodes the frequencies and the distribution of waveguide mode amplitudes. It is shown that the orientation of the electric field experiences an angular shift along a chain of cells having elliptical deformations of different orientation. Here several parameter sets were analyzed in order to gain some experience about the phenomenon. Doing so we took advantage of the analytical description of the cell-to-cell rotation thus leading to very short cpu times.

Hans-Walter Glock, Karsten Rothemund, Ursula van Rienen

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