New approach to characterize RF properties in complex structures
by Margarita Synanidi (CERN) with Thomas Flisgen, Johann Heller, and Ursula van Rienen (Uni Rostock)
Magnitude of the electric field of a higher order multi-cavity eigenmode in a chain of eight superconducting 3.9 GHz cavities (zoom at the bottom), which are connected via beam-pipes and bellows (zoom at the top). Image credit: UniRostock
The computation of RF properties of large and complex accelerating structures is a challenging and important task for the design and operation of particle accelerators. A new method, developed under EuCARD-2 WP12 allows for the characterization of radio frequency properties such as field distributions, quality factors or impedance/scattering parameters of large and complex structures.
Until now direct approaches that determine these properties required expensive supercomputers due to the vast number of degrees of freedom arising from the direct discretization of the complex structure. Moreover, results delivered by direct methods could be unsatisfying due to round-off errors or numerical instabilities arising from ill-conditioned system matrices. The computational demand of the direct computations of the full chain can be avoided by decomposing the structure into single elements. In a next step, the individual segments are described by means of reduced order state-space models, which are combined to a reduced order model of the entire structure. The approach is referred to as state-space concatenations (SSC) and enables the computation of radio frequencies properties of complex structures by using standard workstation computers, rather than supercomputers.
The novelty of this approach is the description of the individual segments by means of state-space models. The state-space models provide a rigorous description of the radio frequency properties of the segments, in particular, they enable to reconstruct field distributions. Computations or radio frequency properties are not only relevant for cavities, but are also performed in other studies, e.g. in case of modelling bunch compressors. The SSC approach is not restricted to a special topology or particle accelerator applications and can be used to other similar calculations as well to avoid the usage of expensive cluster computers.
The next step incorporates the generation of detailed computer models of chains of four and eight third harmonic cavities, which are accommodated in FLASH or the XFEL. Previous studies have shown that the consideration of the entire chain is required rather than restricting to individual cavities. It is planned to generate a modal compendium (list of eigenmodes and their properties) for these structures based on the detailed computer models.
