Scattering parameters of the 3.9 GHz accelerating module in a free-electron laser linac: A rigorous comparison between simulations and measurements

Abstract

This article presents a comparison between measured and simulated scattering parameters in a wide frequency interval for the third harmonic accelerating module ACC39 in the linear accelerator FLASH, located at DESY in Hamburg/Germany. ACC39 is a cryomodule housing four superconducting 3.9 GHz accelerating cavities. Due to the special shape of the cavities (in particular its end cells and the beam pipes) in ACC39, the electromagnetic field in the module is, in many frequency ranges, coupled from one cavity to the next. Therefore, the scattering parameters are determined by the entire string and not solely by the individual cavities. This makes the determination of the scattering properties demanding. As far as the authors can determine, this paper shows for the first time a direct comparison between state-of-the-art simulations and measurements of rf properties of long, complex, and asymmetric structures over a wide frequency band. Taking into account the complexity of the system and various geometrical unknowns, the agreement between experimental measurements and simulations is remarkably good for several distinct measurements, although a variety of effects (e.g. cavity deviations from the ideal shape or interactions with not modeled parts of the structure) is not considered in the computer simulation. After a short introduction, the paper provides detailed descriptions of simulations and experimental measurements performed at the module. In this context, the estimation of the cable properties is discussed as well. As a central part of the article, the comparison between measured and simulated transmission spectra and quality factors is presented. This study represents one of the first detailed comparisons between simulations and measurements for a coupled accelerator cavity system.

Eigenmode compendium of the third harmonic module of the European X-ray Free Electron Laser

“…The HOM couplers have a notch effect at 3.9 GHz so that they do not couple to the TM 01 -π mode (see Fig. 5 in [34]). The HOM couplers of the third harmonic cryomodule of the European XFEL are based on the one-leg design [see Fig.…”

“…The recently proposed state-space concatenation (SSC) [24,[27][28][29][30] algorithm allows for computing eigenmodes of long chains by combining a model-order reduction approach with a domain decomposition technique. The SSC approach is a further development of the coupled S-parameter calculation (CSC) [31][32][33] scheme, which has been employed in [34] to compute the scattering parameters of a four-cavity string with couplers. In contrast to CSC or other concatenation approaches based on sampled scattering matrices such as the GSM technique [35], SSC directly delivers the electromagnetic field information inside the complex rf structure.…”

Computation of lossy higher order modes in complex SRF cavities using Beyn’s and Newton’s methods on reduced order models

“…The speed difference by a few minutes when solving the NLEVP is however insignificant compared to the time consumption of the preceding MOR: the generation of the reduced SSM of the concatenated structure i takes roughly 7 h. For the first monopole band, the results can also be validated against measurements conducted in Ref. 80, which originate from pole-fitting the scattering parameters measured at the Third Harmonic Module at DESY. A comparison between the measurement and the solution of the NLEVP and the linearized problem is shown i The initial FIT mesh has roughly 3 · 10 6 DOF.…”

Comparison of coaxial higher order mode couplers for the CERN Superconducting Proton Linac study

“…However, the spread of those values between the fabricated cavities is of interest. The same is true for the HOM frequencies especially in terms of intercavity mode cross-talk [11]. The Q and frequency spreads also result in design requirements for the HOM coupler in relation to its sensitivity and robustness (Sec.…”