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Geometry Optimization of SPX Bellows for APS-U

Geometry Optimization of SPX Bellows for APS-U
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The Short-Pulse X-ray (SPX) generation project of Advance Photon Source Upgrade (APS-U) is in progress at Argonne National Laboratory (ANL). A method using RF deflecting cavity is proposed, and single-cell superconducting (SC) cavity is chosen for this project. In order to reach the 2-ps x-ray pulse, two cryomodule including four cavities in each cryomodule are needed, and formed bellows are inserted between the cavities. The bellows align the SC deflecting cavities with the electron beam separately. The convolution part of bellows gives mechanical flexibility to the system. However, the wakefield generated by the beam remains inside the structure. Since the bellows are inserted into about 80K cryostats with deflecting cavities, trapped mode at the bellows and its resultant heating can cause the superconductivity quenching. Therefore, the geometry of bellows to have low wake impedance characteristic and high mechanical flexibility has to be considered. In this study, the basic wake and mechanical properties are investigated by geometric parameter scan, first. They are calculated with ABCi and ANSYS workbench codes, respectively. To reduce loss factor and mechanical stress, controlling the pitch radii of convolution part and inserting straight section between convolution sets are needed. However, the wake and mechanical performances have opposite trends according to the geometric parameters. Using these results, the geometry-optimizing strategy is suggested, and the optimized bellows options are obtained. The dissipation power at surface of the option is also calculated with CST Particle Studio (PS), and it does not cause the significant heating issue.
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