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| Book/Report/Dissertation / PhD Thesis | PUBDB-2017-00617 |
; ;
2017
Verlag Deutsches Elektronen-Synchrotron
Hamburg
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Please use a persistent id in citations: doi:10.3204/PUBDB-2017-00617
Report No.: DESY-THESIS-2017-001
Abstract: The European XFEL and the International Linear Collider are based on superconductingrf cavities. In order to reach the theoretical gradient limits of the superconducting cavitiesit is necessary to increase the mechanical quality and chemical composition of the innersurface as well as to understand the reason for performance limitations. This work is basedon the diagnosis of over 100 XFEL and HiGrade cavities whose performance was limitedby several factors: eld emission on dust or surface defects, low-eld thermal breakdowncaused by the defects, Q-slope etc.It was found that some defects were produced during the mechanical production of thecavity and were not removed by electro-chemical polishing, a standard processing tech-nique of the inner cavity surface. On the other hand, some of the defects were producedduring the electro-chemical polishing process as the surface initially had imperfections orinclusions of foreign material.One of the opportunities to overcome the aforementioned drawbacks is to replace the\bulk" electro-chemical polishing process by mechanical centrifugal barrel polishing. Theparameters of the surface after each polishing step were studied using small samples, so-called coupons. An undersurface layer was investigated using metallographic techniquesand cross sectioning. The inuence of centrifugal polishing on the specic parameters ofa 9-cell cavity (eld atness, eccentricity etc.) was investigated. As a result, a single-stepcentrifugal barrel polishing process followed by a standard \light" electropolishing wasproposed for industrial application.Although the performance-limiting mechanisms are understood in general, the origin ofthe quench of the cavity is often unclear. To determine the quench locations, a localisationtool for thermal breakdown using the \second sound" in superuid helium has been used.All components of this tool were improved to increase the accuracy of the measurements.A new program code for quench localisation calculating the path of the second-soundwave was developed. This allows the signals from all sensors to be used, regardlessof their position relative to the quench site. The new approach was validated usingadditional techniques such as a temperature mapping and an optical inspection of theinner cavity surface. Furthermore, a new multi-sensor for second-sound wave detection inthe helium vessel of a cavity was developed and successfully tested on a serial-productionXFEL cavity. The determined quench site location was conrmed by subsequent opticalinspection. The algorithm localises the quench without mode measurements i.e. thereis no need to dismount HOM-antennas which requires special procedures and must beperformed in a clean-room.The mathematical approach described in this paper can be applied for second-sound testsof superconductive cavities of various shapes and dimensions.
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