TY  - THES
AU  - Tamashevich, Yegor
TI  - Diagnostics and treatment of 1.3 GHz Nb cavities
IS  - DESY-THESIS-2017-001
SN  - 1435-8085
PB  - Universität Hamburg
VL  - Dissertation
CY  - Hamburg
M1  - PUBDB-2017-00617
M1  - DESY-THESIS-2017-001
T2  - DESY-THESIS
SP  - 207
PY  - 2017
N1  - Dissertation, Universität Hamburg, 2016
AB  - 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" 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  "  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  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.
LB  - PUB:(DE-HGF)3 ; PUB:(DE-HGF)29 ; PUB:(DE-HGF)11
DO  - DOI:10.3204/PUBDB-2017-00617
UR  - https://bib-pubdb1.desy.de/record/317338
ER  -