Y. LiJ. Pflüger,
   "Control of Gap Dependent Phase Errors on the Undulator Segments for the European XFEL",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper WEP054, pp. 685-689,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-WEP054,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/wep054.pdf, 2015.