C. Emma, K. Fang, C. Pellegrini, and J. Wu,
   "Tapering Studies for TW Level X-ray FELs with a Superconducting Undulator and Built-in Focusing",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper WEP076, pp. 726-729,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-WEP076,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/wep076.pdf, 2015.