A. Mak, F. Curbis, and S. Werin,
   "Further Studies of Undulator Tapering in X-Ray FELs",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper TUP002, pp. 321-326,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-TUP002,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/tup002.pdf, 2015.