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