%\cite{Miginsky:FEL2015-WEB02} \bibitem{Miginsky:FEL2015-WEB02} S.V.~Miginsky, S.~Bae, B.A.~Gudkov, K.H.~Jang, Y.U.~Jeong, H.W.~Kim, \emph{et al.}, % S.V. Miginsky \emph{et al.}, \textquotedblleft{Waveguide THz FEL Oscillators}\textquotedblright, % in \emph{Proc. FEL2015}, WEB02, pp. 576--579, % in \emph{Proc. FEL2015}, Daejeon, Republic of Korea, August 2015, paper WEB02, pp. 576--579, in \emph{Proc. 37th International Free Electron Laser Conference (FEL2015)}, Daejeon, Republic of Korea, August 2015, paper WEB02, pp. 576--579, ISBN: 978-3-95450-134-2, \url{http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/web02.pdf}, doi:10.18429/JACoW-FEL2015-WEB02, 2015.