B.A. Gudkov, S. Bae, K.H. Jang, Y.U. Jeong, H.W. Kim, K. Lee, \emph{et al.}, "Stabilization of Magnetron Frequency for a Microtron-Driven FEL", in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper MOP038, pp. 107-109, ISBN: 978-3-95450-134-2, doi:10.18429/JACoW-FEL2015-MOP038, http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/mop038.pdf, 2015.