K. R. Cohn, J. Blau, W.B. Colson, J.W. Ng, and M.J. Price,
   "Free Electron Lasers in 2015",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper WEP016, pp. 625-629,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-WEP016,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/wep016.pdf, 2015.