S. Di Mitri,
   "On the Importance of Electron Beam Brightness in High Gain Free Electron Lasers",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper MOP079, pp. 227-232,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-MOP079,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/mop079.pdf, 2015.