P.J.M. van der Slot, K.-J. Boller, and A. Strooisma,
   "Threshold of a Mirror-less Photonic Free Electron Laser Oscillator Pumped by One or More Electron Beams",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper TUP003, pp. 327-330,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-TUP003,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/tup003.pdf, 2015.