H.J. Qian, J. Feng, D. Filippetto, R.K. Li, J.R. Nasiatka, H.A. Padmore, \emph{et al.},
   "Alkali Cathode Testing for LCLS-II at APEX",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper MOD03, pp. 280-284,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-MOD03,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/mod03.pdf, 2015.