J.P. MacArthur, Z. Huang, A.A. Lutman, A. Marinelli, and H.-D. Nuhn,
   "Energy Spread Constraints on Field Suppression in a Reverse Tapered Undulator",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper WEP004, pp. 597-601,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-WEP004,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/wep004.pdf, 2015.