I.V. Davidyuk, O.A. Shevchenko, V.G. Tcheskidov, and N. Vinokurov,
   "The Prototype of New Variable Period Undulator for Novosibirsk Free Electron Laser.",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper WEP051, pp. 677-680,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-WEP051,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/wep051.pdf, 2015.