First undulator commissioning with the K-monochromator

Hard X-ray Free Electron Lasers (XFEL) generate intense coherent X-ray beams by passing electrons through very long periodic magnet structures, which extend over hundreds of meters. The SASE1 and SASE2 undulator systems of the European XFEL consist of 35 segments with variable gap planar undulators which are initially tuned to precise on- axis magnetic field strengths in a magnetic measurement lab to keep a quality parameter, the K-value variation from segment to segment below the pierce parameter. After tunnel installation only photon based methods can determine the K-values of undulator segments with a similar accuracy. The synchrotron radiation from single or a few segments can be spectrally filtered by a dedicated crystal monochromator (K-monochromator) and recorded with either a point detector such as a photo diode or with an imager that provides 2D information, tuned for high sensitivity to detect low photon densities from distant single undulator segments. This instrumentation is applied for electron orbit analysis and optimization, and adjustment of individual undulators in terms of their central magnetic axis with respect to the electron beam. Single undulator segments were analyzed by scanning the monochromator crystal angle and detecting the steepest slope of a photo diode signal. Alternatively in the "electron-kick” method, an imager recorded the two radiation cones of electrons passing through one undulator segment, being kicked laterally and radiating a second time in the next downstream undulator.