Girder deformation related phase errors on the undulators for the European X-Ray Free Electron Laser

In long gap tunable undulators, strong magnetic forces always lead to some amount of gap-dependent girder deformation and resulting gap-dependent phase errors. For the undulators for the European XFEL, this problem has been investigated thoroughly and quantitatively. Using the different gap dependencies of suitable shims and pole height tuning, a method is presented which can be applied to reduce the overall gap dependence of the phase error if needed. It is exemplified by tuning one of the undulator segments for the European X-Ray Free Electron Laser back to specs