The undulators in the European XFEL are made of permanent magnets that need to be protected from beam losses that could cause demagnetisation. Under current operating conditions, beam losses in the undulators are prevented by a collimation section downstream of the main Linac and upstream of the switchyard. In the future, a slotted foil may be installed in the European XFEL to reduce the X-ray pulse length; however, the insertion of the foil will spoil the emittance of most of the bunch which increases the probability of particles scraping the collimator and continuing to be transported to the undulator section. In this paper, we report a study to assess the level of the beam losses in the undulators in the European XFEL that would be caused by a slotted foil, and to determine the optimal apertures to use in the collimators to minimise the losses. We also assess whether shielding or an additional collimator in front of the undulator could be added to the beamline to prevent the losses.

Ultra-short X-ray pulses in an XFEL can be generated by means of a slotted foil inserted into a bunch compressor. There is an ongoing study into whether such a technique could be used at the European XFEL. One important factor that must be considered is whether the additional beam losses and radiation load caused by the foil is acceptable with a high repetition rate of up to 4.5MHz at the European XFEL. As there is currently no foil implemented in the European XFEL, experimental investigations were carried out by inserting a screen in the bunch compressor at the location where a foil would be inserted. Simulations have been performed using BDSIM to study losses caused by the insertion of the foil. Neutron radiation measurements and beam loss monitor readings were taken and compared with the simulations to provide validation and calibration of the simulations for the case of a slotted foil.