Scheme for generating and transporting THz radiation to the X-ray   experimental hall at the European XFEL
  The design of a THz edge radiation source for the European XFEL is presented. We consider generation of THz radiation from the spent electron beam downstream of the SASE2 undulator in the electron beam dump area. In this way, the THz output must propagate at least for 250 meters through the photon beam tunnel to the experimental hall to reach the SASE2 X-ray hutches. We propose to use an open beam waveguide such as an iris guide as transmission line. In order to efficiently couple radiation into the iris transmission line, generation of the THz radiation pulse can be performed directly within the iris guide. The line transporting the THz radiation to the SASE2 X-ray hutches introduces a path delay of about 20 m. Since THz pump/X-ray probe experiments should be enabled, we propose to exploit the European XFEL baseline multi-bunch mode of operation, with 222 ns electron bunch separation, in order to cope with the delay between THz and X-ray pulses. We present start-to-end simulations for 1 nC bunch operation-parameters, optimized for THz pump/X-ray probe experiments. Detailed characterization of the THz and SASE X-ray radiation pulses is performed. Highly focused THz beams will approach the high field limit of 1 V/atomic size. 
Winfried Decking, Gianluca Geloni, Vitali Kocharyan, Evgeni Saldin and   Igor Zagorodnov
physics.acc-ph