Beam-based alignment for the LCLS FEL undulator

The linac coherent light source (LCLS) (LCLS Design Study Report, SLAC-R-52 1, 1998) is an X-ray FEL based on a long, permanent magnet undulator compri sed of multiple segments which provide space for electron beam position mon itors (BPMs) and quadrupole focusing magnets. The 15 GeV electron trajector y within the undulator must be straight to similar to 5 mu m over a gain le ngth of 11 m so that the photon beam overlaps efficiently with the electron beam. This alignment precision is not achievable with available mechanical survey methods. For this reason, an empirical beam-based alignment techniq ue is developed that uses BPM readings in conjunction with large, deliberat e variations in the electron energy. We describe here a detailed alignment simulation including BPM and magnet-mover calibration errors, quadrupole gr adient errors, and dipole field errors which demonstrate that the alignment can be achieved with high confidence. (C) 1999 Elsevier Science B.V. All r ights reserved.