Beam-based alignment for the LCLS FEL undulator

The linac coherent light source (LCLS) (LCLS Design Study Report, SLAC-R-521, 1998) is an X-ray FEL based on a long, permanent magnet undulator comprised of multiple segments which provide space for electron beam position monitors (BPMs) and quadrupole focusing magnets. The 15 GeV electron trajectory within the undulator must be straight to ∼5 μm over a gain length 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 technique is developed that uses BPM readings in conjuntion with large, deliberate variations in the electron energy. We describe here a detailed alignment simulation including BPM and magnet-mover calibration errors, quadrupole gradient errors, and dipole field errors which demonstrate that the alignment can be achieved with high confidence.