E. S. LESSNER (ANL)
A free-electron laser (FEL) based on self-amplified spontaneous emission (SASE) is currently under commission at the Advanced Photon Source (APS). The APS SASE FEL  requires a high-brightness, low-emittance, and low-energy spread beam. A photocathode rf gun coupled to the APS linac is the source of the beam. Transverse wakefields generated by misalignments of the accelerating structures can degrade the beam emittance, and result in large transverse trajectory errors. Effects due to random consecutive-cell misalignments, alignment errors of the accelerator components, and long-wavelength distortion on a given structure are studied by simulation. The highest emittance dilution comes from alignment errors (steps) between the rf structures. Also, the high-amplitude centroid oscillations cause large beam losses. It is shown that trajectory corrections help reduce emittance growth and mitigate particle losses. The linac rf-structure misalignment tolerance has been set at 350 microns rms . The emittance dilution of a 5-pi-mm-rad beam due to step errors of the order of 500 microns rms can be reduced to less than 1% by trajectory correction alone. Means to reduce the short-range oscillations by Balakin, Novokhatsky, and Smirnov (BNS) damping are also investigated.
 S. V. Milton et al.,'FEL Development at the Advanced Photon Source,' Proceedings of Free Electron Laser Challenges II, SPIE, January 1999.
 M. Borland et al.,'A Higly Flexible Bunch Compressor for the APS Linac,' these proceedings.
*Work supported by U. S. Department of Energy, Office of Basic Energy Sciences under Contract No. W-31-109-ENG-38.
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