• A.H. Lumpkin
  Fermilab, Batavia
• M. Borland, Y.L. Li, K. Nemeth, S.J. Pasky, N. Sereno
  ANL, Argonne

During the commissioning of the LCLS injector in 2007, unexpected enhancements of the signals in the visible light optical transition radiation (OTR) monitors occurred after compression in a chicane bunch compressor. These were attributed to a microbunching effect of some kind. Explorations of such effects have now been performed on the Advanced Photon Source (APS) linac. The APS injector complex includes an option for photocathode (PC) gun beam injection into the 450-MeV S-band linac. At the 150-MeV point, a 4-dipole chicane was used to compress the micropulse bunch length from a few ps to sub-0.5 ps (FWHM). Noticeable enhancements of the OTR signal sampled after the APS chicane were observed. A FIR CTR detector and interferometer were used to monitor the bunch compression process and correlate the appearance of localized spikes of OTR signal (5-10 times brighter than adjacent areas) within the beam image footprint. We have done spectral dependency measurements at 375 MeV with a series of band pass filters centered from 400 to 700 nm and observed a broad-band enhancement in these spikes. Discussions of the possible mechanisms will be presented.