Observations on COTR due to the Microbunching Instability in Compressed Beams

The observations of the strong enhancements of the optical transition radiation (OTR) signal observed after bunch compression of photocathode beams in the Advanced Photon Source (APS) linac chicane continue to be of interest. These effects have been attributed to a microbunching instability. Recent tests with an Oriel spectrometer with ICCD readout have extended our COTR spectral studies. In this case we used a microchannel plate image intensifier with a GaAs photocathode to explore the NIR regime out to 880 nm. We report for the first time an ∼40% intensity modulation of ∼25-nm period throughout the observed broadband COTR spectra for some localized spatial structures. In addition, the observation of a ring-like COTR image structure in x-y space for a mismatched beam at 355 MeV and a COTR mitigation concept is presented and discussed.