Using coherent optical transition radiation (COTR) techniques, we have observed transverse dependencies, which in some aspects relate to the electron beam microbunching in a visible wavelength (540 nm) self-amplified spontaneous emission (SASE) free-electron laser (FEL). The experimental COTR observations include the zdependent e-beam sizes, the z-dependent angular distributions, and the z-dependent spectra (which show an x-dependence). A 30-40% narrowing of the observed beam size using COTR is explainable by the mechanism's dependence on the square of the number of microbunched particles. However, additional effects are needed to explain beam size reductions by factors of 2-3 at different z locations. Localized e-beam structure in the gun or induced in the bunch compression process may result in microbunching transverse dependence, and hence the observed COTR effects.

At the Advanced Photon Source (APS), an alternate configuration of the injector linacs (low-energy undulator test line) using an rf gun as the source is projected to provide low, normalized emittance electron beams (% -5% mm mrad). In order to characterize such beams, the present intercepting Chromox screens with limited spatihi resolution and temporal response are being complemented by optical transition radiation (OTR) screens at selected positions in the beamline to provide sub-100 pm spatial resolution and sub-ps response times. Both gated cameras and streak cameras will be used to measure the beam properties. Additionally, coherent transition radiation (CTR) and diffraction radiation (DR) b'ased techniques will be evaluated. These beam characterizations will support selfamplified spontaneous emission (SASE) scaling experiments.