First observations of electron-beam microbunching in the deep ultraviolet at 265 nm using COTR in a SASE FEL

We have recently extended our microbunching experiments in a self-amplified spontaneous emission free-electron laser using coherent optical transition radiation (COTR) to the deep ultraviolet wavelengths (265 nm) for the first time. These experiments were performed as a complement to the Advanced Photon Source SASE FEL projectʹs thrust to shorter wavelengths. In order to do this, the optical diagnostics have been modified to include UV-sensitive cameras, and an optical transport has been installed that involves sets of mirrors and UV–visible lens pairs after each undulator. These optics provide transport to the in-tunnel Oriel UV–visible spectrometer. Since this is an imaging spectrometer, both spectral and x/y-plane spatial information are simultaneously available by performing the projections of the images on the x- and y-axis. The initial angular distribution data and beam size data have been obtained in a z-dependent manner by sampling after every other undulator, or every 4.8 m at four z locations. Experiments with sampling of the COTR angular distribution and spectra after each of eight undulators are planned.