First observations of COTR due to a microbunched beam in the VUV at 157 nm

The self-amplified spontaneous emission free-electron laser experiments at the Advanced Photon Source are now operating in the VUV at 157 nm for a user experiment. In conjunction with these runs, we have obtained the first coherent optical transition radiation data due to the microbunching of the electron beam in the VUV. We have used both near- and far-field focusing by selecting the spherical mirror with the appropriate focal length for the distance to the CCD chip. The optics are such that much higher resolution than our visible system is attained with calibration factors of 11 μm/pixel and 10 μrad/pixel, respectively. Localized effects in the distributions in both focal conditions are being addressed.