Observations of z-dependent microbunching harmonic intensities using COTR in a SASE FEL

The nonlinear generation of harmonics in a self-amplified spontaneous emission free-electron laser continues to be of interest. Complementary to such studies is the search for information on the electron beam microbunching harmonic components, which are revealed by coherent optical transition radiation experiments. An initial z-dependent set of data has been obtained with the fundamental at 530nm and the second harmonic at 265nm. The latter data were collected after every other undulator in a nine-undulator string. These results are compared to estimates based on GINGER and an analytical model for nonlinear harmonic generation.