Measurements of Nonlinear Harmonic Generation at the Advanced Photon Source'S Sase Fel

SASE saturation was recently achieved at the Advanced Photon Source's SASE FEL in the low-energy undulator test line at 530nm and 385nm. The electron beam microbunching becomes more and more prominent until saturation is achieved. This bunching causes nonlinear harmonic emission that extends the usefulness of a SASE system in achieving shorter FEL wavelengths for the same electron beam energy. We have investigated the intensity of the fundamental and second harmonic undulator radiation as a function of distance along the undulator line and present the experimental results and compare them to numerical simulations. In addition, we have measured the single-shot second harmonic spectra as well as the simultaneous fundamental and second harmonic spectra and present the experimental results.