We have continued to explore VUV operations on the Advanced Photon Source (APS) self-amplified spontaneous emission (SASE) free-electron laser (FEL). With the installation of a fifth VUV imaging station located after undulator 7 of an eight-undulator series, we have performed our most complete SASE gain curve measurements at 130 nm as well as obtaining beam profile, position, and divergence information. This is the shortest wavelength to date for our complementary coherent optical transition radiation (COTR) measurements. We have also done the first experimental test of Tanaka et al.’s analytical model for the effects of a single-kick error of the e-beam on gain and microbunching in a SASE FEL. In addition, we compared the e-beam image centroid positions with those of the alignment laser at the available cameras and the local rf BPM readings to sort out the effective trajectory and its effect on overall gain. The FEL performance was consistent with GENESIS simulations of the experiment described in detail in a companion paper.

RF photoinjectors produce the highest brightness electron bunches only under nearly ideal illumination by a drive laser. The vacuum window used to introduce the laser beam is an essential element that may potentially degrade any distribution, making it difficult or impossible to know the actual uniformity achieved at the cathode. Because of the necessity to obtain ultrahigh vacuum near the photoinjector, some restrictions are imposed on the fabrication technology available to manufacture distortion-free windows. At the UV wavelengths commonly used for photoinjectors, it is challenging to measure and eliminate degradation caused by vacuum windows. Here, we discuss the initial laser-based measurements of a strain-free, coated, UHV window manufactured by Insulator Seal in collaboration with members of Brookhaven and Argonne National Laboratories.