Lewellen, J.W.
(John W. Lewellen)

MOBOS03 An Experimental Study of the Beam-Steering Effect on the FEL Gain at LEUTL's Segmented Undulators
Yong-Chul Chae, Mark Erdmann, John W. Lewellen, Alex H. Lumpkin, Stephen Milton (ANL/APS, Argonne, Illinois)

The electron trajectories at the LEUTL, a SASE FEL facility at Argonne, were routinely corrected during the user run in order to deliver maximum radiation power to the user. Even though we knew from experience that SASE gain at the segmented undulators was dependent on the trajectory, the quantitative understanding of steering effects associated with the specific trajectory was lacking. Recently Tanaka et al. proposed an analytical model for the single-kick error (SKE) effect. Since LEUTL has eight segmented undulators, we performed the first measurement of SKE on the FEL gain. In the experiments we varied the corrector strength up to the critical angle, and the gain over the undulator was measured for each corrector setting. The results were compared with the analytical model and GENESIS simulations. We also measured the e-beam positions and SASE intensities over the undulators. The experimental data were analyzed and their results were reproduced by GENESIS simulation. The simulation condition, including the measured not-so-ideal trajectory, was used to predict performance enhancements that could be achieved by upgrading e-beam current, e-beam emittance, or trajectory control.

TUPOS50 Use of VUV Imaging to Evaluate COTR and Beam-Steering Effects in a SASE FEL at 130 nm
Alex H. Lumpkin, Yong-Chul Chae, Roger J. Dejus, Mark Erdmann, John W. Lewellen, Yuelin Li (ANL/APS, Argonne, Illinois)

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.

TUPOS52 Development and Measurement of Strain Free RF Photoinjector Vacuum Windows
Sandra G. Biedron (ANL, Argonne, Illinois), John W. Lewellen (ANL, Argonne, Illinois; ANL/APS, Argonne, Illinois), Lahsen Assoufid, Yuelin Li, Jun Qian (ANL/APS, Argonne, Illinois), Marcus Babzien (BNL, Upton, Long Island, New York), Gregory Gill (ISI, Sarasota Florida)

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.