• X.J. Wang, D.A. Harder, J.B. Murphy, H.J. Qian, Y. Shen, X. Yang
  BNL, Upton, Long Island, New York
• H. Freund, W.H. Miner
  SAIC, McLean

Funding: This work is supported in part by the Office of Naval Research (ONR), the Joint Technology Office, and U.S. Department of Energy (DOE) under contract No. DE-AC02-98CH1-886.

We report the first experimental characterization of efficiency and spectrum enhancement in a laser-seeded free-electron laser (FEL) using a tapered undulator. Output and spectra in the fundamental and 3rd harmonic were measured versus distance for uniform and tapered undulators. With a 4% field taper over 3 m, a 300% (50%) increase in the fundamental (3rd harmonic) output was observed. A significant improvement in the spectra with the elimination of side-bands was observed for the first time using a tapered undulator. The experiment is in good agreement with predictions using the MEDUSA simulation code.

• B. Podobedov, L. Ecker, D.A. Harder, G. Rakowsky
  BNL, Upton, Long Island, New York

We investigate AC response of accelerator vacuum chambers to external magnetic field, when the wall thickness is comparable or greater than the skin depth. Good agreement was established between experimental measurements, analytical modeling, and ANSYS simulations. Based on the results we suggest a transfer function model for electrically thick vacuum chambers with arbitrary transverse cross-section.