Abstract

Mirror surface quality requirements are theoretically investigated for XUV FEL oscillators. The resonator configuration considered is a grazing incidence ring using metal coatings. Starting at the end of the wiggler, the optical train consists of (1) a grazing incidence hyperboloid to magnify the optical beam expansion, (2) a multifaceted mirror to retroreflect the beam (the multifaceted mirror is composed of several flats and an off-axis paraboloid to collimate the expanding beam), (3) a scraper to output couple the optical energy, and (4) a second multifaceted mirror and hyperboloid pair to focus the optical beam to the wiggler entrance. For this resonator configuration, we evaluate FEL performance degradation due to mirror surface imperfections which can result from polishing errors and thermal distortion. The concept of mode matching is applied to the resonator design. Mode matching in an FEL oscillator is defined as despacing (translating) the paraboloidal mirrors to match the optical properties of the resonator to the FEL gain region (wiggler).

© 1989 Optical Society of America