Angularly stable ring resonators for high power FELs

Optical cavity configurations for high power FELs (free-electron lasers) must form a focus at the center of the FEL wiggler and simultaneously have large spot sizes on the mirrors to avoid optical damage. Nearly concentric cavities, operating in the TEM(00) mode, satisfy these requirements, but are very sensitive to angular misalignment. In this paper, a stable semiconfocal ring cavity is presented and analyzed. The geometric angular tolerance is shown to be significantly greater than equivalent concentric cavities in the regime of interest. A general overview of the ring cavity design is also presented.