Axisymmetric cw multi-pass diffractive numerical calculations for the free-electron laser (FEL) are presented. The output is extracted around the edge of one mirror in a symmetric resonator of low Fresnel number. Excellent beam quality and small mode area inside the wiggler are obtained when the resonator is in a negative-branch unstable configuration. We suggest the use of a deformable mirror for gain-switching a tapered-wiggler FEL.
Gerald T. Moore,
"Unstable Resonators For The Free-Electron Laser", Proc. SPIE 0453, Free-Electron Generators of Coherent Radiation, (16 February 1984); doi: 10.1117/12.947104; https://doi.org/10.1117/12.947104