Hole Coupling in Free-Electron Lasers

In this paper we present numerical simulations of the performance of a far-infrared Free Electron Laser with an aperture in the upstream cavity mirror. Two different applications of mirror apertures are studied: broadband extraction of radiation and injection of the electron beam. The emphasis is on the effect of the aperture on the amplitude of the higher-order transverse modes. It is shown that the mode distribution at saturation can be drastically influenced by variation of the mirror radius of curvature. A simple formula is derived which is quite useful to predict the dominant higher-order modes. This permits optimization of the cavity with respect to the application in question.