Power Extraction with an on-Axis Aperture

In FELIX, radiation is extracted by means of an aperture in the upstream mirror. In this paper we present results of numerical simulations showing the influence of the aperture on the FEL performance. Several cavity configurations and aperture sizes are studied. The interaction with the electron beam is taken into account in the simulations. The results are compared with measurements on FELIX. Numerical results relevant for the use of holes for injection of the electron beam are also presented. In this case, the radiation field intensity has to be minimized at the aperture position. This is achieved by choosing the resonator geometry such that the second-order Gauss-Laguerre mode has a large amplitude.