Design study of an optical cavity for a future photon-collider at ILC

Hard photons well above 100GeV have to be generated in a future photon-collider which essentially will be based on the infrastructure of the planned International Linear Collider (ILC). The energy of near-infrared laser photons will be boosted by Compton backscattering against a high energy relativistic electron beam. For high effectiveness, a very powerful lasersystem is required that exceeds todays state-ofthe-art capabilities. In this paper a design of an auxiliary passive cavity is discussed that resonantly enhances the peak-power of the laser. The properties and prospects of such a cavity are addressed on the basis of the specifications for the European TeV Energy Superconducting Linear Accelerator (TESLA) proposal. Those of the ILC are expected to be similar.