  Hard photons well above 100 GeV 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-of-the-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.
