Abstract
An Yb-based 78-MHz repetition rate fiber-amplified frequency comb is used to investigate the power scaling limitations of a standard-design bow tie high-finesse enhancement cavity for XUV generation. With a Xenon gas jet in the 22-mu m-radius focus, the 200-fs intra-cavity circulating pulse reaches a maximum of 20 kW of time-averaged power. A novel cavity design is presented, conceived to overcome the observed enhancement limitations and offering the prospect of few-nm high-power high-harmonic generation. Several applications which come into reach for the first time are discussed.