An enhancement cavity includes a plurality of focusing mirrors, at least one of which defines a central aperture having a diameter greater than 1 mm. The mirrors are configured to form an optical pathway for closed reflection and transmission of the optical pulse within the enhancement cavity. Ring-shaped optical pulses having a peak intensity at a radius greater than 0.5 mm from a central axis are directed into the enhancement cavity. Accordingly, the peak intensity of the optical pulse is distributed so as to circumscribe the central apertures in the apertured mirrors, and the mirrors are structured to focus the pulse about the aperture toward a central spot area where the pulse is focused to a high intensity.