Optics challenges in 4GLS: radiation damage

The proposed 4GLS is a suite of temporally synchronised accelerator based light sources, including a low-Q cavity free electron laser operating in the energy range 3 - 10 eV (the VUV-FEL), a seeded XUV-FEL designed to operate in the range 8 - 100 eV, an infra-red cavity FEL (the IR-FEL), as well as spontaneous radiation sources. The output from these sources is summarized. With respect to radiation damage, the two main areas of concern are the high average power from the VUV-FEL and the high peak power from the XUV-FEL. The suitability of different materials for the cavity mirror for the VUV-FEL and beamline optics is investigated. Beamline design strategies to militate against radiation damage are described and an example of a high throughput beamline in the XUV-FEL is given. The need for accurate optical constants in the VUV region, as well as damage threshold measurements, is highlighted and we outline a program of experimental measurements to be undertaken in the near future.