The TTF-FEL radiation can be used for very different types of experiments exploiting the high intensity and brightness to produce extremely high power densities in a small area or to filter out a narrow energy band for spectroscopic studies at ultra-high energy resolution. Most experiments will require optical elements to focus or diffract the photon beam. The FEL beam could be used more efficiently by switching between two separate beamlines using plane mirrors as far upstream as possible. It

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... ppears thus necessary to start with a brief discussion of possible damage to mirrors and other matter due to the extreme power density of the FEL radiation. The results of this discussion will aid in finding appropriate concepts for a possible optical layout of the experimental hall and for a high resolution monochromator.