A Fixed Included Angle Monochromator for the 4(th) generation light source FERMI@Elettra

FERMI@Elettra is a Free Electron Laser (FEL) user facility currently under construction at Sincrotrone Trieste, Italy. It will provide a spatial coherent transform-limited beam in the sub-ps regime, covering the wavelength range from 100 nm to 3 nm with the goal of 1 nm (by using third harmonics). The transform-limited beam is supposed to have a natural energy bandwidth of the order of 50-100 meV. Nevertheless, one of the three future beamlines, the one dedicated to Low Density Matter (LDM) studies, needs a monochromator to clean the signal. It must cover the whole wavelength range with eventual omission of the lower energy part. We will report the design of a fixed included angle monochromator employing three gratings. The optical system will be described, and particular attention will be given to the constraints like the pulse broadening, the focus displacement and, of course, the flux. Engineering constraints and manufacturers tolerances also taken into account will be presented, too.