Wavefront preserving optics for diffraction-limited storage rings and free-electron lasers

Abstract

With the advent of Diffraction Limited Storage Rings (DLSR) and the Free Electron Lasers (FEL), the challenge for optical designers is to achieve diffraction-limited spot in the experimental chamber preserving the wavefront. This improvement permits working out of focus with almost uniform beam. To reach this level of quality on the beam, one should go behind the Marechal Criterion, stating that a Strehl Ratio (SR, e.g. the ratio between the intensity on the spot for a perfect optical system and the actual one) of 0.8 is a synonymous of a well performing optic system. In reality, a Strehl ratio in excess of 0.95 is needed for wavefront preserving purposes. This corresponds having long mirrors polished at a precision of better than 1 nm rms. With the initial upgrade of the photon transport system of LCLS we demonstrated that it is possible to have an “almost” perfect beam out of focus putting proper attention to all the details and, aiming for a SR of 0.97. But, besides the high precision shape error, some other details shall be considered. For instance, how many beam sigma one should consider for the specifying his mirror and, also, does the slope errors play any role in the quality of the beam out of focus? Moreover, with the advent of SXR DLSRs, it’s important to understand the requirements for the gratings, behind the shape and slope errors, e.g. on the precision of the groove placement. Also, in this case, the Strehl Ratio is a good way for assessing this problem.