Wavefront preserving X-ray optics for Synchrotron and Free Electron Laser photon beam transport systems

• Published in: Physics reports Vol. 974; pp. 1 - 40
• Main Authors: Cocco, D., Cutler, G., Sanchez del Rio, M., Rebuffi, L., Shi, X., Yamauchi, K.
• Format: Journal Article
• Language: English
• Published: United States Elsevier B.V 29.08.2022; Elsevier
• Subjects: Diffraction limit; Mirror deformations; OTHER INSTRUMENTATION; Ultra precise mirrors; Wavefront diagnostics; Wavefront preservation; X-ray optic simulations; X-ray optics; Diffraction limit; Ultra precise mirrors; Wavefront diagnostics; X-ray optics; Mirror deformations; X-ray optic simulations; Wavefront preservation
• ISSN: 0370-1573; 1873-6270
• DOI: 10.1016/j.physrep.2022.05.001

In the last two decades, after the first light from the new Free Electron Lasers, either in the UV (FLASH [1] and FERMI@Elettra [2]) or X-ray (LCLS  [3] and SACLA [4]), more and more new diffraction limited sources have been either constructed or planned. Third generation storage rings are upgraded to provide a more collimated, brighter, and coherent light for the next generation experimental techniques. X-ray optics are the bridge between the light sources and the experimental stations. They are the key to the success of advanced experiments but also the potential bottleneck preventing the exploitation of the full characteristics of the source. The beam degradation originated by any mirror defect (either from mirror polishing or from contamination) is amplified with a coherent source. Delivering diffraction-limited spots, including the option for variable spot sizes in and out of focus, requires the control of the surface of the optics at the 1 nm rms level, if not better. At LCLS, only very recently [5] an almost perfectly uniform beam out of focus has been obtained in the hard X-rays. It has been obtained after two 1-m long mirrors with 0.5 nm rms shape precision (after installation). Those mirrors were not readily available just a decade ago. But, thanks to the pioneering work performed at the Osaka University [6,7], those optics are now commercially available with arbitrary tangential profiles. This new generation of mirrors permits achieving unprecedented results. However, they do not remove all road blocks to a perfect photon transport system. Instead, they highlighted the critical importance of mirror mounting, handling of the thermal deformation, and the need of advanced diagnostics to properly exploit all the new potentiality of these optics. Besides the need of “perfect” mirrors, other aspects of the beamline design and elements may impact the quality of the beam in the experimental station, from the lack of blaze gratings to the need of advanced simulation tools, just to cite two. In this article, after a brief historical excursus, we will present the current state of the art of mirrors, gratings, crystals, lenses, diagnostics, and simulation tools. The main problems yet to solve and a look ahead at what would be achievable in the next decade will give the reader an idea on the search for an almost ideal photon transport system and how the path toward experiments, not conceivable today, will unfold.