A Versatile Adaptive Optics System for Alba Beamlines

Abstract

Introduction The development of X-ray photon science has been characterized in the last few years by the development of free electron lasers and the so-called diffraction limited storage rings. These new sources are characterized by very high brilliance and increased transversal coherence. These features open new scientific opportunities, as they allow for higher spatial resolution, increased flux, and extended coherence length for diffraction and imaging techniques. Profiting from these features requires building or upgrading the optical systems of the beamlines employing optical elements, mostly mirrors, of very high quality. Mirrors with exceedingly large surface errors are either limiting the achievable smallest spot size or the highest reachable photon flux density, or causing intensity striations of the beam out of focus [1–3]. Despite the great improvement in the surface accuracy of commercially available mirrors experienced in the last decade, fruit of the development of deterministic polishing [4, 5], obtaining the ideal optical mirror surfaces at a beamline in operation is still challenging. In addition to residual polishing and figuring errors, there are errors that appear only after integration of the mirror at the beamline, or during operation, caused for instance by the heat load or by residual strain induced by the clamps or by the cooling system. More importantly, often beamlines require being able to change or manipulate the wavefront. Examples of this are beamlines where the position of the focused spot is shifted from one station to another, or when it depends on the photon energy. Other examples are beamlines that match the spot size to the sample in order to maintain the total incident flux while minimizing the flux density (to minimize radiation damage or space charge). The simplest example of adaptive optics is given by mechanical mirror benders, existing in many facilities for many years. These devices introduce a mechanical constraint at the ends of the mirror substrate, which results in a change of the mirror curvature profile, thus providing control of the lowest aberration terms of the wavefront. To have better control of the wavefront, one can add more actuators along the mirror (see Figure 1), thus introducing additional degrees of freedom. Of course, for the system to work, the actuators must provide enough resolution to modify the mirror surface with sub-nanometer resolution. Some systems obtain such resolution by taking advantage of the bimorph effect [6, 7] or using piezoelectric actuators [8, 9]. Alternatively, other systems, like the system developed by ALBA, achieve the required resolution by using elastic elements in the actuators [10–12]. These introduce and maintain a force rather than a geometrical constraint. In their case, the ratio between the stiffness of the mirror and of the spring is large enough to allow obtaining sub-nanometer resolution at the surface of the mirror using only conventional UHV mechanics. The use of standard mechanics and materials allows adapting the system to a wide variety of application cases. In this report, we describe the main characteristics of the mirror deformation introduced by a discrete number of force actuators, with the implications they have on the mechanical design. We then describe the mechanical solutions implemented to obtain the required correction.