The past decades have witnessed the development of new X-ray beam sources with brightness growing at a rate surpassing Moore's law. Current and upcoming diffraction limited and fully coherent X-ray beam sources, including multi-bend achromat based synchrotron sources and high repetition rate X-ray free electron lasers, puts increasingly stringent requirements on stability and accuracy of X-ray optics systems. Parasitic motion errors at sub-micro radian scale in beam transport and beam conditioning optics can lead to significant loss of coherence and brightness delivered from source to experiment. To address this challenge, we incorporated optical metrology based on interferometric length and angle sensing and real-time correction as part of the X-ray optics motion control system. A prototype X-ray optics system was constructed following the optical layout of a tunable X-ray cavity. On-line interferometric metrology enabled dynamical feedback to a motion control system to track and compensate for motion errors. The system achieved sub-microradian scale performance, as multiple optical elements are synchronously and continuously adjusted. This first proof of principle measurement demonstrated both the potential and necessity of incorporating optical metrology as part of the motion control architecture for large scale X-ray optical systems such as monochromators, delay lines, and in particular, X-ray cavity systems to enable the next generation cavity-based X-ray free electron lasers.

The name of an author in the article by Walter et al. (2022) [J. Synchrotron Rad. 29, 957–968] is corrected.

In this paper we report on the modeling and characterization of transmission windows for in-situ interferometric measurements of cryogenically cooled mirrors. Specifically, we present a model of the temperature distribution and strain in the transmission window, and the corresponding spatial dependence of the window’s index of refraction. We also present experimental results which characterize the effect of the windows on interferometric measurement.

The LCLS-II HE Project includes the upgrade of the x-ray beam transport line for DXS, CXI and MFX, which requires an additional 10 bendable mirror systems. For meeting the project demand, the LCLS Metrology Laboratory has added another set of instrumentation to perform stitching measurement using a Zygo 6” DynaFiz (Fizeau interferometer). Prior to the upgrade, the lab has one stitching setup for measuring horizontally facing mirrors, up to 1.5 m long. This new setup enables the measurement of vertically facing mirrors up to 1.2 m long. The measuring systems allow the incorporation of environmental logging as well as control of mirror mechanics such as bender actuators. This means the lab can manipulate two mirror systems and perform the measurements simultaneously, independently and fully automatically. The performance and repeatability of the new instrumentation will be presented. An example measurement of a pair of LCLS mirrors will be discussed.

With the nearly full spatial coherence of X-ray free electron lasers comes unprecedented requirements on the precise figure of X-ray mirrors. For example, the Time-resolved AMO Instrument (TMO) requires a bendable Kirkpatrick-Baez (KB) mirror system in order to produce a range of focus spot sizes, while maintaining nm-level figure error across the corresponding range of elliptical shapes. Here we describe the process of TMO KB mirror characterization using optical metrology, the detailed comparison with at-wavelength wavefront sensors during instrument commissioning, and the implementation of an automatic focusing system that ties together the wavefront sensor output with the mirror bender controls.

The newly constructed time-resolved atomic, molecular and optical science instrument (TMO) is configured to take full advantage of both linear accelerators at SLAC National Accelerator Laboratory, the copper accelerator operating at a repetition rate of 120 Hz providing high per-pulse energy as well as the superconducting accelerator operating at a repetition rate of about 1 MHz providing high average intensity. Both accelerators power a soft X-ray free-electron laser with the new variable-gap undulator section. With this flexible light source, TMO supports many experimental techniques not previously available at LCLS and will have two X-ray beam focus spots in line. Thereby, TMO supports atomic, molecular and optical, strong-field and nonlinear science and will also host a designated new dynamic reaction microscope with a sub-micrometer X-ray focus spot. The flexible instrument design is optimized for studying ultrafast electronic and molecular phenomena and can take full advantage of the sub-femtosecond soft X-ray pulse generation program.

Vol. 35, No. 2, 2022, Synchrotron radiation newS Technical RepoRT The X-ray Focusing System at the Time-Resolved AMO Instrument Matthew Seaberg,1 Lance Lee,1 DanieL Morton,1 XinXin cheng,1 JaMeS cryan,1,2 gregorio ivan curieL,1 brenDan DiX,1 taran Driver,1 Kay FoX,1 corey harDin,1 anDrei KaMaLov,1 Kenan Li,1 Xiang Li,1 Ming-Fu Lin,1 yanwei Liu,1 tiM Montagne,1 razib obaiD,1 anne SaKDinawat,1 Peter SteFan,1 ranDy whitney,1 thoMaS woLF,1,2 Lin zhang,1 DaviD Fritz,1 Peter waLter,1 DanieLe cocco,3 anD May Ling ng1 1SLAC National Accelerator Laboratory, Menlo Park, California, USA 2Stanford PULSE Institute, Menlo Park, California, USA 3Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA

This article describes the development and testing of a novel, water-cooled, active optic mirror system (called "REAL: Resistive Element Adjustable Length") that combines cooling with applied auxiliary heating, tailored to the spatial distribution of the thermal load generated by the incident beam. This technique is theoretically capable of sub-nanometer surface figure error control even at high power density. Tests conducted in an optical metrology laboratory and at synchrotron X-ray beamlines showed the ability to maintain the mirror profile to the level needed for the next generation storage rings and FEL mirrors.

An ongoing collaboration among four US Department of Energy (DOE) National Laboratories has demonstrated key technology prototypes and software modeling tools required for new high-coherent flux beamline optical systems. New free electron laser (FEL) and diffraction-limited storage ring (DLSR) light sources demand wavefront preservation from source to sample to achieve and maintain optimal performance. Fine wavefront control was achieved using a novel, roomtemperature cooled mirror system called REAL (resistive element adjustable length) that combines cooling with applied, spatially variable auxiliary heating. Single-grating shearing interferometry (also called Talbot interferometry) and Hartmann wavefront sensors were developed and used for optical characterization and alignment on several beamlines, across a range of photon energies. Demonstrations of non-invasive hard x-ray wavefront sensing were performed using a thin diamond single-crystal as a beamsplitter.

Nine bendable mirrors will be installed as part of the upgrade to Linac Coherent Light Source. To achieve the target performance, accurate elliptical shapes must be generated with these focusing mirrors to an accuracy in the order of 10(4) to 10(5). We briefly summarize the developmental work including surface metrology via stitching and actuator characterization as well as fitting algorithm to achieve shape control of a KB developmental prototype. The height error of the centerline shape generated by the current system is in the order of 3 nm for a one meter long silicon mirror. The most important limiting factor is metrology due to environmental control.

With the onset of high power XFELs and diffraction limited storage rings, there is a growing demand to maintain sub nanometer mirror figures even under high heat load. This is a difficult issue as the optimum cooling design for an optic is highly dependent on the power footprint on the mirror, which can be highly dynamic. Resistive Element Adjustable Length (REAL) cooling can be utilized to change the cooling parameters during an experiment to adapt for changing beam parameters. A case study of the new soft x-ray monochromator for the LCLS L2SI program is presented that utilizes this new capability to allow the beam to translate across the mirror for different operation modes, greatly simplifying the monochromator mechanics. Metrology of a prototype mirror will also be presented.