Diamond Light Source - Annual Review 2022/23

76 77 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 Optics andMetrology Group Kawal Sawhney, Optics and Metrology Group Leader Despite the past year’s challenges, the Optics & Metrology (O&M) group has kept up a steady pace of research and development. Our activities include improvements of the current beamlines, plans for the Diamond-II upgrade, the development of novel X-ray optics, and the implementation of new metrology techniques. Wavefront measurements using knife-edge scans, a novel technique developed by the O&M group, have already been used for rapid optimisation of the VMXm microfocus mirrors. Fast closed-loop operation of bimorph mirrors has now been made possible by a combination of precise in-house metrology and close collaboration with industry. These are the most prominent examples of the wide-ranging capabilities that O&M continues to foster. The Diamond-II upgrade remains a major focus of the O&M group’s activities. We are refining the beamline designs and determining tolerances on optical components. We are providing Diamond’s engineers with power loads on critical components, and we are judging the performance of those components from the engineers’ FEA simulations. To make room for the Diamond-II flagship beamline CSXID at I17, a new optical metrology lab OML2 is being built to house the equipment from the current lab, which will be dismantled. The physical metrology laboratory (PML) is being extensively used in collaboration with Diamond’s engineers for high-precision tests of prototype motion stages in monochromators and other optical components. Fast alignment of X-ray focusing optics using wavefront sensing Mirrors that focus X-rays into sub-micrometre focal spots are used onmany beamlines at Diamond. The resulting X-ray beam can be used for studying small samples or for high spatial resolution mapping of inhomogeneous samples. Precise adjustment of the mirror angle and the sample position is essential to get a small focus. A method for rapid alignment would allow the small spot size to be maintained in the presence of drift in beamline optics. The wavefront is a useful concept for representing the focused X-ray beam. We have developed a new wavefront sensor (“knife-edge wavefront sensor”) that is particularly suited to wavefront measurements in focused beam geometries (Fig. 1). An absorbing knife edge is stepped transversely through the focused beamwhile an area detector positioned downstreammeasures the X-ray intensity. By independently analysing each pixel and projecting back, the wavefront error at the focusing mirror is determined. A single measurement gives both the mirror angle error and the exact position of the focal plane. Using the Test Beamline (B16) microfocus mirror system, the wavefront was measured at a range of mirror angles (±30 µrad) and longitudinal ( z ) positions (±3 mm) and the inset graph shows the remaining wavefront error (caused by mirror figure error) after correction for these. Currently, a single wavefront measurement takes about 100 seconds and the data processing 15 seconds using a multi-core workstation. Both these times could however be reduced and then automatic beamline optics realignment at a rate of one per minute could be feasible. The knife-edge wavefront sensor has been employed already on B16 and theVMXmmolecular crystallography beamline at Diamond to align the optics and achieve a 400 nm focused beam size. Fast closed-loop operation of bimorphmirrors Deformable piezoelectric (“adaptive”) bimorph mirrors at synchrotrons X-ray Technologies at Diamond I t is self-evident that for our instruments to produce world-leading science, we need to have world-class optics, detectors and computing technologies at our fingertips; technological advances never stop but are continually evolving. This section describes the support and advances in the Optics and Metrology Group, Detector Group and Scientific Software Controls and Computation department at Diamond Light Source. Advances which are supporting and enhancing our capabilities today are described, but also developments that will keep us competitive over the next few years. These groups are very active in calculations and specifications for beamlines and instruments being put forward and planned for Diamond-II, an integrated upgrade of the synchrotron, beamlines and computational facilities. These advances continue to keep us competitive worldwide, and Diamond is proud to be on the forefront of many of these technologies. X-ray Technologies have generally been operated at a fixed shape for long periods. Brighter sources now allow larger numbers of samples to be measured in less time. Bimorph mirrors that depend on the piezoelectric effect, which acts instantaneously and generates little heat, can more rapidly match the beam size to the sample. For them to fulfil that potential, however, a concerted effort in many fields has been required: improved designs, strain-free clamping, creep compensation, programmable power supplies, and upgraded metrology capabilities. Closed- loop operation has now been tested at the versatile optics test beamline B16 using feedback from ZPS TM interferometric sensors (Zygo, USA), which can measure mirror figure errors with sub-nanometre resolution at kHz frequencies. In the beamline setup (Fig. 2), the feedback collection does not require any interruption of user operation and permits wavefront errors to be measured simultaneously by X-ray speckle tracking, also developed by O&M. The test mirror was 640 mm long and had 16 electrodes on each side, typical of other mirrors at Diamond. With closed-loop operation at 1 Hz, the focal profile of the test mirror was stabilized within 15 seconds after even a large voltage change, whereas in open loop it took many minutes to stabilize. Beam profiles produced by the test mirror at the focal position were cycled reproducibly at 10 second intervals through a series consisting of the 12 µm FWHM focus, a 55 µm FWHMflattop and a 130 µmflattop. Splitting of the focus into two equal peaks was also demonstrated. This is an important step toward the automated optimisation of adaptive mirrors. Mitigation of fine beam structure produced by reflective X-ray optics X-ray mirrors have always had figure errors that introduce fine structure into the reflected beam off the focal plane. A simple relation of this structure's intensity to parameters of the figure error that manufacturers can measure has long been needed. By combining X-ray speckle tracking wavefront measurements with high-resolution images produced by a modified elliptical mirror and a plane mirror at B16, it has been shown that the intensity at a point in the image is correlated with the curvature error of the wavefront at that point (Fig. 3). A transport of intensity equation showing how the beam profile propagates from the mirror to the imaging detector has been derived. This could allow a given mirror figure error to be tested for acceptably low beam structure in the plane of observation. References: Laundy, D. et al. Refractive optics for modifying X-ray wavefronts. Synchrotron Radiation News 34 (6), 27-31 (2022). https://doi.org/10.1080/08940886.20 21.2022400 Sutter, J. P. et al. Active and adaptive X-ray optics at Diamond Light Source. Synchrotron Radiation News 35 (2), 8-13 (2022). https://doi.org/10.1080/089 40886.2022.2058856 Moxham, T. E. J. et al . Two-dimensional wavefront characterization of adaptable corrective optics and Kirkpatrick-Baez mirror system using ptychography. Opt. Exp. 30, 19185-19198 (2022). https://doi.org/10.1364/ OE.453239 Da Silva, M. B. et al . A Fizeau interferometry stitching system to characterize X-ray mirrors with sub-nanometre errors. Opt. Las. Eng . 161 , 107192 (2022). https://doi.org/10.1016/j.optlaseng.2022.107192 Hu, L. et al. Fast wavefront sensing for X-ray optics with an alternating speckle tracking technique. Opt. Exp. 30 , 33259-33273 (2022). https://doi. org/10.1364/OE.460163 Hu, L. et al . Two-dimensional speckle technique for slope error measurements of weakly focusing reflective X-ray optics. J. Synchrotron Rad. 29 , 1385-1393 (2022). https://doi.org/10.1107/S160057752200916X Khosroabadi, H. et al . Cryo-cooled silicon crystal monochromators: a study of power load, temperature and deformation. J. Synchrotron Rad . 29 , 377-385 (2022). https://doi.org/10.1107/S160057752200039X Sutter, J. P. et al . Calculating temperature-dependent X-ray structure factors of α-quartz with an extensible Python 3 package. J. Appl. Cryst . 55 , 1011-1028 (2022). https://doi.org/10.1107/S1600576722005945 Sutter, J. P. et al . Bimorph mirrors at synchrotron beamlines: from walking to flying . J. Phys.: Conf. Ser. 2380 , 012055 (2022). https://dx.doi. org/10.1088/1742-6596/2380/1/012055 Walters, A. C. et al . An analytical approach to designing a future Nano-ARPES beamline for Diamond-II. J. Phys.: Conf. Ser. 2380 , 012039 (2022). https://doi. org/ 10.1088/1742-6596/2380/1/012039 Figure 1: Knife-edge wavefront sensor. Figure 2: Schematic of beamline setup for testing fast closed-loop operation of deformable piezoelectric bimorph mirror at B16. The double multilayer monochromator (DMM) selected 15.5 keV X-rays. Figure 3: X-ray images produced by reflection from an elliptical mirror modified by the addition of four parabolic sections of equal length. (a) Intensity distribution off the focal plane. (b) Wavefront curvature error at the detector plane. The vertical stripes are caused by the added parabolic arcs. The horizontal stripes in (a) were introduced by the B16 double multilayer monochromator, which selected 9 keV X-rays.