Ion beam figuring for X-ray mirrors: history, state-of-the-art and future prospects.

Synchrotron light sources require X-ray optics with extremely demanding accuracy for the surface profile, with less than 100 nrad slope errors and sub-nanometre height errors. Such errors are challenging to achieve for aspheres using traditional polishing methods. However, post-polishing error correction can be performed using techniques such as ion beam figuring (IBF) to improve optics to the desired quality. This work presents a brief overview of the history of IBF, introduces some of the challenges for obtaining such demanding figure errors, and highlights the work done at several in-house IBF facilities at synchrotron light sources worldwide to obtain state-of-the-art optical quality.

Real-time spatial characterization of micrometer-sized X-ray free-electron laser beams focused by bendable mirrors.

A real-time and accurate characterization of the X-ray beam size is essential to enable a large variety of different experiments at free-electron laser facilities. Typically, ablative imprints are employed to determine shape and size of µm-focused X-ray beams. The high accuracy of this state-of-the-art method comes at the expense of the time required to perform an ex-situ image analysis. In contrast, diffraction at a curved grating with suitably varying period and orientation forms a magnified image of the X-ray beam, which can be recorded by a 2D pixelated detector providing beam size and pointing jitter in real time. In this manuscript, we compare results obtained with both techniques, address their advantages and limitations, and demonstrate their excellent agreement. We present an extensive characterization of the FEL beam focused to ≈1 µm by two Kirkpatrick-Baez (KB) mirrors, along with optical metrology slope profiles demonstrating their exceptionally high quality. This work provides a systematic and comprehensive study of the accuracy provided by curved gratings in real-time imaging of X-ray beams at a free-electron laser facility. It is applied here to soft X-rays and can be extended to the hard X-ray range. Furthermore, curved gratings, in combination with a suitable detector, can provide spatial properties of µm-focused X-ray beams at MHz repetition rate.

1 m long multilayer-coated deformable piezoelectric bimorph mirror for adjustable focusing of high-energy X-rays.

The Diamond Light Source (DLS) beamline I15-1 measures atomic pair distribution functions (PDF) using scattering of 40-80 keV X-rays. A unique focusing element was needed to condense these X-rays from an initial large cross section (11.0 mm H × 4.2 mm V) into a required spot size of FWHM ≈680 µm (H) × 20 µm (V) at a variable position between the sample and the detector. The large numerical aperture is achieved by coating a silicon substrate over 1 m long with three multilayer stripes of Bragg angle 4.2 mrad. One stripe selects X-rays of each energy 40.0, 65.4, and 76.6 keV. Sixteen piezoelectric bimorph actuators attached to the sides of the mirror substrate adjusted the reflecting surface's shape. Focal spots of vertical width < 15 µm were obtained at three positions over a 0.92 m range, with fast, easy switching from one focal position to another. Minimized root mean square slope errors were close to 0.5 µrad after subtraction of a uniform curvature. Reflectivity curves taken along each stripe showed consistent high peaks with generally small angular variation of peak positions. This is the first application of a 1 m long multilayer-coated bimorph mirror at a synchrotron beamline. Data collected with its help on a slice of a lithium ion battery's cathode are presented.

High-speed adaptive optics using bimorph deformable x-ray mirrors.

Recently, the dynamic performance of piezo-electric deformable "bimorph" mirrors for synchrotron radiation and X-ray free electron laser sources has been characterized and significantly improved. This innovation enables high intensity X-ray beams to be rapidly focused or defocused to either match to the size of the sample under test or to select different sized regions of interest in larger samples. In this paper, we extend these results by monitoring a bimorph mirror using a combination of ex situ metrology instruments. Comparison between results from the Diamond-NOM (Nanometre Optical Metrology) slope profiler, a Fizeau interferometer, and Zygo ZPSTM distance measuring probes shows that bimorph X-ray mirrors can reliably and accurately be driven at 1 Hz using advanced features recently added to the high voltage (HV), bipolar "HV-Adaptos" power supply from CAEN.

Dynamic adaptive X-ray optics. Part I. Time-resolved optical metrology investigation of the bending behaviour of piezoelectric bimorph deformable X-ray mirrors.

Piezoelectric bimorph deformable mirrors (`bimorphs') are routinely used on many synchrotron and free-electron laser beamlines to provide active variation in the size and shape of the X-ray beam. However, the time-domain potential of such optics has never been fully exploited. For the first time, the fast dynamic bending response of bimorphs is investigated here using Fizeau interferometry. Automated scripts for acquisition and analysis were developed to collect Fizeau data at a rate of 0.1 Hz to record dynamic changes in the optical surface as voltages were applied to the electrodes of the piezoelectric actuators. It is demonstrated that residual drift in the tangential radius of curvature of a bimorph can be significantly reduced using enhanced opto-mechanical holders and a fast programmable high-voltage power supply. Further improvements are achieved by applying small opposing voltages to compensate for piezoelectric creep. The present study shows that bimorphs can truly be used as high-speed adaptive optics for the X-ray domain, even without closed-loop feedback correction. This opens the possibility for relatively simple real-time tuning of the profile of X-ray bimorphs. Part II of this study [Alcock, Nistea, Signorato, Owen, Axford, Sutter, Foster & Sawhney (2019), J. Synchrotron Rad. 26, 45-51] builds upon these results and demonstrates how bimorphs can rapidly provide customisable sizes and shapes of synchrotron X-ray beams, specifically tailored to suit the experimental samples being investigated.

Dynamic adaptive X-ray optics. Part II. High-speed piezoelectric bimorph deformable Kirkpatrick-Baez mirrors for rapid variation of the 2D size and shape of X-ray beams.

The tangential curvature of actively bent X-ray mirrors at synchrotron radiation and X-ray free-electron laser (XFEL) facilities is typically only changed every few hours or even days. This operation can take tens of minutes for active optics with multiple bending actuators and often requires expert guidance using in situ monitoring devices. Hence, the dynamic performance of active X-ray optics for synchrotron beamlines has historically not been exploited. This is in stark contrast to many other scientific fields. However, many areas of synchrotron radiation and XFEL science, including macromolecular crystallography, could greatly benefit from the ability to change the size and shape of the X-ray beam rapidly and continuously. The advantages of this innovative approach are twofold: a large reduction in the dead time required to change the size of the X-ray beam for different-sized samples and the possibility of making multiple changes to the beam during the measurement of a single sample. In the preceding paper [Part I; Alcock, Nistea, Signorato & Sawhney (2019), J. Synchrotron Rad. 26, 36-44], which accompanies this article, high-speed visible-light Fizeau interferometry was used to identify the factors which influence the dynamic bending behaviour of piezoelectric bimorph deformable X-ray mirrors. Building upon this ex situ metrology study, provided here is the first synchrotron radiation beamline implementation of high-speed adaptive X-ray optics using two bimorphs operating as a Kirkpatrick-Baez pair. With optimized substrates, novel opto-mechanical holders and a next-generation high-voltage power supply, the size of an X-ray beam was rapidly and repeatedly switched in <10 s. Of equal importance, it is also shown that compensation of piezoelectric creep ensures that the X-ray beam size remains stable for more than 1 h after making a major change. The era of high-speed adaptive X-ray optics for synchrotron radiation and XFEL beamlines has begun.