Optics and Metrology Group Update 2023-24

The Optics and Metrology group at Diamond Light Source

Optics for Diamond-II beamlines

The Optics and Metrology group has worked closely with Diamond’s beamline teams to achieve the best possible performance for Diamond-II. They cover the flagship beamlines CSXID, SWIFT and K04, as well as the currently existing beamlines, whose sources and optical components need to be upgraded. Optics and Metrology has carried out simulations of the power load on critical components, particularly white-beam mirrors and monochromators. In collaboration with Diamond’s engineers, they are evaluating the resulting thermal deformations to ensure that these components will be fit for purpose. They are designing beamline layouts by running ray traces and wavefront propagation simulations. The large workloads are being handled efficiently by bundling similar optics on different beamlines into one project, for example the multilayer monochromators on K16, K21 and VMXi. Efficiency has also been gained by producing common formats for FEA simulations and a new theoretical tool for evaluating optics cooling designs rapidly. The Optics and Metrology group is also leading the procurement of all new optics for Diamond-II.  

The OML in Zone 12 was decommissioned and rebuilt in Zone 4. The new OML required special measures to shield the sensitive metrology instruments from the vibrations and acoustic noise produced by the multiple beamlines, peripheral labs, the loading bay, and the plant equipment surrounding it. Wall panels and doors with improved sound absorption were installed, the speed of the fans was lowered, and the ductwork was supplied with vibrational and acoustic damping. In collaboration with the Engineering group, enclosures (Figure 1) were developed to shield the instruments from fluctuations in the surrounding air, and platforms were developed to provide passive and active vibration damping to the instruments mounted on them (Figure 2). Overall, despite stronger external disturbances, the lab achieves exceptional stability in temperature (< 0.01°C) and humidity (<5%), and the metrology instruments are performing better than in the old OML. The Diamond-NOM now achieves a repeatability ~ 9 nrad rms and a high-speed Diamond-VeNOM optical profiler has been developed to speed up the optics metrology measurements. The new OML is now fully operational.

Figure 1: Interior of the new OML, showing the improved isolating enclosures and enhanced damping platforms for the sensitive metrology instruments.

Figure 2: (Left) Accelerometer scans showing vibrational velocity over time during Diamond-NOM fly scans with and without the new active damping. (Right) The closed-loop geophone/voicecoil active damping system.

Reconstruction of B16

The planned construction of the CSXID beamline also required the control room of the B16 Test Beamline to be moved to a new location at the end of its experimental hutch (Figure 3). The experimental hutch therefore also had to be modified with new doors, new chicanes, a reconfigured Personal Safety System, and an altered search route. Contractors managed by the Diamond Installation and Facilities Management group, in collaboration with the B16 technical working group, completed the design and planning stage in December 2023. The reconstruction of B16 began in January 2024 and is scheduled for completion in June 2024. During this period, B16 is not available to users, but other upgrades such as the replacement of several motion axis controllers will be done to prepare for Diamond-II.

Figure 3: Layout of the new control room (K16 CC) at the end of the experimental hutch (K16 EH).

Ion beam figuring of X-ray mirrors

State-of-the-art X-ray mirrors demand height errors below 1 nm and micro-roughness of several Å over lengths of tens of centimetres. These stringent specifications present a formidable challenge. The application of ion beam figuring (IBF) to remove excess material over selected areas on a pre-polished surface shows promise. An in-house IBF system has now been developed at Diamond (Figure 4a). It consists of a large-diameter DC gridded ion source, a 4-axis motion stage equipped with a uniquely designed holder, and a high-resolution camera for precise workpiece alignment. The ion beam remains stationary as the workpiece moves. An onboard laser speckle angular measurement (SAM) system provides real-time metrology feedback. An aperture plate passes ion beams of various shapes and sizes, thus enabling both coarse and fine figure corrections. This IBF apparatus has been used to correct flat Si mirrors of 50 × 20 mm² area. The rms height error of a plane mirror was reduced to below 1 nm and the rms slope error was reduced below 100 nrad (Figure 4b & c). The combination of sub-nanometre rms height error with rms slope error below 100 nrad is a crucial milestone for the Diamond IBF instrument.  The next step is to deliver the same surface quality on samples over 100 mm in size, thus making possible the provision of high-quality beamline optics.

Figure 4: (a) Inside view of the IBF system. (b) Height error map of a Si X-ray mirror before IBF figure correction. (c) Height error map of the same mirror after one iteration of IBF figure correction.

New multilayer fabrication facility

Multilayer monochromators, gratings, and soft X-ray polarimeters are now employed at many synchrotron X-ray beamlines. They can enhance the reflectivity of tender X-ray optics and provide high-aperture mirrors for nanofocusing. Both these applications are in demand for Diamond-II. An in-line multilayer deposition system has been built at Diamond (Figure 5). It is the only one of its kind in the UK, and it will allow advanced and specialised optical elements, including laterally graded and depth-graded multilayers, to be fabricated rapidly. The instrument is engineered to produce both single and multilayer coatings on mirrors up to 1,000 mm long using eight rectangular cathodes. In-situ diagnostic tools measure the thickness uniformity and stress of the coating. Annealing up to 700°C is possible within the load lock chamber. Pumpdown, deposition, and venting are all automated. The power of the sputtering source, the working pressure, and the motor positions are automatically archived. Remarkable lateral film thickness uniformity, with variation as low as 0.06% along the translation direction, was achieved during commissioning. This new fabrication facility is now poised to fabricate periodic, laterally graded and depth-graded multilayers. It will cater to both hard and soft X-ray beamlines that serve diverse scientific applications at Diamond.

Figure 5: Photograph of Diamond’s multilayer deposition system.

X-ray topography

The Optics and Metrology group and the B16 beamline team have built up an X-ray topography program that inspects diffracting crystal optics for both external and internal users. This year’s external users have come from both academic and industrial organisations. X-ray topography serves Diamond as an at-wavelength metrology technique that supplements the OML’s visible-light measurements of surface figure and roughness. Because B16 can admit either white or monochromatic beam, it offers many topographic techniques. Surface scratches and pits, lattice distortions within the bulk and long-range strains caused by clamping have all been measured in the crystal samples of the past year. Samples can be oriented in any direction. It is now possible to measure the evolution of a crystal’s strain “live” as the temperature varies (Figure 6).

Figure 6: Strain relaxation across the centre of a clamped sample crystal during warming from liquid-nitrogen temperature (a) at 83.5 K (b) at 110.2 K. Strain is in µrad (see colour bar) and positions are in mm.

Alvarez X-ray zoom lens

An Alvarez varifocal X-ray lens has been developed and demonstrated at B16 by Optics and Metrology group scientists. This work presents the first demonstration of an Alvarez lens in the X-ray regime which adaptively corrects defocus and astigmatism aberrations of X-ray optics.  An Alvarez lens consists of two refractors with a cubic surface profile that are placed one behind the other (Figure 7). Together they apply a parabolic perturbation to the wavefront that varies as they are shifted transversely by equal but opposite amounts. Therefore, when inserted into a beamline that focuses the beam, an Alvarez lens can move the focal plane. It can also correct the astigmatism and coma of a KB mirror system and compensate chromatic aberrations in a compound refractive lens (CRL). Inserting an Alvarez lens is easy because it does not disturb the beam path. In tests at B16, the focal plane of an elliptical mirror with focal distance 235 mm was displaced by ±2 mm with minimal aberration by a combination of mirror pitch rotation and correction with the Alvarez lens. The Alvarez lens also kept the focal position of the CRL constant while the X-ray energy was varied from 13.8 keV to 16.2 keV.

Figure 7: Schematic of an Alvarez X-ray zoom lens used in conjunction with a vertically focusing mirror (VKB) at B16.

Figure 8: Theoretical plot of critical power (solid curve) with power and power density on DCM first crystals at various Diamond beamlines (individual points). FEA showed acceptable deformation at blue points, and unacceptable deformation at orange and red points.

Assessment of cryo-cooled Si monochromator crystals for Diamond-II

Si double crystal monochromators (DCMs) are key optical components in Diamond’s hard X-ray beamlines. Thermal deformation of the first DCM crystal degrades the performance. A cooling scheme that minimises this deformation must be developed when a DCM is designed or when the power of the incident beam is substantially increased. The latter is true for Diamond-II; therefore, the beamline performance of all Diamond’s DCMs must be reassessed. Finite element analysis (FEA), the usual tool for predicting thermal deformation, is time-consuming, and for each DCM, multiple cooling schemes may need to be trialled. The Optics and Metrology group has therefore developed a simple theoretical model to evaluate and assess the DCM cooling schemes. For each cooling scheme, a curve of critical power versus power density is generated by heat transfer equations. If the expected incident power and power density on the DCM’s first crystal lie below this curve, the cooling scheme passes and may be considered further. Experimental data and FEA simulations on DCMs at several undulator beamlines at Diamond have validated this model (Figure 8).