The Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL is a soft X-ray beamline aiming to unravel electronic, spin and structural properties of materials in ultrafast processes at the nanoscale. Various experimental techniques offered at SCS have different requirements in terms of beam size at the sample. Kirkpatrick-Baez (KB) refocusing optics equipped with mechanical benders allows for independent change of the horizontal and vertical beam size. We report here on the first characterization of the SCS KB mirrors by means of a novel diffraction-based technique which images the beam profile on a 2D pixelated detector. This approach provides a quick characterization of micrometer beam sizes. Results are compared with metrology measurements obtained with a non-contact slope profiler.
For new light sources, like X-ray free-electron lasers (FELs), highly precise diffraction-limited optics are needed, which are leading to ambitious requirements for the X-ray mirrors used in those facilities. For appropriate beam focusing and alignment, a control of the shape on the single-nanometre precision level is necessary, which generates high demands on the manufacturers and on the metrology. To face these questions, the project MooNpics – Metrology On One-Nanometer-Precise Optics was established. With a European-wide round-robin test, the goal is to push the frontiers in mirror metrology in Europe to single-nanometre figure error precision.
Within the MooNpics project, a special mirror holder for long X-ray mirrors was developed that provides reproducible and well defined mounting conditions in each participating laboratory. The goal is to understand mechanical and stress influences and hence to improve mounting methods. Before the actual start of the roundrobin, measurements were done to investigate the influence of the mirror mounting. A procedure was developed to reduce the induced stresses and increase the reproducibility with regard to the planned round-robin.
The European XFEL in Hamburg will be comprised of a linear accelerator and three Free-Electron-Laser beamlines
(SASE1, SASE2 and SASE3) covering the energy range from 250 eV to 24 keV. It will provide up to 2700 pulses in
trains of 600 microsecond duration at a repetition rate of 10 Hz.
SASE3 beamline is the soft X-ray beamline (0.25 - 3 keV) and delivers photon pulses to SQS (Small Quantum System)
and SCS (Spectroscopy & Coherent Scattering) experiments. The beamline is able to operate in both monochromatic and
non-monochromatic mode. The latter provides the inherent FEL bandwidth at higher intensities. The beamline from
photon source to experimental station is about 450 m long. The length of the beamline is related to the optics single-shotdamage
issue. The almost diffraction-limited beam is propagated along the beamline with very long (up to 800 mm clear
aperture), cooled (with eutectic bath) and super-polished (50 nrad RMS slope error and less than 3 nm PV residual height
error) mirrors. The VLS-PG (variable line spacing - plane grating) monochromator covers the entire beamline energy
range and its optical design is guided by the optimization of the energy resolving power, the minimization of the pulse
broadening and the maximization of optics damage tolerance. Grating substrates are 530 mm long, eutectic cooled and
present outstanding surface quality. The VLS parameters of the blazed profile are also a real challenge under
manufacturing and measuring point of view. Adaptive optics in the horizontal (the second offset mirror) and vertical
(monochromator premirror) plane are foreseen in the optical layout to increase the beamline tunability and to preserve
the highly coherent beam properties.
Beamline optical design, expected performance and also mechanical aspects of main beamline components are reported.
The SQS scientific instrument at the European XFEL is dedicated to investigations in the soft X-rays regime,
in particular to studies of non-linear and ultrafast processes in atoms, molecules and clusters using a variety of
spectroscopic techniques. It will be equipped with a Kirkpatrick-Baez (KB) adaptive mirror system enabling
submicron focusing and access to variable focal distances. In this paper we describe the conceptual design of the
beam transport and focusing layout based on the KB system. The design includes a study of feasibility based
on the comparison between the required source and image positions and the theoretical limits for the accessible
mirror profiles.
The European XFEL is a large facility under construction in Hamburg, Germany. It will provide a transversally fully
coherent X-ray radiation with outstanding characteristics: high repetition rate (up to 2700 pulses with a 0.6 milliseconds
long pulse train at 10Hz), short wavelength (down to 0.05 nm), short pulse (in the femtoseconds scale) and high average
brilliance (1.6•1025 photons / s / mm2 / mrad2/ 0.1% bandwidth). Due to the very short wavelength and very high pulse
energy, mirrors have to present high quality surface, to be very long, and at the same time to implement an effective
cooling system. Matching these tight specifications and assessing them with high precision optical measurements is very
challenging. One of the three foreseen beamlines operates in the soft X-ray range and it is equipped with a diffractive
monochromator. The monochromator is using a variable line spacing grating that covers the wavelength range from 4.6nm to 0.41 nm (energies from 270eV to 3000eV). The grating profile is blazed, and due to the small angle and relatively
few lines/mm, it is also very challenging to realize and to be characterized. In this contribution we discuss about the
requirements of the optics involved in the soft X-ray monochromator. We describe mirror and grating specifications, and
the tests that could be carried out during and after the manufacturing in order to ensure the specifications match.
Recently, a technique for optimal tuning and calibration of bendable x-ray optics using surface slope data obtained with a slope measuring long trace profiler (LTP) was developed at the Advanced Light Source (ALS) optical metrology laboratory (OML) [Opt. Eng. 48(8), 083601 (2009)]. In this technique, slope distributions measured at different settings of the bending couples at each end of a flat substrate are used to construct bender characteristic functions. Using regression analysis with the experimental characteristic functions, optimal settings of the benders that best approximate the desired shape in slope are determined. In this work, we describe a method for finding a bender’s characteristic functions based on Finite Element Analysis (FEA) of a complete mirror assembly. The accuracy of the characteristic functions found by simulation is verified by cross comparison with experimental characteristic functions for a long (450 mm) highly curved bendable mirror. The mirror has a sagittally shaped substrate developed for the ALS MERLIN beamline 4.3.0, with a total slope variation 15 mrad. Calculating FEA characteristic functions in the design stage allowed better understanding of the design of the bender’s adjustment mechanism. By calculating FEA characteristic functions a priori, we significantly decrease the time needed in the OML for tuning the mirror. Because the calculated characteristic functions are free of the errors inherent to measurements made in the lab, the tuning is even more accurate.
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