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 (1.6x1025 photons / s / mm2 / mrad2 / 0.1% bandwidth). It will have initially three main beamlines, named SASE1, SASE2 and SASE3. The last one is considered a "soft X-ray" beamline, with energies that will span from 0.25 to 3 keV, delivering photon pulses to SQS (Small Quantum System) and SCS (Spectroscopy and Coherent Scattering) experiments. The optical transport of the almost diffraction- limited beam is done using 950 mm long mirrors, cooled with InGa eutectic bath and super-polished (50 nrad RMS slope error and less than 3 nm PV residual height error). A VLS-PG (Variable Line Spacing - Plane Grating) monochromator is installed to enhance the spectral coherence of the beam. The basic characteristics for the grating substrates are: 530 mm length, InGa eutectic bath cooled and ion-beam polished with gravity sag compensation. For the initial commissioning of the beamline, a shorter grating (150 mm long) will be prepared and installed. We recently received the 150 mm long grating and we present here its characterization performed using Fizeau Interferometry. The VLS parameters are especially investigated and characterized. This grating's study can give an interesting insight in the present status of European XFEL metrology, but also additional information for the future development and characterization of the final 530 mm long grating.
The European XFEL is a large-scale user 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 pulses (in the femtoseconds
scale) and high average brilliance (1.6x1025 photons / s / mm2 / mrad2/ 0.1% bandwidth)1. Due to the short wavelength
and high pulse energies, mirrors need to have a high-quality surface, have to be very long (1 m), and at the same time an
effective cooling system has to be implemented. Matching these tight specifications and assessing them with high precision
optical measurements is very challenging.
The mirrors go through a complicated and long process, starting from classical polishing to deterministic polishing,
ending with a special coating and a final metrology assessment inside their mechanical mounts just before the
installation. The installation itself is also difficult for such big mirrors and needs special care. In this contribution we will
explain how we implemented the installation process, how we used the metrology information to optimize the
installation procedure and we will show some preliminary results with the first mirrors installed in the European XFEL
All the major synchrotron radiation facilities around the world have recently started upgrade projects to go towards the
4th generation of x-ray sources, in the direction of fully "Diffraction Limited Storage Rings" (DLSRs) in order to
produce photon beams with better quality. Several Free Electron Lasers (FELs), also providing diffraction limited beam,
are operating and increasing their performances, while other ones are almost ready to be operational. To fully exploit the
ultimate source properties of these next-generation light sources, the quality requirements for x-ray optics have
significantly increased, especially for reflective optics like mirrors. To maintain the coherence of the beam, such optical
components will need to have shape accuracies in the nanometer regime over macroscopic length scales up to 1 meter. If
we consider the ratio between these two parameters, we can quantify how challenging is not only the manufacturing
process but also the characterization and measurement of such optics. We will outline such challenge taking some
experience from the example case of European XFEL.
The European XFEL will generate extremely short and intense X-ray laser pulses of high coherence and nearly diffraction-limited divergence. Guiding these X-rays beams over a distance of more than 1 km to the experiments requires an extreme precision in pointing stability of the optical beamline components like mirrors and gratings and also a good control of the divergence of the beam. The specifications of the X-ray mirrors that will be used to transport, distribute and focus the beam are high demanding. It will be required for the reflecting surfaces to have a surface quality of better than 2 nm Peak-To-Valley over a 950-mm length: the ratio between these two parameters, on the order of 10-9, makes the requirements very challenging to be accomplished.
In order to account for the real shape of the mirrors and to assist the production with absolute metrology, it is proposed to use a Fizeau interferometer. Being the mirrors much bigger than the interferometer clear aperture, it is however needed to use an angled (“grazing incidence”) cavity setup to be able to measure the mirrors over their entire length. In using this setup, there are some open questions about the reproducibility of the method, the influence of the particular grazing angle that is used and the level of accuracy that could be expected with different averages.
We present a discussion about theory and practical implementation of “grazing incidence” interferometric measurements, with some examples of real measurements at European XFEL on the first beam distribution mirrors.
The European XFEL will generate extremely short and intense X-ray laser pulses of high coherence and nearly diffraction-limited divergence. Guiding these X-rays beams over a distance of more than 1 km to the experiments requires an extreme precision in pointing stability of beamline components like mirrors and gratings and also a control of the divergence of the beam. The specifications of the X-ray mirrors that will be able to transport, distribute and focus the beam are quite challenging. The European XFEL mirrors for the beam transport are 950 mm long and the optical surface specifications are 2 nm Peak-To-Valley. Some of the mirrors will have bending capabilities in order to focus the beam in the right position and with nanometer accuracy. This is implemented using a mechanical bender that will ensure stability of the optics in the nanometer range and will also offer the possibility to correct for mechanical or temperature drifts.
We present here the characterization of a mechanical bender that was done using two instruments, a Large Aperture Fizeau interferometer and a system of three capacitive sensors. The bender is designed in a way that the mirror is hold with clamps on both ends and a symmetric torque is applied on the clamps, inducing a cylindrical shape on the mirror surface. Several long-term stability measurements were done, as well as the characterization of bending capabilities. The parameters retrieved from the measurements are the sagitta and therefore the radius of curvature for different bending positions. The behavior of the variation of the shape of the mirror was also studied. The information gathered from our measurements will be used to optimize the final design of the bender.
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 (1.61025 photons / s / mm2 / mrad2/ 0.1% bandwidth). Due to the very short wavelength and very high pulse energy, all the mirrors need to have 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. In order to measure the mirrors and to characterize their interaction with the mechanical mounts, we equipped a Metrology Laboratory with a Large Aperture Fizeau. The system is a classical 100 mm diameter commercial Fizeau, with an additional expander providing a 300 mm diameter. Despite the commercial nature of the system, special care has been done in the polishing of the reference flats and in the expander quality. In this report, we show the preparation of the instrument, the calibration and the performance characterization, together with some preliminary results. We also describe the approach that we want to follow for the x-rays mirrors measurements. The final goal will be to characterize very long mirrors, almost 1 meter long, with nanometer accuracy.