The ability to split femtosecond free electron laser pulses and recombine them with a precisely adjustable delay has numerous scientific applications such as X-ray Photon Correlation Spectroscopy and X-ray pump X-ray probe measurements. A wavefront-splitting based hard X-ray split-delay system is currently under development at the Linac Coherent Light Source. The design configuration uses a series of Si(220) crystal reflections in the horizontal scattering geometry. It covers an energy range between 6.5 and 13 keV, a delay range from -30 ps up to 500 ps at 8 keV. The design features two planar air bearing based linear stage delay lines for improved stability and accuracy during the delay adjustments in order to maintain spatial overlap of the two branches during a delay scan. We present the basic design concept, tolerance analysis, and estimated performance of the system.
The success of the LCLS led to an interest across a number of disciplines in the scientific community including physics,
chemistry, biology, and material science. Fueled by this success, SLAC National Accelerator Laboratory is developing a
new high repetition rate free electron laser, LCLS-II, a superconducting linear accelerator capable of a repetition rate up
to 1 MHz. Undulators will be optimized for 200 to 1300 eV soft X-rays, and for 1000 to 5000 eV hard X-rays. To
absorb spontaneous radiation, higher harmonic energies and deflect the x-ray beam to various end stations, the transport
and diagnostics system includes grazing incidence plane mirrors on both the soft and Hard X-ray beamline.
To deliver the FEL beam with minimal power loss and wavefront distortion, we need mirrors of height errors below 1nm
rms in operational conditions. We need to mitigate the thermal load effects due to the high repetition rate. The absorbed
thermal profile is highly dependent on the beam divergence, and this is a function of the photon energy. To address this
complexity, we developed a mirror cradle with variable length cooling and first order curve correction. Mirror figure
error is minimized using variable length water-cooling through a gallium-indium eutectic bath. Curve correction is
achieved with an off-axis bender that will be described in details.
We present the design features, mechanical analysis and results from optical and mechanical tests of a prototype
assembly, with particular regards to the figure sensitivity to bender corrections.
Under synchrotron radiation white beam exposure, strong mechanical stress can build up in multilayer optics, caused by
the thermal mismatch between layer material and substrate material. To study the stability and performance of multilayer
optics under heat load, Pd, Cr, and B4C single layers of thicknesses in the nanometer range and [Pd/B4C] multilayers
were prepared in the sputter-depositing facility of the ESRF Multilayer Laboratory. Curvature changes versus
temperature were measured using a Shack-Hartmann wave front sensor. Films coated on 200 μm thin Si wafers induced
significant curvature changes over a temperature range from 60°C to 200°C. A combined parameter K including
Young’s modulus and thermal expansion coefficient (CTE) was defined to describe the thermal deformation properties
of the thin-film layer. The investigation shows that all three materials in thin film cause less thermal expansion than
expected from material properties for bulk material in the literature. In particular, the thermal expansion of B4C films
appears to be close to that of the Si substrate.
The design, manufacture and characterization of a Kirkpatrick-Baez (KB) configuration mirror system for high-throughput
nanofocusing down to 50 nm beam sizes are described. To maximize the system aperture whilst retaining
energy tunability, multilayer coated optics are used in conjunction with 2 dynamically figured mirror benders. This
approach, which has been developed at the ESRF for many years, allows the focusing performance to be optimized when
operating the system in the 13-25 keV photon energy range. Developments in the key technologies necessary for the
production of mirror bending systems with dynamic figuring behavior close to the diffraction limit requirements are
discussed. These include system optimization via finite element analysis (FEA) modeling of the mechanical behavior of
the bender-mirror combination, manufacturing techniques for precisely-shaped multilayer substrates, multilayer
deposition with steep lateral gradients and the stitching metrology techniques developed for the characterization and
figure optimization of strongly aspherical surfaces. The mirror benders have been integrated into a compact and stable
assembly designed for routine beamline operation and results of the initial performance of the system at the ESRF
ID22NI endstation are presented demonstrating routine focusing of 17 keV X-rays to sub-60 nm resolution.
The X-ray power absorption by the Beryllium compound refractive lenses (CRL) installed in the ESRF ID10 front-end reaches 139 W. This non-negligible power leads to an excessive temperature in the lens such that the induced thermal stress is much larger than the yield stress of Beryllium. The thermal fatigue damage of the lens occurred after certain number of operation cycles. Sudden loss of focusing ability was observed recently after 6 ~ 7 years frequent operation. SEM and phase contrast images confirmed the damage of the CRL. Following these observations, optimization of some design parameters (width, and thickness of the thin part between two holes) of the CRL has been carried out as well as some operational parameters (cooling of the lens, vertical aperture of the X-ray beam on the lens). An optimized Beryllium CRL for the ID10 front-end should have a width of 10 mm instead of 2 mm and the thickness of the thin part between two holes should be increased to 0.2 mm. The temperature of the CRL can be reduced by a better cooling of the lens, for instance by improving the thermal contact between the Beryllium and the copper cooling block, or by reducing the vertical aperture of the X-ray beam from 4 mm to 2 mm (eventually to 1 mm).
ID09 is a dual-purpose beamline dedicated to time-resolved and high-pressure experiments. The time-resolved experiments use a high-speed chopper to isolate single pulses of x-rays. The chopper is installed near the sample (focal spot) and the shortest opening time depends on the height of the tunnel in the chopper, i.e. the sharpness of the vertical focus. In the 16-bunch mode, the opening window of the chopper has to be smaller than 0.352 μs in order to isolate single pulses of x-rays. This requires reducing the height of the tunnel to 0.143 mm. To ensure a reasonable transmission though the tunnel, we have designed a very precise toroidal mirror that focuses the beam 22.4 m downstream with a magnification M = 0.677. The 1.0 m long silicon mirror is curved by gravity into a nearly perfect toroid with a meridional radius of 9.9 km. The curvature is fine-tuned by a stepper motor that pushes via a spring from below the mirror. The overall figure error from the gravity sag and the corrective force is less than 0.3 μrad. The polishing error is 0.7 μrad (rms) averaged over the central 450 mm of the 1000 mm long mirror. The measured size of the polychromatic focus is 0.100 mm × 0.070 mm (h x v) in agreement with the prediction from the ESRF long trace profiler data. The small focal spot, which integrates the full central cone of the U17 undulator, is the result of very high optical quality, curvature fine-tuning, strain-free mount, vibration free cooling and careful alignment.
With the aim to improve the reliability of calculating and thus predicting the thermal deformation of cryogenically cooled silicon monochromators for intense synchrotron x-ray beams, we have measured the thermal conductivity of several specimen with different purities: float zone (FZ), Czochralski (CZ) single crystal materials and a Si99.3Ge0.7 (SG) crystal between 85 K and room temperature with 1% accuracy. We have shown by finite element analysis that a measured 30% increase of conductivity between the FZ and the CZ crystals leads to an increase of 30 to 40% of the thermal slope. Whereas the performances of these two materials were just acceptable, the 9 times reduced thermal conductivity of the SG crystal turned out to increase the thermal slope error by a factor 19 to a prohibitive value of 143 (mu) rad, as compared to 7.6 (mu) rad for the FZ crystal. Therefore, the application of SG crystals to cryogenic cooling cannot be recommended. In addition, we determined the thermal conductivity of germanium single crystals in the same temperature range. For the silicon FZ and the germanium materials, excellent agreement with recommended values was found. Moreover, we detected a small, but non-negligible dependence of thermal conductivity on the crystallographic direction (a few % at 85 K) that, to our knowledge, has not been published in previous papers.
We fabricated a water-cooled silicon monochromator crystal with small channels for the special case of a double-crystal fixed-exit monochromator design where the beam walks across the crystal when the x-ray energy is changed. The two parts of the cooled device were assembled using a new technique based on low melting point solder. The bending of the system produced by this technique could be perfectly compensated by mechanical counter-bending. Heat load tests of the monochromator in a synchrotron beam of 75 W total power, 3 mm high and 15 mm wide, generated by a multipole wiggler at SSRL, showed that the thermal slope error of the crystal is 1 arcsec/40 W power, in full agreement with finite element analysis. The cooling scheme is adequate for bending magnet beamlines at the ESRF and present wiggler beamlines at the SSRL.
The high power and/or power density of the X-ray beams of the European Synchrotron Radiation Facility induces engineering constraints for the design of the beamlines, in order to reduce the temperature and the thermal distortion of optical components. The requirements in beam stability, ever more stringent, lead to new engineering constraints, generally in contradiction with high cooling performances: the vibrations created by the cooling fluid -or flow induced vibrations- must now be integrated at the design stage. This document describes the efforts made at the ESRF to better master this aspect, and gives qualitative guidelines which could be used at the design stage of high power optical elements.
A silicon block (typical size 100 X 100 X (20 - 50) mm3) cooled by liquid nitrogen has been studied with various incident power densities and spot sizes on the surface. Gaussian power distribution was assumed. Both bottom cooling and side cooling have been considered. The thermal slope error has been minimized by optimizing the cooling conditions (cooling coefficient and bulk temperature of liquid nitrogen) and the thickness of the silicon block. Finite element analysis has been used because the material properties ((alpha) , k) of silicon are highly non linear. The limits of absorbed total power and power density are estimated for both undulator and wiggler beams with various spot sizes and for the requirement in terms of thermal slope error. Correlations between thermal slope error and power, power density have been established.
The present paper outlines the requirements for the performance of x-ray mirrors at the European Synchrotron Radiation Facility (ESRF), to be built in Grenoble, France. It is shown that present-day surface preparation techniques are about adequate to achieve conservation of the source emittance, although some improvements are needed in special cases. It is much harder to conserve brilliance, where thermal deformation is the major obstacle. Here substantial research and development efforts are
absolutely indispensable. Two possible ways are indicated to solve the heat problem: cryogenic cooling of silicon-based mirrors and adaptive
optical systems. In the first case thermal deformations are drastically reduced, and in the second they can be compensated by mechanical forces.
Our results are based on theoretical considerations of scattering by nonideal surfaces and on a thermomechanical analysis, which are also given.
For layered synthetic microstructures the technological problems appear to be still more difficult. Because the critical photon energy of the ESRF 6 GeV storage ring and of most of its insertion devices is between 10 and 20 keV or even higher, the discussion is limited to hard x-ray optics.