While there are many variations of an Inelastic X-ray Scattering (IXS) spectrometer, the figure of merit is often the energy resolution and the throughput. As part of the LCLS-II-HE project, the DXS team is developing a hard X-ray IXS spectrometer with a resolution of 5 meV at 11.215 keV. The spectrometer relies on a so-called post-sample-collimation scheme, and this high degree of resolution comes with stringent precision and stability requirements. SHADOWOui is used to simulate the setup and analyze the tolerance of 4 optics’ axis (translation, pitch, yaw, roll) and the miscut angle of the channel-cut crystal of the design. The simulation indicates that a 5 meV resolution is achievable by ensuring stringent pitch and vertical translation tolerances. Furthermore, the simulation suggests that a miscut angle of 77 degrees, which necessitates high-quality crystal manufacturing, is optimal.
The goal of the dual KB mirror system for the CXI beamline of the LCLS-HE upgrade is to realize both micro-focusing and nano-focusing functions over a photon energy range of 7-21 keV. This system consists of two pairs of bendable mirrors with the length of 1 m. The dynamical range of the mirror bending is up to 104 due to the fine adjustment of the focusing length to 1 mm over a 10 m range. The prefigured shape and the width profile of each mirror are optimized to have good performance for both functions.
The thermal deformation requirement for wavefront preservation through an X-ray crystal monochromator is found to restrict the standard deviation of the height error to less than 25 pm under certain conditions. By optimizing effective cooling temperature of liquid nitrogen cooled crystals, combined with the compensation of second order component in thermal deformation, we have an approach to reach this unprecedented requirement. As an example, the criteria on thermal deformation of a high heat-load monochromator crystal for a LCLS-II-HE beamline can be achieved for 100 W FEL beam. Wavefront propagation simulations confirm that the reflected beam intensity profile is satisfactory.
The Linac Coherent Light Source (LCLS) is being upgraded to the high repetition rate (up to 1 MHz) mode using cryogenic modules. As the key optical element at the X-ray Pump-probe (XPP) beamline, the goal of the large offset double-crystal monochromator (LODCM) for the High Energy (HE) upgrade is to maintain its x-ray beam multiplexing capability at higher average beam power (up to 200W) for the whole hard x-ray operating range of 6-25 keV. The upgraded LODCM system will use an upstream diamond transmission grating to achieve high power beam multiplexing. It will enable the 0th order 'transmission’ from the grating to pass through the XPP hutch. The +1st order beam, which contains about 20% power of the incident beam of the grating, will be monochromatized at the 1st crystal position, then directed to XPP experiments at the 2nd crystal position. Both crystal positions will provide 111 and 220 Si crystals. The 1 st crystals need to be cooled by Liquide Nitrogen to minimize their thermal deformation under heat load. The second crystals will be controlled close to ambient temperature. The temperature difference between the two crystals leads to a lattice constant mismatch. The corresponding difference in Bragg angles is utilized to compensate the angle between the 1 st order beam and 0th order beam (initial beam propagation axis) from the grating splitter, making it possible to maintain the propagation direction of the monochromatized beam exiting the LODCM parallel to the 0th order incoming beam.
With the LCLS-II high-repetition-rate FEL, the number of pulses on the optics over ten years reaches 20 trillion. The thermal fatigue damage and lifetime of the optics, beam transport components under such large number of FEL pulses are important issues that should be addressed. For the optics, the definition of the damage should be the significant (for instance, 50%) reduction of reflectivity, which is premonitory of damage, and much more stringent than the ablation threshold. Silicon is widely used for X-ray optics both as mirror substrate and crystal monochromator. As the power absorption length of the silicon in visible optical laser wavelength (400 – 800 nm) is comparable as the one in soft X-ray FEL (300 – 1600 eV), we use optical laser of 515 nm wavelength, 200 fs pulse length and up to 0.928 MHz repetition rate for fatigue damage test on silicon. This allows remediating the limited availability or unavailability of high rep-rate FEL beam time. By monitoring the reflectivity of the sample (polished silicon wafer), we can measure the damage versus the number of pulses. Our first results show the damage threshold decreases significantly when the number of pulses increases. We have pushed the number of pulses over 100 billion, which corresponds to several days of MHz operation. The damage here can be explained as thermal mechanical fatigue damage. The outcome of this project will be a fatigue damage model predicting the lifetime of the key components, such as X-ray optics, used for high-repetition-rate X-ray FELs and also sample material under large number of laser pulses.
The x-ray free electron laser facility at SLAC National Accelerator Laboratory (named as LCLS) will be upgraded to LCLS-II in the near future. The high repetition rate light source makes the x-ray optics or components exposed to trillions of pulses over years of operation. Material fatigue properties of x-ray optics are essentially important for their lift-time prediction, optics optimization and opto-mechanical design. In this work, the fatigue properties of typical x-ray optics materials such as single-crystal silicon are experimentally measured by using laser pulses. The laser source can have an average power of 50 W at wavelength of 1.03 μm and repetition rate of 0.928 MHz with pulse duration of ~230 fs. The SHG crystal is used to generate 515 nm laser beam for the test to get an equivalent absorption length to soft x-rays. The maximum single-pulse energy is more than 16 μJ. The numbers of pulses that the optics can survive are measured for different pulse energies (fluences). The definition of the damage of x-ray optics is the significant reduction of reflectivity, which is premonitory of damage, and much more stringent than the ablation threshold.
Nine bendable mirrors will be installed as part of the upgrade to Linac Coherent Light Source. To achieve the target performance, accurate elliptical shapes must be generated with these focusing mirrors to an accuracy in the order of 104 to 105. We briefly summarize the developmental work including surface metrology via stitching and actuator characterization as well as fitting algorithm to achieve shape control of a KB developmental prototype. The height error of the centerline shape generated by the current system is in the order of 3 nm for a one meter long silicon mirror. The most important limiting factor is metrology due to environmental control.
With the onset of high power XFELs and diffraction limited storage rings, there is a growing demand to maintain sub nanometer mirror figures even under high heat load. This is a difficult issue as the optimum cooling design for an optic is highly dependent on the power footprint on the mirror, which can be highly dynamic. Resistive Element Adjustable Length (REAL) cooling can be utilized to change the cooling parameters during an experiment to adapt for changing beam parameters. A case study of the new soft x-ray monochromator for the LCLS L2SI program is presented that utilizes this new capability to allow the beam to translate across the mirror for different operation modes, greatly simplifying the monochromator mechanics. Metrology of a prototype mirror will also be presented.
Development of X-ray Free Electron Lasers (XFEL) opens new era in X-ray science. The full exploitation of unique properties of the XFEL radiation require challenging solutions that preserves radiation properties from a coherent, diffraction limited source under unprecedented instantaneous and average power load. We will present properties of simulated XFEL radiation such as coherence, source shape, divergence and longitudinal location inside the undulator. Recently, a construction of the LCLS II project has started as a major upgrade to the LCLS facility that will increase the average power of the XFEL up to 1 kW level. We will show how the X-ray simulations are used for minimizing thermal distortions on focusing of the LCLS II X-ray beams by 1 meter long Kirkpatrick-Baez mirrors. We will discuss and compare accuracy of simulations using different simulation methods and packages applied to focusing optics. The design of instruments should mitigate the damage to the optics caused by the tremendous instantaneous XFEL power. We will present X-ray simulation of the damage to the diffraction grating coatings and compare it with experimental results obtained at LCLS. The self-seeded mode of the LCLS operation increases temporal coherence and reduces greatly the bandwidth of the X-ray radiation. The results of time dependent X-ray simulations of the LCLS radiation passing through the seeding monochromator will be presented. We will compare two different approaches: Fourier Optics and an approach based on a dispersive system described by 6x6 pulse ray matrixes.
To preserve the full coherence of the FEL, the acceptance of the optics should be at least 2*FWHM of the X-ray beam. The LCLS-II soft X-ray experiments cover a photon energy range from 250 eV to 1300 eV. The photon beam footprint on the flat and KB mirrors varies from 150 mm to 1000 mm. The length of the mirror is chosen as 1 meter. Resistive Element Adjustable Length (REAL) cooling technique has been proposed to minimize the thermal deformation [1] for LCLS-II mirrors when the power FEL is above 200 W. The water cooling of the mirror is applied on the top-up-side [2]. The additional electric heater is adjustable both in length and power density to cope with the variable X-ray beam footprint length. A R&D project including the prototype of this REAL cooling technique is funded by DoE for FY2017 & FY2018.
In this paper, we will present the modeling results of this REAL cooled prototype mirror. The two parameters of the electric heater (length and power density) are optimized for the thermal deformation minimization of the mirror Finite Element Analysis (FEA) with ANSYS. This optimization of two parameters within ANSYS is not straight forward and necessity large number of FEA calculations. SRW software is used for the wavefront propagation simulation to compare the performance of REAL cooled mirror with other frequently used cooling techniques.
1. Zhang L., Cocco D., Kelez N., Morton D.S., Srinivasan V. and Stefan P.M. - Optimizing X-ray mirror thermal performance using matched profile cooling, J. Synchrotron Rad. (2015). 22,1170–1181, doi: 10.1107/S1600577515013090
2. Zhang L. , Barrett R. , Friedrich K. , Glatzel P. , Mairs T. , Marion P. , Monaco G. , Morawe C. , Weng T. - Thermal distortion minimization by geometry optimization for water-cooled white beam mirror or multilayer optics, Journal of Physics : Conference Series 425, 052029-1-052029-4 (2013)
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.
LCLS-2 is a high repetition rate (up to 1 MHz) superconducting FEL and the soft x-ray branch will operate from 0.2 to 1.3 keV. Over this energy range, there is a large variation in beam divergence and therefore, a large variation in the beam footprint on the optics. This poses a significant problem as it creates thermal gradients across the tangential axis of the mirror, which, in turn, creates non-cylindrical deformations that cannot be corrected using a single actuator mechanical bender. To minimize power loss and preserve the wave front, the optics requires sub-nanometer RMS height errors and sub-microradian slope errors. One of the key components of the beam transport in the SXR beamline is the bendable focusing mirror system, operated in a Kirkpatrick-Baez Configuration. For the first time in the Synchrotron or FEL world, the large bending needed to focus the beam will be coupled with a cooling system on the same mirror assembly, since the majority of the FEL power is delivered through every optic leading up to the sample. To test such a concept, we have developed a mirror bender system to be used as a multipurpose optic. The system has been very accurately modeled in FEA. This, along with very good repeatability of the bending mechanism, makes it ideal for use as a metrology tool for calibrating instruments as well as to test the novel cooling/bending concept. The bender design and the tests carried out on it will be presented.
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.
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