Compound refractive lenses (CRL) are widely used to manipulate synchrotron radiation beams. Accurate
modelling of X-ray beam propagation through individual lenses and through "transfocators" composed of a large
number of CRLs is of high importance, since it allows for comprehensive optimization of X-ray beamline designs
for particular user experiments.
In this work we used the newly developed McXtrace ray-tracing package and the SRW wave-optics code to
simulate the beam propagation of X-ray undulator radiation through such a "transfocator" as implemented at ID-
11 at ESRF. By applying two complementary simulation methods, we were able to obtain comparable results
(e.g. the beam's focused properties) and also to provide a complete description of X-ray beam propagation
through the CRLs and other optical components. However, some discrepancies between the results acquired by
both methods (e.g. broader monochromatization degree obtained with the McXtrace code) brought a meaningful
insight into further development strategies for the McXtrace package.
Ultra-low emittance third-generation synchrotron radiation sources such as the NSLS-II offer excellent opportunities for
the development of experimental techniques exploiting x-ray coherence. Coherent light scattered by a heterogeneous
sample produces a speckle pattern characteristic for the specific arrangement of the scatterers. This may vary over time,
and the resultant intensity fluctuations can be measured and analyzed to provide information about the sample dynamics.
X-ray photon correlation spectroscopy (XPCS) extends the capability of dynamic light scattering to opaque and turbid
samples and extends the measurements of time evolution to nanometer length scales. As a consequence XPCS became
crucial in the study of dynamics in systems including, but not being limited to, colloids, polymers, complex fluids,
surfaces and interfaces, phase ordering alloys, etc. In this paper we present the conceptual optical design and the
theoretical performance of the Coherent Hard X-ray (CHX) beamline at NSLS-II, dedicated to XPCS and other coherent
scattering techniques. For the optical design of this beamline, there is a tradeoff between the coherence needed to
distinguish individual speckles and the phase acceptance (high intensity) required to measure fast dynamics with an
adequate signal-to-noise level. As XPCS is a "photon hungry" technique, the beamline optimization requires maximizing
the signal-to-noise ratio of the measured intensity-intensity autocorrelation function. The degree of coherence, as
measured by a two-slit (Young) experiment, is used to characterize the speckle pattern visibilities. The beamline
optimization strategy consists of maximization of the on-sample intensity while keeping the degree of coherence within
the 0.1-0.5 range. The resulted design deviates substantially from an ad-hoc modification of a hard x-ray beamline for
XPCS measurements. The CHX beamline will permit studies of complex systems and measurements of bulk dynamics
down to the microsecond time scales. In general, the 10-fold increase in brightness of the NSLS-II, compared to other
sources, will allow for measurements of dynamics on time-scales that are two orders of magnitude faster than what is
currently possible. We also conclude that the common approximations used in evaluating the transverse coherence
length would not be sufficiently accurate for the calculation of the coherent properties of an undulator-based beamline,
and a thorough beamline optimization at a low-emittance source such as the NSLS-II requires a realistic wave-front
propagation analysis.
we present the developments of the McXtrace project, a free, open source software package based on Monte
Carlo ray tracing for simulations and optimisation of complete X-ray instruments.
The methodology of building a simulation is presented through an example beamline, namely Beamline 811
at MAX-lab, Lund, Sweden - a beamline dedicated to materials science.
Partially-coherent wavefront propagation calculations have proven to be feasible and very beneficial in the design of
beamlines for 3rd and 4th generation Synchrotron Radiation (SR) sources. These types of calculations use the framework
of classical electrodynamics for the description, on the same accuracy level, of the emission by relativistic electrons
moving in magnetic fields of accelerators, and the propagation of the emitted radiation wavefronts through beamline
optical elements. This enables accurate prediction of performance characteristics for beamlines exploiting high SR
brightness and/or high spectral flux. Detailed analysis of radiation degree of coherence, offered by the partially-coherent
wavefront propagation method, is of paramount importance for modern storage-ring based SR sources, which, thanks to
extremely small sub-nanometer-level electron beam emittances, produce substantial portions of coherent flux in X-ray
spectral range. We describe the general approach to partially-coherent SR wavefront propagation simulations and present
examples of such simulations performed using "Synchrotron Radiation Workshop" (SRW) code for the parameters of
hard X-ray undulator based beamlines at the National Synchrotron Light Source II (NSLS-II), Brookhaven National
Laboratory. These examples illustrate general characteristics of partially-coherent undulator radiation beams in low-emittance
SR sources, and demonstrate advantages of applying high-accuracy physical-optics simulations to the
optimization and performance prediction of X-ray optical beamlines in these new sources.
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