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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.
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