The push for high quality x-ray optics is closely linked to improvements in metrology technology. During the last decade, we have seen an ultra-fast progress in x-ray optics performances. This enhancement is directly linked to the development of the necessary tools to control these optical components. These metrology tools are necessary for the fabrication (to guide some polishing deterministic process) and also for the ultimate characterization used to validate surface parameters (often inside their own mechanical support) prior to installation in a beam line. It is now necessary to characterize optical surface figure, slope errors and roughness on meter-long optics over spatial frequencies as short as 0.1 mm and with slope errors reaching less than 100 nrad rms or surface figure errors close to 1 nm in order to not spoiled and preserve the high brightness made available by third and fourth generation synchrotron/FEL sources like NSLSII or LCLS. For this purpose, the new NSLS-II Optical Metrology Laboratory (NSLSII-OML) includes commercial instruments for measuring long spatial frequency figure errors, mid spatial frequencies and high frequency roughness and had started some research and development activities. This paper provides a brief description of the instruments currently available in the laboratory and gives an overview of the very active research and development efforts within the NSLSII-OML.
The coherent soft x-ray and full polarization control (CSX) beamline at the National Synchrotron Light Source -
II (NSLS-II) will deliver 1013 coherent photons per second in the energy range of 0.2-2 keV with a resolving power
of 2000. The source, a dual elliptically polarizing undulator (EPU), and beamline optics should be optimized to
deliver the highest possible coherent flux in a 10-30 μm spot for use in coherent scattering experiments. Using
the computer code Synchrotron Radiation Workshop (SRW), we simulate the photon source and focusing optics
in order to investigate the conditions which provide the highest usable coherent intensity on the sample. In
particular, we find that an intermediate phasing magnet is needed to correct for the relative phase between the
two EPUs and that the optimum phase setting produces a spectrum in which the desired wavelength is slightly
red-shifted thus requiring a larger aperture than originally anticipated. This setting is distinct from that which
produces an on-axis spectrum similar to a single long undulator. Furthermore, partial coherence calculations,
utilizing a multiple electron approach, indicate that a high degree of spatial coherence is still obtained at the
sample location when such an aperture is used. The aperture size which maximizes the signal-to-noise ratio
of a double-slit experiment is explored. This combination of high coherence and intensity is ideally suited for
x-ray ptychography experiments which reconstruct the scattering density from micro-diffraction patterns. This
technique is briefly reviewed and the effects on the image quality of proximity to the beamline focus are explored.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.