The paper presents an overview of the main functions and new application examples of the “Synchrotron Radiation Workshop” (SRW) code. SRW supports high-accuracy calculations of different types of synchrotron radiation, and simulations of propagation of fully-coherent radiation wavefronts, partially-coherent radiation from a finite-emittance electron beam of a storage ring source, and time-/frequency-dependent radiation pulses of a free-electron laser, through X-ray optical elements of a beamline. An extended library of physical-optics “propagators” for different types of reflective, refractive and diffractive X-ray optics with its typical imperfections, implemented in SRW, enable simulation of practically any X-ray beamline in a modern light source facility. The high accuracy of calculation methods used in SRW allows for multiple applications of this code, not only in the area of development of instruments and beamlines for new light source facilities, but also in areas such as electron beam diagnostics, commissioning and performance benchmarking of insertion devices and individual X-ray optical elements of beamlines. Applications of SRW in these areas, facilitating development and advanced commissioning of beamlines at the National Synchrotron Light Source II (NSLS-II), are described.
We present the application of fully- and partially-coherent synchrotron radiation wavefront propagation simulation functions, implemented in the "Synchrotron Radiation Workshop" computer code, to create a ‘virtual beamline’ mimicking the Coherent Hard X-ray scattering beamline at NSLS-II. The beamline simulation includes all optical beamline components, such as the insertion device, mirror with metrology data, slits, double crystal monochromator and refractive focusing elements (compound refractive lenses and kinoform lenses). A feature of this beamline is the exploitation of X-ray beam coherence, boosted by the low-emittance NSLS-II storage-ring, for techniques such as X-ray Photon Correlation Spectroscopy or Coherent Diffraction Imaging. The key performance parameters are the degree of Xray beam coherence and photon flux, and the trade-off between them needs to guide the beamline settings for specific experimental requirements. Simulations of key performance parameters are compared to measurements obtained during beamline commissioning, and include the spectral flux of the undulator source, the degree of transverse coherence as well as focal spot sizes.
High-accuracy physical optics calculation methods used in the “Synchrotron Radiation Workshop” (SRW) allow for multiple applications of this code in different areas, covering development, commissioning, diagnostics and operation of X-ray instruments at light source facilities. This presentation focuses on the application of the SRW code for the simulation of experiments at these facilities. The most complete and most detailed simulation of experiments with SRW is possible in the area of elastic coherent scattering, where the interaction of radiation with samples can be described with the same transmission-type “propagators” that are used for the simulation of fully- and partially-coherent radiation propagation through X-ray optical elements of beamlines. A complete “source-to-detector” simulation of such an experiment for a lithographic sample is described here together with comparisons of the simulated coherent scattering data with actual measurements results, obtained at the Coherent Hard X-ray (CHX) beamline of the National Synchrotron Light Source II (NSLS-II). Particular attention is paid to the analysis of visibility of speckles and intensity levels in the scattered radiation patterns at different degrees of coherence of the radiation entering the sample.
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
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.