Abstract
This paper illustrates the use of spatial filtering with a horizontal slit near the source to enlarge the horizontal coherence in an experimental station and produce a diffraction-limited round focus at an insertion device beamline for X-ray photon correlation spectroscopy experiments. Simple expressions are provided to guide the optical layout, and wave propagation simulations confirm their applicability. The two-dimensional focusing performance of Be compound refractive lenses to produce a round diffraction-limited focus at 11 keV capable of generating a high-contrast speckle pattern of an aerogel sample is demonstrated. The coherent scattering patterns have comparable speckle sizes in both horizontal and vertical directions. The focal spot sizes are consistent with hybrid ray-tracing calculations. Producing a two-dimensional focus on the sample can be helpful to resolve speckle patterns with modern pixel array detectors with high visibility. This scheme has now been in use since 2019 for the 8-ID beamline at the Advanced Photon Source, sharing the undulator beam with two separate beamlines, 8-ID-E and 8-ID-I at 7.35 keV, with increased partially coherent flux, reduced horizontal spot sizes on samples, and good speckle contrast.
Highlights
Synchrotron light sources produce partially coherent light that has enabled new techniques such as X-ray photon correlation spectroscopy (XPCS) to flourish into mature material characterization methods (Sutton, 2008; Livet & Sutton, 2012; Sinha et al, 2014; Sandy et al, 2018)
Beamline 8-ID currently focuses the vertical coherent flux with a 1D parabolic Be lens. It is done at both branches, the 8-ID-I main line which specializes in small-angle XPCS, and the side branch 8-ID-E which focuses on wide-angle XPCS
A Gaussian least-squares fit is shown with a full width at half-maximum (FWHM) of 0.72 mm, and a small background of 1270 counts sÀ1
Summary
Synchrotron light sources produce partially coherent light that has enabled new techniques such as X-ray photon correlation spectroscopy (XPCS) to flourish into mature material characterization methods (Sutton, 2008; Livet & Sutton, 2012; Sinha et al, 2014; Sandy et al, 2018). Due to the very small horizontal source size at this facility, one can focus horizontally 50–100 mm beams coherently to a few mm on the sample without closing any near front-end horizontal slit This type of source spatial filtering allows one to deliver round focused coherent beams at existing synchrotrons with modest horizontal transverse coherent lengths (10 mm or below). The first paper provides an analytical treatment which includes a slit placed just upstream of the lens and shows its impact to control the focal coherence length and spot size (Singer & Vartanyants, 2014). This can be applied to the vertical focusing direction in this paper, it approximates the slit by a Gaussian aperture. We present optical simulations performed with a hybrid coherent diffraction module (Shi et al, 2014)
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