Abstract
An instrument allowing the quantitative analysis of X-ray pulsed wavefronts is presented and its processing method explained. The system relies on the X-ray speckle tracking principle to accurately measure the phase gradient of the X-ray beam from which beam optical aberrations can be deduced. The key component of this instrument, a semi-transparent scintillator emitting visible light while transmitting X-rays, allows simultaneous recording of two speckle images at two different propagation distances from the X-ray source. The speckle tracking procedure for a reference-less metrology mode is described with a detailed account on the advanced processing schemes used. A method to characterize and compensate for the imaging detector distortion, whose principle is also based on speckle, is included. The presented instrument is expected to find interest at synchrotrons and at the new X-ray free-electron laser sources under development worldwide where successful exploitation of beams relies on the availability of an accurate wavefront metrology.
Highlights
At-wavelength metrology is gaining in importance at X-ray large-scale facilities in order to take full advantage of the beams delivered by high-brilliance sources (Berujon, 2013; Sawhney et al, 2013)
The wavefront error was calculated by subtracting the best ellipsoid from the reconstructed wavefront and the wavefront gradient error was obtained by removal of the bestfitted plane
892 Sebastien Berujon et al X-ray pulse wavefront metrology view displayed in the figure corresponds to the common beam area intercepted by the second detector at these two positions d
Summary
At-wavelength metrology is gaining in importance at X-ray large-scale facilities in order to take full advantage of the beams delivered by high-brilliance sources (Berujon, 2013; Sawhney et al, 2013). Since most of them require several acquisitions per wavefront reconstruction, their use becomes confined to synchrotron sources for which the beam remains stable over a time period much longer than the total data acquisition time (Kewish et al, 2010; Brady & Fienup, 2006). These metrology methods can be sometimes tedious to implement since they can require hundreds of images and lownoise detectors. The need for a wavefront metrology capable of analyzing each individual bunch makes many online synchrotron metrology techniques inapplicable to X-FEL sources
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