Abstract
New, high-coherent-flux X-ray beamlines at synchrotron and free-electron laser light sources rely on wavefront sensors to achieve and maintain optimal alignment under dynamic operating conditions. This includes feedback to adaptive X-ray optics. We describe the design and modeling of a new class of binary-amplitude reflective gratings for shearing interferometry and Hartmann wavefront sensing. Compact arrays of deeply etched gratings illuminated at glancing incidence can withstand higher power densities than transmission membranes and can be designed to operate across a broad range of photon energies with a fixed grating-to-detector distance. Coherent wave-propagation is used to study the energy bandwidth of individual elements in an array and to set the design parameters. We observe that shearing operates well over a ±10% bandwidth, while Hartmann can be extended to ±30% or more, in our configuration. We apply this methodology to the design of a wavefront sensor for a soft X-ray beamline operating from 230 eV to 1400 eV and model shearing and Hartmann tests in the presence of varying wavefront aberration types and magnitudes.
Highlights
Recent advances in high-coherent-flux X-ray light sources have spurred the development of beamline optical systems that can preserve the wavefront quality of focused beams
We developed binary, amplitudemodulating reflection gratings to serve both shearing and Hartmann testing with a compact optical element
Our design allows shearing and Hartmann to use the same grating-to-detector distance, with gratings and grids patterned on the same optical element
Summary
Recent advances in high-coherent-flux X-ray light sources have spurred the development of beamline optical systems that can preserve the wavefront quality of focused beams. Hartmann is a non-interferometric approach that commonly uses a grid of holes in an opaque screen to project an array of isolated spots (2D) or lines (1D) onto the detector Both techniques are sensitive to the wavefront slope. We present a solution for adaptive optic feedback in the presence of one-dimensional beam dispersion (upstream of a grating monochromator’s exit slit), narrow beam widths, and high power. On these beamlines, a varied-line-spacing (VLS) planar grating monochromator performs vertical focusing and spectral dispersion [14], while in the horizontal direction, a plane-elliptical adaptive mirror focuses the beam and corrects wavefront aberrations. The creation and metrology of these new elements were described in a previous report [18]
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