Realizing the experimental potential of high-brightness, next generation synchrotron and free-electron laser light sources
requires the development of reflecting x-ray optics capable of wavefront preservation and high-resolution nano-focusing.
At the Advanced Light Source (ALS) beamline 5.3.1, we are developing broadly applicable, high-accuracy, in situ, at-wavelength
wavefront measurement techniques to surpass 100-nrad slope measurement accuracy for diffraction-limited
Kirkpatrick-Baez (KB) mirrors.
The at-wavelength methodology we are developing relies on a series of wavefront-sensing tests with increasing accuracy
and sensitivity, including scanning-slit Hartmann tests, grating-based lateral shearing interferometry, and quantitative
knife-edge testing. We describe the original experimental techniques and alignment methodology that have enabled us to
optimally set a bendable KB mirror to achieve a focused, FWHM spot size of 150 nm, with 1 nm (1.24 keV) photons at
3.7 mrad numerical aperture. The predictions of wavefront measurement are confirmed by the knife-edge testing.
The side-profiled elliptically bent mirror used in these one-dimensional focusing experiments was originally designed
for a much different glancing angle and conjugate distances. Visible-light long-trace profilometry was used to pre-align
the mirror before installation at the beamline. This work demonstrates that high-accuracy, at-wavelength wavefront-slope
feedback can be used to optimize the pitch, roll, and mirror-bending forces in situ, using procedures that are
deterministic and repeatable.
Convenience and cost often lead to synchrotron beamlines where the final bendable Kirkpatrick-Baez focusing pair must
relay the final image to different samples at different image distances e.g., [Proc. FEL2009, 246-249 (2009)] either for
different experimental chambers, or diagnostics. We present an initial analytical approach, starting from, and extending
the work of Howells et al. [OE 39(10), 2748-62 (2000)] to analyze the trade-offs between choice of mirror, bending
couples and the given, shaped sagittal width of the optic. Both experimentally and in simulation, we have found that
after an appropriate re-bending, sagittally shaped optics can perform with high quality at significantly different incidence
angles and conjugate distances. We present one successful demonstration from the ALS Optical Metrology Beamline
5.3.1, and review some new closed form analytical solutions with a view towards understanding our results.
Nano-focusing and brightness preservation for ever brighter synchrotron radiation and free electron laser beamlines
require surface slope tolerances of x-ray optics on the order of 100 nrad. While the accuracy of fabrication and ex situ
metrology of x-ray mirrors has improved over time, beamline in situ performance of the optics is often limited by
application specific factors such as x-ray beam heat loading, temperature drift, alignment, vibration, etc. In the present
work, we discuss the recent results from the Advanced Light Source developing high accuracy, in situ, at-wavelength
wavefront measurement techniques to surpass 100-nrad accuracy surface slope measurements with reflecting x-ray
optics. The techniques will ultimately allow closed-loop feedback systems to be implemented for x-ray nano-focusing. In
addition, we present a dedicated metrology beamline endstation, applicable to a wide range of in situ metrology and test
experiments. The design and performance of a bendable Kirkpatrick-Baez (KB) mirror with active temperature
stabilization will also be presented. The mirror is currently used to study, refine, and optimize in situ mirror alignment,
bending and metrology methods essential for nano-focusing application.
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