Optical design and metrology for wavefront preserving x-ray beamlines
Beamline design and specification have traditionally been based on geometrical optics and ray-tracing. For instance, mirror specification is normally derived from the parameters describing the statistics of ray deflection by its surface. The validity of this description assumes that the source is incoherent. This is valid for 2nd and 3rd generation sources, especially in the horizontal plane, and for high photon energies. The reduction in emittance, and mainly the development of free electron lasers and diffraction limited storage rings invalidates this assumption and pushes for the development of new tools and methods, both in beamline design and in optical metrology.
On the design side, raytracing has to be replaced by wavefront propagation, which is, in comparison, much slower, and therefore less usable during beamline design. Similarly, usual mirror specification based on ray-statistics has to be re-formulated in terms of wavefront-derived quantities.
On the metrology side, the required quality of optical surfaces is considerably more demanding, and surface errors of a fraction of a nanometer are often needed. Although such quality mirrors can be purchased, one must still control errors induced by alignment, vibrations, gravity, the holder (or bender) and cooling system, or by the thermal bump itself. To support all this, the local optics metrology laboratory must provide equipment and methods to characterize these optics: mirrors up to 1 m long, with a wide range of curvatures and with an uncertainty better than the equivalent to λ/600. This can be achieved by using error reduction techniques based on data redundancy. Examples of implementation on different instruments will be given, including a slope measuring instrument like the NOM, and height measuring instruments like the Fizeau interferometer.
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