Development of X-ray refractive optics. New promising prospectives for diffraction limited 4-th generation X-ray sources.
use of X-ray refractive optics has rapidly expanded and they are now in
common use at 15 synchrotrons in 10 countries. This development has
intensified after the successful implementation of transfocators -
tunable devices based on refractive lenses. In addition to traditional
micro-focusing applications, the transfocators can provide the following
beam conditioning functions: condensers with a tunable beam size,
micro-radian collimators, low-band pass filters and high harmonics
After the first successful experimental demonstration 20 years ago , the use of X-ray refractive optics has rapidly expanded and they are now in common use at 15 synchrotrons in 10 countries. This development has intensified after the successful implementation of transfocators - tunable devices based on refractive lenses . In addition to traditional micro-focusing applications, the transfocators can provide the following beam conditioning functions in the energy range from 3 to 100 (200) keV:
- condensers with a tunable beam size,
- micro-radian collimators ,
- low-band pass filters - monochromator 
- high harmonics rejecters 
New advanced parameters of the beam provided by the diffraction limited sources – XFELs and new synchrotrons with the reduced horizontal emittance will open up a unique opportunity to build up a new concept for the loss-free beam transport and conditioning systems based on in-line refractive optics. Taking an advantage of the substantially reduced horizontal source size and the beam divergence these new systems integrated into the front-end can transfer the photon beam almost without losses from the front-end to any further secondary optical systems (mirrors, crystals, lenses etc.) or directly to the end-stations. Evidently, beamlines will benefit from the possibility to include active moveable lens systems in the front-ends. In this regard, development of diamond refractive optics is crucial [4,5]. The implementation of the lens-based beam transport concept will significantly simplify the layout of majority of the new beamlines . It will also allow a smooth beamlines transition from the present beam parameters to the upgraded ones, avoiding major optics modifications .
The field of applications of refractive optics is not limited to beam conditioning, but can be extended into the area of Fourier optics, as well as coherent diffraction and imaging techniques [8-13]. Using the intrinsic property of the refractive lens as a Fourier transformer, the coherent diffraction microscopy and high resolution diffraction methods have been proposed to study 3-D structures of semiconductor crystals and mesoscopic materials [14–15].
Another promising direction of refractive optics development is in-line X-ray interferometry. Recently proposed bi- and multi-lens interferometers can generate an interference field with a variable period ranging from tens of nanometers to tens of micrometers [15,16]. This simple way to create an X-ray standing wave in paraxial geometry opens up the opportunity to develop new X-ray interferometry techniques to study natural and advanced man-made nano-scale materials, such as self-organized bio-systems, photonic and colloidal crystals, and nano-electronics materials. As a classical interferometer it can be used for phase contrast imaging and radiography. Finally it can be useful for the coherence characterization of the X-rays sources and free electron lasers.
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