We develop a new type of X-ray lens system which is achromatic in a limited energy range. For such achromats
we combine different types of refractive and diffractive elements. For example, Fresnel zone plates and planar
parabolic concave SU-8 lenses are combined with lenses with a biconvex parabolic shape and with Fresnel lenses,
respectively. We present numerical results from a theoretical study of such optical systems. We determine the
focal spot size for an energy range of about <i>E</i> ± Δ<i>E</i> with Δ<i>E/E</i> ≈ 17%. Amongst other results we find that, compared with conventional lens systems, the spot size can be reduced by several tens of percent by using such achromatic lens systems.
X-ray prism lenses have been defined with the aim to collimate X-ray radiation emitted from an X-ray tube working as a
condenser lenses. Such a lens must have a large aperture as low absorption as possible. X-ray prism lenses combine low
absorption and large apertures. They are made up of a large array of equilateral triangular prismatic microstructures. The
intent by using these structures is to obtain as many refracting surfaces as possible in the smallest volume. The higher
surface-volume-ratio in comparison to standard lenses reduces absorption significantly at the expense of focus quality.
A first lens has been fabricated by X-ray lithography out of PMMA, with a designed aperture of up to 1.4 mm working
distances of 325 mm to the point source and X-ray energy of 9 keV. The edge-length of the prismatic microstructures is
10 μm. The lenses have been tested at the ESRF in (Grenoble, France) and at ANKA (Karlsruhe, Germany). The results
show an influence of the imperfections of the lens structures (bended prismatic microstructures) on the focal spot along
the focal line. The measured gain was 28 at a focal width of 8 μm at full width at half maximum. Due to these
imperfections the relevant aperture is currently limited to 500 μm.
Over the last decade refractive lenses for monochromatic X-ray radiation have been realized for many different materials by microfabrication technology. All these lens systems are successfully working only for one discrete energy, i.e. the lenses are chromatic. Thus each discrete energy within a certain energy range has a different focal length. While the focal spot size is smaller than a micron for a particular energy at the corresponding focal
distance, it increases up to several tens of microns for a larger energy range. We present results of numerical simulations for a new type of lens system which addresses this problem. We are developing achromats by combining different refractive elements of different materials. Via ray-tracing we determine the parameters of
the lenses by minimizing the focal spot size for an energy range of about E ± ΔE with ΔE = 15%. Thus the spot size of an energy range can be noticeably reduced compared with conventional refractive (chromatic) lens systems.