Refractive x-ray lens
Fig. 1. Refractive x-ray lens

Since the refractive index of x-rays in matter is (slightly) smaller than one, focusing x-ray lenses have a biconcave form which, furthermore, must be parabolic in order to reduce aberrations (see Figure 1). Due to the very weak refraction of x-rays in matter a focal length in the meter range is achieved by choosing a small radius of curvature R at the apex (50 μm to a few mm) and by stacking many individual lenses in a row (see Figure 2). This arrangement is called a compound refractive lens (CRL). The absorption of x-rays in matter being rather high (compared to the absorption of visible light in glass) the lens material must be a low Z material; we mainly employ beryllium and aluminium. Compared to plastics the metallic lens material has the big advantage of avoiding radiation damage and of providing good heat conductivity. Indeed, experience has shown that beryllium lenses are able to withstand both the white beam of an undulator and the intense beam of an x-ray free-electron laser.

Compound refractive lens
Fig. 2. Compound refractive lens

Our optical devices cover a very wide energy range from about 2 keV to about 150 keV. Note that this range is much larger than for any other optic. Moreover, refractive lenses are about a factor 1000 less sensitive to surface roughness and contamination than mirrors. X-ray lenses from RXOPTICS can be used like glass lenses are used for visible light. The only difference is that the numerical aperture is small (of the order 0.001 to 0.0001 as compared to 0.5 to 1 for glass lenses). This makes the optical set-up very slender which, by the way, helps reducing aberrations. The geometric aperture 2R0 (see Figure 1) varies from a few 100 ┬Ám to a few mm. This matches well the dimensions of the x-ray beams of most synchrotron radiation sources.

Refractive x-ray lenses are available from RXOPTICS as rotationally parabolic (2D) and as cylinder parabolic (1D) lenses, focusing in both directions or only in one direction. Figures 3 and 4 show a set of 2D-lenses (R = 50 to 1500 μm) and a 1D-lens (R = 500 μm) in their lens frames, respectively.

2D refractive x-ray lenses
Fig. 3. Set of 2D-lenses
1D refractive x-ray lens
Fig. 4. 1D-lens
Lens casing for a compound refractive lens
Fig. 5. Lens casing

Lenses are much more insensitive to misorientations and vibrations than mirrors. Nevertheless, it is necessary to align the optical axes of the individual lenses in a stack with submicrometer precision. Furthermore, during operation the lens stack must be installed in vacuum. To this end, we provide high precision lens casings that can be integrated in the vacuum of a beam line. Figure 5 shows a lens casing with a stack of 2D-lenses. At both ends of the lens stack should be installed a pinhole or a slit in order to reduce the scattered radiation. Compound refractive lenses from RXOPTICS are robust and compact, easily installed and removed. There is no need for sophisticated benders or order sorting apertures. The focus stays on axis so that there is no need for rearranging the optical components downstream. Alignment of the lenses is done in a short time (typically well below half an hour).

Please read the user instructions of our optical components!

Refractive x-ray lenses show strong chromatic aberration. For a fixed focal length the number of lenses in the stack must be varied if the energy is varied during an experiment. This disadvantage can be avoided by employing a device called transfocator which has been developed at ESRF in Grenoble.