THEORY AND PROBLEMS OF MICROFOCUSING: D. Labergerie, European Synchrotron Radiation Facility, 38043 Grenoble, France.

Because of the unusual index of refraction (n < 1) of X-Rays in matter it is impossible to use classical refractive optics such as lenses to focus such beams. Nevertheless, some optics working either by reflection (mirror) or diffraction (bent crystal, multilayers, Bragg Fresnel lenses) can be successfully used to focus X-ray beam to a few microns.

This presentation will deal with focusing mirror and the optimization of different parameters such as: substrates (energy related), mirror shape (spherical, or aspherical), optical quality (roughness and slope errors), and also selected geometries (Kirkpatrick-Baez, Wolter types) to maximize the flux at the sample.

Using geometrical considerations the focal spot size can be calculated as a function of all these parameters, and this at any plane along the optical path. The comparison between the analytical approach and ray tracing simulations: the program Shadow will be discussed.

As an example results obtained with a novel device developed at the ESRF: bimorph mirror will be presented.Two such mirrors will be soon installed on the High Pressure Beamline in a Kirkpatrick-Baez configuration. This new active mirror made of piezo-electric ceramic: have several advantages over conventional bent mirrors.