Liquid nitrogen cooled Si crystal monochromator: X-ray focusing by controlled heat load

Abstract

The thermal deformation of a liquid nitrogen cooled silicon crystal under increasing absorbed power undergoes five phases: i) no deformation at zero power, ii) concave shape of the crystal, iii) appearance of a central bump, iv) minimum of the slope error averaged over the beam footprint on the crystal due to the balance between the initial concave shape and the growing central bump, and v) rapid growth of the thermal bump. In general, a liquid nitrogen cooled silicon crystal is designed to operate before the appearance of phase v), and mostly in phase ii) (concave shape). This concave shape of the crystal leads to a focusing effect on the beam, which should be considered in combination with other optical elements in the design and optimization of the beamline optical layout.