High heat-load premonochromator

The x-ray beam from the undulator is premonochromatised by a silicon (1,1,1) channel-cut crystal to a relative bandwidth of E/E = 2·10^-4. Due to the high heat load produced by the intense undulator beam, the crystal has to be cooled down to cryogenic temperatures of about 100K where silicon displays a maximum in the thermal conductivity and a mininum in the linear thermal expansion coefficient. This allows to keep the thermal deformation of the crystal below the limits where photon flux losses occur. The crystal is cooling by a closed-loop liquid nitrogen system.

Measured photon flux behind the silicon (1,1,1) premonochromator for the operating points of ID28: Three 1.6 m U32 segments; ring current: 200 mA; primary slits: 1.6 mm (horizontal) x 0.7 mm (vertical); secondary slits: 2.4 mm (hor.) x 1.0 mm (vert.). The number in brackets in the second column indicate the undulator harmonics.