HIGH-THROUGHPUT ASYMMETRIC DOUBLE-CRYSTAL MONOCHROMATOR OF THE SAXS BEAMLINE AT ELETTRA

A new high-flux wiggler beamline for fast time-resolved small-angle X- ray scattering (SAXS) based on double-focusing optics has recently com menced operation at the 2 GeV third-generation storage ring ELETTRA at Trieste, Italy. Its non-dispersive double-crystal monochromator conta ins three pairs of interchangeable asymmetrically cut flat Si(111)-cry stal pairs, each of which is optimized for high throughput at one of t he three fixed energies 5.4, 8 and 16 keV. To cope with the severe the rmal power load produced by a 57-pole wiggler on the first crystal of each pair (up to 5.4 W mm(-2) and 700 W under normal incidence, for 40 0 mA), grazing angles of 2 degrees and optimized back-cooling have bee n chosen. This solution allows simultaneously a gain of 2.5-3.0 in thr oughput and, accordingly, in flux density. Finite-element analysis as well as commissioning tests showed that the cooling layout functions v ery satisfactorily, and that up to 5 x 10(12) photons s(-1) are availa ble at the sample (at 8 keV and 250 mA), as predicted.