Test of a high-heat-load double-crystal diamond monochromator at the Advanced Photon Source

We have tested the first diamond double-crystal monochromator at the Advanced Photon Source (APS), The monochromator consisted of two synthetic type Ib (1 1 1) diamond plates in symmetric Bragg geometry. The single-crystal plates were 6 mm x 5 mm x 0.25 mm and 6 mm x 5 mm x 0.37 mm and showed a combination of mosaic spread/strain of the order of 2-4 arcsec over a central 1.4 mm-wide strip. The monochromator first crystal was indirectly cooled by edge contact with a water-cooled copper holder. We studied the performance of the monochromator under the high-power X-ray beam delivered by the APS undulator A. By changing the undulator gap, we varied the power incident on the first crystal and found no indication of thermal distortions or strains even at the highest incident Power (200 W) and power density (108 W/mm(2) in normal incidence). The calculated maximum power and power density absorbed by the first crystal were 14.5 W and 2.4 W/mm(2), respectively. We also compared the maximum intensity delivered by this monochromator and by a silicon (1 1 1) cryogenically cooled monochromator. For energies in the range 6-10 keV, the flux through the diamond monochromator was about a factor of two less than through the silicon monochromator, in good agreement with calculations. We conclude that water-cooled diamond monochromators can handle the high-power beams from the undulator beamlines at the APS. As single-crystal diamond plates of larger size and better quality become available, the use of diamond monochromators will become a very attractive option.