Abstract
A nearly stigmatic plane grating monochromator under construction for the new undulator beamline at Aladdin will provide a resolving power ≳10000 as it scans from 8 to 240 eV with a single grating. Scanning requires the precise, simultaneous movements of both a plane mirror and a plane grating in close proximity to one another inside a UHV chamber. The mirror, which absorbs up to 16.5 watts, is internally water cooled to minimize thermally induced slope errors. The radiatively cooled grating absorbs less than a watt. Careful examination of the focusing requirements revealed that the monochromator could be scanned either in the conventional mode of rotating-translating the mirror and rotating the grating or in a modified mode of rotating the mirror and rotating-translating the grating. The latter mode was chosen for simplicity of design. The mirror and grating rotate nearly 30 and 40 degrees, respectively, with subarcsec resolution. Both utilize a stepping-motor lead-screw piezoelectric actuator scan drive that is controlled with a feedback loop using a laser interferometer to measure the actual rotation of the optics. The grating mechanism translates nearly 200 mm along a granite surface plate with a positional accuracy and vertical stability of a few micrometers.
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