Performance of a gallium-cooled 85° inclined silicon monochromator for a high power density X-ray beam

Abstract

We have made double crstal rocking curve measurements on a gallium-cooled silicon monochromator in both the normal flat geometry and an 85° inclined geometry on the X-25 focused wiggler beamline at the National Synchrotron Light Source. At 192 mA ring current, the focused wiggler delivers about 37.7 W of power into a spot size of FWHM 0.4 × 0.8 mm 2, resulting in an average power density of about 118 W/mm 2. The inclined crystal geometry spreads the beam footprint on the surface of the crystal while maintaining a b = −1 symmetric Bragg reflection. At an 85° inclination angle, the beam footprint is 11.5 times larger than that for the flat geometry. In the case of the flat geometry at a ring current of 156 mA, we see, via an infrared camera, an increase in temperature of 56°C above the nominal silicon temperature. The rocking curve this case were significantly broadened (FWHM for 15 keV Si(333) = 35 arcsec) due to the thermally induced strain in the silicon. In the inclined crystal, the thermal peak on the crystal was only about 2.7°C above the nominal silicon temperature. In this case, the rocking curve width for the 15 keV Si(333) reflection was measured to be FWHM = 2.7 arc sec compared with the theoretical width of FWHM = 1.0 arcsec. The residual strain is totally due to the mounting of the crystals and not the heating from the X-ray beam.