Carey Shawn Rogers

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 1b (1 1 1) diamond plates in symmetric Bragg geometry. The single-crystal plates were 6 mm × 5 mm × 0.25 mm and 6 mm × 5 mm × 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/mm2 in normal incidence). The calculated maximum power and power density absorbed by the first crystal were 14.5 W and 2.4 W/mm2, 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.

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

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 mm2, resulting in an average power density of about 118 W/mm2. 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.

High heat load monochromator development at the Advanced Photon Source

The Advanced Photon Source (APS) has embarked on a systematic program in high heat load x-ray monochromator optics to mitigate the thermal effects due to the powerful x-ray beams at third-generation synchrotron sources. This program includes both experimental and computational studies. The approaches being studied include the use of new coolants (cryogens and liquid gallium), new crystal geometries (inclined and variable asymmetric), and new materials (diamond). The paper summarizes the high heat load monochromator program at the APS.

Experimental results with cryogenically cooled, thin, silicon crystal x-ray monochromators on high-heat-flux beamlines

A novel, silicon crystal monochromator has been designed and tested for use on undulator and focused wiggler beamlines at third-generation synchrotron sources. The crystal utilizes a thin, partially transmitting diffracting element fabricated within a liquid-nitrogen cooled, monolithic block of silicon. This report summarizes the results from performance tests conducted at the European Synchrotron Radiation Facility (ESRF) using a focused wiggler beam and at the Advanced Photon Source (APS) on an undulator beamline. These experiments indicate that a cryogenic crystal can handle the very high power and power density x-ray beams of modern synchrotrons with sub-arcsec thermal broadening of the rocking curve. The peak power density absorbed on the surface of the crystal at he ESRF exceeded 90 W/mm2 with an absorbed power of 166 W, this takes into account the spreading of the beam due to the Bragg angle of 11.4 degrees. At the APS, the peak heat flux incident on the crystal was 1.5 W/mA/mm2 with a power of 6.1 W/mA for a 2.0 H X 2.5 V mm2 beam at an undulator gap of 11.1 mm and stored current up to 96 mA.

Thermal stress analysis and diffraction simulation of a standard and inclined gallium-cooled high-heat-load X-ray monochromator

Abstract This paper describes the methods used to calculate the thermally induced deformations in symmetrically cut, standardly configured and inclined monochromator crystals using finite element analysis. The results of these analyses are compared to recent undulator experiments conducted at the Cornell High Energy Synchrotron Source (CHESS) using a high-performance, liquid-gallium-cooled silicon crystal. The modeling was carried out for a range of machine currents, and the calculated rocking curve widths were within 10% of the experimental values. The asymmetric shape of the rocking curves at high currents was also predicted. These results lend credibility to our assertion that computer simulations can be used to reliably and accurately predict the performance of high-heat-load X-ray optics for future synchrotron sources.

Performance of cryogenically cooled, high-heat-load silicon crystal monochromators with porous media augmentation

The performance of two Si crystal x-ray monochromators internally cooled with liquid nitrogen was tested on the F2-wiggler beamline at the Cornell High Energy Synchrotron Source (CHESS). Both crystals were (111)-oriented blocks of rectangular cross section having identical dimensions. Seven 6.4-mm-diameter coolant channels were drilled through the crystals along the beam direction. In one of the crystals, porous Cu mesh inserts were bonded into the channels to enhance the heat transfer. The channels of the second crystal were left as drilled. Symmetric, double-crystal rocking curves were recorded simultaneously for both the first and third order reflections at 8 and 24 keV. The power load on the cooled crystal was adjusted by varying the horizontal beam size using slits. The measured Si(333) rocking curve of the unenhanced crystal at 24 keV at low power was 1.9 arcsec FWHM. The theoretical width is 0.63 arcsec. The difference is due to residual fabrication and mounting strain. For a maximum incident power of 601 W and an average power density of about 10 W/MM{sup 2}, the rocking curve was 2.7 arcsec. The rocking curve for the enhanced crystal at low power was 2.4 arcsec. At a maximum incident power of 1803 W and an average power density of about 19 W/mm{sup 2} the rocking curve width was 2.2 arcsec FWHM. The use of porous mesh augmentation is a simple, but very effective, means to improve the performance of cryogenically cooled Si monochromators exposed to high power x-ray beams.

High-heat-flux x-ray monochromators: what are the limits?

First optical elements at third-generation, hard x-ray synchrotrons, such as the Advanced Photon Source, are subjected to immense heat fluxes. The optical elements include crystal monochromators, multilayers and mirrors. This paper presents a mathematical model of the thermal strain of a three-layer (faceplate, heat exchanger, and baseplate), cylindrical optic subjected to a narrow beam of uniform heat flux. This model is used to calculate the strain gradient of a liquid-gallium-cooled x-ray monochromator previously tested on an undulator at the Cornell High Energy Synchrotron Source. The resulting thermally broadened rocking curves are calculated and compared to experimental data. The calculating rocking curve widths agree to within a few percent of the measured values over the entire current range tested (0 to 60 mA). The thermal strain gradient under the beam footprint varies linearly with the heat flux and the ratio of the thermal expansion coefficient to the thermal conductivity. The strain gradient is insensitive to the heat exchanger properties and the optic geometry. This formulation provides direct insight into the governing parameters, greatly reduces the analysis time, and provides a measure of the ultimate performance of a given monochromator.

Cryogenic cooling program in high heat load optics at the Advanced Photon Source

This paper describes some of the aspects of the cryogenic optics program at the Advanced Photon Source (APS). A liquid-nitrogen-cooled, high-vacuum, double crystal monochromator is being fabricated at Argonne National Laboratory (ANL). A pumping system capable of delivering a variable flow rate of up to 10 gallons per minute of pressurized liquid nitrogen and removing 5 kilowatts of x-ray power is also being constructed. This specialized pumping system and monochromator will be used to test the viability of cryogenically cooled, high- heat-load synchrotron optics. It has been determined that heat transfer enhancement will be required for optics used with APS insertion devices. An analysis of a porous-matrix-enhanced monochromator crystal is presented. For the particular case investigated, a heat transfer enhancement factor of 5 to 6 was calculated.

Test results of a diamond double-crystal 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 lb (111) diamond plates in symmetric Bragg geometry. We tested two pairs of single-crystal plates: the first pair was 6 mm by 5 mm by 0.25 mm and 6 mm by 5 mm by 0.37 mm; the second set was 7 mm by 5.5 mm by 0.44 mm. 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. We found no indication of thermal distortions or strains even at the highest incident power (280 watts) and power density (123 W/mm{sup 2} at normal incidence). The calculated maximum power and power density absorbed by the first crystal were 37 watts and 16 W/mm{sup 2} respectively. We also compared the maximum intensity delivered by the diamond monochromator and by a silicon (111) cryogenically cooled monochromator. For energies in the range of 6 to 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 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.