Thermal stress prediction in mirror and multilayer coatings for X-ray optics

Orateur : XianChao CHENG (ESRF)

High heat-load induced thermal deformation in X-ray optics (white beam mirrors, crystal monochromators, and multilayers) is well investigated in the synchrotron community. To minimize this thermal deformation, the routinely adopted methods at ESRF and many other light sources are water cooling combined with smart-cut geometry for white beam mirrors and some multilayer optics liquid nitrogen cooling for silicon crystal monochromators and some multilayer optics on silicon substrates. For the mirror and multilayer optics, the thickness of the coatings is well below one micrometer (tens or hundreds of nanometers). The influences of these coatings on the temperature distribution and thermal deformation are negligible. However, the thermal stress within the coated layers can be very significant due to the different thermal expansion coefficient between the layer and the substrate materials. This is particularly true for liquid-nitrogen cooled mirror and multilayer optics. But the thermal stress within the coatings has not been well addressed due to the difficulties related to the very small thickness of the layers. For example, (1) the huge dimension aspect ratio (> 106) between the size ( 100 mm) and the thickness ( 100 nm) for numerical simulation (by FEA), (2) very limited data on material properties of the thin layer which may be significantly different from those of bulk material. As a part of the results of a PhD study, this presentation reports how we deal with these two difficulties : the FEA of the multilayer opticx experimental tests to extract information on material properties of the thin layers. Single layer coated mirrors and multilayer monochromators cooled by water or liquid nitrogen have been studied with typical heat-load, cooling, and geometrical parameters. The effects of cooling-down of the optics and the X-ray beam heat-load have been detailed. This study gives a first report on the thermal stress issues within the coatings of thin film based X-ray optics.

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