Before the first photon beam was delivered at the SOLEIL synchrotron, scientists tried to anticipate the problem of
carbon contamination on optical components, with for instance the outgasing of chambers by prior exposure to the beam
with dummy optics. In spite of these efforts, deterioration of optical performance by carbon contamination has remained
an outstanding issue: on the low-energy beamlines at SOLEIL. For example, carbon contamination results in significant
photons flux losses (practically at the Carbon K edges), and modifications of the horizontal-to-vertical polarization
transmission ratio, which degrade with time as the thickness of the carbon layer builds up. This contamination is visible
and consists of a gray/black line over the entire photon beam footprint. Addressing the carbon contamination issue, two
cleaning processes have been tested quite successfully on two SOLEIL beamlines (in the UV-VUV and soft X-ray
ranges), namely in-situ oxygen plasma and in-situ ozone generation via UV lamps. A dedicated group is currently
working on the improvement of the cleaning processes, the metrology of the optics before and after cleaning and the
study of the carbon coating in order to propose possible strategies to prevent or slow down the contamination process.
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