Handling the carbon contamination issue at SOLEIL

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.