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SLAC Technical Note: SLAC-TN-20-001
SLAC Release Date: January 6, 2020
Preliminary results for in situ cleaning of B4C-coated FEL mirrors using oxygen plasma.
Peter, Stefan.
An in situ oxygen plasma cleaning process has been applied to LCLS B4C-coated
Kirkpatrick-Baez (KB) mirrors. After a total of five hours treatment, the initial carbonaceous
contamination stripes, acquired over 7 years of FEL beam irradiation, were visibly reduced to barely
detectable. After treatment, the output photon flux showed a reflectivity improvement between ~20% and
~170%, depending upon the photon energy. Further characterization, including detection of possible
sputter-deposited impuri... Show Full Abstract
An in situ oxygen plasma cleaning process has been applied to LCLS B4C-coated
Kirkpatrick-Baez (KB) mirrors. After a total of five hours treatment, the initial carbonaceous
contamination stripes, acquired over 7 years of FEL beam irradiation, were visibly reduced to barely
detectable. After treatment, the output photon flux showed a reflectivity improvement between ~20% and
~170%, depending upon the photon energy. Further characterization, including detection of possible
sputter-deposited impurities and improvement in KB focusing properties, are planned following return of
FEL beams, at the conclusion of the current six-month downtime, in June, 2017. Preparatory, off-line
measurements suggest that the process is both "safe and effective" for B4C-coated mirrors. Test
depositions of 50 nm amorphous carbon could be removed with high selectivity against B4C erosion, with
no change in treated/untreated surface roughness. Nevertheless, sputter-deposited impurities from
adjacent components may be an issue, as seen in soft x-ray reflectivity studies, where surface reflectivity
restoration remained imperfect. Show Partial Abstract
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