Modification of Ultra-High Vacuum Surfaces Using Free Radicals

In ultra-high vacuum systems outgassing from vacuum chamber walls and desorption of surface adsorbates are usually the factors which determine pressure and residual gas composition. In particular in beam vacuum systems of accelerators like the LHC, where surfaces are exposed to intense synchrotron radiation and bombardment by energetic ions and electrons, surface properties like the molecular desorption yield or secondary electron yield can strongly influence the performance of the accelerator. Well-established treatment methods like vacuum bake-out or glow-discharge cleaning have been successfully applied in the past to condition ultra-high vacuum surfaces, but these methods are sometimes difficult to carry out, for example if the vacuum chambers are not accessible. In this work, an alternative treatment method is investigated. This method is based on the strong chemical reactivity of free radicals, electrically neutral fragments of molecules. Free radicals (in the case of this work, nitrogen and oxygen radicals) are generated in a microwave plasma source and pumped through a vacuum chamber where they can react with the surface in various ways, thereby removing surface adsorbates and/or changing the chemical composition of the surface. In order to assess the effect of radical treatment, surfaces of radical treated samples have been analysed using various techniques and compared with surfaces of non-treated samples of the same material. Special emphasis has been put on the measurement of the molecular desorption yield. For this purpose an experimental vacuum system has been set up which permits measurements of the molecular desorption yield induced by electron bombardment (ESD) \emph{in situ}, i.e. without exposing the sample to atmosphere between radical treatment and measurement. In view of a possible application to accelerator beam pipes, the distribution of radicals in cylindrical tubes has been studied in detail by means of a computer simulation whose underlying physical model incorporates concepts of fluid dynamics and chemical kinetics. Radical distributions have also been determined experimentally from measurements of the heat of recombination. Results of these measurements are compared with those from the computer simulation. This work has been carried out in the vacuum group of the LHC division at CERN with the aim to investigate the radical treatment method in general and its application for the conditioning of surfaces in the LHC beam vacuum system in particular. With the experimental and simulation results presented in this work, it is possible to demonstrate capabilities and limitations of the radical treatment method and to specify favourable operating conditions for practical applications.