Photon-stimulated desorption and the effect of cracking of condensed molecules in a cryogenic vacuum system [of LHC]

The design of the Large Hadron Collider (LHC) vacuum system requires a complete understanding of all processes which may affect the residual gas density in the cold bore of the 1.9 K cryomagnets. A wealth of data has been obtained which may be used to predict the residual gas density inside a cold vacuum system exposed to synchrotron radiation. In this study, the effect of cracking of cryosorbed molecules by synchrotron radiation photons has been included. Cracking of the molecular species CO/sub 2/ and CH/sub 4/ has been observed in recent studies and these findings have been incorporated in a more detailed dynamic gas density model for the LHC. In this paper, we describe the relevant physical processes and the parameters required for a full evaluation. It is shown that the dominant gas species in the LHC vacuum system with its beam screen are H/sub 2/ and CO. The important result of this study is that, while the surface coverage of cryosorbed CH/sub 4/ and CO/sub 2/ molecules is limited due to cracking, the coverage of H/sub 2/ and CO molecules may increase steadily during the long-term operation of the machine. (7 refs).