Photon Stimulated Desorption and the Effect of Cracking of Condensed Molecules in a Cryogenic Vacuum System

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 CO2 and CH4 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 H2 and CO. The important result of this study is that while the surface coverage of cryosorbed CH4 and CO2 molecules is limited due to cracking, the coverage of H2 and CO molecules may increase steadily during the long term operation of the machine.