Cold/sticky systems

The understanding of complex and/or large vacuum systems operating at cryogenic temperatures requires a specific knowledge of vacuum science at such temperatures. At room temperature, molecules with a low binding energy to a surface are not pumped. However, at cryogenic temperatures, their sojourn time is significantly increased, thanks to the temperature reduction, which allows a ‘cryopumping’. This pumping mechanism is described by different regimes. Sticking probabilities, capture factor and thermal transpiration concepts are also used to characterize the pumping mechanism. At cryogenic temperature, a gas load into a vacuum system turns into an increase of the surface coverage and of its associated vapour pressure. Some adsorption isotherms of H2 and He which differ with key parameters such as surface nature and temperature are also presented. As an application of this field of vacuum technology, the vacuum system of the CERN Large Hadron Collider is introduced. The implementation of cryosorbers and the consequences of He leaks in the accelerator beam tube are reported.