Electron Stimulated Desorption of Condensed Gases on Cryogenic Surfaces

In ultra-high vacuum systems outgassing from vacuum chamber walls and desorption from surface adsorbates are usually the factors which in°uence pressure and residual gas composition. In particular in beam vacuum systems of accelerators like the LHC, where surfaces are exposed to intense synchro- tron radiation and bombardment by energetic ions and electrons, properties like the molecular desorption yield or secondary electron yield can strongly in°uence the performance of the accelerator. In high-energy particle accelerators operating at liquid helium temperature, cold surfaces are exposed to the bombardment of energetic photons, electrons and ions. The gases released by the subsequent desorption are re-condensed on the cold surfaces and can be re-desorbed by the impinging electrons and ions. The equilibrium coverage reached on the surfaces exposed to the impact of energetic particles depends on the desorption yield of the condensed gases and can a®ect the operation of the accelerator by modifying the secondary electron yield of these surfaces. The desorption yields under electron impact of various gases condensed on a copper surface cooled at 4.2K have been measured and will be presented together with the values of the sticking coe±cient of these gases on a 4.2K condensing surface. A model to explain the variation of the desorption yields with the surface coverage will also be described. In this work the electron stimulated desorption yield (ESDY) at cryogenic temperatures has been measured. This parameter is of importance to un- derstand and predict the vacuum behavior in the LHC, in the presence of an electron cloud, as in that case the electron induced desorption will be the main gas source. Of particular interest is the variation of the electron induced desorption yield with the gas coverage as most gases (with the exception of hydrogen) condense on the beam screen surface, which leads to an increased density of molecules per unit area. The measurement of the ESDY requires a measuring system where ¯rst a known amount of electrons can be accelerated to a target cooled at helium temperature, second the nature and the number of desorbed molecules can be measured and third a predetermined quantity of gas can be injected and condensed on the cold target.