Characterisation of technical surfaces at cryogenic temperature under electron bombardment

The vacuum chambers of the LHC’s arcs operate in a temperature range between 1.9 K, i.e. the temperature of the superconducting magnets, and 20 K. At such low temperatures, most of the residual gas species are efficiently adsorbed on the cold surface. LHC’s proton beam emits synchrotron radiation inside its bending magnets and, consequently, electrons are extracted from the surrounding walls by the photoelectric effect. The successive proton bunches accelerates the photo-electrons, building up an “electron cloud” that generates gas desorption from the vacuum chamber and heat load for the cryogenic system. This phenomenon might become a limiting factor for the operation of the High Luminosity LHC upgrade, where more intense proton bunches will circulate. In order to study the electron interaction with gas adsorbed at cryogenic temperature, a new facility has been designed and built at CERN. It reproduces in the laboratory the typical conditions of a cryogenic ultra-high vacuum surface present in the accelerator. In this paper, the first results obtained with selected accelerator materials at different surface gas coverages are presented.