Vacuum needs of particle accelerators: from discrete pumps to SNEG (Sputtered NEG) coatings

Vacuum systems of particle accelerators have to be designed so as to maintain a pressure in the beam pipe low enough to ensure that beam lifetime is always longer than specified. The pressure is determined by the total gas load and by the available pumping speed. Minimizing the gas load while increasing the pumping speed is therefore key to achieve required pressure levels in high performance machines. SNEG (Sputtered Non Evaporable Getter) technology addresses both issues. A thin layer of getter material deposited onto the internal surface of the accelerator provides the most efficient mean to remove molecules, acting as an in-situ integral pump. The presence of the getter decreases thermal out-gassing from the metal substrate, since molecules are trapped in the getter-substrate interface. Moreover, when bombarded by ions, electrons or photons, NEG coatings are less prone to release gas species than steel, aluminum or copper, showing significantly smaller radiation-induced gas desorption rates. Additionally, they generally have low electron Secondary Emission Yield (SEY), this being particularly beneficial to reduce the risk for resonant electron multiplication or multipacting. For this reasons SNEG films have been selected for adoption in the Large Hadron Collider (LHC) and the European Synchrotron Radiation Facility (ESRF) and are considered in a variety of other machines and accelerators at present under design or construction. (author)