Present and future accelerators' performances may be limited by the electron cloud (EC) effect. The EC formation and evolution are determined by the wall-surface properties of the accelerator vacuum chamber. We present measurements of the total secondary electron yield (SEY) and the related energy distribution curves of the secondary electrons as a function of incident-electron energy. Particular attention has been paid to the emission process due to very low-energy primary electrons (<20 eV). It is shown that the SEY approaches unity and the reflected electron component is predominant in the limit of zero primary incident electron energy. Motivated by these measurements, we have used state-of-theart EC simulation codes to predict how these results may impact the production of the electron cloud in the Large Hadron Collider, under construction at CERN, and the related surface heat load. DOI: 10.1103/PhysRevLett.93.014801 PACS numbers: 29.27.Bd, 41.75.Lx, 79.20.Hx In 1989 an instability driven by photoelectrons was observed at the National Laboratory for High Energy Physics (KEK) Photon Factory. It was not until 1994 that its origin was correctly identified as due to the formation of an electron cloud (EC) [1,2]. Since then several proton-storage rings [3,4], electron-positron colliders [4], and synchrotron radiation (SR) sources, when operating with positrons, have reported similar beam instabilities which are now understood to be due to a coupling between the beam and an EC. Deleterious effects of the EC include interference with diagnostic devices, coupled-bunch coherent beam instabilities, and single-bunch incoherent effects such as emittance increase. In general, the EC is significant in machines with intense, closely spaced, short, positively charged bunches, and vacuum chambers of relatively small transverse dimensions. In the cases of the B factories PEP-II and KEKB, the EC in the positron rings led to important operational limitations and to an intense search for mitigating mechanisms [4 -6]. An EC related effect is the beam-induced electron multipacting, and it can be explained as follows: a few ''seed'' electrons may be generated by ionization of the residual gas or by photoemission. These electrons are accelerated by the bunch electric field in the direction perpendicular to the beam motion. If the bunch charge and the bunch spacing satisfy a certain condition, the traversal time of the electron across the vacuum chamber equals the time interval between successive bunches, and a resonance condition is established. If, in addition, the effective secondary electron yield (SEY) at the chamber is larger than unity, the electron population grows rapidly in time with successive bunch passages, leading to a high electron cloud density. A closely related phenomenon, called trailing-edge multipacting, has also been observed for a single proton bunch at the Los Alamos Proton-Storage Ring (PSR) when the beam intensity exceeds a certain threshold [7]. It could prove important for the future Spallation Neutron...
