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...
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We discuss the choice of bunch length and crossing angle near the beam-beam limit in a storage-ring collider. First, we derive expressions for the tune shifts of either bunched or continuous round beams which are induced by a single collision with arbitrary crossing angle and bunch length and for the associated luminosities. Then, considering two collision points with alternating planes of crossing, we demonstrate that, if the total beam-beam tune shift is held constant, the collider luminosity increases as a function of bunch length and crossing angle. This implies a corresponding increase in the bunch intensity. As an illustration, we present numerical examples for a Large Hadron Collider upgrade and for the Very Large Hadron Collider.
Photoemission and secondary emission are known to give rise to a quasistationary electron cloud inside the beam pipe through a beam-induced multipacting process. We investigate the electron-cloud build up and related effects via computer simulation. In our model, macroparticles representing photoelectrons are emitted synchronously with the passing proton or positron bunch and are subsequently accelerated in the field of the beam. As they hit the beam pipe, new macroelectrons are generated, whose charges are determined by the energy of the incoming particles and by the secondary emission yield of the beam pipe. A quasistationary state of the electron cloud is eventually reached due to space charge. The equilibrium density is used as an input parameter for a second program that analyzes the electron-cloud driven single-bunch instability. The electron cloud simulation also allows the evaluation of the heat load on the cold Large Hadron Collider beam screen, which must stay within the available cooling capacity, and the electron charge deposited on or emitted from the electrodes of the beam-position monitors.
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