Can Low-Energy Electrons Affect High-Energy Physics Accelerators?

Abstract

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...

Monte Carlo computations of the electron backscattering coefficient for bulk targets and surface thin films

“…Some investigators have suggested, on the basis of experimental evidences about Cu, that the reflection coefficient should approach 1 as the energy approaches 0. [16,17] The present computations are consistent with this last suggestion for all the examined elements apart from gold. In order to show the scatter of experimental values and the differences to the Monte Carlo values, the comparison to the present Monte Carlo data with experimental data from various laboratories (taken from the Joy's database [1] ) have been presented in Fig.…”

Simulations of multipactor in circular waveguides

“…Note that within this model, the SEY does not go to zero when the electron impact energy approaches zero, which makes it possible to describe partial reflection of slow electrons from the metal surface. 21,[29][30][31] The number of seed electrons was taken to be N 0 = 25 000. These electrons were launched during the first microwave period from a point with the coordinates x =0, y = R. The maximum number of computer particles was taken to be N th = 50 000.…”

Long-term multipactor discharge in multicarrier systems

“…Several numerical simulations have been carried out using the analytical solution of the electromagnetic threedimensional ͑3D͒ fields in a rectangular waveguide of infinite length for the fundamental mode TE 10 The main reason to choose such signals is twofold. First, both signals have equivalent ON and OFF times in order to be compared.…”

“…Recent SEY measurements undertaken by Cimino et al 10 suggest that Furman and Pivi model underestimates the importance of low-energy electrons, due mainly to the lack of resolution in the experimental measurements of SEY at low energies. Cimino et al conclude that the SEY for most materials tends to one in the limit of zero impact energy; i.e., most of the low-energy electrons are reflected.…”

“…[8][9][10][11] In this work, a recently published SEY model 12 is adopted, taking different values of the curve for low energies. On the other hand, the spread of the emission energy of the secondary emitted electrons is taken into account in both the analytical and the numerical analyses, since it results also in a spread in the electron impacting energy and, therefore, an increase in the range of valid phases at the end of the OFF interval.…”