Abstract
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.
Highlights
Photoemission and electron multiplication on surfaces exposed to an oscillating electromagnetic field cause the phenomenon of multipacting, which can significantly degrade the performance of rf cavities as well as that of storage rings operating with closely spaced positron or proton bunches.Beam-induced multipacting was observed as a pressure rise at the CERN intersection storage ring (ISR) in 1977, after installation of an aluminum test chamber [1]
Crash programs were launched for the positron ring (LER) of the PEP-II B Factory [6,7] and, after simulations and analytical estimates had predicted a serious effect for heat load and beam stability [8,9], for Large Hadron Collider (LHC) [10 –15]
One of the concerns related to the presence of an electron cloud in the beam pipe is the way the electrons can affect the correct functioning of beam position monitors placed along the beam orbit
Summary
Photoemission and electron multiplication on surfaces exposed to an oscillating electromagnetic field cause the phenomenon of multipacting, which can significantly degrade the performance of rf cavities as well as that of storage rings operating with closely spaced positron or proton bunches. If the reflectivity is low, the majority of the photoelectrons impinges on the horizontally outward side of the vacuum chamber. In dipole magnets, these electrons do not approach the beam, and they stay at fairly low energies. The beam-induced electron cloud can produce a serious heat load in the LHC beam screen and it may give rise to a perturbation of the beam position monitor (BPM) signals due to unequal electron bombardment. The application of this model to the low energy ring of the KEK B Factory allows us to discuss the effects of this transverse instability in terms of beam centroid oscillation and emittance growth. IV summarizes the results and draws an outline for future work and development
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