A preliminary comparative study of the electron-cloud effect for the PSR, ISIS, and the ESS (original) (raw)
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ELECTRON CLOUD STUDIES FOR THE UPGRADE OF THE CERN PS
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With the start of the machine studies to characterize the behaviour of the LHC beam in the SPS in 1999, it became evident that electron multipacting was occurring in the SPS vacuum chambers in the presence of this beam. Multipacting induces dramatic pressure increases preventing stable operation, it limits the performance of beam instrumentation and high voltage electrostatic devices (e.g. electrostatic septa) and it induces strong transverse instabilities leading to emittance dilution. Although an increase of the threshold bunch population for multipacting can be obtained by beam conditioning ("scrubbing"), multipacting persists in the arcs for the nominal LHC bunch population and electron cloud instabilities remain an issue for the LHC beam. A programme of studies has been launched since 1999 to study the electron cloud build-up and related instabilities in the SPS and in the PS for the LHC and fixed target beams. The experimental tools and analysis developed so far are presented together with the results of the observations. The countermeasures applied in the PS Complex & SPS against the electron cloud instability are also briefly discussed.
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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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Electron-cloud effects presently limit the performance of several accelerators operating with high beam current, notably the SLAC and KEK B factories, the CERN SPS, the CERN PS, and the Los Alamos PSR. They are a major concern for many future projects, e.g., the CERN LHC and the SNS. An electron cloud is generated in the vacuum chamber by photoemission or beam-induced multipacting and subsequent electron accumulation during a bunch or bunch-train passage. Both coupled and single bunch instabilities, pressure rise, malfunctioning of beam diagnostics and failures of multi-bunch feedback systems have all been attributed to the cloud electrons. We compare observations from various laboratories with computer simulations and analytical estimates, and we address mechanisms by which the electrons may dilute the beam emittance. Possible cures and future research directions are also discussed.
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Electron cloud effects in the CERN PS
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The beam-induced electron cloud build-up is one of the major concerns for the SPS and the design of the future LHC. Recently, this effect has been observed also in the PS with the nominal LHC-type beam, consisting of a batch of 72 bunches of 1.110 11 p/b spaced by 25 ns. The electron cloud induces baseline distortion in electrostatic pickup signals that is observed, both in the last turns of the PS when the full bunch length is reduced to less than 4 ns, and in the transfer line between the PS and the SPS rings. Experimental observations are presented and compared to simulation results and predictions from theory. Furthermore, possible cures, such as variation of the bunch spacing, inserting gaps in the bunch train and applying weak solenoidal fields, are also discussed.
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We present recent simulation results for the main features of the electron cloud in the storage ring of the Spallation Neutron Source (SNS) at Oak Ridge, and updated results for the Proton Storage Ring (PSR) at Los Alamos. In particular, a complete refined model for the secondary emission process including the so called true secondary, rediffused and backscattered electrons has been included in the simulation code.