Electron Cloud in the LHC

Synchrotron radiation from proton bunches in the LHC creates photoelectrons at the beam screen wall. These photoelectrons are pulled toward the positively charged proton bunch. When they hit the opposite wall, they generate secondary electrons which can in turn be accelerated by the next bunch. Depending on several assumptions about surface reflectivity, photoelectron and secondary electron yield, this mechanism can lead to the fast build-up of an electron cloud (the animation shows simulation results by O. Brüning for 10 subsequent bunch passages, during which the pictures become red) with potential implications for beam stability and heat load on the beam screen. In view of the tight deadline for the design of the LHC cryogenic system, a crash program has been set up to measure the relevant physical quantities with and without magnetic field.

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Movies of the single-bunch electron-cloud instability: (1) SPS with space charge, (2) Electron distribution in the SPS during one bunch passage (3) KEKB LER
The animations show the single bunch evolution under the effect of the electron cloud in the SPS as well as in the Low Energy Ring of the KEK B Factory. The dotted line represents the centroid position along the bunch, and the solid lines are located at +/- one bunch rms-size from the centroid. Detailed Description.

Simulations for LHC: Latest news
Simulated LHC arc heat loads increase by a factor 2-3 if elastically scattered electrons are included in addition to the true secondaries.

Electron Cloud in the PS
Since October 2000, apparent electron cloud effects are observed with the LHC beam in the PS to SPS transfer line and also on the last turns prior to ejection from the PS. The observations include dilution of SEM profile measurements, and baseline shifts on BPM pick-up signals. If the LHC beam is stored, and not extracted a single-bunch instability affects the last two thirds of the batch. (R. Cappi, M. Giovannozzi, E. Metral, and G. Metral).

Other News

Computer Codes: ECLOUD (Download), PHOTON, HEADTAIL (Download) , Faktor2 (Downlad)
Simulation Comparison: Communication Web Page
CERN Studies: 2010 MeetingsMeetings , Internal Meetings

Analytic estimates, simulations and measurements on Electron Cloud in the LHC, SPS and PS: Reports and Notes on LHC/SPS/PS Electron Cloud from 2000 and Earlier

Background bibliography and related links

Related Reports and Notes on Electron Cloud from 2000 and Earlier

Workshops


People involved in the crash program on Electron Cloud in the LHC

Synchrotron radiation from the DCI machine, with 3 keV critical energy, after reflection in a circular stainless steel vacuum chamber (O. Gröbner et. al., 29/04/1988). The picture shows light in the visible spectrum collected on a paper screen at the end of the vacuum chamber. The light impinges on the right side and is reflected on the opposite side with an opening angle depending on its wave length and on the surface roughness. Multiple reflections may lead to a nearly uniform illumination of the LHC beam screen.


8 August 2002 Giovanni Rumolo, SL/AP, and Frank Zimmermann, SL/AP