Name Event Date Name Event Date 1 Univ. “La Sapienza”, Rome, 20–24 March 2006 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects in Particle Accelerators fundamental limitations to the ultimate performance of high-luminosity colliders See also slides on Measurements, ideas, curiosities Measurements, ideas, curiosities

Name Event Date Name Event Date 2 CERN F. Ruggiero Electron Cloud and Beam-Beam EffectsOutline electron cloud build-up electron cloud build-up sources of primary electrons sources of primary electrons Secondary Electron Yield Secondary Electron Yield electron pinch and saturation electron pinch and saturation impact on beam quality and accelerator performance impact on beam quality and accelerator performance pressure rise and heat load pressure rise and heat load beam instabilities and emittance growth beam instabilities and emittance growth possible mitigation of electron cloud effects possible mitigation of electron cloud effects beam-beam limit beam-beam limit head-on and parasitic beam-beam encounters head-on and parasitic beam-beam encounters coherent beam-beam effects and tune measurements coherent beam-beam effects and tune measurements

Name Event Date Name Event Date 3 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Observations and importance of electron cloud effects Beam induced pressure rise, multipacting, instabilities, and beam blow-up driven by the electron cloud are observed, e.g., with the LHC proton beam in the CERN SPS, in the PS, at RHIC, PEP-II and KEKB. More recently electron cloud effects have been observed at the Tevatron, Cornell (even with electron beams) and at Daphne. Beam induced pressure rise, multipacting, instabilities, and beam blow-up driven by the electron cloud are observed, e.g., with the LHC proton beam in the CERN SPS, in the PS, at RHIC, PEP-II and KEKB. More recently electron cloud effects have been observed at the Tevatron, Cornell (even with electron beams) and at Daphne. Impact on beam diagnostics and, for the LHC, the heat load on the cold bore are further concerns. Impact on beam diagnostics and, for the LHC, the heat load on the cold bore are further concerns. For future linear collider damping rings or proton drivers the density of the electron cloud may be times higher. For future linear collider damping rings or proton drivers the density of the electron cloud may be times higher. The electron cloud induces large betatron tune shifts and tune spreads, and fast transverse single- and multi-bunch instabilities. The electron cloud induces large betatron tune shifts and tune spreads, and fast transverse single- and multi-bunch instabilities. Also a slow incoherent emittance growth of the LHC beams is predicted by simulations and semi-analytic models. Preliminary observations at the CERN SPS seem to confirm that the driving mechanism is the betatron tune modulation for particles oscillating in the electron cloud with large synchrotron amplitudes. Also a slow incoherent emittance growth of the LHC beams is predicted by simulations and semi-analytic models. Preliminary observations at the CERN SPS seem to confirm that the driving mechanism is the betatron tune modulation for particles oscillating in the electron cloud with large synchrotron amplitudes.

Name Event Date Name Event Date 4 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Electron-cloud build-up in the LHC In the LHC, photoelectrons created at the pipe wall are accelerated by proton bunches up to 200 eV and cross the pipe in about 5 ns Slow or reflected secondary electrons survive until the next bunch. This may lead to an electron cloud build-up with implications for beam stability, emittance growth, and heat load on the cold LHC beam screen. At 7 TeV each proton generates photoelectrons/m, while in the SPS the primary yield is dominated by ionization of the residual gas and at 10 nTorr it is only electrons/m The electron cloud build-up is a non-resonant single-pass effect and may take place also in the transfer lines and in the LHC at injection Most electrons are not trapped in the beam potential, but form a time-dependent cloud extending up to the pipe wall: in field free regions this cloud is almost uniform in the dipoles, electrons spiral along the magnetic field lines and tend to form two stripes at about 1 cm away from the beam axis

Name Event Date Name Event Date 5 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Electron cloud in a dipole magnetic field Electrons spiral in the 8.4 T magnetic field with a typical radius ρ = p/(eB) of 6 μm for 200 eV electrons and perform about 100 rotations during the passage of an LHC proton bunch. Electrons spiral in the 8.4 T magnetic field with a typical radius ρ = p/(eB) of 6 μm for 200 eV electrons and perform about 100 rotations during the passage of an LHC proton bunch. The net effect is therefore a vertical kick, decreasing with the horizontal distance from the bunch. The net effect is therefore a vertical kick, decreasing with the horizontal distance from the bunch.

Name Event Date Name Event Date 6 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Electron-cloud build-up (continued) Depending on the bunch spacing, a significant fraction of secondary electrons is lost in between two successive bunch passages Each bunch passage can be considered as the amplification stage of a photomultiplier: a minimum gain is required to compensate for the electron losses and this corresponds to a critical secondary emission yield typically around 1.3 for nominal LHC beams When the maximum secondary electron yield exceeds this critical value, the electron cloud is amplified at each bunch passage and reaches a saturation value determined by space charge repulsion As a rule-of-thumb, saturation occurs when the electron density approaches the average proton beam density (space charge neutralization)

Name Event Date Name Event Date 7 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Possible Cures against Electron Cloud build-up Reduce bunch intensity or increase bunch spacing/length  lower machine performance Reduce bunch intensity or increase bunch spacing/length  lower machine performance Reduce number of primary electrons Reduce number of primary electrons saw-tooth structure in the LHC dipole beam screen  fewer photo-electrons above/below beam saw-tooth structure in the LHC dipole beam screen  fewer photo-electrons above/below beam better vacuum to reduce ionization electrons better vacuum to reduce ionization electrons Lower Secondary Electron Yield/Amplification Lower Secondary Electron Yield/Amplification special low-emissivity coatings (TiN at SNS, NEG in all LHC warm sections) or surface treatments special low-emissivity coatings (TiN at SNS, NEG in all LHC warm sections) or surface treatments grooved beam pipe surfaces grooved beam pipe surfaces solenoids (KEKB straights) or clearing electrodes solenoids (KEKB straights) or clearing electrodes beam scrubbing  requires circulating beam beam scrubbing  requires circulating beam

Name Event Date Name Event Date 8 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Reduction of SEY by electron dosing (N. Hilleret) SEY variation with the beam energy at 2 different electron doses Material: Colaminated copper on stainless steel

Name Event Date Name Event Date 9 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects schematic of reduced electron cloud build up for a super- Bunch. Most e- do not gain any energy when traversing the chamber in the quasi-static beam potential [after V. Danilov] negligible heat load

Name Event Date Name Event Date 10 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Instabilities & emittance growth caused by the electron cloud 1)Multi-bunch instability – not expected to be a problem can be cured by the feedback system 2)single-bunch instability – threshold electron cloud density  0 ~4x10 11 m -3 at injection in the LHC 3)incoherent emittance growth new understanding! (CERN-GSI collaboration) 2 mechanisms:  periodic crossing of resonance due to e - tune shift and synchrotron motion (similar to halo generation from space charge)  periodic crossing of linearly unstable region due to synchrotron motion and strong focusing from electron cloud in certain regions, e.g., in dipoles

Name Event Date Name Event Date 11 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Effects of the electron cloud Emittance growth below & above electron density threshold    = 1 x m -3    = 2 x m -3    = 3 x m -3 “Transverse Mode Coupling Instability (TMCI)” for e- cloud (  >  thresh ) Long term emittance growth (  <  thresh ) E. Benedetto, F. Zimmermann

Name Event Date Name Event Date 12 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects electron density vs LHC beam intensity typical “TMCI” instability threshold R=0.5 calculation for 1 bunch train  max =1.7  max =1.5  max =1.3  max =1.1

Name Event Date Name Event Date 13 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects LHC working points in collision The beam-beam tune footprint has to be accommodated in between low-order betatron resonances to avoid diffusion and bad lifetime The beam-beam tune footprint has to be accommodated in between low-order betatron resonances to avoid diffusion and bad lifetime

Name Event Date Name Event Date 14 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Transverse emittance growth with random beam-beam offsets g~0.2 feedback gain,  ~0.01 total beam-beam parameter, s 0 ~0.645 since only a small fraction of the energy received from a kick is imparted on the continuum eigen-mode spectrum (Y. Alexahin) 1% emittance growth per hour ↔  x=1.5 nm with feedback ↔  x=0.6 nm w/o feedback

Name Event Date Name Event Date 15 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Coherent Beam-Beam spectra (W. Herr, LHC-Project-Note-356) Head-on collisions in IP 1, 2, 5 and 8. Head-on collisions in IP 1, 2, 5 and 8. Phase advance symmetry restored between IP1 and IP5. Phase advance symmetry restored between IP1 and IP5.

Name Event Date Name Event Date 16 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Tevatron Schottky scan (1.7 GHz) during physics stores (A. Jansson, 2005) The change in tune shift during the store is approximately twice the observed tune spread change, as expected. The change in tune shift during the store is approximately twice the observed tune spread change, as expected. ~0.005

Name Event Date Name Event Date 17 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Minimum crossing angle Beam-Beam Long-Range collisions: perturb motion at large betatron amplitudes, where particles come close to opposing beam cause ‘diffusive’ (or dynamic) aperture, high background, poor beam lifetime increasing problem for SPS, Tevatron, LHC, i.e., for operation with larger # of bunches higher beam intensities or smaller  * require larger crossing angles to preserve dynamic aperture and shorter bunches to avoid geometric luminosity loss  baseline scaling:  c ~1/√  *,  z ~  * dynamic aperture caused by n par parasitic collisions around two IP’s angular beam divergence at IP

Name Event Date Name Event Date 18 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Schematic of a super-bunch collision, consisting of ‘head-on’ and ‘long-range’ components. The luminosity for long bunches having flat longitudinal distribution is ~1.4 times higher than for conventional Gaussian bunches with the same beam-beam tune shift and identical bunch population (see LHC Project Report 627)

Name Event Date Name Event Date 19 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects Long-Range Beam-Beam experiment at the Relativistic Heavy Ion Collider A Virtual Tour of the RHIC Complex Animation courtesy of Brookhaven National Laboratory see

Name Event Date Name Event Date 20 CERN F. Ruggiero Electron Cloud and Beam-Beam Effects E-Cloud and Beam-Beam Effects Summary Electron cloud effects will limit the performance of high intensity accelerators with many closely-spaced bunches. The threshold bunch intensity for electron cloud build-up scales linearly with the bunch spacing. Electron cloud effects will limit the performance of high intensity accelerators with many closely-spaced bunches. The threshold bunch intensity for electron cloud build-up scales linearly with the bunch spacing. If beam parameters can not be adjusted to avoid electron cloud effects, possible cures include beam scrubbing, feedback and increased chromaticity. Incoherent effects may deteriorate the beam quality. If beam parameters can not be adjusted to avoid electron cloud effects, possible cures include beam scrubbing, feedback and increased chromaticity. Incoherent effects may deteriorate the beam quality. Beam-beam effects will limit the performance of high luminosity colliders. For round beams colliding head- on, the beam-beam tune spread depends only on the brightness N b /  n and on the number of IP’s. Beam-beam effects will limit the performance of high luminosity colliders. For round beams colliding head- on, the beam-beam tune spread depends only on the brightness N b /  n and on the number of IP’s. Long-range beam-beam effects with many closely- spaced bunches impose a minimum crossing angle. Higher beam intensities or smaller  * require larger crossing angles to preserve dynamic aperture and shorter bunches to avoid a geometric luminosity loss. Long-range beam-beam effects with many closely- spaced bunches impose a minimum crossing angle. Higher beam intensities or smaller  * require larger crossing angles to preserve dynamic aperture and shorter bunches to avoid a geometric luminosity loss.