Electron Cloud and Beam-Beam Effects in Particle Accelerators

1. Electron Cloud and Beam-Beam Effects in Particle Accelerators fundamental limitations to the ultimate performance of high-luminosity colliders http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ See also slides onMeasurements, ideas, curiosities

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

3. 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. • 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 10-100 times higher. • 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. Electron Cloud and Beam-Beam Effects

4. 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 10-3 photoelectrons/m, while in the SPS the primary yield is dominated by ionization of the residual gas and at 10nTorr it is only 10-7 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 1cm away from the beam axis Electron Cloud and Beam-Beam Effects

5. 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. The net effect is therefore a vertical kick , decreasing with the horizontal distance from the bunch. Electron Cloud and Beam-Beam Effects

6. 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) Electron Cloud and Beam-Beam Effects

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

8. 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 Electron Cloud and Beam-Beam Effects

9. 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 negligible heat load [after V. Danilov] Electron Cloud and Beam-Beam Effects

10. Instabilities & emittance growthcaused by the electron cloud • Multi-bunch instability – not expected to be a problem can be cured by the feedback system • single-bunch instability – threshold electron cloud density r0~4x1011 m-3 at injection in the LHC • 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 Electron Cloud and Beam-Beam Effects

11. Effects of the electron cloud Emittance growth below & above electron density threshold “Transverse Mode Coupling Instability (TMCI)” for e- cloud (r > rthresh) re= 3 x 1011 m-3 Long term emittance growth (r < rthresh) re= 2 x 1011 m-3 re= 1 x 1011 m-3 E. Benedetto, F. Zimmermann Electron Cloud and Beam-Beam Effects

12. electron density vs LHC beam intensity R=0.5 dmax=1.7 dmax=1.5 typical “TMCI” instability threshold dmax=1.3 dmax=1.1 calculation for 1 bunch train Electron Cloud and Beam-Beam Effects

13. 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 Electron Cloud and Beam-Beam Effects

14. Transverse emittance growth with random beam-beam offsets g~0.2 feedback gain, x~0.01 total beam-beam parameter, s0~0.645 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 ↔ Dx=1.5 nm with feedback ↔ Dx=0.6 nm w/o feedback Electron Cloud and Beam-Beam Effects

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

16. 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. ~0.005 Electron Cloud and Beam-Beam Effects

17. 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 dynamic aperture caused by npar parasitic collisions around two IP’s higher beam intensities or smaller b* require larger crossing angles to preserve dynamic aperture and shorter bunches to avoid geometric luminosity loss baseline scaling: qc~1/√b* , sz~b* angular beam divergence at IP Electron Cloud and Beam-Beam Effects

18. 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) Electron Cloud and Beam-Beam Effects

19. 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 http://www.bnl.gov/RHIC/ Electron Cloud and Beam-Beam Effects

20. E-Cloud and Beam-Beam EffectsSummary • 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. • 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 Nb/en 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 b* require larger crossing angles to preserve dynamic aperture and shorter bunches to avoid a geometric luminosity loss. Electron Cloud and Beam-Beam Effects