name event date name event date 1 univ. “la sapienza”, rome, 20–24 march 2006 cern f. ruggiero...

Name Event DateName Event Date11

F. Ruggiero Univ. “La Sapienza”, Rome, 20–24 March 2006CERN

Electron Cloud and Beam-Electron Cloud and Beam-Beam Effects in Particle Beam Effects in Particle

AcceleratorsAcceleratorsfundamental limitations to the fundamental limitations to the ultimate performance of high-ultimate performance of high-

luminosity collidersluminosity colliders

http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/

See also slides on Measurements, ideas, curiosities


F. Ruggiero Electron Cloud and Beam-Beam EffectsCERN

OutlineOutline• electron cloud build-upelectron cloud build-up

• sources of primary electronssources of primary electrons• Secondary Electron YieldSecondary Electron Yield• electron pinch and saturationelectron pinch and saturation

• impact on beam quality and accelerator impact on beam quality and accelerator performanceperformance• pressure rise and heat loadpressure rise and heat load• beam instabilities and emittance growthbeam instabilities and emittance growth

• possible mitigation of electron cloud possible mitigation of electron cloud effectseffects

• beam-beam limitbeam-beam limit• head-on and parasitic beam-beam head-on and parasitic beam-beam

encountersencounters• coherent beam-beam effects and tune coherent beam-beam effects and tune

measurementsmeasurements



Observations and importance of Observations and importance of electron cloud effectselectron cloud effects

• Beam induced pressure rise, multipacting, Beam induced pressure rise, multipacting, instabilities, and beam blow-up driven by the electron instabilities, and beam blow-up driven by the electron cloud are observed, cloud are observed, e.g.e.g., with the LHC proton beam in , with the LHC proton beam in the CERN SPS, in the PS, at RHIC, PEP-II and KEKB. the CERN SPS, in the PS, at RHIC, PEP-II and KEKB. More recently electron cloud effects have been More recently electron cloud effects have been observed at the Tevatron, Cornell (even with electron observed at the Tevatron, Cornell (even with electron beams) and at Daphne.beams) and at Daphne.

• Impact on beam diagnostics and, for the LHC, the heat Impact on beam diagnostics and, for the LHC, the heat load on the cold bore are further concerns.load on the cold bore are further concerns.

• For future linear collider damping rings or proton For future linear collider damping rings or proton drivers the density of the electron cloud may be 10-drivers the density of the electron cloud may be 10-100 times higher.100 times higher.

• The electron cloud induces large betatron tune shifts The electron cloud induces large betatron tune shifts and tune spreads, and fast transverse single- and and tune spreads, and fast transverse single- and multi-bunch instabilities. multi-bunch instabilities.

• Also a slow incoherent emittance growth of the LHC Also a slow incoherent emittance growth of the LHC beams is predicted by simulations and semi-analytic beams is predicted by simulations and semi-analytic models. Preliminary observations at the CERN SPS models. Preliminary observations at the CERN SPS seem to confirm that the driving mechanism is the seem to confirm that the driving mechanism is the betatron tune modulation for particles oscillating in betatron tune modulation for particles oscillating in the electron cloud with large synchrotron amplitudes.the electron cloud with large synchrotron amplitudes.



Electron-cloud build-up in the LHCElectron-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 10 nTorr 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 1 cm away from the beam axis



Electron cloud in a dipole Electron cloud in a dipole magnetic fieldmagnetic field

Electrons spiral in the 8.4 T magnetic field with a Electrons spiral in the 8.4 T magnetic field with a typical radius ρ = p/(eB) of 6 μm for 200 eV typical radius ρ = p/(eB) of 6 μm for 200 eV electrons and perform about 100 rotations during the electrons and perform about 100 rotations during the passage of an LHC proton bunch. passage of an LHC proton bunch.

The net effect is therefore a vertical kick , decreasing The net effect is therefore a vertical kick , decreasing with the horizontal distance from the bunch.with the horizontal distance from the bunch.



Electron-cloud build-up Electron-cloud build-up (continued)(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)



Possible Cures against Possible Cures against Electron Cloud build-upElectron Cloud build-up

• Reduce bunch intensity or increase Reduce bunch intensity or increase bunch spacing/length bunch spacing/length lower machine lower machine performanceperformance

• Reduce number of primary electronsReduce number of primary electrons• saw-tooth structure in the LHC dipole beam saw-tooth structure in the LHC dipole beam

screen screen fewer photo-electrons above/below fewer photo-electrons above/below beam beam

• better vacuum to reduce ionization electronsbetter vacuum to reduce ionization electrons

• Lower Secondary Electron Lower Secondary Electron Yield/Amplification Yield/Amplification • special low-emissivity coatings (TiN at SNS, special low-emissivity coatings (TiN at SNS,

NEG in all LHC warm sections) or surface NEG in all LHC warm sections) or surface treatmentstreatments

• grooved beam pipe surfacesgrooved beam pipe surfaces• solenoids (KEKB straights) or clearing solenoids (KEKB straights) or clearing

electrodeselectrodes• beam scrubbing beam scrubbing requires circulating beam requires circulating beam



Reduction of SEY by electron Reduction of SEY by electron dosing (N. Hilleret)dosing (N. Hilleret)

SEY variation with the beam energy at 2 different electron SEY variation with the beam energy at 2 different electron dosesdoses

Material: Colaminated copper on stainless steelMaterial: Colaminated copper on stainless steel



schematic of reduced electron cloud build up for a super-Bunch. Most e- do not gain any energy when traversingthe chamber in the quasi-static beam potential[after V. Danilov]negligible heat load



Instabilities & emittance Instabilities & emittance growthgrowth

caused by the electron cloud caused by the electron cloud 1) Multi-bunch instability – not expected to be a problem

can be cured by the feedback system2) single-bunch instability – threshold electron cloud

density 0~4x1011 m-3 at injection in the LHC3) 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 generationfrom space charge)

periodic crossing of linearly unstable regiondue to synchrotron motion and strong focusingfrom electron cloud in certain regions, e.g., in

dipoles



Effects of the electron Effects of the electron cloudcloudEmittance growth below & above Emittance growth below & above

electron density thresholdelectron density threshold

= 1 x 1011 m-3

= 2 x 1011 m-3

= 3 x 1011 m-3

“Transverse Mode Coupling Instability (TMCI)” for e- cloud ( > thresh)

Long term emittance growth ( < thresh)

E. Benedetto, F. Zimmermann



electron density vs LHC beam intensity

typical“TMCI”instabilitythreshold

R=0.5

calculation for 1 bunch train

max=1.7

max=1.5

max=1.3 max=1.1



LHC working points in collisionLHC working points in collision

The beam-beam tune footprint has to be The beam-beam tune footprint has to be accommodated in between low-order betatron accommodated in between low-order betatron resonances to avoid diffusion and bad lifetimeresonances to avoid diffusion and bad lifetime



Transverse emittance growth Transverse emittance growth with random beam-beam with random beam-beam

offsetsoffsets 22

20

21

1

4

11

g

xsf

dt

d

xrev

g~0.2 feedback gain, ~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 ↔ x=1.5 nm with feedback↔ x=0.6 nm w/o feedback



Coherent Beam-Beam spectra Coherent Beam-Beam spectra (W. Herr, LHC-Project-Note-356)(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 Phase advance symmetry restored

between IP1 and IP5.between IP1 and IP5.



Tevatron Schottky scan (1.7 GHz) Tevatron Schottky scan (1.7 GHz) during physics stores during physics stores (A. Jansson, (A. Jansson,

2005)2005)

The change in tune shift during the store The change in tune shift during the store is approximately twice the observed tune is approximately twice the observed tune spread change, as expected.spread change, as expected.

~0.

005



Minimum crossing angleMinimum crossing angleBeam-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~*

nθ

c

n11bpar

θ

cda m75.3

A5.036

m75.3

10323

INnd

dynamic aperture caused by npar parasitic collisions around two IP’s

*θ angular beam

divergence at IP



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)



Long-Range Beam-Beam Long-Range Beam-Beam experiment at the experiment at the RRelativistic elativistic

HHeavy eavy IIon on CColliderollider

A Virtual Tour of the RHIC Complex Animation courtesy of Brookhaven National Laboratory

see http://www.bnl.gov/RHIC/



E-Cloud and Beam-Beam EffectsE-Cloud and Beam-Beam Effects SummarySummary

• Electron cloud effects will limit the performance Electron cloud effects will limit the performance of high intensity accelerators with many closely-of high intensity accelerators with many closely-spaced bunches. The threshold bunch intensity spaced bunches. The threshold bunch intensity for electron cloud build-up scales linearly with for electron cloud build-up scales linearly with the bunch spacing.the bunch spacing.

• If beam parameters can not be adjusted to avoid If beam parameters can not be adjusted to avoid electron cloud effects, possible cures include electron cloud effects, possible cures include beam scrubbing, feedback and increased beam scrubbing, feedback and increased chromaticity. Incoherent effects may deteriorate chromaticity. Incoherent effects may deteriorate the beam quality.the beam quality.

• Beam-beam effects will limit the performance of Beam-beam effects will limit the performance of high luminosity colliders. For round beams high luminosity colliders. For round beams colliding head-on, the beam-beam tune spread colliding head-on, the beam-beam tune spread depends only on the brightness depends only on the brightness NNbb//nn and on the and on the number of IP’s.number of IP’s.

• Long-range beam-beam effects with many Long-range beam-beam effects with many closely-spaced bunches impose a minimum closely-spaced bunches impose a minimum crossing angle. Higher beam intensities or crossing angle. Higher beam intensities or smaller smaller * require larger crossing angles to * require larger crossing angles to preserve dynamic aperture and shorter bunches preserve dynamic aperture and shorter bunches to avoid a geometric luminosity loss. to avoid a geometric luminosity loss.

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