Electron Cloud and Beam-Beam Effects in Particle Accelerators

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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 on Measurements, ideas,
curiosities
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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

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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.

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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 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

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Electron cloud in a dipole magnetic field

• Electrons spiral in the 8.4 T magnetic
  field with a typical radius ? 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.

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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)

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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

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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

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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
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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 r04x1011 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

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Effects of the electron cloud

• Emittance growth below above electron density
  threshold

Transverse Mode Coupling Instability (TMCI) for
e- cloud (r gt rthresh)
re 3 x 1011 m-3
Long term emittance growth (r lt rthresh)
re 2 x 1011 m-3
re 1 x 1011 m-3
E. Benedetto, F. Zimmermann
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electron density vs LHC beam intensity
R0.5
dmax1.7
dmax1.5
typical TMCI instability threshold
dmax1.3
dmax1.1
calculation for 1 bunch train
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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

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Transverse emittance growth with random beam-beam
offsets
g0.2 feedback gain, x0.01 total beam-beam
parameter, s00.645 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 ? Dx1.5 nm with
feedback ? Dx0.6 nm w/o feedback
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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.

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Tevatron Schottky scan (1.7 GHz) during physics
stores (A. Jansson, 2005)
0.005

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

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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 IPs
higher beam intensities or smaller b require
larger crossing angles to preserve dynamic
aperture and shorter bunches to avoid geometric
luminosity loss ? baseline scaling qc1/vb ,
szb
angular beam divergence at IP
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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)
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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/
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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.
• 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 IPs.
• 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.