Measurements, ideas, curiosities

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Measurements, ideas, curiosities

• fundamental limitations to the ultimate
  performance of high-luminosity colliders

Complement to the lecture on Electron Cloud and
Beam-Beam Effects
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Electron Cloud build-up and Le Châteliers
principle

• Henri Louis Le Châtelier (1850-1936) was a French
  industrial chemist. In 1888 he made a remarkable
  observation

"If a dynamic equilibrium is disturbed by
changing some conditions, the position of
equilibrium moves to counteract the change."

• In modern accelerators we want to accumulate more
  and more intense, positive beams.
• Nature reacts by the fast build-up of negative
  electron clouds that tend to neutralize the
  system!

Henri Le Châtelier A man of Principle
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Scaling of electron cloud effects
blue e-cloud effect observed red planned
accelerators
experience at several storage rings suggests
that the e-cloud threshold scales as
NbDtsep possible LHC upgrades
consider either smaller Dtsep with constant Nb,
or they increase Dtsep in proportion to Nb
longer fewer more intense bunches
more ultimate bunches
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Electron-cloud build-up in the LHC

• Beam synchrotron radiation is important
• provides source of photo-electrons
• Secondary emission yield (SEY) d(E) is important
  
• characterized by peak value dmax
• determines overall e density
• e reflectivity d(0) is important
• determines survival time of e
• Bunch intensity Nb and beam fill pattern are
  important
• Main concern power deposition by electrons on
  the cold beam screen

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Secondary Electron Yieldfor normal electron
incidence

• R. Cimino et al., Phys. Rev. Lett. 93, 014801
  (2004)

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Electron density m-3 during the passage of an
LHC bunch in a field-free region
HEADTAIL simulation, courtesy E. Benedetto
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Electron density m-3 during the passage of an
LHC bunch in an SPS dipole (top view)

• HEADTAIL simulation, courtesy E. Benedetto

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Electron flux measured in an SPS dipole (strip
detector) with LHC beam at 26 GeV

• courtesy G. Arduini and M. Jimenez

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LHC bunch train at injection in the SPS
Qx26.135 Qy26.185 ?x0.15 ?y0.1 VRF 3
MV dampers on coupling 0.008
rms bunch length (ns)
bunch intensity (au)
time (min)
time (min)

• Evolution of bunch length and bunch
  population for the first and the last bunch in an
  LHC bunch train of 72 bunches. SPS measurements
  with electron cloud in Aug 2004. Courtesy G.
  Rumolo, G. Arduini, and F. Roncarolo.

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Beam-Beam tune spread for round beams
tune shift from head-on collisions (primary IPs)
tune shift from long-range collisions npar
parasitic collisions around each IP
increases for closer bunches or reduced crossing
angle
limit on xHO limits Nb/(ge)
relative beam-beam separation for full crossing
angle qc
xHO / IP no. of IPs DQbb total
SPS 0.005 3 0.015
Tevatron (pbar) 0.01-0.02 2 0.02-0.04
RHIC 0.002 4 0.008
LHC (nominal) 0.0034 3 0.01
conservative value for total tune spread based on
SPS collider experience
high-lumi in IP1 and IP5 (ATLAS and CMS), halo
collisions in IP2 (ALICE) and low-lumi in IP8
(LHC-b)
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Beam-Beam tune footprints

• Comparison of tune footprints, corresponding
  to betatron amplitudes extending from 0 to 6 s ,
  for LHC nominal (red-dotted), ultimate
  (green-dashed), and large Piwinski parameter
  configuration (blue-solid) with alternating H-V
  crossing only in IP1 and IP5. (Courtesy H. Grote)

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Long-Range Beam-Beam Experiment in RHIC, 28 April
2005, Wolfram Fischer et al.

• Effects of long-range beam-beam interaction
  observable at RHIC injection energy (24.3 GeV)
  with a single proton bunch per ring
• Bunch intensity Nb1-2?1011

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Long-Range Beam-Beam Experiment in RHIC, 28 April
2005, Wolfram Fischer et al.
collision at s10.65m
1 bunch per ring Blue beam moved
vertically Tunes B (0.739,0.727) Y
(0.727,0.738)
Beam losses increase when the beam-beam
separation at a single parasitic collision point
is reduced below 5 s.
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Electron Cloud recent links references
http//ab-abp-rlc.web.cern.ch/ab-abp-rlc-ecloud/ (
web site on Electron Cloud Effects in the
LHC) http//icfa-ecloud04.web.cern.ch/icfa-ecloud
04/ (ECLOUD04 workshop, Napa, California, 1923
April 2004) http//www-project.slac.stanford.edu/
ilc/testfac/ecloud/elec_cloud.html (web site of
ILC Damping Ring Task Force 6) Can low energy
electrons affect high energy physics
accelerators?, R. Cimino, I.R. Collins, M.A.
Furmann, M. Pivi, F. Ruggiero, G. Rumolo, and
F. Zimmermann, Phys. Rev. Lett. 93, 014801
(2004) Simulation study of electron cloud
induced instabilities and emittance growth for
the CERN Large Hadron Collider proton beam, E.
Benedetto, D. Schulte, F. Zimmermann, and G.
Rumolo, Phys. Rev. ST Accel. Beams 8, 124402
(2005) Update on electron-cloud power
deposition for the Large Hadron Collider arc
dipoles, M. Furmann and V.H. Chaplin,
LBNL-59062/CBP Note 723/LARP-doc-157, March 2006,
to appear in Phys. Rev. ST Accel. Beams
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Beam-Beam recent links references
http//wwwslap.cern.ch/collective/zwe/lhcbb/ (web
site of LHC Beam-Beam studies) http//www.agsrhic
home.bnl.gov/AP/BeamBeam/Workshop03/ (Beam-Beam
Workshop, Montauk, Long Island, 1923 May
2003) Coherent beam-beam modes in the LHC for
multiple bunches, different collision schemes and
machine symmetries, W. Herr and T. Pieloni,
Proc. CARE HHH-2004 Workshop, CERN, 811
November 2004 Progress of Beam-Beam
Compensation Schemes, F. Zimmermann and
U. Dorda, Proc. LHC-LUMI-05 Workshop, Arcidosso,
31 Aug3 Sept 2005 Beam-beam effects in the
Tevatron, V. Shiltsev, Y. Alexahin, V. Lebedev,
P. Lebrun, R.S. Moore, T. Sen, A. Tollestrup, A.
Valishev, and X.L. Zhang, Phys. Rev. ST Accel.
Beams 8, 101001 (2005) The effects of solenoids
and dipole magnets of LHC experiments, W. Herr,
LHC Project Workshop, Chamonix XV, 23-27 January
2006
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CERN Accelerator Complex (not to scale)