Overview of Electron Cloud Studies and R

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Overview of Electron Cloud Studies
and RD for the
International Linear Collider Mauro Pivi
ILC Division SLAC

13-15 October 2004
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Electron cloud effect (ECE) in a nutshell

• Residual gas ionization and synchrotron radiation
  generate electrons electrons get rattled around
  the chamber
• Number of electrons may increase/decrease from
  collisions against the walls according to the
  secondary electron yield
• Electron cloud couples to the positron beam
• Short range wake
• Long range wake
• Especially strong effect for intense,
  positively-charged beams (e or p). Possible
  consequences
• Fast instabilities single-bunch strong head-tail
  or coupled-bunch and incoherent tune shift
• emittance increase and synchro-betatron coupling
• vacuum pressure rise, excessive heat load
  deposited on the wall (LHC)
• In summary the ECE is a consequence of the
  strong coupling between the beam and its
  environment.
• Electron multiplication conditions depend on
  bunch charge and spacing, vacuum chamber size
  and geometry, secondary emission yield. More
  ingredients chromaticity, photoelectric yield,
  photon reflectivity, electron reflectivity

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Secondary electron yield (SEY or d) for
aluminum (SLAC). Typically dmax3.
Aluminum surface has typically high SEY. Surface
approach. To reduce the SEY to low acceptable
values (peak SEY1.2, see below) in a stable way
under vacuum is a challenge PSR (LANL), PEP-II,
KEKB, Daphne, SPS (CERN), LHC etc.
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• ECE instability in the Linear Collider
• Simulations e- cloud generation (POSINST,
  ECLOUD)
• Simulations single-bunch fast head-tail
  instability (HEAD-TAIL, PEHTS, QUICKPIC,
  CLOUD_MAD) and coupled-bunch instability
  (POSINST)
• Possible remedies present and future needed RD

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Cloud generation simulations benchmark
POSINST/ECLOUD code

• Electron cloud in the DR arcs
• Photo-electrons reach high density. Actually, no
  antechamber design. An antechamber design is
  needed.
• The threshold for the electron cloud formation
  and development in arcs dipole sections is for a
  peak SEY 1.21.3
• In quadrupole magnets an electron trapping
  mechanism (mirror effect) occurs, electrons are
  trapped indefinitely
• Larger beam pipe axis, even few mm, relaxes the
  multiplication conditions and raises the SEY
  threshold 1.41.5
• Electron cloud in the DR wiggler sections
• As severe as in the arcs, more constraints on the
  beam pipe dimensions
• Electron cloud in the DR long straight sections
• Large beam pipe radius electron cloud is not
  expected provided SEY 2.0 achievable with a
  good coating material.
• Electron cloud in the Low Emittance Transport
  (LET) Lines to the IP
• The large bunch spacing 337 ns (176 ns) prevents
  the electron cloud formation

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Electron cloud expectations based on actual
TESLA design
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Electron distribution in wiggler sections
TESLA wiggler
TESLA wiggler
Snapshot of the transverse x-y phase space
electron distribution in the TESLA wiggler

• Equilibrium density in the damping wiggler
  sections for nominal beam conditions. Threshold
  occurs at peak SEY1.25-1.3

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Generation in the DR arcs
Electron line density in units of e/m as a
function of time for a bend (top) and a
field-free region (bottom) of the arcs provided
by an antechamber. The various curves refer to 6
different values of dmax (R. Wanzenberg DESY, D.
Shulte and F. Zimmermann CERN) ECLOUD code. SEY
parameterization assumes a constant Emax240 eV.
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Generation in the DR arcs
Electron density in units of e/cm3 as a function
of time for an arc bend with antechamber, using
POSINST code (SLAC). SEY parameterization assumes
a variable Emax (SPS meas.).
Arc bend simulations. Equilibrium electron
density as a function of the chamber size.
Assuming a peak SEY1.4
Chamber size a,b 22, 18 mm
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Damping Ring options
Electron cloud development in arc bend sections
considering different bunch spacing
configurations. In all cases, assuming a peak
SEY1.4, a larger chamber radius 22mm (18mm) and
including an antechamber design.
In the long straight sections, simulations show
that a bunch spacing as low as 2-3 ns is needed
for an electron cloud to develop.
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• ECE instability in the Linear Collider
• Simulations generation of the cloud (POSINST)
• Simulations single-bunch fast head-tail
  instability (HEAD-TAIL, PEHTS, QUICKPIC,
  CLOUD_MAD) and coupled-bunch instability
  (POSINST)
• Possible remedies RD present and future

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HEAD-TAIL and PEHTS codes description

• Cloud and bunch modeled as ensemble of
  macroparticles. Bunch is also divided in Nsl
  slices.
• typ. 100.000 e- and 300.000 e
• typ. Nsl 70 bunch slices
• Kick approximation assuming electrons induce a
  small perturbation (difference with QUICKPIC)
• cloud localized at n0,1,Nint positions along
  the ring. Used n1 to 10.
• Momentum compaction, chromaticity, (space-charge,
  beam-beam, amplitude detuning) applied on a
  turn-by-turn basis. Impedance represented by the
  broad-band resonator model as a wake function
  kick at each turn, not included in these sim.

Interaction between bunch particles and cloud
electrons
Transverse phase space coordinates of the generic
bunch macrop. are transformed over one turn
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Single-bunch fast head-tail instability TESLA DR
Single-bunch instability occurs for an average
ring electron cloud density 1-2e11 e/m3 assuming
a uniform electron distribution. When assuming a
two stripes electron cloud distribution (typical
in wiggler and bend), the avg. density threshold
is an order of magnitude lower 1-3e10 e/m3.
100 turns
HEAD-TAIL (CERN) code, SLAC simulations
PEHTS code (K. Ohmi, KEK)
Severe instability large beam losses are
expected.
Confirmed by QUICKPIC code (A. Ghalam USC
University) and HEAD-TAIL simul. considering
wiggler sections (F. Zimmermann, CERN)
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• ECE instability in the Linear Collider
• Simulations generation of the cloud (POSINST)
• Simulations single-bunch fast head-tail
  instability (HEAD-TAIL, PEHTS, QUICKPIC,
  CLOUD_MAD) and coupled-bunch instability
  (POSINST)
• Possible remedies RD present and future

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Secondary Electron Yield Measurements and Surface
Analyisis at SLAC
Secondary Electron Yield (SEY) and secondary
energy spectrum measurements. Surface
characterization. F. Le Pimpec, M. P., R. Kirby,
at SLAC, A. Wolski, at LBNL (coatings), P. He,
BNL (coatings).
XPS TiN/Al
TiZrV NEG sample (LBNL)

• Test promising remedies
• TiN coating at LBNL and BNL facilities. Goal
  reproducible results.
• TiZrV non-evaporable getter NEG coatings from
  LBNL, SAES Getter, CERN
• Electron and ion conditioning
• Grooved metal surface profile

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Electron conditioning on aluminum
e- conditioning aluminum at SLAC
e- conditioning aluminum (top curve) at CERN
The electron conditioning on aluminum is not
sufficient to decrease the SEY to low values.
Measurements from different laboratory in good
agreement.
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Electron conditioning for coating materials
TiN, NEG on aluminum substrate
Electron conditioning
Competing effect recontamination under vacuum

• CONCERN We have learned at the SPS (CERN) in
  accelerator environment, the conditioning is
  effective ONLY when an electron cloud is present
  (!) the competing recontamination effect
  re-increases the SEY when the electron cloud is
  NOT present ? high level concern for LHC, ILC,
  Daphne
• Concerns about coating durability ? PEP-II, KEKB,
  PSR

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Rectangular (!) groove surface design
5mm depth samples for PEP-II SEY 0.6
M. Pivi and G. Stupakov SLAC
(triangular concept by
Krasnov CERN)
1 mm
Special surface profile design, Cu OFHC. EDM
wire cutting. Groove 0.8mm depth, 0.35mm step,
0.05mm thickness.
Secondary Yield reduction lt 0.8. More reduction
depending geometry
Preparing to install test chambers with grooves
in PEP-II, to be used next upgrade
Need to verify to what extent the groove profile
design is effective in magnetic field
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Benchmark sim.
Simulations
Lab measurements

• SEY meas. coatings treatments
• Coating durability under vacuum
• Grooved surface design

• e- cloud generation equilibrium
• single and multi-bunch instability
• self-consistent 3D simulations

• e- trapping mechanism in Quad
• e- detector meas. in PEPII
• beam dynamics

• Path
• TiN
• TiZrV
• radius
• groove
• other ?

Requirements
Demonstration I Grooved chamber 6m long section
to be installed in PEP-II
Demonstration II Installation chamber with
coatings in PEPII. Meas. SEY in situ
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• Groups involved in the electron cloud in the ILC
  SLAC DESY CERN KEK Frascati
• Thanks !
• F. Le Pimpec, M. Furman, O. Brüning, Y. Cai, A.
  Chao, O. Gröbner, K. Harkay, N. Hilleret, B.
  Henrist, R. Kirby, R. Macek, K. Ohmi, N. Phinney,
  J. Rogers, R. Rosenberg, G. Rumolo, D. Shulte, G.
  Stupakov, T. Raubenheimer, F. Zimmermann, R.
  Wanzenberg, A. Wolski, ...

http//icfa-ecloud04.web.cern.ch/icfa-ecloud04
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Summary

• The electron-cloud effects through the Damping
  Rings to the Interaction Point of the Linear
  Colliders have been evaluated
• Build up of the electron cloud will be prevented
  by treating the vacuum chambers and increasing
  chamber radius
• Investigations of surface treatments include
• Measurement of the secondary electron yield of
  TiN and TiZrV NEG thin film coatings
• Testing the effectiveness of electron or ion
  conditioning
• Fabrication of specially grooved chamber surfaces
• Demonstration chambers will be installed in PEP-II

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Next slides are back up
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e-cloud in the TESLA Damping Ring
Not an issue in straight sections, provided a
peak SEY lower than 2.0 with a good Al coating
(Tin, NEG)
Threshold in wiggler is for a peak SEY 1.2 1.3
Electron cloud is a serious issue in arc
threshold in dipole section is for a peak SEY
1.2 1.3. Larger chamber size, even few mm, is
beneficial.
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Electron density in units of e/cm3 as a function
of time for an arc bend with antechamber, using
POSINST code (SLAC). SEY parameterization assumes
a variable Emax (SPS meas.).
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Incoherent tune shift in TESLA wiggler
Vertical tune shift after passing through the
TESLA wiggler beam line which has 432 meters of
wiggler in 520 meters of beam line. An electron
cloud density of 6e12 e-/m3 was assumed. No
magnetic field was included.