Electron Cloud simulations, experimental R

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Electron Cloud simulations, experimental RD
work at SLAC and update on code
benchmarking M. Pivi, T. Raubenheimer, R.
Kirby (SLAC) F. Le Pimpec (PSI)
June, 2005
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• The electron-cloud effect (ECE) in a nutshell
• Beam residual gas ionization and photons produce
  primary e-
• Number of electrons may increases/decreases due
  to surface secondary electron yield (SEY)
• Bunch spacing determines the survival of the
  electrons

• Especially strong effect and possible
  consequences
• Single- (head-tail) and coupled-bunch
  instability
• Transverse beam size increase directly affecting
  the Luminosity
• Vacuum pressure and excessive power deposition
  on the walls (LHC cryogenic system)

• In summary the ECE is a consequence of the
  strong coupling between the beam and its
  environment
• many ingredients beam energy, bunch charge and
  spacing, secondary emission yield, chamber size
  and geometry, chromaticity, photoelectric yield,
  photon reflectivity,

The electron cloud has been seen PSR, SPS,
PEP-II, KEKB, DAFNE..
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One of the main limitations to the future
Colliders (LHC, ILC) performances and luminosity
reach is the formation of an electron cloud and
driven collective instabilities
Electron cloud effect occurs mainly in the
Damping Ring of the Linear Collider, due to short
bunch spacing
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Simulation Efforts on ILC

• KEK PEI and PEHTS codes K. Ohmi
• SLAC POSINST code M. Pivi, CLOUDLAND
  code L. Wang
• CERN ECLOUD and HEAD-TAIL codes F.
  Zimmermann, D. Shulte, E.
  Benedetto, G. Rumolo
• USC QUICKPIC code B. Feng, A. Ghalam,
  T. Katsouleas

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Cloud evolution and single-bunch instability
threshold
Simulations electron-cloud using POSINST 17km
long DR arc bend with antechamber. SEY threshold
occurs at peak SEY1.2-1.3. SEY model
parameterization assumes a variable Emax LHC
Proj.Rep-632
Single-bunch simulations using HEAD-TAIL
Evolution of the vertical beam size and a dipole
model for different cloud density (averaged over
ring). Single-bunch instability occurs at 2e11
m-3.
Note ECLOUD code foreseen slightly higher SEY
threshold, different SEY model. Benchmarking in
process.
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Generation in the DR arcs
Fixed SEY dmax1.4 and varied vertical
chamber size
Simulations electron-cloud using POSINST 17km
long DR arc bend with antechamber. SEY threshold
occurs at peak SEY1.2-1.3. SEY model
parameterization assumes a variable Emax LHC
Proj.Rep-632
Arc bend simulations. Equilibrium electron
density as a function of the chamber size.
Assuming a fixed SEY peak dmax1.4
Beam pipe semiaxes Hor, Vert 22, 18 mm
Beam pipe semiaxes Hor 22 , Vert 18 to 30 mm
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SEY thresholds for the DR 6 km and 3 km
Electron density in units of e m3 as a function
of time for an arc bend in the 6 km DR option
(Left) and the 3 km DR option (Right), assuming a
chamber radius 22mm and including an antechamber
design (full height h10mm).
The SEY thresholds for the development of an
electron cloud in the dipole regions are dmax
1.11.2 for the 6 km DR and dmax 1.01.1 (!) for
the 3 km DR option.
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Electron cloud in DR wiggler magnet sections
ILC DR wiggler
17km DR wiggler
threshold for head-tail in wiggler
Snapshot of the transverse x-y phase space
electron distribution in the 3D wiggler field

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

Photoelectrons rate is 0.0007 electrons per meter
per positron. 3D wiggler field is cylindrical
model representation with expansion to first 60
modes (LCC-0113). Simulations are slow!
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e-cloud expectations in the positron DR
Average neutralization levels and single-bunch
(SB) instability electron cloud density
thresholds for various damping ring options in
units of 1012 m-3. The average density
thresholds are for a ring modeled as a dipole
region.
E cloud Color coding

• - Arcs and wiggler sections aiming at SEY 1.2
• not an issue in long straight sections, provided
  a good coating (TiN, TZrV NEG) with SEY lt 1.9.
  Large chamber size.

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The ECE program
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 ex situ
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ILC RD Ecloud program

• RD at KEK. SEY laboratory measurements of
  electron conditioning and coatings studies.
  Installation of dedicated chamber with electron
  and energy spectrum detectors diagnostic in the
  KEKb e ring (ATF?!).
• XPS measurements confirm carbon layer increase
  during e- conditioning H. Kato, KEKb Review, Feb
  2005
• RD at CERN. A large number of electron detectors
  have been installed in quadrupoles, dipoles and
  field free regions of the SPS ring, the LHC
  pre-injector. Laboratory system to measuring SEY
  of technical materials.
• Electron cloud current and energy spectrum
  measurements in SPS
• Dendritic surface reduces SEYlt1 increasing
  roughness Hilleret et al. EPAC 2000
• Electron conditioning in the SPS photon, ion
  conditioning, more.
• RD is focused on reducing the electron cloud
  in the LHC.

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ILC RD Ecloud program

• RD at LANL. Measurement of the electron trapping
  mechanism in quadrupole field developing novel
  electron diagnostics. SLAC collab.
• RD at Frascati. Possibility of important
  measurements to localize the suspected formation
  of electrons in Dafne e ring, in particular, in
  wiggler and dipole regions.
• RD at SNS/BNL. Measurement of the thin film
  coatings, development of new techniques to reduce
  trailing edge multipacting
• Correlation between SEY and Ar pressure during
  TiN thin film coatings H. Hseuh
  ECLOUD04
• Special groove surface design to collect stripped
  500keV e- at injections
• RD focused to reducing the SNS multipacting.
• RD at SLAC. Laboratory measurements of SEY on
  bare metals, TiN and NEG coatings before and
  after processing. Development of grooved surface
  profile and novel TiCN alloy. Construction of
  vacuum chambers for installation in PEP-II to
  verify laboratory measurements.

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Secondary Electron Yield Measurements and Surface
Analysis at SLAC and LBNL
Secondary Electron Yield (SEY) and Surface
characterization R.Kirby,F.Le Pimpec, M.P. SLAC,
LBNL, BNL coatings.
XPS TiN/Al
Electron conditioning
TiZrV NEG sample (LBNL)
Rectangular groove SEY 0.7!
flat surface
Rectangular and triangular grooves
concept
rect. grooves
Preparing to install test chambers with grooves
in PEP-II, to be used next upgrade
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Why not an aluminum chamber?
Al as received
Electron conditioning (bombardment) effect on
the SEY for aluminum. Laboratory measurements at
SLAC and CERN agree very well. The electron
conditioning is not completely effective to
lowering the aluminum SEY as needed
SLAC-PUB-10894.
Most of the Dafne
ring is made of aluminum chambers.
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Electron conditioning (scrubbing or processing)
of thin films TiN, TiZrV.
Laboratory measurements, SLAC.
Residual gas recontamination under vacuum

• Based on laboratory measurements, the required
  conditioning dose in ILC DR would be achievable
  in hours of beam operation during commissioning.
• Concerns about effective e- conditioning time and
  coatings durability in an accelerator environment
  

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Electron conditioning issues

• Electron conditioning Asymptotic behavior
• In an accelerator environment, the electron
  cloud itself is providing the conditioning of the
  vacuum chamber walls (in laboratory conditioning
  is constant by fixed beam)
• When the SEY decreases, the efficiency of the
  electron conditioning will decrease as well

• Recontamination
• - Competing effect residual gas
  recontamination ? e- cloud reappears

• PICTURE at the SEY threshold
  
• Two effects competing against each other

e- (asymptotic) Conditioning
SEY threshold
recontamination
(1) solution (LHC) running at higher current for
a period of time (2) key combined photon/ions
conditioning may keep SEY below threshold (?!)
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RD plans at SLAC installation of test chambers
Project 1

• Installation of dedicated chamber with coated
  samples in the PEP-II Low Energy Ring (LER) to
• Test the efficiency of in situ of electron
  conditioning
• Test the combined photon conditioning effect
• Test thin film coatings durability in
  accelerator environment

Drawings completion. Ready for construction of
dedicated chamber with coated samples
(Left) intended installation PEP-II LER PR12
downstream VAC-PR12-3101 (Right) sample
transferring system
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Rectangular Grooves to Reduce SEY
Rectangular grooves can reduce the SEY without
generating geometric wakefields. The resistive
wall impedance is roughly increased by the ratio
to tip to floor.
Schematic of rectangular grooves Without B field
Schematic of rectangular grooves With B field
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Rectangular (!) groove design
Laboratory tests, SLAC
M.P. and G. Stupakov, SLAC
5mm depth (PEP-II) Same SEY results
Artificially increasing surface roughness.
1 mm
Special surface profile design, Cu OFHC. EDM
wire cutting. Groove 0.8mm depth, 0.35mm step,
0.05mm thickness.
Measured SEY reduction lt 0.8. More reduction
depending geometry.
Triangular groove concept A. Krasnov
LHC-Proj-Rep-617
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Simulation rectangular grooves
grooves parameters
w
a
h
measurements
l
Expected from simulations
Simulations rectangular groove profile.
Reference SEY on a flat copper
surface is 1.7. Also shown measured compared
with expected SEY.
Prototype groove copper sample
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RD plans at SLAC rectangular groove chambers

• Installation of dedicated Tin/Al chambers 6 m
  long sections in the PEP-II LER to
• Test the efficiency of the rectangular groove
  concept in field free region
• PEP-II and ILC collaboration project

Prototype samples at LBNL for TiN and NEG coatings
TiN/aluminum prototype chambers extruded grooved
surface installation in PEP-II LER
Project 2
Does groove concept work in dipole or wigglers
where we needed most ?
Wakefields modeling with MAFIA
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Rectangular grooves in dipole SEY
Sim. parameters rectangular groove period
0.25 mm depth 0.25 mm width 0.025 mm
Simulated secondary yield of a rectangular
grooved surface in a dipole field compared with a
smooth surface (field free reference).

• Possible solution need laboratory and
  accelerator tests in dipole field

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- ILC WG3 Task Force 6 Electron-Cloud
Effects -
Co-ordinatorsKazuhito Ohmi (KEK)
ohmi_at_post.kek.jpM. Pivi (SLAC)
mpivi_at_slac.stanford.eduFrank Zimmermann (CERN)
frank.zimmermann_at_cern.ch
- extract working plan -
Aims A baseline configuration for the ILC will be
selected by the end of 2005. The electron-cloud
effects are among the criteria to be considered
when choosing the positron DR circumference,
bunch charge and bunch spacing, chamber
apertures, wiggler design, antechambers, photon
stops, clearing electrodes etc.
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DR task 6 Specify SEY limits from the electron
cloud - working plan -

• Methodology
• Pertinent parameters for three different rings
  (17 km, 6 km and 3 km circumference)
  For some studies (e.g. electron-cloud build-up)
  it probably is not necessary to study every
  lattice in detail, but pick one in each
  circumference.
• Electron cloud build up is simulated for the
  different regions (arcs, wigglers, straights)
  considering different secondary emission yields.
• For the wigglers simulations the field can be
  modeled at various levels of sophistication, and
  the importance of refined models has to be
  explored
• Single-bunch wake fields and the thresholds of
  the fast single-bunch TMCI-like instability are
  estimated
• Multi-bunch wake fields and growth rates are
  inferred from e-cloud build up simulations
• Electron induced tune shifts will be calculated
  and compared
• Predictions of electron build up from different
  simulation codes are compared
• Implemented in the simulations will be
  countermeasures which may be proposed as the ILC
  DR design evolves.

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Specify SEY limits from the electron cloud -
working plan for task 6 -
Expressions of interest and available
tools Build-up simulation codes are PEI (KEK),
POSINST (LBNL/SLAC), ECLOUD (CERN), and CLOUDLAND
(BNL/SLAC). Instability simulation codes are
PEHTS (KEK) and HEADTAIL (CERN) for single-bunch
instabilities, and PEI-M for multi-bunch
instabilities (KEK). Multi-bunch wake fields can
be extracted from POSINST and ECLOUD. There also
exists a single-bunch instability code written by
Y. Cai at SLAC. DESY, INFN, and CERN are
collaborating in the EUROTeV WP3 ECLOUD subtask,
the goals of which overlap with the ILC WG3
electron-cloud task. Rainer Wanzenberg (DESY)
has started a compilation of ring and beam
parameters. Further contributions are highly
welcome! Comparisons with existing machines A
benchmarking program is ongoing at the CERN SPS
and at DAFNE, in addition to PSR, PEP-II and
KEKB, and can support the predictions.
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STARTED Benchmarking of ECLOUD and POSINST in ILC
DR wiggler (toughest one)
Simulated electron cloud density with POSINST
(SEY params ex. deltamax1.3, Emax190eV).
Photoelectrons rate is 0.007 electrons per meter
per positron. Wiggler field Cartesian model.
Rectangular chamber with semi-axis axb16x9mm and
two antechambers 10mm full size on both sides.
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STARTED Benchmarking of ECLOUD and POSINST in ILC
DR wiggler
e- line density m-1
ECLOUD
dlge/ds0.007 m-1
dlge/ds0.0007 m-1
time s
Simulated electron cloud density with ECLOUD (SEY
params ex. dmax1.3, emax190eV) photoelectron
rates are 0.007 and 0.0007 electrons per meter
per positron wiggler field described by
Cartesian model rectangular chamber with
semi-axis a x b16x9mm Hilleret model for e-
reflection.
Frank Zimmermann, BDIR London, 06/2005
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Future Directions

• ILC DR electron cloud generation simulation
  benchmarking between CERN, SLAC, KEK codes.
• Electron cloud collective instability
  simulations.
• Developing a self-consistent fully dynamical 3D
  code.
• Electron cloud RD program to select possible
  remedy
• Laboratory measurements of the secondary electron
  yield of thin film coatings and testing the
  effectiveness of electron or ion conditioning
• Fabrication of specially grooved chamber surfaces
• Increase few mm chamber aperture beneficial
• Demonstration chambers will be installed in
  PEP-II.
• By October 2005 task force 6 Co-ordinators
  deliver the information that will be necessary
  for making a DR configuration selection.

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PAC05 ROPB001
O. Grobner, M. Furman, J. Seeman, A. Novokhatski,
N. Kurita, G. Stupakov, A. Chao, K. Harkay, B.
McKee, G. Collet, K. Jobe, M. Ross, G. Rumolo, A.
Seryi, A. Variola, P. Bambade, F. Zimmermann, Y.
Cai, R. Cimino, A. Feiz, S. Heifets, U. Irizo, K.
Ohmi, G. Rumolo, G. Vorlaufer, C. Vaccarezza, A.
Wolski, D. Lee, R. Macek, J.M. Laurent, N.
Hilleret, M. Jimenez, A. Rossi, V. Baglin, and
many other colleagues .. Thanks !
Web site www-project.slac.stanford.edu
/ilc/testfac/ecloud/elec_cloud.html
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Secondary electron yield (SEY)
for aluminum (SLAC)
Aluminum etched
Typical secondary yield for as received
aluminum. Peak SEY3, unacceptable for
DR operations. Note it should be Eavg gt E1 to
have average SEY gt1 thus electron multiplication.
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Wake field simulations using
groove profile chamber
MAFIA simulations (A. Novokhatski) indicate that
wake fields are not excited during the beam
passage. Very small losses come from the step
transition from the smooth surface to the grooved
surface and are estimated 1.5E-04 V/pC.
Power losses due to image charge contained (ex.
PEP-II, dP/ds1W/cm).