SLAC HEP Activities: NLC and PEPII Support

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SLAC HEP ActivitiesNLC and PEP-II Support

• Andy Wolski, Christoph Steier,Marco Venturini,
  John Byrd,Daniel Lee, Kurt Kennedy
• DoE HEP Review
• February 2004

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LBNL collaborates closely with SLAC on HEP
projects

• Linear Collider
• LBNL leads the Damping Ring studies
• Lattice design and dynamics studies
• Studies of collective effects (electron cloud
  etc.)
• Oversight of technical and engineering issues
• Contribution to experimental program at KEK-ATF
• Contributes to activities of US Linear Collider
  Steering Group
• LBNL represented on Accelerator Subcommittee
• Contributed to review of US-sited LC options
• Providing information for ITRP
• PEP-II
• Improving present performance through lattice
  studies
• Validation of interaction region optics model
• Optics studies/analysis of closed orbit response
  matrix
• Planning for luminosity upgrade (21034 by July
  2006)
• Low-? lattice designs to benefit from reduced ?
• RF system upgrade to allow stable beam at higher
  currents

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NLC DR lattices revised in 2003 raised
instability thresholds

• Previous lattice design was reworked to increase
  momentum compaction (by reducing dipole field)
  and lengthen the bunch.
• Reduced strength of dipole field compensated by
  additional20 m of wiggler to preserve damping
  time.
• Wiggler has strong nonlinear effects
• Detailed model needed to estimate impact on beam
  dynamics.
• 60 kW mean injected beam power requires 100
  injection efficiency.
• Highly stable operation at high beam current
  needed
• Detailed studies of collective effects have begun.

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Developed new techniques for modeling wiggler
effects

• Problem of modeling wiggler effects breaks down
  into two parts
• Accurate representation of 3D wiggler field is
  needed.
• Effect of wiggler field on beam dynamics must be
  determined.
• Cylindrical mode expansion provides fast and
  accurate field fitting.
• Ideas for fast symplectic integration are being
  explored.
• Continuing collaboration with Cornell to
  benchmark new techniques
• Effects of new wigglers on dynamics in CESR-c to
  be studied.

Field profile of CESR-c 8 pole wiggler.
Error using a 3D multipole field representation
through azimuthal 14-pole term.
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Nonlinearities cause large tune shifts with
amplitude
Frequency map in tune space
Vertical tune shift
Horizontal tune shift

• Large tune shift spreads beam particles through
  a number of resonances
• Control tune shift as a way to make motion more
  regular? Improve dynamic aperture?

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Octupoles can be effective in controlling tune
shifts

• Use two families of octupoles distributed through
  wiggler sections.
• Aim for a dynamic aperture of 15 injected beam
  size.

Dynamic aperture in co-ordinate space. 15
injected beam size is approximately 12 mm 3.8
mm.
Qx2Qy
Dynamic aperture in tune space, using octupoles
to control the tune shifts.
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The wiggler also makes electron cloud more severe
Simulated cloud density in TESLA wiggler.

• Electrons accelerated by positron beam generate
  high numbers of secondaries by impact with wall
  of vacuum chamber.
• Magnetic fields can confine the electrons and
  lead to high densities.
• Electron cloud drives beam instabilities.
• Collaboration with SLAC to research possibility
  of preventing build-up of the cloud using low
  secondary yield coatings.

Cloud density vs chamber SEY in TESLA wiggler.
TiN coatings produced at LBNL for analysis at
SLAC.
Simulation results from Mauro Pivi (SLAC).
TiN coating on Al
Gold coating on stainless
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Resistive wall wake field drives single bunch
instability

• Threshold appears at a few times nominal bunch
  charge.
• Including other sources of impedance will lower
  the threshold.

Longitudinal phase space at nominal, 8 nominal,
and 12 nominal bunch charge.
Growth of quadrupole mode amplitude with bunch
charge.
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Resistive wall also drives multibunch instability

• Tracking simulations to determine growth rates
  include radiation damping and resistive wall wake
  field
• Bunch-by-bunch feedback can be used to suppress
  unstable modes.

t 0 ms
t 10 ms
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Recent experimental results at ATF support IBS
theory

• Results obtained by KEK-ATF group using low
  emittance beam.
• Record low emittance of 4 pm, meets specification
  for NLC DRs.
• LBNL/SLAC contributed through studies of
  beam-based alignment.
• Reliable measurements obtained using advanced
  diagnostics.
• Calculations for present NLC damping ring lattice
  indicate IBS emittance growth will be within
  tolerable limits.

Variation of bunch length, horizontal and
vertical emittance with bunch intensity in
KEK-ATF under various coupling conditions.
Calculations of IBS bunch length and emittance
growth in the NLC MDRs. The red dashed lines
show the design specifications.
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Quality of work on DRs recognized by NLC MAC

• From NLC MAC Report June 2003
• The Committee would like to congratulate the
  damping ring team for the excellent work and the
  important recent achievements.
• The Committee is pleased with the good progress
  the Berkeley group has made with the design of
  the new lattice.
• From NLC MAC Report December 2003
• Study of the damping ring physics has made
  considerable progress since the last review.
• The stated plans for continued study of damping
  ring physics appear to be very complete The
  Committee believes it is entirely appropriate to
  place heavy emphasis on minimizing the production
  of secondary electrons by finding a suitable
  coating for the chamber interior.
• Recent results of emittance measurements at ATF
  are particularly impressive and important.

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PEP-II Luminosity gt 21034 cm-2s-1 by July 2006

• Recently achieved 7.41033 cm-2s-1
• CDR (1993) specified 31033 cm-2s-1 30 fb-1 per
  calendar year.
• Total delivered so far 173 fb-1.
• Luminosity improvement will be achieved by a
  variety of means
• Better reliability.
• Better control over lattice / tuning for optimum
  IP parameters.
• Increased current (bunch charge and number of
  bunches).
• Modified IR to reduce ?
• Modified lattices / increased RF to reduce the
  bunch length.
• At SLACs request, LBNL is already contributing
  to
• Support of existing systems, e.g. bunch-by-bunch
  feedback.
• Modeling of present lattices to improve
  tunability.
• Studies of options for low ? lattices to reduce
  bunch length.
• and we are exploring other opportunities to
  support PEP-II
• Review of RF options and work needed to allow
  stable beam with higher beam current and RF
  voltage.

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Expertise with proven modeling techniques applied
to PEP-II

• LOCO developed by J. Safranek (now at SSRL)
  applied successfully to many machines, including
  the ALS.
• Measure a closed orbit response matrix, and
  adjust parameters in a lattice model to reproduce
  the measured matrix.
• An effective technique for identifying optics
  errors (focusing, coupling).
• Now being applied to PEP-II, with some
  challenges
• Interaction region has complex design.
• Large quantity of data involved (matrices with
  108 components).

Measured response matrix in HER.
Measured - fitted model response matrix.
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Phase advance in fitted model agrees well with
direct measurement
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Fitted model identifies focusing and coupling
errors in HER
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Zero crossing angle makes modeling IR complicated

• PEP-II Interaction Region presently modeled as
  sliced solenoid.
• gt100 slices of length 5 cm.
• Design trajectory through solenoid modeled by
  individual misalignments on solenoid slices and
  vertical kicks.
• Quadrupole and dipole fields inserted as kicks
  between slices.

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Simplifying the IR model requires new maps

• An accurate model of the IR is essential for
  optimizing luminosity.
• Fitting IR magnet parameters to measured data
  would be helped by using a simplified (not
  sliced) model.
• MARYLIE is being used to integrate the trajectory
  through a detailed field model, and to verify the
  maps being used.

Horizontal trajectory through IR.
Vertical trajectory through IR.
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A new LER lattice reducing ? is challenging

• Reasonable low ? solution for HER achieved by
  raising phase advance in arcs from 60º per cell
  to 90º per cell.
• Present LER arcs have 90º phase advance per cell.
• Raising phase advance to 120º in LER reduces ?
  30, and has a severe effect on the dynamic
  aperture.
• Other options require new magnets, for example
• One option is to add a new quadrupole every two
  arc cells.
• Momentum compaction can be reduced from 12.410-4
  to 4.810-4
• Natural emittance also reduced (from 24 nm to 16
  nm).

Present LER arc cell.
Possible new LER arc cell.
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Low ? LER may be possible with hardware changes

• Studies in simplified lattice show reasonable
  dynamic aperture.
• 3-fold symmetry, constructed from modified arcs.
• No errors included.
• Next step is to include IR and adjacent
  straights.
• Other options are still being explored, for
  example
• Low ? insert based on long dipole chicane.

10?x
10??
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Increasing beam current requires RF upgrade

• In heavily beam-loaded large rings, the
  fundamental mode of the accelerating cavities
  becomes the dominant driver of beam
  instabilities.
• RF feedback is used to reduce beam impedance by
  factor 100.

• Existing system is at the edge of stability.
• RF trips are the primary reason for beam loss.

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LBNL has significant expertise in relevant RF
systems

• Increased luminosity calls for
• Factor of 2 increase in the beam currents.
• Factor of 2 increase in the RF voltage.
• Reduction in momentum compaction.
• All the above increase the strain on the
  low-level RF.
• LBNL has strong skills in critical areas
• LBNL is providing RF systems for SNS.
• LBNL can contribute accelerator physics expertise
  for modeling and experimental studies for RF
  systems in PEP-II upgrade.
• Issues identified for possible initial studies
• Evaluate options for high power RF upgrades.
• Redesign of low-level RF system using modern
  technology.

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Summary

• Quality of work and progress with NLC Damping
  Ring studies is recognized by NLC Collaboration
  and MAC.
• Work on wiggler has led to development of
  advanced techniques for modeling field and
  dynamics.
• Collaboration with ALS group has led to
  implementation of powerful techniques for
  studying nonlinear dynamics.
• Electron cloud is recognized as one of the most
  serious issues for the damping rings, and a clear
  path has been defined for RD leading to a
  solution.
• A significant contribution is being made towards
  improving PEP-II luminosity.
• Expertise in use of modeling techniques is being
  applied to improve tuning and optics of present
  lattices.
• Strengths in nonlinear design and optimization
  are being used to find solutions for lower
  momentum compaction lattices, to benefit from
  reduced ? at the interaction point.
• Skills in RF technology are a good match to
  critical systems in the luminosity upgrade.