Novel Schemes for Damping Rings

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Novel Schemes for Damping Rings

• J. Rogers
• Cornell University
• Improving dynamic aperture
• Reducing the circumference of the TESLA damping
  rings

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Alternating bends (P. Raimondi, A. Wolski)

• Problem to be solved limited dynamic aperture of
  conventional damping rings.
• Conventional rings have small dispersion and b
  functions, so chromaticity correction sextupoles
  must be strong.
• Conventional rings also use wiggler magnets,
  which are nonlinear.
• Approach separate damping arcs from chromaticity
  correction sections.
• Chromaticity correction sections are optimized
  to minimize aberration from the sextupoles.
• Arcs use high-field dipole magnets with reverse
  bends to achieve damping, instead of wigglers.

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Alternating bends
Arc cell uses combined function dipoles.
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Alternating bends
Chromaticity correction section dispersion is
generated by weak bends, pairs of sextupoles are
separated by p in phase to cancel the sextupole
nonlinearity.
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Responses to the very large TESLA damping rings

• Problem to be solved large circumference of the
  TESLA damping rings. TESLA uses 2820 bunches per
  train, with a 20 ns spacing (limited by kicker
  rise/fall time).
• Cost
• Share tunnel with main linacs
• Large space-charge tune spread necessitates
  coupling bumps
• Approaches
• Stacked rings
• Fourier series kicker
• RF separation at injection/extraction points
• Injection/extraction from trailing edge of a
  train
• Disadvantage
• Average current is higher in some approaches,
  making multibunch instabilities (e.g., electron
  cloud) more troublesome.

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Fourier series kicker (G. Gollin)
See George Gollins talk this session
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RF separation at injection/extraction points (R.
Helms, D. Rubin)

• A secondary RF system with a different frequency
  is used to separate the beam dispersively, bunch
  by bunch, into different channels.
• One such channel contains the injection/extractio
  n kicker.
• Bunch spacing can be made smaller than the
  kicker rise/fall time (by a factor of 4),
  allowing for a smaller ring.

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• Preliminary optics design of the separation has
  been done.
• The circumference must be made equal for all
  channels. The on-momentum channel must be
  longer than the two off-momentum channels.

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Injection/extraction from trailing edge of a
train (J. Rogers)

• Advantages
• Bunches are always extracted and injected at the
  end of a bunch train, so the injection/extraction
  kickers need only have a fast rise time. The
  damping ring can be much smaller than the dogbone
  design.
• Positron bunch production rate is greatly
  reduced, allowing use of a conventional positron
  source.
• Disadvantage
• An additional small ring is required.

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Injection/extraction from trailing edge of a train

• Two rings (one large, one small) share a common
  RF section.
• As damped bunches are extracted from the large
  ring to the bunch compressor and main linacs,
  bunches are injected into the small ring,
  avoiding RF transients.
• When all of the damped bunches are extracted
  from the large ring, the small ring is full. A
  transfer kicker located in the common straight
  section moves all of the bunches in the small
  ring as a train to the large ring. This requires
  a gap before the stored train, which does not
  appear in the train extracted to the main linac.

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Bunches are ejected from the large ring to the
main linac at bucket passages
Bunches are injected into the small ring at
bucket passages
Trains are transferred from the small to large
ring starting at bucket passages
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Injection/extraction from trailing edge of a train
Simplified timing example 3 trains of 3 bunches
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Design example (timing)
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Summary

• The baseline damping ring designs for the LC
  projects should work, although they are not
  without some risk (2003 ILC-TRC report).
• Alternatives are possible, and there are a
  number of promising concepts being developed.