A' Jansson 1

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GHz Schottky Pick-ups for LHC

• Andreas Jansson
• Fermilab

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Outline

• What we have in the Tevatron.
• Operational experience and results
• What CERN would like.
• Passive tune measurement during stores
• What LARP ( the US taxpayers) want.
• LHC spin-offs for US
• Conclusions.

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What we have in the Tevatron

• Continuous tune, chromaticity and momentum spread
  measurement (for both planes and particle types)
  during stores.
• Extremely useful operational tool, especially
  combined with data logger
• Used routinely.

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Tune during store

• Tune is extracted from location of betatron
  bands.
• Can clearly see tune change due to beam-beam
  effect.
• Bunch-by-bunch tune measurement also possible.

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Momentum spread during stores

• Momentum spread is extracted from the average
  betatron band width.
• Correlates well with dedicated measurements of
  bunch length.

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Chromaticity during stores

• The chromaticity is extracted from the asymmetry
  between upper and lower betatron band width.
• Measured values are in expected range, but not
  yet fully verified by dedicated measurements.

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Emittance measurement

• Emittance is extracted from total betatron band
  power.
• Measurement is fluctuating and correlated with
  noise baseline.
• Noise baseline for pbars very high at start of
  store.

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Attempts to fix emittance

• Changed FE amplifiers to avoid intermittent
  saturation early in store.
• Fixed a software normalization bug responsible
  for some of the correlations.
• Problem is still there, although smaller in
  magnitude

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Ramp measurements

• Mixer tracks RF.
• Less averaging to get time resolution.
• Longitudinal line power varies, with large spike
  at about 250GeV
• Baseline level shifts at specific times (!)
• Betatron band power fluctuates
• Longitudinal or transverse effect?

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Tev Schottky status

• Average tune measurement is routinely used for
  operation.
• Tune measurement can be done bunch-by-bunch for
  protons, and in groups of three for pbars
• Also have momentum spread and chromaticity
  measurement.
• Need to understand power levels (signal and
  baseline), both during stores and on ramp.
• Ongoing

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Beam parameters

• What frequency to use in LHC?
• Bunch length indicates approx x6 higher frequency
• Aperture may not allow more than x2

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Effect of LHC tune

• Strong revolution lines (even with centered
  beam).
• LHC tune is closer to integer (0.2/0.3).
• Potential issue with overlap between betatron
  band and revolution band?

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Effect of LHC size

• Tev S/N 10dB.
• LHC bunch intensity is half of Tevatron.
• LHC radius 4.5 times larger than Tevatron
• Signal power for single bunch will be diluted,
  expected down 19 dB.
• May partly be recuperated by gating out noise,
  need to find out the limits.

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Ramp measurements in LHC?

• LHC ramp is much longer than the Tevatron
• May be possible to track tune on the ramp(?)
• We may not know until it is installed
• Build and test prototype in RHIC?
• LHC prototype would not fit in Tevatron.

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Conclusions

• Passive tune measurement during store should be
  relatively straightforward
• Probably also chromaticity and momentum spread
  measurement.
• In addition, may be able to passively measure
  tune during acceleration due to the long ramp
  time (requires testing, perhaps in RHIC).
• Need to understand spectra in Tevatron better in
  order to extrapolate.

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LARP proposal

• A collaboration to study, develop, build and test
  an LHC Schottky monitor
• Goals
• Passive tune, chromaticity, and momentum spread
  measurement during store (with emphasis on tune).
• The same measurement on the ramp.
• Single bunch gating capability.