Quadrupole Pickups

1
Quadrupole Pick-ups
A. Jansson PS/OP
2
What is a quadrupole pick-up?
A

• A non-invasive instrument sensitive to r.m.s.
  beam size.
• Uses the non-linear terms in electrode response
  to particle position (very small) to measure
  quadrupole moment.

Vacuum chamber
B
D
Pick-up seen along beam path
C
3
PS quadrupole pick-up

• Position contribution can not be avoided, but can
  be measured and subtracted.
• Design suppresses the dominating intensity signal
  by coupling to the radial magnetic field
  component.

Pick-up seen along beam path
D
A
B
C
Induction loop
4
Installation in PS
One pick-up per plane
5
First signals 2001

• Large beam size oscillations detected at start-up
  (due to wrong magnet settings in transfer line).

• First 10-20 turns are used to measure matching.
• Values after filamentation are used to check
  position correction and measure filamented
  emittance.

Beam EASTB Date March 28, 2001
Quadrupole moment evolution after PS
injection Position contribution has been
subtracted
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Measurement of matching

• Simultaneous fit to the two pick-up signals
  gives
• Injected emittances.
• Betatron mismatches.
• Horizontal dispersion mismatch.

• Input parameters
• ?H, ?V, DH, ??H, ??V,?p,qh,qv
• Most input parameters can be checked
  experimentally

10 free parameters, 20 data points
7
Injection matching measurement

• Betatron mismatch
• Dispersion mismatch

8
Measurement of filamented emittance

• For a stable beam, the emittance can be
  calculated from two (constant) pick-up readings.
• Different horizontal/vertical beta function
  ratios at the two pick-ups are required.
• Signal noise is reduced by averaging over many
  turns.

Horizontal
Vertical
Quad PUs
Wire-scanners
Statistical error
9
Beam based calibration

• Comparison Quad PU vs. Wire-scanner on stable
  beam.
• Several different beam types.

• Systematic error from
• Beta function 10
• Dispersion 10
• Mom. spread 3

10
Sensitivity

• Estimated from measurement on stable beam.
• Fluctuation lower than 0.5 mm2 r.m.s. (4 1011
  p/bunch ).
• This corresponds to a 1-2 beta beating at PS
  injection.
• Improvement still possible.

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Can the same pick-ups be used in LHC?

• The pick-ups were optimized for the LHC beam
  parameters at injection into the PS.

• The most important difference between PS and LHC
  is the bandwidth requirement .

Note that a design that works in the SPS would
work in the LHC!
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A pick-up adapted for the LHC

• At high frequencies, the loops essentially behave
  like badly matched strip-line couplers.

• Replace each loop with two matched strip-lines
  and a wideband hybrid.
• Good hybrid required (available).
• Transfer impedances x10 larger than in PS.

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Summary

• Quadrupole pick-ups can measure continuously
  without perturbing the beam.
• Both matching and emittance can be measured, on
  each bunch individually.
• The results in the PS have been very good.
• It can be done in the LHC, e.g. using strip-line
  couplers.