The PS Quadrupolar PickUp

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The PS Quadrupolar Pick-Up
A. JanssonLHC Emittance WorkshopCERN, 3-4 July
2000
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Outline

• Introduction and background
• PS quadrupole pick-up
• Design overview
• Data acquisition and analysis
• Results
• Measured data
• Accuracy and performance
• Future perspectives
• Measurement system
• Non-invasive emittance measurement
• Conclusions

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Introduction History
First proposed by L.L. Goldin (1966)

• PSB (1975)
• electrostatic
• AAC (1989)
• resonant
• electrostatic
• LEAR (1991)
• electrostatic
• CPS (1996)
• stripline
• magnetic

• SLC (1983)
• stripline
• KAON Factory (1993)
• wall current monitor
• SPA/AFEL (1993-1999)
• capacitive
• Somewhere...
• resonant cavity

Pick-up
Electronics
Signal Analysis
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Introduction Working Principle

• Unless the beam is round, the distance from the
  four electrodes to the beam tail is not the same
  even if the beam is centred.
• If (and only if) the pick-up is non-linear, this
  induces a small signal proportional to the
  ellipticity of the beam.
• A good quadrupole pick-up should be very
  non-linear
• The quadrupole signal is formed by taking the
  difference between the total signal induced on
  opposite electrodes

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Introduction Working Principles
1/r2
1/r
1/r3

• The quadrupole antenna moment contains beam width
  information (?H2 - ?V2) ? Measure quadrupole
  field component
• Note that antenna moments decomposition depends
  on definition of centre ? False quadrupole
  component from off-center beam

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Introduction Signals

• Quadrupole pick-up signal components

Emittance and Betatron Matching
Dispersion and Dispersion Matching
Injection Steering and Closed Orbit
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PS Pickup Design Ideas

• Couple to radial magnetic field component!
• Total common mode suppression
• Low impedance
• Such coupling implies a local enlargement of the
  vacuum chamber to allow the flux lines to close
  outside the loop
• Longitudinal impedance
• Ceramic vacuum chamber, loops outside
• Resistive coating

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PS Pickup Prototype
Lab Prototype
Machine Prototype
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PS Pickup Couplings

• Common mode suppression by coupling to radial
  magnetic field component works!
• Antenna loop design key issue.
• Possible to obtain good performance for a large
  bandwidth (100k-100MHz).
• Still room for improvements

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PS Pickup Impedance

• Longitudinal Impedance
• Metal vanes
• In quadrupole symmetry planes
• Cut-off frequency increases with mode number
• Resistive coating
• Screening works
• Ceramic roundness
• Layer homogeneity

Z
R
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PS Pickup Data Acquisition
Hybrid
Amplifier
Oscilloscope
LAN
Pick-up
Intensity
/-
PC with LabView
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PS Pickup Data Analysis

• Historically, frequency domain analysis was used
  (in rings).
• PS All bunches come from different machines.
• Time domain analysis
• Determine bunch shape function from sum signal
  fit.
• Fit to quadrupole signal with fixed bunch shape.

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Results Measurements

• Comparative measurements quadrupole pick-up vs
  SEM-grid (first results).

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Results Measurements

• Injection width oscillations for an elliptic beam
  due to coupling.

• Injection oscillations due to mismatch.

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Results Accuracy

• Estimation of noise floor in treated data gives
  0.5 mm2
• NB. Remember that the prototype was built of
  pieces from the scrap-yard

• Estimation based on
• Amplifier noise 2 nV/Hz1/2
• Amplifier bandwidth 25 MHz
• Coupling impedance 100 ?/m2
• Peak current 2 A
• gives 0.05 mm2 for peak detection without
  accounting for the noise reduction from the fit.
• Cable noise pick-up (from RF etc) more important
  than thermal noise in amplifier
• Pick-up imperfections

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Future Measurement System

• The different components of the quadrupole signal
  can be hard to separate.
• Landau damping
• Small tune separation
• If several pick-ups are available, the signals
  can be combined to achieve this.
• In a ring, two pick-ups are sufficient.
• Optimum configuration
• First pick-up Large horizontal beta
• Second pick-up Large vertical beta
• Betatron phase difference 180? (90? also OK)
• Gives beam sizes turn-by-turn.
• Not possible in the PS.

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Future Emittance Measurement

• For a stable beam, the emittance can be easily
  calculated from
• The matrix equation is well behaved if the beta
  function ratios are very different at the pick-up
  locations.
• Main error contribution from lattice beta
  function, dispersion and momentum spread.
• Comparable to wire-scanner etc.

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Future Emittance Measurement

• From six quadrupole pick-up measurements, the
  emittance and Twiss parameters can be calculated
  (SLAC, LANL).
• Matrix equation to be solved is usually
  ill-conditioned.
• Error propagation calculations show that random
  noise should be below 0.05 mm2 to give 10
  accuracy for LHC emittance (1um at PS injection).
  Pick-up offsets are less dangerous.

Error as a function of tune
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Conclusions

• Quadrupole pick-ups fills a particular
  instrumentation need
• Totally non-invasive
• injection watchdog
• study processes at injection and during cycle
• Fast (bunch resolution can be obtained)
• The PS prototype has given very promising results
• The final PS pick-up design is ready
• Tests of second machine prototype during
  summer/autumn
• Installation of the final system during winter
  shut-down
• Useful in any machine
• Need some modification to fit new machine
  parameters

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