Polarimetry at the ILC

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Polarimetry at the ILC
Ken Moffeit WG4 Beam Instrumentation 15 October
2004---1400-1420

• Compton Polarimetry
• Physics well understood
• Detector backgrounds easy to measure with laser
  off
• Parasitic data taking
• Compton scattering rate is high-small statistical
  errors in short time
• Laser helicity selected pulse-to pulse -gtsmall
  systematics errors
• Laser polarization can be measured with 0.1
  accuracy

• Important Design Considerations
• Angle of e- beam at Compton IP same as e-
  direction at the
• ee- Interaction Point to lt50 microradians.
• Downstream polaimeter
• Require crossing angle at ee- interaction point
  
• Positron polarization -gt polarimeters for e

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References
IPBI TN-2003-1 Status of Linear Collider Beam
Instrumentation Design, by D. Cinabro, E.
Torrence and M. Woods, May 2003.    Also
available as an ALCPG Note  LCD-ALCPG-03-0001
(2003).
IPBI TN-2003-2.  Laser System for a Compton
Polarimeter, by K.C. Moffeit and M. Woods. 
December 2003.
IPBI TN-2004-1.  Luminosity, Energy and
Polarization Studies for the Linear Collider 
Comparing ee- and e-e- for NLC and TESLA, by
M. Woods, A. Florimonte, K.C. Moffeit, T.O.
Raubenheimer, A. Seryi,     and C. Sramek . 
SLAC-PUB-10353.  February 2004. 
IPBI TN-2004-3.  Spin Dynamics at the NLC, K.
Moffeit and M. Woods, June 2004.
STUDIES FOR A DOWNSTREAM COMPTON POLARIMETER AT
THE ILC.By M. Woods, K.C. Moffeit (SLAC),.
SLAC-PUB-10669, Aug 2004.
DET LC-DET-2001-047 The TESLA Compton
Polarimeter 2001-01-26 V.Gharibyan, N.Meyners,
P.Schuler
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Layouts of the ILC
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Location of Polarimeters
Having both an upstream and a downstream
polarimeter will assist in achieving Good
estimates of the systematic errors.

• Extraction-line beam diagnostics are highly
  desirable at the ILC
• More flexibility in the beam optics design than
  upstream
• Independent measurements with different
  systematic errors
• Allows improving systematic errors by sqrt (2)
• Directly measure beam-beam collision effects
• Compare measurements with and without collisions
• Probably requires crossing angle at ee-
  interaction point

• Design Considerations
• Angle of e- beam at Compton IP is the same as the
  e- direction at
• the e e- IP (lt50 microradians).
• Upstream polarimeter needs to be before energy
  slit.
• Downstream polarimeter requires crossing angle at
  
• ee- interaction point
• Downstream polarimeter needs to be compatible
• with the energy spectrometer

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Spin Precession
Change in spin direction for various bend angles
and the projection Of the longitudinal
polarization. Electron beam energy is 250 GeV.
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Upstream Polarimeter

• Direction of beam at Compton IP is the same as at
  the Detector IP (lt50mrad)
• Laser beam access to Compton IP
• Measure laser polarization near Compton IP
• Measure Compton Scattered electrons
  (backscattered 25 GeV)
• momentum analyzed
• Detected with a segmented gas Cerenkov Counter

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Layout of the Beam Delivery System to two
Interaction Regions with Crossing Angles
Should both IR-1 and IR-2 have locations For
upstream Compton Polarimeters? IR-1 has location
with direction okay IR-2 doesnt in plan above!
Upstream Polarimeter before energy slit
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Extraction Line Beam Properties
Angular divergences of the incoming and disrupted
beams at the NLC IP BMT depolarization due to
the angular divergences.
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Extraction Line Polarimeter Elements
Note Downstream polarimeter needs to be
compatible with the energy spectrometer
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Summary of Depolarization Effects
Note The Table entries are for the NLC design.
Need to be redone for ILC design, in particular
the compressor and pre-Linac contribution.
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Compton vs Luminosity weighted polarization
measurement
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Summary

• Accelerator design issues
• Location of polarimeters
• Downstream polarimeter requires ee- crossing
  angle
• No net bend angle in e- beam direction at Compton
  IP and e- direction at ee- IP
• Positron polarization needs e polarimeters
• Polarimeter design issues
• Laser optics (20 hz high power al la SLC, every
  bunch-TESLA design, resonate cavity--HERA)
• Detectors
• Beam test needs before TDR
• Nothing stands out