updating the laser-beam design of the ilc compton polarimeters

Updating the laser-beam design of the ILC Compton polarimeters

A. Martens, K. Cassou, K. Dupraz, D. Nutarelli, C. Pascaud, F. ZomerLAL, Univ Paris-Sud, IN2P3/CNRS

Outline:• Introduction: Compton polarimetry• Current design of polartimeters• Impact of required error budget on design• Proposal for a new laser-systems design

Introduction: polarimetry @ ILC

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Direct measurement at e+e- Interaction Point (IP) of longitudinal polarisation

• Several well-known x-section (total, differential, angular) measurements

Compton polarimeters constraint

useful at low statistics

Combination of Upstream and Downstream Polarimeters:

• Allows infering spin transport effects in the Beam Delivery System

• Allows determination of beam-beam effects at the e+e-IP

LCWS 2018 – MDI, Arlington, TX, USA, 23/10/2018

Compton back-scattering polarimetry

Aurélien MARTENS 3

Used in the '90s at HERA and SLC

Measure scattered productsà e- counters (SLC/ILC)à γ calorimeter (HERA)

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Compton polarimetry performs better compared to alternatives at higher energies

Differential count rate sensitive to transverse and longitudinal polarisation

Most sensitivity at threshold

High-order QED corrections

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QED corrections<0.1% @ 45 GeV

Dedicated studies at each ILC running energy needed ?

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QED corrections about 0.5% @ 500 GeV

Processes: egàeg, egàeee, egàeggCorrections are detector dependent and energy dependent

Introduction: polarimeters @ ILC

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ILC design

Aurélien MARTENS 6LCWS 2018 – MDI, Arlington, TX, USA, 23/10/2018

Current design of laser rooms

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surface rooms: radiation hardness

Similar design assumed for upstream polarimeters

Not necessarily easy:Pressure, temperaturegradients

This design certainly require: • Additional thick optical windows for the laser beam transport of the large beam (few cms)• Careful compensation of pointing and position instabilities of the beam in the accelerator bay• Relatively large optics (cost)


Current design of magnetic chicanes

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4-magnet chicane for (upstream) Compton polarimeters• No e-beam energy dependent field strength and detector acceptance

• The Interaction Point (IP) between laser and electron beams does however

Upstream (@-1800m) : horizontal chicaneLaser-beam energy: about 30µJ per pulse

Upstream


Current design of downstream polarimeter chicane

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6-magnet chicane

Downstream (@150m): vertical chicane ß immunity to synchrotron radiation e+e- IP

About 100mJ laser-beam pulse energy


Current design of detectors

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Permille control of non-linear

LED intensity thanks to

differential LED pulsesQuartz crystals: data driven gain calibration of PMs


Source of uncertainty �P/PAsymmetry prediction (alignment, beam, backgrounds) 0.2%Detector linearity (gains, calibration system) 0.1%Jitters (beam, electronic noise) 0.05%Laser polarization 0.1%Total 0.25%

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Compton polarimeters systematic uncertainties


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Account for this tight constraint in the laser design

Questions:• Does the current laser design seem compliant with this request ?• Strategy to control laser-beam polarisation at this precision ?

Review the laser design for the upstreamcompton polarimeter

Challenging per-mille ellipsometry


Laser-beam ellipsometry


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Extremely difficult to remove wrong polarisation <0.1%

Residual wrong polarisation vs tilt angle

About 1 per mille wrong polarisation

Fine modeling of beam transport is required




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• Minimize amount of material seen by the laser-beam by • use of reflective optics• Reducing optical path length

• Calibrate effects on polarisation from laser-beam transport

Example of fit of PD intensities versus QWP angle• Large number of parameters (misalignements,

thickness defaults,…)• Fraction of per-mille precision




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• Polarisation independent Holographic Beam Sampler• Careful suppression of laser intensity fluctuations• Use of balanced photodiodes and differential electronics

Example of measurements @ HERA

• More frequent measurements ?• Modulation of circular polarisation to

avoid DC fluctuations ?

Temporal fluctuations à precision 0.3% (HERA)

R&D required to reach 0.1%


Upstream Laser design: example of industrial laser


Demanding industrial sector: routinely operating systems

Ultra compact ! à integrate close to Compton IP ?

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Concurrent designs certainly exist à detailed study of the market is required

Viable for 30µJ/pulse in redAllow for continuous ellipsometry at 1.8 MHz J

100W in red à 50w in green


http://www.amplitude-systemes.com/


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http://www.amplitude-systemes.com/

Downstream Laser design: example of industrial laser


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For instance DIRA can deliver 100mJ @ 2kHz

Initial requirement : 100mJ @ 5Hz, commercial laser systems exist

Question: • is it possible to gain in polarimeter performances with existing more powerful laser

• Would it be useful for physics ?


Proposed laser room specifications


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Compton polarimeter IPs

2 mLCWS 2018 – MDI, Arlington, TX, USA, 23/10/2018

Proposed laser-beam transport specifications


Laser room Alternatively: place diagsaround here

Use of lens not ideal

Prefer reflective optics& careful alignement

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ideally ground transport à certainly not possible à requires dedicated studies

Primary vacuum is enough

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Conclusion


Laser beam systems for ILC polarimeters is being reviewed

Optimisation and control of the laser-beam polarisation at the per-mille level is required

• Seem possible to locate laser-room close to compton IPs• Industrial compact and robust lasers that meet requirements are available• First preliminary specifications for optical rooms, laser beam transport drawn• Interfaces to be discussed

• Extremely difficult to cancel residual wrong polarisation at 1‰• Require excellent modelling of the optical system as a whole• Require R&D on detection system, modulation technique ?• Compromise between fast continuous ellipsometry (using e-beam off time too) and

total laser beam power manageable

Actual needs impact final laser-beam system designà Especially true for the downstream polarimeter

Review of laser-beam design, some preliminary statements drawn

R&D is required on precise and robust laser-beam ellipsometry


updating the laser-beam design of the ilc compton polarimeters

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