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Special requirements for Photosources operating at PV electron scattering exp. 

International Workshop PAVI 2006

Milos Island 20/05/2006

by Kurt Aulenbacher

Institut für Kernphysik der Uni Mainz

B2/A4 collaboration

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Outline

• The problem

• HC-intensity asymmetry

• Sources of other HC-fluctuations

• Low energy polarimetry

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Polarized source tasks

1.) Reliable beam production at desired intensity level2.) Provide desired spin orientation3.) High Polarization (>80%)

Necessary, but not specific for PV-Experiment.

I.) Polarization meas. A/A ~P small limiting factor in several PV-exp.

II.) HC-control A always important limiting especially when A<10-6

Source team can provide support for point (I), (II) is more important.

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• Scattering experiments (simplified)

)(xfR

S AT

D measures R+-

RR

RRAexp

+

ZZPVgeIfIf SPA *..

exp....)(....)(

Let x be a vector formed from the relevant parameters: )('......),',,,( iT xyxyxIx

measure P accurately! (I)

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What if ?....)(....)( IfIf

)(

)()(

exp xf

xxdxxdf

PSRR

RRA i

iii

2.) Relative sensitivities have to be determined and are only known with limited accuracy.Higher order coefficients usually not well known. (xi

+-xi-) should be “small” (i.e. sufficiently close to zero).

1.) The (average) values of xi+-xi

- have to be measured with good accuracy. good stability of xi + high spin flip frequency desirable

Goal: Error of HCA should be small against other error contributions. (P, stat)

{

:=HCA

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Source Set-up

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HC-Control schematics (PVA4)

(Almost) no active HC-compensation, except by stabilization!

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Important example: Intensity-HCA (I-HCA)Sketch of polarization optics

Result measuring I-HCA=(I+-I-)/(I++I-)

Adjust to zero crossing & Observe stability!

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Modelling the I-HCA

Description with 4X4 polarization transfer matrices:

in

PockcompopticsISR

KathS

S

S

S

MMMM

S

S

S

S

3

2

1

0

3

2

1

0

****

For ‘thin’ cathodes: I+- ~ S+-0

Assuming analysing power of Photocathode, imperfections in the alignment and in the phase shifts (birefringences) of the optical Elements (similar to Humensky et al. NIM A 521 (2004) 261)

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))2cos()22sin()42sin()( 3

KKCKAISR DAII

II

AISR=Analysing power of cathode, with polarizer axis oriented at 2k,

Measured for several high P cathodes: AISR=0.02-0.05a= f++f-/: Normalized asymmetric phase shift of pockels cell (forced zero crossing!),a=0.03 (typ.) 3=circular stokes component of light at input of Pockels cell, Not measured, est. to <0.003 = diagonal polarisation component at input of Pockels cell,c= deviation of half wave plate from 180 degree retardation0.01 (quote by company) D,=function of birefringence of optical elements between PC and

cathode.(measurable, D~0.01)

Expand Matrix elements to first order in the imperfections:Predicted I-HCA as function of compensator rotation angle

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1.) Stability does not depend on the symmetric phase shift error (f+-f-)- 2.) Parameters extracted from fit in agreement with reasonable values of optical imperfections3.) Introducing an additional half wave plate (General sign changer) will also change I-HCA.

First consequences

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Compensation: Prediction of thermal stability

1.) Absolute value of phase shift does not contribute to IHCA (in first order)2.) Asymmetric phase shift + compensator temperature dependence! 3.) Sensitivity depends on steepness of zero crossing4.) Reduction of sensitivity due to stabilization! (1/G~2-10)

From fit-curve: KppmGT

IHCA/50

1

Realistic only if second order effects (HC-Transmission changes) do not occur

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Compensating the offset term

))2cos()22sin()42sin()( 3

KKCKAISR DAII

II

Offset/(4 amplitude) while varying k:

Offers to reduce Problem by order(s)of magnitude….But

{

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Two questionsAISR is the analyzing power of the photocathode which will depend on the photocathode type (composition, thickness,,,) typically: superlattice 2%, strained layers 4%, GaAs:<0.2%.

1.) Why did PV-experiments before 1990 observe large asymmetries and position fluctuationswith very small analyzing power of the photocathode? 2.) What is the origin of the HC-fluctuation of other parameters like position, angle, energy?

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‘ideal’ Experiment

Pulser

He/Ne Laser

Lock-in

Sw.

PC Detektor with low analysing power

Experiment results in I-HCA of 10000 ppm(no lock in needed!)

Luck! The signal is so large that it´s easy to find a reason….

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Prism

Screen

Backreflexions for the different helicity states.

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Hypothesis

• For scattering centers at different positions the ability to interfere (at an image point) is changed by switching the helicity.

• The interference pattern on the photocathode is therefore also helicity dependent, especcially in the ‘halo’ of the laser beam

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Intensity asymmetry in laser beamPulser

diode-laser

Lock-in

Sw.

PC Movable Detektor with pinhole

Helicity correlated movement of centroid is 1m.

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Causes for HC-fluctuations

Parameter HCA at source HCA at target dominating

Cause

intensity 0-4000ppm 0-4000ppm ISR

position 100nm ~10nm Interference

angle - 10-8 rad Phase space transformation

energy - Few eV Phase space transf.

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Can Polarimetry at low energy help a high energy

experiment?

LOW-E polarimetry provides some support for the experiment if it can be done convienently and fast!

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Moderately ambitious approach: Mott polarimeter at 3.5 MeV

• Goal 1: fast relative measurement at full current with good reproducibility

• Goal 2: accuracy < 2%

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3.5 MeV Mottpolarimeter

Measurement time < 2min @1% stat. Acc. @20 ABeam installation time req: (40min) will be reduced to <15min.

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Asymmetry vs. Spin rotator angle (164 Grad)

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8 hour measurement of asymmetry

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Analyzing power calculation

Theo:

Low energy: Fink et al.: Phys Rev A (38,12), 6055 (1988)

‚High‘ energy: Uginicius et al.:Nucl Phys A 158 418 (1970)

Exp:

Low energy: Gray et al.: Rev. Sci. Instrum. 55,88 (1984)

High energy: Sromicki et al. Phys. Rev. Lett. 82,1, 57 (1999)

Z=79

Analyzing power can be calculated with less than 1% accuracy

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Double scattering effects

Energy variation at fixed scattering angle

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Very ambitious approach

• Low energy may be very accurate (P/P < 1%)

(Mayer et al. Rev.Sci. Inst. (64,952(1993))• Always possible to achieve low set-up time• Spin losses under control <<1%• Spin orientation can be calculated to <1 deg.• Measurement at full exp.current possible and fast.• Calibration check may be handled as accelerator

‚service‘ good calibration tracking.

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Summary

1. HC-effects do contribute to, but do not dominate the error budget (at PVA4).2. Stable operating conditions have to be achieved, if necessary extensive stabilization systems have to be used3. light optical effects are rather complicated but ‘treatable’4. Better understanding + technology offers potential to keep situation acceptable also for future exp.