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Special requirements for Photosources operating at PV electron scattering exp. 

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. Outline. The problem HC-intensity asymmetry

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Special requirements for Photosources operating at PV electron scattering exp. 

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  1. 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

  2. Outline • The problem • HC-intensity asymmetry • Sources of other HC-fluctuations • Low energy polarimetry

  3. Necessary, but not specific for PV- Experiment. Polarized source tasks 1.) Reliable beam production at desired intensity level 2.) Provide desired spin orientation 3.) High Polarization (>80%) I.) Polarization meas. DA/A ~DP 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.

  4. Scattering experiments (simplified) T S A • measure P accurately! (I) D measures R+- q + Let x be a vector formed from the relevant parameters:

  5. What if ? { :=HCA Goal: Error of HCA should be small against other error contributions. (DP, Dstat) 1.) The (average) values of xi+-xi- have to be measured with good accuracy.  good stability of xi +  high spin flip frequency desirable 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).

  6. Source Set-up

  7. HC-Control schematics (PVA4) (Almost) no active HC-compensation, except by stabilization!

  8. Important example: Intensity-HCA (I-HCA) Adjust to zero crossing & Observe stability! Sketch of polarization optics Result measuring I-HCA=(I+-I-)/(I++I-)

  9. Modelling the I-HCA 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) Description with 4X4 polarization transfer matrices: For ‘thin’ cathodes: I+- ~ S+-0

  10. Expand Matrix elements to first order in the imperfections: Predicted I-HCA as function of compensator rotation angle b AISR=Analysing power of cathode, with polarizer axis oriented at 2qk,  Measured for several high P cathodes: AISR=0.02-0.05 fa= f++f-/p: Normalized asymmetric phase shift of pockels cell (forced zero crossing!),fa=0.03 (typ.) e3=circular stokes component of light at input of Pockels cell,  Not measured, est. to <0.003 s~0.999 = diagonal polarisation component at input of Pockels cell, fc= deviation of half wave plate from 180 degree retardation 0.01 (quote by company) D,c=function of birefringence of optical elements between PC and cathode. (measurable, D~0.01)

  11. First consequences 1.) Stability does not depend on the symmetric phase shift error (f+-f-)-p 2.) Parameters extracted from fit in agreement with reasonable values of optical imperfections 3.) Introducing an additional half wave plate (General sign changer) will also change I-HCA.

  12. 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 crossing 4.) Reduction of sensitivity due to stabilization! (1/G~2-10) From fit-curve: Realistic only if second order effects (HC-Transmission changes) do not occur

  13. Compensating the offset term { Offset/(4b amplitude) while varying qk: Offers to reduce Problem by order(s) of magnitude….But

  14. Two questions AISR 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 fluctuations with very small analyzing power of the photocathode? 2.) What is the origin of the HC-fluctuation of other parameters like position, angle, energy?

  15. ‘ideal’ Experiment Lock- in Pulser Sw. He/Ne Laser Detektor with low analysing power PC 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….

  16. Screen Prism Backreflexions for the different helicity states.

  17. 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

  18. Intensity asymmetry in laser beam Lock- in Pulser Sw. diode-laser Movable Detektor with pinhole PC • Helicity correlated movement of centroid is 1mm.

  19. Causes for HC-fluctuations

  20. 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!

  21. Moderately ambitious approach: Mott polarimeter at 3.5 MeV • Goal 1: fast relative measurement at full current with good reproducibility • Goal 2: accuracy < 2%

  22. 3.5 MeV Mottpolarimeter Measurement time < 2min @1% stat. Acc. @20 mA Beam installation time req: (40min) will be reduced to <15min.

  23. Asymmetry vs. Spin rotator angle (164 Grad)

  24. 8 hour measurement of asymmetry

  25. 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

  26. Double scattering effects Energy variation at fixed scattering angle m

  27. Very ambitious approach • Low energy may be very accurate (DP/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.

  28. Summary • 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 used • light optical effects are rather complicated but ‘treatable’ • Better understanding + technology offers potential • to keep situation acceptable also for future exp.

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