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Source Systematics

Source Systematics. Gordon D. Cates, Jr. University of Virginia. PITA - type effects The importance of controlling the analyzer-axis Two Pockels cells Half-wave plate Position asymmetries Lensing effects Phase gradients Other Feedback, and its limitations.

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Source Systematics

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  1. Source Systematics Gordon D. Cates, Jr. University of Virginia • PITA - type effects • The importance of controlling the analyzer-axis • Two Pockels cells • Half-wave plate • Position asymmetries • Lensing effects • Phase gradients • Other • Feedback, and its limitations Parity Workshop - May 10, 2002

  2. Polarization Induced Transport Asymmetries(The PITA effect) Polarization ellipses that result from various phases How imperfect circular polarization translates into helicity correlated intensity asymmetries Right helicity Left helicity

  3. PITA curve from strained GaAs cathode • APITA is plotted as a function of D, where D was adjusted using the voltages on the Pockels cell. • Here, the analyzing power that caused the asymmetry was not the laser transport system, but rather a strained GaAs crystal. PITA curves are routinely used to set feedback parameters in parity experiments.

  4. Matching the polarization ellipse’s to the axis of the analyzing power In a Strained GaAs crystal, there is a preferred axis. Quantum Efficiency is higher for light that is polarized along that axis It is desirable to have a means for orienting your ellipses

  5. SLAC polarization control setup

  6. Ellipses are rotated using the two PC’s The two Pockels cells have their fast axes set at 45o to one another.

  7. The “two-dimensional” PITA effect,formulae for APITA for two Pockels cells Now there is a line in the space spanned by D1 and D2 for which APITA is zero.

  8. The line in phase space that minimizes APITA PITA is suppressed along the line. Circular polarization is only maximized at one point.

  9. Simple PITA-type effects cannot be everything If phase alone were the problem (and not phase gradients), position asymmetries would go to zero at the same time.

  10. Gradients in the phase D and position asymmetries Left helicity, emitted charge vs. position Right helicity, emitted charge vs. position Linear gradient in D Across laser beam

  11. Laser studies of systematics demonstrating sensitivity to gradients of D across Pockels cell Intensity asymmetry vs.position (averaged over photodiode array) Intensity asymmetry vs. position (single photodiode element) Position asymmetry vs. position (sensitive to the gradient of D) Spot size asymmetry vs. position (sensitive to the 2nd derivative of D)

  12. Feedback’s effect on integrated asymmetries (intensity and position) versus time. Envelopes represent the one sigma statistical error for the quantity measured. In the absence of a systematic, this is the error with which one would expect the distribution to be centered on zero. Feedback causes 1/N convergence toward zero.

  13. The effect of inserting and rotating a half-wave plate

  14. Asymmetries while rotating half-wave plate

  15. Systematics beyond the PITA effect

  16. What is needed for the future • Continued empirical work is critical. • Need to focus on passive suppression as well as feedback. • The important problems at JLab could be very different than at SLAC, Bates, Mainz, …

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