source systematics

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Source Systematics 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 Gordon D. Cates, Jr. University of Virginia Parity Workshop - May 10, 2002

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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. - PowerPoint PPT Presentation

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Page 1: Source Systematics

Source Systematics

• 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

Gordon D. Cates, Jr. University of Virginia

Parity Workshop - May 10, 2002

Page 2: Source Systematics

Polarization Induced Transport Asymmetries(The PITA effect)

Polarization ellipses that result from various phases

Right helicity Left helicity

How imperfect circular polarization translates into helicity correlated intensity asymmetries

Page 3: Source Systematics

PITA curve from strained GaAs cathode

• APITA is plotted as a function of , where 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.

Page 4: Source Systematics

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

Page 5: Source Systematics

SLAC polarization control setup

Page 6: Source Systematics

Ellipses are rotated using the two PC’s

The two Pockels cells have their fast axes set at 45 to one another.

Page 7: Source Systematics

The “two-dimensional” PITA effect,formulae for APITA for two Pockels cells

Now there is a line in the space spanned by andfor which APITA is zero.

Page 8: Source Systematics

The line in phase space that minimizes APITA

PITA is suppressed along the line.

Circular polarization is only maximized at one point.

Page 9: Source Systematics

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.

Page 10: Source Systematics

Gradients in the phase and position asymmetries

Linear gradient in Across laser beam

Left helicity,emitted charge vs. position

Right helicity, emitted charge vs. position

Page 11: Source Systematics

Laser studies of systematics demonstrating sensitivity to gradients of across Pockels cell

Intensity asymmetry vs.position(averaged over photodiode array)

Position asymmetry vs. position(sensitive to the gradient of

Intensity asymmetry vs. position(single photodiode element)

Spot size asymmetry vs. position(sensitive to the 2nd derivative of

Page 12: Source Systematics

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.

Page 13: Source Systematics

The effect of inserting and rotating a half-wave plate

Page 14: Source Systematics

Asymmetries while rotating half-wave plate

Page 15: Source Systematics

Systematics beyond the PITA effect

Page 16: Source Systematics

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, …