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Helicity-Correlated effects for SAMPLE Experiment

Helicity-Correlated effects for SAMPLE Experiment M.Farkhondeh, B.Franklin, E. Tsentalovich , T.Zwart MIT-Bates Linear Accelerator Center Middleton, MA 01949, USA. SAMPLE Parity violation experiment, measured asymmetry  1 ppm Demands of the experiment: Current:

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Helicity-Correlated effects for SAMPLE Experiment

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  1. Helicity-Correlated effects for SAMPLE Experiment • M.Farkhondeh, B.Franklin, E. Tsentalovich, T.Zwart • MIT-Bates Linear Accelerator Center • Middleton, MA 01949, USA

  2. SAMPLE • Parity violation experiment, measured asymmetry  1 ppm • Demands of the experiment: • Current: • Average: 40 A (limited by target restrictions) • Peak: 2.5 mA on the target, 7.5 mA from injector ( Rep.rate 600 Hz, pulse duration  30 sec ) • Pulse-to-pulse stability: • better than 0.5 % • Helicity-correlated asymmetry: • Intensity:  0.1 ppm • Beam location on the target < 100 nm

  3. MIT-Bates Polarized Source

  4. Polarization Reversal

  5. PITA

  6. Polarization 100 80 60 40 20 0 P.C. Positional asymmetries: 1. Piezo effect in Helicity Pockels cell. 2. Effects of asymmetric transport system. P(circ.) Distribution of residual linear polarization inside the laser beam P(lin.)

  7. Helicity (+) Helicity (-) Intensity Intensity r r Pcirc.(+) Pcirc.(-) 100% 100% 99% 99% 98% 98% r r Plin.(+) Plin.(-) r r After asymmetric transport system (S and P reflections are balanced imperfectly) Intensity Intensity r r

  8.   /4 Pockels cell  - antisymmetric retardation error  << 1 Rotateable (1/2+) plate  << 1 Other birefringent components (vacuum window) Retardation  << 1 Analyzer Analyzing power   =  [  sin(4 - 2) +  sin(2 - 2) +  sin(2 - 2) ] Analysis by B. Humensky

  9. Analyzing power  • Typical laser transport system  • Strained GaAs crystal (normal incidence)  5-10% • GaAs crystal with the incident angle of  40  20%

  10. TJNAF (HAPPEX) results Strained GaAs crystal, normal incidence

  11. Bates results Strained GaAs crystal, angle of incidence 37

  12. Cross-asymmetries Loading effects in accelerator  (current)  (energy) Dispersion Differential scraping  (current)  (position)

  13. Energy time x x More complications... - time dependence during the pulse - beam size asymmetry - beam shape asymmetry

  14. Handling helicity-correlated asymmetries 1. Minimize existing asymmetries by a) improving circular polarization, b) reducing the analyzing power of transport system c) lining up optical axis of the analyzer with residual linear polarization 2. Minimize residual asymmetries using active feedback systems It is essential to have separate and orthogonal feedback systems for I, X, Y asymmetries

  15. Feedback

  16. V Intensity feedback Polarizer /4 Pockels cell V(PC) V100 V

  17.  10 V Correction Pockels cell Polarizer /4 Pockels cell V0~400 V Slope ~50 ppm/V Adjustable with V0

  18.  10 V Correction Pockels cell /4 Pockels cell Polarizer /10 Slope ~50 ppm/V Adjustable with /10 angle

  19. Pockels cell Polarizer Laser light GaAs V0 Set point Toroid Electron beam T. Zwart’s stabilizer

  20. Piezo-driven optical flat ~1m  > 1 kHz Positional feedback

  21. First use of piezo feedback: SAMPLE -1998 /2 OUT: Average = 5.0  2.5 nm Piezo feedback on /2 in: Average = 31.2  2.6 nm

  22. SAMPLE -1999 /2 OUT: Average = 4.45  1.51 nm /2 in: Average = -2.60  1.44 nm

  23. Summary 1. During the SAMPLE runs, intensity asymmetry averaged over the entire run didn’t exceed 0.1 ppm. 2. Positional asymmetries - much less than 100 nm.

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