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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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slide1

Helicity-Correlated effects for SAMPLE Experiment

  • M.Farkhondeh, B.Franklin, E. Tsentalovich, T.Zwart
  • MIT-Bates Linear Accelerator Center
  • Middleton, MA 01949, USA
slide2

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
positional asymmetries

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.)

slide7

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

slide8

/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

analyzing power
Analyzing power 
  • Typical laser transport system 
  • Strained GaAs crystal (normal incidence)  5-10%
  • GaAs crystal with the incident angle of  40  20%
tjnaf happex results
TJNAF (HAPPEX) results

Strained GaAs crystal, normal incidence

bates results
Bates results

Strained GaAs crystal, angle of incidence 37

cross asymmetries
Cross-asymmetries

Loading effects

in accelerator

 (current)

 (energy)

Dispersion

Differential

scraping

 (current)

 (position)

more complications

Energy

time

x

x

More complications...

- time dependence during the pulse

- beam size asymmetry

- beam shape asymmetry

handling helicity correlated asymmetries
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

intensity feedback

V

Intensity feedback

Polarizer

/4 Pockels cell

V(PC)

V100 V

slide17

 10 V

Correction Pockels cell

Polarizer

/4 Pockels cell

V0~400 V

Slope ~50 ppm/V

Adjustable with V0

slide18

 10 V

Correction Pockels cell

/4 Pockels cell

Polarizer

/10

Slope ~50 ppm/V

Adjustable with /10 angle

t zwart s stabilizer

Pockels cell

Polarizer

Laser light

GaAs

V0

Set point

Toroid

Electron

beam

T. Zwart’s stabilizer
slide21

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

slide22

SAMPLE -1999

/2 OUT: Average = 4.45  1.51 nm

/2 in: Average = -2.60  1.44 nm

summary
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.