Transcript ppt - IASA

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
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Outline
•
•
•
•
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The problem
HC-intensity asymmetry
Sources of other HC-fluctuations
Low energy polarimetry
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Polarized source tasks
1.) Reliable beam production at desired intensity level
2.) Provide desired spin orientation
3.) High Polarization (>80%)
I.) Polarization meas.
II.) HC-control
Necessary, but not specific for PVExperiment.
DA/A ~DP small 
limiting factor in several PV-exp.
A always important
limiting especially when A<10-6
Source team can provide support for point (I),
(II) is more important.
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• Scattering experiments (simplified)
D measures R+-
q
+
S
T
A
Let x be a vector formed from the relevant parameters:
  
R  f (x )  




x  ( I , x, y, x' , y '......)T  ( xi )
Aexp
f ( I ....)  f ( I ....)


 Aexp
R  R
 
R  R
     e. g .PV
 
 PZ * S Z

 
measure P accurately! (I)
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What if
?

df ( x ) 

(
x

x

i
i )


dxi
R R
i
 
 PS 

R R
f ( x)
{
Aexp
f  ( I ....)  f  ( I ....)
:=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).
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Source Set-up
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HC-Control schematics (PVA4)
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(Almost) no active HC-compensation, except by stabilization!
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Important example: Intensity-HCA (I-HCA)
Sketch of polarization optics
Adjust to zero crossing &
Observe stability!
Result measuring I-HCA=(I+-I-)/(I++I-)
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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:
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 S0 
 S0 
 
 
S
 1
 S1 


M
*
M
*
M
*
M
*
ISR
optics
comp
Pock 
S 
S2 
2
 
 
S 
S 
 3  Kath
 3 in
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For ‘thin’ cathodes: I+- ~ S+-0
Expand Matrix elements to first order in the imperfections:
Predicted I-HCA as function of compensator rotation angle 
I I
 AISR ( A   3 ) sin( 2q K  4 )  C sin( 2q K  2 )  D cos( 2q K   )) 


I I
AISR=Analysing power of cathode, with polarizer axis oriented at 2qk,
 Measured for several high P cathodes: AISR=0.02-0.05
a= f++f-/p: Normalized asymmetric phase shift of pockels cell
(forced zero crossing!),a=0.03 (typ.)
3=circular stokes component of light at input of Pockels cell,
 Not measured, est. to <0.003
~0.999 = diagonal polarisation component at input of Pockels cell,
c= deviation of half wave plate from 180 degree retardation
0.01 (quote by company)
D,=function of birefringence of optical elements between PC and
cathode.
(measurable, D~0.01)
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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.
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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:
DIHCA 1
 50 ppm / K
DT
G
Realistic only if second order effects (HC-Transmission changes) do not occur
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Compensating the offset term
Offset/(4 amplitude) while varying qk:
{
I I
 AISR ( A   3 ) sin( 2q K  4 )  C sin( 2q K  2 )  D cos( 2q K   )) 


I I
Offers to reduce
Problem by order(s)
of magnitude….But
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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?
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‘ideal’ Experiment
Pulser
Lockin
Sw.
He/Ne Laser
PC
Detektor with
low analysing power
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….
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Screen
Prism
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Backreflexions for the different helicity states.
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
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Intensity asymmetry in laser beam
Pulser
Lockin
Sw.
diode-laser
PC
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Movable Detektor
with pinhole
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Helicity correlated movement of centroid is 1mm.
Causes for HC-fluctuations
Parameter
HCA at source
HCA at target
dominating
Cause
intensity
0-4000ppm
0-4000ppm
ISR
position
100nm
~10nm
Interference
angle
-
10-8 rad
Phase space
transformation
energy
-
Few eV
Phase space
transf.
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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!
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Moderately ambitious approach:
Mott polarimeter at 3.5 MeV
• Goal 1: fast relative measurement at full
current with good reproducibility
• Goal 2: accuracy < 2%
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3.5 MeV Mottpolarimeter
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Measurement time < 2min @1% stat. Acc. @20 mA
Beam installation time req: (40min) will be reduced to <15min.
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Asymmetry vs. Spin rotator angle (164 Grad)
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8 hour measurement of asymmetry
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Analyzing power calculation
Theo:
Z=79
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)
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Analyzing power can be calculated with less than 1% accuracy
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Double scattering effects
Energy variation
at fixed scattering
angle
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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.
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Summary
1. 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
3. light optical effects are rather complicated but ‘treatable’
4. Better understanding + technology offers potential
to keep situation acceptable also for future exp.
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