pavi ’06 milos may 20, 2006 kent paschke – university of massachusetts controlling...

PAVI ’06 Milos May 20, 2006Kent Paschke – University of

Massachusetts

Controlling Helicity-Correlated Controlling Helicity-Correlated Asymmetries in a Polarized Asymmetries in a Polarized

Electron BeamElectron Beam

Kent PaschkeUniversity of Massachusetts, Amherst

Some slides adapted from G. Cates, PAVI ‘04


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World Data near QWorld Data near Q22 ~0.1 GeV ~0.1 GeV22

Caution: the combined fit is approximate. Correlated errors and assumptions not taken into account

Preliminary

~3% +/- 2.3% of proton magnetic moment

~0.2 +/- 0.5% of proton electric FF

~20 +/- 15% of isoscaler magnetic FF

HAPPEX-only fit suggests something even smaller:

GMs = 0.12 +/- 0.24

GEs = -0.002 +/- 0.017


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Precision goals for PVeS experimentsPrecision goals for PVeS experiments

• HAPPEX: A ~ 1 ppm

• A4: A ~ 300 ppb• G0: A ~ 300 ppb • HAPPEX-II He: A ~ 250 ppb • HAPPEX-II H: A ~ 100 ppb

• SLAC E158: A ~ 15 ppb• PREx: A ~ 15 ppb• QWeak : A ~ 5 ppb

• Moller at 12GeV (e2e)

JLAB

Generation

11

22

33

44

208Pb


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Helicity-Correlated Beam Helicity-Correlated Beam AsymmetriesAsymmetries

• Helicity-correlated intensity (charge) asymmetries

• Helicity-correlated position differences

This is what has been considered for 2nd generation (precision goals at the 10-7 level)See G.D. Cates, PAVI ’04 presentation.

• Helicity-correlated beam spot size asymmetries, x/x’ correlations… in general, higher-order helicity-correlated effects…

If detector is sufficiently symmetric, higher-order effects

will be dominant!

One needs to be careful to focus on the largest problems and develop systems for measuring, removing, and/or estimating corrections for higher order helicity-correlated beam parameters. (Not in this talk.)


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The Polarized eThe Polarized e-- Source Source

Optical Pumping:

HV Extraction and Injection

… of strained GaAs cathode produces highly-polarized e- beam.

Pockels Cell:Rapid Helicity Flip

Preparation of Preparation of Circularly-polarized lightCircularly-polarized light

HC beam asymmetries correspond to differences in preparation of circularly polarized laser light*.


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Feedback is effective… as far as that Feedback is effective… as far as that goesgoes

charge position

Helicity-correlated cut laser power to zero charge asymmetry

Helicity-correlated deflection by a piezoelectric-controlled mirror

This works, but these are heavy hammers for a subtle problem.

Does nothing to fix higher-order problems, may even create them.

Preferred strategy: configure system with care to minimize effects.

If you do it right, all problems get small together*!

If you do your best there, you can use feedback to go the last mile (or nanometer).

*At least, so you hope.

Charge and position feedback successful for G0 forward-angleFigures from K.Nakahara


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Fine Control of Beam Asymmetries in Laser Fine Control of Beam Asymmetries in Laser OpticsOptics

Cates et al., NIM A vol. 278, p. 293 (1989)

T.B. Humensky et. al., NIM A 521, 261 (2004)

G.D. Cates, Proceedings from PAVI ’04

Close Collaboration with the JLab Electron Gun Group in analyzing causes and developing solutions

Recent work:

Lisa Kaufman, Ryan Snyder, Kent Paschke,

T.B. Humensky, G.D. Cates


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Various Causes of Helicity-correlated Beam Various Causes of Helicity-correlated Beam ChangesChanges

• Steering effects – Pockels cell• Imperfect circularly polarized light

– Intrinsic birefringence of the Pockels cell– Other birefringent beamline elements (vacuum

window)– Phase gradient in beam before Pockels Cell– Laser divergence in the Pockels cell– Quantum Efficiency Anisotropy Gradient

• Beam element/helicity electronics pickup• Quantum Efficiency Variation (“QE holes”)• Cross-talk between different beams: cathode

effects or cross-talk in electron-beam transport

(partial list)


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Piezoelectric SteeringPiezoelectric Steering

The piezoelectric Pockels Cell acts as

“active” lens

Translation (inches)

X p

osit

ion

dif

f. (

um

)Y

pos

itio

n d

iff.

(u

m)

Red, IHWP Out

Blue, IHWP IN

Signature of steering:• scales with lever arm• not related to beam polarization• does cancel on slow reversal


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The simplest consequences of The simplest consequences of imperfectly circularly polarized lightimperfectly circularly polarized light

Right helicity Left helicity

Polarization Induced Transport Asymmetry (“PITA” effect)creating intensity (charge) asymmetry AQ

Perfect ±/4 retardation leads to perfect D.o.C.P.

A common retardation offset over-rotates one state, under-rotates the

other

This couples to “asymmetric transport” in the optics system to produce an intensity asymmetry.

22 )DoCP(1)DoLP( Significant DoLP with small change in DoCP

Now L/R states have opposite sign linear components.

In the photocathode, there is a preferred axis:

Quantum Efficiency is higher for light that is polarized along that axis

This is the phase


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Measuring analyzing powerMeasuring analyzing power

Voltage change of 58 Volts, added to both the + and - voltages, would zero the asymmetry.

A rotatable /2 waveplate downstream of the P.C. allows arbitrary orientation of DoLP

A simplified picture: asymmetry=0 corresponds

to minimized DoLP at analyzer

Perfect DoCP

Scanning the Pockels Cell Scanning the Pockels Cell voltage = scanning the voltage = scanning the retardation phase = retardation phase = scanning residual DoLPscanning residual DoLP

Inte

nsit

y A

sym

metr

y

(pp

m)

Pockels cell voltage offset (V)


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maximumanalyzingpower

minimumanalyzingpower

Intensity Asymmetry using RHWPIntensity Asymmetry using RHWP

4 term measures analyzing power*DoLP (from Pockels cell)

Ele

ctr

on

beam

in

ten

sit

y a

sym

metr

y

(pp

m)

Rotating waveplate angle


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What happens if there are phase What happens if there are phase gradients across the laser beam?gradients across the laser beam?

A gradient in the phase results in a DoLP gradient across the beamspot.

Medium charge asymmetry

Large

Medium

Small

Big charge asymmetry

Small charge asymmetry

Gradient in charge asymmetry creates a beam profiles with helicity-dependent centroid.

Same effect (Charge asymmetry gradient -> position difference) can be created by constant linear polarization but gradient in Cathode Analyzing Power


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Evaluating phase gradients and their Evaluating phase gradients and their effectseffects

Intensity asymmetry is proportional to the phase .

Position difference is roughly proportional to the derivative of the intensity asymmetry.

Spot size difference is roughly proportional to the derivative of the position difference.

Optics-table data looking at asymmetries while translating Pockels cell


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Position Differences using RHWPPosition Differences using RHWPEle

ctr

on

beam

posit

ion

d

iffere

nce (

mic

ron

)


4 term measures: analyzing power*(gradient in

DoLP)

+(gradient in analyzing

power)*DoLP

Position differences also follow “2/4” fit.

To minimize all effects, keep DoLP small and stay at small effective analyzing power

Large DoLP -> large position difference

-> Gradient in cathode analyzing power

Ele

ctr

on

beam

posit

ion

d

iffere

nce (

mic

ron

)

With Large voltage offset



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Beam Divergence and Fine Alignment Beam Divergence and Fine Alignment of Cellof Cell

Laser spot centroid difference, after linear polarizer (maximum “analyzing

power”)

Vert

ical p

osit

ion

diff

ere

nce (

m)

Yaw Angle (mrad)

IHWP OUT

IHWP IN

Simultaneous zero position differences for pitch and yaw angles (same for both waveplate states) can be found, representing best average alignment along optic axis.

Higher order: when alignment is complete, this effect will lead to “quadrapole” breathing mode of beam spot.

New!New!• Off-axis beam mixes index of refraction between optic and extraordinary axes• Divergent beam couples -phase to divergence angle• Beam divergence couples angle to position, resulting in a position-sensitive a position-sensitive -phase-phase


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Strategy for successStrategy for success• Well chosen Pockels cells and careful alignment

minimize effects. • Adjust RHWP to get small analyzing power.

– Not large, but not zero. You want to be able to tune DoCP on cathode to counteract vacuum window effect

• Adjust voltage to maximize DoCP on cathode• Use feedback on PC voltage to reduce charge

asymmetry.– Pockels cell voltage feedback maximizes circular

polarization, which is good for both “zeroth” AND higher orders

If you still care about the remaining position differences: use position feedback, keeping in

mind you may just be pushing your problem to the next highest order.

This technique is robust. <300 nm position differences in injector for HAPPEX-H setup, and for G0 back-angle setup

(same algorithm for optics alignment, different personnel)!


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Beam Position Differences, Helium 2005Beam Position Differences, Helium 2005HC beam asymmetries correspond to differences in preparation of circularly polarized laser light*.

*unless you decide to add helicity information to the electron beam after it is generated from the cathode

Problem: Helicity signal deflecting the beam through electronics “pickup”

Large beam deflections even when Pockels cell is off

Helicity signal to driver reversed

Helicity signal to driver removed

• Problem clearly identified as beam steering from electronic cross-talk

• Tests verify no helicity-correlated electronics noise in Hall DAQ at sub ppb level

• Large position differences mostly cancel in average over both detectors, cancels well with slow reversal

X Angle BPM

Raw ALL Asymetry

mic

ron

pp

m

AT 3 GeV!


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Injector Position Differences for Injector Position Differences for 2005 HAPPEX-H2005 HAPPEX-H

Ele

ctr

on

beam

vert

ical

posit

ion

diff

ere

nce

(mic

ron

)

Location in Injector

After configuration:

position differences in injector had maximum around 200 nanometers

Additional suppression from slow reversal and adiabatic damping

200 nm

-200 nm


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Adiabatic Damping Adiabatic Damping

Lower Energy

Higher Energy

position

angle

95keV 335

GeV 3

Area of beam distribution in the phase space (emittence) is

inversely proportional to momentum.

From 100 keV injection energy to 3 GeV at target, one expects helicity-correlated position differences to get smaller

The critical parameter in position difference isn’t sqrt(emittence)

The projection along each axis is sensitive to coupling.

Good match

Bad match

angle

If the coupling develops, it is difficult to remove…

To take advantage of adiabatic damping, keep machine close to design to minimize undesired

correlations.

Design transport

Bad match to design transport

X

X’


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Taking Advantage of Phase Space ReductionTaking Advantage of Phase Space Reduction

Major work invested to controlling beam transport as designed (Yu-Chiu Chao)

• Transport matching design (linacs & arcs) now routine.• Improvements in the 5MeV injector major step forward• Configuration very stable over 2+ months• Next battle: 100 keV injector

X-PZT(Source)

Y-PZT(Source)

X-BPM (mm) Y-BPM (mm)

1C-Line

X-BPM (mm) Y-BPM (mm)

1C-Line

1C-Line

1C-Line

without

with

Factor between 5-30 observed

during HAPPEX-H


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Hydrogen 2005 position differencesHydrogen 2005 position differencesxx x’x’

y’y’ yy

mic

ron

mic

ron

mic

ron

mic

ron

4*p

pm

• Degradation of source setup at end of run but good adiabatic damping

Run Averaged:Energy: -0.25 ppbX Target: 1 nmX Angle: 2 nmY Target : 1 nmY Angle: <1 nm


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What is needed for the futureWhat is needed for the future

• The next generation experiments at JLab (QWeak and PREx) will increase demand to understand and control higher order effects.

• Significant progress has been made by thoroughly understanding the origins of the effects.

• Continued empirical work is critical.• Need to focus on passive suppression, while

exploring what might be gained through (the right kind of) feedback.

• Improvements in beam diagnostics and sensitivity measurements will be required. This may involve new hardware... and new thinking.


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BackupBackup


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Non-linearity in Beam Corrections Non-linearity in Beam Corrections

Magnitude depends on product of: Nonlinear response in apparatus,

and Size modulation at frev in beam <xi

2>

No significant JLab bounds on Δ<I2>, Δ<E2>, Δ<X2>, Δ<X’2>, Δ<Y2>, or Δ<Y’2>. (“Significant” means they haven’t been proven to be smaller than 1 ppm.)

Xi Xi

Simple breathing .

Same <x>, <I>,

Different <x2>

Interaction between scraping and intensity feedback.

Same <x>, <I>,

Different <x2>

time

Differential intensity bounce. Same <x>, <I>,

Different <I 2>

Based on slides by Dave Mack

Examples of currently invisible <xi2>:

How big are these terms?

Do these effects cancel under half-wave plate reversal?

(also, no significant JLab bounds on the 15 unique Δ<xixj>.)


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Phase TrombonePhase Trombone

Constraints:• Preserve beam size at the location of the Compton polarimeter• Preserve large dispersion at center of arc• Preserve ability to independently vary spot size at target

horizontal phase advanced by 60o

while vertical stays fixed

Figures from Beck, PAVI’04

•Goal: vary beta phase• implemented with eight existing quads at the beginning of the Hall A arc• Allows for independent beta fcn phase control in horizontal and vertical planes

• Uses:• Allows one to trade off position and angle differences (10:1 scale between size in accelerator and senstivity for experiment)• Periodic phase changes can be used to randomize or reverse the sign of position differences


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PhaseTrombone, Results from First Test in Hall PhaseTrombone, Results from First Test in Hall AA

Phase TromboneSetpoint (x , y)

x (m)0.3 m

y (m)0.3 m

x(rad)0.01 rad

y (rad)0.02 rad

(0o,0o) 2.9 2.0 -0.08 -0.19

(30o,0o) 2.7 1.2 -0.07 -0.22

(-30o,0o) 2.8 3.2 -0.07 -0.16

(30o,30o) 1.0 1.2 -0.12 -0.21

Data from 2004 (Bogacz and Paschke):

Promising approach, but not applied in 2005Promising approach, but not applied in 2005• “Local” phase trombone undone by over contraints (too few independent quads)

• “Linac” phase trombone promising, but brief test was ambiguous. Diagnositics are probably insufficient.

• Electronics pickup made tests uninterpretable


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Polarized beam without PCPolarized beam without PC

Fast RF phase shifter

/2

/2

/4

atten

attensteering mirror

Fiber-based laser

p-polarized

s-polarized

s and ppolarized

60 degree optical delay line

Fast phase shifter moves beam IN/OUT of slit;Downside: extract 2x required beam current

Slide from M.Poelker


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Position differences at the end of H-Position differences at the end of H-20052005


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Improved measurement of high-Improved measurement of high-frequency beam parametersfrequency beam parameters

Calculated response of slow beam monitors with fast raster

What are the implications of such non-linear response for false asymmetries, or normalization?

pavi ’06 milos may 20, 2006 kent paschke – university of massachusetts controlling...

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