Polarimetry for Qweak

Polarimetry for Polarimetry for QweakQweak

OverviewOverviewStatusStatusPlansPlans

S. Kowalski, M.I.T. (chair)

D. Gaskell, Jefferson Lab

R.T. Jones, U. Connecticut

Chuck Davis, incomingHall C Polarimetry WorkshopNewport News, June 9-10, 2003

Qweak Polarimetry Working Group:

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OverviewOverview

Phase I: Phase I: 8% measurement of A8% measurement of ALRLR

2% combined systematic+statistical error on polarization

sampling measurements with Moller polarimeter

Phase II: Phase II: 4% measurement of A4% measurement of ALRLR

1% systematic+statistical error on polarization

continuous running with Compton polarimeter, combined with periodic Moller samplings

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OverviewOverview: polarimetry goals for : polarimetry goals for QweakQweak

What statistic is relevant for quoting precision?What statistic is relevant for quoting precision?

ALR = + - -

+ + -

r± =( 1 ± P )

2+ + ( 1 + P )

2-

but in terms of measured rates r±

ALR = r+ - r-

r+ + r-

1P( )

the relevantquantityNote: P

P -1

=P

P(1 + 2P

P+ …)

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OverviewOverview: Polarimetry methods : Polarimetry methods for Qweakfor Qweak

Moller polarimeter for QweakMoller polarimeter for Qweak uses existing Hall C Moller spectrometer

incorporates fast kicker to enable operation at high beam currents – pulsed Moller operation

early tests demonstrate operation at 40A, development is ongoing [following slides]

impact on beam and hall backgrounds probably prevents simultaneous running with Qweak

statistics at 1% level obtained in ~40 min.

sub-percent systematic errors (based on experience with standard cw Moller operation at 1-2A)

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Status: the Hall C Moller Status: the Hall C Moller upgrade upgrade Existing Hall C Moller

can do 1% measurements in a few minutes.

Limitations:- maximum current ~10A- at higher currents the Fe target depolarizes due to target heating- measurement is destructive

Goals for the upgrade:- measure beam polarization up to 200A- make measurement quasi-continuously (not for Qweak)

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Target heating limits maximum pulse duration and duty factor

Instantaneous rate limits maximum foil thickness

This can be achieved This can be achieved with a 1 with a 1 m foil m foil

NNrealreal/N/Nrandomrandom≈10 at 200 ≈10 at 200 AA

Rather than moving continuously, beam will dwell at certain point on target for a few s

Status: tests with “half-target” Status: tests with “half-target” foilfoil

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tests by Hall C team during December 2004

measurements measurements consistent at the consistent at the ~2% level~2% level

random coincidence rates were larger than expected

– reals/randoms 10:1 at reals/randoms 10:1 at 4040AA

– mabe due to distorted mabe due to distorted edge of foiledge of foil

– runs at 40A frequently interrupted by BLM trips

Status: tests with 1Status: tests with 1m “half-m “half-target” foiltarget” foil

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Status: kicker + half-foil test Status: kicker + half-foil test summarysummary

Preliminary results look promising.Preliminary results look promising.

Source polarization jumps under nominal run conditions Source polarization jumps under nominal run conditions make it impossible to confirm ~1% stability.make it impossible to confirm ~1% stability.

Running at very high currents may be difficult – problem Running at very high currents may be difficult – problem may have been exacerbated by foil edge distortion.may have been exacerbated by foil edge distortion.

Development is ongoing.Development is ongoing.

Dave Meekins is thinking about improved foil mounting design.Dave Meekins is thinking about improved foil mounting design.

Future tests should be done when Moller already tuned and has Future tests should be done when Moller already tuned and has been used for some period of time so that we are confident we been used for some period of time so that we are confident we understand the polarimeter and polarized source properties.understand the polarimeter and polarized source properties.

The next step is to make 1% polarization measurements The next step is to make 1% polarization measurements at 80at 80A during G0 backward angle run.A during G0 backward angle run.

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ConfiguratConfigurationion Kick widthKick width PrecisionPrecision Max. Max.

CurrentCurrent

NominalNominal -- <1%<1% 2 2 AA

Prototype IPrototype I 20 20 ss few %few % 20 20 AA

Prototype IIPrototype II 10 10 ss few %few % 40 40 AA

G0 Bkwd. G0 Bkwd. (2006)(2006) 3.5-4 3.5-4 ss

Required: Required: 2%2% Goal: Goal: 1%1%

80 80 AA

QQWeakWeak 2 2 ssRequired: Required: 1%1% Goal: Goal: 1%1%

180 180 AA

Plans: kicker + half-foil Moller Plans: kicker + half-foil Moller R&DR&D

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11m foil with kicker should work fine at 1m foil with kicker should work fine at 1A A average current (instantaneous current average current (instantaneous current 180180A)A)

1% measurement will take ~3030 minutesminutes

Conservative heating calculations indicate Conservative heating calculations indicate foil depolarization will be less than 1% in the foil depolarization will be less than 1% in the worst case under these conditions – can be worst case under these conditions – can be checkedchecked

Compton being shaken down during this phase

Plans: operation during Qweak Plans: operation during Qweak phase Iphase I

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To reach 1% combined systematic and statistical To reach 1% combined systematic and statistical error, plans are to operate both Compton and error, plans are to operate both Compton and Moller polarimeters during phase II.Moller polarimeters during phase II.

Duration and frequency of Moller runs can be adjusted to reach the highest precision in average P-1

Can we estimate the systematic error associated Can we estimate the systematic error associated with drifts of polarization between Moller with drifts of polarization between Moller samplings?samplings?

Plans: operation during Qweak Plans: operation during Qweak phase IIphase II

Is there a worst-case model for polarization sampling errors?

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Moller performance during G0 Moller performance during G0 (2004)(2004)

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Plans: estimation of Moller sampling Plans: estimation of Moller sampling systematicssystematics

Worst-case scenario for samplingWorst-case scenario for sampling instantaneous jumps at unpredictable times model completely specified by just two

parameters

maximum effective jump rate is set by duration of a sampling measurement (higher frequencies filtered out)

unpredictability of jumps uniquely specifies unpredictability of jumps uniquely specifies the modelthe model

1. average rate of jumps2. r.m.s. systematic fluctuations in P

y

sampling

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Plans: estimation of Moller sampling Plans: estimation of Moller sampling systematicssystematics

model calculation

Monte Carlo simulation

Inputs:Inputs:

Pave = 0.70

Prms = 0.15

fjump =

1/10min T = 2000hr fsamp =

variable

Rule of Rule of thumb: thumb: Adjust the sample frequency until the statistical errors per sample match P.

sampling systematics only

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Short term plans (2006)Short term plans (2006)– Improve beamline for Moller and Moller Improve beamline for Moller and Moller

kicker operationkicker operation

Long term plans (2008)Long term plans (2008)– Install Compton polarimeterInstall Compton polarimeter

Longer term plans (12 GeV)Longer term plans (12 GeV)– Upgrade Moller for 12 GeV operation Upgrade Moller for 12 GeV operation

Plans: time line for Hall C Plans: time line for Hall C beamlinebeamline

Jlab view:these arenotindependent

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Overview: Compton design Overview: Compton design criteriacriteria

measure luminosity-weighted average measure luminosity-weighted average polarization over period of ~1 hour with polarization over period of ~1 hour with statistical error of 1% under Qweak statistical error of 1% under Qweak running conditionsrunning conditions

control systematic errors at 1% level

coexist with Moller on Hall C beamlinecoexist with Moller on Hall C beamline

be capable of operation at energies 1-11 GeV

fomfomstatstat ~~ E E22 (for same laser and current)(for same laser and current)

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Overview: the Compton Overview: the Compton chicanechicane

10 m

2 m

D1

D2 D3

D4

Comptondetector

Comptonrecoildetector

D

4-dipole design4-dipole design accommodates both gamma and recoil electron accommodates both gamma and recoil electron

detectiondetection nonzero beam-laser crossing angle (~1 degree)nonzero beam-laser crossing angle (~1 degree)

– important for controlling alignment– protects mirrors from direct synchrotron radiation– implies some cost in luminosity

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Alex BogaczAlex Bogacz (CASA) has found a way to fit (CASA) has found a way to fit the chicane into the existing Hall C the chicane into the existing Hall C beamline.beamline.– independent focusing at Compton and target

– last quad triplet moved 7.4 m7.4 m downstream

– two new quads added, one upstream of Moller and one between Moller arms

– fast raster moves closer to target, distance 12 m.

– beamline diagnostic elements also have to move

Focus with Focus with x x yy= 8m near center of = 8m near center of chicanechicane


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20


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3 configurations support energies up to 3 configurations support energies up to 11 GeV11 GeV

Beam energy bend B D xe (=520nm)

(GeV) (deg) (T) (cm) (cm)

1.165 10 0.67 57 2.4 2.0 1.16 4.1 2.5 1.45 5.0 2.5 4.3 0.625 25 2.2 3.0 0.75 2.6 6.0 1.50 4.9 4.0 2.3 0.54 13 1.811.0 1.47 4.5


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Plans: use of a crossing Plans: use of a crossing angleangle

assume a green laserassume a green laser

= 514 nm= 514 nm fix electron and laser fix electron and laser

foci at the same foci at the same pointpoint

= 100 = 100 mm emittance of laser emittance of laser

scaled by diffraction scaled by diffraction limitlimit

= M (= M (/ 4/ 4 scales like scales like 1/1/crosscross

down to scale of down to scale of beam divergencebeam divergence

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Overview: Compton Overview: Compton detectorsdetectors

Detect both gamma and recoil electronDetect both gamma and recoil electron– two independent detectors

– different systematics – consistency checks

Gamma – electron coincidenceGamma – electron coincidence– necessary for calibrating the response of gamma

detector

– marginally compatible with full-intensity running

Pulsed laser operationPulsed laser operation– backgrounds suppressed by duty factor of laser ( few

103 )

– insensitive to essentially all types of beam background, eg. bremsstrahlung in the chicane

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Plans: example of pulsed-mode Plans: example of pulsed-mode operationoperation

detectorsignal

signal gate

background gate

laseroutput

* pulsed design used by Hermes, SLD

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cannot count individual gammas because cannot count individual gammas because pulses overlap within a single shotpulses overlap within a single shot

QQ. . How is the polarization extracted?How is the polarization extracted?

AA.. By measuring theBy measuring the energy-weightedenergy-weighted asymmetry.asymmetry.

Consider the general weighted yield:

For a given polarization, the asymmetry in Y depends in general on the weights wwii used.

i

iw Y

Plans: “counting” in Plans: “counting” in pulsed modepulsed mode

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Problem can be Problem can be solved analyticallysolved analytically

wwii = A( = A(kk))

Solution is statistically optimal, maybe not for systematics.

Standard counting Standard counting is far from optimalis far from optimal

wwii = 1 = 1

Energy weight is better! wwii = = kk


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Define a figure-of-merit for a weighting schemeDefine a figure-of-merit for a weighting scheme

f (ideal) f (wi=1)> f (wi=k)

514nm 2260 9070 3160

248 nm 550 2210 770

193 nm 340 1370 480

N

fp )ˆ(V


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• Systematics of energy-weighted countingSystematics of energy-weighted counting– measurement independent of gamma detector

gain– no need for absolute calibration of gamma

detector– no threshold– method is now adopted by Hall-A Compton team

• Electron counter can use the same techniqueElectron counter can use the same technique– rate per segment must be < 1/shot– weighting used when combining results from

different segments


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Status: Monte Carlo Status: Monte Carlo simulationssimulations

Needed to study systematics fromNeeded to study systematics from– detector misalignment– detector nonlinearities– beam-related backgrounds

Processes generatedProcesses generated– Compton scattering from laser– synchrotron radiation in dipoles (with

secondaries)– bremsstrahlung from beam gas (with

secondaries)– standard Geant list of physical

interactions

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Monte Carlo simulationsMonte Carlo simulationsCompton-geantCompton-geant: based on original Geant3 program by Pat Welch: based on original Geant3 program by Pat Welch

dipole chicane

backscatter exit portgamma detector

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Monte Carlo simulationsMonte Carlo simulations

Example events (several events Example events (several events superimposed)superimposed)

electron beam

Compton backscatter (and bremsstrahlung)

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Monte Carlo simulationsMonte Carlo simulations

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Status: laser optionsStatus: laser options

1.1. External locked cavity (cw)External locked cavity (cw)– Hall A used as reference

2.2. High-power UV laser (pulsed)High-power UV laser (pulsed)– large analyzing power (10% at 180°)

– technology driven by industry (lithography)

– 65W unit now in tabletop size

3.3. High-power doubled solid-state laser High-power doubled solid-state laser (pulsed)(pulsed)

– 90W commercial units available

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laser l P Emax rate <A> t (1%)option (nm) (W) (MeV) (KHz) (%) (min)

Hall A 10641500 23.7 480 1.03 5

UV ArF 193 32 119.8 0.8 5.42 100

UV KrF 248 65 95.4 2.2 4.27 58

Ar-Ion (IC) 514 100 48.1 10.4 2.10 51

DPSS 532 100 46.5 10.8 2.03 54

Status: laser optionsStatus: laser options

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Status: laser Status: laser configurationconfiguration

two passes make up for losses in elementstwo passes make up for losses in elements– small crossing angle: 1small crossing angle: 1°°– effective power from 2 passes: 100 Weffective power from 2 passes: 100 W– mirror reflectivity: >99%mirror reflectivity: >99%– length of figure-8: 100 cmlength of figure-8: 100 cm

laser

electron beam

monitor

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Detector optionsDetector options

Photon detectorPhoton detector– Lead tungstate Lead tungstate – Lead glassLead glass– BGOBGO

Electron detectorElectron detector– Silicon microstripSilicon microstrip– Quartz fibersQuartz fibers

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SummarySummary• Qweak collaboration should have Qweak collaboration should have two independent two independent

methodsmethods to measure beam polarization. to measure beam polarization.• A Compton polarimeter would complement the Moller A Compton polarimeter would complement the Moller

and continuously monitor the and continuously monitor the average polarizationaverage polarization..• Using a Using a pulsed laser systempulsed laser system is feasible, and offers is feasible, and offers

advantages in terms of background rejection.advantages in terms of background rejection.• Options now exist that satisfy to Qweak requirements Options now exist that satisfy to Qweak requirements

with a with a green pulsed lasergreen pulsed laser, that use a simple two-pass , that use a simple two-pass setup.setup.

• Monte Carlo studies are underway to determine Monte Carlo studies are underway to determine tolerances on detector performance and alignment tolerances on detector performance and alignment required for required for 1% accuracy1% accuracy..

• Space obtained at Jlab for a laser test area, together with Hall A.

• Specs of high-power laser to be submitted by 12/2005.

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extra slides(do not show)

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Addendum: recent Addendum: recent progressprogress

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Addendum: recent Addendum: recent progressprogress

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Addendum: laser choicesAddendum: laser choices

• High-power green laser (100 W @ 532 nm)High-power green laser (100 W @ 532 nm)– sold by sold by Talis LaserTalis Laser– industrial applicationsindustrial applications– frequency-doubled solid state laserfrequency-doubled solid state laser– pulsed designpulsed design

• D. Gaskell: visit from D. Gaskell: visit from Talis LaserTalis Laser reps June reps June 20032003– not confident that they could delivernot confident that they could deliver– product no longer being advertised (?)product no longer being advertised (?)

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Addendum: laser choicesAddendum: laser choices• High-power UV laser (50 W @ 248 nm)High-power UV laser (50 W @ 248 nm)

– sold by several firmssold by several firms– industrial applications: industrial applications: micromachiningmicromachining and and

lithographylithography– excimer laser (KrF)excimer laser (KrF)– pulsed designpulsed design

• R. Jones: visit from R. Jones: visit from Lambda PhysikLambda Physik reps reps– sales team has good technical support sales team has good technical support – plenty of experience with excimer lasersplenty of experience with excimer lasers– strong interest in our applicationstrong interest in our application

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• Properties of LPX 220iProperties of LPX 220i– maximum power: 40 W (unstable resonator)maximum power: 40 W (unstable resonator)– maximum repetition rate: 200 Hzmaximum repetition rate: 200 Hz– focal spot size: 100 x 300 focal spot size: 100 x 300 m (unstable resonator)m (unstable resonator)– polarization: should be able to achieve ~90%polarization: should be able to achieve ~90%

• with a second stage “inverted unstable with a second stage “inverted unstable resonator”resonator”– maximum power: 50 Wmaximum power: 50 W– repetition rate unchangedrepetition rate unchanged– focal spot size: 100 x 150 focal spot size: 100 x 150 mm– polarization above 90%polarization above 90%

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• purchase cost for UV laser systempurchase cost for UV laser system– LPX-220i (list):LPX-220i (list): 175 k$175 k$– LPX-220 amplifier (list):LPX-220 amplifier (list): 142 k$142 k$– control electronics:control electronics: 15 k$ 15 k$– mirrors, ¼ wave plates, lenses:mirrors, ¼ wave plates, lenses: 10 k$ 10 k$

• cost of operation (includes gas, maintenance)cost of operation (includes gas, maintenance)– per hour @ full power:per hour @ full power: $35 (single)$35 (single)

$50 (with amplifier)$50 (with amplifier)– continuous operation @ full power:continuous operation @ full power: 2000 hours2000 hours

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Initial tests with kicker and an iron wire target performed in Dec. 2003

Many useful lessons learned– 25 mm wires too thick25 mm wires too thick

– Large instantaneous rate Large instantaneous rate gave large rate of random gave large rate of random coincidencescoincidences

– Duty factor too low – Duty factor too low – measurements would take measurements would take too long too long

Status: tests with iron wire Status: tests with iron wire targettarget

Polarimetry for Qweak

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