Precision M ø ller polarimetry Beam kicker studies for high current polarimetry

New Methods for Precision Møller Polarimetry

Dave Mack Jefferson Lab

(for Dave Gaskell)

May 20, 2006

PAVI06 • Precision Møller polarimetry • Beam kicker studies for high current polarimetry• Final design goals and future plans• Other suggestions for improved Møller polarimetry

Precision Polarimetry • The Standard Model is remarkably successful – but can’t

be the whole story (too many free parameters)

• To search for physics beyond the Standard Model we either need to make

– Measurements at higher energies or,

– Measurements at higher precision -> JLAB

• Knowledge of beam polarization is a limiting systematic in precision Standard Model tests (QWeak, parity violation in Deep Inelastic Scattering )

– Experiments require 1% (or better) polarimetry

• Other, demanding nuclear physics experiments (strange quarks in the nucleon, neutron skin in nuclei) also benefit from precise measurements of beam polarization

Møller Polarimetry

• Møller Polarimeters measure electron beam via polarized electron-electron scattering

• At 90 degrees in the Center of Mass the analyzing power (AMøller) is large = -7/9

• Dominant systematic uncertainty comes from knowledge of target polarization (often use “supermendur” foils in low magnetic fields – systematic uncertainty ~2-3%)

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Detect scattered and recoil electrons

Flip beam spin – measure asymmetry: Ameas. ~ PBeam x PTarget AMøller

Target electron from iron orother easily magnetized atom

Hall C (Basel) Møller Polarimeter at JLab

• Jefferson Lab Hall C Møller replaces in-plane target polarized with low magnetic fields with pure iron polarized out of plane using 4 Tesla solenoid

• Spin polarization in Fe well known, target polarization measurements not needed

• Can use Kerr Effect measurements to verify that Fe is saturated

• Target polarization known to <0.3%

Hall C Møller Polarimeter Properties

• 2-quadrupole optics maintain same event distribution at detector planes (fixed optics)

• Coincidence electron detection to suppress Mott backgrounds, large acceptance to reduce corrections due to Levchuk effect

• Total systematic uncertainty ~ 0.5% (at low currents)• For experiments that run at high currents, extrapolation to

nominal running current still an issue

Levchuk effect 0.3%

Spin Polarization in Fe 0.25%

Beam position 0.16%

Multiple Scattering 0.12%

Quad Setting 0.12%

Total 0.47%

Dominant Systematic Uncertainties

Møller Performance During G0 (2004)

Hall C Møller at High Beam Currents

• Typically, Møller data are taken (during dedicated runs) at 1-2 A

• Higher currents lead to foil depolarization

– Require depolarization effects <<1%

– This limits us to a few A

• However, experiments run at currents of 20-100 (or even 180!) A

Fe Foil Depolarization

Operating Temp.

P ~ 1% for T ~ 60-70 deg.Is Pe @ 2 A = Pe @ 100 A ?

Kicker Magnet for High Current Møller Polarimetry

• We can overcome target heating effects by using a fast kicker magnet to scan the electron beam across an iron wire or strip target

• Kicker needs to move beam quickly and at low duty cycle to minimize time on iron target and beam heating

• First generation kicker was installed in Fall 2003 (built by Chen Yan, Hall C)

Kicker + Møller Layout

• Kicker located upstream of Møller target in Hall C beam transport arc

• Beam excursion ~ 1-2 mm at target

• The kick angle is small and the beam optics are configured to allow beam to continue cleanly to the dump

Accelerator Enclosure Hall C Beamline Enclosure

Kicker and Iron Wire Target• Initial tests with kicker and an

iron wire target were performed in Dec. 2003

• Many useful lessons learned– 25 m wires too thick– Large instantaneous rate

gave large rate of random coincidences

Ncoincidence ~ target thickness

Nrandom ~ (target thickness)2

• Nonetheless, we were able to make measurements at currents up to 20 A (large uncertainties from large random rates)

Target built byDave MeekinsJLab Target Group

Tests With a 1 m “Strip” Target• The only way to keep

random coincidences at an acceptable level is to reduce the instantaneous rate

• This can be achieved with a 1 m foil

– Nreal/Nrandom≈10 at 200 A

• Replaced iron wire target with a 1 m thick iron “strip” target

• Conducted more tests with this target and slightly upgraded kicker in December 2004

• Note: this is 1st generation target – next target holder will reduce material and improve foil flatness

Kicker 2004 Measurements• Run conditions

– 2 A on 4m foil (nominal Møller run conditions), kicker on and off

– Kicker runs at 10, 20, and 40 A– Beam (machine protection ion chamber) trips prevented

us from running at higher currents• Required average current on target less than 1A

to minimize target heating• Measured polarization was reasonably consistent

for all configurations but:– Charge asymmetries were quite large, sometimes 1%!– Some instability, even for “nominal” Møller

configurations (no kicker) – this may be linked to less than optimal laser beam position on polarized source

December 2004 Kicker Test Results

• Short test – no time to optimize polarized source– Tests cannot be used to

prove 1% precision

• Took measurements up to 40 A– Ion chamber trips

prevented us from running at higher currents

– Lesson learned: need a beam tune that includes focus at Møller target AND downstream

• Demonstrated ability to make measurements at high currents – good proof of principle

Optimized Kicker with “Half-Target”

• The ideal kicker would allow the beam to dwell at a certain point on the target for a few s rather than continuously move across the foil

• To reach the very highest currents, the kick duration must be as small as 2 s to keep target heating effects small

• The 1 m target is crucial – we need to improve the mounting scheme to avoid wrinkles and deformations

Kicker R&D

“Two turn” kicker – 2 s total dwell time!

Quasi-flat topkicker interval

Current flow

Magnetic field

Møller + Kicker Performance

ConfigurationKick width achieved

Precision Max. Current

Nominal - <1% 2 A

Prototype I 20 s few % 20 A

Prototype II 10 s few % 40 A

G0 Bkwd. (2006)

3.5-4 sRequired:2%Goal:1%

80 A

QWeak 2 sRequired:1%Goal:1%

180 A

Møller Polarimetry Using “Pulsed” Beam

• The electron beam at JLab can be run in “pulsed” mode– 0.1-1 s pulses at 30 to

120 Hz– Low average current,

but for the duration of the pulse, same current as experiment conditions (10s of A)

• Using a raster (25 kHz) to blow up the effective beam size, target heating can be kept at acceptable levels

Figure courtesy of E. Chudakov

Target Heating vs. Time forone beam pulse

Møller Polarimetry with Atomic Hydrogen Targets

• Replace Fe (or supermendur) target with atomic hydrogen– 100% electron polarization– No Levchuk effect, low Mott background (compared to

iron)– Allows high beam current and continuous measurement

• Atomic Hydrogen Target– Stored in a trap at 300 mK– 5-8 Tesla field separates the low and high ( )

energy states– Density ~ 3 1015 H/cm3

• A <1% (stat) measurement can be done reasonably quickly -> 30 minutes at 30 A for Hall A Møller

• Proposed by E. Chudakov for use in Hall A

B

Summary• Fast kicker magnet and thin iron foil target will allow very precise

(1% syst.) measurements of the beam polarization at full experiment beam current

• R&D is progressing well

– The 2 test runs we’ve had so far have been invaluable in getting the system ready for prime time

– Next round of tests will be during G0 Backward Angle run

• Our goal is to measure the current dependence of the polarization to 1% (up to ~80 A) during G0 Backward Angle run

• For Qweak – we will extend this 180 A

• Alternative methods for reaching high currents also being pursued– Pulsed beam measurements

– Atomic Hydrogen targets

Møller Systematic Uncertainties (G0)Source Uncertainty Effect on A(%)

Beam position:x 0.5 mm 0.15

Beam position:y 0.5 mm 0.03

Beam angle: x 0.15 mr 0.04

Beam angle: y 0.15 mr 0.04

Q1 strength 2% 0.10

Q2 strength 1% 0.07

Q2 position 1 mm 0.02

Multiple Scattering 10% 0.12

Levchuk Effect 10% 0.30

Collimator Positions 0.5 mm 0.06

Target Temperature 5 deg. 0.2

Solenoid field direction 2 deg. 0.06

Spin polarization in Fe 0.19% 0.1

Target Warping 2 deg. 0.37

Leakage Current 0.2

High Current Extrapolation 1.0

Solenoid Simulation 0.1

Electronic deadtime 0.04

Charge asymmetry 0.02

Total Uncertainty 1.2

Spin Dance Checks of Polarimeters (2000)