New Methods for Precision Mller Polarimetry Dave Mack

New Methods for Precision Møller Polarimetry Dave Mack Jefferson Lab (for Dave Gaskell) May 20, 2006 PAVI 06 • 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 Target electron from iron or scattering other easily magnetized atom Flip beam spin – measure asymmetry: Ameas. ~ PBeam x PTarget AMøller Detect scattered and recoil electrons • 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%)

Hall C (Basel) Møller Polarimeter at JLab • Jefferson Lab Hall C Møller replaces inplane 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 Dominant Systematic Uncertainties 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%

Møller Performance During G 0 (2004)

Hall C Møller at High Beam Currents • Typically, Møller data are taken (during dedicated runs) at 1 -2 m. A • Higher currents lead to foil depolarization – Require depolarization effects <<1% – This limits us to a few m. A • However, experiments run at currents of 20 -100 (or even 180!) m. A Is Pe @ 2 m. A = Pe @ 100 m. A ? Fe Foil Depolarization DP ~ 1% for DT ~ 60 -70 deg. Operating Temp.

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 mm 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 m. A (large uncertainties from large random rates) Target built by Dave Meekins JLab Target Group

Tests With a 1 mm “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 mm foil – Nreal/Nrandom≈10 at 200 m. A • Replaced iron wire target with a 1 mm thick iron “strip” target • Conducted more tests with this target and slightly upgraded kicker in December 2004 • Note: this is 1 st generation target – next target holder will reduce material and improve foil flatness

Kicker 2004 Measurements • Run conditions – 2 m. A on 4 mm foil (nominal Møller run conditions), kicker on and off – Kicker runs at 10, 20, and 40 m. A – Beam (machine protection chamber) trips prevented us from running at higher currents • Required average current on target less than 1 m. A 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 m. 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 ms rather than continuously move across the foil • To reach the very highest currents, the kick duration must be as small as 2 ms to keep target heating effects small • The 1 mm target is crucial – we need to improve the mounting scheme to avoid wrinkles and deformations

Kicker R&D Current flow Magnetic field Quasi-flat top kicker interval “Two turn” kicker – 2 ms total dwell time!

Møller + Kicker Performance Configuration Kick width achieved Precision Max. Current Nominal - <1% 2 m. A Prototype I 20 ms few % 20 m. A Prototype II 10 ms few % 40 m. A G 0 Bkwd. (2006) 3. 5 -4 ms QWeak 2 ms Required: 2% Goal: 1% Required: 1% Goal: 1% 80 m. A 180 m. A

Møller Polarimetry Using “Pulsed” Beam • The electron beam at JLab can be run in “pulsed” mode Target Heating vs. Time for one beam pulse – 0. 1 -1 ms pulses at 30 to 120 Hz – Low average current, but for the duration of the pulse, same current as experiment conditions (10 s of m. A) • Using a raster (25 k. Hz) to blow up the effective beam size, target heating can be kept at acceptable levels Figure courtesy of E. Chudakov

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 m. K – 5 -8 Tesla field separates the low and high ( states – Density ~ 3 1015 H/cm 3 ) energy • A <1% (stat) measurement can be done reasonably quickly -> 30 minutes at 30 m. A for Hall A Møller • Proposed by E. Chudakov for use in Hall A

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 G 0 Backward Angle run • Our goal is to measure the current dependence of the polarization to 1% (up to ~80 m. A) during G 0 Backward Angle run • For Qweak – we will extend this 180 m. A • Alternative methods for reaching high currents also being pursued – Pulsed beam measurements – Atomic Hydrogen targets

Møller Systematic Uncertainties (G 0) 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 Q 1 strength 2% 0. 10 Q 2 strength 1% 0. 07 Q 2 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)