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

1. New Methods for Precision Møller PolarimetryDave Mack Jefferson Lab(for Dave Gaskell)May 20, 2006PAVI06 Precision Møller polarimetry Beam kicker studies for high current polarimetry Final design goals and future plans Other suggestions for improved Møller polarimetry

2. 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

3. Møller Polarimetry • Møller Polarimeters measure electron beam via polarized electron-electron scattering Target electron from iron or 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%)

4. 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%

5. 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

6. Møller Performance During G0 (2004)

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

8. 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)

9. 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

10. 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 mA (large uncertainties from large random rates) Target built by Dave Meekins JLab Target Group

11. 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 mA • 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 1st generation target – next target holder will reduce material and improve foil flatness

12. Kicker 2004 Measurements • Run conditions • 2 mA on 4mm foil (nominal Møller run conditions), kicker on and off • Kicker runs at 10, 20, and 40 mA • Beam (machine protection ion chamber) trips prevented us from running at higher currents • Required average current on target less than 1mA 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

13. 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 mA • 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

14. 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

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

16. Møller + Kicker Performance

17. Møller Polarimetry Using “Pulsed” Beam Target Heating vs. Time for one beam pulse • The electron beam at JLab can be run in “pulsed” mode • 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 (10s of mA) • 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

18. 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 mA for Hall A Møller • Proposed by E. Chudakov for use in Hall A

19. 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 mA) during G0 Backward Angle run • For Qweak – we will extend this 180 mA • Alternative methods for reaching high currents also being pursued • Pulsed beam measurements • Atomic Hydrogen targets

20. Møller Systematic Uncertainties (G0)

21. Spin Dance Checks of Polarimeters (2000)