New Methods for Precision M ø ller Polarimetry Dave Mack Jefferson Lab ( for Dave Gaskell ) May 20, 2006 PAVI06 Precision M ø ller polarimetry Beam kicker studies for high current polarimetry Final design goals and future plans Other suggestions for improved M ø ller 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 (Q Weak, 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

M ø ller Polarimetry M ø ller Polarimeters measure electron beam via polarized electron-electron scattering At 90 degrees in the Center of Mass the analyzing power ( A Møller ) 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%) Detect scattered and recoil electrons Flip beam spin – measure asymmetry: A meas. ~ P Beam x P Target A M ø ller Target electron from iron or other easily magnetized atom

Hall C (Basel) M ø ller Polarimeter at JLab Jefferson Lab Hall C M ø ller 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 M ø ller 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 effect0.3% Spin Polarization in Fe0.25% Beam position0.16% Multiple Scattering0.12% Quad Setting0.12% Total0.47% Dominant Systematic Uncertainties

M ø ller Performance During G0 (2004)

Hall C M ø ller at High Beam Currents Typically, M ø ller 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 (or even 180!)  A Fe Foil Depolarization Operating Temp.  P ~ 1% for  T ~ deg. Is P 2  A = P 100  A ?

Kicker Magnet for High Current M ø ller 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 + M ø ller 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 EnclosureHall C Beamline Enclosure

Kicker and Iron Wire Target Initial tests with kicker and an iron wire target were performed in Dec Many useful lessons learned –25  m wires too thick –Large instantaneous rate gave large rate of random coincidences N coincidence ~ target thickness N random ~ (target thickness) 2 Nonetheless, we were able to make measurements at currents up to 20  A (large uncertainties from large random rates) Target built by Dave Meekins JLab 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 –N real /N random ≈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 1 st generation target – next target holder will reduce material and improve foil flatness

Kicker 2004 Measurements Run conditions –2  A on 4  m foil (nominal M ø ller 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 1  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” M ø ller 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 M ø ller 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 top kicker interval Current flow Magnetic field

M ø ller + Kicker Performance Configuration Kick width achieved PrecisionMax. Current Nominal-<1% 2  A Prototype I 20  s few % 20  A Prototype II 10  s few % 40  A G0 Bkwd. (2006)  s Required:2% Goal:1% 80  A Q Weak 2  s Required:1% Goal:1% 180  A

M ø ller 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 for one beam pulse

M ø ller 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 ~ H/cm 3 A 30 minutes at 30  A for Hall A M ø ller 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 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 Q weak – we will extend this 180  A Alternative methods for reaching high currents also being pursued –Pulsed beam measurements –Atomic Hydrogen targets

M ø ller Systematic Uncertainties (G0) SourceUncertaintyEffect on A(%) Beam position:x0.5 mm0.15 Beam position:y0.5 mm0.03 Beam angle: x0.15 mr0.04 Beam angle: y0.15 mr0.04 Q1 strength2%0.10 Q2 strength1%0.07 Q2 position1 mm0.02 Multiple Scattering10%0.12 Levchuk Effect10%0.30 Collimator Positions0.5 mm0.06 Target Temperature5 deg.0.2 Solenoid field direction2 deg.0.06 Spin polarization in Fe0.19%0.1 Target Warping2 deg.0.37 Leakage Current0.2 High Current Extrapolation1.0 Solenoid Simulation0.1 Electronic deadtime0.04 Charge asymmetry0.02 Total Uncertainty1.2

Spin Dance Checks of Polarimeters (2000)